Optical Excitation Research Articles

Room-temperature continuous-wave pumped exciton polariton condensation in a perovskite microcavity.

Microcavity exciton polaritons (polaritons) as part-light part-matter quasiparticles garner considerable attention for Bose-Einstein condensation at elevated temperatures. Recently, halide perovskites have emerged as promising room-temperature polaritonic platforms because of their large exciton binding energies and superior optical properties. However, currently, inducing room-temperature nonequilibrium polariton condensation in perovskite microcavities requires optical pulsed excitations with high excitation densities. Here, we demonstrate continuous-wave optically pumped polariton condensation with an exceptionally low threshold of ~0.53 W cm-2 and a narrow linewidth of ~0.5 meV. Polariton condensation is unambiguously demonstrated by characterizing the nonlinear behavior and coherence properties. We also unveil the trapping potential landscape strategy to facilitate polariton relaxation and accumulation. Our findings lay the foundation for the next-generation energy-efficient polaritonic devices operating at room temperature.

Spatiotemporal observation of surface plasmon polariton mediated ultrafast demagnetization

Surface plasmons offer a promising avenue in the pursuit of swift and localized manipulation of magnetism for advanced magnetic storage and information processing technology. However, observing and understanding spatiotemporal interactions between surface plasmons and spins remains challenging, hindering optimal optical control of magnetism. Here, we demonstrate the spatiotemporal observation of patterned ultrafast demagnetization dynamics in permalloy mediated by propagating surface plasmon polaritons with sub-picosecond time- and sub-μm spatial- scales by employing Lorentz ultrafast electron microscopy combined with excitation through transient optical gratings. We discover correlated spatial distributions of demagnetization amplitude and surface plasmon polariton intensity, the latter characterized by photo-induced near-field electron microscopy. Furthermore, by comparing the results with patterned ultrafast demagnetization dynamics without surface plasmon polariton interaction, we show that the demagnetization is not only enhanced but also exhibits a spatiotemporal modulation near a spatial discontinuity (plasmonic hot spot). Our findings shed light on the intricate interplay between surface plasmons and spins, offer insights into the optimized control of optical excitation of magnetic materials and push the boundaries of ultrafast manipulation of magnetism.

Optical signatures of lattice strain in chemically doped colloidal quantum wells

Lattice strain plays a vital role in tailoring the optoelectronic performance of colloidal nanocrystals (NCs) with exotic geometries. Although optical identifications of lattice strain in irregular-shaped NCs or hetero-structured NCs have been well documented, less is known about optical signatures of the sparsely distributed lattice mismatch in chemically-doped NCs. Here, we show that coherent acoustic phonons (CAPs) following bandgap optical excitations in Cu-doped CdSe colloidal quantum wells (CQWs) offer a unique platform for indirectly measuring the dopant-induced lattice strain. By comparing the behavior of CAPs in Cu-doped and undoped CQWs (i.e., vibrational phase/lifetime/amplitude), we have revealed the driving force of CAPs related to the optical screening of lattice strain-induced piezoelectric fields, which thus allows to determine the strain-induced piezoelectric field of ~102 V/m in Cu-doped CdSe CQWs. This work may facilitate a detailed understanding of lattice strain in chemically-doped colloidal NCs, which is a prerequisite for the design of favorable doped colloids in optoelectronics.

Dynamics of Photoinduced Charge Carrier and Photothermal Effect in Pulse-Illuminated Narrow Gap and Moderate Doped Semiconductors

When a sample of semiconducting material is illuminated by monochromatic light, in which the photon energy is higher than the energy gap of the semiconductor, part of the absorbed electromagnetic energy is spent on the generation of pairs of quasi-free charge carriers that are bound by Coulomb attraction. Photo-generated pairs diffuse through the material as a whole according to the density gradients established, carrying part of the excitation energy and charge through the semiconducting sample. This energy is indirectly transformed into heat, where the excess negatively charged electron recombines with a positively charged hole and causes additional local heating of the lattice. The dynamic of the photoexcited charge carrier is described by a non-linear partial differential equation of ambipolar diffusion. In moderate doped semiconductors with a low-level injection of charge carriers, ambipolar transport can be reduced to the linear parabolic partial differential equation for the transport of minority carriers. In this paper, we calculated the spectral function of the photoinduced charge carrier distribution based on an approximation of low-level injection. Using the calculated distribution and inverse Laplace transform, the dynamics of recombination photoinduced heat sources at the surfaces of semiconducting samples were studied for pulse optical excitations of very short and very long durations. It was shown that the photoexcited charge carriers affect semiconductor heating depending on the pulse duration, velocity of surface recombination, lifetime of charge carriers, and their diffusion coefficient.

Key considerations for effective optical excitation of low-temperature-grown gallium arsenide (LT-GaAs) terahertz emitters

Key considerations for effective optical excitation of low-temperature-grown gallium arsenide (LT-GaAs) terahertz emitters

The Q-Band Energetics and Relaxation of Chlorophylls a and b as Revealed by Visible-to-Near Infrared Time-Resolved Absorption Spectroscopy.

Chlorophyll (Chl) is the most abundant light-harvesting pigment of oxygenic photosynthetic organisms; however, the Q-band energetics and relaxation dynamics remain unclear. In this work, we have applied femtosecond time-resolved (fs-TA) absorption spectroscopy in 430-1,700 nm to Chls a and b in diluted pyridine solutions under selective optical excitation within their Q-bands. The results revealed distinct near-infrared absorption features of the Bx,y ← Qy and Bx,y ← Qx transitions in 930-1,700 nm, which together with the steady-state absorption in 400-700 nm unveiled the Qx(0,0)-state energy that lies 1,000 ± 400 and 600 ± 400 cm-1 above the Qy(0,0)-state for Chls a and b, respectively. In addition, the Qx-to-Qy internal conversion time constants are estimated to be less than 80 fs for Chls a and b. These findings may shed light on understanding the roles of the Chls in the primary excitation energy transfer reactions of photosynthesis.

Three-dimensional photoluminescence imaging of threading dislocations in GaN by sub-band optical excitation

GaN is rapidly gaining attention for implementation in power electronics but is still impacted by its high density of threading dislocations (TDs), which have been shown to facilitate current leakage through devices limiting their performance and reliability. Here, we discuss a novel implementation of photoluminescence (PL) imaging to study TDs in regions within vertically structured p-i-n GaN (PIN) diodes consisting of metalorganic chemical vapor deposition (MOCVD) epitaxial layers grown on ammonothermal GaN (am-GaN) substrates. PL imaging with a sub-bandgap excitation energy (3.1 eV) reveals TDs with excellent clarity in three dimensions within the am-GaN substrate. Galvanometric-driven PL imaging allows the microstructure of hundreds of devices to be characterized in a single session, enhancing the screening process through the addition of device specific TD location tracking and density mapping. The visibility, structural characteristics, luminescent nature and evolution of TDs through the GaN growth process are described, potentially providing the ability to define TD structures associated with leakage current.

Realizing zero-threshold population inversion via plasmonic doping.

Lowering the population inversion threshold is key to leveraging quantum dots (QDs) for nanoscale lasing and laser miniaturization. However, optical realization of population inversion in QDs has an inherent limitation: the number of excited electrons per QD is bound by the absorbed photons. Here we show that one can break this population limit and realize near-zero threshold inversion via plasmonic doping. Specifically, we integrate QDs into a grating-like plasmonic resonator, which, upon optical excitation, can transiently dope the QDs with numerous highly energetic electrons and make excited electrons in the QDs outnumber absorbed photons. This high population under low excitation blocks QD absorption and reduces the population-inversion threshold over 100 times compared to neutrally populated QDs. Our findings not only reveal new understanding of cavity-emitter interactions but also provide practical avenues for zero-threshold lasing, nanolasing and amplification devices.

Femtosecond laser induced ultrafast interface dynamics between single layer graphene and quartz substrate: A theoretical study

SiO2 is the most widely used dielectric substrate for graphene devices. Theoretically investigating the interaction between graphene and SiO2 is vitally important for understanding graphene properties and improving device performance. In recent years, density functional theory (DFT) has been used to investigate the graphene–SiO2 interaction in ground states. However, the strong interface dynamics for an excited graphene–SiO2 system in ultrafast nonequilibrium processes was rarely researched. In this work, a real-time time-dependent density functional theory (rt-TDDFT) method was adopted to study the femtosecond laser induced ultrafast structure evolution and the underlying dynamics mechanism of the interface between a single layer graphene and a Si-terminated quartz substrate. This work indicates that rt-TDDFT is a promising method to study the strong electron dynamics and the coupled nuclear dynamics for graphene-SiO2 interfaces under ultrafast optical excitation, which benefits graphene device designs and mechanism analysis.

Distinguishing local photoinduced forces from global photoacoustic interactions in homodyne infrared photoinduced force microscopy

The rise of tip-based optical force microscopy has transformed nanoscale optical and chemical imaging, enabling sub-10 nm spatial resolution by integrating optical excitation with atomic force microscopy. Photoinduced force microscopy (PiFM) stands out as a key technique, with applications in single-molecule imaging, Raman spectroscopy, and near-field electromagnetic characterization. However, the contrast mechanisms underlying homodyne PiFM, particularly in the infrared regime, remain underexplored. This study provides novel insights into the interpretation of PiF signals, focusing on homodyne IR-PiFM. Contrary to previous assumptions, we demonstrate that the linear dependence of the PiF signal on sample thickness in homodyne mode originates from global photoacoustic forces rather than localized photothermal effects. By minimizing global interactions through power reduction and tip–sample proximity, we achieve a spatial resolution of 11 nm, comparable to heterodyne PiFM. Our findings reveal that both homodyne and heterodyne modes are fundamentally sensitive to tip-enhanced near-field optical intensity, with similar dependencies on sample thickness. This work advances the understanding of the contrast mechanism of PiFM and demonstrates that both homodyne and heterodyne modes can achieve high-resolution imaging, paving the way for broader applications in nanoscale optical and chemical analysis.

An overview of the use of non-titanium MXenes for photothermal therapy and their combinatorial approaches for cancer treatment.

Since the initial publication on the first Ti3C2T x MXene in 2011, there has been a significant increase in the number of reports on applications of MXenes in various domains. MXenes have emerged as highly promising materials for various biomedical applications, including photothermal therapy (PTT), drug delivery, diagnostic imaging, and biosensing, owing to their fascinating conductivity, mechanical strength, biocompatibility and hydrophilicity. Through surface modification, MXenes can mitigate cytotoxicity, enhance biological stability, and improve histocompatibility, thereby enabling their potential use in in vivo biomedical applications. MXenes are also known for their ability to absorb light in the near-infrared (NIR) region and generate heat by localised surface plasmon resonance (LSPR) effects and electron-phonon coupling. Optical excitation laser pulses result in hot photocarrier distribution in MXenes, which quickly transfers surplus energy to the crystal lattice and results in the internal conversion of light into heat with nearly 100% efficiency. The relaxation of hot carrier distribution by electron-phonon interactions leads to the cooling of the lattice by dissipating thermal energy to the surrounding environment. This heating effect of MXenes makes them potential photothermal agents (PTAs), particularly for PTT applications. The adjustable surface of MXenes and their high surface area-to-volume ratios are ideal for the combinatorial approach of PTT along with drug delivery, photodynamic therapy (PDT), bone regeneration and other applications. Since non-Ti MXenes are more biocompatible than Ti MXenes, they are promising candidates for different biomedical applications. This comprehensive review provides a concise overview of the current research patterns, properties, and biomedical applications of non-Ti MXenes, particularly in PTT and its combinatorial approaches.

Constructing Dynamical Symmetries for Quantum Computing: Applications to Coherent Dynamics in Coupled Quantum Dots.

Dynamical symmetries, time-dependent operators that almost commute with the Hamiltonian, extend the role of ordinary symmetries. Motivated by progress in quantum technologies, we illustrate a practical algebraic approach to computing such time-dependent operators. Explicitly we expand them as a linear combination of time-independent operators with time-dependent coefficients. There are possible applications to the dynamics of systems of coupled coherent two-state systems, such as qubits, pumped by optical excitation and other addressing inputs. Thereby, the interaction of the system with the excitation is bilinear in the coherence between the two states and in the strength of the time-dependent excitation. The total Hamiltonian is a sum of such bilinear terms and of terms linear in the populations. The terms in the Hamiltonian form a basis for Lie algebra, which can be represented as coupled individual two-state systems, each using the population and the coherence between two states. Using the factorization approach of Wei and Norman, we construct a unitary quantum mechanical evolution operator that is a factored contribution of individual two-state systems. By that one can accurately propagate both the wave function and the density matrix with special relevance to quantum computing based on qubit architecture. Explicit examples are derived for the electronic dynamics in coupled semi-conducting nanoparticles that can be used as hardware for quantum technologies.

Excitations in layered materials from a non-empirical Wannier-localized optimally-tuned screened range-separated hybrid functional

Accurate prediction of electronic and optical excitations in van der Waals (vdW) materials is a long-standing challenge for density functional theory. The recent Wannier-localized optimally-tuned screened range-separated hybrid (WOT-SRSH) functional has proven successful in non-empirical determination of electronic band gaps and optical absorption spectra for covalent and ionic crystals. However, for vdW materials the tuning of the material- and structure-dependent functional parameters has only been attained semi-empirically. Here, we present a non-empirical WOT-SRSH approach applicable to vdW materials, with the optimal functional parameters transferable between monolayer and bulk. We apply this methodology to prototypical vdW materials: black phosphorus, molybdenum disulfide, and hexagonal boron nitride (in the latter case including zero-point renormalization). We show that the WOT-SRSH approach consistently achieves accuracy levels comparable to experiments and many-body perturbation theory (MBPT) calculations for band structures and optical absorption spectra, both on its own and as an optimal starting point for MBPT calculations.

Nature of the carrier dynamics and contrast formation on the photoactive material surfaces: Insight from ultrafast imaging to DFT calculations.

Precise material design and surface engineering play a crucial role in enhancing the performance of optoelectronic devices. These efforts are undertaken to particularly control the optoelectronic properties and regulate charge carrier dynamics at the surface and interface. In this study, we used ultrafast scanning electron microscopy (USEM), which is a powerful and highly sensitive surface tool that provides unique information about the photoactive charge dynamics of material surfaces selectively and spontaneously in real time and space in high spatial and temporal resolution. Here, time-resolved images of CdTe (110), CdSe (100), GaAs (110), and other semiconductors revealed that the presence of oxide layers on the surface of materials leads to an increase in the work function (WF) and trap state densities upon optical excitation, leading to the formation of dark image contrast in USEM experiments. These findings were further supported by abinitio calculations, which confirmed the reliability of the observed changes in the excited surface WFs. Besides enhancing our understanding of surface charge dynamics, it also offers valuable insights into manipulating these properties. This study paves the way for precise control and the potential to design highly efficient light-harvesting devices.

Ultrafast Control over Stiffening and Softening of Coherent Interlayer Coupling in WSe2/WS2 Heterobilayers.

Twisted van der Waals heterostructures have led to emerging layer-dependent correlated physics in moiré potentials. While optoelectronic controls over interlayer electronic coupling have been reported, the concomitant interlayer vibration has not yet been controlled. Here, we report experimental evidence of ultrafast optical control over the amplitude and oscillation period of interlayer breathing phonons in WSe2/WS2 heterobilayers. Femtosecond optical excitation above the Mott density in gate-tuned devices shows as large as 10% changes of stiffening and softening amplitude of coherent phonons. A theoretical model, incorporating both Buckingham and Hartree energies, is presented to elucidate the impact of charge-separated carriers generated by photoexcitation on phonon dynamics. This work, therefore, provides insights for extending optoelectronic engineering into the coherent phonons in moiré systems.

Mapping and Optically Writing Nanogap Inhomogeneities in 1-D Extended Plasmonic Nanowire-on-Mirror Cavities.

Tightly confined plasmons in metal nanogaps are highly sensitive to surface inhomogeneities and defects due to the nanoscale optical confinement, but tracking and monitoring their location is hard. Here, we probe a 1-D extended nanocavity using a plasmonic silver nanowire (AgNW) on mirror geometry. Morphological changes inside the nanocavity are induced locally using optical excitation and probed locally through simultaneous measurements of surface enhanced Raman scattering (SERS) and dark-field spectroscopy. The increasing molecular SERS intensity and corresponding redshift of cavity plasmon modes by up to 60 nm indicate atomic-scale changes inside the nanocavity. We correlate this to diffusion of silver atoms into the nanogap, which reduces the nanogap size and enhances the optical near-field, enhancing the SERS. These induced changes can be locally excited at specific locations along the length of the nanowire and remain stable and nonreversible. Polymer surface coating on the AgNW affects the power threshold for inducing atom migration and shows that strong polyvinylpyrrolidone (PVP)- Ag binding gives rise to higher power thresholds. Such extended nanogap cavities are an ideal system to provide robust SERS while withstanding high laser powers. These results provide insights into the inhomogeneities of NW nanocavities and pave the way toward spatially controlled NW lithography in ambient conditions.

Dual-channel SPR biosensor for enhanced glioma relapse diagnostics: Blood cell aggregation as a biomarker for tumor malignancy

A biosensor based on the surface plasmon resonance (SPR) phenomenon has been developed for the express diagnosis of brain gliomas relapse by assessing blood cell aggregation indicators. The device features two optical channels, allowing for two studies to be performed simultaneously or enabling one channel to be used as a reference. This approach significantly increases biosensor sensitivity by reducing the impact of external factors. The optical excitation source is a p-polarized semiconductor laser with a 650 nm wavelength. The sensing elements were F1 optical glass plates with a refractive index of 1.61, with sputtered layers of chromium (5 nm) and gold (45…50 nm). As a result of the studies, a correlation was ascertained between the level of peripheral blood cell aggregation in patients and the degree of malignancy of gliomas. A statistically significant difference (p ≤ 0.05) was found between the groups of conditionally healthy individuals and those with grade II-IV gliomas. A decrease in the shift of the SPR curve during blood testing indicates an increase in cell aggregation levels and a decrease in their electric charge on membranes. This trend gradually intensifies with the increasing degree of glioma malig-nancy, reaching minimal values in patients with grade IV gliomas, indicating changes in the physicochemical properties of cell membranes and a reduction in their electric charge.

InSitu Measurements of Light Diffusion in an Optically Dense Atomic Ensemble.

This study introduces a novel method to investigate insitu light transport within optically thick ensembles of cold atoms, exploiting the internal structure of alkaline-earth metals. A method for creating an optical excitation at the center of a large atomic cloud is demonstrated, and we observe its propagation through multiple scattering events. In conditions where the cloud size is significantly larger than the transport mean free path, a diffusive regime is identified. We measure key parameters, including the diffusion coefficient, transport velocity, and transport time, finding a good agreement with diffusion models. We also demonstrate that the frequency of the photons launched inside the system can be controlled. This approach enables direct time- and space-resolved observation of light diffusion in atomic ensembles, offering a promising avenue for exploring new diffusion regimes.

Plasmon-Assisted Electrochemical CO2 Reduction on Copper Nanocubes

Optical excitation of plasmons on metal nanoparticles presents a promising opportunity to enable electrochemical CO2 conversion into hydrocarbons at low overpotentials on stable electrodes. Electrochemical CO2 conversion typically requires high overpotentials, resulting in low energy efficiency and catalyst reshaping. By using light to overcome reaction barriers, plasmon excitation has been shown to lower the required overpotential for electrochemical reactions. Plasmon excitation also facilitates C-C coupling on gold and silver, metals that usually produce no multicarbon products when used as electrocatalysts. While previous studies indicate clear potential that plasmon-based photo-electrocatalysts could enable new electrochemical pathways for CO2 reduction at low overpotentials and stable catalysts, yields and catalytic turnover remained low. To develop industrially viable photo-electrochemical CO2 reduction catalysts, the effect of plasmon excitation at high current density and high mass flux needs to be understood and optimized.Here we investigate the photo-electrochemical CO2 reduction on gold and copper nanoparticles on a gas diffusion electrode. We choose copper nanoparticles because they are the most promising electrocatalysts for C-C coupling and exhibit a strong plasmon resonance around 600 nm, yet have not been explored for plasmonic CO2 reduction. Gold nanoparticles are chosen due to their exceptional stability and because gold has been shown to perform C-C coupling under plasmon excitation, which does not happen electrochemically. Using a gas diffusion electrode allows efficient gas phase mass transport to and from the electrode. We synthesize 50 nm (100) faceted copper nanocubes using copper bromide and oleylamine, and gold nanorods using hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL). In both cases, the nanoparticles have a plasmon resonance of 600 nm. We spray coat these nanoparticles onto a carbon paper gas diffusion membrane and incorporate them into our home-built gas reactor. First, we use real-time GC to investigate electrocatalysis in the dark and find high selectivity for CO on gold and high selectivity for ethylene and CH4 on copper. Then, we vary the optical illumination wavelength from 480 nm to 700 nm and the applied potential, investigating how these affect Faradaic efficiencies for the production of ethylene, CO, H2, and CH4 as well as photothermal contributions using an infrared camera. Finally, we use in-situ liquid cell transmission electron microscopy (TEM) to monitor nanoparticle stability and correlate morphological evolution with CO2 reaction rate. Our work provides the fundamental understanding necessary for building photo-electrocatalysts that can convert CO2 into value-added products under industrially relevant conditions.Figure caption: Schematic of our photo-electrochemical plasmonic gas diffusion electrode converting CO2 into hydrocarbons. Electrochemical CO2 conversion on gold typically leads to CO, under plasmon excitation gold can form hydrocarbons. Figure 1

Delayed luminescence and thermoluminescence in laboratory-grown diamonds

The “blue-green” phosphorescence/thermoluminescence is most commonly observed in diamonds following excitation at or above the indirect band gap and has been explained by a substitutional nitrogen-boron donor-acceptor pair recombination model. “Orange” and “red” phosphorescence have also been frequently observed in laboratory-grown near-colorless high-pressure high-temperature diamonds following optical excitation and their luminescence mechanisms are shown to be different from that of the blue-green phosphorescence. The physics of the orange and red luminescence and phosphorescence bands including the optical-excitation dependency (UV-NIR), temperature dependency (20K–573K), and related charge transfer process are investigated by a combination of self-built time-resolved imaging/spectroscopic techniques. In this paper, an alternative model for long-lived phosphorescence based on charge trapping is proposed to explain the orange phosphorescence/thermoluminescence band. Additionally, the red phosphorescence band is attributed to point defect, which possibly has a three-level phosphorescence system. Published by the American Physical Society 2024