James Heyman

Diffusion and drift in terahertz emission at GaAs surfaces

We study terahertz (THz) emission from GaAs as a function of photon energy and electric field. THz radiation arises from transport of photogenerated charge in an electric field and by hot carrier diffusion (the photo-Dember effect). These mechanisms can be separated by experiments in which either the electric field or the kinetic energy of the carriers is varied. For electric fields E∼4 kV/cm, we find that the electric field controls THz emission for carrier temperatures kBTC⩽0.1 eV, while hot-carrier diffusion dominates for kBTC≈1 eV. Both mechanisms contribute at intermediate fields and carrier temperatures. Our results are consistent with estimates of the relative magnitudes of these two effects.

Time-domain measurement of intersubband oscillations in a quantum well

We report time-domain measurements of electron intersubband oscillations in quantum wells. We use an interferometric technique to measure the change in the profile of a few-cycle THz pulse due to propagation through a modulation-doped Al0.3Ga0.7As/GaAs multiple quantum well structure (10×510 A wells). From this data we obtain the absorption and the index of refraction due to electrons in the quantum well, and due to the GaAs substrate. Unlike existing studies of coherent charge oscillations of electrons and holes in heterostructures excited by ultrashort pulses of band-gap light, our all-THz measurements quantitatively determine the linear optical properties of the quantum well electrons.

Sampling a terahertz dipole transition with subcycle time resolution

We present a time-resolved technique to measure optical excitation processes with a time resolution shorter than the oscillation period of the exciting light. Our terahertz (THz) experiments fully resolve the polarization dynamics of electrons in semiconductor heterostructures when they are excited by a THz pulse. The time resolution of the polarization enables us to deduce the population dynamics of the excited state, which includes the dynamics of a virtual population in the case of off-resonant excitation.

Mid-infrared electroluminescence in GaAs/AlGaAs structures

Design, growth, and operation of an unipolar light emitting diode based on the material system GaAs/AlGaAs is reported. We present mid-infrared transmission, photocurrent, and electroluminescence measurements on a quantum cascade structure with intersubband transition energies greater than the optical phonon energy. Electroluminescence powers up to a few nanowatts at 6.9 μm have been measured.

Far‐infrared pump‐probe measurements of the intersubband lifetime in an AlGaAs/GaAs coupled‐quantum well

We report pump‐and‐probe measurements of the electron intersubband lifetime (T1) in an AlGaAs/GaAs heterostructure using a picosecond pulsed far‐infrared laser. The subband spacing (11 meV) is less than the optical‐phonon energy. Time‐resolved measurements yield intersubband lifetimes ranging from T1=1.1±0.2 ns to T1=0.4±0.1 ns depending on measurement conditions. Results are in agreement with previous lifetime measurements on the same sample using continuous excitation at intensities ≤1 W/cm2. The steady‐state measurements yielded shorter lifetimes at high excitation intensities, possibly due to carrier heating leading to intersubband scattering by optical phonon emission.

Nonlinear quantum dynamics in semiconductor quantum wells

Abstract We discuss recent measurements of the nonlinear response of electrons in wide quantum wells driven by intense electromagnetic radiation at terahertz frequencies. The theme is the interplay of quantum mechanics, strong periodic driving, the electron-electron interaction and dissipation. We discuss harmonic generation from an asymmetric double quantum well in which the effects of dynamic screening are important. Measurements and theory are found to be in good agreement. We also discuss intensity-dependent absorption in a 400A square quantum well. A new nonlinear quantum effect occurs, in which the frequency at which electromagnetic radiation is absorbed shifts to the red with increasing intensity. The preliminary experimental results are in agreement with a theory by Zaluzny, in which the source of the nonlinearity is the self-consistent potential in the Hartree approximation for the electron dynamics.

Polarization of terahertz radiation from laser generated plasma filaments

An analysis of the polarization of terahertz (THz) radiation from a laser-induced plasma source is presented. THz emission is achieved by mixing a laser pulse with its second harmonic after focusing through a β-BaB2O4 (β-BBO) crystal. Numerical calculations, based on the nonlinear four-wave mixing model and the microscopic polarization model, are compared with experimental results. The main focus lies on the study of the dependence of THz polarization on the polarization and relative phase of the incident fundamental and second-harmonic pulses. We show that the modulation of the fundamental pulse by the BBO crystal has to be taken into account in order to describe experimental observations. By including the finite extension of the plasma and considering cross- and self-phase modulation of the two-color pump pulse, we are able to explain the observed ellipticity of the THz pulse as well as the orientation of the polarization axis.

Far-infrared saturation spectroscopy of a single square well

We have performed saturation spectroscopy measurements of the lowest intersubband transition in a single 400 AA GaAs/Al0.3Ga0.7As modulation-doped square quantum well. We couple intense tunable far-infrared radiation from the Santa Barbara free electron laser into our sample using an edge-coupling technique and measure absorption as a function of frequency and intensity. Saturation and frequency shifts in the absorption line are clearly observed. We attribute the frequency shifts to reductions in the many-body depolarization shift. From our preliminary measurements, we estimate the intersubband relaxation time to be 600 ps to within a factor of three.

Materials science in the far-IR with electrostatic based FELs

Abstract A technology gap exists between ∼ 100 GHz and ∼ 10 THz. Free-electron lasers (FELs), driven by high quality, relatively low energy electron beams from electrostatic accelerators, and capable of generating kilowatts of coherent, tunable radiation, are ideally suited to explore the enabling science for future technology in this spectral range. We describe two experiments that use terahertz “optical rectification” to measure i) the intensity and temperature dependent energy relaxation in quantum wells and ii) the intrinsic relaxation of resonant tunneling diodes. Both benefit from the power and tunablilty of the UCSB FELs.

Carrier Heating and Negative Photoconductivity in Graphene

We investigated negative photoconductivity in graphene using ultrafast terahertz techniques. Infrared transmission was used to determine the Fermi energy, carrier density, and mobility of p-type chemical vapor deposition graphene samples. Time-resolved terahertz photoconductivity measurements using a tunable mid-infrared pump probed these samples at photon energies between 0.35 eV and 1.55 eV, approximately one-half to three times the Fermi energy of the samples. Although interband optical transitions in graphene are blocked for pump photon energies less than twice the Fermi energy, we observe negative photoconductivity at all pump photon energies investigated, indicating that interband excitation is not required to observe this effect. Our results are consistent with a thermalized free-carrier population that cools by electron-phonon scattering, but are inconsistent with models of negative photoconductivity based on population inversion.