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At least 163 records · Page 9

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DOE OSTI2025

Long-Lived Population Inversion in Resonantly Driven Excitonic Antiferromagnet

Van der Waals magnets are an emerging material family for investigating light-matter interactions and spin-correlated excitations. Here, we report the discovery of a photo-induced state with a lifetime of 17 ps in the van der Waals antiferromagnet NiPS 3 , which appears exclusively with resonant pumping at 1.476 eV in the antiferromagnetic state. Furthermore, the long-lived state comes with a negative photoconductivity, a characteristic optical response of population inversion. Our findings demonstrate a promising pathway to potentially achieve long-lived lasing at terahertz frequencies in reduced dimensions.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Floquet engineering of polaritonic amplification in dispersive photonic time crystals

In this study, we investigate the dynamics of dispersive photonic time crystals (PTCs) and their potential applications for controlling light-matter interaction. Using the Lorentz-Drude as dispersive material model, we analyze the effects of periodic modulation of dispersion parameters through both theoretical and numerical analysis, revealing the emergence of hybrid bandgaps from the interaction of polaritonic branches with distinctive characteristics. Our study demonstrates that dispersive PTCs offer novel excitation channels and amplification possibilities, that require lower modulation frequencies compared to nondispersive systems thus alleviating experimental challenges for the realization of PTCs in the optical regime. These findings pave the way for advancements in polaritonic lasing and resonant Raman scattering.

Classical optics↗

DOE OSTI2026

Startup regime of high-efficiency tapering-enhanced FEL oscillator

In this paper, we present a design of a high-efficiency high-gain free-electron laser oscillator based on the use of a strongly tapered undulator for extracting energy from high-brightness electron beams. We provide an analytical model of the setup followed by numerical simulations for lasing at the wavelength of 13.5 nm. We discuss the optimization of the system in steady state and the conditions necessary for the pass-per-pass buildup of the power from shot noise level. We propose the use of fast phase shifters as a way to accelerate the buildup. Finally, we present time-dependent simulations of the oscillator and discuss the role of spectral filtering. The optimized working point yields a total energy conversion efficiency from the electron beam to output radiation above 1% at the wavelength of 13.5 nm.

Beam dynamics↗

DOE OSTI2023

Low Threshold, Long Wavelength Interband Cascade Lasers With High Voltage Efficiencies

Here, we report on the substantial advancement of long wavelength InAs-based interband cascade lasers (ICLs) utilizing advanced waveguides formed from hybrid cladding layers and targeting the 10-12 µm wavelength region. Modifications in the hole injector have improved carrier transport in these ICLs, resulting in significantly reduced threshold voltages (V th ) as low as 3.62 V at 80 K. Consequently, much higher voltage efficiencies were observed, peaking at about 73% at 10.3 µm and allowing for large output powers of more than 100 mW/facet. Also, low threshold current densities (J th ) of 8.8 A/cm 2 in cw mode and 7.6 A/cm 2 in pulsed mode near 10 µm were observed; a result of adjustments in the GaInSb hole well composition intended to reduce the overall strain accumulation in the ICL. Furthermore, an ICL from the second wafer operating at a longer wavelength achieved a peak voltage efficiency of 57% at 11.7 µm, with a peak output power of more than 27 mW/facet. This ICL went on to lase beyond 12 µm in both cw and pulsed modes, representing a new milestone in long wavelength coverage for ICLs with the standard W-QW active region.

47 OTHER INSTRUMENTATION↗

DOE OSTI2024

Aging Mechanisms of Broad Area ~800 nm Laser Diodes

Here, this work presents a comprehensive study of early aging behavior (<500 hr) in ~800 nm, phosphide-based laser diodes grown by solid-source MBE with different oxygen concentration levels incorporated into the diode epitaxial layers during growth. The data indicate that lasing characteristics prior to aging are degraded by oxygen introduction, but the gradual power degradation rate after the onset of aging is not a strong function of oxygen at these concentration levels. Devices with oxygen concentrations of ~2.5 × 10 15 cm -3 showed significantly longer delay before the onset of aging (incubation time) than devices with less than 1 × 10 15 cm -3 oxygen. Generation-Recombination current and Laser Beam Induced Current measurements indicate that defect densities and aggregation are suppressed at the facets by oxygen, which can explain longer incubation times. Diagnostic data and parametric fits to diode simulation models show that increased cavity optical loss and defect density are primarily responsible for gradual power degradation during aging, rather than changes in nonradiative recombination. Mechanisms are proposed that explain this behavior, based on density functional theory (DFT) simulations and known recombination-enhanced defect generation phenomena.

47 OTHER INSTRUMENTATION↗

DOE OSTI2024

Thermal Stability of the Dot-in-Well Gain Medium for Photonic Crystal Surface Emitting Lasers

Self-assembled quantum dots (QDs) embedded in InGaAs quantum wells (QWs) are used as active regions for photonic-crystal surface-emitting lasers (PCSELs). An epitaxial regrowth method is developed to fabricate the dot-in-well (DWELL) PCSELs. Here, the epitaxial regrowth starts with the growth of a partial laser structure containing bottom cladding, waveguide, active region, and the photonic crystal (PC) layer. The PC layer is patterned to realize the cavity. Subsequently a top cladding layer is regrown to complete the laser structure. During the regrowth of the top cladding layer, the partial laser structure is subjected to high growth temperatures in excess of 600 °C resulting in an unintentional annealing of the active region. This annealing of the active region can alter the QDs by changing their size resulting in a blue shift in photoluminescence (PL) and narrowing PL emission. This effect results in the misaligning of the gain peak and the cavity resonance, resulting in sub-optimal lasing performance. DWELL active regions are known to have better thermal stability compared to both QDs and QWs and could be an ideal candidate for regrown PCSELs. We successfully demonstrate an optically-pumped epitaxially-regrown DWELL PCSEL with an emission wavelength of 1230 nm operating at room temperature. Furthermore, the DWELL active region shows excellent emission wavelength stability and intensity despite the high temperature regrowth process.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2020

High-Speed Nonpolar InGaN/GaN Superluminescent Diode With 2.5 GHz Modulation Bandwidth

We demonstrate a superluminescent diode fabricated on a nonpolar m-plane GaN substrate by employing a linearly tapered waveguide design. A high electrical -3dB modulation bandwidth ( f\n 3dB \n) of 2.5 GHz at a current density of 30 kA/cm\n 2 \n is achieved. The high modulation bandwidth is attributed to the shorter carrier recombination lifetime, the linear gain curve in the nonpolar m-plane quantum wells, and the ability to operate at high current densities while effectively suppressing lasing. We derive a general expression for the -3dB bandwidth as a function of current density for SLDs using a similar approach to that for laser diodes. The -3dB bandwidth of a nonpolar superluminescent diode increases exponentially with current density. The experimental results are consistent with the derived expression for f\n 3dB \n vs. current density.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

DOE OSTI2022

Solid-state laser refrigeration of core-shell polystyrene microspheres

Microlaser designs based on the coupling of whispering gallery modes (WGMs) with the upconversion processes which take place within lanthanide-doped nanoparticles (UCNPs) have been demonstrated and shown to have many valuable qualities, such as high Q factors and low lasing thresholds. One obstacle that these microlaser designs still face is the challenges caused by photothermal heating of the gain medium, which could be solved through the design of a radiation balanced microlaser. In this work, WGM microresonators composed of 5 µm diameter polystyrene spheres are fabricated with a layer of Yb3+-doped NaYF4 UCNPs in order to test if the anti-Stokes cooling properties of the UCNPs can cool the microresonator and its environment under laser irradiation. We find via calibrated mean fluorescence spectroscopy that the UCNPs can cool their local environment by as much as 23 °C and significantly reduce the heating of the aqueous environment surrounding the microresonator, showing promise for inclusion in a design for a radiation balanced microlaser.

whispering gallery modes, optical refrigeration, m↗

DOE OSTI2021

Semiconductor quantum dots: Technological progress and future challenges

In quantum-confined semiconductor nanostructures, electrons exhibit distinctive behavior compared with that in bulk solids. This enables the design of materials with tunable chemical, physical, electrical, and optical properties. Zero-dimensional semiconductor quantum dots (QDs) offer strong light absorption and bright narrowband emission across the visible and infrared wavelengths and have been engineered to exhibit optical gain and lasing. These properties are of interest for imaging, solar energy harvesting, displays, and communications. Here, we offer an overview of advances in the synthesis and understanding of QD nanomaterials, with a focus on colloidal QDs, and discuss their prospects in technologies such as displays and lighting, lasers, sensing, electronics, solar energy conversion, photocatalysis, and quantum information.

García de Arquer, F. Pelayo↗

DOE OSTI2021

Starting dynamics of a linear-cavity femtosecond Mamyshev oscillator

Mamyshev oscillators can generate high-power femtosecond pulses, but starting a mode-locked state has remained a major challenge due to the suppression of continuous-wave lasing. Here, we study the starting dynamics of a linear Mamyshev oscillator designed to generate high-power femtosecond pulses while avoiding component damage. Reliable starting to stable mode-locking is achieved with a combination of modulation of the pump power and shifting of a filter passband. The starting process is automated, with full electronic control. The laser delivers 21-nJ pulses that are dechirped to 65 fs in duration outside the cavity.

Chen, Yi-Hao (ORCID:0000000244001983)↗

DOE OSTI2020

Multimode description of self-mode locking in a single-section quantum-dot laser

This paper describes a theory for mode locking and frequency comb generation by four-wave mixing in a semiconductor quantum-dot active medium. The derivation uses a multimode semiclassical laser theory that accounts for fast carrier collisions within an inhomogeneous distribution of quantum dots. Numerical simulations are presented to illustrate the role of active medium nonlinearities in mode competition, gain saturation, carrier-induced refractive index and creation of combination tones that lead to locking of beat frequencies among lasing modes in the presence of cavity material dispersion.

Chow, Weng W.↗

DOE OSTI2021

Tunable quantum-cascade VECSEL operating at 19 THz

We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) emitting around 1.9 THz with up to 10% continuous fractional frequency tuning of a single laser mode. The device shows lasing operation in pulsed mode up to 102 K in a high-quality beam, with the maximum output power of 37 mW and slope efficiency of 295 mW/A at 77 K. Challenges for up-scaling the operating wavelength in QC metasurface VECSELs are identified.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2022

O 2 based resonantly ionized photoemission thermometry analysis of supersonic flows

Characterization of the thermal gradients within supersonic and hypersonic flows is essential for understanding transition, turbulence, and aerodynamic heating. Developments in novel, impactful non-intrusive techniques are key for enabling flow characterizations of sufficient detail that provide experimental validation datasets for computational simulations. In this work, Resonantly Ionized Photoemission Thermometry (RIPT) signals are directly imaged using an ICCD camera to realize the techniques 1D measurement capability for the first time. The direct imaging scheme presented for oxygen-based RIPT (O 2 RIPT) uses the previously established calibration data to direct excite various resonant rotational peaks within the S-branch of the C 3 Π, ( v = 2) ← X 3 Σ( v ′ = 0) absorption band of O 2 . The efficient ionization of O 2 liberates electrons that induce electron avalanche ionization of local N 2 molecules generating N 2 + , which primarily deexcites via photoemissions of the first negative band of N 2 + ( B 2 Σ <#comment/> u + − <#comment/> X 2 Σ <#comment/> g + ) . When sufficient lasing energy is used, the ionization region and subsequent photoemission signal is achieved along a 1D line thus, if directly imaged can allow for gas temperature assignments along said line; demonstrated here of up to five centimeters in length. The temperature gradients present within the ensuing shock train of a supersonic under expanded free jet serves as a basis of characterization for this new RIPT imaging scheme. The O 2 RIPT results are extensively compared and validated against well-known and established techniques (i.e., CARS and CFD). The direct imaging capability fully realizes the technique’s fundamental potential and is expected to be the standard of implementation going forward. The direct imaging capability can play instrumental roles in future scientific studies that rely upon acute characterization of thermal gradients within a medium that cannot be easily resolved by a point. Furthermore, the removal of the spectrometer greatly reduces the cost, complexity, and optical alignment associated with prior RIPT measurements.

42 ENGINEERING↗

DOE OSTI2024

Generation of superposed orbital angular momentum beams using a free-electron laser oscillator

With wavelength tunability, free-electron lasers (FELs) are well-suited for generating orbital angular momentum (OAM) beams in a wide photon energy range. We report here the first experimental demonstration of OAM beam generation using an oscillator FEL with the tens of picosecond pulse duration. Lasing around 458 nm, we have produced the four lowest orders of superposed Laguerre-Gaussian beams using a very long FEL resonator of 53.73 m. The produced beams have good beam quality, excellent stability, and substantial average power. We have also developed a pulsed operation mode for these beams with a highly reproducible temporal structure for a range of repetition rate of 1–30 Hz. This development can be extended to short wavelengths, for example to x-rays using a future x-ray FEL oscillator. The OAM operation of such a storage-ring FEL also paves the way for the generation of OAM gamma-ray beams via inverse Compton scattering.

47 OTHER INSTRUMENTATION↗

DOE OSTI2025

femto-PIXAR: a self-supervised neural network method for reconstructing femtosecond X-ray free electron laser pulses

X-ray Free Electron Lasers (X-FELs) operate in a wide range of lasing configurations for a broad variety of scientific applications at ultrafast time-scales such as structural biology, materials science, and atomic and molecular physics. Shot-by-shot characterization of the X-FEL pulses is crucial for analysis of many experiments as well as tuning the X-FEL performance. However, for the weak pulses found in advanced configurations, e.g. those needed for coherent, two-pulse studies of quantum materials, there is no current method for reliably resolving pulse profiles. Here we show that a physics-based U-net model can reconstruct the individual pulse power profiles for sub-picosecond pulse separation without the need for simulations. Using experimental data from weak X-FEL pulse pairs, we demonstrate we can learn the pulse characteristics on a shot-by-shot basis when conventional methods fail.

43 PARTICLE ACCELERATORS↗

DOE OSTI2025

Third-order photon correlations extract single-nanocrystal multiexciton properties in solution

Colloidal semiconductor nanocrystals are considered promising materials for high-flux optical applications, including lasing, light-emitting diodes, biological imaging, and quantum optics. In high-flux applications, multiexcitons can significantly contribute to emission, influencing its brightness, spectral purity, and kinetics. As a result, understanding and controlling multiexciton emission in colloidal nanocrystal materials is of the utmost importance. In the past, single-nanocrystal photon correlation methods have been applied to understand biexciton and triexciton efficiencies, lifetimes, and spectra. While powerful, such methods suffer from user selection bias and require stable emission from single nanocrystals. To compensate for this shortcoming, second-order correlation methods were developed to extract sample-averaged biexciton properties from a solution of nanocrystals. Until now, however, the analogous third-order solution photon correlation methods remained unexplored. In this work, we present a pair of third-order photon correlation techniques to obtain the sample-averaged single-nanocrystal triexciton quantum yield and lifetime in a solution-phase experiment. These techniques derive from the relationship between the Poisson probability of nanocrystal photon absorption and the intrinsic probability of nanocrystal photon emission. We validate the theoretical background of these techniques by creating a numerical model to simulate the diffusion and emission of many nanocrystals in solution. Our simulations confirm that the average triexciton quantum yield and triexciton lifetime can be extracted from a solution of nanocrystals. These techniques will enable researchers to gain a better understanding of the fundamental multiexciton properties of colloidal nanocrystals.

Horowitz, Jonah R. [Massachusetts Institute of Tec↗

DOE OSTI2023

Integrated photonic molecule Brillouin laser with a high-power sub-100-mHz fundamental linewidth

Photonic integrated lasers with an ultra-low fundamental linewidth and a high output power are important for precision atomic and quantum applications, high-capacity communications, and fiber sensing, yet wafer-scale solutions have remained elusive. Here we report an integrated stimulated Brillouin laser (SBL), based on a photonic molecule coupled resonator design, that achieves a sub-100-mHz fundamental linewidth with greater than 10-mW output power in the C band, fabricated on a 200-mm silicon nitride (Si 3 N 4 ) CMOS-foundry compatible wafer-scale platform. The photonic molecule design is used to suppress the second-order Stokes (S2) emission, allowing the primary lasing mode to increase with the pump power without phase noise feedback from higher Stokes orders. The nested waveguide resonators have a 184 million intrinsic and 92 million loaded Q, over an order of magnitude improvement over prior photonic molecules, enabling precision resonance splitting of 198 MHz at the S2 frequency. We demonstrate S2-suppressed single-mode SBL with a minimum fundamental linewidth of 71±18 mHz, corresponding to a 23±6-mHz 2 /Hz white-frequency-noise floor, over an order of magnitude lower than prior integrated SBLs, with an ∼11-mW output power and 2.3-mW threshold power. The frequency noise reaches the resonator-intrinsic thermo-refractive noise from 2-kHz to 1-MHz offset. The laser phase noise reaches −155 dBc/Hz at 10-MHz offset. The performance of this chip-scale SBL shows promise not only to improve the reliability and reduce size and cost but also to enable new precision experiments that require the high-speed manipulation, control, and interrogation of atoms and qubits. Realization in the silicon nitride ultra-low loss platform is adaptable to a wide range of wavelengths from the visible to infrared and enables integration with other components for systems-on-chip solutions for a wide range of precision scientific and engineering applications including quantum sensing, gravitometers, atom interferometers, precision metrology, optical atomic clocks, and ultra-low noise microwave generation.

Liu, Kaikai (ORCID:0000000213609646)↗

DOE OSTI2025

Ink-sprayed transparent ceramic multilayer planar waveguides for ribbon lasers

Multilayer transparent ceramic planar waveguides were fabricated using a newly developed particle-loaded ink spraying method, characterized, and lased. Waveguides with five Yb:YAG gain regions and four Lu:YAG index-matched regions were produced for application as a “ribbon laser,” as well as a homogeneous Yb:YAG waveguide. Elemental mapping revealed that the waveguide thicknesses varied from 40 to 69 μm, comprised of layers of less than 10 µm. The Yb concentration profiles were fit to a diffusion profile. The homogeneous and multilayer waveguides were tested in a laser cavity and produced similar slope efficiencies, with a maximum of 31% achieved. In conclusion, their beam profiles provided evidence of the ribbon structure affecting the preferred mode.

Osborne, Ross [Lawrence Livermore National Laborat↗