Superlattice nonlinearities for Gigahertz-Terahertz generation in harmonic multipliers

Abstract

<jats:title>Abstract</jats:title><jats:p>Semiconductor superlattices are strongly nonlinear media offering several technological challenges associated with the generation of high-frequency Gigahertz radiation and very effective frequency multiplication up to several Terahertzs. However, charge accumulation, traps and interface defects lead to pronounced asymmetries in the nonlinear current flow, from which high harmonic generation stems. This problem requires a full non-perturbative solution of asymmetric current flow under irradiation, which we deliver in this paper within the Boltzmann-Bloch approach. We investigate the nonlinear output on both frequency and time domains and demonstrate a significant enhancement of even harmonics by tuning the interface quality. Moreover, we find that increasing arbitrarily the input power is not a solution for high nonlinear output, in contrast with materials described by conventional susceptibilities. There is a complex combination of asymmetry and power values leading to maximum high harmonic generation.</jats:p>

Combined Structural and Voltage Control of Giant Nonlinearities in Semiconductor Superlattices

“…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: …”

“…Note that typically the interfaces of GaAs over AlAs do not have the same quality as those of AlAs over GaAs. To characterize the quality of interfaces in the superlattice we employ the asymmetry parameter , which effectively depends on the different interface roughness self-energies [ 39 , 41 ]. Therefore, as increases the peak of the current–voltage characteristic for positive bias practically remains the same, whereas for negative bias the peak is significantly reduced.…”

“…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].…”

Homogeneous quantum cascade lasers operating as terahertz frequency combs over their entire operational regime

“…Although relevant models for gigahertz-terahertz generation exploiting nonlinearity in semiconductor superlattices have been recently proposed [7], electrically pumped quantum cascade lasers (QCLs) are the most efficient on-chip sources of optical frequency comb (FC) synthesizers at THz frequencies [8][9][10][11], thanks to the wide frequency coverage [10,12] and the inherently high optical power levels [13,14], which allow continuous-wave (CW) emitting powers per comb tooth in the 3 μW [12] to 60 μW [8] range and the high spectral purity (intrinsic linewidths of ∼100 Hz) [15]. Such a combination of performances makes THz QCL FCs the most suitable choice for the aforementioned applications.…”