Temporally and spectrally resolved imaging of laser-induced plasmas

We report a hybrid imaging technique capable of performing measurements of the spatial, temporal, and spectral emission characteristics of laser-induced plasmas by use of a single detection system. We apply this technique to study the plasma produced by laser ablation of LiNbO3 and observe phenomena not seen in such detail with standard instruments. These include extreme line broadening up to a few nanometers accompanied by self-absorption near the target surface, and expansion dynamics that differ strongly between the different species. Overall, the wealth of quantitative information provided by this novel technique sheds new light on processes occurring during plasma expansion.     

Picosecond laser-induced damage morphology: Spatially resolved microscopic plasma sites

The morphology just above threshold of damage sites caused by picosecond 1.06 μm laser pulses is shown to consist of a collection of micron-sized, spatially distinct vestiges of individual plasmas. From the observed site density, the density of electrons which may initiate breakdown can be inferred. The morphology is consistent with an avalanche ionization model, but not with absorbing inclusion damage.     

Spatially and temporally resolved IR-DFWM measurement of HCN released from gasification of biomass pellets

For the first time, to the best of the authors’ knowledge, nonintrusive quantitative measurement of hydrogen cyanide (HCN) released during the devolatilization phase of straw pellets gasification is demonstrated with high spatial and temporal resolution. Mid-infrared degenerate four-wave mixing (IR-DFWM) measurements of HCN were performed by probing the interference-free P(20) line in the v₁ vibrational band at around 3?µm and the IR-DFWM signal was detected with an upconversion-based detector, providing discrimination of thermal noise and increased sensitivity. A novel single-pellet setup consisting of a multi-jet burner was used to provide hot flue gas environments with an even and well-defined temperature distribution, for single straw pellet gasification at atmospheric pressure. The environments had temperatures of 1380?K, 1540?K and 1630?K with a constant oxygen concentration of 0.5?vol%. In order to quantify the amount of HCN released during the devolatilization of straw pellets, calibration measurements were performed in well-defined HCN gas flows. Selected hot water lines were probed with IR-DFWM in the interrogated volume to obtain the instantaneous temperature, which were used to correct the temperature effect. HCN concentrations up to 1500?ppm were detected during the devolatilization stage, and the results indicate a strong temperature dependence of the HCN release.     

Long-range spectrally and spatially resolved radiation from filaments in air

The distance-resolved spectral intensity distribution of the backscattered light from long filaments generated in air using ultra-short and intense laser pulses is presented. A clean fluorescence spectrum from N₂ molecules and ions, which is produced by the high peak intensity inside the plasma filament of the fundamental pulse, was clearly resolved from the backscattered supercontinuum. The supercontinuum generated by both the fundamental and the third-harmonic pulses developed progressively and became fully developed only at the end of the filamentation.     

Time resolved laser-induced plasma dynamics

The structure and evolution of the laser-induced vapor plume and shockwave were measured from femtosecond time resolved shadowgraph images. By changing the wavelength of the probe beam (400 and 800 nm), differences in the opacity of the vapor plume were measured as a function of delay time from the ablation laser pulse. The evolution of the temperature and electron number density during and after the ablation laser pulse were determined and compared for ablation in argon and helium background gases. A laser supported detonation wave (LSD) observed for ablation in argon, blocks the incoming laser energy and generates a high-pressure region above the vapor plume.     

Spatially resolved measurement of rock core porosity

Density weighted, centric scan, Conical SPRITE MRI techniques are applied in the current work for local porosity measurements in fluid saturated porous media. The methodology is tested on a series of sandstone core samples. These samples vary in both porosity and degree of local heterogeneity due to bedding plane structure. The MRI porosity measurement is in good agreement with traditional gravimetric measurements of porosity. Spatially resolved porosity measurements reveal significant porosity variation in some samples. This novel MRI technique should have applications to the characterization of local porosity in a wide variety of porous media.     

Spatially resolved holographic measurement of the thermal diffusivity in liquids

Phase gratings are recorded holographically with nanosecond light pulses by the interference of two plane waves in colored liquid samples using the thermooptic effect. The samples consist of toluene and distilled water which are immiscible and have different thermal conductivities. The exponential decay of the gratings caused by the thermal conductivity is monitored by diffraction of continuous wave light, and the thermal diffusivity is determined. Scanning of the read-out beam provides spatial resolution. PACS  42.40.My; 44.10.+i; 65.20.-w     

Spatially resolved surface-charge measurement in a planar dielectric-barrier discharge system

We investigate a laterally extended dielectric helium discharge system with plane electrodes. The system is operated in the glow mode and is known to exhibit a rich variety of self-organized lateral patterns in the current distribution, most of them being filamentary. It is known from theory that surface charges on the dielectrics play a major role for the emerging patterns. In this work we present a method to measure the spatial charge distribution on the dielectrics via the Pockels effect of a bismuth-silicon-oxide crystal. The experimental results of the surface-charge distribution measurements are in good agreement with previous numerical solutions of the corresponding transport equations.     

Enhanced laser-induced plasma channels in air

Plasma is a significant medium in high-energy density physics since it can hardly be damaged. For some applications such as plasma based backward Raman amplification(BRA), uniform high-density and large-scale plasma channels are required. In the previous experiment, the plasma transverse diameter and density are 50–200 μm and 1–2 × 10~(19)cm~(-3),here we enhance them to 0.8 mm and 8 × 10~(19)cm~(-3), respectively. Moreover, the gradient plasma is investigated in our experiment. A proper plasma gradient can be obtained with suitable pulse energy and delay. The experimental results are useful for plasma physics and nonlinear optics.     

Sensitive measurement of nonlinear refraction and two-photon absorption by spectrally resolved two-beam coupling

A simple and very sensitive method for determining refractive and absorptive nonlinearities based on spectral resolution of two-beam coupling is demonstrated. Nonlinear phase shifts as small as ~10(-6) rad and two-photon absorption coefficients as small as 10(-4) cm/GW are readily observed by use of this technique with nanojoule pulses from a mode-locked laser.     

Refractive index measurement by spectrally resolved interferometry using a femtosecond pulse laser

We describe a measurement method of refractive indices by way of spectrally resolved interferometry using a femtosecond pulse laser. The method is dispersion insensitive and requires no prior precise knowledge of the geometrical thickness of the specimen. Not only the group but also the phase refractive index can be determined over the wide spectral range covered by the optical comb of the femtosecond pulse laser in use.     

Spatially resolved measurement of femtosecond laser induced refractive index changes in transparent materials

We present a practical method to determine femtosecond laser induced refractive index changes in transparent materials. Based on an iterative Fourier transform algorithm, this technique spatially resolves the refractive index of complex structures by combining the dimensions of the modified region with the corresponding phase change extracted from far-field intensity measurements. This approach is used to characterize optical waveguides written by a femtosecond laser in borosilicate glass.     

Spatially resolved plasma emission and absorption coefficientmeasurements by an optical fiber probe

The plasma emission and absorption coefficients were measured by means of two different techniques using an optical fiber probe in a parallel-plate RF glow discharges. The first technique can, in principle, provide three-dimensional resolution with the movement of the probe, while the second technique provides only a line-average measurement. The results of two different measurement methods and the practical limitations of the techniques are discussed     

Signal detection in temporally modulated and spectrally shaped maskers.

The first part of this paper presents several experiments on signal detection in temporally modulated noise, yielding a general approach toward the concept of comodulation masking release (CMR). Measurements were made on masked thresholds of both long- and short-duration, narrow-band signals presented in a 100% sinusoidally amplitude-modulated (SAM) noise masker (modulation frequency 32 Hz), as a function of masker bandwidth from 1/3 oct up to 13/3 octs, while the masker band was geometrically centered at signal frequency. With the short-duration signals placed in the valley of the masker, a substantial CMR (i.e., a decrease of masked threshold with increasing masker bandwidth) was found, whereas for the long-duration signals CMR was smaller. Furthermore, investigations were carried out to determine whether CMR changes when the bandwidth of the signals, consisting of bandpass impulse responses, is increased. The data indicate that substantial CMR remains even when all masker bands contain a signal component, thus minimizing across-channel differences. This finding is not in line with current models accounting for the CMR phenomenon. The second part of this paper concerns signal detection in spectrally shaped noise. Also investigated was whether release from masking occurs for the detection of a pure-tone signal at a valley or a peak of a simultaneously presented masking noise with a sinusoidally rippled power spectrum, when this masker was preceded and followed by a second noise (temporal flanking burst) with an identical spectral shape as the on-signal noise. Similar to CMR effects for temporal modulations, the data indicate that coshaping masking release (CSMR) occurs when the signal is placed in a valley of the spectral envelope of the masker, whereas no release from masking is found when the signal is placed at a peak of the spectral envelope of the masker. The implications of these experiments for measures of spectral and temporal resolution are discussed.     

Effect of the wavelength dependence of the emissivity on inferred soot temperatures measured by spectrally resolved laser-induced incandescence

Flame-generated soot was heated using a pulsed laser, and temperatures of the irradiated soot were inferred by fitting the Planck function to spectrally resolved laser-induced incandescence with the temperature as an adjustable parameter. The effect of the wavelength dependence of the emissivity on the inferred temperatures of the irradiated soot was studied using selected expressions for the soot emissivity in the fit. Depending upon the choice of the functional form of the emissivity, the maximum temperature reached by the soot during the laser pulse was calculated to span a range of 341 K (3475–3816 K) at a 1064-nm laser fluence of 0.1 J/cm² and 456 K (4115–4571 K) at a 1064-nm laser fluence of 0.4 J/cm² with a 1σ standard deviation about the mean of ∼25 K. Comparison of the present results with temperature measurements from previous studies suggests that the emissivity may depend on flame conditions and that further investigation on the subject is needed. The use of two-color or spectrally resolved LII to infer the soot temperature during or after laser heating requires a careful characterization of the wavelength dependence of the emissivity. The spread in temperature leads to large uncertainties regarding the physico-chemical processes occurring at the surface of the soot during the laser heating.     

On the spectrally resolved thermoluminescence of alumina

Summary The thermoluminescence of α-alumina was investigated by recording light emission simultaneouslyvs. temperature and wavelength. Samples were prepared, starting from amorphous alumina, by means of thermal treatments in the 850 to 1400°C temperature range. The recorded light exhibits two spectral bands at 553 and 706 nm emitted at temperatures close to 500 K, which reappear in the 700 K region with reduced intensity. The 553 nm band fades away in samples treated at temperatures exceeding 1200°C. The results are compared to a kinetic model which explains the most significant experimental features. Some qualitative arguments are advanced which could be useful in identifying the nature of the lattice defects responsible for the thermoluminescent emission observed.

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Riassunto Si è studiata la termoluminescenza dell'α-alumina registrando l'emissione di luce in funzione, simultaneamente, della temperatura e della lunghezza d'onda. Partendo da allumina amorfa, i campioni sono stati preparati mediante trattamenti termici nell'intervallo di temperatura da 850 a 1400°C. La luce registrata mostra due bande spettrali a 553 e 706 nm, emesse a temperature vicine a 500 K. Queste bande riappaiono nella regione attorno a 700 K con intensità ridotta. La banda a 553 nm svanisce nei campioni trattati a temperature al di sopra di 1200°C. I risultati sono paragonati ad un modello cinetico che spiega gli aspetti sperimentali più significativi. Vengono proposti alcuni argomenti qualitativi che potrebbero essere utili per identificare la natura dei difetti reticolari responsabili dell'emissione termoluminescente osservata.

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Резюме Исследуется термолюминесценция α-корунда, регистрируя интенсивность светового излучения в зависимости от температуры и длины волны. Образды изготавливаются из аморфного корунда, используя термическую обработку в области температур от 850°C до 1400°C. Термолюминесценция обнаруживает две спектральных зоны 553 и 606 нм, излучаемые при температурах, близких к 500 K, которые снова появляются в области 700 K с уменьшенной интенсивностью. Зона 553 нм затухает в образцах, обработанных при температуре выше 1200°C. Этот результат сравнивается с кинетической моделью, которая объясняет наиболее сушественные экспериментальные особенности. Приводятся некоторые качественные аргументы, которые могут быть полезны для идентификации природы дефектов решетки, ответственных за наблюдаемое термолюминесцентное излучение.

     

Temporally and spectrally resolved analysis of a copper plasma plume produced by ultrafast laser ablation

We report an experimental analysis of the plasma plume produced during ultrafast laser ablation of a copper target, in high vacuum. The plasma plume optical emission is studied by using a hybrid time-gated imaging technique which allows obtaining simultaneous information on the spectral and spatial characteristics of the emitting species. We used both single and double pulse ablation scheme, observing their influence on the characteristics of the ablated atomic species.     

Mechanism of laser-induced plasma shock wave evolution in air

A theoretical model is proposed to describe the mechanism of laser-induced plasma shock wave evolution in air. To verify the validity of the theoretical model, an optical beam deflection technique is employed to track the plasma shock wave evolution process. The theoretical model and the experimental signals are found to be in good agreement with each other. It is shown that the laser-induced plasma shock wave undergoes formation, increase and decay processes; the increase and the decay processes of the laser-induced plasma shock wave result from the overlapping of the compression wave and the rarefaction wave, respectively. In addition, the laser-induced plasma shock wave speed and pressure distributions, both a function of distance, are presented.