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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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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

High-Density Polyethylene Custom Focusing Lenses for High-Resolution Transient Terahertz Biomedical Imaging Sensors

Transient terahertz time-domain spectroscopy (THz-TDS) imaging has emerged as a novel non-ionizing and noninvasive biomedical imaging modality, designed for the detection and characterization of a variety of tissue malignancies due to their high signal-to-noise ratio and submillimeter resolution. We report our design of a pair of aspheric focusing lenses using a commercially available lens-design software that resulted in about 200 × 200-μm2 focal spot size corresponding to the 1-THz frequency. The lenses are made of high-density polyethylene (HDPE) obtained using a lathe fabrication and are integrated into a THz-TDS system that includes low-temperature GaAs photoconductive antennae as both a THz emitter and detector. The system is used to generate high-resolution, two-dimensional (2D) images of formalin-fixed, paraffin-embedded murine pancreas tissue blocks. The performance of these focusing lenses is compared to the older system based on a pair of short-focal-length, hemispherical polytetrafluoroethylene (TeflonTM) lenses and is characterized using THz-domain measurements, resulting in 2D maps of the tissue refractive index and absorption coefficient as imaging markers. For a quantitative evaluation of the lens effect on the image resolution, we formulated a lateral resolution parameter, R2080, defined as the distance required for a 20–80% transition of the imaging marker from the bare paraffin region to the tissue region in the same image frame. The R2080 parameter clearly demonstrates the advantage of the HDPE lenses over TeflonTM lenses. The lens-design approach presented here can be successfully implemented in other THz-TDS setups with known THz emitter and detector specifications.

47 OTHER INSTRUMENTATION↗
DOE OSTI2025

Efficient Broadband Terahertz Generation by Above‐Band‐Gap Excitation of the Pyroelectric ZnSnN 2

Terahertz (THz) radiation is a powerful probe of low-energy excitations in all phases of matter. However, it remains a challenge to find materials that efficiently generate THz radiation in a broad range of frequencies following optical excitation. Here, we investigate a pyroelectric material, ZnSnN 2 , and find that its above-band-gap excitation results in the efficient formation of an ultrafast photocurrent generating THz radiation. The resulting THz electric field spans a frequency range from below 1 THz to above 30 THz. The results suggest that the photocurrent is primarily driven by an ultrafast pyroelectric effect where the photo-excited carriers screen the spontaneous electric polarization of ZnSnN 2 . Strong structural disorder reduces the photocarrier lifetime significantly and, thus, enables broadband operation. ZnSnN2 shows a similar THz-emitter performance as the best spintronic THz emitters regarding bandwidth and amplitude. The study unveils the large potential of pyroelectric materials as efficient and broadband THz emitters with built-in bias fields.

36 MATERIALS SCIENCE↗
DOE OSTI2023

More Than 30 000-fold Field Enhancement of Terahertz Nanoresonators Enabled by Rapid Inverse Design

The rapid development of 6G communications using terahertz (THz) electromagnetic waves has created a demand for highly sensitive THz nanoresonators capable of detecting these waves. Among the potential candidates, THz nanogap loop arrays show promising characteristics but require significant computational resources for accurate simulation. This requirement arises because their unit cells are 10 times smaller than millimeter wavelengths, with nanogap regions that are 1 000 000 times smaller. To address this challenge, we propose a rapid inverse design method using physics-informed machine learning, employing double deep Q-learning with an analytical model of the THz nanogap loop array. In ~39 h on a middle-level personal computer, our approach identifies the optimal structure through 200 000 iterations, achieving an experimental electric field enhancement of 32 000 at 0.2 THz, 300% stronger than prior results. In conclusion, our analytical model-based approach significantly reduces the amount of computational resources required, offering a practical alternative to numerical simulation-based inverse design for THz nanodevices.

21 terahertz time-domain spectroscopy↗
DOE OSTI2021

Distinguishing bulk and surface recombination in CdTe thin films and solar cells using time-resolved terahertz and photoluminescence spectroscopies

Understanding the nature of recombination and its dependence on defects and interfaces is essential for engineering materials and contacts for higher open circuit voltage (V oc ) and power conversion efficiency in photovoltaic (PV) devices. Time-resolved photoluminescence (TRPL) has conventionally been used to evaluate recombination, but carrier redistribution often dominates the response at short times. In this work, we report on the quantification of carrier dynamics and recombination mechanisms by complementary use of both time-resolved terahertz spectroscopy (TRTS) and TRPL combined with numerical modeling of the continuity equations and Poisson’s equation. We have demonstrated this approach using CdTe thin films. A thin film stack with CdTe fabricated by vapor transport deposition and treated with CdCl 2 exhibited a bulk lifetime of 1.7 ± 0.1 ns, negligible CdTe/CdS interface recombination velocity, and back surface recombination velocity of 6.3 ± 1.3 x10 4 cm/s. In contrast, a film stack without CdCl 2 treatment had a bulk lifetime of only 68 ± 12 ps and a higher interface recombination velocity of 4 ± 2 x10 8 cm/s. By determining the locus and mechanisms of performance-limiting recombination, we can accelerate the development of thin-film PVs with higher V oc and efficiency. While the method has been demonstrated here using CdTe, it is also applicable to perovskites, Cu(InGa)Se 2 , Cu2ZnSn(S,Se) 4, and emerging technologies.

14 SOLAR ENERGY↗
DOE OSTI2025

Monolithic optoelectronic circuit design for on-chip terahertz applications

We demonstrate a monolithic coplanar stripline platform for on-chip terahertz (THz) generation, transmission, and detection, addressing key challenges of mode purity, bandwidth, and referencing. Capacitive coupling of the photoconductive generator switch enforces pure odd-mode propagation, increases THz field strength, and extends the operational frequency range, achieving 0.05–1.4 THz. Our architecture enables fully monolithic fabrication with amorphous silicon switches, in situ field referencing, and galvanic isolation between generation and detection. Finite-element simulations and experiments confirm that suppressing parasitic modes improves signal integrity, providing a robust platform for high-fidelity THz spectroscopy, ultrafast electronics, and nanoscale quantum materials research.

Capacitive coupling↗
DOE OSTI2021

Ultrafast bipolar conductivity driven by intense single-cycle terahertz pulses in a topological insulator Bi 2 Se 3

We present the measurement of ultrafast terahertz (THz) conductivity of a 50 nm thick topological insulator Bi 2 Se 3 at low temperature 5 K by using a THz pump and THz probe spectroscopy scheme. Here, the THz conductivity driven by a single-cycle intense THz pump exhibits a bipolar lineshape and frequency-dependent relaxation dynamics due to the separated surface and bulk charge carriers with distinctly different scattering rates and relaxation times. We also demonstrate THz pump field-dependent excitation of the surface and bulk spectral weights, which suggests one of the optimal THz excitation conditions for generating the most charge carriers from the topology-protected surface state relative to the bulk. This also allows THz control of surface and bulk transport channels in a selective way.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Laser Terahertz Emission Microscope for Imaging Grain Heterogeneity: A Case Study of CH3NH3PbI3 Perovskite Semiconductors

Strong terahertz (THz) emission from the methylammonium lead iodide (MAPbI3) perovskite semiconductors has been observed following above-bandgap photoexcitation, yet local THz responses of crystalline microstructures are absent. We implement laser THz emission microscope (LTEM), yet-to-be applied to the perovskite semiconductors, as a novel and complementary tool to evaluate the electronic and grain heterogeneity of MAPbI3 thin films. Two MAPbI3 samples with different grain sizes are studied. Using this approach, we show that the one with a larger grain size gives more uniform THz radiation. More significant spatial THz intensity fluctuation is observed for the sample with a smaller grain size.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Terahertz Chiral Metamaterials Enabling Broadband Polarization Conversion Using Polarization Guiding Effect

Polarization control plays a vital role in terahertz (THz) photonics, enabling a wide range of applications from imaging and spectroscopy to sensing and wireless communication. However, conventional polarization control methods at THz frequencies are limited by narrow operational bandwidths and excessive absorption losses. In this work, we present a broadband THz polarization control based on the polarization guiding effect. We design and fabricate a THz polarization rotator capable of rotating incident linear polarization by 90°. The device consists of 24 twistingly stacked silicon–air metagrating layers that introduce a large degree of form birefringence, with each metagrating layer being 280 µm thick and sequentially twisted by 3.75°. Numerical simulations using the Berreman 4 × 4 method predict broadband operation of the device. Experimental validation using THz time-domain spectroscopy confirms efficient and broadband 90° polarization rotation from 0.2 to 1.25 THz. Based on this concept, we further propose THz Q-plates for the generation of broadband structured vector beams with radial and azimuthal polarization states. This work offers a scalable, material-agnostic platform for advanced THz polarization manipulation and vector beam engineering, opening new pathways for next-generation THz photonic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2025

High‐Asymmetry Metasurface: A New Solution for Terahertz Resonance via Active Learning‐Augmented Diffusion Model

Terahertz (THz) metamaterials with high‐figure‐of‐merit (high‐FoM) performance resonance are essential for advancing sensors, detectors, and imagers. Conventional designs focus on symmetric or low‐asymmetry geometric structures, leaving high‐asymmetry designs largely unexplored due to the inefficiency of trial‐and‐error‐based rational design. Recent deep learning techniques offer automation and acceleration but are constrained by the need for large datasets inherent to their data‐driven nature. Here, a novel prior knowledge‐guided generative model augmented by a physics‐constrained active learning mechanism to design high‐asymmetry metamaterials. An advanced diffusion model learns features from a small set of classical structures with high‐FoM THz resonance and generates new high‐asymmetry structures. To mitigate the limited number of classical structures, the generated high‐asymmetry structures are actively selected and integrated into the initial training dataset based on their physical characteristics. Experimental results demonstrate the superior resonance performance of the generated high‐asymmetry metamaterials over classical designs, exhibiting improvements exceeding 30% in key resonance metrics. Remarkably, this performance is attained using only 68 classical structures as the initial training dataset, significantly reducing the data requirements for deep learning‐based metamaterial design. The proposed scheme for generating high‐asymmetry structures provides a new effective and efficient solution for high‐FoM resonance, expanding applications in high‐sensitivity THz metadevices.

diffusion model↗
DOE OSTI2026

Ultrabroadband Spacetime Nanoscopy of Terahertz Polaritons in a van der Waals Cavity

Guiding, storing, and processing light at the nanoscale hinges on understanding how polaritons — hybrid quasiparticles of light and matter — propagate and interfere in both space and time. This work introduces a synchrotron-based technique, SYnchrotron SpaceTimE Mapping (SYSTEM), which captures real-time evolution of polariton wave packets with ∼10 nm spatial and sub-100 fs temporal resolution across an ultrabroadband 5–50 THz range. Here, SYSTEM directly visualizes the creation, interference, and decay of multiple high-quality Fabry-Pérot phonon polariton cavity modes in an α-MoO 3 microcavity. These real-space, real-time observations reveal wave-packet dynamics and cavity resonances with record-high quality factors (Q ≈ 100) in the single-digit terahertz regime near 9 THz. SYSTEM thus offers a powerful and broadly applicable platform for probing and engineering ultraslow, deeply subwavelength polaritons, opening new avenues for tailoring light–matter interactions and advancing next-generation THz nanophotonic technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Ultrafast terahertz spectroscopy provides insight into charge transfer efficiency and dynamics in artificial photosynthesis

Terahertz (THz) spectroscopy provides a noncontact method to measure the ultrafast dynamics and photoconductivity of mobile carriers in semiconducting materials. This has proven useful in studying artificial photosynthesis devices which use semiconductor photoelectrodes. In this work, we present a brief introduction to optical-pump THz-probe (OPTP) spectroscopy, a technique that provides unique and useful insight into interfacial electron transfer (from the surface-attached dye to the conduction band of the semiconductor) in dye-sensitized photoelectrochemical cells. Compared with more familiar methods like visible transient absorption spectroscopy, OPTP spectroscopy stands out in offering both sub-picosecond time resolution as well as sensitivity to mobile carriers (electrons and holes) in the semiconductor portion of artificial photosynthesis devices. The mobile carriers are crucial to device performance as only they pass to the other half cell to complete the reaction. In order to highlight these advantages and illustrate the types of questions OPTP spectroscopy can address, we discuss three case studies. In the first, OPTP spectroscopy is used to measure the injection rates from a set of six different dyes, revealing the effect of the energetics and lifetimes of the dye excited states on interfacial electron transfer. The subsequent case studies investigate the influence of varying the moieties which bind to the surface (anchors), as well as the moieties that connect the chromophore with these anchors (linkers). OPTP spectroscopy was used to measure the interfacial electron transfer rate as these moieties were varied.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Terahertz-polarizing effect based on geometric anisotropy of Ti 3 C 2 T x MXene nanosheets

Two-dimensional (2D) nanomaterials have garnered considerable attention due to their unique properties, such as thinness and excellent electronic properties. The inherent anisotropy of 2D nanomaterials, which is characterized by their high aspect ratio, can maximize the directional functionality to enhance the conductivity and utilization of edges. Here, we demonstrate how to utilize the edges of titanium carbide (Ti 3 C 2 T x ) MXene nanosheet to control the linear polarization in the terahertz (THz) frequency range. By a simple and unique vertical alignment exploiting the high surface charge and excellent colloidal dispersibility of MXene nanosheets, MXene nanosheets are vertically oriented along an applied AC electric field. The well-aligned edges justify the THz polarizing effect, revealing outstanding performances such as the broadband coverage in 0.2–1.5 THz and an exceptional extinction ratio reaching 20 dB. Furthermore, we vary the local orientation of the vertical MXene nanosheets by tailoring the applied electric field, enabling spatially controllable polarization capabilities. Our results can provide a generic tool for the utilization of 2D nanomaterials in diverse potential applications in optics and electronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Raman and Terahertz Spectroscopy of Low-Frequency Chiral Phonons in Amino Acids

Chiral phonons are mirror-symmetric vibrations with nonzero angular momenta that correspond to twisting and rotational motions of multiple atoms. In chiral crystals, these include low-energy terahertz (THz)-range vibrations of the molecular segments involving dozens of atoms with energies sensitive to molecular chirality. Here, we present spectral signatures of chiral phonons in circularly polarized Raman optical activity (ROA) spectra from enantiomers of amino acid crystals. Along with complementary THz circular dichroism (TCD) measurements, our ROA data reveal several vibrational bands in enantiomers of valine, alanine, tyrosine, and proline between 30 and 150 cm –1 (∼1–4.5 THz) that exhibit opposite intensities. Density functional theory calculations confirm their assignment to twisting and shearing molecular motions. The simultaneous registration of chiral phonon modes by ROA and TCD demonstrates the necessity of these complementary techniques to identify complex mirror-asymmetric vibrational modes and offers new insights into their interactions with circularly polarized light.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Plasmonic Terahertz Devices and Sensors Based on Carbon Electronics

Tunable terahertz (THz) photonic devices are imperative in a wide range of applications ranging from THz signal modulation to molecular sensing. One of the currently prevailing methods is based on arrays of metallic or dielectric resonators integrated with functional materials in response to an external stimulus, in which for the purpose of sensing the external stimuli may introduce inadvertent undesirable effects into the target samples to be measured. Here we developed an alternative approach by postprocessing nanothickness macro-assembled graphene (nMAG) films with widely tunable THz conductivity, enabling versatile solid-state THz devices and sensors, showing multifunctional nMAG-based applications. The THz conductivities of free-standing nMAGs showed a broad range from 1.2 x 10 3 S/m in reduced graphene oxide before annealing to 4.0 x 10 6 S/m in a nMAG film annealed at 2800 °C. We fabricated nMAG/dielectric/metal and nMAG/dielectric/nMAG THz Salisbury absorbers with broad reflectance ranging from 0% to 80%. The highly conductive nMAG films enabled THz metasurfaces for sensing applications. Taking advantage of the resonant field enhancement arising from the plasmonic metasurface structures and the strong interactions between analyte molecules and nMAG films, we have successfully detected diphenylamine with a limit of detection of 4.2 pg. Those wafer-scale nMAG films present promising potential in high-performance THz electronics, photonics, and sensors.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Origami Terahertz Detectors Realized by Inkjet Printing of Carbon Nanotube Inks

Terahertz (THz) technology has shown promise for several applications, but limitations in sources and detectors have prevented broader adoption. Existing THz detectors are rigid, planar, and fabricated using complex technology, making it difficult to integrate into systems. Here we demonstrate THz detectors fabricated by inkjet printing on submicrometer thick, ultraflexible substrates. By developing p- and n-type carbon nanotube inks, we achieve optically thick p–n junction and p-type devices, enabling antenna-free pixels for THz imaging. By further designing and folding the printed devices, we realize origami-inspired architectures with improved performance over single devices, achieving a noise-equivalent power of 12 nW/Hz 1/2 at room temperature with no voltage bias. Finally, our approach opens avenues for nonplanar, foldable, deployable, insertable, and retractable THz detectors for applications in nondestructive inspection.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Dynamically Tunable Terahertz Emission Enabled by Anomalous Optical Phonon Responses in Lead Telluride

Lead telluride (PbTe), a narrow bandgap semiconductor commonly used in infrared detectors, exhibits anomalous vibrational and structural properties, making it appealing for thermoelectrics. Despite significant fundamental interest in the microscopic origins of its unusual vibrational properties, the optical functionalities stemming from phonons and electron–phonon coupling in PbTe have not been closely investigated. This work reports measurements of terahertz (THz) radiation from a PbTe single crystal following ultrafast optical excitation and investigates IR-active phonon responses as a function of excitation fluence and temperature. We uncover a spectrally tunable THz emission peak enabled by an epsilon-near-zero response of the coupled plasmon–longitudinal optical phonon mode that can be dynamically shifted via tuning photocarrier density. Spectral tunability (Δω/ω = 25%) is significant and beyond what has been achieved by any other THz emitter. In addition, the emitted THz fields reveal signatures of a zone center transverse optical phonon anomaly and unveil a new mode at 0.3 THz that diminishes in amplitude under increasing photocarrier density. Temperature-dependent measurements suggest that the transverse-like modes at 1 and 1.5 THz are possibly of different origins. These results indicate that the unusual phononic properties in PbTe are tunable via photoexcitation and enable new optical functionalities in THz applications, such as spectrally tunable emitters and all-optical modulators.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Terahertz Nanoimaging of Perovskite Solar Cell Materials

Direct visualization and quantitative evaluation of charge filling in grain boundary (GB) traps of hybrid metal halide perovskites require dynamic conductivity imaging simultaneously at the terahertz (THz) frequency and nanometer (nm) spatial scales not accessible by conventional transport and imaging methods used thus far. Here, in this study, we apply a THz near-field nanoconductivity mapping to the archetypal metal halide perovskite photovoltaic films and demonstrate that it is a powerful tool to reveal distinct dielectric heterogeneity due to charge trapping and degradation at the single GB level. Our approach visualizes the filled defect ion traps by local THz charge conductivity and allows for extracting a quantitative profile of trapping density in the vicinity of GBs with sub-20 nm resolution. Furthermore, imaging material degradation by tracking local nanodefect distributions overtime identifies a distinct degradation pathway that starts from the GBs and propagates inside the grains over time. The single GB, nano-THz conductivity imaging demonstrated here can be extended to benchmark various perovskite materials and devices for their global photoenergy conversion performance and local charge transfer proprieties of absorbers and interfaces.

14 SOLAR ENERGY↗
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↗