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Ultralow-frequency ultranarrow-bandwidth coherent terahertz imaging for nondestructive testing of mortar material

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Abstract

Nondestructive testing of concrete materials is essential in civil engineering to maintain social infrastructure such as buildings or bridges. In this study, we constructed an ultralow-frequency, ultranarrow-bandwidth, coherent terahertz (THz) imaging system based on THz time-domain spectroscopy (THz-TDS). Based on its ultralow-frequency-localized THz wave and coherent detection, the present system achieved a wide dynamic range of THz power over 100 dB at 0.046 THz, which is appropriate to measure the mortar material. The achieved dynamic range of the THz power was 59 dB larger than that of a commercialized THz-TDS system and 49 dB larger than that of an ultralow-frequency noncoherent THz imaging system equipped with a high-power electric THz source. Ultimately, the proposed system could visualize the inner structure of a mortar sample with a thickness of 10 mm, and the present system can investigate a mortar sample with a thickness of over 130 mm. The proposed method is an attractive tool for non-destructive testing of thick concrete structures characterized by non-invasiveness and non-contact remoteness.

© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

1. Introduction

Concrete structures are widely used for social infrastructures such as roads, bridges, tunnels or buildings, and is the basis of daily life and industrial activities. Consequently, many of them were built in developed countries during the 1950s to 1970s, which are now aging. However, reconstructing all such structures is realistically challenging. Thus, the continued use of the structures is required along with performing maintenance, as a long lifespan of the structures can be ensured if the damage is detected and repaired at an early stage. Thus, the detection of structural defects is vital for protecting them from fatal collapse.

Conventional nondestructive testing (NDT) for concrete structures includes hammering tests, X-ray inspection, ultrasonic inspection, and infrared thermography [17]. In the hammering test, defects are inspected by distinguishing the variations in the sound produced upon striking an object. Although this test can be conveniently performed, it is subject to contact measurement and the accuracy depends on the skill of the inspector. In contrast, X-ray inspection is advantageous for highly transparent materials, but the harmful exposure should be avoided during outdoor inspection. In context, ultrasonic inspection is a common NDT technique that is less invasive than alternative methods; however, it is based on contact measurements that prevent remote inspection. Additionally, infrared thermography provides noncontact and less invasive measurements using a thermal source and an infrared camera. Nonetheless, this method allows a limited probing depth around the superficial layer.

Recently, terahertz (THz) NDT [8] has garnered attention in the investigation of concrete structures owing to its contactless, non-invasive nature, its moderate transparency, and the possibility for spectroscopic material analysis. For instance, Tanabe et al. investigated a concrete sample with an ultralow-frequency THz wave, which reduced its attenuation in a concrete material containing water [9]. An electric continuous-wave (CW) THz source (GUNN diode, frequency = 0.060 THz) and an incoherent THz detector (Schottky barrier diode) was used to record the point measurement of water content for a 13.5-mm-thick concrete sample. Although such the electric CW THz source benefits from high power, the incoherent THz detector may limit the dynamic range of the THz power and, consequently, reduce the measurable maximum thickness of the concrete sample. Thus, the dynamic range of the THz power should be further increased to expand the scope of THz measurements for thicker and/or more absorptive concrete samples. To this end, the THz time-domain spectroscopy (THz-TDS) [10] is a potential candidate owing to its wide dynamic range of THz power that benefits from the coherent detection of pulsed THz waves as well as various imaging modes such as transmission imaging [11], cross-sectional imaging [1214], and spectral imaging [15,16]. However, typical THz-TDS imaging systems cover a spectral range of 0.1–5 THz, excluding a specific spectral range that is transparent to the concrete materials (typically under 0.1 THz). Moreover, the use of ultralow-frequency-localized THz waves in THz-TDS will be an effective NDT method for concrete materials. Previously, an ultralow-frequency, ultranarrow-bandwidth THz-TDS system was constructed based on a coherent emitter and a detector of bowtie-shaped photoconductive antennas (PCAs) for the sensitive measurement of water content in dry materials [17]. Recently, Tripathi et al. demonstrated a similar THz-TDS to investigate the inner content of chloride ions in concrete samples with thicknesses up to 5 mm [18]. Although a measurable maximum depth in THz-NDT is vital for investigating structural materials, it has not been quantitatively investigated.

In this study, we constructed an ultralow-frequency, ultranarrow-bandwidth THz-TDS imaging system to visualize a thick concrete material. In particular, the coherent THz wave was significantly localized below 0.1 THz by using a pair of bowtie-shaped PCAs for a THz emitter and detector, which is appropriate to measure the mortar sample. To demonstrate the advantages of the constructed system in THz-NDT of a concrete material, we compared its dynamic range of THz power with that of an ultralow-frequency non-coherent CW THz system as well as a commercialized THz-TDS system. Furthermore, the metal inserts of a 10-mm-thick mortar sample were imaged as well.

2. Results

2.1 Basic performance of the ultralow-frequency, ultranarrow-bandwidth THz-TDS imaging system

Figure 1 illustrates a schematic of the experimental setup for the ultralow-frequency ultranarrow-bandwidth THz-TDS imaging system, which is described in the Methods section together with details on the experimental methodology employed for the following measurements. Initially, the temporal and spectral characteristics of the ultralow-frequency, ultranarrow-bandwidth THz-TDS imaging system were investigated without a sample. The time transient of the pulsed THz electric field acquired with a time-window size of 216 ps and a sampling time interval of 422 fs is presented in Fig. 2(a) (lock-in time constants = 100 ms). The corresponding THz power spectrum at a spectral resolution of 4.6 GHz is indicated as red plots in Fig. 2(b), wherein a spectral peak appeared at 0.046 THz with a spectral bandwidth of 28 GHz. The spectral peak frequency corresponded well with the inverse of the time period in the dominant oscillation ( = 22 ps) depicted in Fig. 2(a). Owing to the ultralow-frequency and ultranarrow-bandwidth generation and detection by the bowtie-shaped PCAs, the majority of the THz power spectrum was concentrated at frequencies below 0.1 THz. This spectral characteristic is complementary to that of the typical THz-TDS imaging system equipped with dipole-shaped PCAs covering a spectral range of 0.1–5 THz. For comparison, the pump beam toward the THz emitter was blocked to measure the noise spectrum and evaluate the dynamic range of the THz power of this system, as represented by the blue plots in Fig. 2(b). The dynamic range was defined as the ratio of spectral peak and averaged noise power in frequency domain which averages the fluctuation of the noise power. More importantly, the spectral peak achieved a dynamic range of THz power over 87 dB at lock-in time constants of 10 ms. Furthermore, a dynamic range of THz power was further enhanced over 107 dB at lock-in time constants of 100 ms. The high spectral tail of 0.4–1.0 THz as compared to that of the noise spectrum is discussed later.

 figure: Fig. 1.

Fig. 1. Experimental setup. Ti:S laser, mode-locked Ti:Sapphire laser; PCA emitter, photoconductive antenna for THz generation; PCA detector, photoconductive antenna for THz detection; L1, L2, L3, L4, THz lenses.

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 figure: Fig. 2.

Fig. 2. (a) Temporal waveform of pulse THz electric field and (b) the corresponding THz power spectrum (red plot). For comparison, the blue plot shows the noise spectrum without a pump beam and the green plot shows THz power spectrum when the time delay was fixed at the position indicating the maximum electric field of the pulsed THz radiation [see arrow in Fig. 2(a)]. The power at 0 THz is omitted from the green plot for the visibility. The vertical axis is in dB scale. The time constant of the measurements was 100 ms.

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To highlight the achieved dynamic range of THz power, the dynamic range of the THz power was compared between the proposed THz-TDS imaging system and the ultralow-frequency noncoherent CW THz imaging system. The latter system comprises an Impatt diode (THz source 140 GHz, TeraSense Group, Inc., output frequency = 0.14 THz, output power = 80 mW) for a CW THz emitter, a pair of THz lenses (Tsurupica, PAX. Inc., focal length = 50 mm, diameter = 38.4 mm) for THz optics, and a pyro detector (PHLUXIi RTPO-1, PHLUXI. Inc.) for an incoherent THz detector. In addition, the THz wave was modulated at 14 Hz using an optical chopper and acquired using a digital lock-in amplifier (LI-5640, NF Corp., lock-in time constant = 100 ms). Regardless of the use of a high-power electronic THz source, the resulting dynamic range of THz power remained at approximately 58 dB (not presented), which was 49 dB lower than that of the proposed THz-TDS imaging system. Thus, the proposed system exhibited a higher dynamic range of THz power as compared to the noncoherent CW THz imaging system owing to the coherent generation and detection of ultralow-frequency and ultranarrow-bandwidth THz waves.

Moreover, the imaging performance of the proposed THz-TDS imaging system was characterized using a knife edge measurement, wherein a knife edge profile of the THz focus was obtained for each THz frequency to determine the spatial resolution (beam diameter) based on curve fitting analysis with an error function. The blue and red plots in Fig. 3 depict the spatial resolution of the horizontal and vertical directions with respect to the THz frequency. For reference, the spectral bandwidth of the proposed system was highlighted in the yellow color. For instance, the spatial resolution at the spectral peak of 0.046 THz was 28.0 and 28.2 mm in the horizontal and vertical directions, respectively. In comparison, the theoretical curve of the Rayleigh criterion is indicated with the blue and red dashed lines in Fig. 3. Overall, the experimental plots were in appropriate agreement with the theoretical curve, which is proximate to the Rayleigh criterion. However, the spatial resolution should be further improved, because the beam diameter of the THz beam was smaller than the aperture ( = 38.4 mm) of the focusing and collecting THz lenses (L2 and L3, in Fig. 1). As indicated by the green dashed line in Fig. 3, the theoretical curve of the Rayleigh criterion will be improved upon using the full aperture of those lenses, i.e., a spatial resolution of 10.4 mm can be achieved at 0.046 THz.

 figure: Fig. 3.

Fig. 3. Beam diameter, or the spatial resolution, with respect to THz wavelength. Blue and red circles respectively show the experimental data of beam diameter along the horizontal and vertical direction, measured by the knife-edge measurement. Blue and red dashed lines show the curve-fitting result of the Rayleigh criterion to the experimental beam diameter along the horizontal and vertical direction, respectively. Green dashed line shows the theoretical curve of the Rayleigh criterion when using a full numerical aperture of the focusing lens.

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2.2 THz spectroscopy of the mortar sample

In this article, mortar was used assuming application to concrete without the influence of coarse aggregate. Details of the mortar sample is given in the Methods section. The THz-TDS of the mortar sample was performed to investigate its spectral properties using a commercialized THz-TDS system (TAS-7500, Advantest Corp., spectral range = 0.1–3 THz, spectral resolution = 7.6 GHz). The red and blue plots in Fig. 4(a) depict the transmission spectra of the THz power with and without the sample, and the green plots show the noise spectrum. Based on their comparison, only the THz frequency components under 0.2 THz could pass through the sample with the maximum dynamic range of 23 dB over the noise level, and 99.993% of the broadband THz wave was absorbed. Thus, the commercialized THz-TDS does not have appropriate spectrum for the mortar sample.

 figure: Fig. 4.

Fig. 4. THz power spectra obtained by (a) the commercialized THz-TDS (ADVANTEST TAS-7500) and (b) the ultralow-frequency, ultranarrow-bandwidth THz-TDS. Blue line: THz wave without the mortar sample, red line: THz wave with the mortar sample, green line: no THz wave (noise spectrum). The vertical axis is in dB scale. (c) THz absorption coefficient spectrum of the mortar sample measured by the ultralow-frequency, ultranarrow-bandwidth THz-TDS.

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As the THz power of this THz-TDS system rolled off below 0.2 THz and its dynamic range is insufficient, we further performed ultralow-frequency THz-TDS of the same sample using the proposed THz-TDS imaging system. The red and blue plots in Fig. 4(b) portray the transmission spectra of the THz power with and without the same sample. In contrast to the results presented in Fig. 4(a), 15% of the total power in the ultralow-frequency and ultranarrow-bandwidth THz waves could pass through the sample, and its spectral peak at 0.045 THz achieved a maximum dynamic range of 83 dB in the THz power. Thus, the spectral bandwidth in the proposed THz-TDS imaging system has appropriate spectrum to measure the mortar sample.

Moreover, the absorption spectrum of the mortar sample below 0.15 THz was evaluated from the results presented in Fig. 4(b), as indicated in Fig. 4(c). The absorption coefficient monotonously increased with the THz frequency, implying that the appropriate frequency to measure the mortar sample existed below 0.1 THz.

2.3 THz imaging of the mortar sample without and with the metal inserts

Ultimately, we performed the transmission THz imaging of the mortar sample at 0.046 THz without and with the metal inserts (width and length: 10 mm × 30 mm) [see Figs. 5(a), 5(b) and 5(c)]. The imaging region was set as 48 mm × 60 mm with an interval of 3 mm, corresponding to a pixel size of 16 × 20 pixels. The imaged area is indicated with a red square in Figs. 5(a) and 5(b). The transmission THz images of the mortar samples without and with the metal inserts are illustrated in Fig. 5(d) and 5(e), respectively. The mortar sample without the metal inserts exhibited a uniform spatial distribution of the transmitted THz power of approximately 82 dB over the entire region. In contrast, the metal insert of the mortar sample was prominently visualized as the image contrast of the THz power decreased by 10 dB. Although the THz waves ought to disappear at the region of metal inserts owing to the absence of the THz wave transmission to the metal, moderate THz power appeared at the region. Thus, we consider that the focused THz wave was diffracted by the metal inserts, because the spatial resolution of the imaging system (≈ 28 mm) was comparable to the size of the metal inserts. The size of the visualized metal inserts was in reasonable agreement with the actual size of the metal inserts (size: 10 mm × 30 mm) based on the spatial resolution of the present imaging system. This result implies a potential of the application for the corrosion detection of the reinforcing steel in the concrete structure.

 figure: Fig. 5.

Fig. 5. Visible images of mortar samples (a) without and (b) with the metal inserts. (c) shows the metal insert. Dimensions of the mortar sample is 100 mm in width, 100 mm in height, and 10 mm in thickness. Red square indicates the measured area in the THz imaging ( = 48 mm in width by 60 mm in height). THz transmission image of the mortar samples (d) without and (e) with the metal inserts. The images show intensities at 0.046 THz.

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3. Discussion

In this section, we discuss the reason for which the higher-frequency spectral tail of ultralow-frequency ultranarrow-bandwidth THz waves did not overlap with the noise spectrum [see Fig. 2(b)]. The higher-frequency spectral tail of the ultralow-frequency ultranarrow-bandwidth THz waves should overlap with the noise spectrum because it did not radiate any THz power at this frequency range. However, in Fig. 2(b), the THz power of the spectral tail [see the red plots] was 50 dB higher than that of the noise spectrum [see the blue plots], potentially because of the limited dynamic range of the lock-in amplifier. As the present lock-in amplifier has a dynamic range of 43 dB in the THz amplitude, the dynamic range of the THz power in the present system was estimated as 86 dB. In addition, the difference in the dynamic range of the THz power between the spectral tail and noise spectrum was significantly fewer than the dynamic range of the THz power limited by the lock-in amplifier. Thus, the dynamic range of the present lock-in amplifier is not responsible for the inconsistency of the THz power spectrum between the spectral tail and noise spectrum. An additional cause is the power instability of the pump light. A typical mode-locked Ti:sapphire laser has an optical power instability of 1% within the frequency range < 100 kHz [19]. Thus, the inherent fluctuation of the pump light power was converted into that of the THz electric field via the PCA emitter, and its frequency range existed below 100 kHz as well. However, such inherent fluctuation of the THz electric field appears as a temporal behavior of the femtosecond to picosecond order in the temporal waveform of the THz electric field owing to the pump-probe measurement with the mechanical time-delay scanning. Therefore, we considered that the combined effect of the fluctuating THz electric field with the pump-probe measurement remarkably increased the spectral tail of the THz wave over the noise spectrum in the higher-frequency range. To validate this consideration, we acquired the temporal waveform of the THz electric field while fixing the time delay at the position indicating the maximum electric field of the pulse THz radiation, and thereafter, calculated the corresponding THz power spectrum, as indicated by the green plots in Fig. 2(b). Consequently, the resulting spectrum overlapped significantly with the spectral tail of the THz wave. Therefore, we conclude that the difference between the dynamic range of the THz power for the spectral tail and the noise spectrum [Fig. 2(b)] was primarily caused by the combined effect of the fluctuating THz electric field with the pump-probe measurement, which was first observed in the proposed THz-TDS imaging system with a wide dynamic range of THz power.

Furthermore, the maximum measurable thickness in the mortar sample is discussed herein with comparison of the expected maximum thickness among the three distinct systems: the commercialized THz-TDS system, the ultralow-frequency noncoherent CW THz imaging system, and the proposed THz-TDS imaging system. In the commercialized THz-TDS system, the maximum thickness of the mortar sample was estimated as 19 mm from the absorption coefficient of 5.8 cm−1 and the dynamic range was 48 dB at 0.12 THz. Second, in the ultralow-frequency noncoherent CW THz imaging system, the maximum thickness was estimated as 19 mm from the absorption coefficient of 6.9 cm-1 and the dynamic range was 53 dB at 0.14 THz. Finally, in the proposed ultralow-frequency THz-TDS imaging system, the expected maximum thickness of the mortar sample was 135 mm from the absorption coefficient of 1.7 cm−1 and the dynamic range was 107 dB at 0.046 THz. Even though the absorption coefficient depends on the mortar sample preparation, this comparison demonstrates the advantage of the proposed ultralow-frequency, ultranarrow-bandwidth THz-TDS imaging system for the measurable maximum thickness of the mortar sample, which utilizes the wide dynamic range of THz power. For practical application to the infrastructures, further investigations of absorption coefficients with various sample condition are required.

The wide dynamic range of the system owing to the bowtie-shaped antenna is applicable with the combination with a state-of-the-art asynchronous optical sampling method [20] or dual comb method [21] in a fiber-based optical system provides compactness, robustness, and rapid data acquisition, which are attractive features for practical applications such as on-site NDT of internal defects in concrete infrastructures.

4. Conclusion

We demonstrated the effectiveness of the ultralow-frequency, ultranarrow-bandwidth THz-TDS imaging system to investigate the inner structure of the mortar sample. The simultaneous use of a pair of bowtie-shaped PCAs for coherent emission as well as detection considerably increased the dynamic range of THz power over 107 dB at 0.046 THz, which is appropriate to measure the mortar material. Owing to the wide dynamic range and low absorption coefficient in this frequency band, the proposed THz-TDS imaging system enhanced the measurable maximum thickness in the mortar sample to 135 mm. This thickness is significantly larger than that (19 mm) obtained by the commercialized THz-TDS system and that (19 mm) obtained by the ultralow-frequency noncoherent CW THz imaging system. Moreover, the metal inserts in the mortar sample were prominently visualized with the proposed system using a spatial resolution of 28 mm, indicating its high potential for the THz-NDT of the concrete structure. The application of the proposed ultralow-frequency, ultranarrow-bandwidth THz-TDS imaging system in a reflection configuration (i.e., THz tomography) enabled the cross-sectional imaging of the concrete structure. Thus, the proposed system has the potential to visualize the inner defects and/or water distribution in concrete structures within a depth range of several tens of millimeters from the surface.

Appendix A. Methods

A.1: experimental setup

A schematic of the experimental setup for the ultralow-frequency ultranarrow-bandwidth THz-TDS imaging system is illustrated in Fig. 1. The output light from a mode-locked Ti:sapphire laser (Maitai, Spectra Physics, Inc., center wavelength = 800 nm, output power ∼730 mW, pulse duration = 100 fs, repetition frequency = 80 MHz) was separated into two optical paths to pump the THz emitter and detector of the PCAs as a pump and probe pulses. In this study, we used bowtie-shaped PCAs of low-temperature-grown GaAs for the coherent generation and detection of pulsed THz electric field, which enabled us to localize its spectral energy below 0.1 THz and increase the dynamic range of THz power [17,18]. For lock-in detection, the amplitude of the THz wave was modulated at 12 kHz based on the bias voltage of the PCA emitter. In addition, the radiated THz wave was collimated with the first THz lens (L1, LTA100, Thorlabs Inc., polytetrafluoroethylene, focal length = 100 mm, diameter = 50 mm), and subsequently, focused onto a sample using the second THz lens (L2, Tsurupica, PAX. Inc., focal length = 50 mm, diameter = 38.4 mm). The THz wave passing through the sample was collimated using a third THz lens (L3, Tsurupica, PAX. Inc., focal length = 50 mm, diameter = 38.4 mm), which was incident onto a PCA detector using a fourth THz lens (L4, LTA100, Thorlabs Inc., polytetrafluoroethylene, focal length = 100 mm, diameter = 50 mm). Moreover, the sample position was laterally scanned at a step of 3 mm by a translation stage to conduct two-dimensional transmission imaging. Additionally, the output current from the PCA detector was amplified using a current preamplifier (LI-76, NF Corp.), which was acquired with a digital lock-in amplifier (LI-5640, NF Corp., lock-in time constant = 10 ms or 100 ms). The temporal waveform of the pulsed THz electric field was measured based on a pump-probe measurement with mechanical time-delay scanning (stroke = 32 mm). Finally, we obtained the THz power spectrum of ultralow-frequency THz waves by conducting the Fourier transform of the temporal waveform and the square of the resulting amplitude spectrum.

A.2: mortar sample

The mortar is a building material comprising cement, sand, and water, whereas the concrete is made of cement, sand, gravel, and water. The difference in contained material between them is whether they contain the gravel or not. The gravel function as coarse aggregate in the concrete and increases its strength; on the other hand, it can have some impact on THz spectroscopic property of the concrete via scattering and/or diffraction from a relation between the dimension and the wavelength of THz wave. Although it is expected to be applied to concrete structures in the future, we here used a mortar sample to conduct a basic study in a simple sample that is not affected by the gravel. We here used glass fiber reinforced cement (GRC) mortar, which is a composite material of mortar reinforced with alkali-resistant glass fiber. We prepared a mortar sample (width, 100 mm; height, 100 mm; thickness, 10 mm), as presented in Fig. 5(a). In addition, we prepared a mortar sample containing metal inserts (stainless steel, elliptical shape with a major diameter of 30 mm by a minor diameter of 10 mm) at an unknown depth, as depicted in Fig. 5(b); we used the metal inserts considering the application for the corrosion detection of the reinforcing steel in the concrete structure. Overall, the two mortar samples were indistinguishable from their appearance.

Funding

Japan Society for the Promotion of Science (19H00871, 21K04928); Cabinet Office, Government of Japan (Subsidy for Reg. Univ. and Reg. Ind. Creation).

Acknowledgements

The authors acknowledge Prof. Masahiko Tani, Fukui University, Japan, for fruitful discussions about the dynamic range of THz power in the present system. We would also like to thank Mrs. Hiroshi Ito and Masahito Fujikawa, Nihon Funen Co., LTD. for preparation of mortar samples.

Disclosures

The authors declare no conflicts of interest.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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References

  • View by:

  1. J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
    [Crossref]
  2. P. J. Withers and M. Preuss, “Fatigue and damage in structural materials studied by X-ray tomography,” Annu. Rev. Mater. Res. 42(1), 81–103 (2012).
    [Crossref]
  3. M. Molero, S. Aparicio, G. Al-Assadi, M. J. Casati, M. G. Hernández, and J. J. Anaya, “Evaluation of freeze-thaw damage in concrete by ultrasonic imaging,” NDT E Int. 52, 86–94 (2012).
    [Crossref]
  4. M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
    [Crossref]
  5. C. A. Balaras and A. A. Argiriou, “Infrared thermography for building diagnostics,” Energy Build. 34(2), 171–183 (2002).
    [Crossref]
  6. M. R. Clark, D. M. McCann, and M. C. Forde, “Application of infrared thermography to the non-destructive testing of concrete and masonry bridges,” NDT E Int. 36(4), 265–275 (2003).
    [Crossref]
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  8. D. M. Mittleman, “Twenty years of terahertz imaging,” Opt. Express 26(8), 9417–9431 (2018).
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  9. T. Tanabe, T. Kanai, K. Kuroo, T. Nishiwaki, and Y. Oyama, “Non-contact terahertz inspection of water content in concrete of infrastructure buildings,” World J, Eng. Technol. 6(02), 275–281 (2018).
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  10. M. C. Nuss and J. Orenstein, “Terahertz time-domain spectroscopy,” in Millimeter and Submillimeter Wave Spectroscopy of Solids (Springer, 1998), pp. 7–50.
  11. B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20(16), 1716–1718 (1995).
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  12. D. M. Mittleman, S. Hunsche, L. Boivin, and M. C. Nuss, “T-ray tomography,” Opt. Lett. 22(12), 904–906 (1997).
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  13. K. Fukunaga and I. Hosako, “Innovative non-invasive analysis techniques for cultural heritage using terahertz technology,” Comptes Rendus Phys. 11(7-8), 519–526 (2010).
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  14. C. Seco-Martorell, V. López-Domínguez, G. Arauz-Garofalo, A. Redo-Sanchez, J. Palacios, and J. Tejada, “Goya’s artwork imaging with Terahertz waves,” Opt. Express 21(15), 17800–17805 (2013).
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  16. T. Yasui, K. Sawanaka, A. Ihara, E. Abraham, M. Hashimoto, and T. Araki, “Real-time terahertz color scanner for moving objects,” Opt. Express 16(2), 1208–1221 (2008).
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  17. T. Yasui and T. Araki, “Sensitive measurement of water content in dry material based on low-frequency terahertz time-domain spectroscopy,” Proc. SPIE, 6024, 60240A (2006).
  18. S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
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  19. T. Yasui and T. Araki, “Dependence of terahertz electric fields on electric bias and modulation frequency in pulsed terahertz emissions from electrically-modulated photoconductive antenna detected with free-space electro-optic sampling,” Jpn J. Appl. Phys 44(4A), 1777–1780 (2005).
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  20. Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
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  21. G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
    [Crossref]

2019 (1)

M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
[Crossref]

2018 (3)

D. M. Mittleman, “Twenty years of terahertz imaging,” Opt. Express 26(8), 9417–9431 (2018).
[Crossref]

T. Tanabe, T. Kanai, K. Kuroo, T. Nishiwaki, and Y. Oyama, “Non-contact terahertz inspection of water content in concrete of infrastructure buildings,” World J, Eng. Technol. 6(02), 275–281 (2018).
[Crossref]

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

2016 (2)

A. Redo-Sanchez, B. Heshmat, A. Aghasi, S. Naqvi, M. Zhang, J. Romberg, and R. Raskar, “Terahertz time-gated spectral imaging for content extraction through layered structures,” Nat. Commun. 7(1), 12665–7 (2016).
[Crossref]

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

2014 (1)

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

2013 (1)

2012 (3)

S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
[Crossref]

P. J. Withers and M. Preuss, “Fatigue and damage in structural materials studied by X-ray tomography,” Annu. Rev. Mater. Res. 42(1), 81–103 (2012).
[Crossref]

M. Molero, S. Aparicio, G. Al-Assadi, M. J. Casati, M. G. Hernández, and J. J. Anaya, “Evaluation of freeze-thaw damage in concrete by ultrasonic imaging,” NDT E Int. 52, 86–94 (2012).
[Crossref]

2010 (1)

K. Fukunaga and I. Hosako, “Innovative non-invasive analysis techniques for cultural heritage using terahertz technology,” Comptes Rendus Phys. 11(7-8), 519–526 (2010).
[Crossref]

2008 (2)

T. Yasui, K. Sawanaka, A. Ihara, E. Abraham, M. Hashimoto, and T. Araki, “Real-time terahertz color scanner for moving objects,” Opt. Express 16(2), 1208–1221 (2008).
[Crossref]

C. C. Cheng, T. M. Cheng, and C. H. Chiang, “Defect detection of concrete structures using both infrared thermography and elastic waves,” Autom. Constr. 18(1), 87–92 (2008).
[Crossref]

2005 (1)

T. Yasui and T. Araki, “Dependence of terahertz electric fields on electric bias and modulation frequency in pulsed terahertz emissions from electrically-modulated photoconductive antenna detected with free-space electro-optic sampling,” Jpn J. Appl. Phys 44(4A), 1777–1780 (2005).
[Crossref]

2003 (1)

M. R. Clark, D. M. McCann, and M. C. Forde, “Application of infrared thermography to the non-destructive testing of concrete and masonry bridges,” NDT E Int. 36(4), 265–275 (2003).
[Crossref]

2002 (1)

C. A. Balaras and A. A. Argiriou, “Infrared thermography for building diagnostics,” Energy Build. 34(2), 171–183 (2002).
[Crossref]

1997 (1)

1995 (1)

Abdelsalam, D. G.

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

Abid, A.

M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
[Crossref]

Abraham, E.

Aghasi, A.

A. Redo-Sanchez, B. Heshmat, A. Aghasi, S. Naqvi, M. Zhang, J. Romberg, and R. Raskar, “Terahertz time-gated spectral imaging for content extraction through layered structures,” Nat. Commun. 7(1), 12665–7 (2016).
[Crossref]

Al-Assadi, G.

M. Molero, S. Aparicio, G. Al-Assadi, M. J. Casati, M. G. Hernández, and J. J. Anaya, “Evaluation of freeze-thaw damage in concrete by ultrasonic imaging,” NDT E Int. 52, 86–94 (2012).
[Crossref]

Anaya, J. J.

M. Molero, S. Aparicio, G. Al-Assadi, M. J. Casati, M. G. Hernández, and J. J. Anaya, “Evaluation of freeze-thaw damage in concrete by ultrasonic imaging,” NDT E Int. 52, 86–94 (2012).
[Crossref]

Aparicio, S.

M. Molero, S. Aparicio, G. Al-Assadi, M. J. Casati, M. G. Hernández, and J. J. Anaya, “Evaluation of freeze-thaw damage in concrete by ultrasonic imaging,” NDT E Int. 52, 86–94 (2012).
[Crossref]

Araki, T.

T. Yasui, K. Sawanaka, A. Ihara, E. Abraham, M. Hashimoto, and T. Araki, “Real-time terahertz color scanner for moving objects,” Opt. Express 16(2), 1208–1221 (2008).
[Crossref]

T. Yasui and T. Araki, “Dependence of terahertz electric fields on electric bias and modulation frequency in pulsed terahertz emissions from electrically-modulated photoconductive antenna detected with free-space electro-optic sampling,” Jpn J. Appl. Phys 44(4A), 1777–1780 (2005).
[Crossref]

T. Yasui and T. Araki, “Sensitive measurement of water content in dry material based on low-frequency terahertz time-domain spectroscopy,” Proc. SPIE, 6024, 60240A (2006).

Arauz-Garofalo, G.

Argiriou, A. A.

C. A. Balaras and A. A. Argiriou, “Infrared thermography for building diagnostics,” Energy Build. 34(2), 171–183 (2002).
[Crossref]

Balaras, C. A.

C. A. Balaras and A. A. Argiriou, “Infrared thermography for building diagnostics,” Energy Build. 34(2), 171–183 (2002).
[Crossref]

Barrios, M. A.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Benedetti, L. R.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Boivin, L.

Bradley, D. K.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Casati, M. J.

M. Molero, S. Aparicio, G. Al-Assadi, M. J. Casati, M. G. Hernández, and J. J. Anaya, “Evaluation of freeze-thaw damage in concrete by ultrasonic imaging,” NDT E Int. 52, 86–94 (2012).
[Crossref]

Cheng, C. C.

C. C. Cheng, T. M. Cheng, and C. H. Chiang, “Defect detection of concrete structures using both infrared thermography and elastic waves,” Autom. Constr. 18(1), 87–92 (2008).
[Crossref]

Cheng, T. M.

C. C. Cheng, T. M. Cheng, and C. H. Chiang, “Defect detection of concrete structures using both infrared thermography and elastic waves,” Autom. Constr. 18(1), 87–92 (2008).
[Crossref]

Chiang, C. H.

C. C. Cheng, T. M. Cheng, and C. H. Chiang, “Defect detection of concrete structures using both infrared thermography and elastic waves,” Autom. Constr. 18(1), 87–92 (2008).
[Crossref]

Clark, M. R.

M. R. Clark, D. M. McCann, and M. C. Forde, “Application of infrared thermography to the non-destructive testing of concrete and masonry bridges,” NDT E Int. 36(4), 265–275 (2003).
[Crossref]

Collins, G. W.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Eder, D. C.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Edwards, M. J.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Faramarzi, M.

M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
[Crossref]

Field, J. E.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Forde, M. C.

M. R. Clark, D. M. McCann, and M. C. Forde, “Application of infrared thermography to the non-destructive testing of concrete and masonry bridges,” NDT E Int. 36(4), 265–275 (2003).
[Crossref]

Fukunaga, K.

K. Fukunaga and I. Hosako, “Innovative non-invasive analysis techniques for cultural heritage using terahertz technology,” Comptes Rendus Phys. 11(7-8), 519–526 (2010).
[Crossref]

Hasegawa, T.

S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
[Crossref]

Hashimoto, M.

Hernández, M. G.

M. Molero, S. Aparicio, G. Al-Assadi, M. J. Casati, M. G. Hernández, and J. J. Anaya, “Evaluation of freeze-thaw damage in concrete by ultrasonic imaging,” NDT E Int. 52, 86–94 (2012).
[Crossref]

Heshmat, B.

A. Redo-Sanchez, B. Heshmat, A. Aghasi, S. Naqvi, M. Zhang, J. Romberg, and R. Raskar, “Terahertz time-gated spectral imaging for content extraction through layered structures,” Nat. Commun. 7(1), 12665–7 (2016).
[Crossref]

Hindle, F.

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

Hosako, I.

K. Fukunaga and I. Hosako, “Innovative non-invasive analysis techniques for cultural heritage using terahertz technology,” Comptes Rendus Phys. 11(7-8), 519–526 (2010).
[Crossref]

Hsieh, Y.-D.

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

Hu, B. B.

Hu, G.

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

Hunsche, S.

Ibrahim, S.

M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
[Crossref]

Ihara, A.

Inoue, H.

S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
[Crossref]

Iwata, T.

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

Jones, O. S.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Kanai, T.

T. Tanabe, T. Kanai, K. Kuroo, T. Nishiwaki, and Y. Oyama, “Non-contact terahertz inspection of water content in concrete of infrastructure buildings,” World J, Eng. Technol. 6(02), 275–281 (2018).
[Crossref]

kawagoe, H.

S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
[Crossref]

Kawase, K.

S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
[Crossref]

Khairi, M. T. M.

M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
[Crossref]

Kline, J. L.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Kroll, J. J.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Kuroo, K.

T. Tanabe, T. Kanai, K. Kuroo, T. Nishiwaki, and Y. Oyama, “Non-contact terahertz inspection of water content in concrete of infrastructure buildings,” World J, Eng. Technol. 6(02), 275–281 (2018).
[Crossref]

Landen, O. L.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Li, T.

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

López-Domínguez, V.

Ma, T.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

McCann, D. M.

M. R. Clark, D. M. McCann, and M. C. Forde, “Application of infrared thermography to the non-destructive testing of concrete and masonry bridges,” NDT E Int. 36(4), 265–275 (2003).
[Crossref]

Minamikawa, T.

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

Mittleman, D. M.

Mizuguchi, T.

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

Mizutani, Y.

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

Molero, M.

M. Molero, S. Aparicio, G. Al-Assadi, M. J. Casati, M. G. Hernández, and J. J. Anaya, “Evaluation of freeze-thaw damage in concrete by ultrasonic imaging,” NDT E Int. 52, 86–94 (2012).
[Crossref]

Nakamura, S.

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

Naqvi, S.

A. Redo-Sanchez, B. Heshmat, A. Aghasi, S. Naqvi, M. Zhang, J. Romberg, and R. Raskar, “Terahertz time-gated spectral imaging for content extraction through layered structures,” Nat. Commun. 7(1), 12665–7 (2016).
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T. Tanabe, T. Kanai, K. Kuroo, T. Nishiwaki, and Y. Oyama, “Non-contact terahertz inspection of water content in concrete of infrastructure buildings,” World J, Eng. Technol. 6(02), 275–281 (2018).
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Nitta, K.

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
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Oe, R.

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

Ogura, H.

S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
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Orenstein, J.

M. C. Nuss and J. Orenstein, “Terahertz time-domain spectroscopy,” in Millimeter and Submillimeter Wave Spectroscopy of Solids (Springer, 1998), pp. 7–50.

Oyama, Y.

T. Tanabe, T. Kanai, K. Kuroo, T. Nishiwaki, and Y. Oyama, “Non-contact terahertz inspection of water content in concrete of infrastructure buildings,” World J, Eng. Technol. 6(02), 275–281 (2018).
[Crossref]

Pak, A.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
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Palacios, J.

Peterson, J. L.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Preuss, M.

P. J. Withers and M. Preuss, “Fatigue and damage in structural materials studied by X-ray tomography,” Annu. Rev. Mater. Res. 42(1), 81–103 (2012).
[Crossref]

Pusppanathan, J.

M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
[Crossref]

Raman, K.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Raskar, R.

A. Redo-Sanchez, B. Heshmat, A. Aghasi, S. Naqvi, M. Zhang, J. Romberg, and R. Raskar, “Terahertz time-gated spectral imaging for content extraction through layered structures,” Nat. Commun. 7(1), 12665–7 (2016).
[Crossref]

Redo-Sanchez, A.

A. Redo-Sanchez, B. Heshmat, A. Aghasi, S. Naqvi, M. Zhang, J. Romberg, and R. Raskar, “Terahertz time-gated spectral imaging for content extraction through layered structures,” Nat. Commun. 7(1), 12665–7 (2016).
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C. Seco-Martorell, V. López-Domínguez, G. Arauz-Garofalo, A. Redo-Sanchez, J. Palacios, and J. Tejada, “Goya’s artwork imaging with Terahertz waves,” Opt. Express 21(15), 17800–17805 (2013).
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Romberg, J.

A. Redo-Sanchez, B. Heshmat, A. Aghasi, S. Naqvi, M. Zhang, J. Romberg, and R. Raskar, “Terahertz time-gated spectral imaging for content extraction through layered structures,” Nat. Commun. 7(1), 12665–7 (2016).
[Crossref]

Rygg, J. R.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Sawanaka, K.

Sean, G. P.

M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
[Crossref]

Seco-Martorell, C.

Takeya, K.

S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
[Crossref]

Tanabe, T.

T. Tanabe, T. Kanai, K. Kuroo, T. Nishiwaki, and Y. Oyama, “Non-contact terahertz inspection of water content in concrete of infrastructure buildings,” World J, Eng. Technol. 6(02), 275–281 (2018).
[Crossref]

Tejada, J.

Town, R. P. J.

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Tripathi, S. R.

S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
[Crossref]

Withers, P. J.

P. J. Withers and M. Preuss, “Fatigue and damage in structural materials studied by X-ray tomography,” Annu. Rev. Mater. Res. 42(1), 81–103 (2012).
[Crossref]

Yamamoto, H.

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

Yasui, T.

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

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[Crossref]

T. Yasui and T. Araki, “Dependence of terahertz electric fields on electric bias and modulation frequency in pulsed terahertz emissions from electrically-modulated photoconductive antenna detected with free-space electro-optic sampling,” Jpn J. Appl. Phys 44(4A), 1777–1780 (2005).
[Crossref]

T. Yasui and T. Araki, “Sensitive measurement of water content in dry material based on low-frequency terahertz time-domain spectroscopy,” Proc. SPIE, 6024, 60240A (2006).

Yunus, M. A. M.

M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
[Crossref]

Zhang, M.

A. Redo-Sanchez, B. Heshmat, A. Aghasi, S. Naqvi, M. Zhang, J. Romberg, and R. Raskar, “Terahertz time-gated spectral imaging for content extraction through layered structures,” Nat. Commun. 7(1), 12665–7 (2016).
[Crossref]

Zhao, X.

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

Zheng, Z.

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

Annu. Rev. Mater. Res. (1)

P. J. Withers and M. Preuss, “Fatigue and damage in structural materials studied by X-ray tomography,” Annu. Rev. Mater. Res. 42(1), 81–103 (2012).
[Crossref]

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Comptes Rendus Phys. (1)

K. Fukunaga and I. Hosako, “Innovative non-invasive analysis techniques for cultural heritage using terahertz technology,” Comptes Rendus Phys. 11(7-8), 519–526 (2010).
[Crossref]

Corros. Sci. (1)

S. R. Tripathi, H. Ogura, H. kawagoe, H. Inoue, T. Hasegawa, K. Takeya, and K. Kawase, “Measurement of chloride ion concentration in concrete structures using terahertz time domain spectroscopy (THz-TDS),” Corros. Sci. 62, 5–10 (2012).
[Crossref]

Energy Build. (1)

C. A. Balaras and A. A. Argiriou, “Infrared thermography for building diagnostics,” Energy Build. 34(2), 171–183 (2002).
[Crossref]

Jpn J. Appl. Phys (1)

T. Yasui and T. Araki, “Dependence of terahertz electric fields on electric bias and modulation frequency in pulsed terahertz emissions from electrically-modulated photoconductive antenna detected with free-space electro-optic sampling,” Jpn J. Appl. Phys 44(4A), 1777–1780 (2005).
[Crossref]

Meas. J. Int. Meas. Confed. (1)

M. T. M. Khairi, S. Ibrahim, M. A. M. Yunus, M. Faramarzi, G. P. Sean, J. Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: Principles, design and applications,” Meas. J. Int. Meas. Confed. 146, 490–523 (2019).
[Crossref]

Nat. Commun. (1)

A. Redo-Sanchez, B. Heshmat, A. Aghasi, S. Naqvi, M. Zhang, J. Romberg, and R. Raskar, “Terahertz time-gated spectral imaging for content extraction through layered structures,” Nat. Commun. 7(1), 12665–7 (2016).
[Crossref]

NDT E Int. (2)

M. R. Clark, D. M. McCann, and M. C. Forde, “Application of infrared thermography to the non-destructive testing of concrete and masonry bridges,” NDT E Int. 36(4), 265–275 (2003).
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M. Molero, S. Aparicio, G. Al-Assadi, M. J. Casati, M. G. Hernández, and J. J. Anaya, “Evaluation of freeze-thaw damage in concrete by ultrasonic imaging,” NDT E Int. 52, 86–94 (2012).
[Crossref]

Opt. Express (3)

Opt. Lett. (2)

Phys. Rev. Lett. (1)

J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti, G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L. Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and D. K. Bradley, “2D X-ray radiography of imploding capsules at the national ignition facility,” Phys. Rev. Lett. 112(19), 195001 (2014).
[Crossref]

Sci. Rep. (2)

Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, “Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy,” Sci. Rep. 6(1), 28114 (2016).
[Crossref]

G. Hu, T. Mizuguchi, R. Oe, K. Nitta, X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, “Dual terahertz comb spectroscopy with a single free-running fibre laser,” Sci. Rep. 8(1), 11155 (2018).
[Crossref]

World J, Eng. Technol. (1)

T. Tanabe, T. Kanai, K. Kuroo, T. Nishiwaki, and Y. Oyama, “Non-contact terahertz inspection of water content in concrete of infrastructure buildings,” World J, Eng. Technol. 6(02), 275–281 (2018).
[Crossref]

Other (2)

M. C. Nuss and J. Orenstein, “Terahertz time-domain spectroscopy,” in Millimeter and Submillimeter Wave Spectroscopy of Solids (Springer, 1998), pp. 7–50.

T. Yasui and T. Araki, “Sensitive measurement of water content in dry material based on low-frequency terahertz time-domain spectroscopy,” Proc. SPIE, 6024, 60240A (2006).

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Figures (5)

Fig. 1.
Fig. 1. Experimental setup. Ti:S laser, mode-locked Ti:Sapphire laser; PCA emitter, photoconductive antenna for THz generation; PCA detector, photoconductive antenna for THz detection; L1, L2, L3, L4, THz lenses.
Fig. 2.
Fig. 2. (a) Temporal waveform of pulse THz electric field and (b) the corresponding THz power spectrum (red plot). For comparison, the blue plot shows the noise spectrum without a pump beam and the green plot shows THz power spectrum when the time delay was fixed at the position indicating the maximum electric field of the pulsed THz radiation [see arrow in Fig. 2(a)]. The power at 0 THz is omitted from the green plot for the visibility. The vertical axis is in dB scale. The time constant of the measurements was 100 ms.
Fig. 3.
Fig. 3. Beam diameter, or the spatial resolution, with respect to THz wavelength. Blue and red circles respectively show the experimental data of beam diameter along the horizontal and vertical direction, measured by the knife-edge measurement. Blue and red dashed lines show the curve-fitting result of the Rayleigh criterion to the experimental beam diameter along the horizontal and vertical direction, respectively. Green dashed line shows the theoretical curve of the Rayleigh criterion when using a full numerical aperture of the focusing lens.
Fig. 4.
Fig. 4. THz power spectra obtained by (a) the commercialized THz-TDS (ADVANTEST TAS-7500) and (b) the ultralow-frequency, ultranarrow-bandwidth THz-TDS. Blue line: THz wave without the mortar sample, red line: THz wave with the mortar sample, green line: no THz wave (noise spectrum). The vertical axis is in dB scale. (c) THz absorption coefficient spectrum of the mortar sample measured by the ultralow-frequency, ultranarrow-bandwidth THz-TDS.
Fig. 5.
Fig. 5. Visible images of mortar samples (a) without and (b) with the metal inserts. (c) shows the metal insert. Dimensions of the mortar sample is 100 mm in width, 100 mm in height, and 10 mm in thickness. Red square indicates the measured area in the THz imaging ( = 48 mm in width by 60 mm in height). THz transmission image of the mortar samples (d) without and (e) with the metal inserts. The images show intensities at 0.046 THz.

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