InAs nanowires have been previously shown to be efficient emitters of THz radiation, upon excitation with NIR
pulses. In this work we report complementary measurements of THz radiation and reflectometry together with
DC transport measurements that point to a unifying picture of low-energy acoustic-like longitudinal surface
plasmons as being the origin of this THz emission from the nanowires.
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.
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.
We characterize high quantum efficiency double GaAs/InGaP heterostructures used in semiconductor laser cooling. To
identify potential samples for laser cooling, measuring the nonradiative recombination rate coefficient is necessary. We
describe a technique called power dependent photoluminescence measurement, which when combined with timeresolved
photoluminescence lifetime determines the nonradiative recombination coefficient.
Differential luminescence thermometry (DLT) allows non-contact method of measuring temperature by timedifferencing
luminescence spectra emitted from the material in study. Here, we present a modification to the DLT
technique, termed "two-pixel DLT" (2pixDLT), that combines high temperature and temporal resolutions at the
expense of reduced spectral sampling of the luminescence signal. We showcase our technique by demonstrating
millisecond/millidegree resolution in time and temperature in heating dynamics of GaAs heterostructure sample.
We utilize tnis technique to determine minimum achievable temperature in rare-earth doped fluoride crystal
Yb:YLF to be 170K, when excited at 1030nm.
We discuss recent progress in the laser cooling experiments via resonant cavity. Following analysis of the cooling
efficiency, we highlight importance of wavelength dependence of the minimum achievable temperature for a given
cryocooler. Following the analysis, we utilize pump detuning along with reduction of thermal load on the sample
to achieve absolute temperature of nearly 200K, a 98.5 degree drop, starting from room temperature. Wavelength
dependent analysis suggests that further improvement is possible.
We report the first observation of coherent plasmon emission of THz radiation from arrays of semiconductor
nanowires. The THz signal strength from InAs nanowires is comparable to a planar substrate, indicating the
nanowires are highly efficient emitters. This is explained by the preferential orientation of plasma motion to
the wire surface, which overcomes radiation trapping by total-internal reflection. Using a bulk Drude model,
we identify the average donor density and mobility in the nanowires in a non-contact manner. Contact IV
transconductance measurements provide order of magnitude agreement with values obtained from the THz
We study Landau damping of a coherent, solid-state plasma by means of ultrafast THz spectroscopy. The onset
of this phenomenon occurs when momentum and energy conservation are satisfied for single-particle excitations;
this diminishes collective mode behavior. A series of InSb-based bulk heterostructures have varying amounts of
spatial confinement, allowing direct access to a different range of wave vectors in the electron-electron interaction.
Sufficient confinement leads to disappearance of the plasmon quasi-particle via Landau (collisionless) damping.
The experimental results are quantitatively reproduced by model calculations.
One of the challenges of laser cooling a semiconductor is its typically high index of refraction (greater than 3), which
limits efficient light output of the upconverted photon. This issue is addressed with a novel concept of coupling the
photon out via a thin, thermally insulating vacuum gap that allows light to pass efficiently by frustrated internal
Although silicon technology is mature and inexpensive, the indirect nature of the bandgap of silicon makes it unsuitable
for laser cooling. The material of choice is the binary compound semiconductor GaAs, which can be fabricated with high
quality necessary for laser cooling experiments. Moreover, process technology exists that enables a relatively simple
fabrication of a thin vacuum gap in this material system.
This paper will present an investigation of heat transport and light transmission across a "nanogap" consisting of a thin
epitaxial film supported over a substrate by an array of nanometer-sized posts. The structure is manufactured by crystal
growth of a sacrificial Al0.98Ga0.02As layer on a single crystal GaAs substrate. After lithographically defining holes in the
Al0.98Ga0.02As layer, the holes are filled with GaAs and a top GaAs layer is deposited. Lateral selective etching of the
Al0.98Ga0.02As will create a nanogap between two GaAs layers separated by GaAs posts. We are demonstrating the
successful fabrication of various size nanogaps in this material system, as well as their properties with respect to reduced
heat transfer across the gap. We are also presenting data supporting that the interface quality is high enough to allow
evanescent tunneling of light at angles otherwise forbidden by total internal reflection. The implications for
semiconductor laser cooling will be discussed.
Using a cavity resonant absorption scheme we demonstrate record laser cooling for the rare-earth doped crystalline
solid Yb:YLF. A temperature drop of nearly 70 degrees is obtained with respect to the ambient. Our preliminary
results indicate that minimum achievable temperature in this material/sample is 170 K, as measured using a
modified differential luminescence thermometry technique. This indicates outstanding potential for Yb:YLF as
a cryogenic laser cooler material.
KEYWORDS: Semiconductors, Signal to noise ratio, Microscopes, Luminescence, Gallium arsenide, Single photon, Atomic force microscopy, Semiconductor lasers, Near field scanning optical microscopy, Fluorescence lifetime imaging
We investigate the role of surface defects on semiconductor fluorescence lifetime using near-field scanning optical
microscopy (NSOM) and time correlated single photon counting (TCSPC). A conventional far-field microscope is used
to excite a GaAs sample and subsequent fluorescence is collected with a fiber coupled near-field probe. With the
application of custom fitting algorithms, we find fluorescence lifetimes in the vicinity of surface defects to be
significantly reduced with respect to fluorescence lifetimes measured in defect free regions.
We discuss a cavity enhanced resonant absorption (CERA) approach for increasing optical pump absorption.
This is demonstrated in the context of laser cooling of solids, where the absorption in ytterbium (Yb+3) ion
is increased by over an order of magnitude. This corresponds to more than 90% absorption efficiency. We
demonstrate cooling with Yb:ZBLAN and obtain &Dgr;T = -3 K starting from room temperature.
We present a brief analysis of laser cooling in semiconductor
quantum wells with emphasis on a previous experiment that gave
evidence for local cooling. This work is re-examined in the
context of light management, heat removal, and how increasing
photo-carrier density affects luminescence. Our main conclusion
is that using a single laser to both pump the sample and monitor
temperature may lead to ambiguity in semiconductor cooling
One of the challenges of laser cooling a semiconductor is the typically high index of refraction (greater than 3), which limits efficient light output of the upconverted photon. This challenge is proposed to be met with a novel concept of coupling the photon out via a thin, thermally insulating vacuum gap that allows light to pass efficiently by frustrated total internal reflection. This study has the goal of producing a test structure that allows investigation of heat transport across a 'nanogap' consisting of a thin film supported over a substrate by an array of nanometer-sized posts. The nanogap is fabricated monolithically by first creating a film of SiO2 on a silicon substrate, lithographically defining holes in the SiO2, and covering this structure including the holes with silicon. Selective lateral etching will then remove the SiO2, leaving behind a thin gap between two Si layers spaced apart by nanometer-scale Si posts. Demonstration of this final step by successfully undercutting the a-Si upper layer due to the hydrophobic nature of silicon and the slow etch rate of buffered oxide etch in the small gap has proved to be problematic. Arriving at a feasible solution to this conundrum is the current objective of this project in order to begin investigating the thermal conductivity properties of the structure.
Using an optical cavity, we demonstrate enhanced pump light absorption for laser cooling of solids. A Fabry-Perot cavity containing Yb:ZBLAN glass shows enhancement of resonant absorption by a factor of 11 compared to the double-pass configuration. This corresponds to 85% absorption of the incident laser power.
We demonstrate a non-contact, spectroscopic technique to measure
the temperature change of semiconductors with very high precision.
A temperature resolution of less than 100 μK has been obtained with
bulk GaAs. This scheme finds application in experiments to study
laser cooling of solids. We measure a record external quantum
efficiency of 99% for a GaAs device.
Laser cooling in semiconductor structures due to anti-Stokes luminescence is reviewed. Theoretical background considering luminescence trapping and red-shifting, the effect of free carrier and back-ground absorption, Pauli band-blocking, and the temperature-dependence of various recombination mechanisms are discussed. Recent experimental results demonstrating record external quantum efficiencies (EQE) in GaAs/GaInP heterostructures are described, and conditions favorable for the first observation of laser cooling in semiconductors are discussed.
We extract the chirp of an ultrashort laser pulse accurately in real-time using a simple modified auto-interferometric correlation (MOSAIC) technique. Through the use of our newly developed time-domain algorithm, chirp information is accessible with signal-to-noise levels approaching unity. Correction algorithms have been developed to accommodate signal distortions due to bandwidth limitations, autocorrelator misalignment, and non-quadratic detector response.
We study coherent plasmons in the narrow-gap semiconductor InSb by measuring the far-infrared electromagnetic radiation they emit. These collective oscillations are excited with ultrashort, near-infrared laser pulses having photon energy far above the semiconductor band gap. Coherent plasmon behavior is characterized as a function of temperature, doping density, optically injected carrier density, and spatial confinement.
The chirp of an ultrashort laser pulse can be extracted accurately in real-time using a simple modified autointerferometric correlation (MOSAIC) technique. Our newly developed time-domain algorithm is well suited for low signal-to-noise conditions. We display results revealing high sensitivity to chirp with signal-to-noise levels approaching the noise floor. Correction algorithms have been developed to accommodate signal distortions arising from bandwidth limitations, interferometer misalignment, and non-quadratic detector response.
We present an overview of laser cooling of solids. In this
all-solid-state approach to refrigeration, heat is removed radiatively when an engineered material is exposed to high power laser light. We report a record amount of net cooling (88 K below ambient) that has been achieved with a sample made from doped fluoride glass. Issues involved in the design of a practical laser cooler are presented. The possibility of laser cooling of semiconductor sensors is discussed.
Using the results of previous experiments, CO2 laser induced impact ionization in InSb has
been evaluated using measured dc ionization rate data. The laser field is scaled by 1/w'r, where o is the
laser frequency and t is the momentum relaxation time of the hot electrons. Scaling is done to provide
an ionization rate consistent with the experimental conditions. In this way, a momentum relaxation time
t, 0.34 p5 can be deduced, in agreement with the value previously determined by four wave mixing