Excitation Laser Spot Research Articles

The evolution of micro-photoluminescence spectra of PECVD diamond microcrystals along the vertical growth direction and their dependence on CH4 concentration

The changes in the shape of micro-photoluminescence spectra of PECVD diamond micro-crystal measured, depending on the position of the excitation laser spot along the crystallite height, were analyzed. It was ascertained that the processes of SiV defect formation non-monotonically depend on the distance from the Si substrate. At the distances of 2–20 μm the concentration of SiV defects increases, then at distances larger than 20 μm the number of SiV defects decreases. The concentration of NV− and NV0 defects monotonically increases with the distance from the Si substrate. The predomination of SiV defect formation at the beginning stages of the crystal growth is explained by the substantial concentration of carbon vacancies required for their formation. With the increase in the distance from the substrate, the crystalline perfection increases, the concentration of carbon vacancies decreases and the processes of NV− and NV0 defect formation dominate. The increase in CH4 fraction within 0.75–6 % leads to the increase in the volume fraction of graphite-like carbon, which is the good diffusion channel for Si atoms from the substrate into the plasma. Therefore, the concentration of SiV, NV−, and NV0 defects on the surface of the crystal depends on the volume fraction of graphite-like carbon defined by CH4 content.

Polariton Condensation in Gap-Confined States of Photonic Crystal Waveguides.

The development of patterned multiquantum well heterostructures in GaAs/AlGaAs waveguides has recently made it possible to achieve exciton-polariton condensation in a topologically protected bound state in the continuum (BIC). Polariton condensation was shown to occur above a saddle point of the two-dimensional polariton dispersion in a one-dimensional photonic crystal waveguide. A rigorous analysis of the condensation phenomenon in these systems, as well as the role of the BIC, is still missing. In the present Letter, we theoretically and experimentally fill this gap by showing that polariton confinement resulting from the negative effective mass and the photonic energy gap in the dispersion play a key role in enhancing the relaxation toward the condensed state. In fact, our results show that low-threshold polariton condensation is achieved within the effective trap created by the exciting laser spot, regardless of whether the resulting confined mode is long-lived (polariton BIC) or short-lived (lossy mode). In both cases, the spatial quantization of the polariton condensate and the threshold differences associated to the corresponding state lifetime are measured and characterized. For a given negative mass, a slightly lower condensation threshold from the polariton BIC mode is found and associated to its reduced radiative losses, as compared to the lossy one.

Dual-Color Lasers in Interlayer-Free Solution Processed Polymeric Bilayer Devices.

Multiwavelength organic lasers have attracted considerable interest in recent years due to the cost efficiency, wide luminescence coverage, and simple processability of organics. In this work, by simply spin coating immiscible polymeric gain media in sequence, dual-wavelength (blue-green or blue-red) amplified spontaneous emission (ASE) was achieved in bilayer devices. The blue emission, water/alcohol-soluble conjugated polyelectrolyte, poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), was used as the bottom layer. The commercially available nonpolar solvent soluble polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and its blend with poly(3-hexylthiophene) (P3HT) were used as the top active layers offering green and red emission, respectively. This novel compact configuration, without interlayers between the two active layers, offers potential for developing various applications. The carefully selected top and bottom layer polymers not only meet the conditions of immiscibility and different emission wavelength range but also have a common absorption band in UV, which allows simultaneous blue-green or blue-red dual-color ASE behaviors observed in the bilayer devices under the same 390 nm laser excitation. By introducing two-dimension (2D) square distributed feedback (DFB) gratings with different periods (300 nm for blue, 330 nm for green, and 390 nm for red) as cavities, single mode blue-green (Eth = 245 μJ cm-2) and blue-red (Eth = 189 μJ cm-2) lasers were achieved by focusing the excitation laser spot on different 2D DFB gratings area. Furthermore, we found it possible to gain sufficient light confinement for red emission along its diagonal direction (Λ ∼424 nm), whereas the 2D DFB gratings offer feedback for blue emission from the 300 nm period along the rectangle direction. Therefore, both blue and red lasers were eventually achieved in the same PFN-Br/F8BT:P3HT bilayer device on the single 2D DFB gratings with a period of 300 nm in this work.

Super-resolution imaging of micro- and nanoplastics using confocal Raman with Gaussian surface fitting and deconvolution

Confocal Raman imaging can directly identify and visualise microplastics and even nanoplastics. However, due to diffraction, the excitation laser spot has a size, which defines the image resolution. Consequently, it is difficult to image nanoplastic that is smaller than the diffraction limit. Within the laser spot, fortunately, the excitation energy density behaves an axially transcended distribution, or a 2D Gaussian distribution. By mapping the emission intensity of Raman signal, the imaged nanoplastic pattern is axially transcended as well and can be fitted as a 2D Gaussian surface via deconvolution, to re-construct the Raman image. The image re-construction can intentionally and selectively pick up the weak signal of nanoplastics, average the background noise/the variation of the Raman intensity, smoothen the image surface and re-focus the mapped pattern towards signal enhancement. Using this approach, along with nanoplastics models with known size for validation, real samples are also tested to image microplastics and nanoplastics released from the bushfire-burned face masks and water tanks. Even the bushfire-deviated surface group can be visualised as well, to monitor the different degrees of burning by visualising micro- and nanoplastics. Overall, this approach can effectively image regular shape of micro- and nanoplastics, capture nanoplastics smaller than the diffraction limit, and realise super-resolution imaging via confocal Raman.

Unidirectional Propagation of Spin Waves Excited by Femtosecond Laser Pulses in a Planar Waveguide

Low-energy magnonic logic circuits are an actively developing field of modern magnetism. The potential benefits of magnonics for data processing are vitally dependent on units based on non-reciprocal propagation of spin waves in analogy to semiconductor diodes and transistors in electronics. In this article, we suggest the approach to realize non-reciprocal propagation of spin waves in a ferromagnetic metallic waveguide by exciting them with femtosecond laser pulse. Using micromagnetic modeling, we show that the combination of an external magnetic field and the position of the excitation laser spot across the waveguide leads to unidirectional propagation of the excited spin-wave packet. The results are crucial for the design of hybrid magnonic-photonic circuits in future generations of data processing devices.

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.

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.

Surface optical sensitivity enhanced by a single dielectric microsphere.

Single dielectric microspheres can manipulate light focusing and collection to enhance optical interaction with surfaces. To demonstrate this principle, we experimentally investigate the enhancement of the Raman signal collected by a single dielectric microsphere, with a radius much larger than the exciting laser spot size, residing on the sample surface. The absolute microsphere-assisted Raman signal from a single graphene layer measured in air is more than a factor of two higher than that obtained with a high numerical aperture objective. Results from Mie's theory are used to benchmark numerical simulations and an analytical model to describe the isolated microsphere focusing properties. The analytical model and the numerical simulations justify the Raman signal enhancement measured in the microsphere-assisted Raman spectroscopy experiments.

Bi-chromaticity and tunability of random lasing in mesoporous silica SBA16 doped with rhodamine B

The random laser emission from ordered mesoporous silica SBA16 doped with rhodamine B (RB) organic dye was investigated. Powdered SBA16 with 16 nm average pore diameter have been synthesized and doped with five different concentrations of the organic dye. Typical incoherent feedback random laser behavior was observed. The bi-chromatic emission was characterized for the sample with the highest dye concentration. Tunable random laser emissions from 578 to 618 nm were obtained depending on the excitation laser spot diameter and the rhodamine-B load. The results indicate that mesoporous RB-doped SBA16 is a promising material for the development of solid-state random lasers.

Tunable random lasing in dye-doped mesoporous silica SBA-15

Mesoporous silica, known as SBA-15, possesses a hexagonal pore structure, with 8-nm-diameter pores and high surface area $(\ensuremath{\sim}700\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{2}/\mathrm{g})$. The mesoporosity of SBA-15 is excellent to be filled with luminescent chromophores aiming at optical and photonic applications. SBA-15 powder infiltrated with Rhodamine B (RB) was prepared with different RB loads. Using the second harmonic of a pulsed Nd:yttrium aluminum garnet (YAG) laser (8 ns, 10 Hz) as the excitation source, the light emission spectra of the samples were investigated. Typical incoherent feedback random laser behavior was observed. Tunable random laser emissions from 579 to 585 nm were obtained depending on the excitation laser spot diameter and the RB load. The results indicate that mesoporous SBA-15 infiltrated with RB is a promising material for the development of solid-state random lasers.

Effective Negative Diffusion of Singlet Excitons in Organic Semiconductors.

Using diffraction-limited ultrafast imaging techniques, we investigate the propagation of singlet and triplet excitons in single-crystal tetracene. Instead of an expected broadening, the distribution of singlet excitons narrows on a nanosecond time scale after photoexcitation. This narrowing results in an effective negative diffusion in which singlet excitons migrate toward the high-density region, eventually leading to a singlet exciton distribution that is smaller than the laser excitation spot. Modeling the excited-state dynamics demonstrates that the origin of the anomalous diffusion is rooted in nonlinear triplet–triplet annihilation (TTA). We anticipate that this is a general phenomenon that can be used to study exciton diffusion and nonlinear TTA rates in semiconductors relevant for organic optoelectronics.

Photothermoelectric SnTe Photodetector with Broad Spectral Response and High On/Off Ratio.

A broadband photodetector with high performance is highly desirable for the optoelectric and sensing application. Herein, we report a "photo-thermo-electric" (PTE) detector based on an ultrathin SnTe film. The (001)-oriented SnTe films with the wafer size scale are epitaxially grown on the surface of sodium chloride crystals by a scalable sputtering method. Due to the giant PTE effect under laser spot excitation on the asymmetric position between two terminals, a built-in electrical field is produced to drive bulk carriers for a self-powered photodetector, leading to a broad spectral response in the wavelength range from 404 nm to 10.6 μm far beyond the limitation of the energy band gap. Significantly, the photodetector displays a high on/off photoswitching ratio of over 105 with a suppressed dark current, which is 4-5 orders of magnitude higher than that of other reported SnTe-based detectors. Under zero external bias, the device yields the highest detectivity of ∼1.3 × 1010 cm Hz1/2 W-1 with a corresponding responsivity of ∼3.9 mA W-1 and short rising/falling times of ∼78/84 ms. Furthermore, the photodetector transferred onto the flexible template exhibits excellent mechanical flexibility over 300 bending cycles. These findings offer feasible strategies toward designing and developing low-power-consumption wearable optoelectronics with competitive performance.

Identification and visualisation of microplastics/ nanoplastics by Raman imaging (ii): Smaller than the diffraction limit of laser?

We recently reported (Sobhani et al., 2020) that when a confocal Raman microscope imaged a nanoplastic with the diameter of 100 nm, the imaging lateral size was 300–400 nm, due to the diffraction limit of the laser spot. In this study, we examine the lateral intensity distribution of the Raman signal emitted by nanoplastics (diameters ranging ∼30–600 nm) within the excitation laser spot. We find that the Raman emission intensity, similar to the excitation power density distributed within a laser spot, also follows a lateral Gaussian distribution. To image and visualise individual nanoplastics, we (i) decrease the mapping pixel size, in a hope to generate an image with high-resolution and simultaneously to pick up items from the “blind point”. We can then either (ii) offset the colour to intentionally image only the high-intensity portion of the Raman signal (emitted from the centre of the laser spot), to localise the exact position of the nanoplastic; or (iii) categorise the imaged nanoplastics to different groups via their Raman intensity, to simultaneously and separately visualise large nanoplastics/strong Raman signals, medium nanoplastics and small nanoplastics, in an effort to avoid the shielding and overlooking of weak signals. We (iv) also cross-check multi-images simultaneously mapped at two or three characteristic peaks via either a logic-OR or a logic-AND algorithm. Thus the imaging uncertainty can be significantly reduced from a statistical point of view.

Accurate Clinical Diagnosis of Liver Cancer Based on Simultaneous Detection of Ternary Specific Antigens by Magnetic Induced Mixing Surface-Enhanced Raman Scattering Emissions

Establishing an accurate, simple, and rapid serodiagnosis method aiming for specific cancer antigens is critically important for the clinical diagnosis, therapy, and prognostication of cancer. Currently, surface-enhanced Raman scattering (SERS) readout techniques challenge fluorescent-based detection methods in terms of both optical stability and more importantly multiple detection capability, which become more desirable for clinical diagnostics. We thus started using an interference-free mixing SERS emission (m-SERS) readout to simultaneously indicate, for the first time, three specific liver cancer antigens, including α-fetoprotein (AFP), carcinoembryonic antigen (CEA), and ferritin (FER), even in one clinical serum sample. Here, three triple bonds (C≡N and C≡C) coded SERS tags contribute separate SERS emissions located at 2105, 2159, and 2227 cm-1, respectively; must have one-to-one correspondence from AFP, to FER, to CEA, In the process of detection, the mature double antibody sandwich allows the formation of microscale core-satellite assembly structure between a magnetic bead (MB) and single SERS tags, and therefore a pure and single SERS emission can be observed under the routine excitation laser spot. Because of the action of magnetic force, the uniform 3D packing of SERS tags absorbed MBs will in contrast generate a so-called m-SERS signals. With the help of enrichment and separation by MBs, the proposed m-SERS immunoassay provides an extremely rapid, sensitive, and accurate solution for multiplex detection of antigens or other biomarkers. Herein, the limit of detection (LOD) for simultaneous m-SERS detection of AFP, CEA, and FER was 0.15, 20, and 4 pg/mL, respectively. As expected for 39 clinical serum samples, simultaneous detection of ternary specific antigens can significantly improve the accuracy of liver cancer diagnosis.

Determination of thickness and bulk sound velocities of isotropic plates using zero-group-velocity Lamb waves

We propose a method to simultaneously determine the thickness of an isotropic plate together with the longitudinal and shear elastic wave velocities of its material. The method requires knowledge of the frequencies of two zero-group-velocity Lamb modes and one respective wavenumber. These quantities are defined by the Rayleigh-Lamb equations, which we use in an inverse problem to obtain the properties of the plate. Experimentally, the frequencies of zero-group-velocity points can be obtained at high precision by measuring the elastic response spectrum of a plate, using laser-ultrasound techniques. By shaping the excitation laser spot with a spatial light modulator, we extend this to enable measurements of the corresponding wavenumber. The introduced method is demonstrated for a homogeneous tungsten and an aluminium plate.

Nondestructive Testing of Ceramic Hip Joint Implants with Laser Spot Thermography

AbstractThe paper presents an application of laser spot thermography for damage detection in ceramic samples with surface breaking cracks. The measurement technique is an active thermographic approach based on an external heat delivery to a test sample, by means of a laser pulse, and signal acquisition by an infrared camera. Damage detection is based on the analysis of surface temperature distribution near the exciting laser spot. The technique is nondestructive, non-contact and allows for full-field measurements. Surface breaking cracks are a very common type of damage in ceramic materials that are introduced in the manufacturing process or during the service period. This paper briefly discusses theoretical background of laser spot thermography, describes the experimental test rig and signal processing methods involved. Damage detection results obtained with laser spot thermography are compared with reference measurements obtained with vibrothermography. This is a different modality of active thermography, that has been previously proven effective for this type of damage. We demonstrate that both measurement techniques can be effectively used for damage detection and quality control applications of ceramic materials.

Spatially resolved and time-resolved imaging of transport of indirect excitons in high magnetic fields

We present the direct measurements of magnetoexciton transport. Excitons give the opportunity to realize the high magnetic field regime for composite bosons with magnetic fields of a few Tesla. Long lifetimes of indirect excitons allow the study kinetics of magnetoexciton transport with time-resolved optical imaging of exciton photoluminescence. We performed spatially, spectrally, and time-resolved optical imaging of transport of indirect excitons in high magnetic fields. We observed that increasing magnetic field slows down magnetoexciton transport. The time-resolved measurements of the magnetoexciton transport distance allowed for an experimental estimation of the magnetoexciton diffusion coefficient. An enhancement of the exciton photoluminescence energy at the laser excitation spot was found to anti-correlate with the exciton transport distance. A theoretical model of indirect magnetoexciton transport is presented and is in agreement with the experimental data.

Terahertz generation mechanism in nano-grating electrode photomixers on Fe-doped InGaAsP.

We report the generation mechanism associated with nano-grating electrode photomixers fabricated on Fe-doped InGaAsP substrates. Two different emitter designs incorporating nano-gratings coupled to the same broadband antenna were characterized in a continuous-wave terahertz (THz) frequency system employing telecommunications wavelength lasers for generation and coherent detection. The current-voltage characteristics and THz emission bandwidth of the emitters is compared for different bias polarities and optical polarisations. The THz output from the emitters is also mapped as a function of the position of the laser excitation spot for both continuous-wave and pulsed excitation. This mapping, together with full-wave simulations of the structures, confirms the generation mechanism to be due to an enhanced optical electric field at the grating tips resulting in increased optical absorption, coinciding with a concentration of the electrostatic field.

Exciton-polariton gap soliton dynamics in moving acoustic square lattices

The modulation by a surface acoustic wave (SAW) provides a powerful tool for the formation of tunable lattices of exciton-polariton macroscopic quantum states (MQSs) in semiconductor microcavities. The MQSs were resonantly excited in an optical parametric oscillator configuration. We investigate the temporal dynamics of these lattices using time and spatially resolved photoluminescence (PL). Photoluminescence images of the MQSs clearly show the motion of the lattice at the acoustic velocity. Interestingly, the PL intensity emitted by the MQSs as well as their coherence length oscillate with the position of the lattice sites relative to the exciting laser beam. The coherence length and the PL intensity are correlated. The PL oscillation amplitude depends on both the intensity and the size of the exciting laser spot and increases considerably for excitation intensities close to the optical threshold power for the formation of the MQS. The oscillations are explained by a model that takes into account the combined effects of SAW reflections, which dynamically distort the amplitude of the potential, and the spatial phase of the acoustic lattice within the exciting laser spot. This paper could pave the way to tailor polariton-based light-emitting sources with intensity variations controlled by the SAWs.

Computer Simulation of Metal Surface Micro-Crack Inspection Using Pulsed Laser Thermography

Surface micro cracks are easy to produce in the preparation and service process of metal material, which impacts on the safe operation of metal components. Pulsed laser spot excitation and infrared thermal imaging technology are combined to detect metal surface micro-cracks. The working principle of laser infrared thermal imaging detection technology was described. The three dimensional heat conduction model of pulsed laser excitation flux transfer in metal plate was established, and calculated using finite element method (FEM). The results showed that, thermal flow in the image is a D shape. There are temperature differences between the sound regions and defective regions, and the defects experiences the process of obscure, gradually clear, and gradually obscure. Pulsed infrared thermography sequence was processed by polynomial fitting method, and the coefficient images effectively improve the contrast between defective and non-defective areas, which is beneficial to the determination and of recognition micro cracks.