1. Introduction

Terahertz (THz) imaging has been extensively developed because of its huge variety of practical applications such as packaging inspection, quality control of plastic parts, chemical composition analysis, and biomedical diagnostics [1–3]. However, the diffraction limit restricts the spatial resolution to about a wavelength; thus traditional imaging at 1 THz has a spatial resolution restricted to its equivalent wavelength in vacuum of 300 µm. This limited spatial resolution remains an obstacle for numerous interesting applications. One way to work beyond the diffraction limit is to probe the electric field in the near-field region of the sample. This method has been reported in the THz frequency range using a metal tip or a dynamic aperture to confine the electric field to dimensions smaller than the wavelength [3–8], essentially making a source of subwavelength dimensions.

Alternatively, one can place the sample to be imaged close to the THz electro-optic (EO) sensor crystal [9–12] or photoconductive antennas [13]. In this way, imaging of the near-field distribution of the electric field around metallic structures has been successfully achieved [10–13]. All of these methods work significantly beyond the diffraction limit, thus allowing one to perform THz spectroscopy on extremely small volumes of organic and inorganic materials. Furthermore, the combination of the THz near-field scheme with a CCD camera could ultimately enable us to visualize the dynamics and interactions of biomolecules in mesoscale living cells with label-free probing in real-time. Recent promising studies on label-free probing of deoxyribonucleic acid (DNA) have shown that THz radiation can be used to distinguish between different biomolecules [14,15].

In this paper, we demonstrate a THz near-field microscope using an optical heterodyne detection technique coupled with a CCD camera, and we show that the two-dimensional spatial distribution of the THz electric field of a biological sample can be measured. Our sample is a grain of tyrosine placed directly on the top surface of the EO crystal and lit by high-power THz pulses generated using tilted pulse front excitation technique in a LiNbO₃ crystal [16,17]. The results show that free induction decay (FID) signal from the sample in the near-field region of THz radiation can be observed. The oscillating frequency of the FID signal centered at ~1 THz corresponds to the vibrational mode of the tyrosine crystal. The spatial resolution of better than ~70 μm estimated from this study is smaller than the wavelength of the FID signal (~300 μm), which confirms that our THz near-field microscope can take spectroscopic images with subwavelength spatial resolution.

2. Experimental setup

Our experimental setup is illustrated in Fig. 1 . In order to obtain real-time imaging of a millimeter size sample, an expanded THz beam and an expanded probe beam are needed. We generate high-power THz pulses with the tilted pulse front excitation technique that combines a grating-lens pair to match the noncollinear velocity in the LiNbO₃ crystal [16,17]. In our experiment, an 800-nm and 500-mW laser beam of 100-fs pulse width at a repetition rate of 1 kHz is used to generate high-power, few-cycle, THz pump pulses of 0.1–2 THz bandwidth. Using an off-axis parabolic mirror with a focal length of 152.4 mm, the emitted THz radiation is focused down to ~2 mm in diameter onto a 1-mm-thick (110) ZnTe crystal for EO detection. The incident THz field is polarized in the y direction and the peak electric field of the THz pulse is estimated to be ~30 kV/cm at the sample position.

 

Fig. 1 Schematic of the experimental setup. NPBS: non-polarized beam splitter, QWP: quarter-wave plate, Pol.: analyzing polarizer. The focal lengths of the three lenses are f₁ = 200 mm, f₂ = 36 mm, and f₃ = 200 mm. Top and bottom surfaces of the EO crystal have a high-reflection and anti-reflection coating for a probe pulse, respectively. The black dotted line implies that the combination of lenses (f2 and f3) makes an image of the EO crystal surface on the CCD camera. Inset shows an optical micrograph of the tyrosine grain sample on a 1-mm-thick (110) ZnTe crystal.

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To visualize the near-field spatial distribution of THz waves behind a 140-μm-thick tyrosine grain sample (980 × 250 μm²), the sample is placed directly on the top surface of the EO crystal (as diagrammed in Fig. 1 and imaged in its inset). To cover the whole THz beam, the probe beam is collimated with diameter of over 2 mm and reflected at the top surface of the EO crystal. The probe pulse energy is around 10 μJ. Notice that the EO sampling crystal used in this experiment has an anti-reflection coating (AR) on the bottom surface and a high-reflection coating (HR) on the top (sample side) surface. In this way, the probe beam propagates with the THz beam collinearly inside the EO crystal. The THz waveform measured in this reflection geometry is the same as the one obtained in the transmission geometry [9,18]. The subpicosecond THz pulse incident on the tyrosine sample coherently excites its molecular vibration mode and causes it to reemit the energy through FID.

The polarization of the probe beam is modulated by the THz electric field through the EO effect. Then an analyzing polarizer converts this polarization modulation into intensity modulation of the EO sampling pulse that can later be detected by the CCD camera [19]. The camera used in this experiment is a 512 × 512 pixel EM-CCD camera (model C9100-12, Hamamatsu Photonics K. K.) operating at 2.7 frames per second (fps). To obtain the optical heterodyne signal proportional to the THz electric field, an analyzer is set with an axis perpendicular to the incident probe light polarization and a quarter-wave plate (QWP) is oriented at a small angle (~10 degrees) from the crossed position [20,21] (see Fig. 1). Finally, the background signal (without THz pulses) is subtracted from the one with THz pulses, thus allowing the detection of signal images corresponding to the THz radiation waveform. The CCD camera records the 5.5-fold image of the 2D spatial distribution of the THz-modulated probe beam magnified by the lenses (f₁, f₂, and f₃).

3. Results

We use our THz near-field microscope to show the time-dependent electric field polarized in the y-direction measured underneath the tyrosine grain sample on a 1-mm-thick (110) ZnTe EO sensor crystal. Figure 2(a) is the sample observed using a conventional reflection type visible microscope, whereas Fig. 2(b) to Fig. 2(j) are successive images showing the THz field evolution after irradiation of the sample; note that each image has been measured by the CCD camera operating at 2.7 frames per second (fps) and individually normalized to show its maximum amplitude range. (A movie showing intermediate times is available online (2.7 Mbyte).) The electric field oscillates even after the passing of the main peak of the incident THz pulse at least for as long as the sample shapes can be spatially resolved. Figures 2(d)–(g) in particular show that the THz spatial distribution inside the sample is inhomogeneous and changing as time passes. This result may imply that the cavity modes of the sample, which are determined by its refractive index and its shape, could be resonantly excited with a THz pulse impinging at normal incidence to the sample.

 

Fig. 2 (a) Optical micrograph of the grain of tyrosine sample on a 1-mm-thick (110) ZnTe crystal. Dotted circle shows the location where the time profiles presented in Fig. 3(a) were extracted. Figure 2(b)–(j) (Media 1) show the two-dimensional mapping of the THz electric-field component E _y measured with increasing delay times after the irradiation of the sample. Scale bars show the relative strength of the measured fields. Movie showing the electric field at intermediate times is shown online (2.7 Mbyte).

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In Fig. 3(a) , we compare the temporal profile obtained from the CCD camera with the one obtained using balanced EO detection at the same position indicated by the dotted circle in Fig. 2(a). The time origin (0 ps) is set equal to when the reference peak THz pulse is measured without sample (see inset of Fig. 3(a)). As shown in Fig. 3(a), after 3.5 ps the electric field waveforms measured by these two methods are consistent. This means that the oscillation with small electric field amplitudes after the passing of the main peak of the incident THz pulse can be detected well by the optical heterodyne method with a QWP inserted before the analyzing polarizer. However, for the phase retardation Γ of the probe pulse due to birefringence of the EO crystal induced by the THz electric field (corresponding to the region before 3.5 ps in Fig. 3(a)), the electric field retrieved from the CCD camera shows a different behavior when compared to the one obtained from balanced EO detection. This result indicates that optical heterodyne detection failed in the case of high electric field measurement, thus resulting in a distorted THz waveform profile. This situation occurs when the phase retardation Γ is much larger than the sum of the retardation of residual birefringence of the EO crystal θ and the intrinsic birefringence of the QWP δ. For the linear THz detection regime, the condition expressed by |θ + δ | >> |Γ | should be maintained [20,21]. In this experimental condition, the THz electric field ranging from ~0.1 to 10 kV/cm can be measured without the distortion of THz waveform as shown in Fig. 3(a) (a total integration time of ~0.8 s per point).

 

Fig. 3 (a) Time dependent electric field transients measured by the CCD camera (solid red line) and balanced EO detection (dotted blue line) at the location indicated by the dotted circle in Fig. 2(a). Dashed line shows the reference THz pulse without sample measured by the balanced EO detection. (b) Red and blue lines show the Fourier transformation of the THz transients taken 3.5 ps after the main peak position of Fig. 3(a) measured by the CCD camera and balanced EO detection, respectively. Dashed line shows the absorption spectra of the tyrosine pellet.

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In Fig. 3(b), we present the Fourier transformation of the periodic modulation of the THz response obtained by both the CCD camera and the balanced EO detection. Only data corresponding to a temporal position greater than 3.5 ps in Fig. 3(a) are taken in the Fourier transform. Figure 3(b) also shows the absorption spectra of the tyrosine pellet measured by the transmission-type THz time-domain spectroscopy [22]. From Fig. 3(b), one can notice that the center frequency (0.98 THz) of the periodic modulation is almost the same as the one of the absorption peak line (0.97 THz) obtained for the pellet sample of tyrosine. The good agreement between these two peaks confirms that the periodic modulation of the THz response shown in Fig. 3(a) corresponds to the FID signal from the vibrational mode of the tyrosine crystal.

Figure 4 shows a line profile of the CCD signal which indicates the spatial distribution of THz electric field due to the cavity mode of the tyrosine sample. The line profile is evaluated across the x-axis and each value is obtained by integrating along the y-direction within the rectangular region of the inset of Fig. 4. We characterized the spatial resolution by looking at the electric field spatial distribution varying from 10% to 90% in amplitude [6] and found a resolution of greater than 70 µm. Peak frequency (0.97 THz) of the FID signal corresponds to a vacuum-wavelength of ~300 μm; thus, the spatial resolution of ~70 µm is equivalent to ~λ/4, suggesting that the EO near-field method allows us to measure the FID signal in the near-field region.

 

Fig. 4 Line profile of the CCD signal across the tyrosine sample for the x-axis direction. Each value is estimated by integrating along the y-axis direction within the rectangular region shown in the inset.

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4. Conclusions

We demonstrated two-dimensional THz near-field imaging of the FID signal behind a crystalline tyrosine sample with dimensions of the order of the wavelength. The oscillating frequency of the FID signal centered at ~1 THz is consistent with that of the absorption peak line obtained from a pellet tyrosine sample and corresponds to the vibrational mode of the tyrosine crystal. The spatial resolution of better than ~70 μm estimated from this study is smaller than the wavelength of the FID signal (~300 μm) by a factor of ~4, suggesting that the EO near-field method allows us to measure the FID signal in the near-field region. These results confirm that the THz near-field microscope can take spectroscopic images with subwavelength spatial resolution at 2.7 frames per second.