We compare an InAs quantum dot (QD) vertical external-cavity surface-emitting laser (VECSEL) design consisting of 4
groups of 3 closely spaced QD layers with a resonant periodic gain (RPG) structure, where each of the 12 QD layers is
placed at a separate field antinode. This increased the spacing between the QDs, reducing strain and greatly improving
device performance. For thermal management, the GaAs substrate was thinned and indium bonded to CVD diamond. A
fiber-coupled 808 nm diode laser was used as pump source, a 1% transmission output coupler completed the cavity. CW
output powers over 4.5 W at 1250 nm were achieved.
In this paper, we demonstrate a high operating temperature (HOT) quantum dot-in-a-well (DWELL) infrared
photodetector with enhanced normal incidence (s-polarization) radiation photocurrent. The s-to-p polarization ratio
was increased to 50%, compared to the 20% in conventional quantum dot detectors. This improvement was achieved
through engineering the dot geometry and the quantum confinement via post growth capping materials of the
quantum dots (QDs). The effect of the capping procedures was determined by examining the dot geometry using
transmission electron microscopy (TEM) and s-to-p polarization induced photocurrent in the DWELL structure
photodetector. The TEM image shows a quantum dot with a reduced base of 12 nm and an increased height of 8 nm.
The infrared photodetectors fabricated from this material shows a peak photodetectivity of 1×109 cmHz1/2/W at 77K
for a peak wavelength of 4.8 μm and 1×107 cmHz1/2/W at 300K for a peak wavelength of 3.2 μm. The dark current
density is as low as 2×10-4A/cm2 and the photocurrent gain is 100 at the optimal operating bias.
In this paper, we report some of our recent results on improving the operating temperature of dots-in-a-well
(DWELL) infrared photodetectors. This was achieved by engineering the dot geometry and the interrelated quantum
confinement by varying the growth conditions and composition of the subsequent capping of the quantum dots
(QDs). The influence of these conditions was determined by examining the optical properties of the QDs directly
and indirectly with their function in a DWELL IR photodetector. Spectral response was observed until 250K with
spectral response peak at 3.2μm, and the peak detectivity is 1×109 cmHz1/2/W at 77K and ~ 1e8 cmHz1/2/W at 250K.
By varying the external bias, the DWELL heterostructure allows for the manipulation of the operating wavelength.
This tunability is a critical stepping stone towards creating multicolor imaging systems that can be used to take
images at multiple wavelengths from each pixel in a focal plane array.
Investigation of pulse shortening by passive negative feedback in mode locked train from 2.4 at. % crystalline
Czochralski grown Nd:YAG in a bounce geometry under QCW diode pumping is reported. For passive mode locking a
semiconductor saturable absorber with 33 quantum wells grown on GaAa substrate acting also as nonlinear element for
passive negative feedback by beam defocusing was used. Temporal diagnostics was performed with high speed digital
oscilloscope with bandwidth of 9 GHz combined with fast photodiode which enabled direct observation of pulse
shortening along extended pulse trains from single laser shot. Efficient pulse shortening from 120 ps in the beginning of
the train to 35 ps for pulses at the end of the extended train containing more than 100 pulses was achieved.
Laser cooling of a semiconductor has been an elusive but highly desirable goal for several years. Although it is
theoretically possible, tedious and often time-consuming sample preparation, processing and testing has slowed
the progress on the experimental end. The work presented here focuses on a new approach to the first step, the
growth of high quality starting samples by molecular beam epitaxy (MBE).
MBE is believed to have an inherent advantage over chemical vapor deposition techniques since typically
material with higher purity can be grown by MBE, thereby reducing the chance for parasitic absorption and nonradiative
recombinations to occur. Additionally, with MBE very precise control over interfaces is possible,
where a significant portion of the non-radiative traps are usually located. The most promising material for laser
cooling is the binary compound GaAs. The lattice-matched material Ga0.515In0.485P is chosen for passivating the
surface as it has shown much longer radiative lifetimes in GaAs than, for example, AlxGa1-xAs. The present
study focuses on growth optimization of Ga0.515In0.485P/GaAs/Ga0.515In0.485P heterostructures and the influence of
growth conditions on sample suitability for laser cooling as measured by non-radiative lifetimes in GaAs. In
particular, parameters such as growth temperature, group V:III overpressure, substrate orientation, doping, and
interface composition on a monolayer length scale are varied and analyzed. The suitability of an optimized
sample for semiconductor laser cooling is discussed.
The operation of pulsed diode pumped Nd:GdVO4 and Nd:YVO4 slab lasers in a bounce geometry in a free running
and a passively mode-locked regime using a semiconductor saturable absorber was demonstrated. Higher efficiency of
Nd:GdVO4 in both regimes was achieved. In the free running regime the optical to optical efficiency of 28.3% was
obtained with Nd:YVO4 and 39.3% with Nd:GdVO4 crystal in spatial mode close to TEM00. In the passively mode
locked regime the Q-switched and mode locked single trains containing 5 pulses were generated for a pump energy of 11
mJ from Nd:YVO4 while the pump energy for Nd:GdVO4 was only 5.5 mJ. The pulse duration was 48 ps and 65 ps
respectively. Our results clearly demonstrate the advantage of using Nd:GdVO4 in a pulsed free running and also a
passively mode locked diode pumped regime in a bounce geometry.
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
reflection.
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.
In our research group, we develop novel dots-in-a-well (DWELL) photodetectors that are a hybrid of the quantum dot
infrared photodetector (QDIP). The DWELL detector consists of an active region composed of InAs quantum dots
embedded in InGaAs quantum wells. By adjusting the InGaAs well thickness, our structure allows for the manipulation
of the operating wavelength and the nature of the transitions (bound-to-bound, bound-to-quasibound and bound-to-continuum)
of the detector. Based on these principles, DWELL samples were grown using molecular beam epitaxy and
fabricated into 320 x 256 focal plane arrays (FPAs) with Indium bumps using standard lithography at the University of
New Mexico. The FPA evaluated was hybridized to an Indigo 9705 readout integrated circuit (ROIC) in collaboration
with QmagiQ LLC and tested with a CamIRaTM system manufactured by SE-IR Corp. From this evaluation, we report
the first two-color, co-located quantum dot based imaging system that can be used to take multicolor images using a
single FPA. We demonstrated that we can operate the device at an intermediate bias (Vb=-1.25 V) and obtain two color
response from the FPA at 77K. Using filter lenses, both MWIR and LWIR responses were obtained from the array at the
same bias voltage. The MWIR and LWIR responses are thought to be from bound states in the dot to higher and lower
lying states in the quantum well respectively. Temporal NEDT for the DWELL FPA was measured to be 80mK at 77K.
Operation of Nd:YAG triangle slab laser side pumped by 600 W quasiicontinuous laser diode passively mode locked using semiconductor saturable absorber is reported. Pulse trains with energy up to 2 mJ and pulse duration of 65 ps were generated and pulse shortening by passive negative feedback introduced due to the beam defocusing in GaAs saturable absorber substrate was measured.
In this work, the optical characteristics of monolithic passively mode-locked lasers (MLLs) fabricated from 1.24-&mgr;m
InAs dots-in-a-Well (DWELL), 1.25-&mgr;m InGaAs single quantum well (SQW), and 1.55-&mgr;m GaInNAsSb SQW
structures grown using elemental source molecular beam epitaxy (MBE) are reported. 5 GHz optical pulses with sub-picosecond
RMS jitter, high pulse peak power (1W) and narrow pulse width (< 10 ps) were demonstrated in monolithic
two-section InAs DWELL passive MLLs. With the 42% indium InGaAs SQW MLL, a record high-temperature
performance for a monolithic passively mode-locked semiconductor laser is found. Compared with the typical operating
range of the InAs DWELL devices (<60°C), the operation is in excess of 100 °C. The first 1.55-&mgr;m GaInNAsSb SQW
MLL operates at a repetition rate of 5.8 GHz and has a 3-dB bandwidth of 170 kHz in the RF spectrum indicating
respectable jitter.
Doping of the clad layers in thin GaAs/GaInP heterostructures, displaces the band energy discontinuity, modifies
the carrier concentration in the active GaAs region and changes the quality of the hetero-interfaces. As a result,
internal and consequently external quantum efficiencies in the double heterostructure are affected. In this paper,
the interfacial quality of GaAs/GaInP heterostructure is systematically investigated by adjusting the doping
level and type (n or p) of the cladding layer. An optimum structure for laser cooling applications is proposed.
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.
All solid state mode-locked flashlamp pumped Nd:YAG laser system with selectable pulse duration was developed based on the oscillator where a single semiconductor structure containing a multiple-quantum-well was used as a saturable absorber for mode-locking, and energy limiter for passive negative feedback. Single pulse selection from various parts of extended 200 ns long Q-switched pulse train enables the changing of pulse duration before entering into three stages of laser amplifiers. Using of additional acousto-optic mode-locker, stability enhancement of the output pulses was obtained and the amplitude fluctuations were reduced below 5%. The exploitation of the solid state saturable absorber and limiter integrated in the single element improved significantly the long term characteristics of the laser system which can be therefore used for various applications as a satellite laser ranging, spectroscopy, or medicine.
Various solid state lasers such as Cr:LISAF, Yb:YAG, Nd:Vanadate, Ti:sapphire and Nd:YAG have in common a long lifetime of the laser level, which results in a tendency to Q-switching rather than pure mode-locking. These lasers are being used in a linear or ring cavity for intracavity sensing applications (displacements, rotation, electric and magnetic fields), and for applications in spectroscopy. The requirements for these applications are that the pulses be centered at a specific wavelength, and be of a specific pulse duration. Multiple Quantum Wells (MQW) typically used for ultrashort pulse generation have often a high defect concentration which causes losses incompatible with the large number of intracavity elements required by the applications. We have established for all these lasers a composition curve for the MQW, that enables one to tune to a specific wavelength. These saturable absorbers have excellent optical quality both in reflection and transmission.
The approach to prevent Q-switching has generally been to use very low loss modulation (single quantum wells). With a large number of intracavity elements, a larger loss modulation is desirable, hence the use of multiple QW (4 to 100). We have successfully demonstrated stable continuous model-locked operation by using passive energy limiters in the cavity. Two-photon generated carriers induce lensing in the cavity, resulting in power dependent losses through an aperture in the cavity. We show that the attenuation is proportional to the square of pulse intensity, resulting in a steep energy limiter. We demonstrate theoretically and experimentally that the presence of two intracavity pulses required for sensor applications can be satisfied with multiple quantum wells appropriately positioned in the cavity. Examples of applications include rotation sensors (ring cavity) or acceleration sensors (linear cavity), magnetic filed sensor, displacement sensors.
Results of experimental investigation of diode pumped Nd:YAG laser passively mode locked using either second harmonic nonlinear mirror or semiconductor saturable absorber are reported. As an active element the 10 mm long Nd:YAG rod end pumped by 20 W fiber coupled laser diode was used. The linear folded resonator has on the other end either saturable absorber with single 15 nm thick In 0.25 Ga 0.75 As quantum well layer integrated on the top of Bragg mirror (SESAM) either nonlinear mirror (NLM) consisting of a dichroic dielectric mirror and crystal for second harmonic generation (SHG). With SESAM the 2.5 W of average output power and pulse duration of 21 ps was achieved, using the 3.5 mm long type II KTP crystal we obtained 1.5 W of output power with single pulse duration of 26 ps. Substantial pulse shortening to 9 ps was achieved with a 10 mm long critically phase-matched type I LBO crystal.
Using of single semiconductor element containing multiple quantum well saturable absorber both for mode-locking and as passive negative feedback element in the resonator of flashlamp pumped Nd:YAG laser is reported. In the Q-switched and mode locked regime of generation, 40 ns long trains containing 5 pulses (at FWHM) were generated; the energy of the whole train was 1.5 mJ, single pulse duration was 50 ps. In the regime of passive negative feedback the pulse trains were stretched to 160-280 ns, and effective pulse shortening along the train from 50 ps to 25 ps was observed.
Flashlamp pumped oscillator - three amplifiers Nd:YAG picosecond laser system mode-locked with multiple quantum well (MQW) saturable absorber was developed and investigated. 80 ps long pulses with the energy of 120 mJ were generated.
A three color normal incidence quantum dots in a well (DWELL) operating in the mid-wave infrared (MWIR), long wave infrared (LWIR) and very long wave infrared (VLWIR) are reported. The peak operating wavelengths are at ~ 6 mm, ~10.5 mm and ~ 23.2 mm. We believe that the shorter wavelength response (6 mm and 10.5 mm) are due to bound-to-continuum and bound-to-bound transitions between the states in the dot and states in the well, whereas the longer wavelength response (23.2 mm) is due to intersubband transition between dot levels. A bias dependent activation energy ~ 100 meV was extracted from the Arrhenious plots of the dark currents, which is a factor of three larger than that observed in quantum well infrared photodetectors operating at comparable wavelengths.
Passively mode locked ring vanadate laser pumped by 1 W laser diode was developed. Laser threshold for free running regime was 55 mW, the bidirectional mode locked operation was obtained for incident pump power of only 630 mW. The repetition rate of the laser was 226 MHz, pulse duration 53 ps. New saturable absorber and configuration with two lenses inside the resonator improved stability of mode locking regime.
We report on flashlamp pumped oscillator - three amplifiers Nd:YAG picosecond laser system in which the liquid saturable dye used for passive mode locking is replaced by semiconductor saturable absorber with multiple quantum well (MQW) structure. This element placed at Brewster angle inside a laser resonator had 100 layers of absorber and therefore it has high nonlinearity and is suitable for high power Q-switched and mode locked operation. The short pulse train from oscillator contained only 5-6 pulses with total energy of 3 mJ in single transversal mode, the pulse duration was 80 ps. After amplification, the maximum energy of the pulse train was 180 mJ. In the regime of the amplification of a single selected pulse the energy on the output of the third amplifier was 50 mJ. Operation of the oscillator in active-passive regime of mode locking using an additional acousto-optic mode-locker leads to improvement of reproducibility and stability of output parameters.
Using the segmented contact method, we have measured the passive modal absorption, modal gain and spontaneous emission spectra of an InAs “dot-in-well” (DWELL) system where the inhomogeneous broadening is sufficiently small that the ground and excited state transitions can be spectrally resolved. The modal optical gain from the ground state saturates with current at a maximum value of one third of the magnitude of the measured absorption. The population inversion factor spectrum, obtained from the measured gain and emission spectra, shows that the carrier distributions cannot be described by a single global Fermi distribution. However, the inversion factor spectrum can be described by a system where the ground state and excited state occupancies are each described by a Fermi distribution but with different quasi-Fermi energy separations.
Photoluminescence spectra are investigated of InAs/InGaAs QD structures prepared be MBE on GaAs substrates in a range of pumping power density up to 0.6 kW/cm2. Multiple spectra band are observed corresponding to electron shells in atom-like dots. Identification of shells is proposed on the basis of spherical oscillator model. Energy diagram of dots is proposed taking into account identical temperature dependence of PL intensity in three lowest spectral bands.
Operation of laser diode and flash lamp pumped Nd:YAG lasers mode locked with two different types of semiconductor saturable absorbers is reported. In the first type that is used mainly in diode pumped systems the absorber layers are integrated on highly reflective Bragg mirror. The second type is for use in transmission mode inside the resonator. Different design of semiconductor elements, pumping geometries and resonator configurations were investigated and characteristics of laser operation in mode-locked regime are presented.
A theoretical model for the dependence on temperature of the carrier behavior in a semiconductor structure containing InAs quantum dots grown inside a Ga0.85In0.15As quantum well is presented. The conditions, that have to be imposed in order to obtain analytical solutions with obvious physical interpretation are kept to minimum. Two temperature domains are approached in this model. In the low temperature case the equation system that describes the carrier behavior can be reduced to a cubic equation. One of the solutions of the equation represents the quantum dot photoluminescence yield. Also, a solution is obtained for the dot emission yield in the high temperature domain, where the carrier thermal escape from dots cannot be neglected. The solution depends, on the probabilities for electron and hole capture and reemission, and on the number of dot states occupied by electrons and holes. Temperature dependent measurements of the quantum dot photoluminescence are performed and the results are fit with the theoretical model.
Optical characteristics are investigated and compared of nanostructure semiconductor lasers with quantum dots and quantum dashes. Spectra of optical gain and of linewidth enhancement factor are obtained. Optical anisotropy in quantum dash structures is investigated.
We report our progress on the design and fabrication of electrostatically-actuated microelectromechanical (MEM) tunable wavelength filters and vertical cavity surface-emitting lasers (VCSELs). We investigate both an all-semiconductor monolithic approach and a hybrid approach based on the combination of conventional polysilicon microelectromechanical systems (MEMS) and III-V semiconductor thin-film distributed Bragg reflector (DBR) and VCSEL structures. In the tunable hybrid structures the III-V semiconductor layers are flip-bonded onto specially designed polysilicon foundry MEMS structures and separated from their lattice-matched parent substrates by a novel post-bonding lift-off process.
Normal incidence InAs/In0.15Ga0.85As dots-in-a-well detectors operating at T=78K with λcut-off ~8.2 μm and a spectral width (Δλ/λ) of 35% are reported. The peak at 7.2 μm is attributed to the bound-to-bound transitions between the ground state of the dot and the states within the InGaAs well. A broad shoulder around 5 μm, which is attributed to the bound to continuum transition, is also observed. Calibrated blackbody measurements at a device temperature of 78K yield a peak responsivity of 3.58 A/W (Vb=-1V), peak detectivity= 2.7x109cmHz1/2/W (Vb=-0.3V), conversion efficiency of 57% and a gain ~25.
Quantum dots laser diodes using the dots-in-a-well (DWELL) structure (InAs dots in an InGaAs quantum wells) have exhibited significant recent progress. With a single InAs dot layer in In0.15Ga0.85As quantum well, threshold current densities are as low as 26 A cm-2 at 1.25 micrometer. Quantum dot laser threshold current densities are now lower than any other reported semiconductor laser. In this work, the threshold current density is reduced to 16 A cm-2 by HR coatings on the same device. Further investigation of performance reveals that use of multiple DWELL stacks improves the modal gain and internal quantum efficiency. It is suggested that carrier heating out of the quantum dots limits the TO value of these DWELL lasers.
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