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Gallium arsenide shows excellent promise for terahertz generation using mid infrared.
This is for two reasons. First, the indices of refraction for the terahertz (nTHz=3.61 at 1 THz) and mid infrared (nopt=3.431 at 2 μm) are close allowing a long interaction length.
Second, the linear absorption is low at terahertz frequencies (αTHz=.5 to 4.5 cm⁻¹ for 1 to 3 THz). Since gallium arsenide is a direct bandgap material, multiphoton absorption and nonlinear refraction are issues for efficiency and system design in the mid infrared. In fact, linear absorption makes this material opaque at or below 870 nm. Additionally, two photon absorption is quite pronounced between 870 nm and 1.74 μm.
I will present the theory then the experimental data for two and three photon
absorption (870 nm to 1.74 μm and 1.74 μm to 2.61 μm, respectively) as well as for
nonlinear refraction. The three photon absorption has a minimum at 2 μm in the spectral range of 1.74 to 2.61 μm thus it is the preferred wavelength for terahertz generation. At 2 μm the anisotropy in the three photon absorption was almost 50 %. The nonlinear refraction remains fairly constant in this range as expected. However at 2 μm the anisotropy in nonlinear refraction was only 16% as compared with the predicted factor of 2. Qualitatively, the anisotropic behavior of the nonlinear refraction still conforms to the expected symmetry class of zincblende crystals. Terahertz time domain spectroscopy will be discussed on both theoretical and experimental levels. The results will show that terahertz generation is promising in the mid infrared range for wavelengths 2 μm and above. At 2 μm I demonstrate the advantages of a quasi-phase matched structure. One, the inverting structure generates a narrow band source. Additionally, shaped domains map to the terahertz electric field allowing shaped pulses. Also, of benefit is the increase in power production from having a longer effective interaction path in the crystal. |
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