Pulsed Laser Illumination Research Articles

Modelling of picosecond timing signals from fast vacuum photodiodes

Very high bandwidth vacuum photodiodes with 10 s of picoseconds rise time are utilised in Cherenkov detectors for inertial confinement fusion diagnostics. Experimental measurements of the pulse shape and time resolution of a fast Photek PD010 photodiode tube, undertaken using very fast pulsed laser illumination at the ORION facility, Atomic Weapons Establishment, Aldermaston UK, were compared with a computer model to understand detector performance with the aim of improving the time resolution. The physical processes defining the performance of the photodiode detector were modelled using CST Studio Suite software. This package combines very high frequency electromagnetic field modelling, particle tracking, space charge effects and secondary electron emission simulation, and has enabled a realistic simulation of detector behaviour and performance predictions to be made. We present a comparison between experimental measurements of time resolution and pulse shape over a range of device operating voltages, with simulations obtained with CST particle tracking software using the device CAD design file to accurately model the PD010 device. These results demonstrate the remarkable correlation that can be achieved between experiment and simulation, even at picosecond time-scales. • Time resolution of fast vacuum photodiodes is limited by detector geometry. • Impedance matching between anode and signal cable is critical. • Physics of the detector were modelled using CST Studio Suite software. • Remarkable correlation between experiment and simulation is demonstrated.

Experimental study of mode shifting in an asymmetrically heated rectangular plate

Modal shifting and jumping in thermally buckled plates has been investigated previously using computational models; however, relatively few experimental explorations have been reported. In this study, the modal shifts and jumps in a simple rectangular plate or panel were investigated using digital image correlation to obtain detailed mode shapes. One face of the planar panel, which was 219 × 146 mm, was speckled and viewed with a stereoscopic digital image correlation system using pulsed-laser illumination and with a laser vibrometer at a single point. The other face was heated using a set of 1 kW quartz lamps to produce a non-uniform temperature distribution which progressively increased from room temperature to around 760 K. An infra-red camera recorded the time-varying temperature distribution on the panel while it was excited with an electrodynamic shaker. A waterfall plot, or time-frequency spectrogram, showing natural frequencies as a function of time was generated, which highlighted the shifting response of the panel with temperature. The heating sequence was repeated and the panel excited at the natural frequencies identified in the waterfall plot, which permitted the corresponding mode shapes to be measured. These data showed that mode shifting and jumping was present with asymmetric heating, but not with spatially uniform heating, and was associated with thermally-induced buckling.

Clinical optoacoustic imaging combined with ultrasound for coregistered functional and anatomical mapping of breast tumors

Optoacoustic imaging, based on the differences in optical contrast of blood hemoglobin and oxyhemoglobin, is uniquely suited for the detection of breast vasculature and tumor microvasculature with the inherent capability to differentiate hypoxic from the normally oxygenated tissue. We describe technological details of the clinical ultrasound (US) system with optoacoustic (OA) imaging capabilities developed specifically for diagnostic imaging of breast cancer. The combined OA/US system provides co-registered and fused images of breast morphology based upon gray scale US with the functional parameters of total hemoglobin and blood oxygen saturation in the tumor angiogenesis related microvasculature based upon OA images. The system component that enabled clinical utility of functional OA imaging is the hand-held probe that utilizes a linear array of ultrasonic transducers sensitive within an ultrawide-band of acoustic frequencies from 0.1 MHz to 12 MHz when loaded to the high-impedance input of the low-noise analog preamplifier. The fiberoptic light delivery system integrated into a dual modality probe through a patented design allowed acquisition of OA images while minimizing typical artefacts associated with pulsed laser illumination of skin and the probe components in the US detection path. We report technical advances of the OA/US imaging system that enabled its demonstrated clinical viability. The prototype system performance was validated in well-defined tissue phantoms. Then a commercial prototype system named Imagio™ was produced and tested in a multicenter clinical trial termed PIONEER. We present examples of clinical images which demonstrate that the spatio-temporal co-registration of functional and anatomical images permit radiological assessment of the vascular pattern around tumors, microvascular density of tumors as well as the relative values of the total hemoglobin [tHb] and blood oxygen saturation [sO2] in tumors relative to adjacent normal breast tissues. The co-registration technology enables increased accuracy of radiologist assessment of malignancy by confirming, upgrading and/or downgrading US categorization of breast tumors according to Breast Imaging Reporting And Data System (BI-RADS). Microscopic histologic examinations on the biopsied tissue of the imaged tumors served as a gold standard in verifying the functional and anatomic interpretations of the OA/US image feature analysis.

Strain and distortion monitoring during arc welding by 3D digital image correlation

ABSTRACTA novel three-dimensional (3D) digital image correlation (DIC) approach using a stereovision system was developed to measure the evolution of strain and distortion near the fusion zone during the gas tungsten arc welding process. Unlike the previously reported two-dimensional (2D) DIC approach using a single camera, the 3D DIC method was immune to the out-of-plane displacement and was capable of measuring 3D deformation. Both 2D and 3D DIC approaches in welding applications were based on the utilisation of the novel high-temperature speckle and the pulsed laser illumination plus bandpass filters. However, the speckle pattern was partially specular reflective causing issues in subset matching in the 3D approach. A new algorithm and experimental procedure was incorporated to solve this problem.

Photoexcitation in Solids: First-Principles Quantum Simulations by Real-Time TDDFT (Adv. Theory Simul. 8/2018)

Real-Time TDDFT Solid-state materials can exhibit highly nonlinear electronic and optical behaviors under laser excitation. However, a unified theoretical framework has not been established to explain the underlying mechanisms. In article number 1800055, Sheng Meng and co-workers develop an efficient real-time time-dependent density functional theory (TDDFT) approach for accurate simulations of excited state dynamics in solids. The photoexcitation process of the generation of hot electron-hole pairs under laser pulse illumination in a layered solid is shown schematically.

Thermal Effect on Plasmon-induced Electron Transfer System under Intense Pulsed Laser Illumination

The effect of plasmon excitation on metal nanostructures was investigated under electrochemical potential control with intense pulsed laser illumination. The well-defined Au nano-rod structures showed a distinct thermal effect in the form of shape transformation, in the absence of the plasmon-induced electron transfer reaction. It was found that the shape transformation only occurred under the resonance condition of the localized surface plasmon mode excitation.

Two-color multiphoton emission for comprehensive reveal of ultrafast plasmonic field distribution

We experimentally demonstrate comprehensive reveal of the ultrafast plasmonic field distribution in a bowtie nanostructure by two-color photoemission electron microscopy (PEEM). We attribute the comprehensive reveal of the field distribution to an effective opening of the two-color quantum channel in multiphoton photoemission, which leads to a dramatic reduction of the nonlinear order (from 4.07 down to 2.01) of the plasmon-assisted photoelectrons and a huge increment of the photoemission yields (typically 20-fold enhancement). Furthermore, we have found that opening extent of the quantum channel strongly related with the photoemission yields generated from one-color 400 and 800 nm laser pulse illumination, and the optimized ratio between the yields for effective opening of two-color quantum channel in our experiment is also achieved. Additionally, benefiting from the high spatial resolution of PEEM, we found there exists a large difference in the nonlinear order of two-color photoemission under the plasmonic excitation within a nanostructure, which has not been reported yet. This work introduces multicolor quantum channel photoemission into the PEEM imaging and offers new way to flexibly control the nonlinear order of the plasmon-assisted photoemission, and it will enable PEEM as a versatile tool in many potential applications.

Integrated catheter for simultaneous radio frequency ablation and optoacoustic monitoring of lesion progression

Radio frequency (RF) catheter ablation is commonly used to eliminate dysfunctional cardiac tissue by heating via an alternating current. Clinical outcomes are highly dependent on careful anatomical guidance, electrophysiological mapping, and careful RF power titration during the procedure. Yet, current treatments rely mainly on the expertise of the surgeon to assess lesion formation, causing large variabilities in the success rate. We present an integrated catheter design suitable for simultaneous RF ablation and real-time optoacoustic monitoring of the forming lesion. The catheter design utilizes copper-coated multimode light guides capable of delivering both ablation current and near-infrared pulsed-laser illumination to the target tissue. The generated optoacoustic responses were used to visualize the ablation lesion formation in an ex-vivo bovine heart specimen in 3D. The presented catheter design enables the monitoring of ablation lesions with high spatiotemporal resolution while the overall therapy-monitoring approach remains compatible with commercially available catheter designs.

Optical shaping of a nano-scale tip by femtosecond laser assisted field evaporation

We have investigated the morphology of a nanotip under femtosecond laser pulse illumination and a high electric field. We show that both the symmetry and the local radius of the tip change with the direction of laser polarization as against the tip axis. The experiments were performed on the very same GaN nanotip by laser-assisted atom probe tomography and electron tomography. This allowed an accurate assessment of the tip features by following the order of evaporation of single atoms from the surface. A change of atom emission sites was observed when a change of the angle between the tip axis and the linearly polarized electric field of the laser was imposed. This enables an optical control of field-evaporation sites. A close optical control of the tip morphology on a scale below 10 nm is thus achievable. Calculations of the field at nanotip apex and absorption maps support the experimental observations. Based on the present study, methods can be developed for reshaping nanotips at the nanometer level. This finding opens perspectives for numerous applications, making use of nanotips as probes or field emitters, and for plasmonic devices.

High-resolution depth profiling using a range-gated CMOS SPAD quanta image sensor.

A CMOS single-photon avalanche diode (SPAD) quanta image sensor is used to reconstruct depth and intensity profiles when operating in a range-gated mode used in conjunction with pulsed laser illumination. By designing the CMOS SPAD array to acquire photons within a pre-determined temporal gate, the need for timing circuitry was avoided and it was therefore possible to have an enhanced fill factor (61% in this case) and a frame rate (100,000 frames per second) that is more difficult to achieve in a SPAD array which uses time-correlated single-photon counting. When coupled with appropriate image reconstruction algorithms, millimeter resolution depth profiles were achieved by iterating through a sequence of temporal delay steps in synchronization with laser illumination pulses. For photon data with high signal-to-noise ratios, depth images with millimeter scale depth uncertainty can be estimated using a standard cross-correlation approach. To enhance the estimation of depth and intensity images in the sparse photon regime, we used a bespoke clustering-based image restoration strategy, taking into account the binomial statistics of the photon data and non-local spatial correlations within the scene. For sparse photon data with total exposure times of 75 ms or less, the bespoke algorithm can reconstruct depth images with millimeter scale depth uncertainty at a stand-off distance of approximately 2 meters. We demonstrate a new approach to single-photon depth and intensity profiling using different target scenes, taking full advantage of the high fill-factor, high frame rate and large array format of this range-gated CMOS SPAD array.

Theoretical analysis and simulation of pulsed laser heating at interface

Quantitative yet simple analytical solutions of surface temperature under pulsed laser illumination are presented for a quick estimation in optical spectroscopy studies. Dependence of steady state surface temperature as well as its temporal evolution on laser parameters, such as repetition rate and beam radius, together with medium properties is thoroughly investigated using the analytical solution, which is supported by numerical simulation. It is found that when the pulse number is larger than 100 within the heat diffusion time, the steady-state temperature rise reaches more than 85% of the temperature rise induced by CW laser heating of the same power. We provide a summary of the results to allow their use for a quick estimate of surface temperature evolution from pulse laser heating if laser parameters and medium properties are known.

Gold nanoparticle nucleated cavitation for enhanced high intensity focused ultrasound therapy

High intensity focused ultrasound (HIFU) or focused ultrasound surgery is a non-invasive technique for the treatment of cancerous tissue, which is limited by difficulties in getting real-time feedback on treatment progress and long treatment durations. The formation and activity of acoustic cavitation, specifically inertial cavitation, during HIFU exposures has been demonstrated to enhance heating rates. However, without the introduction of external nuclei its formation an activity can be unpredictable, and potentially counter-productive.In this study, a combination of pulse laser illumination (839 nm), HIFU exposures (3.3 MHz) and plasmonic gold nanorods (AuNR) was demonstrated as a new approach for the guidance and enhancement of HIFU treatments. For imaging, short duration HIFU pulses (10 μs) demonstrated broadband acoustic emissions from AuNR nucleated cavitation with a signal-to-noise ranging from 5–35 dB for peak negative pressures between 1.19–3.19 ± 0.01 MPa. In the absence of either AuNR or laser illumination these emissions were either not present or lower in magnitude (e.g. 5 dB for 3.19 MPa). Continuous wave (CW) HIFU exposures for 15 s, were then used to generate thermal lesions for peak negative pressures from 0.2–2.71 ± 0.01 MPa at a fluence of 3.4 mJ . Inertial cavitation dose (ICD) was monitored during all CW exposures, where exposures combined with both laser illumination and AuNRs resulted in the highest level of detectable emissions. This parameter was integrated over the entire exposure to give a metric to compare with measured thermal lesion area, where it was found that a minimum total ICD of a.u. was correlated with the formation of thermal lesions in gel phantoms. Furthermore, lesion area (mm2) was increased for equivalent exposures without either AuNRs or laser illumination.Once combined with cancer targeting AuNRs this approach could allow for the future theranostic use of HIFU, such as providing the ability to identify and treat small multi-focal cancerous regions with minimal damage to surrounding healthy tissue.

Children's Urinary Environmental Carbon Load. A Novel Marker Reflecting Residential Ambient Air Pollution Exposure?

Ambient air pollution, including black carbon, entails a serious public health risk because of its carcinogenic potential and as climate pollutant. To date, an internal exposure marker for black carbon particles that have cleared from the systemic circulation into the urine does not exist. To develop and validate a novel method to measure black carbon particles in a label-free way in urine. We detected urinary carbon load in 289 children (aged 9-12 yr) using white-light generation under femtosecond pulsed laser illumination. Children's residential black carbon concentrations were estimated based on a high-resolution spatial temporal interpolation method. We were able to detect urinary black carbon in all children, with an overall average (SD) of 98.2 × 105 (29.8 × 105) particles/ml. The urinary black carbon load was positively associated with medium-term to chronic (1 mo or more) residential black carbon exposure: +5.33 × 105 particles/ml higher carbon load (95% confidence interval, 1.56 × 105 to 9.10 × 105 particles/ml) for an interquartile range increment in annual residential black carbon exposure. Consistently, children who lived closer to a major road (≤160 m) had higher urinary black carbon load (6.93 × 105 particles/ml; 95% confidence interval, 0.77 × 105 to 13.1 × 105). Urinary black carbon mirrors the accumulation of medium-term to chronic exposure to combustion-related air pollution. This specific biomarker reflects internal systemic black carbon particles cleared from the circulation into the urine, allowing investigators to unravel the complexity of particulate-related health effects.

Three-photon-induced upconversion luminescence of lead bromide CH3NH3PbBr3 perovskites

In this paper, upconversion luminescence from CH3NH3PbBr3 perovskite under pulsed laser illumination is reported. Excitation of bromide perovskite powders with a UV wavelength results in green PL bands at 562 nm. Infrared-to-visible upconversion luminescence is demonstrated in CH3NH3PbBr3 hybrid perovskites under 1064 nm nanosecond pulsed laser irradiation at room temperature. The shapes of both emission spectra are nearly identical, with a maximum at 562 nm. The green fluorescence emission is discernible by the naked eye. A remarkable feature of CH3NH3PbBr3 perovskite is the fact that fluorescence emission can be induced UV or NIR wavelength through different absorption processes. Upconversion luminescence is identified by measuring the NIR pump beam intensity dependence of luminescence which illustrates that the upconverted emission is induced by three-photon absorption process. The quadratic dependency of PL peak position on incident intensity is also another verification for the three-photon absorption process.

Thermomechanical characterization of a nanoscale copper thin-film using picosecond ultrasonics

Measurements were performed using the picosecond ultrasonic technique to evaluate thermal and elastic properties of a nanoscale copper (Cu) thin-film in the present study. A 50-nm thick Cu film was deposited using an e-beam evaporator, and the reflectance changes induced by thermal diffusion and acoustic wave propagation in the film upon illumination of femtosecond laser pulses were recorded. A numerical analysis was conducted using the finite difference method in order to extract the thermal conductivity and Young’s modulus of the material. Results showed that the film’s thermal conductivity decreased compared to the bulk property while its elastic modulus increased due to the scale effect originating from changes in microstructural contributions to the resultant characteristics. Results were compared to literature values and validated. The present study demonstrates an advanced measurement technique that can effectively and simultaneously analyze thermomechanical characteristics of nanoscale materials in a nondestructive way.

Nanostructures with periodic heating–cooling cycles for photoacoustic imaging using continuous-wave illumination

Over the last few years, there is a growing interest in photoacoustic imaging using nanoparticles techniques due to the improved penetration depth and resolution. Working with such nanoparticles usually requires pulsed laser illumination to generate an acoustic signal in the right frequencies. However, these pulsed lasers are considered expensive and complicated with respect to continuous-wave (CW) illumination. We design and simulate a special nanostructure with overall dimensions of 190×50× (26–34) nm, which blinks with fast temporal periodicity of 20 to 40 ns, under CW illumination and can be used for the generation of acoustic signals. This blinking is done using the enhanced optical absorption of metallic nanoparticles due to localized surface plasmon resonance (SPR) and the thermal expansion to generate heating–cooling cycles of the nanostructure. The CW laser wavelength is adapted to the localized SPR of the metallic nanostructure at the NIR region, which provides maximum penetration depth of light into biological tissues.

Comparing photoporation and nucleofection for delivery of small interfering RNA to cytotoxic T cells

The success of cancer immunotherapy through the adoptive transfer of cytotoxic T lymphocytes (CTLs) is highly dependent on the potency of the elicited anti-tumor responses generated by the transferred cells, which can be hindered by a variety of upregulated immunosuppressive pathways. Downregulation of these pathways in the T cells via RNA interference (RNAi) could significantly boost their capacity to infiltrate tumors, proliferate, persist, and eradicate tumor cells, thus leading to a durable anti-tumor response. Unfortunately, it is well known that primary T cells are hard-to-transfect and conventional non-viral transfection agents are generally ineffective. Viral transduction and electroporation are more efficient but their use is restricted by high cost, safety issues, and cytotoxicity. Photoporation has recently gained interest as a more gentle alternative physical approach to deliver membrane-impermeable macromolecules into cells. By attaching gold nanoparticles (AuNPs) to the cell surface followed by pulsed laser illumination, transient membrane pores can be generated that allow the diffusion of macromolecules directly into the cell cytosol. Here, we evaluated this technique for the non-toxic and effective delivery of small interfering RNA (siRNA) and subsequent silencing of target genes in activated CTLs. We compared photoporation with nucleofection, the current standard physical technique for T cell transfection, and demonstrated a significantly reduced cytotoxicity and higher average dose per cell for the photoporation technique.

Experimental study on the effect of injector nozzle K factor on the spray characteristics in a constant volume chamber: Near nozzle spray initiation, the macroscopic and the droplet statistics

In this work, the effects of the injector K factor on the spray characteristics have been investigated by using a high pressure common rail injection system and a constant volume chamber. The near nozzle region spray tip behavior, the spray macroscopic characteristics, and the droplet statistics in a fixed spray location is sequentially examined. Specifically, in the near nozzle region, laser pulse illumination and microscopic imaging technique was used to capture the very initial spray evolution morphology and it is found to take on the form of a mushroom like structure which quickly breaks up. Increasing K factor advances the spray tip evolution and favors the formation of the shockwave which might be caused the accelerated nozzle inner flow. For different K factor nozzles, the spray tip penetration (STP) evolution before the primary breakup is all found to include two sub-stages: in the very initial stage when the needle valve open is relatively small, the breakup of the mushroom like tip results in a slightly decelerated STP evolution. Subsequently, when the needle valve is fully opened, the STP evolution is gradually accelerated by the increased mass flux. The classic Hiroyasu and Arai [1] empirical model, which shows constant STP evolution speed before the primary breakup, is then refined to account this unsteady STP evolution behavior. Secondly, the spray macroscopic behavior data such as the spray tip penetration and the spray cone angle from varying K factor nozzles were collected by high speed visualization. It is found that as the K factor is increased, the STP evolution is accelerated and spray cone angle decreases. The classic model for the STP by Hiroyasu and Arai [1] which is applied K=0 nozzle was extended to accommodate the effect of K factor on the STP evolution after the primary breakup and the revised empirical model show perfect performance in predicting the STP behavior after the primary breakup for all different K factor nozzles. Finally, by using the particle/droplet image analysis (PDIA) system, the droplet statistics and the Sauter Mean Diameter (SMD) that characterize the atomization behavior at a certain spray location were obtained. It is seen that high K factor nozzle yields a slightly smaller SMD and it’s believed to enhance the atomization process. However, our experimental results show that this K nozzle effect is relatively weak and it vanishes for nozzles with larger outlet diameter.

Study of the laser melting of pre-deposited intermetallic TiAl powder by comprehensive optical diagnostics

The objective of this study is to develop intermetallic TiAl coatings through the laser melting of Ti–48Al–2Cr–2Nb pre-deposited powder on Ti6Al4V substrate. The coating formation is studied inside an insulated box filled with a protective gas using a high-speed single-wave pyrometer, an IR-camera, and a high-speed CCD camera with pulsed laser illumination. The relationship between the IR-camera signal and the powder melted track width measured by microscopy was used to determine the signal level corresponding to the molten pool shape. The geometry of the molten pool and the heat affected zone versus processing parameters are analysed.It is found that undesirable Al losses through its volumetric boiling prevent TiAl formation. By analysing the evolution of the pyrometer signal, it is possible to assess the size of the zone of the laser radiation transfer in the powder bed which is found larger than the length of the molten pool.The process visualization has shown that the powder particles are melted and form relatively large size liquid droplets, 30–100μm, that later on are integrated into the molten pool because of surface tension. Several small droplets are always detected well before the molten pool at the distance of about 200–300μm. Their motion may be explained by the reactive pressure of evaporation and by the pressure of the expanding heated gas filling the space in between the particles.

Optically and acoustically triggerable sub-micron phase-change contrast agents for enhanced photoacoustic and ultrasound imaging

We demonstrate a versatile phase-change sub-micron contrast agent providing three modes of contrast enhancement: 1) photoacoustic imaging contrast, 2) ultrasound contrast with optical activation, and 3) ultrasound contrast with acoustic activation. This agent, which we name ‘Cy-droplet’, has the following novel features. It comprises a highly volatile perfluorocarbon for easy versatile activation, and a near-infrared optically absorbing dye chosen to absorb light at a wavelength with good tissue penetration. It is manufactured via a ‘microbubble condensation’ method. The phase-transition of Cy-droplets can be optically triggered by pulsed-laser illumination, inducing photoacoustic signal and forming stable gas bubbles that are visible with echo-ultrasound in situ. Alternatively, Cy-droplets can be converted to microbubble contrast agents upon acoustic activation with clinical ultrasound. Potentially all modes offer extravascular contrast enhancement because of the sub-micron initial size. Such versatility of acoustic and optical ‘triggerability’ can potentially improve multi-modality imaging, molecularly targeted imaging and controlled drug release.