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Abstract
This work demonstrates terahertz (THz) line imaging that acquires broadband spectral information by combining echelon-based single-shot THz spectroscopy with high-sensitivity phase-offset electrooptic detection. An approximately 40 dB signal-to-noise ratio is obtained for a THz spectrum from a single line of the camera, with a detection bandwidth up to 2 THz at the peak electric-field strength of 1.2 kV/cm. The spatial resolution of the image is confirmed to be diffraction limited for each spectral component of the THz wave. We use the system to image sugar tablets by quickly scanning the sample, which illustrates the capacity of the proposed spectral line imaging system for high-throughput applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz (THz) imaging is one of the most important applications of THz science and technology and can be used to map materials that are difficult to distinguish using other frequency regimes [1,2]. Material properties that can be detected using THz imaging include polymorphisms of organic materials [3], strain in rubbers [4], refractive indices of chemicals and proteins [5,6], pigments in the artificial paints [7], carriers in semiconductor [8], and foams in the polymeric materials [9]. In addition, THz imaging coupled with THz spectroscopy has recently been used to investigate elementary excitations such as soft-modes in ferroelectrics [10], collective pseudospins in superconductors [11], magnons [12], and plasmons [13–16]. For these imaging applications, researchers use THz cameras from microbolometers [17], THz CMOS cameras [18], up-conversion to the near-infrared or visible range [19], etc. Although spectral information is fundamentally the crucial information on which this technology is based, it remains difficult or time-consuming to obtain complete spectral information using these techniques.
One advanced technique for THz imaging is THz time-domain spectroscopy (THz-TDS), which uses ultrashort laser pulses and offers important advantage of competing techniques [20,21]. Since THz-TDS acquires the full temporal waveform of THz transients, it can provide a precise, broadband spectrum. However, increasing the measurement throughput is difficult because the temporal domain must be scanned to acquire the full waveform of the THz pulses in addition to the spatial dimensions normally required for THz imaging.
Recently, significant efforts have been devoted to circumvent this difficulty by developing single-shot THz time-domain spectroscopy [22–27]. Yasui et al. used oblique crossings of THz waves and probe pulses in an electro-optic (EO) crystal to demonstrate single-shot THz line imaging [28]. The oblique-crossing technique, however, requires a large EO crystal to map out sufficient temporal information and, more importantly, temporal information can be distorted by spatial inhomogeneities in the EO crystal and/or by scattered THz waves because different temporal information comes from different parts of the crystal.
The reflective echelon technique uses a stair-step mirror to map the temporal information to spatial positions on the mirror and allows us to focus the probe pulses onto the EO crystal, which can reduce the distortion of temporal waveforms [29,30]. THz-TDS in a pulsed magnetic field and Kerr-gate spectroscopy have already been demonstrated using this method, illustrating the precise acquisition of the temporal waveform and the corresponding spectrum [31–33]. In the present work, we combine this technique based on an echelon mirror with a phase-offset method to enhance electric-field detection [34–37], and we implement the system in an imaging system to realize high-throughput line imaging.
2. Experiments
Figure 1 summarizes the system used in this work. We used a Ti:sapphire regenerative amplifier with an output power of 1 mJ, a center wavelength of 800 nm, and a repetition rate of 1 kHz to generate and detect THz waves. Part of the laser output served to pump the LiNbO3 prism with a wavefront tilt to efficiently generate an intense THz wave [38,39], which was then collimated by using a spherical lens placed after the LiNbO3 prism and linearly focused onto the sample with a cylindrical lens. The THz image of the sample was then transferred to the detection electro-optic crystal that detected the birefringence induced by the THz electric field and the spatial distribution.
Fig. 1. Experimental setup for THz spectral line imaging using an echelon mirror. TL: THz Tsurupika lenses. TCL: THz cylindrical lens, CL: cylindrical lens, EOC: electro-optic crystal, Pol: polarizer, QWP: quarterwave plate.
The remaining laser output of about 50 mW was used to probe the electric-field distribution on the EO crystal (1-mm-thick (110) ZnTe). The probe was reflected from an echelon mirror with a step width of 25 µm and step height of 2.5 µm and linearly focused onto the crystal to cover the focal line of the THz wave. The number of steps on the echelon surface was 750. Since each segment of the echelon mirror reflects the probe pulse at different delay times (see inset of Fig. 1), the temporal information is mapped to the horizontal axis of the camera (pco.edge, 16 bit, 2560X2160 pixels with a pixel size of 6.5X6.5 µm2. Full-well capacity of the camera was 30000 e-) upon imaging the echelon surface onto the camera. To image the spatial information in the vertical direction on the EO crystal, we used a set of cylindrical lenses after the EO crystal.
Because the THz pulses are linearly focused in our system, the electric-field strength at the focus is less than that in other THz-TDS systems that use tightly focused THz pulses. To compensate for the reduced electric-field strength, we enhanced the sensitivity of the EO sampling to obtain line imaging with a large signal-to-noise ratio (SNR). Here, we used the phase offset method in the detection setup [35]. In this setup, we placed between the crossed polarizers an EO crystal and a quarterwave plate (QWP) with a slight offset rotation.
To describe the detection mechanism, we first write the Jones vector ${E_{in}}$ of the incident probe pulse as
3. Results
Figures 2(a) and 2(b) shows the images obtained with the THz pulses at phase-offset angles of +2° deg. and −2° deg. with the exposure time of 50 ms. The images show clearly the formation of the THz pulses into a stripe pattern, with the two offset angles producing opposite polarities. Figures 2(c) and 2(d) shows linear profiles taken along the 700th line of the images shown in Fig. 2(a) and 2(b), respectively, produced by calculating $\Delta I/I = ({{I_{\textrm{THz on}}} - {I_{\textrm{THz off}}}} )/{I_{\textrm{THz off}}}$. Using Eq. (6) to estimate the electric-field strength, we subtract the waveform with negative phase offset from that with positive phase offset as shown in Fig. 2(e). Even though the data are from a single line of the camera with no spatial averaging, the THz waveform obtained has reasonable SNR.
Fig. 2. THz spectral line imaging. Images taken with phase offset of (a) +2° and (b) −2°. (c), (d) Horizontal line profiles of images shown in panels (a) and (b), respectively. (e) THz waveform obtained by subtracting the data of panel (d) from that of panel (c). (f) Fourier transform of panel (e).
Thanks to the phase-offset method, we could use the full dynamics range of the camera for terahertz detection to enhance the sensitivity. In contrast, if we use a QWP set at 45° deg. for detection as in the normal electro-optic sampling method, we could only use 5% of the dynamic range, resulting in a much worse SNR (typically 20 dB lower than the phase offset method if the terahertz electric field strength is similar to this work). It might be possible to enhance further the SNR if we simultaneously obtain signals with plus and minus phase offsets [26,32]. The singleshot acquisition of the image can be demonstrated if we use much higher power for the probe, although the data are not shown here.
Figure 2(f) shows the Fourier transform of the line shown in Fig. 2(e). The spectrum of the THz wave indicate clearly that spectroscopy is possible in each pixel of the camera up to 2 THz. The SNR of this line reaches 40 dB, which is very promising for THz line imaging with spectroscopic resolution. This SNR is mainly determined by the shot noise of the camera. Maximum count of the image can be set around several 10000, leading to the SNR of 100 for the electric field strength, and thus 40 dB for the intensity spectrum. Note that averaging vertically over several lines within the diffraction limit will further increase the SNR. The results clearly demonstrate the possibility to enhance the throughput of the terahertz imaging because we only need to scan one dimension (horizontal direction in our setup) for obtaining full 2D information on the transmitted terahertz waveform at each point of the sample. The spectroscopic information acquired by this system allows us not only to analyze materials with different characteristic THz absorptions but also to investigate the modifications of the spectrum in the THz range.
To demonstrate and characterize the imaging capability of the system, we also measured the spatial resolution of the imaging system by blocking part of the object plane with a metal plate. Figure 3(a) shows a typical result of such an experiment with the metal plate blocking part of the vertical extent of the image to characterize the spatial resolution in the vertical direction. In addition to the vertical lines that correspond to the incident THz pulses, we observe several nonvertical lines that originate from the edges of the metal plate. These correspond to the THz pulses scattering from the edge of the metal plate and forming interference patterns. The appearance of these scattered waves is one important advantage offered by the proposed method based on an echelon mirror over the oblique-crossing method of single-shot detection of THz pulses [28]. The echelon mirror can detect the correct waveform of the scattered waves as well, whereas they may distort the THz waveform in the oblique crossing method.
Fig. 3. (a) THz line image acquired with a metal plate blocking the bottom half of the object plane. (b) Vertical line profile of the measured terahertz wave at 1 THz. (c) Vertical and (d) horizontal spatial resolutions as a function of frequency estimated from the fitting shown in (b). Blue triangles, green rectangles and red circles are the estimated spatial resolutions when the metal plate was placed just at the focus of the terahertz wave (0 mm), 5 mm and 10 mm closer to the TCL, respectively. The solid line is the calculated spatial resolution considering the numerical aperture (0.3) of our setup.
The spatial resolution is estimated by fitting the vertical profile for each frequency component by the error function ${I_0}\left\{ {1 + \textrm{erf}\left[ {2\sqrt {\textrm{ln}2} ({x - {x_0}} )/d} \right]} \right\}/2$, as shown in Fig. 3(b), where ${x_0}$ is the central position of the THz wave, ${I_0}$ is the THz intensity, and d is the spatial resolution of the system. To characterize the horizontal resolution, we horizontally scanned the metal plate and examined the decrease in spectral intensity as a function of the position of the metal plate. The data were fit to an error function to estimate the horizontal spatial resolution.
Figure 3(c) and 3(d) show, as a function of frequency, the vertical and horizontal spatial resolution estimated from the width of the error function. The black curve is the diffraction limit of the system calculated by assuming the estimated numerical aperture of 0.3. Also plotted are the data obtained with the metal plate not at the focal plane. The results are consistent with the measured spatial resolution of the system, although the data for the vertical direction are slightly worse compared with the diffraction limit. The difference may be caused by the slight misalignment of the system especially the focusing lens before the sample. Since the imaging system are designed with spherical lenses, the spatial resolutions for horizontal and vertical directions are the same although the sample was illuminated with a cylindrical lens.
Finally, to demonstrate the spectral line imaging, we prepared two sugar tablets with different spectral characteristics at the THz region as shown in Fig. 4(a). The lactose has a pronounced absorption at 0.53 THz whereas the sucrose only has a smooth absorption up to 1 THz. We scan the sample shown in Fig. 4(b) at the horizontal direction and the data for the width of about 36 mm and the height of about 5 mm are obtained. The vertical direction (y-axis) is imaged using our line-imaging system. The obtained full data of the spectral imaging plotted as a 3D image is also shown in the lower part of Fig. 4(b). Figure 4(c) shows the slices of the data at several frequencies, at 0.48 THz, 0.53 THz, 0.58 THz, 1.06 THz and 1.63 THz. The data clearly show the difference of the absorption at 0.53 THz between two tablets where the strong absorption of lactose exists. We can also distinguish the difference at 1.06 THz where the sucrose has stronger absorption.
Fig. 4. (a) Transmittance spectra of the sucrose and lactose tablets. (b) Picture of the sugar tablets under investigation. The latter part shows the full spectral absorption image plotted in color for 3D. (c) Some slices of the 3D transmittance data taken at indicated frequencies.
The results shown in Fig. 4 indicate that the spectral line imaging using an echelon mirror can be very promising for future applications in industry and sciences. The 3D color image of the sample is of essential importance for comparing and distinguishing materials with different THz responses. Using our imaging system, the spectral information is obtained on a single-shot basis, which can avoid the distortion of the THz waveform due to the temporal fluctuation such as changes of the spectrum or position of the sample. It is also possible to combine our system with a compressive sampling technique to expand the spatial dimensions of the imaging [40]. Full and reliable spectral information in the THz region that is obtainable in our system can be the key to realize such applications in the future.
4. Summary
In summary, we demonstrated the spectral line imaging by combining the single-shot THz detection technique by using an echelon mirror with the phase offset method. The obtained signal to noise ratio of the single line reaches 40 dB, offering enough dynamic range for spectroscopy applications. The spatial resolutions for both horizontal and vertical directions are comparable to the diffraction limit of the THz waves at each frequency components. The obtained results of the imaging of sugar tablets demonstrate the high potential of the spectral line imaging for the future applications.
Funding
Ministry of Education, Culture, Sports, Science and Technology (KAKENHI 17H06124, KAKENHI 20H05662).
Acknowledgments
We thank Z. Liu and Y. Takigawa of Nikon Corporation, J. Takeda, K. Asakawa, M. Kobayashi, K. Izumi, and K. Kaneshima of Yokohama National University for helpful discussions.
Disclosures
The authors declare no conflicts of interest.
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