Scanning laser terahertz near-field imaging system - OSA Publishing

Scanning laser terahertz near-field imaging system Kazunori Serita,1,* Shori Mizuno,1 Hironaru Murakami,1 Iwao Kawayama,1 Yoshinori Takahashi,2 Masashi Yoshimura,2 Yusuke Mori,2 Juraj Darmo,1,3 and Masayoshi Tonouchi1 1

Institue of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka,565-0871, Japan Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka,565-0871, Japan 3 Photonics Institute, Vienna University of Technology, Gusshausstrasse 27-29, 1040 Vienna, Austria * [email protected] 2

Abstract: We have proposed and developed a scanning laser terahertz (THz) near-field imaging system using a 1.56µm femtosecond fiber laser for high spatial resolution and high-speed measurement. To obtain the two-dimensional (2D) THz images of samples, the laser pulses are scanned over a 2D THz emitter plate [DASC: 4’-dimenthylamino-N-methyl-4stilbazolium p-chlorobenzenesulfonate] by a galvano meter. In this system, THz wave pulses locally generated at the laser irradiation spots transmit through the sample set on the emitter, and the amplitude of the transmitted THz wave pulse is detected by using a typical THz time-domain spectroscopy (THz-TDS) technique. Using this system, we have succeeded in obtaining THz transmission images of a triangle shaped metal sheet of millimeter-size and a human hair sample with a spatial resolution of sub-wavelength order up to ~27µm (~λTHz/28) at an imaging speed of about 47 seconds/image for 512 x 512 pixels. ©2012 Optical Society of America OCIS codes: (110.6795) Terahertz imaging; (300.6495) Spectroscopy, terahertz.

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#163639 - $15.00 USD Received 27 Feb 2012; revised 20 Apr 2012; accepted 21 Apr 2012; published 24 May 2012

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1. Introduction Recent terahertz (THz) imaging technology has attracted much attention for various applications from biology to security, such as inspection of illegal drugs, explosive substances, and evaluation of historical cultural assets, bio tissue, and diagnosis of early cancer [1–9]. For further practical use of the THz imaging technique, there are several problems to be overcome in terms of imaging speed, spatial resolution, and convenience. In conventional THz imaging technique, samples were set at the focal position of THz waves and moved two-dimensionally by using mechanical stages, so it takes several ten minutes or hours to obtain an image [10,11]. As for the imaging speed, a real-time THz imaging had been achieved by an electro optical (EO) sampling technique using a CCD camera [12], but it needs a high power optical source, which costs a lot. On the other hand, regarding the spatial resolution, it is limited at most several hundreds of µm due to the diffraction limit of the THz waves. There are many researches for improving the spatial resolution, for instance, by using a metallic tip and small apertures for generating near-field THz waves [13,14], and by setting samples close to detector to obtain THz electric fields in near-field [15]. As one of the simplest methods to improve the spatial resolution, a sample is adjacent to the THz generation points, which can be moved two-dimensionally. It is based on local emission of the THz radiation in the waist of the pumping near-infrared beam [16]. Using this technique, a spatial resolution of sub-wavelength order, which is shorter than the center wavelength of generated THz waves (λTHz), is achieved [17–19]. In this study, by introducing this simple technique to achieve high spatial resolution THz imaging, we developed a scanning laser terahertz near-field imaging system using 1.56µm femtosecond fiber laser as an optical source. In this system, a galvano meter was employed to scan the pump laser beam over a two dimensional (2D) THz emitter at high-speed. This system configuration enables us to observe high-resolution THz transmission images of thin and flat samples directly set on the emitter without interspaces. Using the developed system, we have tried high-speed and high-resolution THz transmission imaging of various samples, and evaluated the system performance.


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2. System setup and evaluation Figure 1 shows a schematic drawing of the scanning laser THz near-field imaging system. In this system, we used a 1.56µm femtosecond fiber laser (TOPTICA FFS.SYS.HP: maximum power 350 mW, pulse width 110 fs, repetition rate 80 MHz), which is compact, robust, stable and low-cost, compared to Ti-sapphire laser sources. At first, the laser beam is divided into pump and trigger beams using a beam splitter. The pump beam is modulated by means of an acousto-optic modulator (AOM) and scanned over the 2D THz emitter plate with a galvano meter. As for the THz emitter, we employed an organic nonlinear crystal, DASC (4’-dimenthylamino-N-methyl-4-stilbazolium p-chlorobenzenesulfonate) of 200-µm-thick [20,21]. It was grown by slope nucleation method (SNM) [22] by which we can obtain large and flat crystals with high quality up to about 10 mm x 10 mm. The THz radiation is provided by optical rectification in the DASC crystal, and it is known as one of the appropriate THz generation sources for 1.5µm wavelength laser excitation. The optical characteristics of DASC are almost similar to those of DAST crystal in THz spectral region [23]. THz beams that are locally generated at the pump beam irradiation spots transmit through a sample that is directly set on the emitter and focused onto a detector by using a pair of parabolic mirrors. Therefore, the spot size of the THz source approximately corresponds to the pump beam size. As for a detector, we use a bow-tie shaped photoconductive antenna (PCA) that fabricated on a low temperature grown GaAs (LT-GaAs). On the other hand, the trigger beams are converted to 780 nm by a periodically poled LiNbO3 (PPLN) crystal, and then irradiated to the PCA. Therefore, we can observe a THz transmission image of the sample by monitoring the amplitude of the THz pulses. In addition, reflected laser beam from the emitter is detected by a photodiode, so we can observe a laser reflection image of emitter surface or a sample and compare it with the THz images at the same time. The pump beam size estimated from the laser reflection image is about φ 20µm, so the Rayleigh length is estimated to be about 400µm, which implies that the pump beam size is almost constant in the whole DASC crystal. By scanning such localized THz point light sources two dimensionally, and also by setting the sample in vicinity of such light sources, the sample interacts with THz waves and allow us to observe THz transmission images at a spatial resolution of sub-wavelength order.

Fig. 1. Schematic drawing of a scanning laser THz near-field imaging system and optical image of DASC.


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Fig. 2. THz radiation images of DASC when the detector was set at (a) focused and (b) unfocused position. (c) and (d) show the frequency spectra of THz amplitude when the THz beam was focused and unfocused on the detector, respectively. They are calculated by their time domain waveforms as shown in each inset.

Prior to the experiment, we carried out pre-experiment to find the best optical alignment condition to obtain enough THz imaging area. Because we must consider the fact that imaging results are strongly affected by the thickness and quality of the DASC crystal. Most especially, the anisotropic refractive index along each crystal axis may cause a different distribution of THz intensity radiated from the crystal in relation to the plane of polarization of excitation laser. Therefore, we made the optical axis perpendicular to the crystal surface. We also tried to optimize the detector position. Since we are using a PCA as a detector, the observable THz imaging area might be limited to narrow area. To broaden the THz imaging area, we adjusted the PCA position. Figure 2(a) shows a THz radiation image from the DASC without a sample when the PCA was positioned at the focal point of THz beam. To obtain a 2D THz image, we fixed the delay stage at the maximum amplitude of THz waveforms and scanned the pump laser beam by the galvano meter. The pump beam is modulated at 100 kHz using the AOM, and it took 47 seconds/image for 512 x 512 pixels image (2 mm x 2 mm) by sampling the signals at 100µs/pixel. As you can see, we succeeded in detecting the THz signals from the DASC. However, the imaging area is limited to narrow region. To broaden the imaging area, we also tried to optimize the detector position. Figure 2(b) shows the THz radiation image from the DASC when the THz beam was unfocused on the detector. Here, we set the detector and the objective lens at the half of the focal length of the parabolic mirror. It can be seen that the THz imaging area for the unfocused position is about 5 times wider than that for focused one. Furthermore, a uniform area of THz intensity of about 1.5 mm long, which can be used as a canvas in the THz imaging, can be obtained. This is because the generated THz waves still have opportunity to hit the gap of the PCA even if the scanning pump laser beam is deviated away from the center of the light axis. However, the THz amplitude as shown in Fig. 2(d) is reduced to one-fourth, and the bandwidth is decreased by half in this condition, compared with that of #163639 - $15.00 USD Received 27 Feb 2012; revised 20 Apr 2012; accepted 21 Apr 2012; published 24 May 2012

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focused one as shown in Fig. 2(c). On the other hand, the center frequency of generated THz waves is located around 0.4 THz for both the conditions. Through the pre-experiments, it was found that the appropriate detector position can be determined depending on the sample size. The broad absorption feature in spectra at 1 THz is due to phonon absorption in the DAST crystal [24]. 3. THz imaging results Figure 3(a) and 3(b) show a THz transmission image of a 2-mm-equilateral-triangle-shaped copper sheet sample of 100-µm-thick and a difference image obtained by subtracting THz image without the sample in order to remove crystal artifacts, respectively. As shown here, the triangle shape of the sample is clearly visible in these images.

Fig. 3. (a) THz transmission image and (b) 2-mm-equilateral-triangle-shaped copper sheet sample

difference

THz

image

of

a

Furthermore, it is considered that this imaging system is available to smaller, flat and thin samples, because the THz waves are locally generated from the emitter. Therefore, we tried to measure THz transmission images of a human hair (φ 80µm) as shown in Fig. 4(a). Figure 4(b) and 4(c) show a THz transmission image and a difference image of the human hair sample, respectively. In this measurement, the detector was set at the focused position. We could observe a clear thin shape of the hair sample. It is noticed that several weak THz transmission spots exist inside the identical hair sample. This result indicates the possibility that the transmitted THz waves correlate with the inner structures or internal constituent such as water of the human hair. Generally, a human hair consists of keratin protein and includes 11-13% of water, so further measurements by spectroscopic method will clear these points in the future. As for the system performance, those THz transmission images are composed of 512 x 512 pixels, and the imaging speed is 47 seconds/frame. The spatial resolution reaches up to 27µm from the line profile in Fig. 4(d), which is approximately corresponding to the pump beam size. This spatial resolution is also corresponding to 1/28 times of the center wavelength of 0.4 THz in the broadband spectra of the generated THz waves shown in Fig. 2(c). This spatial resolution is achieved due to near-field effect that occurs between the surface of the DASC and the sample. To obtain such a high spatial resolution, we must pay attention to THz divergence inside the DASC crystal. Since the THz waves are generated along the pump beam path, the spatial resolution should be limited [18]. The THz beam waist W(z) inside an organic crystal, which depends on the propagation distance z, is described as [25]

W ( z ) = W0 1 +

z2 , z (ω ) 2 0

(1)

where W0 and z0(ω) are the initial THz beam waist and the frequency-dependent Rayleigh length, respectively. Here, z0(ω) is also described as


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z0 (ω ) =

ω 2c

W02 .

(2)

In the case of, W0