Generation and modulation of circularly polarized terahertz electromagnetic radiation have been demonstrated by using a four-contact photoconductive antenna and a total-reflection Si prism. The quality of the circularly polarized terahertz pulsed radiation has been evaluated by using a polarization sensitive terahertz time-domain spectroscopy system. The characteristic of the dynamic modulation between the left and right circularly polarized states of the THz radiation is also evaluated. The ellipticity of the modulated circularly polarized THz radiation without a polarizer is not as good as that of the non-modulated because of the non-uniform bias field distribution and the asymmetric pump laser intensity profile on the photoconductive gap.
©2006 Optical Society of America
Recently, much attention is attracted to the vibrational circular dichroism (VCD) spectroscopy based on the circular dichroism (CD) of biological molecules in the vibrational frequency range . Since VCD is sensitive to the stereochemical structures of chiral molecules, the VCD spectroscopy in the infrared and mid-infrared region focused on biomolecules are actively carried out. For example, the conformational analysis of proteins [2, 3] and the chirality investigation of pharmaceutical molecules [4, 5] are possible with VCD spectroscopy. However, the frequency range of VCD spectroscopy is limited to above ~20 THz (~650 cm-1) at present . VCD signal is equivalent to the difference of the absorption between the left-handed circularly polarized light and the right-handed circularly polarized light in the vibrational frequency region. However, the VCD signal is generally very weak, with a typical magnitude about 10-4, against the background vibrational absorption. In order to detect such small VCD signals, the polarization state of the light is modulated and the difference signal is lock-in detected. The circular-polarization modulation in the infrared and mid-infrared region is usually achieved by using a photo-elastic modulator. However, no such a modulator with a broad bandwidth at the THz frequency range is available. Therefore, the development of the polarization modulator is essential in realization of a VCD spectrometer in the THz frequency region, which motivated us to construct the THz polarization modulator reported in this paper.
2. Principle of THz polarization modulation
There were several attempts as for the polarization modulation and polarization sensitive detection of THz radiation. Chen and Zhang  successfully demonstrated polarization modulation of THz radiation generated by optical rectification in a (110)-cut ZnTe crystal; they rotated the polarization direction of the pump femtosecond laser so that the direction of THz polarization was rotated according to the tensor property of the nonlinear optical crystal. Shimano et al  reported a polarization modulation scheme using an electro-optic THz emitter (a ZnTe crystal) and an interferometer: the two orthogonally polarized femtosecond optical pulses were generated with a time delay by the interferometer and the two pump pulses generated two orthogonally polarized THz radiation pulses with a phase-retardation through the optical rectification effect in the (110)-cut ZnTe crystal with an appropriate crystal orientation. The polarization state of the combined THz radiation is modulated by changing the optical delay in the interferometer. However, in this scheme the ellipticity spectrum of such a broadband THz radiation can not be flat but periodic with the frequency because the phase retardation depends not only on the path difference in the interferometer but also the frequency of the THz radiation. Castro-Camus et al  reported a polarization sensitive photoconductive detector, which has three terminal contacts to detect the orthogonal polarization component of the THz radiation simultaneously. They observed a cross-polarized extinction ratio better than 100:1 for vertically or horizontally polarized THz radiation.
In this paper, we report a novel scheme of polarization modulation of THz radiation by using a four-contact photoconductive antenna, aiming for realization of a VCD spectrometer in the THz frequency region. To authors’ knowledge this is the first demonstration of generation and modulation of a broadband circularly polarized THz radiation.
We fabricated a four-contact photoconductive antenna, which has four metal contacts (as shown in Fig. 1), on a low-temperature-grown GaAs (LT-GaAs) substrate by a standard photolithographic and chemical etching method. By applying a bias voltage to two adjacent contacts with the other two are grounded, the bias electric field in the photoconductive gap is directed to +45° or -45° from the horizontal axis. When a short pulse light (pump beam) is irradiated to the biased photoconductive gap, the transient photocurrent generates THz radiation which is linearly polarized in the direction of the bias field . By rotating the bias voltages by 90°, we can generate THz radiation with the same amplitude but with the polarization orthogonal to the previous one. Since the bias voltage is switched easily and quickly by using an appropriate electronic circuit, the polarization direction of THz radiation is alternated between the +45° and -45° direction at a rate of more than several kHz. These linearly polarized THz radiations are converted to circularly polarized radiations by using the total reflection within a high-resistivity Si prism (see Fig. 2): the s-polarized component of electromagnetic radiation is phase-retarded against p-polarized one by the total reflection and its phase-shift depends on the reflection angle. The internal reflection angle of the Si prism is designed to give rise to a π/2 phase-shift so that the linearly polarized radiations at +45° becomes left-handed circularly polarized radiations, and, in the same way, the ones at -45° become right-handed circularly polarized radiations. This linear-to-circular-polarization conversion method is effective for a broadband THz pulsed radiation because the total reflection phase-shift is independent of the wavelength due to the flat dispersion of Si in the THz frequency range. In this way, the combination of a four-contact photoconductive antenna and a Si prism enables us to build a broadband, circularly polarized THz radiation source and modulator.
The schematic diagram of the experimental setup is shown in Fig. 3. We used a mode-locked Ti:sapphire laser operated at 80 MHz (λ~770 nm, δλ~10 nm) as the light source. A polarization beam splitter split the laser beam into a pump beam and a probe beam. The power balance between the pump and probe beam was controlled by rotating a half waveplate placed in front of the polarization beam splitter. The pump beam with an averaged power of 20 mW was focused on the gap of the four-contact photoconductive emitter. The bias voltages on the antenna were modulated at 33 kHz. For generation of THz radiation with the polarization axis directed to +45°, the antenna contacts at the top, right, left and bottom (see Fig. 1) were biased to 0 V, 0 V, 30 V and 30V, respectively, and in the case of -45° polarization, these electrode were biased to 30 V, 0 V, 30 V and 0V, respectively (the bias field was rotated by 90°). Thus, the polarization of the emitted THz radiation was alternated between +45° and -45° directions at the bias modulation frequency. The THz radiation was collimated and focused on to a dipole-type photoconductive detector antenna using a pair of parabolic mirrors. The probe beam of 20 mW was focused on the gap of the detector antenna for the photoconductive sampling of the THz field. Since the direction of the detector photoconductive antenna was oriented to horizontal direction, the polarization components of the THz field at 0 or 180° from the horizontal axis are detected. By moving the optical-delay-line and detecting the photoconductive signal step by step, the time domain THz waveforms were measured. Since we used a lock-in amplifier synchronized with the alternating bias on the emitter antenna, the detected signal was the difference of the amplitude between the two switched radiations. With additionally modulating the pump beam by a mechanical chopper at 3.3 kHz and using the chopper modulation frequency as the reference for the lock-in amplifier we also detected the average of the amplitude of the two switched radiations: Since the amplifier detects the difference of the signals with the pump and without the pump, while the bias was modulated at a much higher frequency (33 kHz), the average of the amplitude of the two switched radiations was able to be detected. By rotating the polarizer inserted between the two parabolic mirrors, we detected the polarized components of the THz radiation in the +45° and -45° directions. From these difference and average data we can construct the trajectories of the THz electric field vectors in time and space, that is, the THz waveform presented in the three dimensional space which consists of the time axis and the polarization plane, for the two switched radiations.
4. Results and discussion
The trajectory of the THz electric field vectors for the +45° and -45° directional bias voltage are shown in Fig. 4(a) and 4(b), respectively. In Fig. 5 and Fig. 6, the frequency dependent ellipticity (the ellipticity spectra) and the azimuthal angle (the direction of polarization) of the two linearly polarized THz electromagnetic radiations are shown (the solid and open squares), respectively. In these figures, we can see that each THz radiation is almost linearly polarized in the corresponding bias direction. However, the ellipticity of the +/-45° radiations was not exactly zero (corresponding to the perfect linear polarization), but has values between -0.05 to +0.1, indicating slightly elliptical polarization. In addition, the direction of polarization is also slightly deviated from the expected +/-45° directions, as shown in Fig. 6.
The trajectory and the ellipticity spectra of the THz radiations transmitted through the total reflection Si prism with the bias voltage modulation in directions to +45° and -45° are shown in Figs. 4(c), (d) and Fig. 5 respectively. In these results, we can see that two switched radiations are nearly left and right-handed circularly polarized radiations, indicating the success of the generation of the circularly modulated THz pulsed radiation. However, the ellipticities of the modulated radiations were not exactly 1.0 (that corresponds to a perfect left-handed circular radiation) or -1.0 (a right-handed one), indicating the circular polarization of the modulated radiations was not perfect.
When THz radiation was prepared in a linear polarization state at +45° or -45° by using a wire-grid THz polarizer and transmitted through the Si prism, we observed a nearly perfect circular polarization (the ellipticity very close to +1 or -1) (see Fig. 5). This proves that the Si prism worked very well as a π/2-phase retarder in the whole spectrum range of the THz radiation. Therefore, the main reason of the imperfect circular polarization is attributed to the distortion from the linear polarization or the deviation of the polarization direction from +/-45° directions in the THz radiation emitted by the four-contact PC antenna. There are several factors which may cause the distortion of the polarization of THz radiation from the PC antenna; The spatial inhomogeneity of the photoelectric property of the photoconductive substrate (LT-GaAs), the inhomogeneous distribution of the pump beam intensity and the deviation of the bias field distribution from the ideal symmetry such as due to an imperfect patterning of the contact electrodes. When an inhomogeneous or asymmetric pump beam is incident on the PC gap with an inhomogeneous bias electric field distribution, the polarization of the emitted THz radiation will significantly distorted from the linear polarization.
To estimate the influence of inhomogeneous distribution of the bias field and pump power, we simulated the bias field distribution applied to the gap of the photoconductive antenna. To simulate the static field in the photoconductive gap of the antenna, we have to solve the Laplace equation,
for the two-dimensional potential, u(x, y), with an appropriate boundary condition. The charge density, ρ(x, y), is assumed to be zero since the substrate semiconductor is semi-insulating before the optical excitation. We used the following difference equation  to approximate Eq. (1):
Here, ui,j indicates the potential of the i-row-j-column cell. As the boundary condition, we assumed an insulating area surrounded by perfect conductors with the shape of the four-contact antenna (Fig. 1) and the +45°-bias condition. A 101-µm by 101-µm region centering on the square gap was divided into 202 by 202 cells and the potential of each cell was successively calculated using Eq.(2) until it converged. Using the calculated potential, ui,j, the direction and the magnitude of the electric field vector were calculated by using the following equations:
Here, Δ(=0.5 µm) is the grid interval.
The calculated result is shown in Fig. 7. In the experiment, the pump beam was broadly focused inside the circle indicated in Fig. 7 and the carriers inside this region were photo-excited. Although the field vectors are not oriented in the same direction within the circle, as can be seen in Fig. 7, the THz radiation will be linearly polarized in the expected direction (+45° or -45°) due to the averaging effect when the distribution of pump beam intensity is centrosymmetric (not necessarily homogenous). On the other hand, if the distribution of the pump beam intensity is off-centrosymmetric, the polarization direction of the emitted THz radiation will deviate from the +/-45° direction. To improve the polarization quality, a careful control of the pump beam profile is necessary to make the intensity distribution on the photoconductive gap centrosymmetric.
In the above simulation of the bias field distribution, we neglected the field-screening effect and the charge accumulation effect by the long-lived photo-excited carriers. Since the diffusion velocity of the electrons is much higher than the holes, positively biased electrodes (anodes) are screened more effectively than the grounded electrodes (cathodes). This may cause asymmetric field distribution and result in distortion of the linear polarization of THz radiation.
In conclusion, we built a circular-polarization modulator of broadband THz radiation, which consisted of a four-contact photoconductive antenna and a Si prism, aiming to realize the VCD spectrometer in THz frequency region based on the THz-TDS. We succeeded in generation of the circularly polarized THz radiations with a broadband spectrum from 0.1 THz to around 2.5 THz. In addition, we also succeeded in modulation of the helicity of the circularly polarized THz radiation at several tens of kHz. However, the quality of the circular-polarization was relatively poor when it is modulated, compared to that of non-modulated case. We simulated the distribution of the electric field vector on the photoconductive gap to investigate the reason of the imperfect circular polarization, and found that the bias electric field is not homogenous and the distortion of the polarization will occur if the distribution of the pump beam intensity on the PC gap is off-centrosymmetric. The quality in the circular-polarization modulation of THz radiation from the four-contact photoconductive antenna will be improved by a careful adjustment of the pump laser beam profile on the photoconductive gap.
We believe our circular-polarization modulation method using the bias modulation on a four-contact photoconductive antenna and the total reflection in a Si prism is potentially useful for the VCD spectroscopy for chiral molecules in the THz frequency range. The four-contact photoconductive antenna alone is also useful for the THz ellipsometry . If it is used to generate linearly polarized THz radiation with its polarization direction alternated by the bias switching and another four-contact photoconductive antenna is used to detect its orthogonal polarization components of the polarization-modulated THz radiation simultaneously, the ratio of p- and s-polarized components of THz radiation reflected from samples can be measured with a high precision, giving rise to high SNR ellipsometric information on the samples.
We acknowledge the help of Dr. Shigehisa Tanaka with Central Research Laboratory, Hitachi Ltd., Japan, in fabrication of the PC antenna. This work was supported by the grant with Scientific Research (B) (subject # 17360030) under the Grant-in-Aid for Scientific Research program of the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).
References and links
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