1. Introduction

Terahertz (THz) frequency-range radiation (0.3–10 THz) has been used to demonstrate imaging of objects that are opaque at optical frequencies. There are many applications of coherent THz sources, including medical imaging, security screening, heterodyne receivers, spectroscopy, and trace gas detection [1,2]. Work on THz imaging technology based on THz-QCLs [3,4] has been performed widely for raster-scanning two-dimensional (2D) imaging [5–7], 3D imaging [8], real-time imaging with a microbolometer focal-plane array [9–11], and imaging by self-mixing [12,13]. However, the maximum operating temperature of THz-QCLs has been limited to 200 K so far [14].

As an alternative approach, THz-QCLs based on intra-cavity difference-frequency generation (DFG) have been demonstrated [15]. Currently, these are the only electrically pumped monolithic THz semiconductor sources operable at room temperature [16,17]. The performance of THz DFG-QCLs has rapidly improved by adopting a Cherenkov emission scheme [18–20], and as a result, a peak output power of nearly 2 mW in pulsed operation (duty cycle 1%) and continuous-wave output power of 14 μW both at room temperature have been demonstrated [21,22]. Recently, we have developed THz DFG-QCLs [23] based on the dual-upper-state active region design [24–27]; these devices exhibit broadband emission between 1.6 THz and 3.8 THz with a peak output power of approximately 300 μW in pulsed operation (duty cycle 1%) and a high mid-IR to THz conversion efficiency of 1.2 mW/W² [27]. In addition, an epi-up-mounted continuous-wave DFG-QCL with very low threshold current density (1.3 kA∕cm²) at room temperature has been achieved [28]. Although the device performance of THz DFG-QCLs has significantly improved, the potential for practical applications of these devices has not been investigated so far.

Currently, various room temperature compact THz sources have been reported [2,29,30]. However, the operation frequencies of these sources were basically below 1 THz. Although several devices demonstrate THz emission above 1THz, output powers are still quite low; thus, it is very difficult to apply to practical THz applications. On the other hand, THz DFG-QCLs are able to operate above 1 THz, and the average THz output powers have exceeded 10 μW (duty cycle >5%) at room temperature [27], which can potentially be applied to THz applications [31]. Since THz DFG-QCLs show a better THz far-field pattern [19,21] compared to that of THz-QCLs with metal-metal waveguides [32], these devices could be suitable for non-destructive inspection and quality control [1]. Thus, state-of-the-art DFG-QCL devices could lead to significant size reductions in THz systems. Here, we present the first demonstration of THz non-destructive imaging with room temperature monolithic semiconductor THz light sources. Our THz DFG-QCLs are based on nonlinear mixing between a single mode selected by a distributed feedback (DFB) grating and Fabry-Perot (FP) modes selected by the laser cavity [27], and we demonstrated a Gaussian-like beam pattern as well as ultra-broadband THz emission in the pulsed-current mode. By building a compact imaging system with a high spatial resolution, we obtained highly resolved images of objects obscured by opaque materials.

2. Experimental setup

We performed THz imaging experiments with a broadband THz DFG-QCL device using a DFB/FP configuration operated in pulsed-current mode. The THz DFG-QCL device was operated at –30 °C in order to obtain higher THz output power, which can be achieved by increasing the mid-IR pump power product when the device is cooled. The output power of the device at –30 °C was about twice as high as that at room temperature. The device was driven at a current of 1.2 A with a 100 kHz repetition rate and 2% duty cycle. THz spectra were taken in rapid scan mode at a resolution of 0.2 cm⁻¹ with a Fourier transform infrared (FTIR) spectrometer equipped with a helium-cooled bolometer. The THz light-current characteristic measured with the DFB/FP device is shown in the inset in Fig. 1(a). The device provided an ultra-broadband multi-mode THz emission spanning from 1.5 to 3.3 THz, as shown in Fig. 1(a), in which multi-mode emission lines at 2.6 and 2.8 THz are observed to be attenuated by water vapor absorption.

 

Fig. 1 Properties of THz DFG-QCL at –30 °C: (a) Spectrum, (b) far-field profile, (c) horizontal and vertical sections of the beam profile.

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Thanks to the adoption of Cherenkov phase matching scheme [19,21], THz DFG-QCLs have exhibited single-lobed beam distributions. To map the 2D far-field emission patterns of the devices, we used a set-up consisting of two motorized XY translation stages. A Golay-cell detector with an active area diameter of 6 mm was mounted on the translation stages and was placed about 59 mm away from the device to scan laser beam profiles along the emission plane. Our measurements were performed in a two-dimensional (2D) scan (62.5 mm × 62.5 mm) at intervals of 1.25 mm. Figure 1(b) shows a THz beam pattern of our device operating with 1% duty cycle and 100 kHz repetition rate at –30 °C. From the vertical and horizontal profiles (Fig. 1(c)), we estimated beam FWHM values of 30.8 mm along the slow axis (perpendicular to the epi-growth direction) and 25.3 mm along the fast axis (parallel to the epi-growth direction); the corresponding divergence angles were 29.3° and 24.2°, respectively. The narrow FWHM along the fast axis was due to the large THz aperture corresponding to the thickness of the InP substrate (~350 µm). In edge-emitting metal-metal THz QCLs, ring-like fringe patterns in their far-field beams are frequently observed due to far-field interference of coherent radiation in deep sub-wavelength apertures [32]. In contrast, our DFG-QCL emitting THz light through the large substrate aperture showed a Gaussian-like beam pattern that is favorable for THz imaging applications. It should be noted that in most terahertz imaging systems, the terahertz beam is refined by passing it through a spatial filter [33,34], which is used for enhancing image features. On the other hand, in our imaging system, a spatial filter is not necessary thanks to the superior beam profile.

Figure 2 depicts the experimental setup of our transmission imaging system. The THz beam from our device was collimated with an off-axis parabolic mirror (f = 50 mm) and was focused onto a test object, using an aspheric plastic lens (f = 40 mm; Tsurupica, Pax Co.). Then, the terahertz beam transmitted through the object was collimated using a Tsurupica lens (f = 40 mm) and was collected on the Golay-cell detector using another off-axis parabolic mirror (f = 100 mm). Terahertz images were acquired by the raster-scanning method. The object was mounted on a computer-controlled two-axis translation stage. As shown in Fig. 2, our imaging system was compact thanks to the lack of an optical path for inserting a small-aperture spatial filter. The imaging experiment was performed in an ambient air environment without being purged with dry nitrogen or dry air.

 

Fig. 2 Schematic representation of the transmission imaging setup: OAP1, off-axis parabolic mirror for collimating THz beam; L1, focusing lens; L2, collimating lens; OAP2, off-axis parabolic mirror for focusing on detector.

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3. Results and discussion

To evaluate the performance of our imaging system, the focused beam waist at the test object position was obtained by means of the knife-edge technique. The result is shown in Fig. 3. The distance between the positions of the blade where 11.9% and 88.1% of the full intensity were transmitted is defined as the beam waist [35,36]; it is equal to the full width at half maximum (FWHM) of a Gaussian beam. From the result of the knife-edge technique, the horizontal and vertical beam waists of the spot were estimated to be 0.6 mm and 0.5 mm, respectively. The Airy disc diameter, a, which corresponds to theoretical minimum spot size, is defined as:

 

Fig. 3 Knife-edge scanning of the terahertz beam at the focus position.

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Otherwise, using λ = 136 μm (corresponding to the center of frequency of the broadband terahertz emission from this device) and NA of our imaging system, the theoretical focused spot sizes for the x- and y-directions are 0.5 mm and 0.4 mm, respectively, as estimated by Eq. (1). Here, NA values in horizontal and vertical direction are calculated to be 0.33 and 0.45 because horizontal and vertical collimated beam diameters at L1 are approximately 26 mm and 36 mm. Considering the theoretical focused spot size, the actual focused spot size reached almost the theoretical limit. Thus, we found that our measurement system was capable of good imaging despite the lack of a spatial filter; it was difficult for conventional THz-QCLs with metal-metal waveguides because of their poor beam quality [32,33].

Imaging was demonstrated using a stainless-steel test object having slits formed periodically along the horizontal and vertical directions. Figure 4(a) shows a photograph of the test object. As shown in Fig. 4(a), the upper part of the test object had 1 mm-wide stripes, and the lower part had 0.5 mm-wide stripes. The scanning step length during the imaging was 0.2 mm. The imaging result is shown in Fig. 4(b). Both vertical and horizontal stripes of 0.5 mm were observed. Figures 4(c)-4(d) depict the line profile extracted from the Figs. 4(b) along the horizontal solid lines (1-1’, 2-2’). Figures 4(e)-4(f) depict the line profile extracted from the Fig. 4(b) along the vertical solid lines (3-3′, 4-4’). Here, the modulation depth M is defined as:

 

Fig. 4 (a) Photograph of test object whose thickness was 0.3 mm. (b) Terahertz image of test object obtained by our terahertz imaging system. The line profile of terahertz intensity along the solid line 1-1’, 2-2’, 3-3′ and 4-4’ ((c), (d), (e) and (f) respectively).

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where I_max and I_min are maximal and minimal intensities of the image profile. For 0.5 mm width strips along the horizontal direction (x axis) and the vertical direction (y axis), M were about 0.23 and 0.38. From these results, we found that spatial resolution of the vertical direction is better than that of the horizontal direction. Because the focused beam is elliptical the beam size along the vertical direction is smaller.

Subsequently, we demonstrated the imaging of the test object obscured by opaque materials. The object (a test object, made of a stainless-steel plate partially etched with the letters “HPK”) was placed in a paper envelope, and the imaging was performed by the raster-scanning method (100 × 180 pixels, scan step of 200 μm along both horizontal and vertical directions) [5–7]. Sample images taken by a visible-light camera and by our THz imaging system with the DFG-QCL are shown in Figs. 5(a) and 5(b), respectively.

 

Fig. 5 (a) Photograph of stainless-steel plate with openings in the shape of the letters “HPK”. (b) Terahertz image of plate in paper envelope obtained with THz DFG-QCL. (c) Photograph of a rubber band, a plastic button, a clip, and a cutter knife blade. (d) Terahertz image of small articles in paper envelope obtained with THz DFG-QCL.

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Furthermore, as other test objects, we placed a rubber band, a plastic button, a paper clip, and a cutter knife blade in a paper envelope (Fig. 5(c)) and obtained the terahertz image shown in Fig. 5(d), acquired with the THz DFG-QCL (100 × 125 pixels, 400 μm steps). The high-quality THz images confirmed that our DFG-QCL, showing good beam quality and high THz output power, can be employed in various THz applications. The plastic button, paper clip and cutter knife blade appeared dark in Fig. 5(d), whereas the double-sided adhesive which was used to fix the paper clip to the envelop appeared as neutral tints (grey) because the adhesive was moderately transparent to terahertz waves. For the images of the plastic button and metallic materials, we observed that signal value reaching the detector is equivalent to the noise level of the detector. In general, plastic buttons are made of Nylon or ABS (acrylonitrile butadiene styrene), which have high absorption coefficient (α $\ge$ 40 cm⁻¹ at 2 THz) [37,38]. According to our estimation, the transmitted terahertz power decreases more than six orders of magnitude. Therefore, because the terahertz intensity was attenuated below noise level of the detector, the plastic button appeared dark, as is the case in metallic materials of the terahertz image. Acquisition time which was limited to a time constant of the Golay-cell detector was about 2 s per a point. For the Fig. 5(d), it took about 6 hours to obtain the image (12500 points). The cause of the long acquisition time is not due to a terahertz light source but to a slow response time of the Golay-cell detector. Using the high-speed detector, faster terahertz imaging with THz DFG-QCL will be possible; in fact, high-speed terahertz detector have recently been investigated, for example, a bolometer based on microelectromechanical (MEMS) resonator [39]. In this paper, we demonstrated the ability to perform non-destructive imaging using a THz DFG-QCL.

Finally, we evaluated the imaging performance with the THz DFG-QCL operating at room temperature (15 °C). Figure 6(a) shows a photograph of the THz DFG-QCL package. The same device was assembled in a fully hermetic, butterfly-style package that incorporated a thermoelectric cooler (TEC). The THz output was coupled out via a polyethylene window. The package was designed to operate on an air-cooled heat sink. The THz spectrum shown in Fig. 5(b) was basically similar to the result observed at –30 °C. Figure 6(c) shows a terahertz image of the stainless steel plate with nothing on it (80 × 180 pixels, 200 μm steps). We found that terahertz images were obtained using our THz DFG-QCL at room temperature. Since we confirmed that the far-field pattern is virtually independent of temperature between –30 °C and room temperature, the difference between–30 °C and room temperature can be considered only in terms of THz output power.

 

Fig. 6 (a) Photograph of hermetic butterfly-style package. (b) Room-temperature spectrum of the THz DFG-QCL. Inset shows the current–voltage–THz output power characteristic at room temperature. (c) Terahertz image of stainless steel plate obtained with room temperature THz DFG-QCL.

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4. Conclusion

We demonstrated THz imaging with a broadband THz DFG-QCL. Imaging of objects obscured by opaque materials was demonstrated with a DFG-QCL operating at –30 °C, which can be attained using thermoelectric coolers. Furthermore, we successfully performed terahertz imaging with a THz DFG-QCL operating at room temperature (15 °C). Due to the Gaussian-like beam profile of the THz DFG-QCL, our imaging system using a Golay-cell detector provided high-quality images without any extra optical components, and ring-like fringe patterns were not observed in the images. As a next step, we will further improve the output power in order to realize real-time imaging with an uncooled 2D micro-bolometer camera, which may widen the range of applications in the terahertz spectral region.