We report on our research in power scaling VECSEL around 1 μm to exceed 100W per chip. Recently, we have
utilized these optimized VECSEL chips to achieve a new record for a mode-locked VECSEL. The output power
of the laser was 3.4W. This corresponds to a pulse energy of 7.5nJ and a pulse peak power of 13.3kW. Both are
record values for a semiconductor laser in the femtosecond regime. These optimized structures have also been
used to demonstrate high power operation with a highly coherent TEM00 mode and to demonstrate a record
single frequency output power of 15W.
We demonstrate the utility of optically pumped semiconductor lasers (OPSLs) in the eld of precision atomic spectroscopy. We have constructed an OPSL for the purpose of laser-cooling and trapping neutral Hg atoms. The OPSL lases at 1015 nm and is frequency quadrupled to provide the trapping light for the ground state cooling transition. We report up to 1.5 W of stable, single-frequency output power with a linewidth of < 70 kHz with active feedback. From the OPSL we generate deep-UV light at 253.7 nm used to form a neutral Hg magneto-optical trap (MOT). We present details of the MOT. We also report initial results for spectroscopy of the 61S0 - 63P0 clock transition in the Hg199 isotope.
We report on our research in power scaling OPSL around 1 μm to exceed 100W per chip by combining a rigorous quantum design of an optimized MQW epitaxial structure, highly accurate and reproducible wafer growth and an efficient thermal management strategy. Recently we have utilized these state-of-the-art optimized OPSL chips to achieve a new record for a mode-locked OPSL with an intra-cavity SESAM. The average output power of the laser in the optimum operation point of mode-locked operation was 5.1W while being pumped with 25W of net pump power. This corresponds to a pulse energy of 3 nJ and a pulse peak power of 3.8 kW.
We investigate experimentally and theoretically the influence of non-radiative carrier losses on the performance of
VECSELs under pulsed and CW pumping conditions. These losses are detrimental to the VECSEL performance
not only because they reduce the pump-power to output-power conversion efficiency and lead to increased
thresholds, but also because they are strong sources of heat. This heating reduces the achievable output power
and eventually leads to shut-off due to thermal roll-over. We investigate the two main sources of non-radiative
losses, defect recombination and Auger losses in InGaAs-based VECSELs for the 1010nm-1040nm range as well
as for InGaSb-based devices for operation around 2μm. While defect related losses are found to be rather
insignificant in InGaAs-based devices, they can be severe enough to prevent CW operation for the InGaSb-based
structures. Auger losses are shown to be very significant for both wavelengths regimes and it is discussed how
structural modifications can suppress them. For pulsed operation record output powers are demonstrated and
the influence of the pulse duration and shape is studied.
KEYWORDS: Quantum wells, Absorption, Diamond, Reflectivity, Chromium, Temperature metrology, High power lasers, Semiconducting wafers, Finite element methods, Gallium arsenide
Strategies for power scaling VECSELs, including improving thermal management, increasing the quantum well
gain/micro-cavity detuning that increases the threshold but increases roll-over temperature, and double-passing the
excess pump via reflection from a metalized reflector at the back of a transparent distributed Bragg reflector (DBR) were
studied. The influence of the heat spreader thickness and the pump profile on the temperature rise inside the active
region was investigated using commercial finite element analysis software. Improvement was observed in optical
efficiency of the VECSEL devices with a transparent DBR by double passing the pump light. Higher dissipated power at
maximum output power was found in devices with larger spectral detuning between the quantum well gain and the
micro-cavity detuning.
We demonstrate a novel epitaxial process for the growth of low-dislocation density GaSb on GaAs. The
growth mode involves the formation of large arrays of periodic 90° misfit dislocations at the interface
between the two binary alloys which results in a completely strain relieved III-Sb epi-layer without the
need for thick buffer layers. This epitaxial process is used for the growth of antimonide active regions
directly on GaAs/AlGaAs distributed Bragg Reflectors (DBRs) resulting in 2 μm VECSELs on GaAs
substrates.
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.
An approach based on fully microscopically computed material properties like gain/absorption, radiative
and Auger recombination rates are used to design, analyze and develop optimization strategies for Vertical
External Cavity Surface Emitting Lasers for the IR and mid-IR with high quantitative accuracy. The microscopic
theory is used to determine active regions that are optimized to have minimal carrier losses and
associated heating while maintaining high optical gain. It is shown that in particular for devices in the
mid-IR wavelength range the maximum output power can be improved by more than 100% by making rather
minor changes to the quantum well design. Combining the sophisticated microscopic models with simple onedimensional
macroscopic models for optical modes, heat and carrier diffusion, it is shown how the external
efficiency can be strongly improved using surface coatings that reduce the pump reflection while retaining the
gain enhancing cavity effects at the lasing wavelength. It is shown how incomplete pump absorption can be
reduced using optimized metallization layers. This increases the efficiency, reduces heating and strongly improves
the maximum power. Applying these concepts to VECSELs operating at 1010nm has already resulted
in more than twice as high external efficiencies and maximum powers. The theory indicates that significant
further improvements are possible - especially for VECSELs in the mid-IR.
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.
Design of optimized semiconductor optically-pumped semiconductor lasers (OPSLs) depends on many ingredients
starting from the quantum wells, barrier and cladding layers all the way through to the resonant-periodic gain (RPG) and
high reflectivity Bragg mirror (DBR) making up the OPSL active mirror. Accurate growth of the individual layers
making up the RPG region is critical if performance degradation due to cavity misalignment is to be avoided.
Optimization of the RPG+DBR structure requires knowledge of the heat generation and heating sinking of the active
mirror. Nonlinear Control Strategies SimuLaseTM software, based on rigorous many-body calculations of the
semiconductor optical response, allows for quantum well and barrier optimization by correlating low intensity
photoluminescence spectra computed for the design, with direct experimentally measured wafer-level edge and surface
PL spectra. Consequently, an OPSL device optimization procedure ideally requires a direct iterative interaction between
designer and grower. In this article, we discuss the application of the many-body microscopic approach to OPSL devices
lasing at 850nm, 1040nm and 2μm. The latter device involves and application of the many-body approach to mid-IR
OPSLs based on antimonide materials. Finally we will present results on based on structural modifications of the
epitaxial structure and/or novel material combinations that offer the potential to extend OPSL technology to new
wavelength ranges.
A frequency-converted optically pumped semiconductor laser (OPSL) is described. The 976-nm OPSL is frequency
doubled intracavity and is forced to operate in single longitudinal mode. An external resonator, containing a cesium
lithium borate crystal is locked to the 488-nm fundamental, generating the second harmonic at 244 nm. Continuous
wave output in excess of 200 mW is generated.
SERS method for biomolecular analysis has several potentials and advantages over traditional biochemical approaches, including less specimen contact, non-destructive to specimen, and multiple components analysis. Urine is an easily available body fluid for monitoring the metabolites and renal function of human body. We developed surface-enhanced Raman scattering (SERS) technique using 50nm size gold colloidal particles for quantitative human urine creatinine measurements. This paper shows that SERS shifts of creatinine (104mg/dl) in artificial urine is from 1400cm-1 to 1500cm-1 which was analyzed for quantitative creatinine measurement. Ten human urine samples were obtained from ten healthy persons and analyzed by the SERS technique. Partial least square cross-validation (PLSCV) method was utilized to obtain the estimated creatinine concentration in clinically relevant (55.9mg/dl to 208mg/dl) concentration range. The root-mean square error of cross validation (RMSECV) is 26.1mg/dl. This research demonstrates the feasibility of using SERS for human subject urine creatinine detection, and establishes the SERS platform technique for bodily fluids measurement.
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