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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Inverse design for waveguide dispersion with a differentiable mode solver

Inverse design of optical components based on adjoint sensitivity analysis has the potential to address the most challenging photonic engineering problems. However, existing inverse design tools based on finite-difference-time-domain (FDTD) models are poorly suited for optimizing waveguide modes for adiabatic transformation or perturbative coupling, which lies at the heart of many important photonic devices. Among these, dispersion engineering of optical waveguides is especially challenging in ultrafast and nonlinear optical applications involving broad optical bandwidths and frequency-dependent anisotropic dielectric material response. In this work, we develop gradient back-propagation through a general-purpose electromagnetic eigenmode solver and use it to demonstrate waveguide dispersion optimization for second harmonic generation with maximized phase-matching bandwidth. This optimization of three design parameters converges in eight steps, reducing the computational cost of optimization by ∼100x compared to exhaustive search and identifying new designs for broadband optical frequency doubling of laser sources in the 1.3–1.4 µm wavelength range. Furthermore, we demonstrate that the computational cost of gradient back-propagation is independent of the number of parameters, as required for optimization of complex geometries. This technique enables practical inverse design for a broad range of previously intractable photonic devices.

Gray, Dodd (ORCID:000000030469599X)↗
NASA NTRS2005

Proper Criteria of Nonlinear Optical Crystals for Space Laser Systems and the Possible Causes for Space Laser Failures

NASA is striving to develop a scientific understanding of the universe and the Earth-Sun System and its response to natural or human-induced changes. Space lasers are vital tools for NASA's missions to advance our understanding of space research and improving our prediction capability for climate, weather, and natural hazards. Unfortunately, several past space missions that utilized lasers proved to be short-lived and unreliable. In this paper, we are reporting the results of our investigations on several nonlinear optical crystals, which are vital components in space lasers. Examples of these investigations are: The correlation of the phase diagrams of nonlinear crystals and its durability, the effect of radiating these crystals by high-energy beams of protons and gamma on their second harmonic efficiency, and measurements of the high-energy and low-energy thresholds for each crystal before and after irradiation. A set of proper criteria for these crystals will be presented. We will also discuss the possible causes of failures in a space laser and propose a solution to a contamination problem in all future space lasers.

Abdeldayem, Hossin A.↗
NASA NTRS1998

Syntheses of Octasubstituted Metal Phthalocyanines for Nonlinear Optics

Many organic materials can be used as nonlinear optical media. Phthalocyanines are of special interest because they show an unusually large third order nonlinear response, they are thermally and photochemically stable and they can be formed into oriented thin films (Langmuir-Blodgett films). They also can be easily complexed by a large variety of metals, which place them at the interface between organics and organometallics, and allows for fine tuning of the macro cycle electronic properties by the coordinated metal and substituent groups. A series of 1,4,8,11,15,18,22,25-octaalkoxy metal-free and metal phthalocyanines and 2,3,9,10,16,17,23,24-octaalkoxy metal phthalocyanines has been synthesized. Their nonlinear optical properties have been measured. The physical properties of all the phthalocyanines synthesized in this work are subject to both acid and solvent effects.

Guo, Huaisong↗
NASA NTRS2016

CLARREO: Reference Inter-Calibration on Orbit With Reflected Solar Spectrometer

The CLARREO approach for reference intercalibration is based on obtaining coincident highly accurate spectral reflectance and reflected radiance measurements, and establish an on-orbit reference for existing Earth viewing reflected solar radiation sensors: CERES and VIIRS on JPSS satellites, AVHRR and follow-on imagers on MetOp, and imagers on GEO platforms. The mission goal is to be able to provide CLARREO RS reference observations that are matched in space, time, and viewing angles with measurements from the aforementioned instruments, with sampling sufficient to overcome the random error sources from imperfect data matching and instrument noise. The intercalibration method is to monitor over time changes in targeted sensor response function parameters: effective offset, gain, nonlinearity, spectral degradation, and sensitivity to polarization of optics.

Lukashin, C.↗
DOE OSTI2021

Comprehensive anisotropic linear optical properties of the Weyl semimetals TaAs and NbAs

We report that TaAs and NbAs are two of the earliest identified Weyl semimetals that possess many intriguing optical properties, such as chirality-dependent optical excitations and giant second harmonic generation (SHG). Linear and nonlinear optics have been employed as tools to probe the Weyl physics in these crystals. Here, we extend these studies to address two important aspects: determining the complete anisotropic dielectric response and exploring if and how they can reveal essential Weyl physics. We determine the complete anisotropic dielectric functions of TaAs and NbAs by combining spectroscopic ellipsometry and density functional theory (DFT). Parameterized Lorentz oscillators are reported from 1.2 - 6 eV (experiment) and 0 - 6 eV (DFT), and good agreement is shown between them. Both linear and nonlinear optical properties have been previously reported to reveal Weyl physics. We suggest that strong optical resonances from trivial bands are the likely origin of the large optical SHG previously reported at these energies. Furthermore, by comparing the contribution of a small k -space centered around the Weyl cones with the total linear dielectric function, we find that these contributions are highly anisotropic and are < 25% of the total dielectric function below 0.5 eV; abov1 eV, these contributions are negligible. Thus, the study of Weyl physics using optical techniques requires very low energies, and even there, a careful assessment is required in distinguishing the much smaller contributions of the Weyl bands from the dominant contributions of the trivial bands and Drude response to the total dielectric function.

36 MATERIALS SCIENCE↗
NASA NTRS2013

Chaos and Beyond in a Water Filled Ultrasonic Resonance System

Finite amplitude ultrasonic wave resonances in a one-dimensional liquid-filled cavity, formed by a narrow band transducer and a plane reflector, are reported. The resonances are observed to include not only the expected harmonic and subharmonic signals (1,2) but chaotic signals as well. The generation mechanism requires attaining a threshold value of the driving amplitude that the liquid-filled cavity system becomes sufficiently nonlinear in response. The nonlinear features of the system were recently investigated via the construction of an ultrasonic interferometer having optical precision. The transducers were compressional, undamped quartz and lithium niobate crystals having the frequency range 1-10 MHz, driven by a high power amplifier. Both an optical diffraction system to characterize the diffraction pattern of laser light normally incident to the cavity and a receiving transducer attached to an aligned reflector with lapped flat and parallel surfaces were used to assess the generated resonance response in the cavity. At least 5 regions of excitation are identified.

Lazlo, Adler↗
DOE OSTI2021

Measurement and control of optical nonlinearities in dispersive dielectric multilayers

Dispersive dielectric multilayer mirrors, high-dispersion chirped mirrors in particular, are widely used in modern ultrafast optics to manipulate spectral chirps of ultrashort laser pulses. Dispersive mirrors are routinely designed for dispersion compensation in ultrafast lasers and are assumed to be linear optical components. In this work, we report the experimental characterization of an unexpectedly strong nonlinear response in these chirped mirrors. At modest peak intensities <2 TW/cm 2 —well below the known laser-induced damage threshold of these dielectric structures—we observed a strong reflectivity decrease, local heating, transient spectral modifications, and time-dependent absorption of the incident pulse. Through computational analysis, we found that the incident laser field can be enhanced by an order of magnitude in the dielectric layers of the structure. The field enhancement leads to a wavelength-dependent nonlinear absorption, that shows no signs of cumulative damage before catastrophic failure. The nonlinear absorption is not a simply two-photon process but instead is likely mediated by defects that facilitate two-photon absorption. To mitigate this issue, we designed and fabricated a dispersive multilayer design that strategically suppresses the field enhancement in the high-index layers, shifting the high-field regions to the larger-bandgap, low-index layers. This strategy significantly increases the maximum peak intensity that the mirror can sustain. However, our finding of an onset of nonlinear absorption even at ‘modest’ fluence and peak intensity has significant implications for numerous past published experimental works employing dispersive mirrors. Additionally, our results will guide future ultrafast experimental work and ultrafast laser design.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Directionally asymmetric nonlinear optics in ferrorotational MnTiO 3

Ferrorotationally ordered materials possess a planar chiral lattice of electric dipoles that are a promising platform for exhibiting directionally asymmetric response functions. A key characteristic of such order is that enantiomorph conversion occurs when the solid is flipped by 180° about an in-plane axis. Using second-harmonic interferometry, we show here that when circularly polarized light is incident on MnTiO 3 , the generated harmonic intensity depends on whether the incident light is parallel or antiparallel to the ferrorotational axis. For a particular photon helicity, constructive interference occurs in one direction while destructive interference occurs in the other. Here, our observations represent a fundamentally new optical effect, associated with circularly polarized light in ferrorotational solids, that is distinct from conventional optical activity observed in three-dimensional chiral and Faraday media.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

A strategy to identify materials exhibiting a large nonlinear phononics response: tuning the ultrafast structural response of LaAlO 3 with pressure

We use theory and first-principles calculations to investigate how structural changes induced by ultrafast optical excitation of infrared-active phonons change with hydrostatic pressure in LaAlO 3 . Our calculations show that the observed structural changes are sensitive to pressure, with the largest changes occurring at pressures near the boundary between the cubic perovskite and rhombohedral phases. We rationalize our findings by defining a figure of merit that depends only on intrinsic materials quantities, and show that the peak response near the phase boundary is dictated by different microscopic materials properties depending on the particular phonon mode being excited. Our work demonstrates how it is possible to systematically identify materials that may exhibit particularly large changes in structure and properties due to optical excitation of infrared-active phonons.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

High-pressure control of optical nonlinearity in the polar Weyl semimetal TaAs

The transition metal monopnictide family of Weyl semimetals recently has been shown to exhibit anomalously strong second-order optical nonlinearity, which is theoretically attributed to a highly asymmetric polarization distribution induced by their polar structure. We experimentally test this hypothesis by measuring optical second harmonic generation (SHG) from TaAs across a pressure-tuned polar-to-nonpolar structural phase transition. Despite the high-pressure structure remaining noncentrosymmetric, the SHG yield is reduced by more than 60% by 20 GPa as compared to the ambient pressure value. By examining the pressure dependence of distinct groups of SHG susceptibility tensor elements, we find that the yield is primarily controlled by a single element that governs the response along the polar axis. Furthermore, our results confirm a connection between the polar axis and the giant optical nonlinearity of Weyl semimetals and demonstrate pressure as a means to tune this effect in situ.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

The ultrafast Kerr effect in anisotropic and dispersive media

The ultrafast optical Kerr effect (OKE) is widely used to investigate the structural dynamics and interactions of liquids, solutions, and solids by observing their intrinsic nonlinear temporal responses through nearly collinear four-wave mixing. Non-degenerate mixing schemes allow for background free detection and can provide information on the interplay between a material’s internal degrees of freedom. Here, we show a source of temporal dynamics in the OKE signal that is not reflective of the internal degrees of freedom but arises from a group index and momentum mismatch. It is observed in two-color experiments on condensed media with sizable spectral dispersion, a common property near an optical resonance. In particular, birefringence in crystalline solids is able to entirely change the character of the OKE signal via the off-diagonal tensor elements of the nonlinear susceptibility. We develop a detailed description of the phase-mismatched ultrafast OKE and show how to extract quantitative information on the spectrally resolved birefringence and group index from time-resolved experiments in one and two dimensions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Novel vertically aligned nanocomposite of Bi 2 WO 6 -Co 3 O 4 with room-temperature multiferroic and anisotropic optical response

A review of a new vertically aligned nanocomposite (VAN) structure based on two-dimensional (2D) layered oxides has been designed and self-assembled on both LaAlO 3 (001) and SrTiO 3 (001) substrates. The new VAN structure consists of epitaxially grown Co 3 O 4 nanopillars embedded in the Bi 2 WO 6 matrix with a unique 2D layered structure, as evidenced by the microstructural analysis. Physical property measurements show that the new Bi 2 WO 6 -Co 3 O 4 VAN structure exhibits strong ferromagnetic and piezoelectric response at room temperature as well as anisotropic permittivity response. This work demonstrates a new approach in processing multifunctional VANs structure based on the layered oxide systems towards future nonlinear optics, ferromagnets, and multiferroics.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Active and tunable nanophotonic metamaterials

Abstract Metamaterials enable subwavelength tailoring of light–matter interactions, driving fundamental discoveries which fuel novel applications in areas ranging from compressed sensing to quantum engineering. Importantly, the metallic and dielectric resonators from which static metamaterials are comprised present an open architecture amenable to materials integration. Thus, incorporating responsive materials such as semiconductors, liquid crystals, phase-change materials, or quantum materials (e.g., superconductors, 2D materials, etc.) imbue metamaterials with dynamic properties, facilitating the development of active and tunable devices harboring enhanced or even entirely novel electromagnetic functionality. Ultimately, active control derives from the ability to craft the local electromagnetic fields; accomplished using a host of external stimuli to modify the electronic or optical properties of the responsive materials embedded into the active regions of the subwavelength resonators. We provide a broad overview of this frontier area of metamaterials research, introducing fundamental concepts and presenting control strategies that include electronic, optical, mechanical, thermal, and magnetic stimuli. The examples presented range from microwave to visible wavelengths, utilizing a wide range of materials to realize spatial light modulators, effective nonlinear media, on-demand optics, and polarimetric imaging as but a few examples. Often, active and tunable nanophotonic metamaterials yield an emergent electromagnetic response that is more than the sum of the parts, providing reconfigurable or real-time control of the amplitude, phase, wavevector, polarization, and frequency of light. The examples to date are impressive, setting the stage for future advances that are likely to impact holography, beyond 5G communications, imaging, and quantum sensing and transduction.

36 MATERIALS SCIENCE↗
NASA NTRS1984

Nonlinear self-contraction of electron waves

Laboratory evidence is presented of modulationally unstable electron wave packets which can be described by a nonlinear geometrical optics theory. Growth times for self-contraction are found to be much faster than ion response times and the bursts do not appear to be related to Zakharov Langmuir-wave collapse.

Intrator, T.↗
NASA NTRS1995

Absorbing Boundary Conditions For Optical Pulses In Dispersive, Nonlinear Materials

This paper will present results in computational nonlinear optics. An algorithm will be described that provides absorbing boundary conditions for optical pulses in dispersive, nonlinear materials. A new numerical absorber at the boundaries has been developed that is responsive to the spectral content of the pulse. Also, results will be shown of calculations of 2-D electromagnetic nonlinear waves computed by directly integrating in time the nonlinear vector Maxwell's equations. The results will include simulations of "light bullet" like pulses. Here diffraction and dispersion will be counteracted by nonlinear effects. Comparisons will be shown of calculations that use the standard boundary conditions and the new ones.

Goorjian, Peter M.↗
NASA NTRS2011

Subharmonics, Chaos, and Beyond

While studying finite amplitude ultrasonic wave resonance in a one dimensional liquid-filled cavity, which is formed by a narrow band transducer and a plane reflector, subharmonics of the driver's frequency were observed in addition to the expected harmonic structure. Subsequently it was realized that the system was one of the many examples where parametric resonance takes place and in which the observed subharmonics are parametrically generated. Parametric resonance occurs in any physical system which has a periodically modulated natural frequency. The generation mechanism also requires a sufficiently high threshold value of the driving amplitude so that the system becomes increasingly nonlinear in response. The nonlinear features were recently investigated and are the objective of this presentation. An ultrasonic interferometer with optical precision was built. The transducers were compressional undamped quartz and Lithium Niobate crystals ranging from 1-10 Mhz, and driven by a high power amplifier. Both an optical diffraction system and a receive transducer attached to an aligned reflector with lapped flat and parallel surfaces were used to observe the generated frequency components in the cavity.

Adler, Laszlo↗
DOE OSTI2023

Uncovering extreme nonlinear dynamics in solids through time-domain field analysis

Time-domain analysis of harmonic fields with sub-cycle resolution is now experimentally viable due to the emergence of sensitive, on-chip techniques for petahertz-scale optical-field sampling. We demonstrate how such a time-domain, field-resolved analysis uncovers the extreme nonlinear electron dynamics responsible for high-harmonic generation within solids. Time-dependent density functional theory was used to simulate harmonic generation from a solid-state band-gap system driven by near- to mid-infrared waveforms. Particular attention was paid to regimes where both intraband and interband emission mechanisms play a critical role in shaping the nonlinear response. We show that a time-domain analysis of the harmonic radiation fields identifies the interplay between intra- and interband dynamical processes underlying the nonlinear light generation. With further analysis, we show that changes to the dominant emission regime can occur after only slight changes to the peak driving intensity and central driving wavelength. Furthermore, time-domain analysis of harmonic fields also reveals, for the first time, the possibility of rapid changes in the dominant emission mechanism within the temporal window of the driving pulse envelope. Finally, we examine the experimental viability of performing time-domain analysis of harmonic fields with sub-cycle resolution using realistic parameters.

74 ATOMIC AND MOLECULAR PHYSICS↗