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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Multifrequency stimulated Raman scattering of light in a calcite single crystal

Using pulses of multifrequency stimulated Raman scattering (SRS) in calcite, nonlinear photoluminescence in a stilbene molecular crystal is excited. When multifrequency SRS is excited by the radiation of a Nd{sup 3+} : YAG laser with a wavelength of 1064 nm, eleven anti-Stokes components are observed in the visible spectrum with an average frequency shift of 1086 cm{sup −1} between them. Using the second harmonic of the Nd {sup 3+} : YAG laser radiation allowed three anti-Stokes and four Stokes components to be recorded. The large spectral range of the frequency comb facilitated effective reduction in the duration and increase in the intensity of the radiation pulse of multifrequency SRS. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

BaGa{sub 2}GeS{sub 6} and BaGa{sub 2}GeSe{sub 6} crystals for nonlinear optical frequency conversion

We analyse the functional capabilities of new crystals, BaGa{sub 2}GeS{sub 6} (BGGS) and BaGa{sub 2}GeSe{sub 6} (BGGSe), which are used for nonlinear optical frequency conversion in their transparency range. The wavelengths at which maximum conversion efficiencies can be obtained and the tuning range for difference-frequency generation are found. It is shown that there are wavelength combinations at which the effective nonlinearity coefficient varies only slightly in a wide frequency band. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

457-nm radiation source based on a diode laser for precision spectroscopy of magnesium atoms

We report on the development of a highly stable source of 457-nm radiation based on a diode laser and a tapered amplifier operating in a double-pass scheme. The diode laser frequency was stabilised by a high-Q reference Fabry – Perot interferometer and doubled in a PPSLT nonlinear crystal placed in an enhancement cavity. At a maximum output power of 200 mW at a wavelength of 457 nm, the laser linewidth was less than 5 kHz. The radiation source operation was demonstrated in experiments on precision spectroscopy of cold magnesium atoms in a magneto-optical trap. (lasers)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

Influence of growth temperature of KTiOAsO{sub 4} single crystals on their physicochemical parameters and formation of domain structures

A potassium titanyl arsenate (KTiOAsO{sub 4}, KTA) crystal 50 × 80 × 60 mm in size has been grown by upgraded Czochralski method from flux (TGGS) with a decrease in temperature from 900 to 770 °C during pulling. It is shown that the spectroscopic properties of the parts of KTA crystals grown at 900 and 770 °C are close, whereas the electrical conductivity of the low-temperature (770 °C) KTA part turned out to be an order of magnitude lower than that of the high-temperature part. Visualisation of the domain structure by second-harmonic generation microscopy revealed a more efficient domain intergrowth (throughout the sample) in the low-temperature KTA, which is important for forming a regular domain structure (RDS) in a KTA-based nonlinear optical element. It is established that the quantum efficiency of parametric generation of light in the RDS formed in low-temperature KTA is several times higher than in the case of high-temperature KTA. The results obtained are important for optimising RDS parameters. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Numerical simulation of dispersion and nonlinear characteristics of microstructured silica fibres with a thin suspended core in a wide range of their parameters

Dispersion and nonlinear characteristics of microstructured silica fibres with a thin suspended core surrounded by three, four or six air holes have been studied theoretically in the wavelength range 1 – 2 μm. It has been shown that, owing to strong fundamental mode confinement near the core, the Kerr nonlinearity coefficient can exceed the nonlinearity coefficient of standard telecom fibre SMF28e by two orders of magnitude. The large waveguide contribution allows for effective group velocity dispersion management. Estimates are presented that demonstrate the feasibility of using suspended core fibre exhibiting Kerr nonlinearity for generating non-classical light: a state with squeezed quantum fluctuations in one of the quadrature components of a cw laser signal at a wavelength near 1.55 μm. (paper)

36 MATERIALS SCIENCE↗
DOE OSTI2020

Degradation of the contrast of a short light pulse in a CPA system

We have calculated the effect of four-wave mixing of the main radiation pulse with the background radiation components on the value of the output radiation contrast in a chirped pulse amplification system of petawatt laser facilities. Formulae are presented that make it possible to quantify the contributions of two mechanisms, i.e. Kerr nonlinearity of the refractive index of the amplifying medium and gain saturation. For typical values of the nonlinear refractive index, both contributions are comparable. The possibility of mutual quenching of nonlinear contributions is noted. The influence of spectral filtering of radiation on the output pulse contrast is considered. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Self-compression of laser pulses in a discrete medium

Using the discrete nonlinear Schrödinger equation, we study the laser radiation self-action dynamics in a nanostructured waveguide system. It is shown that for a laser pulse energy exceeding the critical value, the nonlinear evolution of the wave field markedly differs from the corresponding process in a continuous medium. While the pulse propagates in a discrete medium, its length decreases to values comparable to the scale of the structure and the velocity decreases to zero. The system parameters are determined at which the wave field after the laser pulse termination begins to propagate in the opposite direction in a compressed form. The process of slowing down and stopping the pulse is accompanied by strong radiation losses, so that about a third of the energy of the initial field distribution remains in the final compressed state. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Two-stage nonlinear compression of high-power femtosecond laser pulses

Two-stage compression of laser pulses with a power of 250 TW is experimentally realised by broadening their spectrum during self-phase modulation in fused silica and subsequent dispersion compensation upon reflection from chirping mirrors. A five-fold decrease in the duration is demonstrated, from 75 to 15 fs, with a B-integral value of about 5 at each stage. It is possible to avoid small-scale self-focusing due to self-filtering of the laser beam during free propagation in vacuum. With optimal parameters of the dispersive mirror, the pulse can be compressed to a duration of less than 5 fs. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Decreasing the amplitude of macroscopic quantum fluctuations in the case of transient SRS

It is shown experimentally that, under the conditions of transient stimulated Raman scattering, pumping by two successive orthogonally polarised laser pulses makes it possible to reduce the amplitude of macroscopic quantum energy fluctuations of a Stokes pulse by a factor of 4 in comparison with single-shot pumping. An energy dispersion of 0.9 % for the first Stokes component is obtained in hydrogen at a relative energy dispersion of ytterbium laser pulses of 0.4 %. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Generation of few-cycle pulses in media with alternating sign of effective cubic nonlinearity

An original technique is developed for spectral broadening of femtosecond pulses with compensation for the nonlinear spatial phase during the propagation through nonlinear media with effective cubic nonlinearity of different signs. It was shown that in the region of 1.5 μm, the proposed scheme with BBO crystals at the first stage and NaCl at the second stage allows, using chirped mirrors, the formation of few-cycle pulses of about 7 fs duration with a small B-integral. The possibility of focusing of pulses compressed in the proposed scheme with a large Strehl ratio is demonstrated. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Localised electromagnetic waves in a rhombic waveguide array with competing nonlinearities

We consider a model of a discrete photonic system representing a quasi-one-dimensional rhombic array of waveguides, where, in addition to the positive cubic nonlinearity, the negative quintic nonlinearity is taken into account for the material of which the central chain of the waveguides is made. The other two waveguide chains are made of an optically linear material. A continual approximation is used to obtain a solution for a system of coupled waves, which describes a wave localised in the transverse direction. In a certain special case, the competition of nonlinearities leads to the formation of a step-shaped distribution of the field intensities over the waveguides. (integrated optics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Interference in between the acts of pre- and postselection

As an alternative approach for measuring the weak effects associated with the artificial preparation of rare events in quantum metrology, we propose the study of the interference pattern generated by acts of pre- and postselection of a quantum system. An example of two Mach – Zehnder interferometers connected by a cross-Kerr nonlinearity is considered. Postselection of photon states at the output of one of the interferometers and the application of a controlled phase shift in one of its arms induces interference phenomena in the photodetection statistics at the output of the second interferometer. The nonlinearity parameter determines the shift and width of the structures in the interference pattern. The main features of this pattern are studied depending on the magnitude of the Kerr nonlinearity and the number of photons at the input of the interferometers. (paper)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Study of saturated-absorption resonances on the {sup 3}P{sub 0, 1, 2} – {sup 3}D{sub 1, 2, 3} transitions of magnesium atoms in a hollow-cathode discharge cell

The saturated-absorption resonances on the {sup 3}P{sub 0, 1, 2} – {sup 3}D{sub 1, 2, 3} transitions of magnesium atoms in a hollow-cathode discharge cell are studied. The line width (FWHM) for the observed saturated-absorption resonance on the 3{sup 3}P{sub 0} → 3{sup 3}D{sub 1} transition turns out to be ∼220 MHz. Experiments are performed using a system based on a 766-nm diode laser with amplification and frequency doubling in a nonlinear BiBO crystal. The results obtained are of interest for sub-Doppler cooling on the 3{sup 3}P{sub 2} → 3{sup 3}D{sub 3} transition. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Effect of CO{sub 2}-laser power density on the absorption coefficient of polycrystalline CVD diamonds

The effect of 1-s irradiation of an uncooled polycrystalline CVD diamond plate by a focused cw CO{sub 2} laser beam with a power density of 300 – 800 kW cm{sup −2} is investigated. The absorption coefficient of the sample at a power density of 800 kW cm{sup −2} is found to be 0.035 cm{sup −1} larger as compared with that at 300 kW cm{sup −2}, which is related to the temperature dependence of its phonon-induced absorption with a change in temperature from 44 to 100 °C. It is shown that polycrystalline diamond, in contrast to other optical materials, does not exhibit nonlinear (avalanche-like) rise in absorption at high CO{sub 2}-laser power densities, at least up to 800 kW cm{sup −2}. (paper)

36 MATERIALS SCIENCE↗
DOE OSTI2020

Nonlinear waves in a thin dielectric film on the surface of a topological insulator

Based on the dispersion relation for a wave localised in a thin film of a nonlinear dielectric, which is located on the surface of a topological insulator, we have derived a system of equations that describes the propagation of a surface wave. It is shown that the longitudinal and transverse tangential components of the electric field vector are related due to the nonlinearity of the film and change periodically during propagation. It is found that the rotation period of this vector is determined by the axion charge of the topological dielectric and the nonlinear susceptibility of the thin film. (paper)

36 MATERIALS SCIENCE↗
DOE OSTI2024

Communication Lower Bounds and Optimal Algorithms for Multiple Tensor-Times-Matrix Computation

Multiple tensor-times-matrix (Multi-TTM) is a key computation in algorithms for computing and operating with the Tucker tensor decomposition, which is frequently used in multidimensional data analysis. Here, we establish communication lower bounds that determine how much data movement is required (under mild conditions) to perform the Multi-TTM computation in parallel. The crux of the proof relies on analytically solving a constrained, nonlinear optimization problem. We also present a parallel algorithm to perform this computation that organizes the processors into a logical grid with twice as many modes as the input tensor. We show that, with correct choices of grid dimensions, the communication cost of the algorithm attains the lower bounds and is therefore communication optimal. Finally, we show that our algorithm can significantly reduce communication compared to the straightforward approach of expressing the computation as a sequence of tensor-times-matrix operations when the input and output tensors vary greatly in size.

HBL-inequalities↗
DOE OSTI2023

Parallel Memory-Independent Communication Bounds for SYRK

In this paper, we focus on the parallel communication cost of multiplying a matrix with its transpose, known as a symmetric rank-k update (SYRK). SYRK requires half the computation of general matrix multiplication because of the symmetry of the output matrix. Recent work (Beaumont et al., SPAA '22) has demonstrated that the sequential I/O complexity of SYRK is also a constant factor smaller than that of general matrix multiplication. Inspired by this progress, we establish memory-independent parallel communication lower bounds for SYRK with smaller constants than general matrix multiplication, and we show that these constants are tight by presenting communication-optimal algorithms. The crux of the lower bound proof relies on extending a key geometric inequality to symmetric computations and analytically solving a constrained nonlinear optimization problem. Here, the optimal algorithms use a triangular blocking scheme for parallel distribution of the symmetric output matrix and corresponding computation.

Communication costs↗
DOE OSTI2025

Communication Lower Bounds and Optimal Algorithms for Symmetric Matrix Computations

In this article, we focus on the communication costs of three symmetric matrix computations: (i) multiplying a matrix with its transpose, known as a symmetric rank-k update (SYRK) (ii) adding the result of the multiplication of a matrix with the transpose of another matrix and the transpose of that result, known as a symmetric rank-2k update (SYR2K) (iii) performing matrix multiplication with a symmetric input matrix (SYMM). All three computations appear in the Level 3 Basic Linear Algebra Subroutines (BLAS) and have wide use in applications involving symmetric matrices. We establish communication lower bounds for these kernels using sequential and distributed-memory parallel computational models, and we show that our bounds are tight by presenting communication-optimal algorithms for each setting. Our lower bound proofs rely on applying a geometric inequality for symmetric computations and analytically solving constrained nonlinear optimization problems. As a result, the symmetric matrix and its corresponding computations are accessed and performed according to a triangular block partitioning scheme in the optimal algorithms.

Al Daas, Hussam [Rutherford Appleton Laboratory, D↗