We are in the midst of the second quantum revolution. Research institutes and companies worldwide are working toward harnessing the power of quantum physics for technological applications. Gapless surface states on topological insulators are protected from elastic scattering on nonmagnetic impurities, which makes them promising candidates for low-power electronic applications. Conventional III–V infrared (IR) materials have the flexibility to engineer topologically protected surface states that can be resistant to ambient environments. In particular, largely hybridized band structures provide thermodynamically stable edge currents at the higher operating temperatures, which are important for IR sensing applications. Hence, we focused on optimizing two critical components for establishing ambient topological insulator; one for enlarging the hybridization gap, Δ, and the other for reducing bulk conduction in InAsSb/InGaSb structures. We performed a modelling study, and achieved an approximately 79 meV from InAs/InGaSb superlattices (SLs) lattice matched to AlSb, which is one of the largest reported value by far. Based on this modeling study, we selected a baseline SL design of InAsSb/GaSb on GaSb with Δ of ~62 meV to address key material issues such as finite bulk carrier conduction in undoped region of SLs. Systematic growth/processing optimization was performed in order to reduce the bulk charge carriers. The origin of constrained carrier dynamics in largely hybridized SL system and their effects on the designed topological structure were discussed.
We discuss control of thermally-induced focal shifts via engineering the metalens construction and show that metalenses offer additional degree of freedom in controlling the thermal stability of optical systems, compared to standard refractive and diffractive lenses.
We experimentally analyze the simultaneous processes of difference frequency generation (DFG) and second harmonic (SH) generation in multilayer structures exhibiting an effective epsilon-near-zero (ENZ) response and ENZ properties in one of their layers. The structures consist of subwavelength-thin tri-layer periods of 75% ITO, 12.5% Al2O3, and 12.5% BaTiO3, with the total thickness kept near 120 nm. The number of periods and ITO layer thickness (3-30 nm) vary between samples, allowing to tune the effective ENZ wavelength over 1000 nm. We demonstrate that the level of DFG and SH enhancement can be increased by over two orders of magnitude with multilayer composition, with the highest enhancement in samples having 12-15 nm thick ITO layers. The peak enhancement wavelength follows the effective ENZ wavelength, while the relative enhancement levels of DFG and SH depend on sample composition. Our findings are supported by COMSOL simulations, TEM analysis, and ellipsometry data.
Metasurfaces offer flexibility for expanding functionality and reducing the size of optical systems by providing optical functionality from a flat surface. Previous work has demonstrated a rapid fabrication and testing process for wafers containing multiple 1-centimeter diameter metalenses that can be applied towards mass manufacturing. However, quality feedback was limited to analyzing imaging performance parameters such as the modulation transfer function and focal length. These techniques do not give direct feedback about specific manufacturing errors. Currently, getting this feedback still requires expensive, time-intensive processes such as scanning electron microscope (SEM) measurements or local area interferometry, which tend to have a small field of view. Theoretical investigation suggests that phase errors in the metasurface phase profile result in a shift in diffraction efficiency away from the first order and into the other diffraction orders, zero order, second, third, etc. We exploit this concept to comprehensively characterize metalens performance, including the analysis of standard image quality parameters and extending the study to multiple diffraction orders. An extensive set of measurements of the relative efficiency of the diffraction orders is presented for a set of fabricated metalenses alongside SEM measurements to cross-validate the presence of manufacturing defects. This will establish the extent to which current conventional CMOS processing and manufacturing techniques can be applied to metasurface optics by indicating uniformity and yield characteristics across positions and wafers.
One of the most promising candidates to use as compact high sensitivity magnetometers is the Nitrogen-Vacancy (NV) center, however traditional implementations of this technology are plagued by low collection efficiencies or poor signal contrasts of the Optically Detected Magnetic Resonance (ODMR). Laser Threshold Magnetometry (LTM) offers a path towards both efficient signal collection and high signal contrasts by taking advantage of near threshold laser dynamics. We demonstrate an infrared LTM using a Vertical External Cavity Surface Emitting Laser (VECSEL) with an intra cavity diamond plate doped with NV centers. The VECSEL was tuned to the spin dependent absorption line of the NV centers, which tied the VECSEL output power to the magnetic field sensed by the NV centers. Furthermore, the contrast and the projected sensitivity limit are shown to improve when operating close to the lasing threshold. We measure a sensitivity of 7.5 nT/√ Hz between 10-50 Hz with a contrast of 18.4% and a projected Photon Shot Noise Limited (PSNL) sensitivity of 26.6 pT/√ Hz close to threshold. We also observe a saturable absorption-like effect near threshold, which further enhances the signal contrast and projected PSNL near threshold. A rate equation model for the VECSEL threshold magnetometer is described and is fit to mimic the observed threshold dynamics.
To test whether conventional infrared materials can be used to control the electronic wavefunction to form a topological state, a 6.2 Å metamorphic (InAs/InGaSb/InAs) quantum well (QW) absorber with ~60 meV of hybridization gap (Δ) was investigated. We developed a thick metamorphic InGaSb buffer layer on GaAs wafer to create a 6.2 Å lattice constant for the QW growths. The lattice constant of virtual substrates (VSs) was very close to the target value of 6.2 Å, however the resulting crystalline quality of the VSs was inefficient for topological insulator. The cross-sectional transmission electron microscopy image revealed that the dislocation density in the InGaSb buffer layer was high closer to the GaAs substrate and gradually reduced upon continued growth. However some mismatch-related defects were propagated into the absorber region, consequently degraded the transport quality of absorber. The QW absorber grown on VS had a low mobility. The mobility was dramatically improved by selecting pseudomorphic QW or superlattice absorber with a small Δ that was grown on a lattice-matched GaSb substrate. Hence, in order for the proposed 6.2 Å materials to be viable for sensing applications, a critical effort will be the development of better optimized metamorphic buffers for the design or of highlyhybridized psedomorphic designs that can be grown on lattice-matched substrates.
We investigate the effect of fabrication tolerances on photonic multimode waveguides operating in the vicinity of a third-order exceptional point degeneracy (EPD), known as a stationary inflection point (SIP). An EPD is a point in the parameter space where two or more Bloch eigenmodes coalesce in an infinite periodic waveguide, and at an SIP three modes coalesce to form the frozen mode. Waveguides operating near an SIP exhibit slow-light behavior in finite-length waveguides with anomalous cubic scaling of the group delay with waveguide length. The frozen mode facilitates stronger light-matter interactions in active media, resulting in a significant increase in the effective gain within the cavity. However, systems operating near an EPD are also exceptionally sensitive to fabrication deviations. In this work, we explore wave propagation and the impact of various fabrication imperfections in analytic models and in fabricated photonic chips for three mirrorless devices operating near an SIP. To advance the concept of the SIP laser, we also analyze how the addition of gain and loss affects the SIP performance. Our results show that while minor deviations from the ideal parameters can prevent perfect mode coalescence at the EPD, the frozen mode remains resilient to small perturbations and a significant degree of mode degeneracy prevails. These findings provide critical insights into the design and fabrication of passive and active photonic devices operating near high-order EPDs, paving the way for their practical implementation in a wide range of applications.
A single transverse mode high pulse-energy GaSb VECSEL emitting at 2030 nm was studied. The peak power exceeds 500 W while maintaining good beam quality throughout the operation range. The cavity employs a Pockels cell combined with a low-loss thin film polarizer to selectively dump the intracavity energy in a 10 ns pulse. Thermal mitigation of the gain chip is achieved by both gain-switching and utilizing a long wavelength pump laser at 1470 nm compared to the traditional 980 nm pump for GaSb VECSELs. The laser has promise for incoherent LIDAR, materials processing, gas sensing, and nonlinear optics.
We report on the simulation of cavity-dumped semiconductor disk lasers utilizing an intracavity Pockels cell. This technique is used to generate high peak power pulses with pulse lengths of nominally one cavity round-trip. These results are compared to experiments demonstrated using InGaAs quantum-well gain region operating at approximately 1 μm to generate micro-Joule level nanosecond pulses.
The modal characteristics of nonresonant five-element phase-locked arrays of 4.7-μm emitting quantum cascade lasers (QCLs) have been studied using spectrally resolved near- and far-field measurements and correlated with results of device simulation. Devices are fabricated by a two-step metal-organic chemical vapor deposition process and operate predominantly in an in-phase array mode near threshold, although become multimode at higher drive levels. The wide spectral bandwidth of the QCL’s core region is found to be a factor in promoting multispatial-mode operation at high drive levels above threshold. An optimized resonant-array design is identified to allow sole in-phase array-mode operation to high drive levels above threshold, and indicates that for phase-locked laser arrays full spatial coherence to high output powers does not require full temporal coherence.
To date high power, high energy pulses in the few ns rage have been unobtainable in semiconductor lasers due to the short carrier lifetime and long cavity buildup times. In this paper we show a wavelength and pulse-width tunable semiconductor laser that is able to achieve pulses in the range of a few ns at power levels above 1 kW leading to several μJ pulse energies. This was done by inserting a polarizing beam splitter (PBS) and a λ/4 Pockels Cell (PC) into the cavity of a vertical external cavity surface emitting laser (VECSEL) allowing access to the high energy stored in the VECSEL cavity. The PC is used to electronically control the cavity polarization and with proper tailoring, all the photons built up within the cavity may be completely dumped within a single photon round trip. After this the PC is switched off and the light in the cavity is allowed to build up once again. Once the light has built back up, the VECSEL is ready to be dumped again. This has been demonstrated in both single gain chip and dual gain chip setups. We record a maximum pulse energy of 7.78 μJ and peak power of 1.7 kW at a wavelength of 1019 nm with a tunability of 16 nm.
Due to its unique zero-bandgap structure, linear disperion of electrons and compatibility with various optoelec- tronic platforms, graphene has become one of the principal materials of interest with strong development for many nonlinear optical devices. Functionalized graphene-composites exhibit excellent optical limiting properties while single layer graphene speci cally, has shown great promise as a saturable absorber in mode locking ber and solid state lasers from the visible to infrared regime. However, more recently work has been done to in- tegrate graphene in a vertical-external-cavity-surface-emitting-lasers (VECSELs). Currently VECSELs employ semiconductor-based saturable absorbers which have a narrow tuning range and require complex fabrication procedures. By developing a graphene-based saturable absorber, one can take advantage of its zero bandgap structure and therefore its frequency-independent absorption as well as its thermal and mechanical stability to passively modelocking lasers over a wide frequency range { potentially through the terahertz regime. Here we report on recent developments in graphene mode-locking of VECSELs, speci cally for high power operation. Further, work done in studying the nonlinear optical properties of graphene pertaining to the development of sat- urable absorbers as well as optical limiters will be presented. Finally preliminary fabrication and characterization work conducted to integrate the graphene-based materials in a VECSEL will be presented.
Orientation-patterned GaP is a promising nonlinear material for frequency conversion in the mid and longwave IR (2-5
μm and 8-12 μm) by quasi-phase matching. As an alternative to OPGaAs, OPGaP has the advantage of having lower
two-photon absorption in the convenient pumping range 1 – 1.7 μm. We report recent results on development of thick QPM GaP for high power tunable laser sources radiating in the mid IR. Two are the major challenges to producing OPGaP: development of simple techniques for preparation of patterned templates and a technology for fast epitaxial growth of thick, high quality GaP on these templates. The focus was to adapt/simplify the wafer fusion process for OPGaP template preparation. Then increase the growth rate and layer thickness of regrown material, while maintaining vertical domain propagation. The growth experiments were conducted in a horizontal quartz reactor, using a standard hydride vapor phase epitaxial process. The growth was performed on: (i) plain (100) GaP; (ii) half-patterned (HP) and (iii) orientation-patterned (OP) templates, fabricated on (100) GaP. Up to 370 μm thick layers with high crystal and optical quality were reproducibly grown on plain material. Growth on HP templates resulted in up to 470 μm thick layers with rectangular mesa‟s shape. These results were used to determine the optimal substrate and pattern orientations. HVPE growth on OP templates achieved stable growth rates of 50-70 μm/h with domain walls propagating vertically, following the periodicity of the initial pattern, and resulted in the first 350 μm thick device quality OPGaP.
The antimonide based vertical external cavity surface emitting lasers (VECSELs) operating in the 1.8 to 2.8 Tm wavelength range are typically based on InGaAsSb/AlGaAsSb quantum wells on AlAsSb/GaSb distributed Bragg reflectors (DBRs) grown lattice-matched on GaSb substrates. The ability to grow such antimonide VECSEL structures on GaAs substrates can take advantage of the superior AlAs based etch-stop layers and mature DBR technology based on GaAs substrates. The growth of such III-Sb VECSELs on GaAs substrates is non-trivial due to the 7.78% lattice mismatch between the antimonide based active region and the GaAs/AlGaAs DBR. The challenge is therefore to reduce the threading dislocation density in the active region without a very thick metamorphic buffer and this is achieved by inducing 90 ° interfacial mist dislocation arrays between the GaSb and GaAs layers. In this presentation we make use of cross section transmission electron microscopy to analyze a variety of approaches to designing and growing III-Sb VECSELs on GaAs substrates to achieve a low threading dislocation density. We shall demonstrate the failure mechanisms in such growths and we analyze the extent to which the threading dislocations are able to permeate a thick active region. Finally, we present growth strategies and supporting results showing low-defect density III-Sb VECSEL active regions on GaAs.
Vertical external cavity surface emitting lasers (VECSELs) have proven themselves to be a suitable semiconductor
answer to many solid-state lasers. Their simplicity makes them a very versatile platform for accessing wavelengths
from the UV through the THz through direct and frequency-converted emission. This wavelength flexibility,
combined with an optical cavity accommodating additional tuning or nonlinear elements, make the VECSEL a
uniquely suited solution to a variety of applications. We will present recent AFRL progress in VECSELs and
potential applications for these lasers.
We present an overview of the quantum design, growth and lasing operation of both IR and mid-IR OPSL
structures aimed at extracting multi-Watt powers CW and multi-kW peak power pulsed. Issues related to
power scaling are identified and discussed. The IR OPSLs based on InGaAs QW bottom emitters targeted at
wavelengths between 1015nm and 1040nm are operated in CW mode (yielding a maximum power of 64W)
and pulsed (peak power of 245W). The mid-IR top emitter OPSLs designed to lase at 2μm are based on a
novel lattice mismatched growth using InGaSb QWs and yield a maximum peak power of 350W pulsed.
Vertical external cavity surface emitting lasers (VECSELs) are attractive for many applications due to their high-power,
high-brightness outputs. In order to power scale the devices, the pump spot size should be increased. However, the large
pump area greatly amplifies the guided spontaneous emission in the epitaxial plane. In order to efficiently power scale
the devices, amplified spontaneous emission (ASE) and lateral lasing must be reduced. We begin by first reporting on
the temperature dependence of the phenomena. Particularly, since the quantum well gain and bandgap are functions of
temperature, ASE and lateral lasing are greatly dependent on the operating temperature as well as the pump power. The
easiest method of quantifying the affect of ASE and lateral lasing is by removing the Fabry-Perot cavity formed by the
chip edges. We have chosen two different methods: Reducing the Fresnel reflections by patterning the edges of the
sample, and depositing a layer of Ge on the edges of the VECSEL chip as the high index of refraction for Ge should
reduce the Fresnel reflections and the absorption properties in the NIR regime should also act to prevent feedback into
the pump area. Our research shows both of these methods have increased the performance and visibly decreased the
amount of lateral lasing seen in the devices.
Vertical external cavity surface emitting lasers (VECSELs) provide a laser design platform in order to explore
a variety of systems, and their flexibility eases this exploration. Moreover, their high-brightness operation
makes them attractive for many applications. In considering the methods of coupling VECSELs as well as
their potential uses, we begin by reporting on the development of a gain coupled VECSEL for use in optical
switching. In particular, two VECSEL cavities share a common gain region; the competition for a common set
of carriers dictate how these cavities interact. The easiest manifestation to realize gain coupling is to utilize
a linear cavity as well as a v-cavity, built around a single half-vertical cavity surface-emitting laser (VCSEL)
chip. The cavity gain/loss of each cavity can be controlled independently through use of birefringent filters,
allowing us to explore the design space, which can be divided up into coarse behavior, easy to analyze through
comparing the two uncoupled lasers, and a fine behavior, where one cavity will affect the other and each cavity
can lase simultaneously, sometimes at dramatically different wavelengths. These two regions may be explained
with simple rate equations, and it will be shown that if prepared properly, spontaneous emission plays a large
role in balancing the two laser cavities within the fine regime, while may be completely neglected in the coarse
regime.
Vertical external cavity surface emitting lasers (VECSELs) have captured the interest of high-brightness semiconductor
researchers, primarily due to their simplicity in design, power scalability, and "open cavity architecture,"
wherein it is simple to integrate nonlinear elements into the cavity. Through direct emission and indirect
(frequency-converted) means, wavelengths from the UV through to the mid-wave infrared regimes have been
demonstrated, increasing the suitability of the VECSEL platform for multiple applications. This presentation
outlines recent progress in VECSELs, measurements, novel cavities, and potential applications for these lasers.
The small signal modulation of a vertical external cavity surface emitting laser (VECSEL) is examined. The
modulation transfer function (MTF) of the cavity is measured for multiple photon lifetimes operating between
Class A and Class B regimes, where the photon and carrier lifetimes are of the same order. Three coupled ordinary
differential equations with similarities to an electrically-injected quantum-well laser with a separate confinement
heterostructure are used to mathematically describe the time-dependant VECSEL response. We present a series
of measurements that provide important laser parameters such as internal device losses and differential gain.
The VECSEL operating in this regime is an overdamped oscillator and has free-running characteristics that are
not unlike quantum-dot and quantum-cascade lasers.
We report the influence of bonding temperature on SU-8 to SU-8 bonding and report fabrication of a hybrid microelectromechanical-tunable filter (MEM-TF) using SU-8 bond pads. We demonstrate use of 2-µm-thick 50×50-µm2 SU-8 bond pads to attach 4.92-µm-thick 250×250-µm2 Al0.4Ga0.6As-GaAs distributed Bragg reflectors (DBR) to polysilicon MUMPs® piston actuators. Advantages of this process include compatibility with hydrofluoric-acid-release chemistry, low-temperature/low-pressure bonding, simple bond-pad photolithography, 57% flip-bonded DBR yield, and 30% electrostatically actuatable hybrid MEM-TF yield.
Vertical external cavity surface emitting lasers (VECSELs) have been considered the “ultimate disk-laser” due
to their extremely thin active regions and because they take advantage of the high gain found in semiconductor
material. This paper discusses power scaling limitations, including heating effects, surface roughness losses, and
laterally guided amplified spontaneous emission (ASE).
Tapered unstable resonator lasers incorporating laterally finite mirrors are reported. By reducing the lateral extent of mirrors, cavity losses may be shifted from internal losses (contributing to scattering and absorption) to mirror losses (contributing to output power). Design of these cavities will be discussed and evaluated. Experimental data substantiates theoretical analysis, showing an increase in slope efficiency of 40% over conventional infinite aperture tapered lasers.
We present the design and fabrication of miniaturized light sources for micro-optical systems using organic light emitting diodes (OLEDs). These devices can be integrated on a micro-optical table (MOT) using various backplanes. Acceptable angular uniformity of emitted radiance, and a brightness of more than 30,000 cd/m2 can be readily achieved with OLEDs having areas ranging from 0.0004 cm2 to 0.0363 cm2.
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