Mark Yu-Tsu Su

Generation of first-order terahertz optical sidebands in asymmetric coupled quantum wells

We have generated first-order tetrahertz (THz) optical sidebands on a near-infrared (NIR) probe beam by driving an excitonic intersubband resonance with THz electric fields. We use THz radiation polarized along the noncentrosymmetric axis of a quantum well system to generate a comb of sidebands ωsideband=ωNIR+nωTHz. The n=1 process offers an efficient means of modulating a NIR carrier beam at THz frequencies and yields new spectroscopic information on excitonic intersubband transitions.

Voltage-controlled wavelength conversion by terahertz electro-optic modulation in double quantum wells

An undoped double quantum well (DQW) was driven with a terahertz (THz) electric field of frequency ωTHz polarized in the growth direction, while simultaneously illuminated with a near-infrared (NIR) laser at frequency ωNIR. The intensity of NIR upconverted sidebands ωsideband=ωNIR+ωTHz was maximized when a dc voltage applied in the growth direction tuned the excitonic states into resonance with both the THz and NIR fields. There was no detectable upconversion far from resonance. The results demonstrate the possibility of using gated DQW devices for all-optical wavelength shifting between optical communication channels separated by up to a few THz.

Terahertz dynamics in confined magnetoexcitons

Abstract We have investigated the linear and nonlinear response of confined magnetoexcitons to intense terahertz (THz) radiation. By monitoring photoluminescence from THz-driven GaAs quantum wells, we have observed for the first time internal transitions in direct excitons. The spectrum of excitations is enriched by the complexities of the valence band and is well explained by an effective-mass theory. At high THz-field strengths, the emission properties of the driven quantum wells are completely dominated by new near-band-gap features, or optical sidebands , which appear at frequencies ω NIR ±2 nω THz , where ω NIR is the exciton-creation frequency, ω THz is the driving frequency, and n is an integer. The intensity of the sidebands exhibits pronounced enhancement when ω THz coincides with one- and two-photon exciton internal transitions, providing new and accurate information on the internal dynamics of excitons.

Odd terahertz optical sidebands from asymmetric excitonic intersubband excitation

Abstract We have generated terahertz (THz) optical sidebands on a near-infrared probe beam by driving an excitonic intersubband resonance with THz electric fields. We use THz radiation polarized along the non-centrosymmetric axis of a quantum well system to generate a comb of sidebands ωsideband=ωNIR+nωTHz. In exploring the processs rich polarization, power, and NIR frequency dependences we encounter both an efficient perturbative regime modeled by a χ(2) non-linear susceptibility, and a non-perturbative regime which has not been previously explored in driven quantum systems.

Near-infrared spectroscopy of terahertz-driven semiconductor nanostructures

We have explored near-infrared (NIR)--far-infrared (FIR) two-color optical experiments in quantum-confined semiconductor systems, using NIR radiation from a tunable cw Ti:Sapphire laser and intense and coherent FIR radiation from the UCSB Free-Electron Lasers. In this paper two recent experiments are discussed, both of which provide new insight into the internal structure and dynamics of confined excitons: (1) We have observed for the first time FIR internal transitions associated with the direct exciton in GaAs/AlGaAs quantum wells. The spectrum of excitations is enriched by the complexities of the valence band and differ significantly from simple reduced-mass, hydrogenic models. We provide a critical test of detailed calculations including the valence-band mixing of Bauer and Ando. (2) We have discovered resonant nonlinear optical mixing of NIR and FIR radiation, which results in strong near-bandgap emission lines, or optical sidebands. The sidebands appear when optically-created excitons are driven strongly by intense FIR fields. The frequencies of the sidebands are (omega) NIR +/- 2n(omega) FIR, where (omega) NIR is the interband exciton-creation frequency, (omega) FIR is the frequency of the driving field, and n is an integer. The intensity of the sidebands exhibits pronounced resonances as a function of applied magnetic field, which are well- explained in terms of virtual transitions between magnetically-tuned energy levels in the excitons.

First-order coherent THz optical sideband generation from asymmetric QW intersubband transitions

Abstract We have generated terahertz (THz) optical sidebands on a near-infrared probe beam by driving an excitonic intersubband resonance with THz electric fields. We use THz radiation polarized along the non-centro-symmetric axis of a quantum well system to generate a comb of sidebands ω sideband = ω NIR + nω THz . In exploring the rich polarization of the processs power, and NIR frequency dependences we encounter both an efficient perturbative regime modeled by a X (2) non-linear susceptibility, and a non-perturbative regime which has not been previously explored in driven quantum systems.

Coherent terahertz mixing spectroscopy of asymmetric quantum well intersubband transitions

Since terahertz electric fields can couple strongly to quantum well intersubband transitions we expect interband optical properties of a semiconductor heterostructure to change resonantly under a THz driving field. By driving the excitonic intersubband resonance of an asymmetric quantum well with intense THz electric fields from a free electron laser, we modulate the transmission of a near-IR (NIR) laser beam at terahertz frequencies. This process manifests itself as the emission of optical sidebands on the NIR probe. In previous THz electro-optical studies in semiconductors, only even sidebands of frequency (omega) sideband equals (omega) NIR + 2n(omega) THz had been observed. BY breaking inversion symmetry we are able to generate a comb of even and odd sidebands. The sidebands obey both THz and near-IR polarization selection rules and are enhanced when the NIR energy is resonant with a peak in the excitonic density of states. The ability to generate THz optical sidebands of all orders is important for the future application of THz EO effects in nonlinear spectroscopy and in ultrafast optical phase and amplitude modulation.

Photo-thermal transitions of magneto-excitons in quantum wells

By monitoring changes in excitonic photoluminescence (PL) that are induced by terahertz (THz) radiation, we observe resonant THz absorption by magnetoexcitons in GaAs/AlGaAs quantum wells. Changes in the PL spectrum are explored as a function of temperature and magnetic field, providing insight into the mechanisms which allow THz absorption to modulate PL. The strongest PL-quenching occurs at the heavy hole

Photothermal transitions of magnetoexcitons inGaAs/AlxGa1−xAsquantum wells

1s\to 2p^+

Terahertz Linear and Nonlinear Dynamics in Confined Magnetoexcitons

resonance where heavy hole excitons are photo-thermally converted into light hole excitons.