GaAs/AlAs coupled multilayer cavity structures are proposed as terahertz emission devices. Two cavity modes with an optical frequency difference in the terahertz region can be realized when two cavity layers are coupled by an intermediate distributed Bragg reflector multilayer. Interference between the enhanced light fields of the cavity modes has been demonstrated by the simultaneous injection of two cavity-mode lights using an ultrashort pulse laser. Such coupled multilayer cavity structures are promising for use as compact and room temperature operable terahertz emission devices based on difference frequency generation by the cavity-mode lights.
IntroductionEfficient wavelength conversion devices using second-order optical nonlinearity of semiconductor materials are attractive for new light sources in the undeveloped frequency region. Especially, difference frequency generation (DFG) methods with high conversion efficiency enable us to develop compact terahertz emitters operating at roomtemperature. A half-wavelength (λ/2) cavity structure sandwiched between two GaAs/AlAs distributed Bragg reflector (DBR) multilayers is useful for nonlinear optical devices such as all-optical Kerr gate switches [1,2] because extremely strong light field is realized in the λ/2 cavity layer. Recently, we have proposed a GaAs/AlAs coupled multilayer cavity structure, where two λ/2 cavity layers are coupled by the intermediate DBR multilayer, grown on a high-index GaAs substrate.[3] Two cavity modes are realized in the center of the high reflection band, and its frequency difference can be precisely defined in the terahertz region by the number of periods of the coupling DBR multilayer. We can expect strong frequency mixed signal from the two cavity-mode lights with different fundamental frequencies, since the light electric field of each cavity mode is strongly enhanced in both of the two λ/2 cavity layers. Growth of the coupled cavity structure on a non-(001) substrate is essential for the frequency mixing because the effective second-order nonlinear coefficient is zero on the conventional (001) orientation due to crystal symmetry. [4] The growth on the (113)B substrate would be beneficial for the high conversion efficiency with keeping good crystalline quality of the epitaxial layers. In fact, Kaneko et al. demonstrated blue vertical-cavity surface-emitting lasers (VCSELs) based on second-harmonic generation (SHG) grown on the (113)B GaAs substrate. [5] In this study, the GaAs/AlAs coupled multilayer cavity structure was grown on the (113)B-oriented GaAs substrates by molecular beam epitaxy (MBE). Strong sum frequency generation (SFG) using the two cavity modes were demonstrated by the ultrashort pulse laser excitation.
We have investigated optical anisotropy of sum frequency generation (SFG) of two cavity modes realized in a (113)B GaAs/AlAs coupled multilayer cavity. The SFG signals were measured by simultaneous excitation of two cavity modes with an optical frequency difference of 3.3 THz using 100-fs laser pulses. The peak intensity of the SFG signal was more than 400 times larger than that of the second-harmonic generation signal from the (113)B GaAs bulk substrate. Large optical anisotropy depending on the polarization angle of the excitation laser was observed in the strongly enhanced SFG in the (113)B coupled cavity sample. The observed anisotropic behavior was well explained by the in-plane anisotropy of the second-order nonlinear polarization on the (113)B GaAs substrate. #
We observed terahertz (THz) radiation using difference frequency generation (DFG) of two cavity modes in a (113)B GaAs/AlAs coupled multilayer cavity. 100 fs laser pulses were used to simultaneously excite two cavity modes, and oscillations with a period of 0.45 ps were clearly observed in the temporal waveform of THz time-domain measurements. The oscillation period agrees well with the difference frequency of the two cavity modes (2.2 THz). This clearly indicates that the observed signal originates from THz radiation generated by DFG of the two cavity modes in the GaAs/AlAs coupled multilayer cavity.
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