Excitation Spot Research Articles

Condensation of Exciton-Polaritons in a Bound State in the Continuum: Effects of the Excitation Spot Size and Polariton Transport.

We report the formation of polariton condensates from strongly coupled molecules to bound states in the continuum with quadrupolar character in a metasurface of silicon nanoparticles. Our experiments demonstrate a strong dependence of the condensation threshold on the excitation spot size. The condensation threshold decreases as the excitation spot size increases, achieving thresholds below 3 μm cm-2 for spot sizes of around 1 mm2 and condensate lifetimes exceeding 20 ps. The strong dependence of the condensation threshold on the spot size is caused by the long propagation length of the polaritons. We reproduce this dependence in simulations by including a term for the ballistic transport of exciton-polaritons in the rate equations describing the condensation. These results illustrate the critical role that polariton transport plays in condensation and highlight the relevance of considering the size of the excitation in condensation experiments.

Laser ultrasound wave pattern analysis for efficient defect detection in samples with curved surfaces

Many production processes involve curved sample surfaces, such as welding or additive manufacturing. These pose new challenges to characterization methods for quality inspection, which are usually optimized for flat extended sample geometries. In this paper, we present a laser ultrasound (LUS) method that can be used to efficiently detect defects (e.g., voids), without extensive scanning effort and without a prior knowledge of the defect location, in finite samples with curved surfaces. The developed method starts with generalized simulations of the LUS wave patterns in samples with varying radii of curvature and width as well as varying excitation size and mechanism (thermoelastic or ablative). Based on the wave pattern analysis, it is possible to predict how every point in the weld can be reached with only few excitation spots. In a second step, we assume a grid of finite size defects at locations at which such voids are most likely formed and perform a thorough simulation analysis that is based on B-Scans to find a few pairs of excitation–detection points most favorable for finding defects anywhere in the weld seam. These results are then compared to the wave pattern analysis, discussing similarities and deviations from the predictions. In a final step, the simulations are compared to experimental results, verifying the almost threefold increase in the detectability of defects by choosing the predicted optimal excitation–detection positions. It is expected that this method will significantly improve the reliability and time efficiency of detecting internal defects in samples with curved surfaces in potential industrial applications.

Raman characterization of lunar highlands simulants for in-situ resource utilization in a lunar setting

Lunar regolith simulants from the NU-LHT series were characterized using different configurations for performing Raman spectroscopy measurements, including with different excitation wavelengths and spot sizes. This testing was performed to explore various Raman measurement configurations for analyzing lunar regolith, especially in terms of capturing distinctive fluorescence features that can provide more information about the composition of the regolith. Results obtained utilizing configurations having a 785 nm laser for excitation showed relatively narrow, intense fluorescence signals between 870 and 890 nm, which, to the best of our knowledge, has not been observed before in lunar regolith or lunar simulant samples. These distinctive fluorescence features, attributed to a specific rare earth element (REE) impurity, adds further support that these types of features can be utilized for identification and potentially quantification of the amount of REE or transition metal impurities in lunar regolith. Further, other aspects of these Raman instrument configurations and their advantages were explored, especially those applicable for use in a lunar environment in support of in-situ resource utilization (ISRU).

Predesigned perovskite crystal waveguides for room-temperature exciton-polariton condensation and edge lasing.

Perovskite crystals-with their exceptional nonlinear optical properties, lasing and waveguiding capabilities-offer a promising platform for integrated photonic circuitry within the strong-coupling regime at room temperature. Here we demonstrate a versatile template-assisted method to efficiently fabricate large-scale waveguiding perovskite crystals of arbitrarily predefined geometry such as microwires, couplers and splitters. We non-resonantly stimulate a condensate of waveguided exciton-polaritons resulting in bright polariton lasing from the transverse interfaces and corners of our perovskite microstructures. Large blueshifts with excitation power and high mutual coherence between the different edge and corner lasing signals are detected in the far-field photoluminescence, implying that a spatially extended condensates of coherent polaritons has formed. The condensate polaritons are found to propagate over long distances in the wires from the excitation spot and can couple to neighbouring wires through large air gaps, making our platform promising for integrated polaritonic circuitry and on-chip optical devices with strong nonlinearities.

Microlens Hollow-Core Fiber Probes for Operando Raman Spectroscopy.

We introduce a flexible microscale all-fiber-optic Raman probe which can be embedded into devices to enable operando in situ spectroscopy. The facile-constructed probe is composed of a nested antiresonant nodeless hollow-core fiber combined with an integrated high refractive index barium titanate microlens. Pump laser 785 nm excitation and near-infrared collection are independently characterized, demonstrating an excitation spot of full-width-half-maximum 1.1 μm. Since this is much smaller than the effective collection area, it has the greatest influence on the collected Raman scattering. Our characterization scheme provides a suitable protocol for testing the efficacy of these fiber probes using various combinations of fiber types and microspheres. Raman measurements on a surface-enhanced Raman spectroscopy sample and a copper battery electrode demonstrate the viability of the fiber probe as an alternative to bulk optic Raman microscopes, giving comparable collection to a 10 objective, thus paving the way for operando Raman studies in applications such as lithium battery monitoring.

Design of GaAs microcavity on SiN waveguide for efficient single-photon generation by resonant excitation

Silicon nitride (SiN) photonic circuits are attracting significant interest as a platform for photonic quantum information processing. Integration of deterministic single photon sources (SPSs) is required for large-scale single-photon-based quantum applications. InAs/GaAs quantum dots (QDs) have been demonstrated to be state-of-the-art deterministic SPSs under resonant excitation. However, InAs/GaAs QD SPSs integrated on chip often suffer from unwanted crosstalk from scattering of resonant excitation laser. Moreover, the mismatch in refractive indices of SiN and GaAs impedes efficient coupling of single photons into the photonic circuit. In this work, we design a GaAs photonic crystal (PhC) nanobeam cavity with an embedded QD on top of a SiN waveguide in SiO2 that can suppress crosstalk from resonant excitation and realize high coupling efficiency at the same time. The crosstalk is reduced by employing a carefully designed nanobeam cavity that removes complex structures around the excitation spot. The high coupling efficiency is achieved with a weak hybridized mirror formed by proximity of GaAs PhC nanobeam and SiN waveguide that makes the cavity and helps transferring photons into the waveguide. This enables more than 90% coupling efficiency. The designed device is expected to be a bright source of indistinguishable photons.

Breaking the mirror symmetry of photonic spin-orbit interaction using a geometrically symmetric chiral resonator

The symmetries of photonic spin-orbit interaction (PSOI) at waveguide interfaces provide flexible modulation capability but limit their practical implementation due to the stringent requirements of excitation conditions. This limitation can be mitigated by intentionally breaking local symmetries, offering a novel platform for achieving directional coupling and optical isolation with PSOI-based interfaces. For example, breaking the inversion symmetry of a nanofiber PSOI interface using a nanosphere scatterer reduces the required accuracy in the size and position of excitation spots. This study introduces a novel approach to break the mirror symmetry of a PSOI-based nanofiber waveguide by coupling it with a geometrically symmetric chiral gold nanohelicoid (GNH) resonator, which relaxes the original requirement of circularly polarized excitations. Finite-difference time-domain simulations demonstrate unidirectional light coupling and propagation under both circularly and linearly polarized excitations, showcasing the versatility of this hybrid symmetry-broken system. The Fano-like features observed in directionality spectra align with the GNH’s circular dichroism spectrum, emphasizing an intricate correlation between plasmonic near-field chirality and far-field scattering dichroism. This work paves the way for enhancing the functionalities of PSOI-based waveguide interfaces through locally coupling them with nanoscale chiral resonators, thereby expanding their application in quantum photonics, information transport and plasmonic nanophotonics.

Speckle-enabled in vivo demixing of neural activity in the mouse brain.

Functional imaging of neuronal activity in awake animals, using a combination of fluorescent reporters of neuronal activity and various types of microscopy modalities, has become an indispensable tool in neuroscience. While various imaging modalities based on one-photon (1P) excitation and parallel (camera-based) acquisition have been successfully used for imaging more transparent samples, when imaging mammalian brain tissue, due to their scattering properties, two-photon (2P) microscopy systems are necessary. In 2P microscopy, the longer excitation wavelengths reduce the amount of scattering while the diffraction-limited 3D localization of excitation largely eliminates out-of-focus fluorescence. However, this comes at the cost of time-consuming serial scanning of the excitation spot and more complex and expensive instrumentation. Thus, functional 1P imaging modalities that can be used beyond the most transparent specimen are highly desirable. Here, we transform light scattering from an obstacle into a tool. We use speckles with their unique patterns and contrast, formed when fluorescence from individual neurons propagates through rodent cortical tissue, to encode neuronal activity. Spatiotemporal demixing of these patterns then enables functional recording of neuronal activity from a group of discriminable sources. For the first time, we provide an experimental, in vivo characterization of speckle generation, speckle imaging and speckle-assisted demixing of neuronal activity signals in the scattering mammalian brain tissue. We found that despite an initial fast speckle decorrelation, substantial correlation was maintained over minute-long timescales that contributed to our ability to demix temporal activity traces in the mouse brain in vivo. Informed by in vivo quantifications of speckle patterns from single and multiple neurons excited using 2P scanning excitation, we recorded and demixed activity from several sources excited using 1P oblique illumination. In our proof-of-principle experiments, we demonstrate in vivo speckle-assisted demixing of functional signals from groups of sources in a depth range of 220-320 µm in mouse cortex, limited by available speckle contrast. Our results serve as a basis for designing an in vivo functional speckle imaging modality and for maximizing the key resource in any such modality, the speckle contrast. We anticipate that our results will provide critical quantitative guidance to the community for designing techniques that overcome light scattering as a fundamental limitation in bioimaging.

Multicolor two-photon light-patterning microscope exploiting the spatio-temporal properties of a fiber bundle.

The development of efficient genetically encoded indicators and actuators has opened up the possibility of reading and manipulating neuronal activity in living tissues with light. To achieve precise and reconfigurable targeting of large numbers of neurons with single-cell resolution within arbitrary volumes, different groups have recently developed all-optical strategies based on two-photon excitation and spatio-temporal shaping of ultrashort laser pulses. However, such techniques are often complex to set up and typically operate at a single wavelength only. To address these issues, we have developed a novel optical approach that uses a fiber bundle and a spatial light modulator to achieve simple and dual-color two-photon light patterning in three dimensions. By leveraging the core-to-core temporal delay and the wavelength-independent divergence characteristics of fiber bundles, we have demonstrated the capacity to generate high-resolution excitation spots in a 3D region with two distinct laser wavelengths simultaneously, offering a suitable and simple alternative for precise multicolor cell targeting.

Negative diffusion of excitons in quasi-two-dimensional systems.

We show how two different mobile-immobile type models explain the observation of negative diffusion of excitons reported in experimental studies in quasi-two-dimensional semiconductor systems. The main reason for the effect is the initial trapping and a delayed release of free excitons in the area close to the original excitation spot. The density of trapped excitons is not registered experimentally. Hence, the signal from the free excitons alone includes the delayed release of not diffusing trapped particles. This is seen as the narrowing of the exciton density profile or decrease of mean-squared displacement which is then interpreted as a negative diffusion. The effect is enhanced with the increase of recombination intensity as well as the rate of the exciton-exciton binary interactions.

Annular pupil confocal Brillouin-Raman microscopy for high spectral resolution multi-information mapping.

Brillouin-Raman combined confocal spectroscopy is a novel and powerful technique for providing non-contact and direct readout of the micro-regional chemical and mechanical properties of a material, and thus used in a broad range of applications, including material characterization in manufacturing and biological imaging. However, the inadequate spectral and spatial resolution restricts the further development of combined spectral technology. In this paper, an annular pupil confocal Brillouin-Raman microscopy (APCBRM) scheme is proposed to achieve high-spectral-resolution Brillouin spectral detection and high-lateral-resolution Brillouin, Raman, and 3D topography mapping. The use of an annular pupil significantly suppresses the spectral broadening caused by a high-numerical-aperture objective lens and compresses the full width at half maximum of the Brillouin spectrum by 22.1 %, effectively improving the Brillouin spectral resolution. In addition, the size of the excitation spot is compressed, and the lateral resolutions in Brillouin and Raman spectroscopy increased to about 353.2 nm and 347.1 nm, respectively. Thus, high lateral resolution and Brillouin spectral resolution are achieved simultaneously. Furthermore, the high-precision confocal focusing system based on reflected light realizes real-time focusing during scanning and three-dimensional topography mapping. These results demonstrate that APCBRM has excellent potential for applications in the fields of novel materials, precision machining, and biomedicine.

Near-Field Coupling with a Nanoimprinted Probe for Dark Exciton Nanoimaging in Monolayer WSe2.

Tip-enhanced photoluminescence (TRPL) is a powerful technique for spatially and spectrally probing local optical properties of 2-dimensional (2D) materials that are modulated by the local heterogeneities, revealing inaccessible dark states due to bright state overlap in conventional far-field microscopy at room temperature. While scattering-type near-field probes have shown the potential to selectively enhance and reveal dark exciton emission, their technical complexity and sensitivity can pose challenges under certain experimental conditions. Here, we present a highly reproducible and easy-to-fabricate near-field probe based on nanoimprint lithography and fiber-optic excitation and collection. The novel near-field measurement configuration provides an ∼3 orders of magnitude out-of-plane Purcell enhancement, diffraction-limited excitation spot, and subdiffraction hyperspectral imaging resolution (below 50 nm) of dark exciton emission. The effectiveness of this high spatial XD mapping technique was then demonstrated through reproducible hyperspectral mapping of oxidized sites and bubble areas.

Effect of interface quality on photoluminescence of encapsulated MoSe<sub>2</sub> monolayers

Photoluminescence spectra of excitons and trions in MoSe2 monolayers encapsulated with hexagonal boron nitride under nonresonant laser excitation were studied. When the size of the laser excitation spot decreases from 8 to 3 nm, individual peaks with a line width of ~2 meV emerge in the photoluminescence spectra, which were unresolved with a larger spot. Studies of the sample surface using a scanning electron microscope revealed the existence of a large number of features at the interfaces of structures with characteristic sizes ranging from submicron to micron and more. It was expected that the lines appearing in the spectrum at small excitation spot sizes were associated with similar submicron inhomogeneities. A study of a specially made heterostructure covered by a metal mask with holes of 1.6 microns in diameter confirmed this assumption.

High-Sensitivity Visualization of Ultrafast Carrier Diffusion by Wide-Field Holographic Microscopy

Ultrafast transient microscopy is a key tool to study the photophysical properties of materials in space and time, but current implementations are limited to ≈1-μm fields of view, offering no statistical information for heterogeneous samples. Recently, we demonstrated wide-field transient imaging based on multiplexed off-axis holography. Here, we perform ultrafast microscopy in parallel around a hundred diffraction-limited excitation spots over a ≈60-μm field of view, which not only automatically samples the photophysical heterogeneity of the sample over a large area but can also be used to obtain a 10-fold increase in signal-to-noise ratio by computing an average spot. We apply our microscope to study the carrier diffusion processes in methylammonium lead bromide perovskites. We observe strong diffusion due to the presence of hot carriers during the first picosecond and slower diffusion afterward. We also describe how many-body kinetics can be misleadingly interpreted as strong diffusion at high excitation densities, while at weak excitation, real diffusion is observed. Therefore, the vast increase in sensitivity offered by this technique benefits the study of carrier transport not only by reducing data acquisition times but also by enabling the measurement of the much smaller signals generated at low carrier densities.

Crack detection in metallic components with varying surface characteristics using laser spot scanning thermography

Cracks are the most common cause of failure in the manufacturing of metal parts, hence technologies for detecting them are essential for defect-free production. In this study, a local laser spot excitation combined with robotic scanning is used to identify vertical cracks in titanium. It is feasible to observe anisotropies in the lateral diffusivity by capturing temporal temperature data with an infrared camera utilizing local thermal stimulation. The crack parameter may then be quantified based on the regional transient behavior of temperature distribution. In doing so, we present an advanced technique for distinguishing between contrast created by the surface oxide layer and contrast caused by the vertical crack. In addition, we provide results from a numerical simulation that looked at the notion of local heat excitation for crack parameter quantification. Thermal conduction, radiation, and convection all play a role in the simulation. The experimental and theoretical findings were found to be quite consistent.

Nanoscale Electric Field Probing in a Single Nanowire with Raman Spectroscopy and Elastic Strain.

In this work we investigate the Raman response of extremely strained gallium phosphide nanowires. We analyze new strain-induced spectral phenomena such as 2-fold and 3-fold phonon peak splitting which arise due to nontrivial internal electric field distribution coupled with inhomogeneous strain. We show that high bending strain acts as a probe allowing us to define the electric field distribution with deep subwavelength resolution using the corresponding changes of the Raman spectra. We investigate the nature of the localization with respect to nanowire diameter, excitation spot position, and light polarization, supporting the experiment with 3D numerical modeling. Based on our findings we propose a research tool allowing to precisely localize the electric field in a certain subwavelength region of the nanophotonic resonator.

Cathodoluminescence of Ultrathin InAs Layers Embedded in GaAs Matrix

Ultrathin InAs layers with different thicknesses, from 0.75 to 1.4 monolayer, are grown in the GaAs matrix by molecular beam epitaxy on GaAs (001) substrates. For sub-monolayer heterostructures, islands or segregations exist during the growth process. Taking advantage of the high spatial resolution of focused electron beams, cathodoluminescence measurements obtain a smaller excitation spot than conventional photoluminescence. Based on the change on the peak position, line width, and intensity, cathodoluminescence spectra indicate that the size, geometry, and roughness develop with the InAs content. Moreover, spatial discontinuities of ultrathin InAs layers are observed on spectrum images and transmission electron microscopy images. This research reveals the correlation between the optical and structural properties of ultrathin InAs layers.

Doping of Al/CuO with microwave absorbing Ti3C2 MXene for improved ignition and combustion performance

Nanothermites have a wide application in many civil and defense applications, including propulsion and detonations systems, micro-electromechanical (MEMs) and other systems that require fast energy release rate and low ignition temperature. Traditional ignition methods are commonly based on surface excitation (spot ignition), possessing a risk of undesired ignition. Safe and selective microwave (MW)-based igniting systems can interact with an ignitable material (typically a thermite) over its full volume at once. Unfortunately, thermites have low susceptibility to MW due to the presence of Al2O3 oxide shell on Al particles, which would commonly require a high ignition power of MW radiation and long ignition delay times. In this work, Ti3C2 MXene was introduced as a MW susceptor into Al/CuO energetic nanocomposites. The samples were ignited using a MW probe, showing that the introduction of MXene into Al/CuO/MXene could significantly reduce the power required for its MW ignition, as well as shorten the ignition delay time versus the parent Al/CuO nanothermite. Besides, presence of MXene could modulate and control a heat release, gas production and combustion performance of the Al/CuO/MXene composites, substantially extending safety, flexibility and adaptability of new nanothermites to various operational requirements.

Side‐viewing handheld confocal Raman probe coupled with an off‐axis parabolic mirror for superficial epithelial Raman measurements of luminal organs

AbstractWe report on the development of a side‐viewing handheld confocal Raman probe coupled with an off‐axis parabolic (OAP) mirror for superficial epithelial Raman measurements of luminal organs. The OAP mirror realizes an effective numerical aperture of 0.36, a measured Raman excitation spot diameter of 130 μm and a depth of focus of 128 μm, allowing real time Raman spectra acquisition. The OAP mirror tilts the Raman optical path by 90°, making it well suited for superficial epithelium Raman interrogation of luminal organs. In addition, the probe results in no Raman interferences that may arise from transmission optics. The superficial epithelium Raman interrogation capability of the probe developed is validated through a two‐layer tissue phantom experiment. This work suggests the potential of the side‐viewing handheld confocal Raman probe coupled with an OAP mirror for enhancing the diagnosis of epithelial precancer and cancer of luminal organs.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging