atomic-layer-deposition-vanadium-dioxide-temperature-dependent

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with ...

Naval Research Laboratory

We demonstrate a mode-locked erbium-doped fiber laser with a 1-microm InGaAs saturable absorber that produces 84-ps, 1-nJ transform-limited pulses. Measurements of the InGaAs multiple quantum well revealed a slow saturable absorber that is useful for passive mode locking. Optical fiber was added to extend the cavity and vary the repetition rate from 51 kHz to 5.4 MHz. The narrow spectral width of the laser output (<0.04 nm) permits amplification to 0.2 microJ/pulse with minimal pulse broadening. Pulse energies as large as 1.7 microJ can be achieved with pulse widths of <330 ps. Average powers of 0.5 W at megahertz repetition rates are demonstrated.

High-energy saturable absorber mode-locked fiber laser system.

We demonstrate a solitary pulse output from an 8.3-MHz mode-locked Yb-doped fiber laser, operating entirely in the normal dispersion regime. The typical output hyperbolic-secant pulses have a 14-ps pulse width and a 1.2-mW average output power. The spectrum has steep band edges with a 6.1-nm width and a tunable center wavelength between 1050 and 1080 nm. Using a frequency-resolved optical gating setup, we show that the pulse intensity and phase profiles are consistent with a chirped soliton. Energy quantization is observed, thus demonstrating the non-parabolic nature of these pulses. The laser output is compressed to near the transform limit (approximately 430 fs).

Experimental measurements of solitary pulse characteristics from an all-normal-dispersion Yb-doped fiber laser.

We report results of scattering measurements using femtosecond pulses to collect a wealth of information in a single experiment. Potential issues with particle scattering, such as variation in particle size, were avoided by using 9 and 50 microm diameter glass fibers. We first establish an angular scattering intensity baseline, and we show that the spectral width of very short pulses leads to smoothing of the angular scattering pattern, consistent with continuous broadband illumination. We then measure the angular scattering pattern from short pulses with a spectrometer and reveal an underlying spectral periodicity of broadband scattered light that is consistent with narrowband cw scattering. Our experimental results compare well with existing theory. We show that such two-dimensional experimental data and derived analytic solution can provide robust characterization of scattering objects even in the presence of noise.

Broadband optical pulse scattering from a glass fiber.

Optical pulse propagation in water is experimentally investigated using an evolutionary algorithm (EA) to control the shape of an optical pulse. The transmission efficiency (ratio of output to input optical power) is maximized by searching the combined amplitude and phase space governing an optical pulse shaper. The transmission efficiency of each tested pulse is physically determined by experiment during the course of the optimization. Combining the EA with an experiment in this manner is a powerful means of improving some figure of merit because no analytical or computational model is required-we optimize directly given the physics of the experiment. In addition, the EA is capable of efficiently searching a large parameter space. Here, we demonstrate improved linear optical pulse propagation near 800nm. Our results demonstrate a pulse with a dramatically narrower bandwidth that coincides with a local absorption minimum (near 800 nm) implying that the transmission efficiency is dominated by water's absorption spectrum.

Improved optical pulse propagation in water using an evolutionary algorithm.

Low temperature growth of GaAs (LT-GaAs) near 200 °C results in a recombination lifetime of nearly 1 ps, compared with approximately 1 ns for regular temperature ~600 °C grown GaAs (RT-GaAs), making it suitable for ultra high speed detection applications. However, LT-GaAs detectors usually suffer from low responsivity due to low carrier mobility. Here we report electro-optic sampling time response measurements of a detector that employs an AlGaAs heterojunction, a thin layer of LT-GaAs, a channel of RT-GaAs, and a vertical electric field that together facilitate collection of optically generated electrons while suppressing collection of lower mobility holes. Consequently, these devices have detection efficiency near that of RT-GaAs yet provide pulse widths nearly an order of magnitude faster--~6 ps for a cathode-anode separation of 1.3 μm and ~12 ps for distances more than 3 μm.

Performance enhancement of a GaAs detector with a vertical field and an embedded thin low-temperature grown layer.

Avalanche photodiodes (APDs) are essential components in quantum key distribution systems and active imaging systems requiring both ultrafast response time to measure photon time of flight and high gain to detect low photon flux. The internal gain of an APD can improve system signal-to-noise ratio (SNR). Excess noise is typically kept low through the selection of material with intrinsically low excess noise, using separate-absorption-multiplication (SAM) heterostructures, or taking advantage of the dead-space effect using thin multiplication regions. In this work we demonstrate the first measurement of excess noise and gain-bandwidth product in III-V nanopillars exhibiting substantially lower excess noise factors compared to bulk and gain-bandwidth products greater than 200 GHz. The nanopillar optical antenna avalanche detector (NOAAD) architecture is utilized for spatially separating the absorption region from the avalanche region via the NOA resulting in single carrier injection without the use of a traditional SAM heterostructure. 

Plasmonic field confinement for separate absorption-multiplication in InGaAs nanopillar avalanche photodiodes.

Luminescent gold nanocrystals (AuNCs) are a recently-developed material with potential optic, electronic and biological applications. They also demonstrate energy transfer (ET) acceptor/sensitization properties which have been ascribed to Förster resonance energy transfer (FRET) and, to a lesser extent, nanosurface energy transfer (NSET). Here, we investigate AuNC acceptor interactions with three structurally/functionally-distinct donor classes including organic dyes, metal chelates and semiconductor quantum dots (QDs). Donor quenching was observed for every donor-acceptor pair although AuNC sensitization was only observed from metal-chelates and QDs. FRET theory dramatically underestimated the observed energy transfer while NSET-based damping models provided better fits but could not reproduce the experimental data. We consider additional factors including AuNC magnetic dipoles, density of excited-states, dephasing time, and enhanced intersystem crossing that can also influence ET. Cumulatively, data suggests that AuNC sensitization is not by classical FRET or NSET and we provide a simplified distance-independent ET model to fit such experimental data.

Energy Transfer Sensitization of Luminescent Gold Nanoclusters: More than Just the Classical Förster Mechanism.

Monolayer transition metal dichalcogenides are promising materials for valleytronic operations. They exhibit two inequivalent valleys in the Brillouin zone, and the valley populations can be directly controlled and determined using circularly polarized optical excitation and emission. The photoluminescence polarization reflects the ratio of the two valley populations. A wide range of values for the degree of circularly polarized emission, P, has been reported for monolayer WS, although the reasons for the disparity are unclear. Here, we optically populate one valley and measure P to explore the valley population dynamics at room temperature in a large number of monolayer WS samples synthesized via chemical vapor deposition. Under resonant excitation, P ranges from 2 to 32%, and we observe a pronounced inverse relationship between photoluminescence (PL) intensity and P. High-quality samples exhibiting strong PL and long exciton relaxation time exhibit a low degree of valley polarization, and vice versa. This behavior is also demonstrated in monolayer WSe samples and transferred WS, indicating that this correlation may be more generally observed and account for the wide variations reported for P. Time-resolved PL provides insight into the role of radiative and nonradiative contributions to the observed polarization. Short nonradiative lifetimes result in a higher measured polarization by limiting opportunity for depolarizing scattering events.

Marc Currie

Naval Research Laboratory

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model