Measurement of Gaussian laser beam radius using the knife-edge technique: improvement on data analysis

Broadband and efficient third-harmonic generation from black phosphorus-hybrid plasmonic metasurfaces in the mid-infrared

Black phosphorus (BP), with a mid-infrared (MIR) bandgap of 0.34 eV, presents itself as a promising material for MIR nonlinear optical applications. We report the realization of MIR third-harmonic generation (THG) in both BP and BP-hybrid plasmonic metasurfaces (BPM). BP exhibits a high third-order nonlinear susceptibility ([Formula: see text]) exceeding 10-18 m2/V2 in the MIR region with a maximum value of 1.55 × 10-17 m2/V2 at 5000 nm. The BP flake achieves a THG conversion efficiency of 1.4 × 10-5, surpassing that of other 2D materials by over one order of magnitude. To further enhance this nonlinear performance, a BPM is designed and fabricated to achieve a two-order-of-magnitude enhancement in THG, leading to a record conversion efficiency of 6.5 × 10-4, exceeding the performance of previously reported metasurfaces by more than one order of magnitude. These findings establish BP as a promising platform for next-generation MIR nonlinear optical devices.

Design and commissioning of a THz time-domain spectro-goniometer in a cryogenic comet simulation chamber

We present our newly developed laboratory setup, called COCoNuT (Characteristic Observation of Cometary Nuclei using THz-spectroscopy), consisting of a THz spectro-goniometer in a vacuum chamber along with a cryo-cooler intended for cooling down samples rich in ices expected in cometary environments. This setup allows the use of THz time domain spectroscopy on analogs including refractory materials in the spectral range from 0.1 to 5.5 THz with a spectral resolution of up to 0.005 THz. Since the optical setup is mounted on a precision controlled x/y-stage, imaging or spatial averaging can be performed. In this work, we present the design decisions and selection of components. Furthermore, the commissioning of the setup is described and the capabilities are shown. The system can reach pressures of 10-7 mbar and temperatures of 50 K. For 2D scanning, a spatial resolution of 0.3 line-pair per mm is obtained.

Fast Terahertz Reflection Imaging for In-Line Detection of Delaminations in Glass Fiber-Reinforced Polymers

Terahertz (THz) is an emerging technology particularly well suited for the non-destructive investigation of inner structures in polymers. To realize its full potential, THz imaging systems adapted to industrial constraints as well as more application studies in areas of interest are needed. In this work, we present a fast and flexible THz imaging system comprising hardware and software and demonstrate its capabilities for the investigation of defects in glass fiber-reinforced polymers (GFRPs), particularly for the detection of drilling-induced delaminations. Measurement data obtained by raster scanning of GFRP samples are gathered in 3D volumetric images. THz images of the drilled holes are then compared to reference images of the same holes obtained from X-ray computed tomography measurements. We show that THz imaging is capable of identifying not only artificial defects in the form of aluminum and Teflon inlays, but also real defects such as delaminations generated by drilling operations, and is suitable for non-destructive testing in industrial conditions.

Sensitivity Chronicles in Molecular Properties: Unraveling the Nexus of Dual-Beam Z-Scan and Time-Resolved Thermal Lens Spectroscopy

Ultrasensitive thermal lens (TL) spectroscopy, where the interplay between conduction and convection is crucial, provides profound insights into molecular behavior. This work focuses on the critical role of molecular convection by using the dual-beam z-scan method and time-resolved TL spectroscopy. We specifically investigated the correlation between the detection iris and the spot size of the pump beam. Additionally, we address detection limitations and their influence on the perception of convection timing. We introduce an experimentally derived ratio optimized for higher sensitivity, enhancing the reliability of TL spectroscopy. This refined approach to TL spectroscopy allows for a deeper exploration of molecular characteristics in liquids.

Application of the Knife-Edge Technique on Transition Metal Dichalcogenide Monolayers for Resolution Assessment of Nonlinear Microscopy Modalities

We report application of the knife-edge technique at the sharp edges of WS2 and MoS2 monolayer flakes for lateral and axial resolution assessment in all three modalities of nonlinear laser scanning microscopy: two-photon excited fluorescence (TPEF), second- and third-harmonic generation (SHG, THG) imaging. This technique provides a high signal-to-noise ratio, no photobleaching effect and shows good agreement with standard resolution measurement techniques. Furthermore, we assessed both the lateral resolution in TPEF imaging modality and the axial resolution in SHG and THG imaging modality directly via the full-width at half maximum parameter of the corresponding Gaussian distribution. We comprehensively analyzed the factors influencing the resolution, such as the numerical aperture, the excitation wavelength and the refractive index of the embedding medium for the different imaging modalities. Glycerin was identified as the optimal embedding medium for achieving resolutions closest to the theoretical limit. The proposed use of WS2 and MoS2 monolayer flakes emerged as promising tools for characterization of nonlinear imaging systems.

Impact of excitation pulse width on the upconversion luminescence lifetime of NaYF4:Yb3+,Er3+ nanoparticles

The upconversion luminescence (UCL) lifetime has a wide range of applications, serving as a critical parameter for optimizing the performance of upconversion nanoparticles (UCNPs) in various fields. It is crucial to understand that this lifetime does not directly correlate with the decay time of the emission level; rather, it represents a compilation of all the physical phenomena taking place in the upconversion process. To delve deeper into this, we analyzed the dependence of the UCL lifetime on the excitation pulse width for β-NaYF4:Yb3+,Er3+ nanoparticles. The results revealed a significant increase in the UCL lifetime with both the excitation pulse width and the excitation intensity. The laser fluence was identified as the parameter governing the UCL decay dynamics. We showcased the universality of the pulse-width-dependent UCL lifetime phenomenon by employing UCNPs of various sizes, surface coatings, host matrices, Yb3+ and Er3+ ratios, and dispersing UCNPs in different solvents. Theoretical explanations for the experimental findings were derived through a rate equation analysis. Finally, we discussed the implications of these results in UCNP-FRET (Förster resonance energy transfer)-based applications.

Power Density Titration of Reversible Photoisomerization of a Fluorescent Protein Chromophore in the Presence of Thermally Driven Barrier Crossing Shown by Quantitative Millisecond Serial Synchrotron X-ray Crystallography

We present millisecond quantitative serial X-ray crystallography at 1.7 Å resolution demonstrating precise optical control of reversible population transfer from Trans-Cis and Cis-Trans photoisomerization of a reversibly switchable fluorescent protein, rsKiiro. Quantitative results from the analysis of electron density differences, extrapolated structure factors, and occupancy refinements are shown to correspond to optical measurements of photoinduced population transfer and have sensitivity to a few percent in concentration differences. Millisecond time-resolved concentration differences are precisely and reversibly controlled through intense continuous wave laser illuminations at 405 and 473 nm for the Trans-to-Cis and Cis-to-Trans reactions, respectively, while the X-ray crystallographic measurement and laser illumination of the metastable Trans chromophore conformation causes partial thermally driven reconversion across a 91.5 kJ/mol thermal barrier from which a temperature jump between 112 and 128 K is extracted.

Highly efficient continuous-wave solid-state Raman crystal lasers at 555 and 559 nm

High-power efficient continuous-wave Nd:YVO4/KGW Raman lasers at 555 and 559 nm are achieved by using a double-sided dichroic coating output coupler to improve the resonance quality factor. The Np-cut potassium gadolinium tungstate (KGW) is used to generate the Stokes waves at 1159 and 1177 nm by placing the polarization of the1064 nm fundamental wave parallel to the Ng and Nm axes, respectively. The lithium triborate (LBO) crystal with the cut angle in the XY plane for the type-I phase matching is used to perform the intracavity sum frequency generation for yielding the green light at 555 nm and the lime light at 559 nm at the optimal phase matching temperature. Experimental results were systematically accomplished to comprehend the optimal cavity length for the conversion efficiency. Under the optimal cavity length, the output powers can reach 6.6 and 6.3 W at a pump power of 22 W for the wavelengths of 555 and 559 nm, respectively. The conversion efficiencies can be up to 30% and 28.6% for 555 and 559 nm, respectively.

Electrically tunable plasmonic metasurface as a matrix of nanoantennas

We report the fabrication and characterization of a plasmonic metasurface comprising electrically-contacted sub-wavelength gold dipole nanoantennas, conformally coated by a thin hafnia film, an indium tin oxide layer and a backside mirror, forming metal-oxide-semiconductor (MOS) capacitors, for use as an electrically-tunable reflectarray or metasurface. By voltage biasing the nanoantennas through metallic connectors and leveraging the carrier refraction effect in the MOS capacitors, our measurements demonstrate phase control in reflection over a range of about 30°, with a constant magnitude of reflection coefficient of 0.5, and the absence of secondary lobes. Comprehensive electromagnetic and quantum carrier models of the structure are developed and are in excellent agreement with the measurements. The metasurface holds promise for use as an optical phased array.

Laser induced damage threshold of GaSe with antireflection microstructures at a wavelength of 5 µm

Large GaSe crystals were grown and various antireflection microstructures (ARMs) were fabricated on their cleaved surfaces using optimized femtosecond laser ablation, which provided the antireflection effect in a wide wavelength range of 4-16 µm. The influence of ARMs created on the GaSe surface on the change of the laser-induced damage threshold (LIDT) of the crystal at a wavelength of 5 μm was evaluated. The 5-µm Fe:ZnMgSe laser with the pulse duration of 135 ns was used for the LIDT test in conditions close to single pulse exposure. The measured values of LIDT of 56 ± 6 MW/cm2 and 51 ± 9 MW/cm2 for two GaSe substrates, respectively, were comparable with the known data of single pulse LIDT of GaSe. The average LIDT intensities of 54 ± 6 MW/cm2 and 52 ± 7 MW/cm2 for the ARMs at two GaSe plates, respectively, were close to LIDT intensities for the corresponding GaSe substrates. The ARMs with lower structural quality had lower LIDT (50-52 MW/cm2) in comparison with the high-quality ARMs (58-60 MW/cm2). High LIDT for high-quality ARMs can be caused by increased selenium content in the ARMs. In any case, all the tested ARMs on the GaSe plates with different surface quality are workable for development of widely tunable mid-infrared nonlinear optical converters.

Bimodal optical and optoacoustic multiview microscope in the frequency-domain

Hybrid fluorescence and optoacoustic microscopy systems have recently emerged as powerful imaging modalities concurrently capturing both radiative and non-radiative molecular relaxations in biological tissues. Nevertheless, such approaches provide limited information as specimens are imaged exclusively from one side, not permitting the acquisition of their full anatomical, structural, or functional features in multiple views of interest. Herein we present a bimodal optical and optoacoustic multiview (BOOM) cost-efficient microscope operating in the frequency-domain for the comprehensive label-free imaging of established and emerging model organisms. Thus, the capabilities of BOOM microscopy have been proven suitable for highly demanding observations in developmental biology and embryology.

Anion Tuned Structural Modulation and Nonlinear Optical Effects of Metal-Ion Directed 310 -Helix Networks

Metals play an important role in the structure and functions of various proteins. The combination of metal ions and peptides have been emerging as an attractive field to create advanced structures and biomaterials. Here, we are reporting the anion-influenced, silver ion coordinated diverse networks of designed short tripeptide 310 -helices with terminal pyridyl groups. The short peptides adopted classical right-handed, left-handed and 310 EL -helical conformations in the presence of different silver salts. The peptides have displayed conformational flexibility to accommodate different sizes and interactions of anions to yield a variety of metal-coordinated networks. The complexes of metal ions and peptides have shown different porous networks, right- and left-handed helical polymers, transformation of helix into superhelix and 2 : 2 metal-peptide macrocycles. Further, the metal-peptide crystals with inherent dipoles of helical peptides gave striking second harmonic generation response. The optical energy upconversion from NIR to red and green light is demonstrated. Overall, we have shown the utilization of short 310 -helices for the construction of diverse metal-coordinated helical networks and notable non-linear optical effects.

Optimized terahertz pulse generation with chirped pump pulses from an echelon-based tilted-pulse-front (TPF) scheme

We successfully demonstrated the generation of single-cycle terahertz (THz) pulses through tilted-pulse-front (TPF) pumping using a reflective echelon in a lithium niobate crystal. By optimizing the pump pulse duration using a chirp, we achieved a maximum pump-to-THz conversion efficiency of 0.39%. However, we observed that the saturation behavior began at a relatively low pump energy (0.37 mJ), corresponding to a pump intensity of 22 GW/cm2. To elucidate this behavior, we measured the near- and far-field THz beam profiles and found variations in their beam characteristics, such as the beam size, location, and divergence angle in the plane of the tilted pulse direction, with the pump energy (intensity). This nonlinear behavior is attributed to the reduced effective interaction length, which ultimately leads to the saturation of THz generation. The results obtained from our study suggest that it is feasible to develop an effective THz source using echelon-based TPF pumping while also considering the impact of nonlinear saturation effects.

Accuracy of Measurements of Thermophysical Parameters by Dual-Beam Thermal-Lens Spectrometry

Thermal-lens spectrometry is a sensitive technique for determination of physicochemical properties and thermophysical parameters of various materials including heterogeneous systems and nanoparticles. In this paper, we consider the issues of the correctness (trueness) of measurements of the characteristic time of the thermal-lens effect and, thus, of the thermal diffusivity determined by dual-beam mode-mismatching thermal lensing. As sources of systematic errors, major factors-radiation sources, sample-cell and detector parameters, and general measurement parameters-are considered using several configurations of the thermal-lens setups, and their contributions are quantified or estimated. Furthermore, with aqueous ferroin and Sudan I in ethanol as inert colorants, the effects of the intermolecular distance of the absorbing substance on the correctness of finding the thermophysical parameters are considered. The recommendations for checking the operation of the thermal-lens setup to ensure the maximum accuracy are given. The results obtained help reducing the impact of each investigated factor on the value of systematic error and correctly measure the thermophysical parameters using thermal-lens spectrometry.

Versatile Confocal Raman Imaging Microscope Built from Off-the-Shelf Opto-Mechanical Components

Confocal Raman microscopic (CRM) imaging has evolved to become a key tool for spatially resolved, compositional analysis and imaging, down to the μm-scale, and nowadays one may choose between numerous commercial instruments. That notwithstanding, situations may arise which exclude the use of a commercial instrument, e.g., if the analysis involves toxic or radioactive samples/environments; one may not wish to render an expensive instrument unusable for other uses, due to contamination. Therefore, custom-designed CRM instrumentation-being adaptable to hazardous conditions and providing operational flexibility-may be beneficial. Here, we describe a CRM setup, which is constructed nearly in its entirety from off-the-shelf optomechanical and optical components. The original aim was to develop a CRM suitable for the investigation of samples exposed to tritium. For increased flexibility, the CRM system incorporates optical fiber coupling to both the Raman excitation laser and the spectrometer. Lateral raster scans and axial profiling of samples are facilitated by the use of a motorized xyz-translation assembly. Besides the description of the construction and alignment of the CRM system, we also provide (i) the experimental evaluation of system performance (such as, e.g., spatial resolution) and (ii) examples of Raman raster maps and axial profiles of selected thin-film samples (such as, e.g., graphene sheets).

Electro-Optical Sampling of Single-Cycle THz Fields with Single-Photon Detectors

Electro-optical sampling of Terahertz fields with ultrashort pulsed probes is a well-established approach for directly measuring the electric field of THz radiation. This technique usually relies on balanced detection to record the optical phase shift brought by THz-induced birefringence. The sensitivity of electro-optical sampling is, therefore, limited by the shot noise of the probe pulse, and improvements could be achieved using quantum metrology approaches using, e.g., NOON states for Heisenberg-limited phase estimation. We report on our experiments on THz electro-optical sampling using single-photon detectors and a weak squeezed vacuum field as the optical probe. Our approach achieves field sensitivity limited by the probe state statistical properties using phase-locked single-photon detectors and paves the way for further studies targeting quantum-enhanced THz sensing.

Assessment of a 3050/3200 nm fiber laser system for ablative fractional laser treatments in dermatology

Background and Objectives

Mid‐infrared (IR) ablative fractional laser treatments are highly efficacious for improving the appearance of a variety of dermatological conditions such as photo‐aged skin. However, articulated arms are necessary to transmit the mid‐IR light to the skin, which restricts practicality and clinical use. Here, we have assessed and characterized a novel fiber laser‐pumped difference frequency generation (DFG) system that generates ablative fractional lesions and compared it to clinically and commercially available thulium fiber, Erbium:YAG (Er:YAG), and CO₂ lasers.

Materials and Methods

An investigational 20 W, 3050/3200 nm fiber laser pumped DFG system with a focused spot size of 91 µm was used to generate microscopic ablation arrays in ex vivo human skin. Several pulse energies (10–70 mJ) and pulse durations (2–14 ms) were applied and lesion dimensions were assessed histologically using nitro‐blue tetrazolium chloride stain. Ablation depths and coagulative thermal damage zones were analyzed across three additional laser systems.

Results

The investigational DFG system‐generated deep (>2 mm depth) and narrow (<100 µm diameter) ablative lesions surrounded by thermal coagulative zones of at least 20 µm thickness compared to 13, 40, and 320 µm by the Er:YAG, CO₂, and Thulium laser, respectively.

Conclusion

The DFG system is a small footprint device that offers a flexible fiber delivery system for ablative fractional laser treatments, thereby overcoming the requirement of an articulated arm in current commercially available ablative lasers. The depth and width of the ablated microcolumns and the extent of surrounding coagulation can be controlled; this concept can be used to design new treatment procedures for specific indications. Clinical improvements and safety are not the subject of this study and need to be explored with in vivo clinical studies.

Highly accurate THz-CT including refraction effects

The principles of algebraic image reconstruction are applied to THz computed tomography (THz-CT) in order to account for refraction within the sample. Using the nominal sample geometry as a priori knowledge, a highly accurate and robust image reconstruction algorithm based on the physics of geometric optics is presented. The validity of the geometric forward model is verified by a numerical simulation of Maxwell's equations. Furthermore, the developed method is experimentally tested using measurements performed with a fast THz-CT system based on a THz time-domain spectrometer in transmission mode. Automated evaluations of the reconstructed sample cross sections showed an accuracy of <150 μm.

Nanoparticle Fragmentation Below the Melting Point Under Single Picosecond Laser Pulse Stimulation

Understanding the laser-nanomaterials interaction including nanomaterial fragmentation has important implications in nanoparticle manufacturing, energy, and biomedical sciences. So far, three mechanisms of laser-induced fragmentation have been recognized including non-thermal processes and thermomechanical force under femtosecond pulses, and the phase transitions under nanosecond pulses. Here we show that single picosecond (ps) laser pulse stimulation leads to anomalous fragmentation of gold nanoparticles that deviates from these three mechanisms. The ps laser fragmentation was weakly dependent on particle size, and it resulted in a bimodal size distribution. Importantly, ps laser stimulation fragmented particles below the whole particle melting point and below the threshold for non-thermal mechanism. We propose a framework based on near-field enhancement and nanoparticle surface melting to account for the ps laser-induced fragmentation observed here. This study reveals a new form of surface ablation that occurs under picosecond laser stimulation at low fluence.

Evaluation of the role of beam homogeneity on the mechanical coupling of laser-ablation-generated impulse

The material emitted from a target surface during laser ablation generates a net thrust (propulsion) in the opposite direction. The momentum generation efficiency of this laser-driven propulsion is given by the mechanical coupling coefficient (C_m). In this work, we considered nanosecond UV laser ablation of the aluminum 6061 alloy to study the C_m behavior with different irradiating conditions. This is done by systematically changing fluence, uniform/nonuniform intensity, and incident angle of the laser beam. In particular, we found that when dealing with nonuniform laser intensity, characterizing C_m exclusively in terms of fluence is not fully satisfactory because the energy distribution over the irradiated area plays a key role in the way material is removed-interplay between vaporization and phase explosion-and thrust is generated.

Simple and robust method for determination of laser fluence thresholds for material modifications: an extension of Liu's approach to imperfect beams

The so-called D-squared or Liu's method is an extensively applied approach to determine the irradiation fluence thresholds for laser-induced damage or modification of materials. However, one of the assumptions behind the method is the use of an ideal spatial Gaussian beam that can lead in practice to significant errors depending on beam imperfections. In this work, we rigorously calculate the bias corrections required when applying the same method to Airy-disk like profiles. Those profiles are readily produced from any beam by insertion of an aperture in the optical path. Thus, the correction method gives a robust solution for exact threshold determination without any added technical complications as for instance advanced control or metrology of the beam. Illustrated by two case-studies, the approach holds potential to solve the strong discrepancies existing between the laser-induced damage thresholds reported in the literature.  It provides also an appropriate tool for new studies with the most extreme laser radiations.

Precise determination of the focal point on a glass sample using spectroscopy analysis

Laser marking of various materials is commonly used nowadays in commercial products. In the past two decades, CO₂ lasers have been used extensively to mark insulators such as glass and wood during industrial production. Usually, a system of mirrors is used during a marking procedure. Although a laser beam can be characterized accurately using well-known methods, it is desirable to identify where the focal point is after reflecting on the scanning mirrors. The positioning of a motorized stage with a knife-edge and a sensing device to characterize a beam after reflecting through a mirror scanner system is impractical due to limited space. The method described here to determine the focal point accurately requires using only a large numerical aperture fiber connected to a motorized stage. In this paper, we investigate how spectroscopy of typical emission lines can be used to identify the position of the focal point in real time during laser marking procedures.

Study of laser actions by bird's feathers with photonic crystals

Random lasers had been made by some biomaterials as light scattering materials, but natural photonic crystals have been rarely reported as scattering materials. Here we demonstrate the ability of natural photonic crystals to drive laser actions by sandwiched the feathers of the Turquoise-Fronted Amazon parrot and dye between two plastic films. Parrot feathers comprise abundant photonic crystals, and different color feathers compose of different ratios of the photonic crystal, which directly affect the feather reflectance. In this study, the multi-reflection scattering that occurred at the interface between the photonic crystal and gain media efficiently reduce the threshold; therefore, the more photonic crystal constitutes in the feathers; the lower threshold can be obtained. The random lasers can be easily made by the integration of bird feather photonic crystals and dye with a simple and sustainable manufacturing approach.

Performance Evaluation of a THz Pulsed Imaging System: Point Spread Function, Broadband THz Beam Visualization and Image Reconstruction

Terahertz (THz) technology is a promising research field for various applications in basic science and technology. In particular, THz imaging is a new field in imaging science, where theories, mathematical models and techniques for describing and assessing THz images have not completely matured yet. In this work, we investigate the performances of a broadband pulsed THz imaging system (0.2–2.5 THz). We characterize our broadband THz beam, emitted from a photoconductive antenna (PCA), and estimate its point spread function (PSF) and the corresponding spatial resolution. We provide the first, to our knowledge, 3D beam profile of THz radiation emitted from a PCA, along its propagation axis, without the using of THz cameras or profilers, showing the beam spatial intensity distribution. Finally, we evaluate the THz image formation on a test-sample composed by a regular linen natural pattern.

Simple and effective coupling technique for polarization maintaining fibers

We present a simple and effective technique for coupling free-space laser beams into polarization maintaining fibers (PMFs) with high coupling efficiency. We measure both input and output laser beam sizes near the PMF by using the knife-edge method and build a suitable two-lens system for beam shaping according to the difference between those two beam sizes. For tapered amplifiers, we achieve high coupling efficiency above 70% with the help of the seeding mirrors. For external cavity diode lasers, we obtain high coupling efficiency above 80%. In addition, we demonstrate that theoretical maximum coupling efficiency can be approached by using a mode-filtered beam. Our technique is easy to implement and suitable for many applications such as coherent optical communication, atomic physics experiments, and precision measurements.

Thermal Analysis of Infrared Irradiation-Assisted Nanosecond-Pulsed Tumor Ablation

Nanosecond Pulsed Electric Fields (nsPEF) have the potential to treat a variety of cancer types including melanoma, pancreatic and lung squamous cancers. Recent studies show that nsPEF-based cancer therapy may be improved further with the assistance of moderate heating of the target. A feedback-looped heating system, utilizing a 980-nm fiber optic laser, was integrated into nsPEF electrodes for tumor ablation. The laser beam profile was determined to be Gaussian using a knife-edge technique. Thermal properties of the biological target were evaluated based on the treatment area, penetration depth and thermal distribution due to laser irradiation with or without nsPEF. Synergistic effects between nsPEF and the moderately elevated temperature at the target was observed, resulting in enhanced overall survival tumor regression up to 50% in the treatment of lung squamous cell cancer in mice.

Thermal properties of thin films made from MoS2 nanoflakes and probed via statistical optothermal Raman method

A deep understanding of the thermal properties of 2D materials is crucial to their implementation in electronic and optoelectronic devices. In this study, we investigated the macroscopic in-plane thermal conductivity (κ) and thermal interface conductance (g) of large-area (mm2) thin film made from MoS2 nanoflakes via liquid exfoliation and deposited on Si/SiO2 substrate. We found κ and g to be 1.5 W/mK and 0.23 MW/m2K, respectively. These values are much lower than those of single flakes. This difference shows the effects of interconnections between individual flakes on macroscopic thin film parameters. The properties of a Gaussian laser beam and statistical optothermal Raman mapping were used to obtain sample parameters and significantly improve measurement accuracy. This work demonstrates how to address crucial stability issues in light-sensitive materials and can be used to understand heat management in MoS2 and other 2D flake-based thin films.

High-intensity and low-divergence THz laser with 1D autofocusing symmetric Airy beams

Single-mode THz quantum-cascade lasers (QCLs) have been realized using a wide variety of techniques to obtain a combination of large power output, good beam quality with single-lobed beams, and low far-field divergence. Beam shaping using external components has not been previously exploited due to limited commercial availability of THz optical components and also the accompanying large loss from most THz optical materials. Here, we demonstrate that excellent beam characteristics could be obtained for a THz QCL by integration of a surface-emitting distributed-feedback (DFB) QCL with a simple lens within the vacuum chamber of a cryocooler. Plano-convex lenses are made from inexpensive plastic balls and integrated in proximity with a 3.4 THz DFB QCL. With appropriately chosen lens parameters, dual-lobed Airy beams are generated that autofocus into a high-intensity single-lobed beam with large focusing efficiency. A simple and general method to generate one-dimensional autofocusing Airy beams is thus demonstrated that is applicable at any wavelength. THz laser beams with high peak intensity (57 mW/mm² in a spot-size of 1 mm²) or low-divergence (1.4^∘×1.8^∘ for a beam with 118 mW peak power) are realized at 62 K in a compact electrically operated Stirling cooler. A high brightness of 5.4×10⁶ Wsr ^-1m ^-2 is estimated for the focused beams by measuring the beam propagation ratios (M ² parameters).

Development of a subsurface LIBS sensor for in situ groundwater quality monitoring with applications in CO2 leak sensing in carbon sequestration

Sub-surface activity such as geologic carbon sequestration (GCS) has the potential to contaminate groundwater sources with dissolved metals originating from sub-surface brines or leaching of formation rock. Therefore, a Laser Induced Breakdown Spectroscopy (LIBS) based sensor is developed for sub-surface water quality monitoring. The sensor head is built using a low cost passively Q-switched (PQSW) laser and is fiber coupled to a pump laser and a gated spectrometer. The prototype sensor head was constructed using off the shelf components and a custom monolithic, PQSW laser and testing has verified that the fiber coupled design performs as desired. The system shows good calibration linearity for tested elements (Ca, Sr, and K), quick data collection times, and Limits of Detection (LODs) that are comparable to or better than those of table top, actively Q-switched systems. The fiber coupled design gives the ability to separate the PQSW LIBS excitation laser from the pump source and spectrometer, allowing these expensive and fragile components to remain at the surface while only the low-cost, all optical sensor head needs to be exposed to the hostile downhole environment.

Low-cost Gaussian beam profiling with circular irises and apertures

The fundamental Gaussian TEM₀₀ mode is the most common mode of propagation within various optical devices, modules, and systems. Beam profilers are widely used in accurately ascertaining the cross-sectional irradiance profile of a TEM₀₀ mode for free-space optical communication systems as well as tracking beam evolution when propagating within optical submodules. We demonstrate beam profiling methods that use low-cost, off-the-shelf, widely available circular apertures such as circular irises and spatial filters. In order to demonstrate beam profiling with any circular aperture, we first derive exact analytical expressions for power transmittance of the TEM₀₀ mode through a decentered circular aperture and then use this mathematical derivation to estimate the irradiance profile of a Gaussian beam by 1) fixing the location of a circular aperture and changing its radius, and 2) scanning the entire area of the beam profile by translating a circular aperture of a fixed radius across the region of interest. This method is fast and easily reproducible and simply puts to use circular irises/circular spatial filters, which are commonly available in most optical laboratories. Consequently, the proposed method provides cheap and convenient means to estimate the profile of a Gaussian beam with simple optical components. Our experimental results demonstrate a performance that is comparable to a standard knife-edge-based estimate of beam profile. Moreover, a strong agreement with presented theory validates the analytical expressions derived in this paper.

Study on Optical Properties and Internal Quantum Efficiency Measurement of GaN-based Green LEDs

In this paper, the optical properties of GaN-based green light emitting diode (LED) are investigated and the internal quantum efficiency (IQE) values are measured by temperature dependent photoluminescence (TDPL) and power dependent photoluminescence (PDPL) methods. The "S-shaped” shift of peak wavelength measured at different temperature disappears gradually and the spectra broadening can be observed with increasing excitation power. The IQE calculation results of TDPL, which use the integrated intensity measured at low temperature as unity, can be modified by PDPL in order to acquire more accurate IQE values.

Ultrafast dynamics of the antibiotic Rifampicin in solution

Ultrafast time-resolved studies demonstrate that intra- and intermolecular H-bonds with water molecules act synergistically to stabilize the active zwitterionic form of Rifampicin, an effective antibiotic against mycobacterial infections.

Assessment of skin lesions produced by focused, tunable, mid-infrared chalcogenide laser radiation

BACKGROUND

Traditionally, fractional laser treatments are performed with focused laser sources operating at a fixed wavelength. Using a tunable laser in the mid-infrared wavelength range, wavelength-dependent absorption properties on the ablation process and thermal damage formation were assessed with the goal to obtain customizable tissue ablations to provide guidance in finding optimized laser exposure parameters for clinical applications.

METHODS

Laser tissue experiments were carried out on full thickness ex vivo human abdominal skin using a mid-infrared tunable chromium-doped zinc selenide/sulfide chalcogenide laser. The laser has two independent channels: a continuous wave (CW) output channel which covers a spectrum ranging from 2.4 μm to 3.0 μm with up to 9.2 W output power, and a pulsed output channel which ranges from 2.35 μm to 2.95 μm. The maximum pulse energy of the pulsed channel goes up to 2.8 mJ at 100 Hz to 1,000 Hz repetition rate with wavelength-dependent pulse durations of 4-7 ns.

RESULTS

Total ablation depth, ablation efficiency, and coagulation zone thickness were highly correlated to wavelength, pulse width, and pulse energy. Using the same total radiant exposure at 2.85 μm wavelength resulted in 10-times smaller coagulation zones and 5-times deeper ablation craters for one hundred 6 ns pulses compared to one 100 ms pulse. For a fixed pulse duration of 6 ns and a total radiant exposure of 2.25 kJ/cm² the ablation depth increased with longer wavelengths.

CONCLUSION

The tunable laser system provides a useful research tool to investigate specific laser parameters such as wavelength on lesion shape, ablation depth and thermal tissue damage. It also allows for customization of the characteristics of laser lesions and therefore facilitates the selection of suitable laser parameters for optimized fractional laser treatments. Lasers Surg. Med. 50:961-972, 2018.© 2018 Wiley Periodicals, Inc.

Automated translating beam profiler for in situ laser beam spot-size and focal position measurements

We present a simple and convenient, high-resolution solution for automated laser-beam profiling with axial translation. The device is based on a Raspberry Pi computer, Pi Noir CMOS camera, stepper motor, and commercial translation stage. We also provide software to run the device. The CMOS sensor is sensitive over a large wavelength range between 300 and 1100 nm and can be translated over 25 mm along the beam axis. The sensor head can be reversed without changing its axial position, allowing for a quantitative estimate of beam overlap with counter-propagating laser beams. Although not limited to this application, the intended use for this device is the automated measurement of the focal position and spot-size of a Gaussian laser beam. We present example data of one such measurement to illustrate device performance.

Intense broadband mid-infrared pulses of 280 MV/cm for supercontinuum generation in gaseous medium

We produce extremely bright mid-infrared (mid-IR) pulses with a tunable wavelength of 7 μm to 15 μm through difference frequency generation. Optimization of beam quality and beam focusing results in an intense mid-IR field spatiotemporally confined in the lambda-cubic volume. A near planar wavefront is achieved through manipulating the wavefront curvature of the pumping pulse in the frequency downconversion process. Coherent mid-IR pulses are produced with the peak field of 280 MV/cm at 10 μm, and its intensity exceeds 100  TW/cm², estimated from measured pulse energy, and spatial and temporal pulse profiles. Interaction of such an intense mid-IR field with Xe and Kr gas forms plasma and generates a supercontinuum in the visible range.

Non-invasive perturbations of intracellular flow reveal physical principles of cell organization

Recent advances in cell biology enable precise molecular perturbations. The spatiotemporal organization of cells and organisms, however, also depends on physical processes such as diffusion or cytoplasmic flows, and strategies to perturb physical transport inside cells are not yet available. Here, we demonstrate focused-light-induced cytoplasmic streaming (FLUCS). FLUCS is local, directional, dynamic, probe-free, physiological, and is even applicable through rigid egg shells or cell walls. We explain FLUCS via time-dependent modelling of thermoviscous flows. Using FLUCS, we demonstrate that cytoplasmic flows drive partitioning-defective protein (PAR) polarization in Caenorhabditis elegans zygotes, and that cortical flows are sufficient to transport PAR domains and invert PAR polarity. In addition, we find that asymmetric cell division is a binary decision based on gradually varying PAR polarization states. Furthermore, the use of FLUCS for active microrheology revealed a metabolically induced fluid-to-solid transition of the yeast cytoplasm. Our findings establish how a wide range of transport-dependent models of cellular organization become testable by FLUCS.

Ferroelectric domain states of a tetragonal BiFeO3 thin film investigated by second harmonic generation microscopy

We investigate the ferroelectric state of a tetragonal BiFeO₃ thin film grown on a LaAlO₃ (001) substrate using an optical second harmonic generation (SHG) microscope. Whereas the ferroelectric state of this material hosts nanometer-sized domains which again form micrometer-sized domains of four different configurations, we could figure out the characteristic features of each domain from the SHG mapping with various sizes of the probe beam, i.e., from 0.7 to 3.9 μm in its diameter. In particular, we demonstrate that a single micrometer-sized domain contributes to the SHG as a coherent summation of the constituent nanometer-sized domains, and multi-micrometer-sized domains contribute to the SHG as an incoherent summation of each micro-domain.

Experimental Comparison of Photothermal Conversion Efficiency of Gold Nanotriangle and Nanorod in Laser Induced Thermal Therapy

An experimental comparison of the photothermal conversion efficiency (PCE) for gold nanotriangles (GNTs) and nanorods (GNRs) was carried out in the present work. The discrete dipole approximation method was applied to identify the spectral characteristic of GNTs and GNRs with different aspect ratios. On this basis, the PCE of GNTs and GNRs in photothermal therapy were compared theoretically. Afterwards, an in vitro experiment was adopted to investigate the thermal effect of porcine muscle induced by laser irradiation, with and without injected GNTs and GNRs. The influences of laser total power, nanoparticle concentration, and nanoparticle type were investigated. It was found that for the commonly-used wavelengths for photothermal therapy, the PCE of GNTs is higher than that of the GNRs. Furthermore, for GNRs loaded in tissue in vitro, high laser power and high concentration of nanoparticles leads to the degeneration and even carbonization of tissue. However, for the GNTs with the same situation (laser power, nanoparticle volume concentration, and heating time), it could lead to the tissue's evaporation instead of carbonization.

Terahertz saturable absorbers from liquid phase exfoliation of graphite

Saturable absorbers (SA) operating at terahertz (THz) frequencies can open new frontiers in the development of passively mode-locked THz micro-sources. Here we report the fabrication of THz SAs by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z-scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies. Fourier-transform infrared spectroscopy provides evidence of intraband-controlled absorption bleaching. These results pave the way to the integration of graphene-based SA with electrically pumped THz semiconductor micro-sources, with prospects for applications where excitation of specific transitions on short time scales is essential, such as time-of-flight tomography, coherent manipulation of quantum systems, time-resolved spectroscopy of gases, complex molecules and cold samples and ultra-high speed communications, providing unprecedented compactness and resolution.

Graphene shows promise for saturable absorption, a key property for ultrafast lasing, yet graphene saturable absorbers operating in the terahertz region suffer from low absorption modulation. Here, the authors report terahertz saturable absorbers based on inkjet printed graphene with 80% transparency modulation.

Method for measuring the area of radiometric apertures using the ratio of Gaussian beams

I propose and demonstrate a method for determining the area of radiometric apertures using the power ratio between Gaussian beams. The result relies on measuring the power of an optical beam of known radius with and without the radiometric aperture, this with area smaller than the beam spot. The impact of the characterization of the laser beam and of the radiometric measurements on the area estimation is discussed and a 3-mm in-diameter sample is measured for validation. The contactless method is fast and simple and results in a relative uncertainty of 0.12% for the sample studied.

Effects of deviation from focal plane on lesion geometry for ablative fractional photothermolysis

BACKGROUND AND OBJECTIVE

Fractional Photothermolysis (FP) is a method of skin treatment that generates a thermal damage pattern consisting of multiple columns of thermal damage, also known as microscopic treatment zones (MTZs). They are very small in diameter and are generated by application of highly focused laser beams. In order to obtain the smallest spot size, the treatment should be performed in the focal plane. Any deviation from the focal plane (DFP) results in an increase of spot size. FP devices typically utilize distance holders in order to facilitate exposures at this specific location. In spite of the use of distance holders, DFP can occur. In particular, variations of contact pressure to the skin surface and anatomical treatment areas of high surface curvature may be prone to DFP during FP treatments. The impact of such distance variation on lesion geometry, such as depth and diameter of the thermal injury, has not previously been evaluated. The objective of this study was to investigate the relation between DFP and the resulting lesion geometry for a selected ablative fractional device.

MATERIAL AND METHODS

A handpiece of an ablative fractional laser (DeepFX, UltraPulse Encore, Lumenis, Yokneam, Israel) was mounted to a rigid stand. Full thickness human skin obtained from abdominoplasty was mounted to a separate stand perpendicular to the handpiece. The tissue stand allowed the distance between the handpiece and the tissue to be adjusted to produce a variation up to ±3 mm from the focal plane. A 1 × 1 cm(2) scanning area of 169 MTZs, 50 mJ energy per MTZ, 120 μm nominal spot size, was applied at -3, -2, -1, 0, +1, +2, and +3 mm deviated from the focal plane. Minus (-) and plus (+) signs indicate decreasing and increasing distance between the handpiece and the tissue, respectively. Depth and diameter of the laser induced tissue lesions were assessed and quantified.

RESULTS

DFPs produced a significant alteration of the lesion geometry. DFPs of -3, -2, -1, 0, +1, +2, +3 mm resulted in average lesion depths of 1,020 (-40%), 1,180 (-31%), 1,400 (-18%), 1,700 (0%), 1,620 (-5%), 780 (-55%), 680 (-60%) µm, and average lesion diameters of 314 (+26%), 311 (+25%), 273 (+10%), 248 (0%), 256 (+3%), 316 (+27%), 359 (+44%) µm, respectively. The underlined values represent the focal plane. The percentage changes relative to values at focal plane are in parentheses.

CONCLUSIONS

A relatively minor DFP has a marked impact on the thermal injury profile, including lesion depth and diameter, of the laser-exposed tissue. Such marked changes of the thermal injury profile might affect the wound healing, safety, and efficacy of ablative fractional resurfacing procedures. Clinicians should carefully maintain the focal plane during ablative fractional treatment for reproducible results. The presented data are device specific and the clinical impact of such alteration of thermal injury profile warrants further investigation. Lasers Surg. Med. 48:555-561, 2016. © 2016 Wiley Periodicals, Inc.

Rapid spontaneous Raman light sheet microscopy using cw-lasers and tunable filters

We perform rapid spontaneous Raman 2D imaging in light-sheet microscopy using continuous wave lasers and interferometric tunable filters. By angularly tuning the filter, the cut-on/off edge transitions are scanned along the excited Stokes wavelengths. This allows obtaining cumulative intensity profiles of the scanned vibrational bands, which are recorded on image stacks; resembling a spectral version of the knife-edge technique to measure intensity profiles. A further differentiation of the stack retrieves the Raman spectra at each pixel of the image which inherits the 3D resolution of the host light sheet system. We demonstrate this technique using solvent solutions and composites of polystyrene beads and lipid droplets immersed in agar and by imaging the C-H (2800-3100cm(-1)) region in a C. elegans worm. The image acquisition time results in 4 orders of magnitude faster than confocal point scanning Raman systems, allowing the possibility of performing fast spontaneous Raman·3D-imaging on biological samples.

Arbitrary state of polarization with customized degree of polarization generator

An optical setup able to generate arbitrary states of polarization (SOPs) with customized degree of polarization is presented in this Letter. Compared with the few alternatives existing in literature, it presents an easy-to-build optical setup and leads to a superior performance. In fact, experimental results are presented, providing an accurate control for the generation of SOPs (maximum error of 1.7% and 3.3% for ellipticity and azimuth, respectively) as well as for the associated degree of polarization (full experimental variation from 1 up to 0.003, with a 1.7% maximum error). The system proposed may be useful for different applications, for example, for polarimeters testing, speckle metrology, and biological applications.

Temperature-dependent thermal properties of supported MoS2 monolayers

Thermal properties can substantially affect the operation of various electronics and optoelectronics devices based on two-dimensional materials. In this work, we describe our investigation of temperature-dependent thermal conductivity and interfacial thermal conductance of molybdenum disulfide monolayers supported on SiO2/Si substrates, using Raman spectroscopy. We observed that the calculated thermal conductivity (κ) and interfacial thermal conductance (g) decreased with increasing temperature from 62.2 W m(-1) K(-1) and 1.94 MW m(-2) K(-1) at 300 K to 7.45 W m(-1) K(-1) and 1.25 MW m(-2) K(-1) at 450 K, respectively.

Sub-wavelength terahertz beam profiling of a THz source via an all-optical knife-edge technique

Terahertz technologies recently emerged as outstanding candidates for a variety of applications in such sectors as security, biomedical, pharmaceutical, aero spatial, etc. Imaging the terahertz field, however, still remains a challenge, particularly when sub-wavelength resolutions are involved. Here we demonstrate an all-optical technique for the terahertz near-field imaging directly at the source plane. A thin layer (<100 nm-thickness) of photo carriers is induced on the surface of the terahertz generation crystal, which acts as an all-optical, virtual blade for terahertz near-field imaging via a knife-edge technique. Remarkably, and in spite of the fact that the proposed approach does not require any mechanical probe, such as tips or apertures, we are able to demonstrate the imaging of a terahertz source with deeply sub-wavelength features (<30 μm) directly in its emission plane.

Direct monitoring of ultrafast electron and hole dynamics in perovskite solar cells

Electron and hole dynamics, and internal and surface trap states in perovskite-based solar cells are explored by fs transient absorption spectroscopy.

Ultrahigh precision nonlinear reflectivity measurement system for saturable absorber mirrors with self-referenced fluence characterization

Measurement of nonlinear optical reflectivity of saturable absorber devices is discussed. A setup is described that enables absolute accuracy of reflectivity measurements better than 0.3%. A repeatability within 0.02% is shown for saturable absorbers with few-percent modulation depth. The setup incorporates an in situ knife-edge characterization of beam diameters, making absolute reflectivity estimations and determination of saturation fluences significantly more reliable. Additionally, several measures are discussed to substantially improve the reliability of the reflectivity measurements. At its core, the scheme exploits the limits of state-of-the-art digital lock-in technology but also greatly benefits from a fiber-based master-oscillator power-amplifier source, the use of an integrating sphere, and simultaneous comparison with a linear reflectivity standard.

Quantum dots fluorescence quantum yield measured by Thermal Lens Spectroscopy

An essential parameter to evaluate the light emission properties of fluorophores is the fluorescence quantum yield, which quantify the conversion efficiency of absorbed photons to emitted photons. We detail here an alternative nonfluorescent method to determine the absolute fluorescence quantum yield of quantum dots (QDs). The method is based in the so-called Thermal Lens Spectroscopy (TLS) technique, which consists on the evaluation of refractive index gradient thermally induced in the fluorescent material by the absorption of light. Aqueous dispersion carboxyl-coated cadmium telluride (CdTe) QDs samples were used to demonstrate the Thermal Lens Spectroscopy technical procedure.