Time-resolved Optical Transmission Research Articles

Effect of temperature and illumination conditions on the photochromic performance of yttrium oxyhydride thin films

Transparent yttrium oxyhydride (YHO) thin films exhibit photochromic effects under ambient conditions. The optical transmission in such materials can be strongly modulated upon light absorption. This switchable optical property is interesting for technological applications, such as energy-saving smart windows and optical sensors. For the practical use of photochromic YHO materials, it is crucial to understand how the kinetics of coloration and the resulting photochromic contrast are influenced under different illumination conditions at both room and elevated temperatures. In the present study, we have prepared yellow, transparent, photochromic YHO thin films on glass substrates by using reactive magnetron sputtering. We investigated the photochromic performance of both as-deposited and annealed YHO thin films using illumination sources with varying photon energies and intensities. The photochromic contrast and the coloration were found to decrease considerably with increasing annealing temperature and with decreasing intensity and photon energy of the illumination sources. Moreover, time-resolved optical transmission measurements revealed a nearly logarithmic time dependence of the coloration of YHO thin films, indicating rapid coloration in the beginning of light exposure, followed by slow coloration with increasing illumination time.

Initial experimental demonstration of the principles of a xenon gas shield designed to protect optical components from soft x-ray induced opacity (blanking) in high energy density experiments

The design principles of a xenon gas shield device that is intended to protect optical components from x-ray induced opacity (“x-ray blanking”) have been experimentally demonstrated at the OMEGA-60 Laser Facility at the Laboratory for Laser Energetics, University of Rochester. A volume of xenon gas placed in front of an optical component absorbs the incoming soft x-ray radiation but transmits optical and ultra-violet radiation. The time-resolved optical (532 nm) transmission of samples was recorded as they were exposed to soft x-rays produced by a gold sphere source (1.5 kJ sr−1, 250–300 eV). Blanking of fused silica (SiO2) was measured to occur over a range of time-integrated soft x-ray (<3 keV) fluence from ∼0.2–2.5 J cm−2. A shield test device consisting of a 30 nm silicon nitride (Si3N4) and a 10 cm long volume of 0.04 bar xenon gas succeeded in delaying loss of transmission through a magnesium fluoride sample; optical transmission was observed over a longer period than for the unprotected sample. It is hoped that the design of this x-ray shield can be scaled in order to produce a shield device for the National Ignition Facility optical Thomson scattering collection telescope, in order to allow measurements of hohlraum plasma conditions produced in inertial confinement fusion experiments. If successful, it will also have applications in many other high energy density experiments where optical and ultra-violet measurements are desirable.

Measuring excited state lifetime of Rb atoms with pump-probe technique

A technique for measuring the excited state lifetime τex of optical transitions in alkali atoms is presented. It is a form of pump-probe technique based on time-resolved optical transmission through the atomic vapor cell. This technique can serve as an alternative to the traditionally used time-resolved photofluorescence methods when measuring alkali vapor cells with heavily quenched fluorescence, where the τex is expected to be on the order of a few nanoseconds, and the highly sensitive fluorescence detectors with sub-nanosecond temporal resolution that are required may not be available. We use this technique to measure the τex of Rb atoms in vapor cells with different buffer gas pressures.

Hot electron injection driven phase transitions

We report on a general mechanism for photo-induced phase transitions. The process relies on the photo-injection of hot electrons from an adjacent metallic layer to trigger the structural dynamics of the materials of interest. This mechanism is demonstrated for the semiconductor-to-metal phase transition of VO${}_{2}$ using a 20 nm Au injection layer. The nature of the phase transition is demonstrated by time-resolved optical transmission measurements, as well as a well defined bias dependence that illustrates that the Au film is the source of nonequilibrium electrons driving the phase transition.

在冲击波纳秒加载过程中水的结晶相变及其动力学现象

The dynamic process of the liquid to solid phase transition of water under multiple shock wave compressions was monitored using time-resolved optical transmission measurements.When the water sample was compressed to its second time of 2.21 GPa,near the border of the liquid-solid phase transition line,the metastable water nuclearyed crystal rapidly under the effect of the interface effects,and the size of the crystal was produced to the scattering of incident light effect in dozens of nanoseconds.The volume of the solid phase was found to increase with increasing pressure.The rate of crystallization decreased when pressure tended towards saturation.

Time-resolved imaging of laser-induced refractive index changes in transparent media

We describe a method to visualize ultrafast laser-induced refractive index changes in transparent materials with a 310 fs impulse response and a submicrometer spatial resolution. The temporal profile of the laser excitation sequence can be arbitrarily set on the subpicosecond and picosecond time scales with a pulse shaping unit, allowing for complex laser excitation. Time-resolved phase contrast microscopy reveals the real part of the refractive index change and complementary time-resolved optical transmission microscopy measurements give access to the imaginary part of the refractive index in the irradiated region. A femtosecond laser source probes the complex refractive index changes from the excitation time up to 1 ns, and a frequency-doubled Nd:YAG laser emitting 1 ns duration pulses is employed for collecting data at longer time delays, when the evolution is slow. We demonstrate the performance of our setup by studying the energy relaxation in a fused silica sample after irradiation with a double pulse sequence. The excitation pulses are separated by 3 ps. Our results show two dimensional refractive index maps at different times from 200 fs to 100 μs after the laser excitation. On the subpicosecond time scale we have access to the spatial characteristics of the energy deposition into the sample. At longer times (800 ps), time-resolved phase contrast microscopy shows the appearance of a strong compression wave emitted from the excited region. On the microsecond time scale, we observe energy transfer outside the irradiated region.

A model for the study of the optical transmission dynamics of liquid crystals dispersions under the influence of an electric field

In this paper, mathematical models are developed to correlate the time-resolved optical transmission experiments of liquid crystals dispersion devices under the influence of an electric field. The dispersion of liquid crystal in the confining matrices is supposed to be composed of identical spherical microdroplets containing the nematic liquid crystal phase. It is proposed that the confining surface induces the formation of a gradient of the torsional molecular velocities (switching velocities) in the response to an electric field or relaxation by the removal of an electric field, along the radius of the liquid crystal microdroplets. A method is proposed to determine the probability density function of the switching velocities. Applying the Laplace transform, the time-dependent functions for the optical transmission behavior of the liquid crystal dispersions are obtained. The model was applied to the study of the dynamics of glass dispersed liquid crystals devices, showing good correlation with the experimental data.

Characterization of in-situ time-resolved optical spectra during excimer-laser crystallization

A real-time in-situ time-resolved (~1 ns) optical reflectivity and transmission (TRORT) measurement system combining two He–Ne probe lasers, a digital oscilloscope, and three fast photodiodes is developed to investigate the rapid phase-change processes of Si thin films during the excimer-laser crystallization (ELC). The changes in both reflectivity and transmission of Si thin films during ELC are recorded by the TRORT measurement system. Melting and resolidification behaviors of Si thin films during ELC are interpreted. The fall time of liquid Si is reduced with increase in the excimer-laser energy density, while the rise time of liquid Si remains approximately constant at 5 ns. The first small peak in the reflectivity spectrum is proved to be not a phenomenon of explosive crystallization.

Self-guided glass drilling by femtosecond laser pulses

Straight through-holes of high aspect ratio have been fabricated in glass by femtosecond laser pulses, utilizing unique characteristics of ultrafast lasers such as volumetric multi-photon absorption and nonlinear self-focusing. In this study, interestingly, the drilling process was initiated and progressed in a self-regulated manner, while the laser focus was fixed through the specimen at the neighborhood of the rear surface that was in contact with liquid during the entire drilling process. The deposition of laser energy along the nonlinearly extended focal range and the guided drilling along the pre-defined region are explained based on time-resolved optical transmission and emission measurements.

Observation of explosive crystallization during excimer laser annealing using in situ time-resolved optical reflection and transmission measurements

XeF excimer laser-induced melting and recrystallization of amorphous silicon was investigated using in situ on-line time-resolved reflection and transmission measurements with a nanosecond time resolution. Explosive crystallization was observed for 900 Å thick amorphous silicon on SiO 2 deposited on non-alkali glass substrate upon 25 ns pulse duration of excimer laser by continuous-wave He-Ne probe laser. Three distinct regrowth regimes were investigated on the basis of various excimer laser fluences. Scanning electron microscopy, Raman spectroscopy and atomic force microscopy were used to evaluate the excimer laser-irradiated region of the sample. Grain size, surface roughness and melt duration as a function of various laser fluences are also determined.

Characterization of polycrystalline Ge thin films fabricated by short-pulse XeF excimer laser crystallization

XeF excimer laser-induced melting and recrystallization dynamics of amorphous germanium are investigated using time-resolved optical reflection and transmission measurements with a nanosecond time resolution, field-emission scanning electron microscopy, and micro-Raman spectroscopy. It is found that the disc-shaped grain with a diameter of approximately 0.8 µm is located in the complete melting regime with a melt phase duration of approximately 141–200 ns. The significant change of transmissivity is a key phenomenon revealing the excessive excimer laser fluence during excimer laser crystallization by in-situ optical measurements. Differences between the melting and recrystallization phenomenon for Si and Ge thin films are also discussed.

Characterization of polycrystalline silicon thin films fabricated by excimer laser crystallization

Phase transformation mechanisms during excimer laser crystallization and analysis of the resulting microstructures of polycrystalline silicon have been investigated in detail using in-situ time-resolved optical reflection and transmission measurements, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy, and micro-Raman spectroscopy. Grain size, surface roughness, crystallinity, and melt-phase duration as functions of various excimer laser fluences are determined. A detailed microstructure development model of Si thin films upon complete melting using excimer laser irradiation is proposed. The results of this work can help in the development and fabrication of singular-grain Si thin film transistors.

Monitoring explosive crystallization phenomenon of amorphous silicon thin films during short pulse duration XeF excimer laser annealing using real-time optical diagnostic measurements

Melting and crystallization scenario of amorphous silicon (a-Si) thin films have been investigated using in situ time-resolved optical reflection and transmission measurements. The explosive crystallization phenomenon is observed using a single-mode continuous wave He–Ne probe laser for thickness of 50 nm and 90 nm a-Si thin films upon 25 ns pulse duration of XeF excimer laser irradiation, respectively. The explosive crystallization phenomenon is easier to observe in the large thickness of a-Si thin films, a sample with pure a-Si microstructure and under longer pulse duration of excimer laser irradiation by time-resolved optical reflection and transmission measurements.

Piezoelectric characterization of ejecta from shocked tin surfaces

Using piezoelectric diagnostics, we have measured densities and velocities of ejected particulate as well as “free-surface velocities” of bulk tin targets shock loaded with high explosive. The targets had finely grooved, machined finishes ranging from 10 to 250μin. Two types of piezoelectric sensor (“piezopins”), lithium niobate and lead zirconate titanate, were compared for durability and repeatability; in addition, some piezopins were “shielded” with foam and metal foil in order to mitigate premature failure of the pins in high ejecta regimes. These experiments address questions about ejecta production at a given shock pressure as a function of surface finish; piezopin results are compared with those from complementary diagnostics such as x-ray radiography and time-resolved optical transmission techniques. The mass ejection shows a marked dependence on groove characteristics and cannot be described by a groove defect theory alone.

Revealing two-competing processes in carrier dynamics of single-walled carbon nanotube films

Ultrafast time-resolved optical transmission in single-walled carbon nanotubes was measured as a function of pump fluence at a temperature of 200 K employing degenerate pump–probe experiment. We observed a relaxation dynamics with two components characterized by 0.1 and 1 ps, respectively. The signal amplitude of the slow component also shows a crossover from negative to positive when the pump fluence decreased. It is suggested that the fast component is related to the intraband relaxation and the slow component is attributed to two-competing processes.

Crystallization kinetics of amorphous Ga–Sb–Te films: Part II. Isothermal studies by a time-resolved optical transmission method

Isothermal crystallization kinetics of amorphous Ga–Sb–Te films was studied by means of a time-resolved optical transmission method. Thin films with compositions along the pseudo-binary tie-lines Sb7Te3–GaSb and Sb2Te3–GaSb in the ternary phase diagram were prepared by the co-sputtering method. Crystallization of GaSbTe films reveals a two-stage process: an initial surface nucleation and coarsening (Stage 1) followed by the one-dimensional grain growth (Stage 2). The kinetic exponent (n) value in Stage 1 shows strong dependence on film compositions, while that of Stage 2 is less dependent. The activation energy in Stage 1 increases with increasing GaSb content and reaches the maximum values at compositions close to GaSb, but a decreasing trend was observed in Stage 2. Kinetics parameters between isothermal crystallization of thin films and non-isothermal crystallization of powder samples analyzed by differential scanning colorimetry [J. Mater. Res. 19, 2929 (2004)] are compared. The kinetic parameters in Stage 1 show much correspondence with those of non-isothermal cases in comparable kinetic exponents but with lower activation energies. The discrepancies between nonisothermal and isothermal kinetics are attributed to the sample morphology and the constraint effects.

The Franz–Keldysh effect in shocked GaN:Mg

The band gap of uniaxially strained semi-insulating GaN:Mg has been examined using time-resolved optical transmission measurements in shock-wave experiments. For longitudinal stresses between 2.9 and 4.6 GPa, the absorption edge broadened significantly. Such a broadening is consistent with the presence of large piezoelectric fields (Franz–Keldysh effect) generated due to shock compression. For stresses greater than 4.6 GPa, however, the absorption edge remained relatively sharp. The sharp absorption onset suggests the presence of free charge carriers that screen the piezoelectric field, thereby suppressing the Franz–Keldysh effect. These observations indicate that electrical breakdown occurs at a field of 1 MV/cm. One possible mechanism that could result in free carriers is the dissociation of Mg–H complexes under the very high piezoelectric fields.

Band gap changes of GaN shocked to 13 GPa

The band gap of GaN under uniaxial-strain compression was determined using time-resolved optical transmission measurements in shock-wave experiments. Shock waves were generated by impacting the GaN samples with c-cut sapphire impactors mounted on projectiles fired by a gas gun. Impact velocities were varied to provide longitudinal stresses ranging from 4.5 to 13 GPa. An abrupt increase of the band gap is observed upon shock-wave compression, followed by a slower increase. By measuring the absorption threshold before and during shock compression of the GaN layer, the band-gap shift for a particular longitudinal stress was obtained. A linear fit to the data yields a band-gap shift of 0.02 eV/GPa. Comparison with ab initio calculations show that this slope lies between the calculated bounds for isotropic and uniaxial compression. Potential reasons for the differences are indicated.

A prototype device for standardized calibration of pulse oximeters II.

There is no commonly accepted in vivo calibration method for pulse oximeters available up to now. On the basis of a prototype device for the calibration of pulse oximeters which was introduced recently, a second approach based on the same concept was tackled in order to design a reliable method for standardized calibration of pulse oximeters. An extensive clinical database of time-resolved optical transmission spectra of patient fingers is used to simulate the behavior of patients. A device which is capable of playing back these spectroscopic data to pulse oximeters, and a database where the oxygen status measured with the reference method (Co-Oximetry) is stored, are the main parts of the concept. The playback device has an artificial finger as interface to the pulse oximeters and serves to collect light from the pulse oximeter for analysis and to playback simulated light to the pulse oximeter. The light intensity emitted by two LEDs which illuminates the pulse oximeter detector is controlled via a computer in such a way that it is the same as if the pulse oximeter light had passed the finger. The pulse oximeter display during the data playback can thus be compared to the true SaO2 of the patient. The device is tested with 4 pulse oximeters based on 100 patient spectra. For the four pulse oximeters used in this investigation, an Agilent Technologies CMS monitor (formerly Hewlett-Packard), an Ivy 2000 with Masimo Set technology and Nellcor N-3000 and N-395, there is good correlation between SPO2 and SaO2, and mean and standard deviation of in vivo SpO2-SaO2 and playback SpO2-SaO2 are in good agreement. For two instruments, Nellcor N3000 and Agilent CMS Monitor, a quantitative comparison between the in vivo and in vitro SpO, results was derived. A mean of the deviation playback vs. in vivo SpO2 is less than 0.5% SpO2. The error limits are comparable with the calibration error of the conventional calibration routine. The device is also capable of data playback even in situations with rapid desaturation changes, as displayed in Figure 2. For the other tested pulse oximeters the results are comparable. Compared to the first prototype the current version is simpler and less expensive in production. Many of previously existing problems are solved and the applicability to a large variety of pulse oximeters and sensors is given. The novel concept for the calibration of pulse oximeters is a tool for assessing the performance of pulse oximeters.

A prototype device for standardized calibration of pulse oximeters.

To develop and test a method for standardized calibration of pulse oximeters. A novel pulse oximeter calibration technique capable of simulating the behavior of real patients is discussed. It is based on an artificial finger with a variable spectral-resolved light attenuator in conjunction with an extensive clinical database of time-resolved optical transmission spectra of patients fingers in the wavelength range 600-1000 nm. The arterial oxygen saturation of the patients at the time of recording was derived by analyzing a corresponding blood sample with a CO-oximeter. These spectra are used to compute the modulation of the light attenuator which is attached to the artificial finger. This calibration method was tested by arbitrarily playing back recorded spectra to pulse oximeters and comparing their display to the value they displayed when the spectra were recorded. We were able to demonstrate that the calibrator could generate physiological signals which are accepted by a pulse oximeter. We also present some experience of playing back recorded patient spectra. The mean difference between the original reading of the pulse oximeters and the display when attached to the calibrator is 1.2 saturation points (displayed oxygen saturation SpO2) with a standard deviation of 1.9 saturation points. The tests have shown the capabilities of a spectral light modulator for use as a possible calibration standard for pulse oximeters. If some improvements of the current prototype can be achieved we conclude from the experience with the device that this novel concept for the calibration of pulse oximeters is feasible and that it could become an important tool for assessing the performance of pulse oximeters.