Time-domain terahertz spectroscopy in high magnetic fields.

There are a variety of elementary and collective terahertz-frequency excitations in condensed matter whose magnetic field dependence contains significant insight into the states and dynamics of the electrons involved. Often, determining the frequency, temperature, and magnetic field dependence of the optical conductivity tensor, especially in high magnetic fields, can clarify the microscopic physics behind complex many-body behaviors of solids. While there are advanced terahertz spectroscopy techniques as well as high magnetic field generation techniques available, a combination of the two has only been realized relatively recently. Here, we review the current state of terahertz time-domain spectroscopy (THz-TDS) experiments in high magnetic fields. We start with an overview of time-domain terahertz detection schemes with a special focus on how they have been incorporated into optically accessible high-field magnets. Advantages and disadvantages of different types of magnets in performing THz-TDS experiments are also discussed. Finally, we highlight some of the new fascinating physical phenomena that have been revealed by THz-TDS in high magnetic fields.

Observation of Photoinduced Terahertz Gain in GaAs Quantum Wells: Evidence for Radiative Two-Exciton-to-Biexciton Scattering.

We have observed photoinduced negative optical conductivity, or gain, in the terahertz frequency range in a GaAs multiple-quantum-well structure in a strong perpendicular magnetic field at low temperatures. The gain is narrow band: it appears as a sharp peak (linewidth <0.45 meV) whose frequency shifts with applied magnetic field. The gain has a circular-polarization selection rule: a strong line is observed for hole-cyclotron-resonance-active polarization. Furthermore, the gain appears only when the exciton 1s state is populated, which rules out intraexcitonic transitions to be its origin. Based on these observations, we propose a possible process in which the stimulated emission of a terahertz photon occurs while two free excitons scatter into one biexciton in an energy and angular-momentum conserving manner.

The effect of luminescent coupling on modulated photocurrent measurements in multijunction solar cells.

Luminescent coupling in multijunction solar cells has a major impact on device response, and its impact on current-voltage and quantum efficiency measurements is well established. However, the role of luminescent coupling in more advanced characterization techniques such as modulated photocurrent spectroscopy is virtually unknown. Here we present measurements of the frequency-dependent photocurrent of a triple junction solar cell with significant coupling between adjacent junctions. We develop an equivalent circuit model that includes luminescent coupling which shows good agreement with the measured frequency response. The model also shows how the system response can elucidate the type of charge carrier recombination in these III-V semiconductor materials.

Frequency response of the external quantum efficiency in multijunction solar cells.

The frequency dependence of the external quantum efficiency (EQE) of high-quality multijunction solar cells was examined by the modulated photocurrent spectroscopy method via an optical setup comprised of a light-pipe-coupled compact LED array. The optical excitation was achieved through sinusoidal electrical modulation of an appropriate LED by a custom-designed, high bandwidth amplifier. We observed unique features in the amplitude and phase data of the EQE frequency sweeps that are very sensitive to various subcell parameters and light bias conditions. These features are discussed extensively within the context of an AC equivalent circuit model, showing remarkable agreement between the experimental data and the proposed model.