CO2 laser ablative etching of polyethylene terephthalate

Films of polyethylene terephthalate (PET) can be successfully etched with 9 m radiation from a pulsed TEA CO₂ laser. The relationship between etch depth and fluence is broadly similar to that observed for excimer laser etching but with a less well-defined threshold. Time-resolved photoacoustic measurements of stress waves generated in the interaction show that at a fluence of 1.8 J cm^–2 ablation occurs 100–200 ns after the start of the laser pulse, a time which is consistent with the rate of thermal decomposition of PET. The volatile products of ablation are carbon monoxide, carbon dioxide, methane, ethyne, ethene, benzene, ethanal, and small quantities of other products. For fluences close to and appreciably above the threshold the ablated material consists predominantly of involatile species of relatively high molecular weight, whereas at higher fluences substantial fragmentation of the polymer to small molecules occurs.School of Chemistry     

Efficient ablation of organic polymers polyether sulphone and polyether ether ketone by a TEA CO2 laser with high perforation ability

Organic polymer (PES: PolyEther Sulphone and PEEK: PolyEther Ether Ketone) ablation with oscillation-line selected TEA CO₂ lasers is successfully demonstrated. With different irradiation conditions the ablative etch-rate slopes were varied, which means that the ablation process is dependent on the ablation conditions such as incident laser intensity and ambient gas. In perforation processing of the PEEK film, the TEA CO₂ laser had a higher etch rate of 42 m/pulse at a fluence of 70 J/cm² in vacuum than the XeCl laser.     

On submillimeter laser lines from optically pumped12CH3OH

Several new submillimeter laser lines have been observed in a CH₃OH submillimeter laser, optically pumped by CO₂—laser radiation in a low power pulsed mode.Assignments have been suggested for a number of the lines. A few lines have wavelengths close to atmospheric water vapor absorption peaks, and can be observed only when the air path from the submillimeter resonator exit mirror to the detector is kept short or flushed with dry nitrogen.     

Laser ablation parameters of atherosclerotic arteries

Summary The threshold of atherosclerotic plaque ablation in water by the XeCl laser radiation was measured to be of (1.5±0.2) J/cm². Within the simple thermal model this value corresponds to the mean temperature of about 373 K in the irradiated volume as for other angioplasty lasers. Time-of-flight probing of ablation in air by the XeCl and CO₂ lasers revealed microsecond lifetimes of ablated volumes that confirm the mechanism of explosive boiling of superheated tissue water. So the local temperature must be about 582 K which can be due to inhomogeneous light distribution and absorption. The short lifetime leads to the absence of heat diffusion and, hence, to the ablative character of tissue destruction.     

The characteristics of multiple-photon absorption of SF6 molecules cooled in free jet expansion from a pulsed supersonic nozzle

The multiple-photon absorption of pulsed TEA CO₂ laser radiation by SF₆ molecules cooled toT _R40K andT _v160K in the free jet expansion from a pulsed supersonic nozzle has been investigated at energy fluences of 0.1 to 3.0 J·cm^–2.For practically all laser lines which coincide with the linear absorption spectrum of thev ₃ vibrational mode of SF₆ [P(12)...P(28), 10,6 m], the dependence of the absorbed energyE _ab on the exciting energy fluence was found to be steeper than linearE _abⁿ, wheren=(1.1 to 1.8). Considerable increases of the absorption cross sections with increasing energy fluence were observed.The fraction of the molecules interacting with the laser radiation is estimated.     

The absorption of pulsed CO2-laser radiation by ethylene at total pressures from 25 to 3000 Torr

The absorption of pulsed CO₂-laser radiation by ethylene has been measured at total pressures from 25 to 3000 Torr, using the P(12) and P(14) lines in the 10.6 m band, with incident fluences from 0.1 to 0.7 J/cm². Marked deviations from the Beer-Lambert absorption law were observed, with the effective absorption coefficient varying with pressure, fluence, absorption path-length and the addition of non-absorbing gas. Pressure broadening of the rotational lines of the ethylene absorption spectrum was shown to be the major cause of these deviations, together with lesser effects which can be attributed to the rise in temperature of the absorbing gas during the laser pulse.     

Comparison of the transmission behavior of a triazeno-polymer with a theoretical model

The threshold fluence,F _Th, of ablation of a triazeno-polymer was measured in the low fluence range for thin films using conventional UV-spectroscopy. It was found that there is a clearly definedF _Th for 308 nm irradiation between 20 and 25 mJ cm^–2. In the case of 248 nm irradiation, a threshold fluence range between 16 and 32 MJ cm^–2 was found. The ablation rate for both irradiation wavelengths depends on film-thickness. For the XeCl excimer-laser, the point at which the rate becomes independent of thickness was observed to lie at a value which did not correspond to the calculated laser penetration depth, whereas for the KrF laser the independence was not reached within the applied thickness range (up to 0.35 m). Additional transmission measurements have been performed showing that the target transmission at 248 nm increases only slightly, whereas for 308 nm the transmission increases by a factor of approximately 4. This result shows that dynamic target absorption properties are very important for describing the ablation process. The results derived from the transmission studies and etch rates were analyzed theoretically with a two-level model of chromophore absorption. For 248 nm irradiation this model can describe the transmission behavior and the ablation rate. In the case of 308 nm irradiation, it was only possible to match one data set. A good agreement with the experimental transmission ratio does not match the ablation rate and vice versa.     

Surface modifications of TiN coating by pulsed TEA CO2 and XeCl lasers

Interactions of a transversely excited atmospheric (TEA) CO₂ laser and an excimer XeCl laser, pulse durations ∼2 μs (initial spike FWHM ∼100 ns) and ∼20 ns (FWHM), respectively, with polycrystalline titanium nitride (TiN) coating deposited on high quality steel AISI 316, were studied. Titanium nitride was surface modified by the laser beams, with an energy density of 20.0 J/cm² (TEA CO₂ laser) and 2.4 J/cm² (XeCl laser), respectively. The energy absorbed from the CO₂ laser beam is partially converted to thermal energy, which generates a series of effects such as melting, vaporization of the molten material, shock waves, etc. The energy from the excimer XeCl laser primarily leads to fast and intense target evaporation. The calculated maximum temperatures on the target surface were 3770 and 6300 K for the TEA CO₂ and XeCl lasers, respectively. It is assumed that the TEA CO₂ laser affects the target deeper, for a longer time than the XeCl laser. The effects of the XeCl laser are confined to a localized area, near target surface, within a short time period.Morphological modifications of the titanium nitride surface can be summarized as follows: (i) both lasers produced ablation of the TiN coating in the central zone of the irradiated area and creation of grainy structure with near homogeneous distribution; (ii) a hydrodynamic feature, like resolidified droplets of the material, appeared in the surrounding peripheral zone; (iii) the process of irradiation, in both cases, was accompanied by appearance of plasma in front of the target.Target color modifications upon laser irradiation indicate possible chemical changes, possibly oxidation.     

Absorption of pulsed CO2 laser radiation by air and carbon dioxide

An optoacoustical method is used in experimental study of nonlinearity in absorption of pulsed CO₂ laser radiation by air with CO₂ and H₂O added and pure CO₂. The laser pulse consists of frequencies related to several rotational transitions, generated simultaneously. Nonlinearity in absorption was detected for laser radiation levels above 0.2 MW/cm².Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 49–51, October, 1982.     

Laser separation of oxygen isotopes by IR multiphoton dissociation of (CH3)2O

Isotopically selective (with respect to ¹⁸O) one- and two-frequency multiphoton dissociation of dimethyl ether (CH₃)₂O by pulsed TEA CO₂ laser radiation has been studied. The maximum primary selectivity, 16, is attained with the dissociation yields of the desired component (CH₃) ₂ ¹⁸ O ₁₈=5×10^–4 and 1.7×10^–2 for one- and two-frequency excitation, respectively. The dependences of MPD yields and selectivity on laser radiation frequency, (CH₃)₂O pressure, buffer gas (N₂) pressure and temperature have been measured. Multiphoton absorption coefficients have been measured and the average number of absorbed quanta calculated. The laser photon energy consumed for separating one ¹⁸O atom has been estimated: 11 and 4 keV/¹⁸O atom for one- and two-frequency excitation, respectively.     

Bone-ablation mechanism using CO2 lasers of different pulse duration and wavelength

Bone ablation using different pulse parameters and four emission lines of 9.3, 9.6, 10.3, and 10.6 m of the CO₂ laser exhibits effects which are caused by the thermal properties and the absorption spectrum of bone material. The ablation mechanism was investigated with light- and electron-microscopy at short laser-pulse durations of 0.9 and 1.8 s and a long pulse of 250 s. It is shown that different processes are responsible for the ablation mechanism either using the short or the long pulse durations. In the case of short pulse durations it is shown that, although the mineral components are the main absorber for CO₂ radiation, water is the driving force for the ablation process. The destruction of material is based on explosive evaporation of water with an ablation energy of 1.3 kJ/cm³. Histological examination revealed a minimal zone of 10–15 m of thermally altered material at the bottom of the laser drilled hole. Within the investigated spectral range we found that the ablation threshold at 9.3 and 9.6 m is lower than at 10.3 and 10.6 m. In comparison the ablation with a long pulse duration is determined by two processes. On the one side, the heat lost by heat conduction leads to carbonization of a surface layer, and the absorption of the CO₂ radiation in this carbonized layer is the driving force of the ablation process. On the other side, it is shown that up to 60% of the pulse energy is absorbed in the ablation plume. Therefore, a long pulse duration results in an eight-times higher specific ablation energy of 10 kJ/cm³.     

Influence of infrared radiation on the excitonic absorption edge of CuCl

We report changes in the excitonic absorption edge of CuCl caused by intense CO₂ laser radiation at 10.6 m, a wavelength which lies in the infrared (ir) transparency region of CuCl. With an ir intensity of 0.4 GW/cm² we observe a 100% absorption increase for the Kr⁺ laser probe wavelength of 406.7nm. The effect scales linearly with ir intensity but does not depend on relative polarization. We explain the effect by laser field induced electroabsorption of the exciton. The magnitude of the effect is closely related to electroabsorption induced by static external fields and by internal electric fields from optical phonons.     

Weak antilocalization in a three-dimensional topological insulator based on a high-mobility HgTe film

The up-conversion luminescence of Er³⁺ from the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels in nanocrystals of Y_0.95(1?x)Yb_0.95xEr_0.05PO₄ (x = 0, 0.3, 0.5, 0.7, 1) orthophosphates activated with Er³⁺ ions has been studied under the excitation of Yb³⁺ ions to the ²F_5/2 level by 972-nm cw laser radiation. Broadband radiation in the wavelength range of 370–900 nm has been observed at certain power densities of exciting laser radiation; this broadband radiation is absent in the case of excitation of the powders under study by pulsed laser radiation with a wavelength of 972 nm at a pulse repetition frequency of 10 Hz and a duration of a pulse of 15 ns. Experimental data indicating that this radiation is thermal in nature have been presented.     

On-line monitoring of cyclotron target-gas output by means of tunable lead salt diode laser absorption spectroscopy

We demonstrate what is, to our knowledge, the first use of mid-infrared laser absorption spectroscopy for trace-gas measurements of cyclotron target outputs used for the generation of radioactive carbon-11 in positron emission tomography (PET). The spectrometer was based upon a liquid-nitrogen-cooled lead salt diode laser generating single-mode radiation in the wavenumber range of 2230–2240 cm^?1. The sample flowed to a multiple-pass optical cell with a total path length of 15.23 m and the laser radiation was detected by two liquid-nitrogen-cooled InSb photodetectors. We present the results of CO, N₂O and CO₂ measurements on PET trace cyclotron output and discuss future work on ¹¹CO and ¹¹CO₂ detection.     

Thermooptical Mechanism of Sound Generation by High-Power Laser Radiation Propagating in the Atmosphere

In the present paper, the results of experimental and theoretical investigations into the thermooptical mechanism of sound generation by high-power pulsed laser radiation (HPLR) propagating in the atmosphere are given. Experiments were performed with collimated and weakly-focused beams of a pulsed CO₂ laser with a wavelength of 10.6 m. A sound pulse was recorded with omnidirectional microphones placed at distances of 0.2–0.5 m from the laser beam axis and spaced along the laser beam propagation path whose length was a few hundred meters. A sound pressure level was measured with a precision impulse sound level meter, and time histories of acoustic pulses were also displayed on the screen of an oscilloscope. Based on these investigations, the methods of thermoacoustic sounding of the effective HPLR beam radius and divergence angle, the laser energy absorption coefficient, and the total laser energy have been suggested and realized together with the method of the diagnostics of HPLR propagation regime from the waveform or spectrum of the received acoustic signal.     

A simple current pulsed low-pressure CO2 laser with passive Q-switching

We report on a simple and stable pulse tunable CO₂ laser suitable for many investigations in the region around 10m. The pulsed discharge when combined with the passiveQ-switching technique provides pulses of 100 ns duration with a peak power of few kilowatts.Due to the interest in pulsed low-pressure CO₂ lasers as useful irradiation sources, extensive investigations have been carried out. In particular, various Q-switching techniques have been developed to generate short CO₂ laser pulses [1–4].     

Pulsed laser deposition: metal versus oxide ablation

We present experimental results of pulsed laser interaction with metal (Ni, Fe, Nb) and oxide (TiO₂, SrTiO₃, BaTiO₃) targets. The influence of the laser fluence and the number of laser pulses on the resulting target morphology are discussed. Although different responses for metal and oxide targets to repetitive laser irradiation could be expected due to the different band structures of metals and oxides, the optical response is quite similar for 248-nm laser irradiation. Therefore, the difference in response is largely caused by differences in thermal properties. Metal targets show periodic structures of the order of micrometers after consecutive pulses of laser radiation, while the SrTiO₃ and BaTiO₃ targets show a flat surface after ablation for relatively low fluences (1.0 Jcm^-2). The observed TiO₂ target ablation characteristics fall in between those of the ablated metals and perovskites, because ablation results in the presence of Ti-rich material, which shields the underlying stoichiometric target material from ablation. The final target morphology is dependent on fluence, number of pulses, and the movement of the target itself (rotating, scanning, or stationary). It can take between 15 and 75 pulses to reach a steady-state target morphology on a stationary target. PACS 79.20.Ds; 52.38.Mf; 81.15.Fg     

CO2 laser cleaning of black deposits formed during the excimer laser etching of polyimide in air

Pulsed CO₂ laser cleaning of black debris formed during the excimer laser ablation of polyimide in air is demonstrated. The 10.6 m CO₂ laser radiation is strongly absorbed in the debris but only weakly absorbed in polyimide thus enabling the clean removal of the debris without damaging the polyimide.     

FIR Stark spectroscopy on optically pumped CH3OH laser

An extensive theoretical and experimental analysis of the absorption and emission spectrum of a CH₃OH FIR-laser excited by a conventional CO₂ laser is presented. Particular interest is devoted to the Stark shifts of the pump and lasing lines and to the electric field dependence of the Fir-laser output of the various lines. The offsets with respect to the exciting radiation and the Stark shifts of the IR absorption (pump) lines are measured by means of the transferred Lamb dip technique. The theoretical behaviours of the Stark patterns are calculated for several choices of the quantum numbers and selection rules involved in the transitions. A large variety of experimental results are reported and compared to theory. Non-linear Stark shifts have been observed for the 37.5m FIR laser line and for the IR-pump transitions excited by the 9-P(38) and 10-R(38) CO₂ laser Lines. Line assignments are proposed and new FIR laser lines are reported.