Giant controllable gigahertz to terahertz nonlinearities in superlattices

Abstract

Optical nonlinearities are of perpetual importance, notably connected with emerging new materials. However, they are difficult to exploit in the gigahertz–terahertz (GHz–THz) range at room temperature and using low excitation power. Here, we present a clear-cut theoretical and experimental demonstration of real time, low power, room temperature control of GHz–THz nonlinearities. The nonlinear susceptibility concept, successful in most materials, cannot be used here and we show in contrast, a complex interplay between applied powers, voltages and asymmetric current flow, delivering giant control and enhancement of the nonlinearities. Semiconductor superlattices are used as nonlinear sources and as mixers for heterodyne detection, unlocking their dual potential as compact, room temperature, controllable sources and detectors. The low input powers and voltages applied are within the range of compact devices, enabling the practical extension of nonlinear optics concepts to the GHz–THz range, under controlled conditions and following a predictive design tool.

Combined Structural and Voltage Control of Giant Nonlinearities in Semiconductor Superlattices

“…This is consistent with the predictions and measurements in Ref. [ 42 ]. We also expect that current flow asymmetry improves the even harmonics, as predicted in Ref.…”

“…This may make the observation of high harmonic ‘petals’ easier, since the available samples, such as those used in the experiments described in Ref. [ 42 ], cannot sustain high voltages without permanent sample damage. This can also lead to devices with a more uniform higher power output at high-order even harmonic, leading to a wide range of useful frequencies in the GHz–THz range.…”

“…where the speed of light is c 2 = 1/µ 0 0 . Simplifying the Boltzmann equation for carrier transport with relaxation approximation, one can show that if the superlattice is illuminated with combined static and monochromatic fields [1,28,[38][39][40][41][42],…”

“…If there are no bound charges and no magnetization in the medium, we obtain the following wave equation: where the speed of light is . Simplifying the Boltzmann equation for carrier transport with relaxation approximation, one can show that if the superlattice is illuminated with combined static and monochromatic fields [ 1 , 28 , 38 , 39 , 40 , 41 , 42 ], the nonlinear generation of harmonics and current rectification, with the current I , related to the current density by j = I / A , is described by the following: …”

“…Strong excitonic effects can also play a role under well-defined conditions [35][36][37]. All these processed can be somehow be combined to enhance HHG in SSLMs; however, in this paper we focus on the combined effect of a uniform bias and Nanomaterials 2021, 11, 1287 2 of 8 current-voltage asymmetry on the Bloch oscillations modulation, which has successfully predicted giant voltage control of HHG in unbiased SSLMs [28,[38][39][40][41][42].…”