Active control of terahertz wave polarization is important for various applications, such as terahertz polarization measurements for chirality detections of biomolecules, identification of enantiomers, and terahertz wireless communications. In particular, a terahertz polarization modulator should improve terahertz polarization measurements as photoelastic modulators in the visible region increased the sensitivity of circular dichroic measurements. Subwavelength artificial structures or metamaterials have strong potential for realizing such active terahertz devices that control their optical responses by external stimulus [1]. Terahertz metamaterials actively controlled by photoexcitation [2], electrical methods [3], and microelectromechanical systems techniques [4] have been reported.

Enhanced optical activity in artificial chiral structures is useful for controlling the polarization of terahertz waves. In chiral media, which have nonsuperimposable mirror forms, the polarization plane of a propagating electromagnetic wave rotates. Strong optical activity in planar metal chiral structures has been demonstrated in the visible [5–7] and terahertz regions [8,9]. Active control of such artificial chiral structures has also been demonstrated [10–13]. However, the prefabrication of structures limits the controllability of the polarization states.

All-photoinduced terahertz metamaterials in which patterned photocarriers in semiconductors without any built-in structure behave as quasi-metallic metamaterials have been attracting attention [14–16]. Intensity profiles of the optical pump beam are controlled in the subwavelength scale of terahertz waves using a spatial light modulator (SLM), which can generate arbitrarily patterned optical beams. These all-photoinduced devices enable a systematic and on-demand control of their terahertz responses. In particular, active control of terahertz polarization has been attempted with a birefringence effect in a photoexcited wire grid pattern [17]. Optical activity is a nonlocal optical response resulting from the first-order spatial dispersion, which is usually smaller than the local response, for instance, birefringence. Therefore, observation of optical activity in all-optically induced chiral structures without the combination of metal structures is the next challenge. Such photoimprinted chiral structures can switch the signs of polarization rotation, which is difficult in conventional active metamaterials [10–13]. The modulation of polarization rotation by all-photoinduced optical activity would pave the way for high-sensitivity terahertz polarization measurements, such as sensing of vibration circular dichroism of chiral biomolecules [18].

In this Letter, we propose and demonstrate a new method to arbitrarily and actively control terahertz optical activity without using chiral metal gratings, simply by irradiating the excitation beam on a bare Si wafer. The chiral patterned photocarrier distributions are directly generated by a spatially intensity-modulated pump beam. The intensity profiles of the optical pump beam can be modulated using the SLM within the subwavelength scale of terahertz waves.

Figure 1 shows the schematic of the experimental setup. The synchronized optical pump and terahertz probe measurements are performed using a Ti:sapphire regenerative amplifier with 1 kHz repetition rate and 100 fs pulse duration. Terahertz pulses are generated by optical rectification in a ${\mathrm{LiNbO}}_3$ crystal [19] and detected via electro-optic sampling using a ZnTe crystal [20]. The polarization states of transmitted terahertz waves are measured with two wire grid polarizers [9].

 

Fig. 1. Schematic of the experimental setup. Chiral patterned photoexcitation is realized with a reflection-type two-dimensional SLM, and the polarization states of the transmitted terahertz pulse are measured.

Download Full Size | PDF

The spatial profile of the pump beam is controlled by the SLM, which is based on a liquid crystal on Si (LCOS-SLM, Hamamatsu Photonics Inc.), as shown in Fig. 2. This apparatus can modulate only phases of light with a certain polarization direction, while the intensity of the light is not modulated. Therefore, if this device is tilted 45° from the horizontal axis, the horizontal polarization is converted to vertical polarization with $\pi$ phase modulation. Using the device with normal incidence and picking out the reflected vertical polarization component with a polarization beam splitter (PBS), the on–off switching of light can be controlled pixel by pixel. The SLM image is transferred to a high-resistance Si wafer (resistivity $\rho \sim 10\mathrm{k}\mathrm{\Omega }\mathrm{cm}$) by a lens pair with a magnification of 0.5. Using a pellicle beam splitter, the optical pump and terahertz probe pulses become coaxial and irradiate the wafer at normal incidence. Because the pixel size of the SLM is $20\mathrm{\mu m}\times 20\mathrm{\mu m}$, the spatial profile at the Si substrate can be controlled at a resolution of 10 μm, which is enough to generate the photocarrier spatial distribution in the subwavelength order. An intensity profile image of the pump beam at the position of the Si wafer is shown in the inset of Fig. 2, which is captured by placing a complementary metal-oxide-semiconductor (CMOS) camera instead of the Si wafer. The design pattern is successfully generated in an area several millimeters wide. In this experiment, the excitation density reaches a maximum of $750\mathrm{\mu J}/{\mathrm{cm}}^2$. The photocarriers in Si show quasi-metallic optical responses in the terahertz frequency region because the plasma frequency is of the order of 10 THz at this excitation density. In addition, photocarriers are excited with a density gradient along the normal to the surface because the penetration depth of the pump pulse is approximately 10 μm. This photocarrier layer gives rise to three-dimensional chirality, which is essential for inducing optical activity [5,10].

 

Fig. 2. Setup for patterned photoexcitation and terahertz polarization measurements. CMOS image of the patterned pump beam is shown in the inset.

Download Full Size | PDF

The terahertz probe pulse passes through the sample approximately 50 ps after the photoexcitation. This delay time is much longer than the rising time of the photocarrier response and much shorter than the time constant of the carrier diffusion effect or carrier lifetime [21]. Thus, the terahertz pulse can probe the responses of carriers in the quasi-equilibrium state.

Figure 3 shows the experimental results for the transmittance, chirality-induced polarization rotation ($\theta$), and ellipticity ($\eta$) spectra. Because there is polarization rotation from birefringence, which derives from the imperfect imaging of the SLM patterns, the differences from the polarization spectra of the achiral cross patterns $({\theta }_c,{\eta }_c)$ are plotted in Figs. 3(c) and 3(d). The spectra of ${\theta }_c$ and ${\eta }_c$ are also shown at the bottom of Fig. 3(b). The period of the gammadion chiral pattern is 180 μm, and the excitation density is $450\mathrm{\mu J}/{\mathrm{cm}}^2$, as shown in Fig. 3(e). The intensity profiles of the pump beam are also shown on the right side. As can be seen in this figure, the signs of the differential polarization rotation are opposite for the right and left gammadion patterns, whereas the transmission spectra are almost identical. The results clearly indicate that the polarization rotation is sensitive to the handedness of the chiral patterns, and enantiomer chirality switching is successfully realized.

 

Fig. 3. Experimental results. (a) Transmittance for chiral pattern excitation, (b) polarization rotation and ellipticity for achiral (cross) pattern excitation, (c) differential polarization rotation for chiral pattern, (d) differential ellipticity, and (e) chiral patterns of excitation beam captured by CMOS camera.

Download Full Size | PDF

Figure 4(a) shows terahertz polarization rotation spectra with different periodicities and photoexcitation densities. Gammadion patterns with periodicities of 60, 120, and 180 μm are generated by the SLM. Half of the difference between the measured spectra of left- and right-handed patterns are plotted in Fig. 4(a). ${\theta }_r$ and ${\theta }_l$ are the polarization rotation of the right- and left-handed patterns. The excitation density varied from 150 to $750\mathrm{\mu J}/{\mathrm{cm}}^2$.

 

Fig. 4. (a) Experimental results for the periodicity and excitation density dependence of terahertz polarization rotation spectra. The insets are CMOS images of pump patterns. (b) Numerical simulation results for polarization rotation spectra. Frequencies of surface modes are also shown as arrows.

Download Full Size | PDF

The polarization rotation angle increased with the excitation density. In addition, the polarization rotation spectra shifted to higher frequencies as the periodicities decreased. The results suggest that the magnitude, sign, and spectral features of the optical activity can be controlled by changing the excitation density and SLM patterns.

To validate the experimental results, numerical simulations were performed using commercially available software employing the rigorous coupled-wave analysis method (DiffractMOD, Rsoft Design Group, Inc). The refractive index of Si with photocarriers is described using the Drude model [10]. In this model, plasma frequency (${\omega }_p$) is proportional to the square root of the excitation density. Calculation results for 9.8 μm penetration depth are shown in Fig. 4(b).

In the calculated spectra, we can see kinks or peaks, depending on the periodicities. They originated from surface modes on periodic structures [22], which are indexed by integers of the order $(i,j)$. The frequencies of the modes are expressed as $f_{i,j}={(i^2+j^2)}^{1/2}c/(na)$, where $c$ is the speed of light, $a$ is the pattern periodicity, and $n$ is the refractive index of Si ($n=3.42$) or air ($n=1$). The abovementioned frequencies are shown in Fig. 4(b) using arrows with numbers of $i^2+j^2$. They completely agree with the kink or peak frequencies in the calculated spectra. Only the 1.67-THz mode in the periodicity of 180 μm originates from the air side as indexed in the figure by (1), whereas the others originate from the Si side. We see that the optical activity is enhanced at these frequencies. Therefore, the tuning of the periodicity is important for designing polarization rotation spectra.

We see from Fig. 4 that the polarization rotation increases as the periodicity decreases. If the structures are three-dimensionally similar, the spectra are simply scaled by the periodicity because of the scaling law [23]. However, in the present case, the thickness of the photocarrier layer is determined by the penetration depth of the pump beam, which is independent of periodicity. Thus, the ratio of the thickness to periodicity is larger in the case of shorter periodicity. Therefore, the optical activity is enhanced for shorter periodicity because of the enhanced three-dimensionality [6,7,9].

The dependence of the calculated excitation density and periodicity is in good agreement with the experimental data, as shown in Fig. 4. Note that in the case of periodicity of 60 μm, the difference between the experiment and calculations is larger than the rest. This is because the deviation from the ideal alignment of the optical components becomes more critical in the case of smaller patterns.

In conclusion, we demonstrated a new scheme for active control of terahertz optical activity by patterning the photocarrier distributions. Using an SLM, the spatial profile of the pump beam is controlled to form a chiral shape of the photocarriers in the semiconductor substrate, and thus the chirality-sensitive polarization rotation is clearly observed. The polarization rotation spectra are tuned by the pattern of the SLM and excitation density. Numerical simulations show good agreement with experimental results, which suggests that we can design the spectra of the polarization rotation. Because an SLM can generate arbitrary chiral patterns, it can realize unique optical functionality, such as active enantiomer switching by the modulation of the complete mirror copy of chiral patterns. This technique can be applied to terahertz polarization modulation devices and chirality-sensitive measurements.