We report the observation of unusual thermal radiation at elevated temperatures (T=400-900K) from a three-dimensional metallic photonic-crystal composite that includes a micro-cavity. Upon thermal excitation by a heating element of a large heat-mass and a constant temperature (heat bath), its emissive power at resonant wavelengths exceeds a blackbody’s at nominally the same surface temperature. The possible explanations include, but are not limited to, angular concentration of light emission, slightly lower lattice-temperature for a reference blackbody and also a significant pumping of hot electrons at resonance such that our sample’s electron-temperature is higher than its latticetemperature.
The use of metallic nanostructures for enhanced transmission and near field phenomena have been a topic of extensive
research. Here we present integration of active media, consisting of InAs quantum dots (QD) embedded in quantum wells,
with 2 dimensional metallic hole arrays (2DHA) leading to a strong interaction between resonant surface plasmons, excited
at the metal-semiconductor interface, and intersubband transitions of quantum dots. The presence of a low-loss absorber
within the enhanced near field region of 2DHA leads to an enhancement of photoresponse. The parameters of 2DHA
were designed to overlap with absorption peaks of QDs. We present techniques of fabrication, accurate characterization of
enhancement and efforts to optimize the 2DHA-QD coupling. Over an order of magnitude enhancement in photoresponse
is observed due to spectral matching of intersubband absorption of quantum dots to that of 2DHA resonance, optimal
placement of QD within the structure, and improved interaction lengths due to lateral propagation. This enhancement is also
accompanied by significant narrowing of linewidth and the ability to tune the resonance by varying the 2DHA parameters.
A hexagonal lattice with periodic circular holes on a thin gold film is used as the 2DHA. With further optimizations, these
structures have significant applications in the mid-wave infrared (3-5 μm) and long-wave infrared (8-12 μm) regions for
multispectral and polarization sensitive sensing
The incorporation of semiconductor quantum dots into different heterostructures for applications in nanoscale
photodetection, lasing and amplification has been an active area of research in recent years. Here, we use ultrafast
differential transmission spectroscopy to temporally and spectrally resolve density-and-temperature-dependent carrier
dynamics in an InAs/InGaAs quantum dots-in-a-well (DWELL) heterostructure. In our experiments, electron-hole pairs
are optically injected into the three dimensional GaAs barriers, after which we monitor carrier relaxation into the two
dimensional InGaAs quantum wells and the zero dimensional InAs quantum dots by tuning the probe photon energy.
We find that for low photoinjected carrier densities, carrier capture and relaxation are dominated by Auger carrier-carrier
scattering at low temperatures, with thermal emission playing an increasing role with temperature. At low temperatures
we also observe excitation-dependent shifts of the quantum dot energy levels. In contrast, high density measurements
reveal an anomalous induced absorption at the quantum dot excited state that is correlated with quantum well population
dynamics. Our experiments provide essential insight into carrier relaxation across multiple spatial dimensions and reveal
unique Coulomb interaction-induced phenomena, with important implications for DWELL-based lasers and amplifiers.
Next generation infrared photodetector technology will require focal plane array (FPA) systems that have multi-spectral
imaging capabilities. One proposed approach to realizing these multicolor devices is to use plasmonic resonators.
However, device development and characterization are commonly addressed with large front side illuminated single
pixel detectors on a supporting epitaxial substrate. The focal plane arrays on the other hand are backside illuminated.
Moreover, in a front side illuminated device, there is significant substrate scattering of the incident light. Here, we
propose a method for the accurate measurement of device performance by using a hybridized chip design (hybrid chip)
that is similar to the fabrication of an FPA system, with the substrate completely removed through a combination of
mechanical polishing and subsequent wet etching techniques. The hybrid chip was also designed to precisely
characterize the effects of varying mesa size by incorporating square mesa structures that range from 25 to 200 μm in
width. This approach offers an advantage over conventional device characterization because it incorporates mesas that
are on the same scale as those normally used in FPA systems, which should therefore provide a fast transition of new
photodetector technology into camera based systems. The photodetector technology chosen for this work is a multi-stack
quantum dots-in-a-well (DWELL) structure designed to absorb electromagnetic radiation in the mid-infrared spectral
range.
We report on the performance of multi-stack quantum dots in a well (DWELL) detectors. Present-day QD detectors are
limited by low responsivity and quantum efficiency (QE). This can be attributed to the low absorption efficiency of
these structures due to the small number of QD stacks in the detector. In this paper we examine the effect of the number
of stacks on the performance of the detector. In particular, we investigate the InAs/GaAs/AlGaAs D-DWELL (Dots-in-double-well) design, which has a lower strain per DWELL stack than the InAs/InGaAs/GaAs DWELLs thereby enabling
the growth of many more stacks in the detector. The purpose of the study detailed in this paper is to examine the effects
of varying the number of stacks in the InAs/InGaAs/GaAs/AlGaAs D-DWELL detector, on its device performance. The
numbers of stacks grown using solid source molecular beam epitaxy (MBE), were 15, 30, 40, 50, and 60. Once
fabricated as single pixel devices, we carried-out a series of device measurements such as spectral response, dark current,
total current, responsivity along with computing the photoconductive gain and the activation energies. The goal of these
experiments is to not only study the single pixel detector performance with varying number of stacks in a D-DWELL
structure, but to also understand the effect of the transport mechanism in these devices.
μWe report the demonstration of multi-spectral quantum dots-in-a-well infrared photo-detectors through the coupling of
incident light to resonant modes of surface plasmons. The integration of a surface plasmon assisted cavity with the detector
results in shifting the peak wavelength of absorption of the detector to that of the resonant wavelength of the cavity. The
cavity consists of a square lattice structure with square holes in it. A wavelength tuning of 8.5 to 9 μm was observed,
by changing the pitch of the fabricated pattern forming the cavity. Polarization sensitive detectors can be fabricated by
breaking the symmetry of the lattice. This is achieved by stretching the lattice constants along the x and y directions. A
DWELL detector with resonant frequency at 6.8 μm where the response of the 0 ° polarization is twice as strong as the 90°
polarization is reported. This technique, in principle, is detector agnostic and shows promise in fabrication of multi-spectral
focal plane arrays (FPA).
We report the fabrication of low strain quantum-dots-in-a-double-well (DDWELL) infrared photodetector where the net
strain on the system has been reduced by limiting the total indium content in the system. The detector consists of InAs
dots embedded in In0.15Ga0.85As and GaAs wells with a Al0.1Ga0.9As barrier, as opposed to In0.15Ga0.85As wells and a
GaAs barrier in standard dots-in-a-well (DWELL) detector. The structure was a result of multilevel optimization involving
the dot, well layers above and below the dot for achieving the desired wavelength response and higher absorption, and
the thickness of the barriers for reduction in dark current. Detector structures grown using solid source molecular beam
epitaxy (MBE) were processed and characterized. The reduction in total strain has enabled the growth of higher number
of active region layers resulting in enhanced absorption of light. The detector shows dual color response with peaks in
the mid-wave infrared (MWIR) and the long-wave infrared (LWIR) region. A peak detectivity of 6.7×1010 cm.√
Hz/W
was observed at 8.7μm. The detector shows promise in raising the operating temperature of DWELL detectors, thereby
enabling cheaper operation.
This paper discusses recent and future advancements in the field of quantum dots-in-a-well (DWELL) focal plane arrays (FPAs). Additionally, for clarity sake, the fundamentals of FPA figures of merit are reviewed. The DWELL detector represents a hybrid between a conventional quantum well photodetector (QWIP) and a quantum dot (QD) photodetector (QDIP). This hybridization, where the active region consists of QDs embedded in a quantum well (QW), grants DWELLs many of the advantages of its components. This includes normally incident photon sensitivity without gratings or optocoupers, like QDIPs, and reproducible control over operating wavelength through 'dial-in recipes' as seen in QWIPs. Conclusions, drawn by the long carrier lifetimes observed in DWELL heterostructures using femtosecond spectroscopy, have recently backed up by reports of high temperature operation results for DWELL FPAs. This paper will conclude with a preview of some upcoming advances in the field of DWELL focal plane arrays.
We report Quantum Dot Infrared Detectors (QDIP) where light coupling to the self assembled quantum dots
is achieved through plasmons occurring at the metal-semiconductor interface. The detector structure consists
of an asymmetric InAs/InGaAs/GaAs dots-in-a-well (DWELL) structure and a thick layer of GaAs sandwiched
between two highly doped n-GaAs contact layers, grown on a semi-insulating GaAs substrate. The aperture of
the detector is covered with a thin metallic layer which along with the dielectric layer confines light in the vertical
direction. Sub-wavelength two-dimensional periodic patterns etched in the metallic layer covering the aperture
of the detector and the active region creates a micro-cavity that concentrate light in the active region leading
to intersubband transitions between states in the dot and the ones in the well. The sidewalls of the detector
were also covered with metal to ensure that there is no leakage of light into the active region other than through
the metal covered aperture. An enhanced spectral response when compared to the normal DWELL detector
is obtained despite the absence of any aperture in the detector. The spectral response measurements show
that the Long Wave InfraRed (LWIR) region is enhanced when compared to the Mid Wave InfraRed (MWIR)
region. This may be due to coupling of light into the active region by plasmons that are excited at the metal-semiconductor
interface. The patterned metal-dielectric layers act as an optical resonator thereby enhancing the
coupling efficiency of light into the active region at the specified frequency. The concept of plasmon-assisted
coupling is in principle technology agnostic and can be easily integrated into present day infrared sensors.
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