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Real-time terahertz (THz) wave imaging has wide applications in areas such as security, industry, biology, medicine, pharmacy, and the arts. This report describes real-time room-temperature THz imaging by nonlinear optical frequency up-conversion in an organic 4-dimethylamino-N’-methyl-4’-stilbazolium tosylate (DAST) crystal, with high resolution reaching the diffraction limit. THz-wave images were converted to the near infrared region and then captured using an InGaAs camera in a tandem imaging system. The resolution of the imaging system was analyzed. Diffraction and interference of THz wave were observed in the experiments. Videos are supplied to show the interference pattern variation that occurs with sample moving and tilting.
© 2015 Optical Society of America
1. Introduction
Terahertz (THz) imaging has important applications in many areas such as security, science, the arts, agriculture, and industry. Extensive studies have been reported for explosive, weapons and drugs detection [1,2], integrated circuit inspection [2], cancer diagnosis [3,4], semiconductor wafer characterization [5,6], plant breeding [7], art painting investigation [8], non-destructive quality monitoring of products such as foods [7], tablet film [9], plastic weld joints [10], car body painting [11], space shuttle foam [12], and so on. Real-time imaging at room temperature is usually desirable in both scientific studies and practical applications. Continuous efforts have been undertaken to achieve THz real-time imaging.
With electro-optic (E-O) imaging [13], a pioneer approach to obtain real time THz imaging, one can achieve real-time two-dimensional (2D) imaging or spatiotemporal imaging [14] by one shot.
Another means is using a focal plane array (FPA) device. Various types of FPA detectors have been developed, such as arrays based on micro-antenna [15], micro-bolometer [16], thermocouple [17], field-effect-transistor (FET) [18,19], Schottky barrier diode [20], heterostructure backward diode [21], and high-temperature superconducting Josephson junction [22]. These devices are important for specific applications where extreme performance characteristics are required.
Nonlinear optical wavelength up-conversion is another promising technique for wave detection because, theoretically, any wavelength can be converted to a new one that is within the sensitivity range of a photon detector that incorporates mature technology [23,24]. In single-point detection, excellent sensitivity has been reported using lithium niobate (LN) [25], 4-dimethylamino-N’-methyl-4’-stilbazolium tosylate (DAST) [26,27], ZnGeP2 [28] and GaAs [29] in the THz-wave region. Now, commercial FPA detectors in the visible and near-infrared (IR) region show sufficient sensitivity for applications in frequency up-conversion imaging. In the mid-IR region, recent reports show excellent sensitivity for frequency up-conversion imaging at the level of a single or few photons [30,31]. In the THz region, our previous report described real-time THz imaging by optical frequency up-conversion in a DAST crystal [32], with a higher sensitivity than a commercial micro-bolometer THz camera, thanks to the high nonlinearity of the DAST crystal [33]. Recently, a commercial real-time THz imaging system by frequency up-conversion in a nonlinear crystal utilizing a high-power quasi-continuous-wave (quasi-CW) THz source is also presented [34].
Real-time imaging with high resolution is important because more useful information can be obtained in one shot. Generally, in real-time THz wave imaging, diffraction-limited resolution is the best achievement and is sufficient for general applications.
As described herein, we developed a real-time room-temperature THz imaging by nonlinear optical frequency up-conversion in a DAST crystal, with high resolution mainly limited by the THz diffraction. Using a tandem imaging system, a THz wave image was converted to the near-infrared region and then captured using an InGaAs camera. Resolution of the imaging result is shown. Results agree well with the calculation result based on the diffraction limit theory. Images of THz diffraction after an alphabetic aperture and THz interference are readily apparent in a thin silicon wafer. We also supply videos showing the interference pattern variation along with the wafer moving and tilting.
2. Experiment system
We modified the projection imaging system reported previously [32] to the current tandem imaging system, as presented in Fig. 1. Most parameters of this system are the same as those described in our previous report. They will not be repeated here.
Fig. 1 Experiment setup: FL, focal lens; PBS, polarizing beam splitter; OAP, off-axis parabolic mirror; HWP, half-wavelength plate; and OBJ, object plane. ITO glass is a glass plate coated with an indium tin oxide film. Filter 1 absorbs the green laser and is clear for the near IR laser. Filter 2 is a Ge plate. Filters 3 are bandpass filters that block the pumping laser for up-conversion.
In this system, THz waves at 19.3 THz were generated by difference-frequency generation (DFG) in a DAST crystal (DAST 1) pumped by 1409 nm and 1292 nm laser with total energy of 0.76 mJ from the dual-wavelength OPO system with dual-KTP crystals. The THz wave was then collimated by an off-axis parabolic mirror (OAP 1) with 50.8 mm focal length.
The collimated THz-wave by OAP1 was irradiated to the object at the OBJ plane. Furthermore, the THz-wave transmitted through the object or attenuated was reflected by ITO glasses. Then imaging was done on a second DAST crystal (DAST 2) by an OAP 2 mirror. ITO glass 1 was a 0.7-mm-thick glass plate coated with 90 nm indium tin oxide (ITO) film. ITO glass 2 plates were two 1-mm-thick glass plates coated with 200 nm ITO film. The OAP 2 focal length was 50.8 mm. The DAST 2 crystal was set at the image plane of the OAP 2 mirror. The 1292-nm pumping laser for frequency up-conversion comes from the single KTP-OPO, with pulse energy of 0.4 mJ. ITO glass 1 has a transmission of 50% for the IR laser.
To obtain good resolution, the projection imaging system depends greatly on the beam collimation condition, whereas in an imaging system, collimation is not a problem of resolution but one of energy conversion efficiency. To obtain sufficiently high up-conversion efficiency, a much smaller THz image on the nonlinear crystal is required. It means, according to the thin lens formula, a long object distance is unavoidable for a lens with focal length of 50.8 mm. Therefore, we designed a folded THz wave scheme to minimize the system dimension. Consequently, the current system has almost identical dimensions to those of the previous projection imaging system.
The THz image size on the DAST 2 is determined after evaluating the whole system. A smaller image size will increase the THz wave power density and the up-conversion pumping laser, and therefore results in a higher conversion efficiency. Because of the long wavelength of the THz wave, a smaller image engenders worse resolution as a result of diffraction. Considering the diffraction limit of THz wave on the DAST 2, for this system that has a smaller image than the object, we have
where r stands for the resolution on the object plane, λ represents the wavelength of the THz wave, DB signifies the collimated THz wave beam diameter, and Do denotes the object distance of this imaging system. According to Eq. (1), when a larger THz beam size and a smaller object distance are used, better resolution is achieved. The best possible resolution might reach the order of λ. However, it also entails a larger THz image size and lower efficiency of frequency up-conversion. Therefore, a tradeoff of the resolution and the up-conversion efficiency should be made carefully according to the desire. In our case, the distance from the OBJ plane to OAP 2, i.e. the object distance, was chosen as about 1 m. The image plane was about 53.5 mm distant from the OAP 2 mirror.By selecting a proper focal lens for the IR beam, the pumping laser beam size approximately matched the size of the THz-wave image on DAST 2, to get conversion for the full THz image, following the phase-matching condition of DFG. Then the up-converted image was imaged again into the FPA of an InGaAs camera using a lens set with two lenses with a focal length of 50 mm and 100 mm. Therefore, the THz-wave up-conversion and imaging part compose a tandem imaging system, which transfers the THz image information to the IR camera. The magnification of the imaging sub-system for the near-infrared should also be considered carefully, so that the resolution of the CCD array does not limit that of the whole system.
3. Results and discussion
The imaging results are presented in Fig. 2. Figure 2(a-1) portrays the object we used at the OBJ plane. It is an aperture with the RIKEN logo shape cut from an aluminium foil, adhered with a paper to hold the shape. A ruler is shown to indicate that its size is about 6 mm × 6 mm. Figure 2(a-2) is the imaging result of Fig. 2(a-1). The imaging is extremely clear with good resolution. Figure 2(b-1) is a larger aperture with a size of about 15 mm × 22 mm. Furthermore, Fig. 2(b-2) is its THz image. It is apparent from Fig. 2(b-2) that the THz beam on the object is about 20 mm in diameter. Actually, the THz beam diameter can be controlled by changing the focal length of OAP 1 mirror, and the divergence angle of the pumping laser for THz generation. A larger imaging area is possible for real-time imaging in case of sufficient THz wave energy supplied.
Fig. 2 Imaging results: (a-1) a metal ruler with a self-made sample with a RIKEN logo shape aperture cut from aluminium foil; (a-2) imaging result of (a-1); (b-1) another sample with a larger RIKEN logo shape aperture; (b-2) the imaging result of (b-1), showing the THz beam size.
For quantitative analysis of the resolution, we show intensity profiles of the imaging result of Fig. 2(a-2) in Fig. 3. Figure 3(a) is a colour version of Fig. 2(a-2) showing more clearly the intensity distribution inside the image. Figures 3(b) and 3(c) respectively show profiles of Fig. 2(a) along the green dashed line and the orange dotted line in Fig. 3(a). The centre parts were enlarged to show details clearly. It is apparent that the normalized intensity rises from 10% to 90% in about 800–900 μm. One pixel in the IR camera corresponds to about 160 μm in the object plane. Therefore, the measured resolution is about 900 ± 160 μm, considering the reading error of one pixel.
Fig. 3 Resolution analysis: (a) colour-map of the imaging results presented in Fig. 2(a-1); (b) and (c) respectively show the profiles along the dashed green and the dotted orange line shown in (a). Centre parts were enlarged to clarify details.
The THz wave image in DAST 2 crystal was about 2 mm in diameter for higher conversion efficiency. Also, the THz wave wavelength is much longer than the IR laser. We can know that the diffraction limit at DAST 2 limited the resolution of the whole system. In our system, λ is 15.5 μm, DB is about 20 mm, and Do is about 1 m. According to Eq. (1), we can obtain the resolution at the objective plane to be 945.5 μm. Therefore, the calculated resolution agrees quite well with the experimental one, considering the error of measurement, which means that the resolution is quite close to the diffraction limit of this system.
In previous reports, we described that a commercial micro-bolometer THz camera is not sufficiently sensitive to obtain an image in our system [32]. Considering that the diffraction patterns and the interference patterns usually have much weaker intensity than the source, it is more difficult to see them. By virtue of the high sensitivity and the resolution of this system, we observed diffraction patterns and interference patterns in the THz region.
A small 5 mm × 6 mm “K”-shaped aperture, resembling that portrayed in Fig. 4(a-1), was put at the object plane. Furthermore, the DAST 2 was moved back slightly from the OAP 2 mirror, to produce an image of the THz distribution immediately behind the aperture, it is possible to view the diffraction pattern. Figure 4(a-2) presents the originally obtained diffraction pattern image. The diffraction pattern is visible, but it is not very clear. Therefore, we tuned the brightness and contrast. Figure 4(a-3) shows the refined results. It is apparent that more than three diffracted stripes were recorded.
Fig. 4 Diffraction and interference results: (a-1) sample; (a-2) original image of the diffraction pattern; (a-3) enhanced image to show the diffraction more clearly; and (b-1) silicon wafer with both surfaces polished. Interference between the two surfaces is shown in (b-2). Videos of interference pattern moving along with the wafer (Media 1) and changing along with tilting of the wafer (Media 2) are supplied. (b-3) Enhanced interference pattern after procession, as described in the text.
When a thin silicon wafer with double sides polished was inserted to the THz beam, and the system was tuned to image the THz distribution in the wafer, a clear interference pattern was observed. Figure 4(b-1) depicts the silicon wafer we used. It is a Φ2 inch × 0.5 mm p-type silicon wafer with both sides polished. Figure 4(b-2) portrays the original interference pattern image. To show clearly that the interference comes from this wafer, we supply two videos: Media 1 and Media 2. Media 1 shows the wafer moved up and down by hand and the interference pattern moved along with the wafer. It means the pattern came from the interference of the surfaces of the wafer which are not perfectly parallel to each other. Media 2 shows the hand-held wafer, which was then tilted to change the effective thickness of the wafer in the THz beam. Thereby, the interference order was changed. The centre changed from bright to dark and vice versa when the wafer was tilted. Pockmarks in the beam derive from the low-quality tiny area in the DAST crystal. The video flickers slightly because of the asynchronism of the laser source (100 Hz) and the camera capture rate (60 Hz). Figure 4(b-3) presents an enhanced image of Fig. 4(b-2), which was obtained by subtracting the reference THz beam image without the sample. Considering that the transmission of the silicon wafer is not 100%, we scaled the reference THz beam image with an attenuation ratio of 0.8 before subtraction, to clarify the interference pattern image. We also tried a single-surface-polished silicon wafer with other conditions completely identical with the wafer portrayed in Fig. 4. No interference pattern was observed, confirming that the pattern derives from THz-wave interference inside the wafer with both surfaces polished. By measuring and analysing the interference patterns, it is possible to use this system to check the semiconductor wafer surface parallelism, or the parameters inside, such as the carrier uniformity, material uniformity and refractive index distribution.
This system is a practical real-time direct-imaging system for online monitoring of the quality of the semiconductor wafers, car body coating, bubbles or rust under painting, and so on. For example, Fig. 5 shows one single-frame excerpts from a video (Media 3) recording the real-time THz imaging for doping uniformity of a hand-held n-type GaAs wafer. The Φ2 inch × 0.35 mm GaAs wafer was doped by silicon with a carrier concentration in the order of 1018 cm−3. Localized high doping parts resulted in lower transmission and became shadow areas in the video. No interference was observed for only one side of the wafer was polished. Another sample from the same company was also tried and found to be much clearer inside.
Fig. 5 One single-frame excerpts from a video (Media 3) recording the real-time THz imaging for doping uniformity of a hand-held n-type GaAs wafer.
In addition, by virtue of the imaging design of the system and the collimated THz beam, it has possible extended advanced applications by combination with interferometers, such as the Mach–Zehnder or Michelson interferometers, to obtain localized interference fringes at the desired plane to measure information carried by the THz phase.
It is worthwhile to mention that besides the 19.3 THz used in this report, 16.5 THz and 4.3 THz were also proved to work well in this system. The wide-range frequency tunability of the DAST crystal makes it possible for this system to work from a few THz to more than twenty THz.
4. Conclusion
This report describes real-time room-temperature THz imaging by nonlinear optical frequency up-conversion in a DAST crystal, with a high-resolution limited by diffraction. Resolution of the imaging result is shown to be close to the calculation based on diffraction limit theory. Furthermore, results showing THz diffraction after passage through an alphabetic aperture and THz interference in a thin silicon wafer are shown. We also supply videos showing the interference pattern change along with the wafer moving and tilting.
Acknowledgments
The authors would like to thank Prof. H. Ito of RIKEN and Prof. K. Kumano of Tohoku University for excellent discussions and comments. The authors are grateful Ms. M. Saito in RIKEN for providing the DAST crystals, and Ms. Y. Kamata in RIKEN for the FTIR measurement, which helped greatly during our experiments. This work was partially supported by the Strategic International Cooperative Program (Japan–Singapore) and (Japan–France), Collaborative Research Based on Industrial Demand of the Japan Science and Technology Agency (JST), and the JSPS Sakura-project (I2012651), KAKENHI (23360045), (23560053), (24560535), (25286075), (25400436), (25220606), (26246046), (26287067), (50302237).
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