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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Onset of surface stimulated emission at 260 nm from AlGaN multiple quantum wells

We demonstrated onset of deep-ultraviolet (DUV) surface stimulated emission (SE) from c-plane AlGaN multiple-quantum well (MQW) heterostructures grown on a sapphire substrate by optical pumping at room temperature. The onset of SE became observable at a pumping power density of 630 kW/cm{sup 2}. Spectral deconvolution revealed superposition of a linearly amplified spontaneous emission peak at λ ∼ 257.0 nm with a full width at half maximum (FWHM) of ∼12 nm and a superlinearly amplified SE peak at λ ∼ 260 nm with a narrow FWHM of less than 2 nm. In particular, the wavelength of ∼260 nm is the shortest wavelength of surface SE from III-nitride MQW heterostructures to date. Atomic force microscopy and scanning transmission electron microscopy measurements were employed to investigate the material and structural quality of the AlGaN heterostructures, showing smooth surface and sharp layer interfaces. This study offers promising results for AlGaN heterostructures grown on sapphire substrates for the development of DUV vertical cavity surface emitting lasers (VCSELs)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Multi-dimensional dynamics of stimulated Brillouin scattering in a laser speckle: Ion acoustic wave bowing, breakup, and laser-seeded two-ion-wave decay

Two- and three-dimensional particle-in-cell simulations of stimulated Brillouin scattering (SBS) in laser speckle geometry have been analyzed to evaluate the relative importance of competing nonlinear processes in the evolution and saturation of SBS. It is found that ion-trapping-induced wavefront bowing and breakup of ion acoustic waves (IAW) and the associated side-loss of trapped ions dominate electron-trapping-induced IAW wavefront bowing and breakup, as well as the two-ion-wave decay instability over a range of ZTe/Ti conditions and incident laser intensities. In the simulations, the latter instability does not govern the nonlinear saturation of SBS; however, evidence of two-ion-wave decay is seen, appearing as a modulation of the ion acoustic wavefronts. This modulation is periodic in the laser polarization plane, anti-symmetric across the speckle axis, and of a wavenumber matching that of the incident laser pulse. A simple analytic model is provided for how spatial “imprinting” from a high frequency inhomogeneity (in this case, the density modulation from the laser) in an unstable system with continuum eigenmodes can selectively amplify modes with wavenumbers that match that of the inhomogeneity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Enhancement of iron content in spinach plants stimulated by magnetic nano particles

In our previous study, the iron content in spinach plants could be detected by magnetic susceptibility values. In the present work, magnetic nano particles were found from the iron sand. The magnetic nano particles are synthesis by using co-precipitation process and sol-gel technique. The stimulation of magnetic nano particles in the plant has been done by the provision of magnetic nano particles in growing media. After certain time, plant samples was characterized using susceptibility-meter MS2B and atomic absorption spectroscopy to measure the magnetic susceptibility and the amount of iron content that absorbed of the plant, respectively. The iron content in the spinach plants was increased when the magnetic nano particles was injected in the growing media.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Flat-topped ultrabroad stimulated emission from chirped InAs/InP quantum dot laser with spectral width of 92 nm

We demonstrate the generation of 92 nm (−3 dB bandwidth) flat-topped ultrabroad stimulation emission from a chirped InAs/InP quantum dot (QD) laser. A greatly enhanced bandwidth of the gain spectrum is achieved, which is attributed to the additionally broadened quantum dot energy levels utilizing gradually changed height of QDs in the stacked active layers. The laser exhibits a maximum output power of 0.35 W under pulsed conditions, and the average spectral power density of above 3.8 mW/nm is obtained. The ultrabroad lasing spectrum in the wavelength interval of 1.49–1.61 μm covering S-C-L bands makes such a laser potentially useful as an optical source for various applications being compatible with silica fibers.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Composite implants coated with biodegradable polymers prevent stimulating tumor progression

In this experiment we studied oncologic safety of model implants created using the solution blow spinning method with the use of the PURASORB PL-38 polylactic acid polymer and organic mineral filler which was obtained via laser ablation of a solid target made of dibasic calcium phosphate dihydrate. For this purpose the implant was introduced into the area of Wistar rats’ iliums, and on day 17 after the surgery the Walker sarcoma was transplanted into the area of the implant. We evaluated the implant’s influence on the primary tumor growth, hematogenous and lymphogenous metastasis of the Walker sarcoma. In comparison with sham operated animals the implant group demonstrated significant inhibition of hematogenous metastasis on day 34 after the surgery. The metastasis inhibition index (MII) equaled 94% and the metastases growth inhibition index (MGII) equaled 83%. The metastasis frequency of the Walker sarcoma in para aortic lymph nodes in the implant group was not statistically different from the control frequency; there was also no influence of the implant on the primary tumor growth noted. In case of the Walker sarcoma transplantation into the calf and the palmar pad of the ipsilateral limb to the one with the implant in the ilium, we could not note any attraction of tumor cells to the implant area, i.e. stimulation of the Walker sarcoma relapse by the implant. Thus, the research concluded that the studied implant meets the requirements of oncologic safety.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2016

Super-resolution for scanning light stimulation systems

Super-resolution (SR) is a technique used in digital image processing to overcome the resolution limitation of imaging systems. In this process, a single high resolution image is reconstructed from multiple low resolution images. SR is commonly used for CCD and CMOS (Complementary Metal-Oxide-Semiconductor) sensor images, as well as for medical applications, e.g., magnetic resonance imaging. Here, we demonstrate that super-resolution can be applied with scanning light stimulation (LS) systems, which are common to obtain space-resolved electro-optical parameters of a sample. For our purposes, the Projection Onto Convex Sets (POCS) was chosen and modified to suit the needs of LS systems. To demonstrate the SR adaption, an Optical Beam Induced Current (OBIC) LS system was used. The POCS algorithm was optimized by means of OBIC short circuit current measurements on a multicrystalline solar cell, resulting in a mean square error reduction of up to 61% and improved image quality.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Label-free DNA imaging in vivo with stimulated Raman scattering microscopy

Significance Microscopic imaging of DNA has to rely on the use of fluorescent staining, an exogenous labeling in biological and biomedical studies, which often leads to uncertainty with respect to the quality and homogeneity of the staining. Label-free imaging of DNA will enable noninvasive visualization of live cell nuclei in both human and animals. Spontaneous Raman microspectroscopy offers label-free chemical contrast for DNA imaging; however, its slow imaging speed hampers its wide application for in vivo and dynamic studies. Here we developed a novel and simple approach with multicolor stimulated Raman scattering microscopy to evaluate rapid DNA imaging, which can be applied to both in vivo DNA dynamic studies and instant label-free human skin cancer diagnosis.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

Coherent control of long-range photoinduced electron transfer by stimulated X-ray Raman processes

Significance Electron transfer is ubiquitous in chemical reactions and many biophysical processes such as respiration and photosynthesis. In this work, we show how broadband X-ray pulses can be used to change the local electronic structures of donor–bridge–acceptor (DBA) molecular systems, thus enabling and controlling electron transfer between designated sites in the molecule. An X-ray–stimulated Raman process can control electron transfer with atomic specificity and ultrafast timescale. With an appropriate X-ray pulse setting, electron pathways can be selectively enhanced. The control can be implemented at the donor, bridge, or acceptor sites in a DBA system.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Mitigation of stimulated Raman scattering in the kinetic regime by external magnetic fields

Here, we show via particle-in-cell simulations that small normalized magnetic fields (ω c /ω p << 1) can significantly modify the evolution of backward stimulated Raman scattering (SRS) in the kinetic regime due to the enhanced dissipation of nonlinear electron plasma waves propagating perpendicular to magnetic fields. A magnetic field applied perpendicularly to the electron plasma wave (and driving light wave) increases the SRS threshold for kinetic inflation and decreases the amount of reflectivity when SRS is driven significantly above threshold. Analysis indicates that this arises because trapped electrons are accelerated as they surf across the wave, leading to the continual dissipation of the electron plasma waves over a wider range of wave amplitudes. The reduction in SRS reflectivity is most significant for a purely perpendicular field, although reduction also occurs for other angles; a parallel field can slightly increase single-speckle SRS but decreases multispeckle SRS. These simulations demonstrate the significance of magnetic-field contributions to nonlinear electron plasma wave damping with respect to nonlinear parametric decay instabilities; the simulation parameters are directly relevant for SRS in inertial confinement fusion devices and indicate that approximately 30 tesla magnetic fields might significantly reduce SRS backscatter.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

An all-electronic fail-safe for stimulated Brillouin scattering suppression

The mitigation of transverse stimulated Brillouin scattering (SBS) via phase modulation is mandatory to avoid damage in high-energy laser systems. A novel fail-safe system that indirectly monitors the optical bandwidth applied to suppress SBS by monitoring the input, reflected, and through rf power of the SBS-suppression lithium-niobate phase modulator is demonstrated. The fail-safe system has high sensitivity and reliability provided by optimized redundant circuits for power monitoring and fail-safe decision logic. Calibration of the fail-safe’s thresholds is straightforward. Furthermore, the fail-safe circuitry reacts within 45 ns to stop an incorrectly modulated pulse by blocking the propagation of an electronic timing trigger.

fail-safe↗
DOE OSTI2024

Squeezed Light Stimulated Raman Scattering (SRS) Microscopy for Bioenergy Applications

Stimulated Raman scattering (SRS) microscopy using picosecond near-IR pulses have provided a great penetration depth with reduced fluorescence interference when imaging biological samples for bioenergy applications. These tools have provided insight into 1) tracking the degradation of chemical composites in biomass feedstocks to investigate the recalcitrant factors during the deconstruction processes, 2) monitoring the production of chemicals in photosynthetic plants and wood-digesting microorganisms, and 3) probing plant-bacteria interactions. However, the above processes are usually slow and require continuous imaging for an extended period. This is challenging for classic SRS because the laser power needed to achieve enough sensitivity causes photodamage in the samples during such long experiments. Quantum-squeezed light with reduced noise in the intensity quadrature can improve the sensitivity of classic SRS microscopy beyond the shot noise limit. The successful squeezing of one of the picosecond pulses in the above SRS will improve sensitivity and reduce photodamage, greatly expanding the range of studies available to SRS microscopy.

BIOMASS FUELS↗
DOE OSTI2016

SU-G-IeP4-04: DD-Neutron Source Collimation for Neutron Stimulated Emission Computed Tomography: A Monte Carlo Simulation Study

Purpose: To optimize collimation and shielding for a deuterium-deuterium (DD) neutron generator for an inexpensive and compact clinical neutron imaging system. The envisioned application is cancer diagnosis through Neutron Stimulated Emission Computed Tomography (NSECT). Methods: Collimator designs were tested with an isotropic 2.5 MeV neutron source through GEANT4 simulations. The collimator is a 52×52×52 cm{sup 3} polyethylene block coupled with a 1 cm lead sheet in sequence. Composite opening was modeled into the collimator to permit passage of neutrons. The opening varied in shape (cylindrical vs. tapered), size (1–5 cm source-side and target-side openings) and aperture placements (13–39 cm from source-side). Spatial and energy distribution of neutrons and gammas were tracked from each collimator design. Parameters analyzed were primary beam width (FWHM), divergence, and efficiency (percent transmission) for different configurations of the collimator. Select resultant outputs were then used for simulated NSECT imaging of a virtual breast phantom containing a 2.5 cm diameter tumor to assess the effect of the collimator on spatial resolution, noise, and scan time. Finally, composite shielding enclosure made of polyethylene and lead was designed and evaluated to block 99.99% of neutron and gamma radiation generated in the system. Results: Analysis of primary beam indicated the beam-width is linear to the aperture size. Increasing source-side opening allowed at least 20% more neutron throughput for all designs relative to the cylindrical openings. Maximum throughput for all designs was 364% relative to cylindrical openings. Conclusion: The work indicates potential for collimating and shielding a DD neutron generator for use in a clinical NSECT system. The proposed collimator designs produced a well-defined collimated neutron beam that can be used to image samples of interest with millimeter resolution. Balance in output efficiency, noise reduction, and scan time should be considered to determine the optimal design for specific NSECT applications.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Picosecond buildup and relaxation of intense stimulated emission in GaAs

In support of the idea developed previously based on circumstantial evidence, we have found that stimulated emission emerges in GaAs and its intensity increases with a picosecond delay relative to the front of powerful picosecond optical pumping that produced a dense electron-hole plasma. The emission intensity relaxes with decreasing pumping with a characteristic time of {approx}10 ps. We have derived the dependences of the delay time, the relaxation time, and the duration of the picosecond emission pulse on its photon energy. The estimates based on the fact that the relaxation of emission is determined by electron-hole plasma cooling correspond to the measured relaxation time.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Oscillations of absorption of a probe picosecond light pulse caused by its interaction with stimulated picosecond emission of GaAs

The self-modulation of absorption of a picosecond light pulse was observed earlier [1] in a thin (∼1-μm thick) GaAs layer pumped by a high-power picosecond pulse. Analysis of the characteristics of this self-modulation predicted [5] that the dependences of the probe pulse absorption on the pump pulse energy and picosecond delay between pump and probe pulses should be self-modulated by oscillations. Such self-modulation was experimentally observed in this work. Under certain conditions, absorption oscillations proved to be a function of part of the energy of picosecond stimulated emission of GaAs lying above a certain threshold in the region where the emission front overlapped the probe pulse front. Absorption oscillations are similar to self-modulation of the GaAs emission characteristics observed earlier [4]. This suggests that the self-modulation of absorption and emission is determined by the same type of interaction of light pulses in the active medium, the physical mechanism of which has yet to be determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Self-synchronization of the modulation of energy-levels population with electrons in GaAs induced by picosecond pulses of probe radiation and intrinsic stimulated emission

Picosecond optical pumping leads to the initiation of intrinsic picosecond stimulated emission in GaAs. As was established previously, due to the interaction of pulses of probe radiation with those of intrinsic emission, the dependence of the absorption α of the probe pulse on its delay τ with respect to the pump pulse is modulated with oscillations. It is found that the oscillatory dependences α(τ) have a similar shape only in the case of certain combinations of energies of the interacting pulses. As a result, it is assumed that the above interaction is, in fact, a synchronization of modulations (formed by pulses) of charge-carrier populations at energy levels; this synchronization occurs in the direction of the reconstruction of detailed equilibrium. The real-time picosecond self-modulation of the absorption α is measured for the first time. The characteristics of this self-modulation as well as absorption α and intrinsic emission self-modulation characteristics measured previously by correlation methods are now accounted for by the concept of synchronization.

36 MATERIALS SCIENCE↗
DOE OSTI2017

On the stimulated emission of InGaAs/GaAs/AlGaAs laser structures grown by MOCVD on exact and inclined Ge/Si(001) substrates

GaAs/AlGaAs laser structures with InGaAs quantum wells are grown by metal-organic chemical vapor deposition (MOCVD) on exact Si(001) substrates and substrates inclined by 4° to the [011] axis with a relaxed Ge buffer layer, emitting in the transparency region of bulk silicon (the wavelength is longer than 1100 nm at room temperature). The threshold power densities of stimulated emission, observed for the structures grown on exact and inclined substrates are 45 and 37 kW/cm{sup 2}, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2018

THz Stimulated Emission from Simple Superlattice in Positive Differential Conductivity Region

Narrow band emissions at 2.6–2.8 THz are observed out of liquid helium cooled 1 mm disk chips prepared of a wafer with the very low n type doped weak barrier GaAs–GaAlAs superlattice of 1000 periods. The emissions are at about 8.0–18.0 V pulsed voltage applied to the chips in region of the chips positive DC differential conductivity that guaranties absence of inhomogeneous electric field domains in the chips. The emission frequency bands are estimated with a cyclotron resonance filter; the measurements show that the band width is of about that of the THz quantum cascade laser. By using long voltage pulses the chip heating above 100 K is achieved without substantial change in emission power. We speculate that the emission is super luminescence (amplification) of whispering gallery modes in the chips as a result of inverted Wannier-Stark level transitions under bias. The results are the first world demonstration of THz stimulated emission in a simple superlattice within region of positive DC differential conductivity; they give strong impetus for development of THz and higher frequency sources based on such simple superlattices; the sources should well compete with the THz quantum cascade lasers in particular at elevated temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Magnetooptical Studies and Stimulated Emission in Narrow Gap HgTe/CdHgTe Structures in the Very Long Wavelength Infrared Range

We investigate the prospects of HgTe/HgCdTe quantum wells for long-wavelength interband lasers (λ = 15–30 μm). The properties of stimulated emission (SE) and magnetoabsorbtion data of QWs structures with wide-gap HgCdTe dielectric waveguide provide an insight on dominating non-radiative carrier recombination mechanism. It is shown that the carrier heating under intense optical pumping is the main factor limiting the SE wavelength and intensity, since the Auger recombination is greatly enhanced when carriers populate high energy states in the valence band.

36 MATERIALS SCIENCE↗