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We carried out terahertz (THz)-wave generation from the GaP planar waveguides under collinear phase-matched difference-frequency mixing of two near-infrared sources. TE- and TM-mode of THz-waves were generated simultaneously by adjusting the polarization direction of two incident infrared sources. The phase shift between TE- and TM-mode of THz-wave in the waveguide was dependent on the waveguide length and contributed to the generation of the elliptical polarized THz-wave. The ellipticity of generated THz-wave increased as waveguide length increased. We indicated the possibility of control of rotational direction of elliptical polarization of emitted THz wave.
©2012 Optical Society of America
1. Introduction
In recent years, terahertz (THz)-wave technology has made great progress with the development of THz sources, THz detectors, and THz optical components because of their promising potential for many important applications, such as THz spectroscopy for the identification of biomolecules and THz imaging for medical diagnosis [1,2]. A variety of THz-wave sources have been developed for these applications, and most sources are based on the use of photoconductive antennas [3–5], quantum-cascade lasers [6], or nonlinear optical effects such as difference-frequency mixing (DFM) [7–17], optical parametric oscillations [18] and optical rectification [19,20]. In particular, the DFM technique can produce tunable and narrow line-width ns-pulsed THz waves, as well as CW infrared lasers [11], at room temperature. Several nonlinear optical materials (e.g., GaP [7–14], GaSe [15], LiNbO3 [16], and DAST [17]) have been used for THz-wave generation through the DFM technique.
GaP crystal are attractive for coherent THz-wave generation due to its second-order nonlinear susceptibility (d36 = 70.6 pm V–1 at 1.064 μm) [21] and low absorption coefficients in both the THz-wave (α ~3 cm–1 at 1 THz) and the infrared range (α < 0.1 cm–1 at 1.064 μm). Our previous work demonstrated the generation of THz waves using bulk GaP crystals under a small-angle noncollinear phase-matched DFM scheme.
In order to achieve efficient THz-wave generation, THz-wave generation from waveguide structure on the scale of order of THz-wave length itself has been conducted by our and other research groups [22–27]. The guiding mode can be existed in the THz waveguide structures. For example, TE and TM guiding mode for THz-wave are existed in the planar, ridge, channel waveguides. GaP crystal with optical isotropic property in infrared regime possesses the birefringence feature in the THz region. Therefore, the collinear phase matching condition is satisfied in the GaP THz waveguide structures by utilizing the THz-wave guiding modes. THz optical components to control the THz-wave properties such as wave plate and polarizer have been developed actively by using quarts plate, wire grids, and metamaterials [28–30] and so on. However, THz wave absorption is exist and cannot be ignored in any THz components. When we focus on the control of polarization of THz, we can propose one of the possible ways by utilizing the THz birefringence property. GaP THz waveguides can possess the both functions of THz-wave generator and wave plate. The realization of polarization controlled THz-wave generator is attractive for the application for the polarization sensitive THz spectroscopy, THz imaging, and vibrational circular dichroism to discriminate the chiral in biomolecules [31].
In this work, we demonstrate the elliptically polarized THz-wave generation under the collinear phase-matched difference frequency mixing inside the planar GaP waveguide. And we investigate the possibility of the control of rotational direction of elliptical polarization..
2. Experiment
We prepared the planar waveguides by using a semi-insulating 290-μm-thick GaP (001) substrate with mechanically polished on top and bottom surface. The samples were cleaved into a 10 mm-wide and 5, 10, and 15 mm long of rectangle along <110> crystalline direction, respectively. The schematic of experimental configuration is shown in Fig. 1 . A 1064-nm Q-switched Nd:YAG laser (11 ns, rep. 10 Hz, 50μJ / pulse, line width~3GHz) was used as the signal light for the DFM technique, and a 355-nm third-harmonic beam was used to pump a β-BaB2O4 (BBO)-based optical parametric oscillator (OPO, 6 ns, 50μJ / pulse, line width~50MHz). The OPO wavelength was tuned in the 1058–1062 nm range, which corresponds to a frequency difference between 0.5 and 1.5 THz. The input beams were focused to a diameter of 300 μm at the incident surface of the waveguide, and the spot sizes of the incident beams were measured by a beam profiler.
Fig. 1 Schematic diagram of THz wave generation under the difference frequency generation. The polarization of the pump and signal source was set to along [11] crystalline direction. The polarization of generated THz wave was measured by wire-gird polarizer.
In the difference frequency mixing process, the nonlinear polarization vibration of difference frequency PTHz is expressed by
where d14and Ei,m (i = 1, 2; m = x, y, z) denotes nonlinear optical susceptibility and vector component of electric field for incident light sources (1 and 2 indicates Nd:YAG and OPO).The polarization of two incident light sources was set to parallel along [11] crystalline direction, as shown in Fig. 1. In this configuration, the strength of excited nonlinear polarization vibration of difference frequency along [11] crystalline direction should be the strongest in the GaP crystal and TE- and TM-modes THz-wave can be generated simultaneously inside the planar waveguide. The polarization characteristics were measured by using the wire-grid polarizer. The generated wave was collected by a polyethylene lens and detected by a liquid helium-cooled Si bolometer.
3. Result
Figure 2 shows THz-wave output characteristics from 10 mm-long GaP planar waveguide as a function of difference frequency of two infrared sources. Under our present experimental conditions, highest absolute power of the generated THz wave will be about 1W (6nJ/pulse) with the signal to noise ratio of about 14dB. This results in that the conversion efficiency is about 10−9. The Fabri-Perot interferometer revealed that the line width of generated THz wave was about 3GHz under present experimental conditions. Black solid line shows the total THz output power distribution. Blue and red solid line illustrate TE- and TM-mode component separated by setting the wire-grid polarizer. The center frequency of total THz-wave output located at 1.0 THz and its band width was 300 GHz. For TE- and TM-mode, the center frequency positions were at 0.95 THz and 1.05 THz, respectively.
Fig. 2 Frequency dependence of the THz power for GaP planar waveguides. Bule and red solid line illustrates the transmitted THz power through the wire-grid polarizer for TE and TM-mode component.
This discrepancy of the center frequency between TE- and TM-mode is explained by considering the phase matching condition inside the THz planar waveguide theoretically, as follows. The phase-matching condition in the planar waveguide is expressed as
where ni (i = p, s, or eff, THz) and λi (i = p, s, or THz) correspond to the refractive indices and wavelengths of the pump, signal, and guided mode of THz wave, respectively, We estimate neff,THz for the fundamental TE- and TM-mode by effective index method. The effective refractive index of TE-mode is obtained from the following equation,where t and nTHz are the thickness of planar waveguide and refractive index of bulk GaP in THz frequency region. For the case of TM-mode THz-wave, the effective refractive index is given byThe calculated results of effective indices by Eqs. (3) and (4) are shown in Fig. 3 . Blue and red solid lines express the modal dispersion curve of TE- and TM-mode. Dotted line illustrated the refractive index required for phase matching, which is calculated from left side of Eq. (2) The phase-matching condition is satisfied at the intersection of the modal-dispersion curve (neff,THz) and the dotted line. Therefore, thefrequency position difference between TE- and TM-mode is expressed theoretically, which was consistent with the experimental results The spectral overlapping between generated TE- and TM-mode was confirmed over 0.8 THz, which indicates that both simultaneous generations of both guiding modes are possible.Fig. 3 The calculated results for the modal-dispersion relationships of the guiding modes in the GaP planar waveguide. The bule and red solid lines indicate the effective refractive index in the waveguide for TE- and TM-modes. The dotted line shows the refractive index which is required for the collinear phase matching condition in difference frequency mixing.
The polarization properties of THz-wave generated from the GaP planar waveguide were investigated by using the wire grid polarizer. We measured the transmission intensity of THz-wave by rotating the wire-grid polarizer. Figure 4(a) illustrates the polar plot of the measured transmission intensity of THz-wave with different waveguide length of 5, 10, and 15 mm, respectively. The elliptical polarization of THz-wave was confirmed for any waveguide lengths. The longer axis direction of the ellipse was parallel to [11] crystalline direction for 5-mm-long waveguide, which is coincide with the theoretical estimation of the direction of nonlinear polarization of difference frequency PTHz. The ellipticity increased as waveguide length became long. We also conducted the same experiment by using bulk GaP crystals with the cross sectional size is much greater than the THz wavelength itself. Their polarization properties are shown in Fig. 4(b). Lineally polarized THz-wave along <111> crystalline direction was indicated for any crystal lengths. Therefore the mechanism of ellipticalpolarized THz-wave generation was suggested due to the relative phase difference between TE- and TM-modes produced during the propagation through waveguide. From the point of view of the initial polarization direction of [11] and effective index for both modes, the rotation direction of generated elliptical polarization can be considered as clockwise direction because the effective index for TE-mode is larger than that for TM-mode, as illustrated.
Fig. 4 The polar plot for the THz radiation power from GaP planar waveguides (a) and bulk GaP crystals (b) with <111> excitation by the incident sources as a function of rotation angle of wire-grid polarizer for several crystal lengths of 5(square), 10(circle), and 15mm(triangle).
If the polarization direction of two incident sources are aligned along [11] crystalline direction, the vibrational direction of PTHz should be along the same direction as [11]. The phase difference of the polarization direction between along [11] and [11] is π. Thus the rotation direction of elliptical polarization generated by the [11] configuration should be counter clockwise direction.
Figure 5 shows the polarization measurement results under the excitation by the [11] and [11] crystalline configuration for 10-mm-long GaP planar waveguide at frequency of 1 THz. Black and red plots indicate the [11] and [11] excitation configuration. Both configurationshave shown that long axis direction of elliptical polarization are parallel to [11] and [11] crystalline directions. The rotation directions of generated elliptical polarization of THz-wave under both configurations are suggested to be opposite direction each other.
Fig. 5 The polar plot for the THz radiation power from GaP planar waveguides (a) as a function of rotation angle of wire-grid polarizer for 10-mm-long planar waveguide under the polarization direction of two incident sources along the [11] (black) and [11] (red) crystalline direction.
4. Summary
We carried out THz-wave generation from the GaP planar waveguides under collinear phase-matched difference-frequency mixing of two near-infrared sources. TE- and TM-mode of THz-wave were generated simultaneously by adjusting the polarization direction of two incident infrared sources to be parallel to [11] crystalline direction around 1 THz. We confirmed that the phase shift between quasi-TE and TM-mode in the waveguide depended on the waveguide length and contributed to the generation of the elliptical polarized THz-wave. The ellipticity of generated THz-wave increased as waveguide length increased. We indicated the possibility of change of rotational direction of elliptical polarization from clockwise to counterclockwise by adjusting the polarization direction along to the [11] and [11] crystalline direction. These results are attractive for the possible application for the polarization sensitive THz spectroscopy, THz imaging, and vibrational circular dichroism to discriminate the chiral molecules.
Acknowledgments
The authors would like to thank to T. Sasaki, K. Suto, and J. Nishizawa of the Semiconductor Research Institute for the discussion on the experimental results. We also would like to thank to T. Kimura of the Semiconductor Research Institute for help with waveguide fabrication.
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