Chia-Yeh Li

Precision, all-optical measurement of external quantum efficiency in semiconductors

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed.

Broadband field-resolved terahertz detection via laser induced air plasma with controlled optical bias.

We report a robust method of coherent detection of broadband THz pulses using terahertz induced second-harmonic (TISH) generation in a laser induced air plasma together with a controlled second harmonic optical bias. We discuss a role of the bias field and its phase in the process of coherent detection. Phase-matching considerations subject to plasma dispersion are also examined.

Characterization of external quantum efficiency and absorption efficiency in GaAs/ InGaP double heterostructures for laser cooling applications

The state of current research in laser cooling of semiconductors is reviewed. Emphasis is placed on the characterization of external quantum efficiency and absorption efficiency in GaAs/InGaP double heterostuctures. New experimental results will be presented that characterize device operation as a function of laser excitation power and temperature. Optimum carrier density is obtained independently and used as a screening tool for sample quality. The crucial importance of parasitic background absorption is discussed.

Observation of strong and broadband terahertz induced electroabsorption in multiple quantum wells

We report the observation of strong terahertz-induced electroabsoption (EA) modulation in in multiple double quantum well (MDQW) structures Broadband terahertz generated by two-color laser induced plasma is focused (~1MV/cm) onto the MQW and spectro-temporal response is probed in transmission geometry, where up to 60% modulation signals are observed. EAS signal is attributed to several mechanisms, including observed qualitative agreement with the two-dimensional Franz-Keldysh response. Utilizing strong EA signals, we present a simple THz imaging scheme using conventional imagers.

Air-breakdown coherent detection of terahertz using controlled optical bias

Detection of the electric field induced second harmonic (EFISH) signal has been used in gas plasma for measurement of the symmetry-breaking THz transients. Previously, detection linearity with the THz field was accomplished by either mixing EFISH signal with a second harmonic component of an octave-spanning plasma supercontinuum or by addition of a high voltage DC bias across the plasma. Here we report a new method where controlled injection of the second harmonic signal provides the necessary bias for the coherent signal detection. This is accomplished simply by insertion of the BBO crystal in an optical path. The absence of high intensity or high voltage makes the detection scheme more viable for remote sensing.

GaAs/GaInP double heterostructure characterization for laser cooling of semiconductors

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent photoluminescence. Differential power-dependent photoluminescence measurement is used to quickly screen the sample quality before processing.

Broadband THz imaging in gas and multiple quantum-well media

We discuss two schemes of ultrafast THz imaging, both constituting non-perturbative response of either gas or solidstate media to the THz bias fields and thus offering very sensitive detection of the latter. In the first approach, we utilize air-breakdown plasma for space-time mapping of the THz field. In the second approach, we THz-induce strong electroabsorption response in the multiple quantum-well sample of thickness much smaller than the wavelength of the THz bias. As such, ultrabroadband imaging of the quasi single-cycle THz pulses can be possible.

Coherent detection of Terahertz via laser induced plasma with controlled optical bias

Common air-breakdown coherent detection techniques rely on either large DC bias across plasma or second harmonic contribution from the supercontinuum to provide a local oscillator field. Here we report coherent detection by purposely injecting second harmonic.

Broadband Field-Resolved Terahertz Detection in Laser-Induced Plasma with Controlled Optical Bias

Broadband coherent THz detection in laser-induced plasmas is a well-established detection technique. We report new modification, where injection and control of a second harmonic local bias field allows for simpler implementation particularly for stand-off applications.

Detection of ultrafast THz pulses via electro-absorption in coupled asymmetric quantum wells

We utilize quantum-confined Stark-effect in asymmetric double quantum wells (ADQW) to realize coherent detection of broadband THz pulses. For that, broadband THz transients formed by a two-color air plasma are focused onto ADQW, in turn dynamically shifting the ADQW bands, with the bandedge at ~ 825 nm. Spectrally-resolved detection scheme analyzes absorption modulation signatures imprinted onto the transmitted NIR probe spectrum. Use of only few micron thick samples ensures large detection bandwidth, currently demonstrated up to ~ 15 THz. Time-domain analysis of this signal shows pronounced bi-polar (coherent) as well as small unipolar components of the signal.