Resonant Crossover of Terahertz Loss to the Gain of a Bloch OscillatingInAs/AlSbSuperlattice

Abstract

Terahertz absorption in waveguides loaded with InAs/AlSb super-superlattice mesas reveals a frequency dependent crossover from loss to gain that is related to the Stark ladder produced by an applied dc electric field. Electric field domains appear to be suppressed in the super-superlattice composed of many very short segments of superlattice, interrupted by heavily doped InAs regions. Resonant crossover is indicated by an increase in terahertz transmission as the Stark splitting or Bloch frequency determined by the applied dc electric field exceeds the measurement frequency.

Loss and gain in Bloch oscillating super-superlattices: THz Stark ladder spectroscopy

“…7 shows the change in transmission vs. voltage at 1.98 THz. [6] The voltage required to open the transmission through the guide is about 80% less than expected which implies that ~ 20% of the superlattice experience little or no field. This result is obtained at each frequency between 1.5 and 2.5 THz.…”

“…2. [6] It is clear that we should set aside naive models of a Bloch oscillator device. A Bloch oscillator device does not oscillate at the Bloch frequency, it oscillates at a frequency determined by the external resonator or circuit but only at frequencies below the Bloch frequency.…”

On the possibility of a shunt-stabilized superlattice terahertz emitter

“…Current investigations of possible SL-based THz sources focus on a variety of schemes for stabilizing structures in the NDC region, e.g., using modulated bias 4 or stacking SLs with small number of periods. 5 The possibility of using a conductive shunt layer to stabilize a SL was recently suggested by Daniel et al 6 Using a distributed nonlinear circuit model, they identified values of SL width and shunt resistivity for which static electric field domains are suppressed. However, this model is not capable of describing instability to moving field domains, 3 since it does not include physical effects such as the dependence of tunneling currents on local charge density, or diffusive currents that flow parallel to the quantum wells of the SL.…”

“…5 The value of peak drift velocity v M is variable and depends on well and barrier widths of the SL. 14,18 We bias the structures in the NDC region, with an average field U / l = 6.67ϫ 10 6 V / m, where the total applied voltage U = 12 V is held constant and l = 1.8 m is the length of the SL in z direction.…”

Sub-terahertz amplification in a semiconductor superlattice with moving charge domains

“…[14][15][16][17][18][19] SLs comprise multiple alternating layers of different semiconductor materials, 12,13 which form a periodic modulation of the conduction band. This creates a tunable, quantum mechanical environment that is suitable for the realization of Bloch gain, 7,[20][21][22][23] which can be utilized both for the generation and amplification of sub-THz and THz signals. However, the same quantum mechanisms that give rise to Bloch gain result in electric instability, which leads to the formation of moving high-field charge domains.…”