Repetition Rate Dependence Research Articles

Accurate Determination of the Low-Light-Level Absorption of DUV-Fused Silica at 193 nm with Laser Calorimetry

The low-light-level absorption coefficient of OH-contained and H2-impregnated synthetic fused silica material in 193 nm optical lithography application is determined via a laser calorimetry measurement. The fluence and repetition rate dependences of the absorptances of the deep ultraviolet (DUV)-fused silica samples with different thickness are measured. The measured dependences are fitted to a theoretical model, taking into consideration the generation and annealing of laser irradiation induced defects. The surface absorption, the low-light-level linear absorption coefficient, as well as the nonlinear absorption coefficient of the fused silica material are accurately determined via the fitting. The low-light-level linear absorption coefficients determined via the fluence dependence and the repetition rate dependence are in good agreement, demonstrating the reliability of the measured low-light-level absorption coefficient, which is the key parameter to the determination of the internal transmission of the DUV-fused silica material used in the 193 nm optical lithography.

THE USE OF THE EXRADIN W1 PLASTIC SCINTILLATOR FOR MEASUREMENTS IN EXTERNAL RADIOTHERAPY.

The Exradin W1 plastic scintillator (Standard Imaging) was examined for measurement suitability in high-energy photon beams, high-energy electron beams and tomotherapy in terms of dose to water for reference fields. For photon beams, pulse repetition rate dependence, calibration stability, noise from the photodiode enclosure, detector motion during treatment, output factors, off-axis doses and percentage depth doses were tested. For electron and tomotherapy beams, energy non-dependence was verified. All features make the detector suitable in small and non-standards fields, and for electron beams.

Temperature Rise under Two-Photon Optogenetic Brain Stimulation

In recent decades, optogenetics has been transforming neuroscience research, enabling neuroscientists to drive and read neural circuits. The recent development in illumination approaches combined with two-photon (2P) excitation, either sequential or parallel, has opened the route for brain circuit manipulation with single-cell resolution and millisecond temporal precision. Yet, the high excitation power required for multi-target photostimulation, especially under 2P illumination, raises questions about the induced local heating inside samples. Here, we present and experimentally validate a theoretical model that makes it possible to simulate 3D light propagation and heat diffusion in optically scattering samples athigh spatial and temporal resolution under the illumination configurations most commonly used to perform 2P optogenetics: single- and multi-spot holographic illumination and spiral laser scanning. By investigating the effects of photostimulation repetition rate, spot spacing, and illumination dependence of heat diffusion, we found conditions that make it possible to design a multi-target 2P optogenetics experiment with minimal sample heating.

Initial clinical experience with ArcCHECK for IMRT/VMAT QA.

Many devices designed for the purpose of performing patient‐specific IMRT/VMAT QA are commercially available. In this work we report our experience and initial clinical results with the ArcCHECK. The ArcCHECK consists of a cylindrical array of diode detectors measuring entry and exit doses. The measured result is a cumulative dose displayed as a 2D matrix. The detector array requires both an absolute dose calibration, and a calibration of the detector response, relative to each other. In addition to the calibrations suggested by the manufacturer, various tests were performed in order to assess its stability and performance prior to clinical introduction. Tests of uniformity, linearity, and repetition rate dependence of the detector response were conducted and described in this work. Following initial testing, the ArcCHECK device was introduced in the clinic for routine patient‐specific IMRT QA. The clinical results from one year of use were collected and analyzed. The gamma pass rates at the 3%/3 mm criterion were reported for 3,116 cases that included both IMRT and VMAT treatment plans delivered on 18 linear accelerators. The gamma pass rates were categorized based on the treatment site, treatment technique, type of MLCs, operator, ArcCHECK device, and LINAC model. We recorded the percent of failures at the clinically acceptable threshold of 90%. In addition, we calculated the threshold that encompasses two standard deviations (2 SD) (95%) of QAs (T95) for each category investigated. The commissioning measurements demonstrated that the device performed as expected. The uniformity of the detector response to a constant field arc delivery showed a 1% standard deviation from the mean. The variation in dose with changing repetition rate was within 1 cGy of the mean, while the measured dose showed a linear relation with delivered MUs. Our initial patient QA results showed that, at the clinically selected passing criterion, 4.5% of cases failed. On average T95 was 91%, ranging from 73% for gynecological sites to 96.5% for central nervous system sites. There are statistically significant differences in passing rates between IMRT and VMAT, high‐definition (HD) and non‐HD MLCs, and different LINAC models (p‐values <<0.001). An additional investigation into the failing QAs and a comparison with ion‐chamber measurements reveals that the differences observed in the passing rates between the different studied factors can be largely explained by the field size dependence of the device. Based on our initial experience with the ArcCHECK, our passing rates are, on average, consistent with values reported in the AAPM TG‐119. However, the significant variations between QAs that were observed based on the size of the treatment fields may need to be corrected to improve the specificity and sensitivity of the device.PACS number(s): 87.55.Qr, 87.56.Fc

Repetition rate dependence of absorption of fused silica irradiated at 193 nm

Repetition rate-dependent absorbance measurements of synthetic fused silica at 193-nm irradiation are performed in the range of 50-1 000 Hz with an ArF laser calorimeter. The single- and two-photon absorption coefficients are determined by measuring the laser fluence-dependent absorbance of fused silica samples with different thicknesses to separate the surface absorption and bulk absorption. The measurement results indicate a reversible nonlinear increase of both apparent single- and two-photon absorption coefficients with increasing repetition rate for the synthetic fused silica, whereas the surface absorption shows no dependence on the repetition rate.

Repetition Rate Dependence of Ferroelectric Properties of Polycrystalline BiFeO3 Films Prepared by Pulsed Laser Deposition Method

Polycrystalline BiFeO3 (BFO) films on Pt/TiO2/SiO2/Si substrate have been prepared by a pulsed laser deposition (PLD) method with various repetition rates. The X-ray diffraction (XRD) pattern observes single-phase perovskite structure with a small secondary phase. While the film-thickness is increased with increasing repetition rates, the grain size increases, and the leakage current reduces. Also, good polarization versus electric field (P-E) hysteresis loops of all polycrystalline BFO films were obtained at room temperature (RT). It is seen that the polarization value (Pr ) practically remains almost constant for all the BFO films, and the coercive field (Ec ) decreases with increasing the film thickness.

Optimizing the accuracy of a helical diode array dosimeter: A comprehensive calibration methodology coupled with a novel virtual inclinometer

The goal of any dosimeter is to be as accurate as possible when measuring absolute dose to compare with calculated dose. This limits the uncertainties associated with the dosimeter itself and allows the task of dose QA to focus on detecting errors in the treatment planning (TPS) and/or delivery systems. This work introduces enhancements to the measurement accuracy of a 3D dosimeter comprised of a helical plane of diodes in a volumetric phantom. We describe the methods and derivations of new corrections that account for repetition rate dependence, intrinsic relative sensitivity per diode, field size dependence based on the dynamic field size determination, and positional correction. Required and described is an accurate "virtual inclinometer" algorithm. The system allows for calibrating the array directly against an ion chamber signal collected with high angular resolution. These enhancements are quantitatively validated using several strategies including ion chamber measurements taken using a "blank" plastic shell mimicking the actual phantom, and comparison to high resolution dose calculations for a variety of fields: static, simple arcs, and VMAT. A number of sophisticated treatment planning algorithms were benchmarked against ion chamber measurements for their ability to handle a large air cavity in the phantom. Each calibration correction is quantified and presented vs its independent variable(s). The virtual inclinometer is validated by direct comparison to the gantry angle vs time data from machine log files. The effects of the calibration are quantified and improvements are seen in the dose agreement with the ion chamber reference measurements and with the TPS calculations. These improved agreements are a result of removing prior limitations and assumptions in the calibration methodology. Average gamma analysis passing rates for VMAT plans based on the AAPM TG-119 report are 98.4 and 93.3% for the 3%/3 mm and 2%/2 mm dose-error/distance to agreement threshold criteria, respectively, with the global dose-error normalization. With the local dose-error normalization, the average passing rates are reduced to 94.6 and 85.7% for the 3%/3 mm and 2%/2 mm criteria, respectively. Some algorithms in the convolution/superposition family are not sufficiently accurate in predicting the exit dose in the presence of a 15 cm diameter air cavity. Introduction of the improved calibration methodology, enabled by a robust virtual inclinometer algorithm, improves the accuracy of the dosimeter's absolute dose measurements. With our treatment planning and delivery chain, gamma analysis passing rates for the VMAT plans based on the AAPM TG-119 report are expected to be above 91% and average at about 95% level for γ(3%/3 mm) with the local dose-error normalization. This stringent comparison methodology is more indicative of the true VMAT system commissioning accuracy compared to the often quoted dose-error normalization to a single high value.

A trial of Fe(Se1−xTex) thin film fabrication by pulsed laser deposition using ArF excimer laser

We fabricated Fe(Se1−xTex) thin films on LSAT(100), MgO(001), R-Al2O3 substrates by ArF excimer pulsed laser deposition (ArF-PLD) and investigated pulse repetition rate dependence on film growth of Fe(Se1−xTex) thin films in ArF-PLD. Through x-ray diffraction measurements of Fe(Se1−xTex) thin films grown by ArF-PLD, 00l peaks of Fe(Se1−xTex) were confirmed in Fe(Se1−xTex) thin films grown by pulse repetition rate of 10 Hz but the 00l peaks were not confirmed in Fe(Se1−xTex) thin films grown at 5 Hz. Atomic force microscopy (AFM) revealed that 100 ~ 250 nm sized grains were formed on surface of the thin films grown at 10 Hz. It was found that the thin films grown at 5 Hz were formed thinner than those grown at 10 Hz, in spite of the same pulses. Energy dispersive x-ray spectroscopy (EDX) analysis revealed that composition elements of the thin films grown at 5 Hz were re-evaporated from them more than those grown at 10Hz. In ρ-T measurements of the thin films grown at 10 Hz, it was confirmed that the thin films has TConset = 6.5 ~ 10.5 K and TC0 of the Fe(Se1−xTex) thin film on an MgO substrate is 3.9 K.

ArF laser induced pulse absorption in fused silica (type III): Modeling the fluence and repetition rate dependence

Abstract At 193 nm, weak stationary bulk absorption coefficients αstat in standard and experimental grade fused silica (type III) are measured in dependence on the laser fluence H and repetition rate f. The samples show non-linear increases αstat(H) for 0.2 ⩽ H ⩽ 5 mJ cm−2 pulse−1 (f = const.) and αstat(f) for 100 ⩽ f ⩽ 1000 Hz (H = const.). An absorption model, focussing on ArF laser induced E′ center generation and annealing, and the associated rate equations are applied to simulate the experimental data quantitatively. From the simulations, material parameters like the 2-photon absorption (TPA) coefficient, the E′ center absorption cross section σE′ and the hydrogen related E′ annealing rate are calculated. TPA coefficients values of 9.7 · 10−9 cm/W (standard grade material) and 1.4 · 10−8 cm/W (experimental grade material), E′ center cross sections of 4.5 · 10−18 and 3.6 · 10−18 cm2 and hydrogen annealing rates of 1.5 s−1 (standard grade) and 3.4 s−1 (experimental grade) are found.

Optical Heterodyne Detected Accumulated Acoustic Grating Responses in Near Supercritical Fluids

A novel third-order polarization effect due to an accumulated optical heterodyne detected (OHD) transient acoustic grating response in near critical fluids was observed and experimentally characterized. Femtosecond pump-probe responses in near critical CO2 and CHF3 illustrate this phenomenon. This large optically generated acoustic response due to electrostrictive coupling appears only when pump and probe pulses are temporally overlapped and is pi out-of-phase with the normal optical Kerr effect (OKE) birefringent signal. The local oscillator, the laser intensity, and the modeled experimental repetition rate dependence identify the accumulated heterodyne origin of these responses. The observed OHD accumulated acoustic birefringent signal is inversely dependent on sound velocity to the fifth power. A corresponding sound velocity dependent dichroic (in-phase) response was also observed for these electronically nonresonant samples. The accumulated effect described here may have applications for the design of efficient modulators and as a simple and sensitive experimental technique for the measurement of near critical fluid thermodynamic and acoustic parameters.

On the damage behavior of dielectric films when illuminated with multiple femtosecond laser pulses

The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated. The measured temperature increase of a Ta2O5 film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that depends quadratically on the pulse fluence and is attributed to two-photon excitation of self-trapped excitons that form after band-to-band excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant pulse number. The repetition rate dependence of the breakdown threshold is also measured for TiO2, HfO2, Al2O3, and SiO2 films. A theoretical model is presented that explains these findings.

Correlation Between Oxygen Consumption and Photobleaching During In Vitro Photodynamic Treatment with ATX‐S10‐Na(II) Using Pulsed Light Excitation: Dependence of Pulse Repetition Rate and Irradiation Time¶

ABSTRACTWe revealed that in ATX‐S10‐Na(II)(13,17‐bis (1‐carboxypropionyl) carbamoylethyl‐8‐etheny‐2‐hydroxy‐3‐hydroxyiminoethylidene‐2,7,12, 18‐tetraethyl porphyrin sodium)‐mediated photodynamic therapy using 667 nm nanosecond‐pulsed light excitation at a peak intensity of 2.0 MW/cm2, photoxicity increased with decreasing pulse repetition rate in the range of 5‐30 Hz for A549 cell cultures. To examine the relation between the reaction mechanism and measured phototoxicity, we carefully measured the kinetics of photochemical oxygen consumption and photobleaching during irradiation of ATX‐S10‐Na(II)‐sensitized A549 monolayer cultures. Measurements of oxygen consumption with a microelectrode, which was performed just above the cells, showed that there was no significant difference between the magnitudes of decrease in oxygen at the three repetition rates at the same cumulative fluence. Loss of ATX‐S10‐Na(II) fluorescence intensity also exhibited little repetition rate dependence when compared at the same cumulative fluence. We investigated the correlation between oxygen consumption and photobleaching during irradiation and obtained “fluorescence‐oxygen diagrams.” The diagrams showed dynamic changes between oxygen‐dependent and oxygen‐independent photobleaching at the higher repetition rates of 10 and 30 Hz, whereas such change was not clearly seen over the whole irradiation time at 5 Hz. These results suggest that the reduced phototoxicity at high repetition rates might be due to an oxygen‐independent reaction. We presumed that the change in the reaction mechanism was associated with the local concentrations of the photosensitizer and oxygen in cells during irradiation.

Correlation between oxygen consumption and photobleaching during in vitro photodynamic treatment with ATX-S10.Na(II) using pulsed light excitation: dependence of pulse repetition rate and irradiation time.

We revealed that in ATX-S10.Na(II)(13,17-bis (1-carboxypropionyl) carbamoylethyl-8-etheny-2-hydroxy-3-hydroxyiminoethylidene-2,7,12,18-tetraethyl porphyrin sodium)-mediated photodynamic therapy using 667 nm nanosecond-pulsed light excitation at a peak intensity of 2.0 MW/cm(2), phototoxicity increased with decreasing pulse repetition rate in the range of 5-30 Hz for A549 cell cultures. To examine the relation between the reaction mechanism and measured phototoxicity, we carefully measured the kinetics of photochemical oxygen consumption and photobleaching during irradiation of ATX-S10.Na(II)-sensitized A549 monolayer cultures. Measurements of oxygen consumption with a microelectrode, which was performed just above the cells, showed that there was no significant difference between the magnitudes of decrease in oxygen at the three repetition rates at the same cumulative fluence. Loss of ATX-S10.Na(II) fluorescence intensity also exhibited little repetition rate dependence when compared at the same cumulative fluence. We investigated the correlation between oxygen consumption and photobleaching during irradiation and obtained "fluorescence-oxygen diagrams." The diagrams showed dynamic changes between oxygen-dependent and oxygen-independent photobleaching at the higher repetition rates of 10 and 30 Hz, whereas such change was not clearly seen over the whole irradiation time at 5 Hz. These results suggest that the reduced phototoxicity at high repetition rates might be due to an oxygen-independent reaction. We presumed that the change in the reaction mechanism was associated with the local concentrations of the photosensitizer and oxygen in cells during irradiation.

Two-photon-absorption-induced accumulated thermal effect on femtosecond Z-scan experiments studied with time-resolved thermal-lens spectrometry and its simulation

Accumulated thermal effect (ATE), which can be an artifact in femtosecond closed-aperture Z-scan measurements, was investigated by varying the repetition rate of the incident pulse for a dye solution in CHCl3. It was found that the measurement is affected by two-photon-absorption-induced ATE, even at a low repetition rate of 1 kHz. The relaxation time of the ATE was found to be tens of milliseconds by time-resolved thermal-lens experiments and simulations, which is consistent with the observed repetition-rate dependence of the Z-scan measurements. The simulations for various commonly used solvents also exhibited that the ATE can be prominent in hydrocarbon and halogenated hydrocarbon solvents and inconspicuous in alcohols and water.

Self-trapped exciton luminescence and repetition rate dependence of two-photon absorption in CaF2 at 193 nm

Abstract Repetition rate-dependent measurements of nonlinear absorptance and self-trapped exciton luminescence ( π -luminescence) of high-purity CaF 2 crystals at 193 nm irradiation were performed in the range between 25 and 300 Hz. The CaF 2 samples showed different (effective) two-photon absorption coefficients β eff , which are attributed to different impurity or defect concentrations inside the crystals. For samples with low β eff , no dependence on repetition rate could be observed, while samples with high values reveal a strong increase of β eff for increasing repetition rates. In contrast, the intensity of self-trapped exciton luminescence is nearly the same for all samples and constant for all repetition rates. Different two-photon absorption mechanisms were derived from these results and a simple kinetic model for the absorption behaviour was developed, including the formation of transient absorbing states. In addition, laser induced bleaching of Ce 3+ emission during irradiation with a total dose up to 900 mJ/cm 2 could be observed in the luminescence spectra.

Pulse repetition rate dependence of hydroxylapatite deposition by laser ablation

Hydroxylapatite (HA) is used as coating on metallic dental and orthopaedical implants due to its similarity to the mineral part of bone. As an alternative to the commercial plasma spraying coating technique, pulsed laser deposition has been applied. In order to study the possibility of obtaining high-deposition rates by varying the pulse repetition rate, a HA target was ablated with an ArF laser at an energy density of 0.9 J cm −2 in a water vapour pressure of 45 Pa , and the material was deposited on a titanium substrate held at 460°C. The coating thickness was measured by profilometry. The phases present in the coating were detected by X-ray diffraction. Their Ca/P ratios were determined by energy dispersive X-ray spectrometer, while their OH − and CO 3 2− contents were evaluated by Fourier transform infrared. As the pulse repetition rate is increased from 20 to 80 Hz, the structural order of the coatings decreases, and, eventually, at 100 Hz the coatings become amorphous. When the time interval between pulses is too short there is not enough time for the material to diffuse within the surface and accommodate in the structure before the material from the next pulse arrives. A long time between pulses also promotes the desorption of species from the substrate surface, preferentially CO 3 2−.

Repetition-rate dependence of thermal loading in diode-end-pumped Q-switched lasers: influence of energy-transfer upconversion

The fractional thermal loading in diode-end-pumped Q-switched lasers was estimated from the theory of cavity stability. The experimental results show that the thermal loading depends on the pulse-repetition rate. This dependence is attributed to the effect of energy-transfer upconversion (ETU). A theoretical model including the ETU effect was developed to analyze the fractional thermal loading in end-pumped Q-switched lasers. The theoretical predictions were found to be in good agreement with the experimental data.

Repetition-rate dependence of the saturation power of gain-clamped semiconductor optical amplifiers

We show theoretically that, when using gain-clamped semiconductor optical amplifiers (GCSOAs) for the amplification of a train of pulses, the saturation power of the GCSOAs strongly depends on the repetition rate of the pulse train. This saturation power is 3 dB higher at very high repetition rates than at low repetition rates. It reaches a minimum for intermediate repetition rates, where it can even be 6 or more decibels smaller than the maximum saturation power level.

Repetition rate dependence of gray-tracking in KTiOPO4 during second-harmonic generation at 532 nm

The aim of this work is to study gray tracking during type II second-harmonic-generation 1064→532 nm in halide flux grown KTiOPO4 crystals. The repetition rate of the laser varies from 1 to 20 kHz. We experimentally demonstrate that the gray-tracking threshold, expressed by the harmonic peak power density, is a decreasing exponential function of the Q-switch frequency. The corresponding 532 nm average power density, however, remains constant.

Photodegradation and nonlinear effects in optical fibers induced by pulsed uv-laser radiation

Photodegradation of fused silica optical fibers was investigated during irradiation with pulsed uv-light for different laser parameters and coupling conditions. Colour center formation was detected by measuring absorption and fluorescence spectra between two subsequent laser pulses. E′- and NBOH-centers were detected after irradiation with 266 nm laser pulses, with the result of higher absorption of laser light by NBOH-centers. In addition, stronger degradation with distinct repetition rate dependence was observed when the core-cladding interface region had been irradiated. This effect results in a dramatic decrease in transmission and has to be avoided for optimum transmission by proper choice of coupling conditions. Nonlinear absorption and stimulated Raman-scattering were measured and their influence on uv-transmission for sensor applications was determined.