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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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450 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Highly efficient small molecule multi-junction organic photovoltaic cells

A highly efficient multi junction photovoltaic device, such as a two, three, or four junction device, is disclosed. The multi-junction device may include a first subcell comprising a first photoactive region and a second subcell comprising a second photoactive region. The first and second photoactive regions are designed to minimize spectral overlap and maximize photocurrent across a broad absorption spectra, such as wavelengths ranging from 400 nm to 900 nm. The device may further include an inter-connecting layer, disposed between the first subcell and the second subcell, that is at least substantially transparent. By introducing a transparent interconnecting layer, a dual element (tandem) cell achieves a power conversion efficiency of 10.0±0.5%. By adding an additional (3rd) sub-cell that absorbs at the second order optical interference maximum within the stack. The triple junction cell significantly improves the quantum efficiency at shorter wavelengths, achieving a power conversion efficiency of 11.1±0.5%. Adding additional sub-cells has been shown to increase power conversion efficiency above 12%.

Che, Xiaozhou↗
DOE OSTI2023

Observing Charge Separation Distance in Organic Donor-Acceptors with Photoinduced Absorption Detected Magnetic Resonance Spectroscopy

The debatable role of charge transfer states in photoinduced charge separation to useful free carriers has been a long-standing controversy in the field of organic solar cells. In this work, varying charge-pair distances are measured in sensitized donor-acceptor thin films using a photoinduced absorption-detected magnetic resonance (PADMR) technique where nearest-neighbor charge transfer states are identified by their strong magnetic dipole-coupling relative to more distant charges. Correlation with the direct measurement of free carriers using time-resolved microwave conductivity has corroborated a distributed-range electron transfer model in which charges generated at longer distances become free carriers, while shorter transfer distances give only bound charge transfer states that cannot separate to generate photocurrent.

charge transfer↗
DOE OSTI2026

Calibration and Data Analysis of a Frequency Selectable Laser Source for CMB Detector Characterization

Cosmic Microwave Background (CMB) experiments study faint radiation left over from the early universe. The CMB was created when the universe became cool enough for light to travel freely through space, and today it gives scientists one of the earliest images of the universe. One important goal of modern CMB experiments is to measure this radiation with higher precision in order to search for evidence that supports the theory of cosmic inflation. To do this, scientists use extremely sensitive detectors that must be calibrated accurately. The Frequency Selectable Laser Source, or FLS, is a new calibration tool that can send selected frequencies to detectors and help measure their response. During my internship, I worked on the FLS after it returned to Fermilab from Chile, where it had been used to characterize detectors at the Simons Observatory. The system came back in parts, so the first part of my project was helping rebuild the optical and mechanical setup. After the system was rebuilt, we performed alignments to maximize the receiver photocurrent. We then collected calibration measurements over different frequency ranges, including 543 GHz to 568 GHz, 740 GHz to 766 GHz, and 60 GHz to 500 GHz. These measurements were used to check waterline calibration and reflectivity features and compare new data with previous data. Another major part of my project was learning Python so I could understand previous analysis code, modify it for new files, and write my own code to compare the mean response between datasets. The results showed that the new data was close to previous measurements and that waterline features near 556 GHz and 752 GHz were found within less than 1.5 GHz of the expected values. I also completed the reflectivity analysis for five prisms in two polarization orientations. In the original orientation, the results were consistent between the five prisms and close to values measured on a different system at the University of Chicago. I then collected a second set of measurements on my own with the polarization of the laser rotated by 90 degrees and compared them with the original data using the same Python workflow. The measured reflectivity increased for all five prisms in the new orientation, showing that the prism reflectivity depends on polarization. Future work will focus on using the FLS to characterize real CMB detectors.

Pumarino Meza, Rafael [Unlisted, US; Fermilab]↗
DOE OSTI2026

Calibration and Data Analysis of a Frequency Selectable Laser Source for CMB Detector Characterization

Cosmic Microwave Background (CMB) experiments study faint radiation left over from the early universe. The CMB was created when the universe became cool enough for light to travel freely through space, and today it gives scientists one of the earliest images of the universe. One important goal of modern CMB experiments is to measure this radiation with higher precision in order to search for evidence that supports the theory of cosmic inflation. To do this, scientists use extremely sensitive detectors that must be calibrated accurately. The Frequency Selectable Laser Source, or FLS, is a new calibration tool that can send selected frequencies to detectors and help measure their response. During my internship, I worked on the FLS after it returned to Fermilab from Chile, where it had been used to characterize detectors at the Simons Observatory. The system came back in parts, so the first part of my project was helping rebuild the optical and mechanical setup. After the system was rebuilt, we performed alignments to maximize the receiver photocurrent. We then collected calibration measurements over different frequency ranges, including 543 GHz to 568 GHz, 740 GHz to 766 GHz, and 60 GHz to 500 GHz. These measurements were used to check waterline calibration and reflectivity features and compare new data with previous data. Another major part of my project was learning Python so I could understand previous analysis code, modify it for new files, and write my own code to compare the mean response between datasets. The results showed that the new data was close to previous measurements and that waterline features near 556 GHz and 752 GHz were found within less than 1.5 GHz of the expected values. I also completed the reflectivity analysis for five prisms in two polarization orientations. In the original orientation, the results were consistent between the five prisms and close to values measured on a different system at the University of Chicago. I then collected a second set of measurements on my own with the polarization of the laser rotated by 90 degrees and compared them with the original data using the same Python workflow. The measured reflectivity increased for all five prisms in the new orientation, showing that the prism reflectivity depends on polarization. Future work will focus on using the FLS to characterize real CMB detectors.

Pumarino Meza, Rafael [Unlisted, US; Fermilab]↗
DOE OSTI2020

Monolayer Semiconductor Auger Detector

Auger recombination in semiconductors is a many-body phenomenon in which the recombination of electrons and holes is accompanied by excitation of other charge carriers. The excess energy of the excited carriers is normally rapidly converted to heat, making Auger processes difficult to probe directly. In this paper, we employ a technique in which the Auger-excited carriers are detected by their ability to tunnel out of the semiconductor through a thin barrier, generating a current. We use vertical van der Waals heterostructures with monolayer WSe 2 as the semiconductor, with hexagonal boron nitride as the tunnel barrier, and a graphite collector electrode. The Auger processes combined with resonant absorption produce characteristic negative photoconductance. We detect holes Auger-excited by both neutral and charged excitons and find that the Auger scattering is surprisingly strong under weak excitation. Our work expands the range of techniques available for probing relaxation processes in 2D materials.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Phase Transition in a Memristive Suspended MoS 2 Monolayer Probed by Opto- and Electro-Mechanics

Semiconducting monolayers of a 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H–1T' phase transition in a suspended 2D monolayer membrane of MoS 2 . Electronic transport shows unexpected memristive properties in the MoS 2 membrane, in the absence of any external dopants. A strong mechanical softening of the membrane is measured concurrently and may only be related to the 2H–1T' phase transition, which imposes a 3% directional elongation of the topological 1T' phase with respect to the semiconducting 2H. We note that only a few percent 2H–1T' phase switching is sufficient to observe measurable memristive effects. Our experimental results combined with first-principles total energy calculations indicate that sulfur vacancy diffusion plays a key role in the initial nucleation of the phase transition. Our study clearly shows that nanomechanics represents an ultrasensitive technique to probe the crystal phase transition in 2D materials or thin membranes. Lastly, a better control of the microscopic mechanisms responsible for the observed memristive effect in MoS 2 is important for the implementation of future devices.

2D materials↗
DOE OSTI2023

Ultrafast Carrier Drift Transport Dynamics in CsPbI3 Perovskite Nanocrystalline Thin Films

We study the early time carrier drift dynamics in CsPbI3 nanocrystal thin films with a sub 25 ps time resolution. Prior to trapping, carriers exhibit band-like transport characteristics, which is similar to those of traditional semiconductor solar absorbers including Si and GaAs due to optical phonon and carrier scattering at high temperatures. In contrast to the popular polaron scattering mechanism, the CsPbI3 nanocrystal thin film demonstrates the strongest optical phonon scattering mechanism among other inorganic-organic hybrid perovskites, Si, and GaAs. This ultrafast dynamics study establishes a foundation for understanding the fundamental carrier drift properties that drive perovskite nanocrystal optoelectronics.

CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS,S↗
DOE OSTI2025

Carrier-Envelope Phase Control in Terahertz Pulse Generation Using InAs Ribbon Metasurfaces

Generation of broadband terahertz (THz) pulses with variable polarization and carrier-envelope phase can enable the tailoring of THz beam wavefronts for advanced applications in THz imaging and spectroscopy and for strong THz field optics. While metasurfaces composed of deeply subwavelength THz emitters have recently been demonstrated to define the polarization and spatial profile of the generated THz fields, precise phase control or synthesis of THz pulse waveforms remains a challenging problem. Here, we propose and demonstrate metasurfaces composed of indium arsenide (InAs) nanoscale ribbon arrays capable of generating THz pulses with variable carrier-envelope phase. We show that different THz generation mechanisms, each contributing distinct phases, can be activated in the ribbons, enabling carrier-envelope phase control spanning a range of π over a wide band of frequencies (∼1–3 THz). This is achieved solely through the ribbon array geometry using linearly polarized optical excitation of the ribbons. The arrays enable precise control of the THz phase and amplitude, opening the door to advanced structured THz wavefront synthesis using ultrathin dielectric metasurfaces.

carrier-envelope phase↗
DOE OSTI2022

Photoinduced electric effects in various plasmonic materials

Photoinduced voltages associated with surface plasmon polariton excitations are studied both theoretically and experimentally in various plasmonic systems as the function of material, wavelength, and type of structure. Experimental photovoltage normalized to the absorbed power shows a general decrease upon an increase in the wavelength, enhancement in the nanostructured samples, and a strong variation in the magnitude as a function of the material, which are not in line with the theoretical predictions of the simple plasmonic pressure approach. Furthermore, the results can be used for clarification of the mechanisms and further development of an adequate theoretcial approach to the plasmon drag effect.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Franck-Condon electron emission from polar semiconductor photocathodes

An analytical formulation of (optical-)phonon-mediated and momentum-resonant Franck-Condon emission of photoexcited electrons from polar semiconductors is shown to be very consistent with (i) the observed emission properties of a cesiated Ga⁢As⁢(001) photocathode at 808 nm [J. Phys. D: Appl. Phys. 54, 205301 (2021)] and (ii) the measured spectral emission properties of a Ga⁢N(0001) photocathode from just below its bandgap energy to 5 eV. The theoretical analysis in the parabolic band approximation predicts the form of both the quantum efficiency and mean transverse energy of photoemission as a function of the photocathode’s electron affinity and the electron temperature in the vicinity of its emission face. The good agreement between theory and experimental data also suggests that sub-10-nm rms surface roughness effects are not significant for polar semiconductor photocathodes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Spectral emission properties of a nitrogen-doped diamond (001) photocathode: Hot electron transport and transverse momentum filtering

The electron emission properties of a single-crystal nitrogen-doped diamond(001) photocathode inserted in a 10 kV DC photoelectron gun are determined using a tunable (235–410 nm) ultraviolet laser radiation source for photoemission from both the back nitrogen-doped substrate face and the front homoepitaxially grown and undoped diamond crystal face. Here, the measured spectral trends of the mean transverse energy and quantum efficiency of the emitted electrons are both anomalous and nonmonotonic, but are shown to be consistent with (i) the known physics of electron photoexcitation from the nitrogen substitution states into the conduction bands of diamond, (ii) the energy position and dispersion characteristics of the conduction bands of diamond in the (001) emission direction, (iii) the effective electron affinity of the crystal faces, (iv) the strong electron-(optical)phonon coupling in diamond, and (v) the associated hot electron transport dynamics under energy equipartition with the optical phonons. Notably, the observed hot electron emission is shown to be restricted parallel to the photocathode surface by the low transverse effective masses of the emitting band states—a transverse momentum filtering effect.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Absence of quantization in the circular photogalvanic effect in disordered chiral Weyl semimetals

The circularly polarized photogalvanic effect (CPGE) is studied in chiral Weyl semimetals with short-range quenched disorder. Without disorder, the topological properties of chiral Weyl semimetals lead to quantization of the CPGE, which is a second-order optical response. Furthermore, using a combination of diagrammatic perturbation theory in the continuum and exact numerical calculations via the kernel polynomial method on a lattice model, we show that disorder perturbatively destabilizes the quantization of the CPGE.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Strong-Field Bloch Electron Interferometry for Band-Structure Retrieval

When Bloch electrons in a solid are exposed to a strong optical field, they are coherently driven in their respective bands where they acquire a quantum phase as the imprint of the band shape. If an electron approaches an avoided crossing formed by two bands, it may be split by undergoing a Landau-Zener transition. We here employ subsequent Landau-Zener transitions to realize strong-field Bloch electron interferometry (SFBEI), allowing us to reveal band structure information. In particular, we measure the Fermi velocity (band slope) of graphene in the vicinity of the K points as 1.07±0.04 nm fs –1 . As a result, we expect SFBEI for band structure retrieval to apply to a wide range of material systems and experimental conditions, making it suitable for studying transient changes in band structure with femtosecond temporal resolution at ambient conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1996

Improved Saturation Performance in High Speed Waveguide Photodetectors at 1.3 ??sing an Asymmetric InA1GaAs/InGaAsP Structure

Waveguide photodetector (WGPD) results have recently been presented demonstrating the very large bandwidth-efficiency product potential of these devices. Improved saturation and linearity characteristics are realized in waveguide p-i-n photodetectors at 1.3 ??y using an asymmetric cladding structure with InA1GaAs/InGaAsP in the anode and InGaAsP in the cathode.

multi-mode structures photocurrent saturation wave↗
NASA NTRS2013

On Using Intensity Interferometry for Feature Identification and Imaging of Remote Objects

We derive an approximation to the intensity covariance function of two scanning pinhole detectors, facing a distant source (e.g., a star) being occluded partially by an absorptive object (e.g., a planet). We focus on using this technique to identify or image an object that is in the line-of-sight between a well-characterized source and the detectors. We derive the observed perturbation to the intensity covariance map due to the object, showing that under some reasonable approximations it is proportional to the real part of the Fourier transform of the source's photon-flux density times the Fourier transform of the object's intensity absorption. We highlight the key parameters impacting its visibility and discuss the requirements for estimating object-related parameters, e.g., its size, velocity or shape. We consider an application of this result to determining the orbit inclination of an exoplanet orbiting a distant star. Finally, motivated by the intrinsically weak nature of the signature, we study its signal-to-noise ratio and determine the impact of system parameters.

quantum optics↗