Hunter Banks

High-order sideband generation in bulk GaAs

When an intense THz field at frequency fTHz is applied to excitons resonantly created in bulk GaAs by a near infrared laser at frequency fNIR, sidebands are observed at frequencies fsideband = fNIR + 2nfTHz, where n is an integer. At temperature T = 10 K, sidebands of order −4 ≤ 2n ≤ 16 are observed. Sidebands up to 10th order persist at 170 K.

Terahertz electron-hole recollisions in GaAs/AlGaAs quantum wells: robustness to scattering by optical phonons and thermal fluctuations.

Electron-hole recollisions are induced by resonantly injecting excitons with a near-IR laser at frequency fNIR into quantum wells driven by a 10  kV/cm field oscillating at fTHz=0.57  THz. At T=12  K, up to 18 sidebands are observed at frequencies fsideband=fNIR+2nfTHz, with -8≤2n≤28. Electrons and holes recollide with total kinetic energies up to 57 meV, well above the ELO=36  meV threshold for longitudinal optical (LO) phonon emission. Sidebands with order up to 2n=22 persist up to room temperature. A simple model shows that LO phonon scattering suppresses but does not eliminate sidebands associated with kinetic energies above ELO.

Dynamical birefringence: Electron-hole recollisions as a probe of Berry curvature

The direct measurement of Berry phases is still a great challenge in condensed matter systems. The bottleneck has been the ability to adiabatically drive an electron coherently across a large portion of the Brillouin zone in a solid where the scattering is strong and complicated. We break through this bottleneck and show that high-order sideband generation (HSG) in semiconductors is intimately affected by Berry phases. Electron-hole recollisions and HSG occur when a near-band gap laser beam excites a semiconductor that is driven by sufficiently strong terahertz (THz)-frequency electric fields. We carried out experimental and theoretical studies of HSG from three GaAs/AlGaAs quantum wells. The observed HSG spectra contain sidebands up to the 90th order, to our knowledge the highest-order optical nonlinearity observed in solids. The highest-order sidebands are associated with electron-hole pairs driven coherently across roughly 10% of the Brillouin zone around the \Gamma point. The principal experimental claim is a dynamical birefringence: the sidebands, when the order is high enough (> 20), are usually stronger when the exciting near-infrared (NIR) and the THz electric fields are polarized perpendicular than parallel; the sideband intensities depend on the angles between the THz field and the crystal axes in samples with sufficiently weak quenched disorder; and the sidebands exhibit significant ellipticity that increases with increasing sideband order, despite nearly linear excitation and driving fields. We explain dynamical birefringence by generalizing the three-step model for high order harmonic generation. The hole accumulates Berry phases due to variation of its internal state as the quasi-momentum changes under the THz field. Dynamical birefringence arises from quantum interference between time-reversed pairs of electron-hole recollision pathways.

High-order sideband generation in semiconductors: Beyond the three step model

Measurements of high-order sideband generation and absorption as a function of near-ir frequency in strong monochromatic sub-THz fields highlight opportunities to explore recollision physics in regimes that are difficult to access in atomic systems.

High-order sideband generation: Effect of optical polarization

Continuous optical excitation of excitons in quantum wells driven by intense, monochromatic terahertz fields leads to high-order sideband generation. With optical and terahertz fields perpendicular, sideband generation is enhanced, leading to 60th-order and higher processes.

Antenna-boosted mixing of terahertz and near-infrared radiation

Using moderate terahertz intensities of approximately 20 kW/cm2 near 0.6 THz, together with simple antennas, we have observed up to 12 sidebands on a near IR laser. The high-order sidebands were generated at room temperature in a membrane containing GaAs/AlGaAs quantum wells. The antennas were rectangular apertures ∼0.2 mm long in a gold film evaporated onto the membrane. Comparing the intensities required to generate comparable sideband spectra with and without antennas, we estimate the local terahertz field was enhanced by a factor of 5 ± 1, in agreement with finite difference time domain calculations.

Near infrared frequency dependence of high-order sideband generation

The near infrared frequency dependence of high order sideband generation in InGaAs quantum wells is discussed. The NIR frequency dependence of the sidebands indicates that the HSG phenomenon is excitonic in nature.