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Abstract
We report the demonstration of imaging of a single human hair with a terahertz quantum cascade laser (THz-QCL) source based on intracavity difference-frequency generation. A single human hair whose diameter was about 100 µm was detected using the THz-QCL source operating at 240 K, of which the THz beam had a linear polarization. The results show that the THz image of a human hair clearly depends on the polarization direction of the THz beam. The THz QCL sources that are capable of room temperature operation will be useful for detection of small foreign objects like human hairs.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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 [1,2]. There are many imaging applications, including biomedical [3–6], security screening [7] and the study of historical artifacts [8–11]. Recent research on THz nondestructive testing applications based on polarization imaging has made impressive progress; for example, internal deformation and internal filler alignment in elastomers [12–14] and anisotropy of birefringent materials [15–17] have been measured.
On the other hand, techniques for detecting foreign objects in food or medical products play an important role in manufacturing. THz waves are expected to be capable of detecting foreign bodies in industrial products [18], and they may be particularly useful for the detection of human hairs, since the THz transparency of human hairs decreases with increasing frequency [19].
However, widespread utilization of such THz technology has not yet been realized due to the lack of compact light sources. THz quantum cascade lasers (THz-QCLs) are the most promising light source in the THz region. Work on THz imaging technology based on THz-QCLs [20,21] has been performed widely for raster-scanning two-dimensional (2D) imaging [22–24], 3D imaging [25], real-time imaging with a microbolometer focal-plane array [26–28], and imaging by self-mixing [29,30]. However, the maximum operating temperature of THz-QCLs has been limited to 210.5 K so far [31]. In addition, when an image of a small object is obtained with a coherent, single-mode THz-QCL, it is difficult to resolve the object because fringes or speckle patterns appear in the THz image.
An alternative approach is based on intracavity difference-frequency generation (DFG) in a dual-wavelength mid-IR QCL in which two different laser active regions are integrated [32]. These devices, known as THz DFG-QCLs or THz NL-QCLs (THz nonlinear quantum cascade lasers), use mid-IR active regions engineered to exhibit a large intersubband nonlinear susceptibility, $\chi ^{(2)}$, in order to achieve an efficient THz DFG process. Currently, these are the only electrically pumped monolithic THz semiconductor sources operable at room temperature in the 0.6 to 6 THz range [33–36]. The performance of THz NL-QCLs has been improved by adopting a Cherenkov emission scheme [37,38]. In addition, there have been recent efforts in wave-function engineering using a dual-upper-state (DAU) active region [39,40], which possesses a broad gain bandwidth. In this approach, instead of integrating two QCL active regions for dual-wavelength emission, THz generation is achieved in mid-IR QCLs based on the DAU active region with a device structure almost identical to that of commercialized, typical mid-IR QCLs [41,42]. In fact, the DAU active region leads to a significant improvement in terms of device performance, as well as the optical nonlinearity of the active region for efficient THz generation. As a result of the enhanced optical nonlinearity, this approach expands the frequency range down to subterahertz [36].
Since THz NL-QCLs are compact semiconductor light sources that do not need a cooling system, they have great potential for practical applications. Furthermore, by adopting the Cherenkov phase-matching scheme, THz NL-QCLs have good beam quality compared with THz-QCLs using metal–metal waveguides. Therefore, THz NL-QCLs are suitable for imaging applications. In fact, we performed high-quality nondestructive imaging using a THz NL-QCL [43].
In this paper, we demonstrate imaging of a single human hair using a THz NL-QCL. Although THz imaging of a single human hair based on near-field imaging has been reported [44], THz imaging of a single human hair based on far-field imaging has been rarely reported because a human hair in a THz image would be hidden by noise-like fringes or speckle patterns. Since the THz beam from a THz NL-QCL has a single-lobed Gaussian-like pattern, no fringe pattern or speckle noise is expected to appear on the image [43]. Therefore, a THz NL-QCL could be suitable for THz imaging of small objects such as human hairs. In this experiment, we obtained an image of a 100 µm-diameter single hair with the beam from a THz NL-QCL while taking into account the beam polarization, because a human hair has polarization dependency.
2. EXPERIMENTAL SETUP
We performed THz imaging experiment with a broadband multimode THz NL-QCL by using a DFB/FP configuration operated in pulsed mode; details of the THz NL-QCL are described in Ref. [41]. The THz NL-QCL device was operated at 240 K, which is a temperature that can be achieved with thermoelectric coolers (TECs), 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. This device exhibited a broadband THz emission spectrum between about 1.0 THz and about 3.3 THz [43]. The THz NL-QCL was driven at a current of 1.7 A with a 2% duty cycle and a 100 kHz repetition rate. Figure 1(a) shows the current–voltage–THz output power characteristic at 240 K. We measured a THz peak output power of ${\sim}{0.4}\;{\rm{mW}}$ at 240 K in front of the device, using a pyroelectric detector. Figure 1(b) shows the THz spectrum obtained at 240 K, which indicates ultrabroadband, multimode THz emission spanning from 1.0 to 3.5 THz. Figure 2 depicts the experimental setup of our transmission imaging system, in which the THz NL-QCL was placed at the focal point of an off-axis parabolic mirror (OAP1, $f = {{50}}\;{\rm{mm}}$). The THz beam from our device, in which the electric field of the light generated in the QCL was aligned along the growth direction of the heterostructure, was collimated with OAP1 and was focused onto a test object using an aspheric plastic lens ($f = {{40}}\;{\rm{mm}}$; Tsurupica, Pax Co.). Then, the THz beam transmitted through the object was collimated using a Tsurupica lens ($f = {{40}}\;{\rm{mm}}$) and was collected on the Golay-cell detector using another off-axis parabolic mirror (OAP2, $f = {{100}}\;{\rm{mm}}$). THz images were acquired by the raster-scanning method. The object was mounted on a computer-controlled two-axis translation stage. The Airy disc diameter, $a$, which corresponds to theoretical minimum spot size, is defined as
where $\lambda$ is the wavelength of the light, and NA is the numerical aperture. In the case of broadband light source, we calculate theoretical beam spot size using central wavelength [45]. Using $\lambda = {{136}}\;\unicode{x00B5}{\rm m}$ (2.2 THz), the theoretical focused spot size is calculated to be 0.4–0.5 mm. In a previous work, the results of modulation depth of test object (line pair pattern having 0.5 mm width stripe) were over 20%. According to the 20% modulation threshold criterion, the spatial resolution was better than 0.5 mm [43].
Fig. 1. Properties of THz NL-QCL at 240 K. (a) Current–voltage–THz output power characteristic; (b) THz spectrum.
In an experiment to measure the polarization properties and imaging with a polarizer, we placed a wire-grid polarizer between OAP1 and the focusing lens. Since mid-IR laser emission from the THz NL-QCL was TM-polarized, the THz beam was expected to be horizontally polarized, as shown in Fig. 2.
Fig. 4. (a) Photograph of hair with sample holder; (b) magnified photograph of the hair obtained by laser scanning microscope.
Fig. 5. THz image of single human hair. (a) Oriented in vertical direction; and (b) oriented in horizontal direction; (c) THz image without human hair.
3. RESULT AND DISCUSSION
We investigated the polarization properties of the THz beam from the THz NL-QCL. Figure 3 shows the polarization properties of the THz NL-QCL obtained using a commercially available wire-grid polarizer (PW010-025-075, Purewave Polarizers Ltd.). From Fig. 3, we clarified that the THz beam from the THz NL-QCL was linearly polarized in the horizontal plane, as expected from the characteristics of the QCL, in which the emitted mid-IR radiation (pump) is inherently TM-polarized.
Accordingly, we obtained THz imaging of a single human hair with the THz NL-QCL. Figure 4 shows (a) a photograph of a hair sample in a sample holder and (b) a magnified photograph of the hair obtained by a laser scanning microscope. The diameter of the single human hair was found to be about 100 µm. We obtained a THz image (${{60}}\;{\rm{pixels}} \times {{60}}\;{\rm{pixels}}$, 0.2 mm steps) using our transmission imaging system without a polarizer. Figures 5(a) and 5(b) show THz images of the human hair oriented in the vertical direction and the horizontal direction. Because the polarization of the THz beam was in the horizontal direction, the hair oriented in the vertical direction [Fig. 5(a)] corresponds to a hair axis perpendicular to the polarization direction, and the hair oriented in the horizontal direction [Fig. 5(b)] corresponds to a hair axis parallel to the polarization direction. Figure 5(c) shows a THz image without a human hair, for comparison. Although the diameter of the hair was small enough compared to the focused beam diameter, we could observe both the hair oriented in the vertical direction and the hair oriented in the horizontal direction. In addition, the hair diameter in the vertical direction was observed to be larger than that of the hair in the horizontal direction.
To produce highly resolved THz images, we chose an area of ${{2}}\;{\rm{mm}} \times {{2}}\;{\rm{mm}}$ [solid line in Fig. 5(a) and 5(b)], and the scan was performed in steps of 0.05 mm with four integrations in each step. Figures 6(a) and 6(b) show highly resolved images of the samples in the vertical and horizontal directions. Compared with the hair oriented in the horizontal direction, the intensity of the hair oriented in the vertical direction was much less, and the width of the hair silhouette was larger. We considered that this is because the THz beam was attenuated by scattering effects, which depended on the relationship between hair direction and the polarization direction of the incident wave. In the case of investigating a single human hair using visible light, the intensity of the scattering light with a hair axis perpendicular to the polarization direction is much larger than that of light with a hair axis parallel to the polarization direction [46]. The THz intensity is also considered to be strongly scattered by hair oriented in the vertical direction when illuminated by a horizontally polarized beam. Therefore, in the case of the THz wave, hair oriented in the vertical direction was highly attenuated and diffused compared with hair oriented in the horizontal direction.
Fig. 6. (a) Highly resolved THz image of single human hair oriented in vertical direction; (b) line profile of the human hair oriented in vertical direction along the horizontal broken line; (c) highly resolved THz image of single human hair oriented in horizontal direction; (d) line profile of the human hair oriented in horizontal direction along the vertical broken line.
We obtained THz images of a hair without a polarizer. For comparison, we obtained a THz image of the hair using the THz NL-QCL with a polarizer. Figure 7 shows THz images of single human hairs oriented in the vertical direction and the horizontal direction (${{60}}\;{\rm{pixels}} \times {{60}}\;{\rm{pixels}}$, 0.2 mm steps) using a polarizer (PW010-025-075, Purewave Polarizers Ltd.). In addition, Figs. 8(a) and 8(b) show line profiles of the normalized THz intensity along the vertical axis and the horizontal axis, respectively. We found that the line profile obtained from the THz image of the hair with the polarizer was in good agreement with the line profile obtained from the THz image of the hair without a polarizer. These results indicate that THz imaging of a single human hair, showing polarization dependency, could be performed by using a THz NL-QCL without a polarizer.
Fig. 7. THz image of single human hair using polarizer. (a) Oriented in vertical direction and (b) oriented in horizontal direction.
Fig. 8. Line profile of THz normalized intensity without polarizer (black) and with polarizer (red). (a) Along horizontal axis and (b) along vertical axis.
4. CONCLUSION
In this work, we acquired a THz image of a single human hair by using a THz NL-QCL. Although THz imaging of single human hairs based on near-field imaging has been reported, THz imaging of single human hairs based on far-field imaging has been rarely reported because a human hair in a THz image would be hidden by noise-like fringes or speckle patterns. The THz beam from our THz NL-QCL had a single-lobed Gaussian-like pattern, and therefore no fringe pattern or speckle noise appeared on the image. Therefore, THz NL-QCLs will be suitable for THz imaging of small objects such as human hairs. In the case of observation of a collection of hair, it would be difficult to distinguish each hair. However, we consider that it is important to recognize whether there are foreign objects (hairs) rather than distinguishing each hair.
In addition, we investigated the polarization properties of the THz emission from the THz NL-QCL (for the first time, we believe), and we obtained THz images of a human hair by changing the relationship between the sample and the polarization direction of the incident THz waves. We found that a different THz image was obtained by changing the relationship between the hair axis and the polarization direction of the incident THz wave. Also, we confirmed the human hairs are detectable despite the direction of hair axis.
We expect that polarization imaging with THz NL-QCLs will become a useful technique for nondestructive testing. This work is an important step towards the realization of nondestructive testing based on polarization imaging with a THz NL-QCL.
Funding
Ministry of Internal Affairs and Communications (MIC/SCOPE) (195006001).
Acknowledgment
The authors express their gratitude to A. Ito for sample preparation and device fabrication, and to S. Hayashi for his technical support in the THz measurements. The authors also wish to acknowledge T. Edamura for his valuable comments.
Disclosures
The authors declare no conflict of interest.
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