Optical Fluence Research Articles

High-resolution 3D light fluence mapping for heterogeneous scattering media by localized sampling.

We demonstrate an innovative concept for three-dimensional optical fluence mapping in heterogeneous highly scattering media as, e.g.,biomedical tissues. We propose to use the relative light extinction analysis principle together with a miniaturized collection fiber in a direct fluence measurement setup as a method to obtain the spatially resolved light intensity distribution under transversally inhomogeneous light propagation conditions and provide local characterization of the transport medium. System performance is validated in two extreme conditions: an optically thin scattering medium and an absorption-dominated light transport. Both extremes demonstrate good agreement to theoretical expectations. Finally, we successfully prove the ability of the system to deliver high-resolution fluence maps through a model study of the light distribution induced in a scattering medium by a vertical diode laser stack with individual bars pitched only 500μm apart.

Graphics processing units-accelerated adaptive nonlocal means filter for denoising three-dimensional Monte Carlo photon transport simulations.

.The Monte Carlo (MC) method is widely recognized as the gold standard for modeling light propagation inside turbid media. Due to the stochastic nature of this method, MC simulations suffer from inherent stochastic noise. Launching large numbers of photons can reduce noise but results in significantly greater computation times, even with graphics processing units (GPU)-based acceleration. We develop a GPU-accelerated adaptive nonlocal means (ANLM) filter to denoise MC simulation outputs. This filter can effectively suppress the spatially varying stochastic noise present in low-photon MC simulations and improve the image signal-to-noise ratio (SNR) by over 5 dB. This is equivalent to the SNR improvement of running nearly more photons. We validate this denoising approach using both homogeneous and heterogeneous domains at various photon counts. The ability to preserve rapid optical fluence changes is also demonstrated using domains with inclusions. We demonstrate that this GPU-ANLM filter can shorten simulation runtimes in most photon counts and domain settings even combined with our highly accelerated GPU MC simulations. We also compare this GPU-ANLM filter with the CPU version and report a threefold to fourfold speedup. The developed GPU-ANLM filter not only can enhance three-dimensional MC photon simulation results but also be a valuable tool for noise reduction in other volumetric images such as MRI and CT scans.

Impact of depth-dependent optical attenuation on wavelength selection for spectroscopic photoacoustic imaging

An optical wavelength selection method based on the stability of the absorption cross-section matrix to improve spectroscopic photoacoustic (sPA) imaging was recently introduced. However, spatially-varying chromophore concentrations cause the wavelength- and depth-dependent variations of the optical fluence, which degrades the accuracy of quantitative sPA imaging. This study introduces a depth-optimized method that determines an optimal wavelength set minimizing an inverse of the multiplication of absorption cross-section matrix and fluence matrix to minimize the errors in concentration estimation. This method assumes that the optical fluence distribution is known or can be attained otherwise. We used a Monte Carlo simulation of light propagation in tissue with various depths and concentrations of deoxy-/oxy-hemoglobin. We quantitatively compared the developed and current approaches, indicating that the choice of wavelength is critical and our approach is effective especially when quantifying deeper imaging targets.

Dual photoacoustic monitoring in laser ablation synthesis of silver nanoparticles to find in situ their fluence threshold formation

Abstract In this study, a dual pulsed photoacoustic technique was applied to monitor, in real time, the laser ablation synthesis in solution (LASiS) of silver nanoparticles. It was found that, at low optical fluence values, the ultrasound signal associated to the laser ablation process exhibited constant amplitude, independently of the ablation time. Further analysis confirmed that, under these conditions, the ablation process provided only suspended micro-sized Ag fragments. However, fluence values above 400 mJ cm−2 promoted the formation of nanoparticles, accompanied by amplitude changes in the photoacoustic signal. Determination of this threshold condition establishes a fingerprint in the photoacoustic signal that can be used to identify, in situ and in real time, the beginning of nanoparticle synthesis. Supplementary, the nanoparticle formation was also photoacoustically monitored during the ablation process introducing an additional laser pulse train of low fluence value propagating perpendicularly to the pulse train used for ablation. These laser pulses did not interfere with the nanoparticle LASiS process, but renders an ultrasound signal enhancement due to the surface plasmon resonance of metal nanoparticles. The characterization of the LASiS process through the photoacoustic technique can provide information about nanoparticle concentration and other synthesis features of interest.

Temperature-dependence of hypersound dynamics in SrRuO3/SrTiO3 heterostructures

We studied GHz hypersound in heterostructures of ${\mathrm{SrRuO}}_{3}$ (SRO) and ${\mathrm{SrTiO}}_{3}$ (STO) by using femtosecond pulses with fluence as low as several tens of $\ensuremath{\mu}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{2}$. We found that the efficiency of generating hypersound in SRO, through thermal expansion, is linear up to optical fluence of a few $\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^{2}$. We further investigated the propagation of acoustic pulses in the STO substrate. The acoustic properties of STO dramatically changed around the temperature of structural transition (105 K) because the generated hypersound was coupled to the softening mode. Nevertheless, our experimental results revealed that the quadratic frequency dependence of acoustic attenuation in STO, predicted by the mode-coupling theory, breaks down at tens of GHz. Finally, the refractive index of STO was experimentally obtained between 365 and 400 nm, for which there was disagreement in previous results.

Combination of virtual point detector concept and fluence compensation in acoustic resolution photoacoustic microscopy.

We propose a hybrid approach to image enhancement in acoustic resolution photoacoustic microscopy. The developed technique is based on compensation for nonuniform spatial sensitivity of the optoacoustic (OA) system in both optical and acoustic domains. Spatial distribution of optical fluence is derived from full three-dimensional Monte Carlo simulations accounting for conical geometry of tissue laser illumination at the wavelength of 532nm. Approximate nonuniform spatial response of acoustic detector with numerical aperture of 0.6 is derived from the two-dimensional k-Wave modeling. Application of the developed technique allows to improve the spatial resolution and to balance in-depth signal-level distribution in OA images of phantom and in-vivo objects.

Revealing the nature of morphological changes in carbon nanotube-polymer saturable absorber under high-power laser irradiation

Composites of single-walled carbon nanotubes (SWNTs) and water-soluble polymers (WSP) are the focus of significant worldwide research due to a number of applications in biotechnology and photonics, particularly for ultrashort pulse generation. Despite the unique possibility of constructing non-linear optical SWNT-WSP composites with controlled optical properties, their thermal degradation threshold and limit of operational power remain unexplored. In this study, we discover the nature of the SWNT-polyvinyl alcohol (PVA) film thermal degradation and evaluate the modification of the composite properties under continuous high-power ultrashort pulse laser operation. Using high-precision optical microscopy and micro-Raman spectroscopy, we have examined SWNT-PVA films before and after continuous laser radiation exposure (up to 40 hours) with a maximum optical fluence of 2.3 mJ·cm−2. We demonstrate that high-intensity laser radiation results in measurable changes in the composition and morphology of the SWNT-PVA film due to efficient heat transfer from SWNTs to the polymer matrix. The saturable absorber modification does not affect the laser operational performance. We anticipate our work to be a starting point for more sophisticated research aimed at the enhancement of SWNT-PVA films fabrication for their operation as reliable saturable absorbers in high-power ultrafast lasers.

Ultrafast frequency-agile terahertz devices using methylammonium lead halide perovskites.

The ability to control the response of metamaterial structures can facilitate the development of new terahertz devices, with applications in spectroscopy and communications. We demonstrate ultrafast frequency-agile terahertz metamaterial devices that enable such a capability, in which multiple perovskites can be patterned in each unit cell with micrometer-scale precision. To accomplish this, we developed a fabrication technique that shields already deposited perovskites from organic solvents, allowing for multiple perovskites to be patterned in close proximity. By doing so, we demonstrate tuning of the terahertz resonant response that is based not only on the optical pump fluence but also on the optical wavelength. Because polycrystalline perovskites have subnanosecond photocarrier recombination lifetimes, switching between resonances can occur on an ultrafast time scale. The use of multiple perovskites allows for new functionalities that are not possible using a single semiconducting material. For example, by patterning one perovskite in the gaps of split-ring resonators and bringing a uniform thin film of a second perovskite in close proximity, we demonstrate tuning of the resonant response using one optical wavelength and suppression of the resonance using a different optical wavelength. This general approach offers new capabilities for creating tunable terahertz devices.

Reconstruction of optical absorption coefficient distribution in intravascular photoacoustic imaging

Intravascular photoacoustic (IVPA) imaging can discriminate between normal arterial tissues and atheromatous plaques in images, particularly images of lipid-rich plaques with high spatial resolution and optical contrast. However, conventional IVPA only recovers the deposited optical energy that is the product of the tissue optical absorption coefficient and local optical fluence. Herein, a one-step model-based method for single-wavelength IVPA imaging system is proposed. The proposed method directly reconstructs the optical absorption coefficient from the boundary measurement of acoustic pressure. To obtain the theoretical optical deposition, light illumination and transport in multilayered vessel-wall tissues are numerically modelled with the diffusion approximation to radiative transfer equation. Then, the generation and propagation of photoacoustic (PA) waves in acoustically homogeneous vessel-wall tissues are modelled by using the PA wave equation. The theoretical acoustic pressure series is thus obtained. Finally, the optical absorption coefficient is iteratively updated from an initial guess by minimising a nonlinear least-square error function to quantify the difference between the measured and theoretical acoustic pressure with split Bregman optimisation based on total-variation regularisation. The numerical simulation experiments demonstrated that optical absorption coefficient maps can be directly recovered on the basis of the acoustic boundary measurement of vascular cross-sections. Compared with the optical deposition map, the optical absorption coefficient map can provide more reliable qualitative and quantitative information on the morphology and optical properties of the imaged arteries. The proposed method enables the accurate and reliable evaluation of atheromatous lesions and the early identification of vulnerable plaques.

A Bayesian Approach to Eigenspectra Optoacoustic Tomography.

The quantification of hemoglobin oxygen saturation (sO2) with multispectral optoacoustic (OA) (photoacoustic) tomography (MSOT) is a complex spectral unmixing problem, since the OA spectra of hemoglobin are modified with tissue depth due to depth (location) and wavelength dependencies of optical fluence in tissue. In a recent work, a method termed eigenspectra MSOT (eMSOT) was proposed for addressing the dependence of spectra on fluence and quantifying blood sO2 in deep tissue. While eMSOT offers enhanced sO2 quantification accuracy over conventional unmixing methods, its performance may be compromised by noise and image reconstruction artifacts. In this paper, we propose a novel Bayesian method to improve eMSOT performance in noisy environments. We introduce a spectral reliability map, i.e., a method that can estimate the level of noise superimposed onto the recorded OA spectra. Using this noise estimate, we formulate eMSOT as a Bayesian inverse problem where the inversion constraints are based on probabilistic graphical models. Results based on numerical simulations indicate that the proposed method offers improved accuracy and robustness under high noise levels due the adaptive nature of the Bayesian method.

Deep-tissue temperature mapping by multi-illumination photoacoustic tomography aided by a diffusion optical model: a numerical study.

Temperature mapping during thermotherapy can help precisely control the heating process, both temporally and spatially, to efficiently kill the tumor cells and prevent the healthy tissues from heating damage. Photoacoustic tomography (PAT) has been used for noninvasive temperature mapping with high sensitivity, based on the linear correlation between the tissue's Grüneisen parameter and temperature. However, limited by the tissue's unknown optical properties and thus the optical fluence at depths beyond the optical diffusion limit, the reported PAT thermometry usually takes a ratiometric measurement at different temperatures and thus cannot provide absolute measurements. Moreover, ratiometric measurement over time at different temperatures has to assume that the tissue's optical properties do not change with temperatures, which is usually not valid due to the temperature-induced hemodynamic changes. We propose an optical-diffusion-model-enhanced PAT temperature mapping that can obtain the absolute temperature distribution in deep tissue, without the need of multiple measurements at different temperatures. Based on the initial acoustic pressure reconstructed from multi-illumination photoacoustic signals, both the local optical fluence and the optical parameters including absorption and scattering coefficients are first estimated by the optical-diffusion model, then the temperature distribution is obtained from the reconstructed Grüneisen parameters. We have developed a mathematic model for the multi-illumination PAT of absolute temperatures, and our two-dimensional numerical simulations have shown the feasibility of this new method. The proposed absolute temperature mapping method may set the technical foundation for better temperature control in deep tissue in thermotherapy.

Simulating Light Diffusion in Human Brain Tissues Using Monte-Carlo Simulation and Diffusion Equation

Medical diagnosis with optical techniques is favorable due to its safe and painless features. Every tissue type can be distinguished by its optical absorption and scattering properties that are related to many physiological changes and considered to be very important signs for tissue heath. Characterizing light propagation in the human brain tissues is a vital issue in many diagnostic and therapeutic applications. In this work, light propagation in different brain tissues in normal and coagulated state was investigated. A Monte-Carlo simulation model was implemented to obtain spatially resolved steady state diffuse reflectance profiles of the examined tissues. Furthermore, the diffusion equation was solved to create images presenting the optical fluence rate distribution at the tissue surface using the finite element method. The proposed diffuse reflectance curves and fluence rate images show different features regarding tissue type and condition that promises to be effective in medical diagnosis.

Self-assembled nanotextures impart broadband transparency to glass windows and solar cell encapsulants

Most optoelectronic components and consumer display devices require glass or plastic covers for protection against the environment. Optical reflections from these encapsulation layers can degrade the device performance or lessen the user experience. Here, we use a highly scalable self-assembly based approach to texture glass surfaces at the nanoscale, reducing reflections by such an extent so as to make the glass essentially invisible. Our nanotextures provide broadband antireflection spanning visible and infrared wavelengths (450–2500 nm) that is effective even at large angles of incidence. This technology can be used to improve the performance of photovoltaic devices by eliminating reflection losses, which can be as much as 8% for glass encapsulated cells. In contrast, solar cells encapsulated with nanotextured glass generate the same photocurrent as when operated without a cover. Ultra-transparent windows having surface nanotextures on both sides can withstand three times more optical fluence than commercial broadband antireflection coatings, making them useful for pulsed laser applications.

A Method for Medical Diagnosis Based on Optical Fluence Rate Distribution at Tissue Surface.

Optical differentiation is a promising tool in biomedical diagnosis mainly because of its safety. The optical parameters’ values of biological tissues differ according to the histopathology of the tissue and hence could be used for differentiation. The optical fluence rate distribution on tissue boundaries depends on the optical parameters. So, providing image displays of such distributions can provide a visual means of biomedical diagnosis. In this work, an experimental setup was implemented to measure the spatially-resolved steady state diffuse reflectance and transmittance of native and coagulated chicken liver and native and boiled breast chicken skin at 635 and 808 nm wavelengths laser irradiation. With the measured values, the optical parameters of the samples were calculated in vitro using a combination of modified Kubelka-Munk model and Bouguer-Beer-Lambert law. The estimated optical parameters values were substituted in the diffusion equation to simulate the fluence rate at the tissue surface using the finite element method. Results were verified with Monte-Carlo simulation. The results obtained showed that the diffuse reflectance curves and fluence rate distribution images can provide discrimination tools between different tissue types and hence can be used for biomedical diagnosis.

Picosecond laser fabrication of nanostructures on ITO film surface assisted by pre-deposited Au film

With greater optical penetration depth and lower ablation threshold fluence, it is difficult to directly fabricate large scales of laser-induced periodic surface structures (LIPSSs) on indium–tin–oxide (ITO) films. This study proposed an approach to obtain optimized LIPSSs by sputtering an Au thin film on the ITO film surface. The concept behind the proposal is that the upper layer of the thin Au film can cause surface energy aggregation, inducing the initial ripple structures. The ripples deepened and become clear with lower energy due to optical trapping. The effective mechanism of Au film was analyzed and verified by a series of experiments. Linear sweep, parallel to the laser polarization direction, was performed using a Nd:VAN laser system with 10-ps Q-switched pulse, at a central wavelength of 532 nm, with a repetition rate of 1 kHz. The complete and clear features of the nanostructures, obtained with the periods of approximately 320 nm, were observed on ITO films with proper laser fluence and scanning speed. The depth of ripples was varying in the range of 15–65 nm with clear and coherent ITO films. The preferred efficiency of fabricating nanostructures and the excellent results were obtained at a scanning speed of 2.5 mm/s and a fluence of 0.189 J/cm2. In this way, the ablation and shedding of ITO films was successfully avoided. Thus, the proposed technique can be considered to be a promising method for the laser machining of special nonmetal films.

Silver nanoparticle-decorated graphene oxide for surface-enhanced Raman scattering detection and optical limiting applications

Silver nanoparticle-decorated graphene oxide was prepared using a simple green chemical reduction method, and its performance being used for surface-enhanced Raman scattering detection and optical limiting was studied. TEM images, photoluminescence quenching effect and the characteristic absorption peaks of GO and silver nanoparticles confirm the successful preparation of GO–Ag nanocomposite. The Raman spectra results show that the vibration fingerprints of RhB and R6G moleculars are detected only on the GO–Ag substrate, revealing good potential of being used as surface-enhanced Raman scattering substrate for detecting organic molecules. Additionally, nonlinear optical absorption of GO–Ag was measured using z-scan technique with a nanosecond laser operating at the wavelength of 532 nm. The z-scan results show that the transmittance of GO–Ag aqueous suspension has a large decline of 90%, and the nonlinear optical absorption onset input fluence of GO–Ag dispersed PVA film is as low as 1.3 J/cm2, indicating that GO–Ag is a good potential optical limiting material for the commonly used high-power 532 nm nanosecond pulsed laser.

Stability investigation of laser darkened metal surfaces

Pulsed laser irradiation-induced reflectivity decrease of metal surfaces is a well-established phenomenon, which is extensively utilized in numerous applications. Since the stability of these black surfaces is often a demand, we investigated the resistance of darkened copper and titanium surfaces against optical and mechanical damages. For optical stability studies, samples were irradiated by a probe laser beam (λ = 775 nm, FWHM = 150 fs, f = 1 kHz) at different fluences (0–300 mJ/cm2), while the mechanical stability was studied with scratch test using 2.5 µm radius tip and applying normal force in 29.4–147 µN range. The observed reflectivity and morphological changes indicated that the optical damage threshold fluence is 130 and 160 mJ/cm2 for copper and titanium surfaces, respectively. Mechanical damage only in case of copper could be detected in the applied parameter range indicating a scratch hardness of 21.5 MPa.

A search for optical bursts from the repeating fast radio burst FRB 121102

We present a search for optical bursts from the repeating fast radio burst FRB 121102 using simultaneous observations with the high-speed optical camera ULTRASPEC on the 2.4-m Thai National Telescope and radio observations with the 100-m Effelsberg Radio Telescope. A total of 13 radio bursts were detected, but we found no evidence for corresponding optical bursts in our 70.7-ms frames. The 5-sigma upper limit to the optical flux density during our observations is 0.33 mJy at 767nm. This gives an upper limit for the optical burst fluence of 0.046 Jy ms, which constrains the broadband spectral index of the burst emission to alpha < -0.2. Two of the radio pulses are separated by just 34 ms, which may represent an upper limit on a possible underlying periodicity (a rotation period typical of pulsars), or these pulses may have come from a single emission window that is a small fraction of a possible period.

Recombination at high carrier density in methylammonium lead iodide studied using time-resolved microwave conductivity

Time-resolved microwave conductivity (TRMC) is a highly versatile method to rapidly evaluate the electronic properties of semiconducting compounds without the need to construct and optimize electronic devices. In this report, we study how bimolecular and Auger recombination mechanisms affect TRMC measurements. In particular, we investigate how recombination reduces the measured value of the TRMC figure-of-merit: ϕΣμ, at a high incident optical fluence. Using a numerical model, we calculate how these higher-order recombination processes reduce experimentally measured values of ϕΣμ relative to a regime of low carrier concentration with little recombination. By fitting this model to experimentally obtained data for the hybrid halide perovskite compound, methylammonium lead iodide, we are able to extract the bimolecular and Auger rate constants and provide a clear determination of the sum of the hole and electron mobilities for these films.

Optical fluence compensation for handheld photoacoustic probe: An in vivo human study case

Integrating photoacoustic (PA) and ultrasound (US) into a handheld probe to perform PA/US dual-modal imaging has been widely studied over the past few years. However, optical fluence decreases quickly in deeper tissue due to light scattering and absorption, which would significantly affect the quantitative PA imaging. In this paper, we performed a fluence compensation for a PA imaging study of human breast. The comparison of PA/US image with and without optical fluence compensation demonstrated that the fluence compensation could effectively improve imaging quality for handheld probe.