Absorption Of CO2 Laser Radiation Research Articles

The absorption of CO2 laser radiation by ethylene in mixtures with methane

A computational-experimental method for the investigation of photochemical and thermal effect on a methane-ethylene flow was developed to determine the saturation intensity of ethylene in the high-temperature region up to 1000-1200 K at a pressure near 1 atm. This method included experiments on radiation absorption and a developed numerical model of the cell. Saturation intensity Is found at 700 K for three power values, 60, 35 and 12 W, was equal to 55, 40 and 20 W/cm2, respectively. The analytical relation was proposed for calculating the saturation intensity from experimental data. The calculation results obtained within both approaches are well consistent with each other.

CO2 laser-induced micro-plume treatment of silicon: technique and application

A method for treating the surface of crystalline silicon (α-Si) covered with a fused silica (SiO2) plate by a laser-induced plume arising from the absorption of CO2 laser radiation in glass is presented. The mechanism of micro-plume formation and the effect of this micro-plume on α-Si sample are considered. The α-Si processing regimes were determined that relate the depth of the formed relief to the radiation power density, the temperature of the micro-plume formed during SiO2 irradiation was measured and reached 3000 K. The diffraction phase gratings were designed, manufactured on silicon, and tested, they successfully split 10.6 μm laser beam at 0 and ± 1 diffraction orders. The method belongs to the group of direct writing methods and is easily adapted to different types of diffraction elements on the surface of Si and other materials transparent to 10 μm irradiation.

On the laser lift-off of lightly doped micrometer-thick n-GaN films from substrates via the absorption of IR radiation in sapphire

The intense absorption of CO2 laser radiation in sapphire is used to separate GaN films from GaN templates on sapphire. Scanning of the sapphire substrate by the laser leads to the thermal dissociation of GaN at the GaN/sapphire interface and to the detachment of GaN films from the sapphire. The threshold density of the laser energy at which n-GaN started to dissociate is 1.6 ± 0.5 J/cm2. The mechanical-stress distribution and the surface morphology of GaN films and sapphire substrates before and after laser lift-off are studied by Raman spectroscopy, atomic-force microscopy, and scanning electron microscopy. A vertical Schottky diode with a forward current density of 100 A/cm2 at a voltage of 2 V and a maximum reverse voltage of 150 V is fabricated on the basis of a 9-μm-thick detached n-GaN film.

Thermoacoustic amplification of photoacoustic signal

The thermoacoustic effect is used to amplify the photoacoustic signal induced by trace gas absorption of CO2 laser radiation. The acoustic wave pattern in a thermoacoustic amplifier coupled to a photoacoustic cell is represented in terms of electric transmission lines. Predictions of this model have resulted in a prototype thermoacoustic–photoacoustic (TAPA) detector to get a better understanding of this combination. The photoacoustic signal strength of the TAPA cell was linear with the trace gas density in the cell. Within this study we observed for the TAPA cell a higher PA signal than generated by a normal PA cell. Design criteria for better thermoacoustic amplification of photoacoustic signal are discussed.

Characteristics of Radiation Absorption in Metallic Particles

Thermal radiation absorption in metallic particles is an important phenomenon in many contemporary laser-processing techniques, including laser cladding of coating materials and laser cleaning of particulate contaminations. In this work, the Drude free-electron theory and electromagnetic wave theory are utilized to characterize the internal absorption of CO2 laser radiation in aluminum, chromium, and nickel particles. The results show that metallic particles have unique radiation properties. Radiation absorption in large particles occurs only in a very narrow region of the front particle surface, which results in inefficient radiation absorption. On the other hand, micron and submicron particles can absorb radiation very efficiently, due to the strong diffraction effect at the particle surface. For extremely small particles (e.g., nanometer particles), radiation absorption becomes less efficient. The particle absorption efficiency is found to increase with temperature, and this temperature dependence can be determined from those of flat metal surfaces at the normal incidence.

Photoexcited hot electron relaxation processes in n-HgCdTe through impact ionization into traps

In this article we report on a new type of spectroscopy for impurity and/or defect levels in the energy gap of narrow-gap semiconductors using the near-band-gap photon energies from a laser. This spectroscopy is done under the conditions of intense laser photoexcitation and is associated with the Auger relaxation processes of hot electrons involving impact ionization of valence electrons into impurity or defect levels. Wavelength-independent structure in the photoconductive response versus magnetic field is observed at high intensities in samples of Hg1−xCdxTe with x≊0.22 and 0.24. This structure arises from hot electrons photoexcited high into the conduction band by sequential absorption of CO2 laser radiation. The hot electrons lose their energy by impact ionizing valence electrons into impurity/defect levels in the gap. For the sample with x≊0.22 and an energy gap of 95 meV, three levels are found at 15, 45, and 59 meV above the valence band. A level at 61 meV is found for the sample with x≊0.24 and a gap of 122 meV.

Absorption of CO2laser radiation by photocarriers in a germanium crystal

The absorption of CO2 laser radiation in a germanium crystal, excited previously by a pulse from an Nd laser, was investigated experimentally and numerically. An analytic dependence of the optical transmission coefficient on the intensity of the exciting radiation was derived for a semiconductor in its transparency range. This analytic dependence was in good agreement with the experimental data obtained for a germanium crystal in the investigated range (of practical importance) of the photoexcitation rates. A high density of free carriers in photoexcited germanium demonstrated that this material is suitable for modulators of CO2 laser radiation.

Laser-induced chemical vapor deposition of silicon nitride films: Film and process characterizations

We have deposited hydrogenated amorphous silicon-nitride (a-SixN1−x:H) films from NH3-SiH4-Ar gas mixtures, heated by gas-phase absorption of CO2 laser radiation. For the first time, stoichiometric (Si/N=0.75) a-Si3N4:H films were obtained for NH3/SiH4 flow ratios of the order of 1000 and substrate temperatures Ts of about 500 °C. Growth rates as high as 13 Å/min were observed, depending on the partial pressure of SiH4, P(SiH4), the gas temperature Tg and Ts. Tg was calculated from a derived energy-balance equation, which depends sensitively on process parameters. To model the deposition process, the experimental film growth rate is separated into the Si growth rate G(Si) and the N growth rate G(N). Film stoichiometry is interpreted as the ratio G(Si)/G(N). The rate-limiting steps for Si growth are the gas-phase decomposition of SiH4 for Tg below about 750 °C and the SiH4 flow rate at higher Tg. G(N) is affected by both Ts and Tg. The NH3/SiH4 flow ratio must be kept large to ensure a sufficient concentration of N to attain stoichiometry. The reactions are likely to occur both in the gas phase and on the film surface.

Two-frequency excitation of NH3under fast rotational relaxation conditions

An experimental investigation was made of the absorption of CO2 laser radiation in the course of two-stage excitation of ammonia to the 2ν2 state. An estimate was obtained of the vibrational–translational relaxation time of ammonia mixed with nitrogen. A considerable fraction of the ammonia molecules was excited to the 2ν2 level. It was concluded that two-stage excitation of ammonia provided a convenient method for the conversion of CO2 laser radiation to the 16–20 μ range.

Applications of Pulsed Co2 Laser Processing of Silicon

ABSTRACTThe absorption of CO2 laser radiation in doped silicon is dominated by free-carrier transitions, which is an extrinsic absorption property. For applications where one would like to preferentially deposit the pulse energy in heavilydoped layers, the CO2 laser is found to be preferable to other laser systems that oscillate at a photon energyexceeding the bandgap.The advantages of laser processing with below-bandgap radiation are illustrated by three different applications that areeither impossible or much more difficult to achieve with a visible or ultraviolet laser: (1) the melting of buried layerswithout melting the material that encapsulates the molten layer, (2) the melting of shallow layers at the surface, and(3)the annealing of lattice imperfections caused by high-temperature diffusion of phosphorus dopants into small diffusion wells.

CO2 laser spectroscopy of shallow and deep levels in n-InSb by photo-Hall techniques

We report the first use of laser-induced photo-Hall techniques to investigate shallow and deep levels in n-InSb. From these measurements we show that photo-Hall techniques provide a very sensitive measure of the photoinduced changes in the free-electron concentration of InSb due to weak transitions from impurity or defect levels. As a result a new class of impurity transitions, referred to as LO phonon-assisted impurity cyclotron resonance harmonics, have been identified. In addition, time-resolved and intensity-dependent photo-Hall studies performed for temperatures between 3.6 and 20 K have resulted in the determination of absorption coefficients and recombination rates associated with the weak absorption of CO2 laser radiation by shallow and deep levels in n-InSb.

Thermal effects of the absorption of CO2laser radiation by water vapor

An analysis is made of the cooling of a gas containing water vapor as a result of absorption of CO2 laser radiation of 9.4–10.4μ wavelengths. A numerical study is reported of the influence of the parameters of the gas and of the radiation intensity on the magnitude of the cooling effect and its lifetime.

Generation and detection of very high frequency acoustic phonons in diamond

Generation and detection of very high frequency acoustic phonons in diamond is reported. We generate phonons at a frequency of 28 THz by defect-induced one-phonon absorption of CO2 laser radiation and observe, after pulsed excitation, phonon decay products in the frequency range from 1 THz to 7 THz. For detection vibronic sideband spectroscopy is used. We find strongly frequency dependent lifetimes for frequencies above 4 THz which we attribute to spontaneous phonon decay.

Nonlinear optical studies and CO2 laser-induced melting of Zn-doped GaAs

The absorption of CO2 laser radiation in p-type GaAs is dominated by direct free-hole transitions between states in the heavy- and light-hole bands. For laser intensities on the order of 10 MW/cm2, we report that the absorption associated with these transitions in moderately Zn-doped GaAs (∼1017 cm−3) begins to saturate in a manner predicted by an inhomogeneously broadened two-level model. At higher laser intensities surface melting occurs initially at localized sites in moderately doped material and more uniformly in heavily Zn-doped samples (≳1018 cm−3). As the energy density of the CO2 laser radiation is progressively increased further, the surface topography of the samples shows signs of ripple patterns, high local stress, vaporization of material, and exfoliation of solid GaAs fragments. Electron-induced x-ray emission data taken on the laser-melted samples show that there is a loss of As, compared to Ga, from the surface during the high-temperature cycling. By irradiating the samples in air, argon, and vacuum, we find that the vaporization rates are directly influenced by the ambient environment, particularly by the interaction of oxygen with the molten GaAs. Secondary ion mass spectrometry measurements are used to study the diffusion of oxygen from the native oxide and the incorporation of oxygen in the near-surface region of the GaAs samples that have been melted by a CO2 laser pulse. We find that oxygen incorporation does occur, and that the amount and depth of the oxygen incorporation depends on the laser energy density, number of laser shots, and ambient environment. For samples that are irradiated in argon or vacuum, we find that removal of the native oxide can be accomplished with CO2 laser pulses. Similar measurements are performed on Si-implanted GaAs, and results are reported for the redistribution of the implanted silicon atoms, the deviations from stoichiometry, and the incorporation of oxygen in the resolidified layer.

Channel cooling by turbulent convective mixing

Results from a series of experiments are described which show that hot, reduced-density channels in the atmosphere usually cool by a process of turbulent convective mixing. Five different types of channels were created: (a) by the interaction of a pulsed CO2 laser with aerosols in the atmosphere, (b) by electric discharges in the atmosphere, (c) by laser-guided electric discharges in the atmosphere, and (d) and (e) by the absorption of CO2 laser radiation in nitrogen doped with sulfur hexafluoride. For channels in which the energy deposition was almost cylindrically symmetric and axially uniform, (e), the rate of cooling, after reaching pressure equilibrium, was within an order of magnitude of thermal conduction. But for channels in which the energy deposition was asymmetric and/or axially nonuniform, the rate of cooling was typically one thousand times faster than thermal conduction (for channels whose radius at pressure equilibrium was ∼1 cm). These channels were seen to be turbulent and to cool by mixing cold surrounding air into the hot channel. Such turbulence has been explained by Picone and Boris [Phys. Fluids 26, 365 (1983)] in terms of a residual vorticity that is caused by the noncylindrical energy deposition. A simple empirical formula is deduced relating the rate of cooling (growth of channel envelope) to the radius of the channel at pressure equilibrium and to the ambient sound speed, which indicates that the effect of vorticity/turbulence saturates for variations in the energy deposition of greater than about 2 to 1.

CO2-laser radiation absorption by metal gratings

It is shown that CO2-laser radiation-induced periodic structures on the surface of metallic targets result in a dramatic increase (by up to one order of magnitude) in the sample absorptivity. This behavior has made it possible to gain new insight into the thermal coupling of powerful CO2-laser radiation with the initially highly reflectant metallic surfaces.

Statistics of an ensemble of oscillators under excitation. V. Analysis of experiments on the absorption of CO2-laser radiation by SF6 based on a triply degenerate oscillator model

A numerical analysis is given of experiments on the absorption of intense CO2-laser radiation by SF6 molecules in the band in the presence of a buffer gas. It is shown that from the dependence of the steady-state absorption on the buffer gas pressure one can obtain the relaxation constants Kr+1-r and the transition cross sections for levels up to r - 4. At some pressures of the buffer gas a competition between excitation and relaxation can result in an inversion population of the v6 vibrational state of SF6 and in imitation of linear absorption by SF6 and by other polyatomics.

Compensation densities in n-type Hg1−xCdxTe from transport properties of optically generated free carriers

Improvements to a photoneutralization technique for accurately determining compensation densities in narrow-gap Hg1−xCdxTe are discussed. A more comprehensive theoretical understanding of the electron mobility as a function of optically excited carrier density has been established, incorporating a theory for dynamic dielectric screening in the treatment of electron-hole scattering. In addition, uniform carrier generation has been demonstrated using two-photon absorption of CO2 laser radiation for compositions in the range 0.24<x<0.27. This eliminates the need for nonuniformity corrections to the photo-Hall data for appropriate compositions of Hg1−xCdxTe. Good agreement between theory and experiment verifies the accuracy of the technique.

Investigation of the absorption of radiation emitted from a tunable CO2laser by water vapor in the range 9–11 μ

The results of measurements of the transmission of radiation emitted from a tunable CO2 laser by the water vapor continuum in the range 9–11 μ are reported. The results for 23 laser lines are used to derive an approximate expression for the calculation of the continuous absorption of radiation by water vapor as a function of its temperature and pressure, as well as a function of the radiation wavelength and the pressure of the broadening gas. It is shown that this expression can be used to calculate the absorption of CO2 laser radiation propagating in the atmosphere.

Changes in fluorescence induced by infrared multiphoton excitation of optically excited SO2

Photophysical properties of excited SO2 were studied by the UV+MPE technique: single-photon UV excitation together with IR multiphoton excitation (MPE). Compared to the fluorescence in the absence of IR irradiation, the fluorescence following MPE showed a decrease of the main fluorescence peaks, accompanied by the appearance of broad fluorescence features to the high-energy side of the excitation frequency. Different CO2 laser lines were about equally effective. The dependence on laser fluence can be accounted for quantitatively using a simple stepladder model for MPE. The cross section for absorption of CO2 laser radiation by SO2, excited to around 32 000 cm−1, was found to be σ = 1.5×10−21 cm2. The effective decay time of the fluorescence with IR-on is increased twofold, compared to that with IR-off. This increase seems to parallel the reported dependence of the lifetimes on excitation frequency. It may be interpreted as due to more efficient coupling of excited vibronic states with highly-excited vibrational states in the ground electronic state.