Pawan Gogna

Low-threshold photonic crystal laser

We have fabricated photonic crystal nanocavity lasers, based on a high-quality factor design that incorporates fractional edge dislocations. Lasers with InGaAsP quantum well active material emitting at 1550 nm were optically pumped with 10 ns pulses, and lased at threshold pumping powers below 220 μW, the lowest reported for quantum-well based photonic crystal lasers, to our knowledge. Polarization characteristics and lithographic tuning properties were found to be in excellent agreement with theoretical predictions.

High-performance InAs quantum-dot lasers near 1.3 μm

High-performance quantum dot (QD) lasers near 1.3 μm were fabricated using four stacks of InAs QDs embedded within strained InGaAs quantum wells as an active region and a reactive-ion-etched 5-μm-ridge waveguide design. For a 1.5-mm-long cavity QD laser, ground-state continuous-wave (cw) lasing has been achieved with a single facet output power of 15 mW at temperatures as high as 100 °C, while at room temperature having a differential quantum efficiency of 55% and a single facet output power of 50 mW. The characteristic temperature T0 for ground-state cw lasing is 78 K up to our temperature measurement limit of 100 °C.

Near-field scanning optical microscopy of photonic crystal nanocavities

Near-field scanning optical microscopy was used to observe high-resolution images of confined modes and photonic bands of planar photonic crystal (PPC) nanocavities fabricated in active InGaAsP material. We have observed the smallest optical cavity modes, which are intentionally produced by fractional edge dislocation high-Q cavity designs. The size of the detected mode was roughly four by three lattice spacings. We have also observed extended dielectric-band modes of the bulk PPC surrounding the nanocavity by geometrically altering the bands in emission range and eliminating localized modes out of the emission range.

NANOCOMPOSITES TO ENHANCE ZT IN THERMOELECTRICS

The concept of using “self-assembled” and “force-engineered” nanostructures to enhance the thermoelectric figure of merit relative to bulk homogeneous and composite materials is presented in general terms. Specific application is made to the Si-Ge system for use in power generation at high temperature. The scientific advantages of the nanocomposite approach for the simultaneous increase in the power factor and decrease of the thermal conductivity are emphasized along with the practical advantages of having bulk samples for property measurements and a straightforward path to scale-up materials synthesis and integration of nanostructured materials into thermoelectric cooling and power generation devices.

Progress Towards an Optimization Methodology for Combustion-Driven Portable Thermoelectric Power Generation Systems

There is enormous military and commercial interest in developing quiet, lightweight, and compact thermoelectric (TE) power generation systems. This paper investigates design integration and analysis of an advanced TE power generation system implementing JP-8 fueled combustion and thermal recuperation. In the design and development of this portable TE power system using a JP-8 combustor as a high-temperature heat source, optimal process flows depend on efficient heat generation, transfer, and recovery within the system. The combustor performance and TE subsystem performance were coupled directly through combustor exhaust temperatures, fuel and air mass flow rates, heat exchanger performance, subsequent hot-side temperatures, and cold-side cooling techniques and temperatures. Systematic investigation and design optimization of this TE power system relied on accurate thermodynamic modeling of complex, high-temperature combustion processes concomitantly with detailed TE converter thermal/mechanical modeling. To this end, this paper reports integration of system-level process flow simulations using CHEMCAD™ commercial software with in-house TE converter and module optimization, and heat exchanger analyses using COMSOL™ software. High-performance, high-temperature TE materials and segmented TE element designs are incorporated in coupled design analyses to achieve predicted TE subsystem-level conversion efficiencies exceeding 10%. These TE advances are integrated with a high-performance microtechnology combustion reactor based on recent advances at Pacific Northwest National Laboratory (PNNL). Predictions from this coupled simulation approach lead directly to system efficiency–power maps defining potentially available optimal system operating conditions and regimes. Further, it is shown that, for a given fuel flow rate, there exists a combination of recuperative effectiveness and hot-side heat exchanger effectiveness that provides a higher specific power output from the TE modules. This coupled simulation approach enables pathways for integrated use of high-performance combustor components, high-performance TE devices, and microtechnologies to produce a compact, lightweight, combustion-driven TE power system prototype that operates on common fuels.

High Temperature Thermoelectric Properties of Nano-Bulk Silicon and Silicon Germanium

Point defect scattering via the formation of solid solutions to reduce the lattice thermal conductivity has been an effective method for increasing ZT in state-of-the-art thermoelectric materials such as Si-Ge, Bi 2 Te 3 -Sb 2 Te 3 and PbTe-SnTe. However, increases in ZT are limited by a concurrent decrease in charge carrier mobility values. The search for effective methods for decoupling electronic and thermal transport led to the study of low dimensional thin film and wire structures, in particular because scattering rates for phonons and electrons can be better independently controlled. While promising results have been achieved on several material systems, integration of low dimensional structures into practical power generation devices that need to operate across large temperature differential is extremely challenging. We present achieving similar effects on the bulk scale via high pressure sintering of doped and undoped Si and Si-Ge nanoparticles. The nanoparticles are prepared via techniques that include high energy ball milling of the pure elements. The nanostructure of the materials is confirmed by powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and dynamic light scattering. Thermal conductivity measurements on the densified pellets show a drastic 90% reduction in the lattice contribution at room temperature when compared to doped single crystal Si. Additionally, Hall effect measurements show a much more limited degradation in the carrier mobility. The combination of low thermal conductivity and high power factor in heavily doped n-type nanostructured bulk Si leads to an unprecedented increase in ZT at 1275 K by a factor of 3.5 over that of single crystalline samples. Experimental results on both n-type and p-type Si are discussed in terms of the impact of the size distribution of the nanoparticles, doping impurities and nanoparticle synthesis processes.

Nanophotonics based on planar photonic crystals

Summary form only given. By creating different types of defects in the photonic crystal lattice, various nanophotonics components, such as cavities and waveguides, can be realized. The quest for a compact and efficient nano-cavity, with high quality factor (Q) and small mode volume (V/sub mode/), has been a central part of research in integrated optics. Recently, we have proposed a systematic method to design optical nano-cavities that satisfy both of these requirements. The cavity consists of a defect hole that is smaller than surrounding holes arranged in the triangular lattice photonic crystal. In order to test our design we have fabricated high-Q cavities in the InGaAsP material system.

Development of New High Temperature Power Generating Couples for the Advanced Thermoelectric Converter (ATEC)

Radioisotope Thermoelectric Generators (RTGs) have served as a reliable source of space power for decades, enabling robotic spacecraft to explore regions of the Solar System where photovoltaic systems are impractical. The increased power requirements for future missions, combined with the reduced availability of radioisotope fuel, has prompted the development of a higher specific power, higher efficiency converter system employing thermocouples with advanced thermoelectric materials. The challenges in incorporating these advanced materials into power generating thermocouples suitable for operation in a space-rated RTG are discussed herein.

High temperature continuous wave operation of InAs quantum dot lasers near 1.3 /spl mu/m

We have demonstrated high performance temperature insensitive narrow ridge waveguide QD lasers near 1.3 /spl mu/m using four stacks of InAs QD layer embedded within strained InGaAs quantum wells as an active region. For a 1.5 mm long cavity QD laser, ground state CW lasing has been achieved with single facet output power of 15 mW and a differential slope efficiency of 35% at temperature as high as 100C, while at room temperature having a differential quantum efficiency about 55% and single facet output power of 50 mW. The characteristic temperature for ground state CW lasing is 78 K at temperatures ranging from 200C to 1000C.

Thermoelectric properties of Si/Ge nano-composite

In this study, we developed a nano-composite approach to make bulk materials with nanostructures that have lower thermal conductivity than their bulk alloy counterparts. Room temperature measurement results of Si/Ge composites with nano-particle shows lower thermal conductivity than that of Si/Ge composites made with micro-sized particles. For high density sample, we also observed thermal conductivity reduction without deterioration of electrical conductivity so that higher ZT than bulk alloy is achieved.