Pulsed waveguide CO laser operating at room temperature

A pulsed waveguide CO laser operating at room temperature is described. Experimental results of the laser output power variation with discharge parameters and its spectral distribution are given and discussed. In contrast to other CO lasers the best performance has been obtained without diluent gases. A small amount of oxygen added to the CO laser gas enhances the output energy and prevents carbon deposition at the walls of the waveguide channel. So far a laser pulse energy of 40 J has been obtained at an efficiency of 0.4%. Possible ways to increase the laser output energy are discussed.     

CO-laser side-band spectrometer: Sub-Doppler heterodyne frequency measurements around 5 µm

We report the realization of a tunable sub-Doppler heterodyne spectrometer with high absolute accuracy, employing side-band generation with a CO laser. The fixed-frequency CO-gas laser, working from 4.7 to 8.4µm, is made partially tunable by the use of microwave side-band generation in a CdTe Electro-Optical Modulator (EOM). This leads to tunable radiation of high spectral purity. We describe the design of the microwave EOM, adapted to the CO laser, its performance and its first application to highly accurate frequency measurements. The side-band radiation is used for sub-Doppler stabilization of the CO laser, while the carrier frequency is mixed with the frequencies of two CO₂ reference lasers. As a first result, we present measurements of OCS transitions in the 4.9µm (61 THz) region, reaching an absolute accuracy of 30 kHz (/ = 5×1O^–10). Further application of our spectrometer to calibration gases will establish a variety of InfraRed (IR) calibration standards with a new quality of accuracy. On visit from: Fachbereich Physik, Humboldt-Universität zu Berlin     

Frequency stabilization of CO laser using RF optogalvanic Lamb-dip

The Lamb dip of CO rovibrational transition is detected by a room temperature extracavity RF optogalvanic cell and employed to stabilize the frequency of a CO laser. The S/N ratio of optogalvanic signal is about 2000  at optical power < 1 W. The relative depth of Lamb dip is 2.3%. The S/N ratios of first and third harmonic demodulated saturation signals are about 40  and 10  , respectively. The CO laser is stabilized using the first harmonic demodulated signal, and the frequency stability is better than 300 kHz. Concurrently, the influences of operational parameters, which include the coil current, partial pressures of gas mixture, are investigated. A simple model for the influence of coil current is presented, and further improvements are addressed as well.     

Saturation stabilization of the CO fundamental-band laser

We recently reported the extension of CO laser operation to a variety of single-line transitions in the fundamental band (v=1 0). Now saturation stabilization of these lines could be achieved using an intracavity absorption cell with CO at low pressure. A 3f-detection technique allowed reliable frequency locking on transitions from P(8) to P(14). A frequency stability of better than 10kHz was indicated. The frequency of the P(14) transition was measured in a heterodyne experiment to be 62, 546, 358.324 (50) MHz.     

Master-oscilltor-amplifier electroionization carbon monoxide laser system and propagation of its radiation through atmosphere

Conclusion Thus, we have created an EI carbon monoxide laser systems, operating by the scheme master-oscillator-amplifier. We have experimentally studied the amplification and absorption of IR (5–6 m) by multilevel active medium of EI CO laser. Conditions of saturated amplification of EI CO laser radiation have been found. By using various methods we have performed the formation of the EI CO laser system spectrum in accordance with atmospheric transparency windows. It should be noted that EI CO laser system with selected spectral lines can be used in laser ranging, laser chemistry, and laser isotope separation.     

The CO fundamental-band laser as secondary frequency standard at 5 µm

We present a very high-resolution heterodyne spectrometer based on a CO laser which operates down to fundamental-band transitions of the molecule. This allows us to detect saturated absorption signals on these transitions at very low pressure (0.4 Pa) and laser intensity (< 1 mW/cm²), yielding a linewidth of about 250 kHz. With the CO fundamental-band laser stabilized to these saturation signals we have measured the transition frequencies of the fundamental bands of three isotopic species to an accuracy of typically 20 kHz (v/v 3 × 10^–10), referenced to the CO₂ frequency standard. Together with additional frequency measurements of the first hot bands, these provide the first heterodyne frequency data of sub-Doppler accuracy for transitions in low lying bands of CO. They now represent the most accurate secondary frequency standard in the spectral region around 5 µm (60 THz).     

Incubation/ablation patterning of polymer surfaces with sub-μm edge definition for optical storage devices

By exposure to low fluence UV laser radiation, the optical absorption coefficient of subsurface polymer material can be increased (incubation) with spatial control, using a suitable contact mask, proper imaging of the mask, or laser direct writing. Spatially selective ablation of polymethylmethacrylate (PMMA) is then achieved with large area XeCl excimer laser pulses at 308 nm. In this way, the transfer of spatial information to the material can be decoupled from the high laser fluence removal (ablation) step. The advantages are: The mask is exposed to only low fluence laser radiation — damage is avoided. Since the mask can be removed before the ablation step, mask contamination by the ablated plume cannot occur. Using this incubation/ablation method, PMMA surfaces can be patterned (248 nm/308 nm) with submicrometer spatial control and edge contrasts better than 0.2 m. This has impact on optical storage technology and laser surface processing techniques in general. The smallest single structure obtained was somewhat smaller than 0.5 m in diameter up to now, given by the mask.Presented at Laserion '91, June 12–14, 1991     

Laser projection-patterned etching of (100) GaAs by gaseous HCl and CH3Cl

Laser projection-patterned etching of GaAs in a HCl and CH₃Cl atmosphere performed using a pulsed KrF-excimer laser (=248 nm, =15 ns) and deep-UV projection optics (resolution 2 m) is reported. The etching process carried out in a vacuum system having a base pressure of 10^–6 mbar is shown to result from a purely thermochemical reaction. Etching takes place in two steps: (i) between the laser pulses, the etchant gas reacts with the GaAs surface-atomic layer to form chlorination products (mainly As and Ga monochlorides), (ii) local laser surface heating results in the desorption of these products allowing further reaction of the gas with the surface. The influence of the etching parameters (laser energy density, gas pressure and pulse repetition rate) on the etch rate and the morphology of the etched features was studied. Etch rates up to 0.15 nm per pulse, corresponding to the removal of 0.5 GaAs molecular layer, are achieved. The spatial resolution of the etching process is shown to be controlled by the heat spread in the semiconductor and by the nonlinear dependence of the etch rate on the surface temperature. As a result, etched features smaller or larger than the projected features of the photomask are achieved depending on the laser energy density. Etched lines having a width of 1.3 m were obtained at low fluences by the projection of 2 m wide lines onto the GaAs surface.     

Theory for the laser-induced femtosecond phase transition of silicon and GaAS

We analyze he femtosecond instability of the chamond lattice of silicon and GaAs, which is induced by a dense electron-hole plasma after excitation by a very imense laser pulse. We obtain that the electron-hole plasma causes an instability of both transverse acoustic and longitudinal optical phonons. So, within less than 200fs, the atoms are displaced more than 1 Å from their equilibrium position. The gap between the conduction and the valence band then vanishes and the symmetries of the diamond structure are destroyed, which has important effects on the optical reflectivity and second-harmonic generation. After that, the crystal melts very rapidly because of the high kinetic energy of the atoms. Note that mis is in good agreement with recent experiments done on Shand GaAs using a pump laser to excite a dense electron hole plasma and a probe laser to observe the resulting changes in the atomic and electronic structure.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Spectrophonic humidity-trace detection in hydrogen and air

A spectrophone with low resonant frequency compatible with the repetition frequency of a pulsed discharge CO laser is described. The spectrophone is used for humidity detection in hydrogen and air. The simple and inexpensive system provides a detection capacity of 10–100ppb/ . The responsivity of the resonant spectrophone is interpreted taking the dependence of the acoustical quality factor on the geometry into account.     

Morphology and composition on Al surface irradiated by femtosecond laser pulses

We study the surface chemicals and structures of aluminum plates irradiated by scanning femtosecond laser pulses in air for a wide range of laser fluence from 0.38 to 33.6 J/cm². X-ray photoelectron spectroscopy and X-ray diffraction analyses indicate clearly that crystalline anorthic Al(OH)₃ is formed under femtosecond laser pulse irradiation. Besides aluminum hydroxide, crystalline Al₂O₃ is also found in the samples irradiated at high laser fluence. Field emission scanning electron microscopy demonstrates that the surfaces of the samples irradiated with low laser fluence are colloidal-like and that nanoparticles with a few nanometers in size are embedded in glue-like substances. For high laser fluence irradiated samples, the surfaces are highly porous and covered by nanoparticles with uniform size of less than 20 nm.     

Gain dynamics of the 4.3 μm CO2 laser

A detailed study of the gain dynamics of the pulsed, optically pumped 4.3 m CO₂ laser is described. Small-signal gain coefficients as high as 14%/cm are measured in a 4.3 m amplifier using low-power pulses from a 4.3 m probe laser. The measurements are compared with a rate-equation model and good quantitative agreement is obtained. The model, which uses no adjustable parameters, is described in detail. Gain is studied as a function of optical pumping power, gas mixture, gas pressure and discharge excitation of the 4.3 m amplifier. Optimization of the gain is discussed.     

Threshold behavior in polyimide photoablation: Single-shot rate measurements and surface-temperature modeling

The ablation rate of Kapton-type polyimide has been measured as a function of incident fluence and excimer laser wavelength using a sensitive quartz-crystal microbalance (QCM). The experiments were performed such that the fluence and the ablated depth were known for each laser pulse, avoiding the need to average rate and fluence data over many pulses. By limiting the investigations to the low-fluence regimes near ablation threshold, high precision and detailed curve shapes were obtained. It was found that the ablation rate increases smoothly and exponentially with increasing fluence for 248, 308, and 351 nm wavelengths. This exponential behavior was modeled using an Arrheniustype thermal rate equation. In contrast, the 193 nm curve is linear in fluence, displays a sharp threshold, and is consistent with a possible photochemical ablation mechanism. Using a sophisticated surface temperature modeling code, the maximum laser induced surface temperature at the fluence at which ablation can first be detected is found to be the same, 850° C, for all four wavelengths. This ablation temperature is significantly higher than the approximately 500° C temperature at which Kapton starts to degrade under isothermal heating conditions.     

Resonant electronic nonlinearity and laser heating induced nonlinearity of chlorophosphonazo I

The nonlinear refraction of chlorophosphonazo I (CPA I) was investigated using the Z-scan technique with a pulse Nd:YAG laser and a cw HeNe laser as excitation sources. Positive and large nonlinear refractive index attributed to resonant electronic nonlinear effect was observed under pulse 532 nm excitation. Under cw HeNe 633 nm excitation, the origin of optical nonlinearity was discussed in term of laser heating effect. The experimental results show that CPA I will have potential applications in nonlinear optical devices.     

Lateral growth rates in laser CVD of microstructures

Lateral growth rates of Ni spots deposited on absorbing substrates by decomposition of Ni(CO)₄ with visible Kr⁺ laser light have been measured. The experimental data are consistent with the calculated temperature distributions. The mechanism of decomposition is thermal with an apparent chemical activation energy of 22±3 kcal/mole for the temperature range 350 KT500 K.     

High power single-longitudinal-mode operation in a twisted-mode-cavity laser with a c-cut Nd:GdVO4 crystal

A twisted-mode-cavity laser was established by using a piece of c-cut Nd:GdVO₄ crystal as the lasers active material. Output spectra were scanned by a scanning Fabry–Perot interferometer, which demonstrated that the single-longitudinal-mode laser operation was realized in the twisted-mode-cavity laser configuration. A maximum single-longitudinal-mode laser output power of 2.1 W was obtained when the pump power was 11.5 W. The pump slope efficiency was about 20.0%. A passively Q-switched single-longitudinal-mode laser was also achieved in a twisted-mode cavity by inserting a piece of Cr⁴⁺:YAG as an intracavity saturable absorber. The Q-switched single-longitudinal-mode laser pulse duration was measured to be 100 ns and the single-pulse energy was about 40.0 J.     

Two-dimensional diffraction calculations for a fast-flow CO laser

Conclusion We have developed, a two-dimensional diffractive model of a free-flow CO:N₂ laser, using a detailed kinetic model of the active medium.We have shown that the constant-gain model and the diffractive model yield practically the same calculated energy characteristics of a free-flow CO laser. Calculation of the spectral characteristics has shown that the diffractive model yield the same numbers and intensities of the rotational-vibrational transitions on the cavity axis as the model with constant gain. We have determined the errors incurred in calculations by the constant-gain model.At an active-medium density 0.25 Amagat the angular divergence of the radiation is determined by the large-scale inhomogeneity of the subsonic gas flow while the radiation directivity in the far zone is manifested by a noticeable angular shift and is split.Translated from Preprint IAE-5724/11, Kurchatov Institute of Atomic Energy, Moscow, 1990.     

Tunable te-CO2 laser-pumped formaldehyde far-infrared lasers: Offsets, assignment, and superfluorescence

Sixty and three absorption transitions in D₂CO and H₂CO, respectively, have produced a number of far infrared laser lines when they are pumped by an etalon-tuned TE-CO₂ laser. Almost all the absorption transitions pumped previously by a free runing TE-CO₂ laser have been efficiently pumped by the etalon tuned CO₂ laser and found to have offset within ±500 MHz from the line-center of the relevant CO₂ pump lines. 22 (1) absorption and 63 (4) emission lines of D₂CO (H₂CO) are assigned. Some of these lines have generated superfluorescence. In paticular, the D₂CO 319-m line pumped by CO₂-9P(32) delivered an output energy of approximately one half that of the well-known D₂O 66-m. It is shown that a large electric dipole moment and an appreciable amount of fractional population in the lower level of the pump transition of this line are responsible to the superfluorescence.     

Defects in pulsed laser and thermal processed ion implanted silicon

The evolution of the nature and concentration of the defects produced by 100 or 300 keV As ions at fluences 1 to 4×10^–12 cm^–2 inn-type, Fz Silicon doped with 10¹⁵ to 10¹⁶ cm^–3 has been studied as function of thermal treatments (in the range 500°–900 °C) and of the energy density (in the range 0.3–0.6 J cm^–2) of a light pulse from a ruby laser (15 ns, 0.69 m). Deep-level transient spectroscopy (DLTS) combined with capacitance — voltage (C-V) measurements were used to get the characteristics (energy level, crosssection for the capture of majority carriers) of the defects and theirs profiles. The difficulties encountered in the analysis of the results, due to the large compensation of free carriers in the implanted region and to the abrupt defect and free carrier profiles, are discussed in detail and the corrections to apply on the C-V characteristics and the DLTS spectra are described. The defects resulting from the two types of treatments are found to be essentially the same. Only, for laser energies higher than 0.5 J cm^–2, the laser treatment appears to introduced new defects (atE0.32 eV) which should result from a quenching process. The fact that a laser energy smaller than the threshold energy for melting and recrystallization is able to anneal, at least partially, the defects produced by the implantation, demonstrates that the annealing process induced by the laser pulse is not a purely thermal process but is enhanced by a mechanism involving ionization.     

Optical characterization of laser-induced crystallized silicon films

Optical absorption coefficient spectra of thin silicon films were precisely investigated using a simple reflectance system with total reflectance mirrors placed on the rear side of samples in order to cancel an interference effect in a range between 1.1 eV and 3 eV. The absorption coefficient decreased according to crystallization as the laser energy increased and it got similar to that of single crystalline silicon in the range of 1.7 eV 3 eV. However, the absorption coefficient was higher than 10² cm^–1 in the photon energy lower than 1.3 eV. This probably results from band tail states caused by defect states localized at grain boundaries in the crystallized films. 2.5%-phosphorus doped laser crystallized silicon films had a high optical absorption coefficient ( > 10⁴ cm^–1) in the low photon energy range (1.1 eV 1.7 eV) caused by free carriers produced from the dopant atoms activated in the silicon films. The experimental results gave the carrier density of 1.3 × 10²¹ cm³ and the carrier mobility of 20 cm²/Vs.