Gene Tsvid

Low-threshold thin-film III-V lasers bonded to silicon with front and back side defined features.

A III-V thin-film single-quantum-well edge-emitting laser is patterned on both sides of the epitaxial layer and bonded to silicon. Injected threshold current densities of 420 A/cm(2) for gain-guided lasers with bottom p-stripes and top n-stripes and 244 A/cm(2) for index-guided bottom p-ridge and top n-stripe lasers are measured with a lasing wavelength of approximately 995 nm. These threshold current densities, among the lowest for thin-film edge-emitting lasers on silicon reported to date (to our knowledge), enable the implementation of integrated applications such as power-efficient portable chip-scale photonic sensing systems.

MOCVD-Grown Dilute Nitride Type II Quantum Wells

Dilute nitride Ga(In)NAs/GaAsSb ldquoWrdquo type II quantum wells on GaAs substrates have been grown by metal-organic chemical vapor deposition (MOCVD). Design studies underscore the importance of nitrogen incorporation to extend the emission wavelength into the 1.5 mum region as well as increase the electron confinement, given the material strain relaxation limitations. These studies also indicate that the Sb content of the GaAs1-xSbx hole well is required to be greater than x ~ 0.2, to provide adequate hole confinement (i.e., DeltaEnu > 150 meV). Photoluminescence (PL) and electroluminescence (EL) studies are used to characterize the optical transitions and compare with a ten-band \bm k.p simulation. We find that the lowest energy type II transition observed is in good agreement with theory. Preliminary results are presented on diode lasers with two- and three-stage ldquoWrdquo-active regions that exhibit emission that is blue-shifted from the PL, due to charge separation and carrier band-filling of higher energy transitions. Further structure optimization, including multiple-stage (eight to ten W-stages) active regions is required to lower the threshold carrier density and minimize carrier band-filling and built-in electric field effects resulting from charge separation. Dilute nitride materials, such as GaAs1- y-z Sby Nz /InP, are also under development offering potential for wavelength extension into the mid-IR employing InP substrates.

Spontaneous Radiative Efficiency and Gain Characteristics of Strained-Layer InGaAs–GaAs Quantum-Well Lasers

The optical gain spectra, unamplified spontaneous emission spectra, and spontaneous radiative efficiency are extracted from the measurement of amplified spontaneous emission (ASE) on a single pass, segmented contact 0.98-mum-emitting aluminum-free InGaAs-InGaAsP-GaAs quantum-well (QW) laser diode. These measurements provide a baseline for which to compare higher strain InGaAs QW lasers emitting near 1.2 mum. The peak gain-current relationship is extracted from gain spectra and the peak gain parameter go is found to agree within 25% of the value extracted using conventional cavity length analysis for 0.98-mum-emitting devices. The spontaneous radiative current is extracted using the fundamental connection between gain and unamplified spontaneous emission, which in turn gives an estimate of the amount of nonradiative recombination in this material system. The spontaneous radiative efficiency, the ratio of spontaneous radiative current to total current, at room temperature of 0.98-mum-emitting InGaAs QW laser material is found to be in the range of 40%-54%, which is 2.5-3.5 times larger than that of highly strained InGaAs QW laser emitting near lambda = 1.2 mum. Whereas the gain parameter, g0 = dg/d(ln j), was measured to be 1130 and 1585 cm-1 for the 0.98-mum- and 1.2-mum-emitting materials, respectively. From the calculated below threshold current injection efficiency of 75%-85%, we deduce that the internal radiative efficiency of the QW material is ~ 20% higher than the ratio of internal radiative current to external injected current extracted directly from ASE measurements.

Electrostatic confinement and manipulation of DNA molecules for genome analysis

Significance Repeated sequences make up approximately two-thirds of the human genome, which become fully accountable when very large DNA molecules are analyzed. Long, single DNA molecules are problematic using common experimental techniques and fluidic devices because of mechanical considerations that include breakage, dealing with the massive size of these coils, or the huge length of stretched DNAs. Accordingly, we harness analyte “issues” as exploitable advantages by invention and characterization of the “molecular gate,” which controls and synchronizes formation of stretched molecules as DNA dumbbells within nanoslit geometries that may also offer new routes to separation. This was accomplished by theoretical studies and experiments leveraging a series of electrical forces acting on DNA molecules, device walls, and the fluid flows within our devices. Very large DNA molecules enable comprehensive analysis of complex genomes, such as human, cancer, and plants because they span across sequence repeats and complex somatic events. When physically manipulated, or analyzed as single molecules, long polyelectrolytes are problematic because of mechanical considerations that include shear-mediated breakage, dealing with the massive size of these coils, or the length of stretched DNAs using common experimental techniques and fluidic devices. Accordingly, we harness analyte “issues” as exploitable advantages by our invention and characterization of the “molecular gate,” which controls and synchronizes formation of stretched DNA molecules as DNA dumbbells within nanoslit geometries. Molecular gate geometries comprise micro- and nanoscale features designed to synergize very low ionic strength conditions in ways we show effectively create an “electrostatic bottle.” This effect greatly enhances molecular confinement within large slit geometries and supports facile, synchronized electrokinetic loading of nanoslits, even without dumbbell formation. Device geometries were considered at the molecular and continuum scales through computer simulations, which also guided our efforts to optimize design and functionalities. In addition, we show that the molecular gate may govern DNA separations because DNA molecules can be electrokinetically triggered, by varying applied voltage, to enter slits in a size-dependent manner. Lastly, mapping the Mesoplasma florum genome, via synchronized dumbbell formation, validates our nascent approach as a viable starting point for advanced development that will build an integrated system capable of large-scale genome analysis.

Progress Towards Intersubband Quantum-Box Lasers for Highly Efficient Continuous Wave Operation in the Mid-Infrared

Intersubband quantum-box (IQB) lasers, which are devices consisting of 2-D arrays of ministacks (i.e., 2-4 stages) intersubband QB emitters have been proposed as alternatives to 30-stage quantum-cascade (QC) devices, for efficient room-temperature (RT) emission in the mid-infrared (4-6 µm) wavelength range. Preliminary results include: 1) the design of devices for operation with 50% wallplug efficiency at RT; 2) realization of a novel type of QC device: the deep-well (DW) QC laser, that has demonstrated at λ µm low temperature sensitivity of the threshold current, a clear indication of suppressed carrier leakage; 3) the formation of 2-D arrays at nanopoles by employing nanopatterning and dry etching; 4) the formation of 40 nm-diameter, one-stage IQB structures on 100 nm centers by preferential regrowth via metal-organic vapor phase epitaxy (MOVPE).

Top-Bottom Stripe Thin Film InGaAs/GaAsP Laser integrated on Silicon

Integration of photonic active and passive components - and ultimately, full systems - directly onto Si and Si CMOS are a critical step toward chip scale photonic system integration. Thin film compound semiconductor lasers heterogeneously integrated onto silicon open up a wide range of applications, including portable sensing systems, optical interconnects, and chip scale ultrafast optical signal processing. Thin film lasers on silicon are a critical component for these applications, and a low threshold current density is especially desirable for low power, portable applications. In this paper, a thin film InGaAs/GaAsP single quantum well (SQW) laser with strain compensation and a unique top/bottom stripe contact structure for efficient current distribution is presented. The thin film (3.8 mum thick), and the ability to perform fabrication processes on both sides of this thin film laser, enable this new contact structure.

Native-oxide-confined high-index-contrast λ=1.15 μm strain-compensated InGaAs single quantum well ridge waveguide lasers

High performance native-oxide-confined high-index-contrast (HIC) ridge waveguide (RWG) diode lasers are fabricated in a strain-compensated In0.4Ga0.6As single quantum well structure by employing a deep dry etch plus nonselective O2-enhanced wet thermal oxidation process. The thermal native oxide grown on the etch-exposed RWG sidewalls of the Al0.74Ga0.26As waveguide cladding layers and GaAs core with GaAsP–InGaAs quantum well provides both strong optical and electrical confinements for the active region. Due to a smoothing of sidewall roughness by the O2-enhanced oxidation, the lasers exhibit a low internal loss in αi=7.2 cm−1 for a w=7.2 μm narrow stripe HIC RWG structure, only 53% larger than that of w=87.2 μm broad-area devices, enabling their room temperature operation at a low 300 A/cm2 threshold current density.

Strain Compensated InGaAs/GaAsP Single Quantum Well Thin Film Lasers Integrated onto Si Substrates

Thin film InGaAs/GaAsP lasers with strained single quantum well active regions have been bonded to Si and tested. The thin film laser structure was designed and grown with no net strain using strain compensation.

Towards intersubband quantum box lasers: Electron-beam lithography update

We report on the progress in the patterning and fabrication of the intersubband quantum-box (QB) laser structure. From a patterning point of view our goal is to make 30-nm-diameter SiO2 and/or hydrogen silsesquioxane (HSQ) disks on 60–80nm centers on a GaAs surface to serve as masks for in situ etch and regrowth of QBs. Electron-beam lithography with high-resolution negative resist HSQ was used, and two processes have been investigated. The first process is to pattern HSQ directly on the GaAs surface, while the second one involves putting down an intermediate oxide layer first, followed by the e-beam lithography and the transfer of the pattern into the oxide. Problems were encountered with the e-beam patterning of HSQ directly on the GaAs surface because of the broad scattering from the substrate and not very good adhesion. Excellent patterning was demonstrated when the intermediate oxide layer was present between the GaAs substrate and the HSQ resist.

Radiative Efficiency of InGaAs/InGaAsP/GaAs Quantum Well Lasers

The gain, spontaneous emission spectra, and radiative efficiency of InGaAs quantum well laser structures are determined using single pass, segmented contact amplified emission measurements. The relationship between injection efficiency and radiative efficiency is also considered.