Luminescence of rare gas crystals at high excitation densities for VUV laser applications

Exciton densities of the order of 10¹⁸ cm^–3 are generated in 0.1–0.3 mm thick surface layers in an area of 10×20 mm² of optically clear rare gas crystals. The quantum efficiencies at 126 nm (Ar), 145 nm (Kr), and 172 nm (Xe) remain near 0.5 even for the highest excitation densities. The corresponding gain coefficients of 2.6 cm^–1 (Ar) to 18 cm^–1 (Xe) exceed those of high pressure gas lasers by a factor of 20. Stimulated emission is inferred by observing the line narrowing, the dependence of intensities and time courses on excitation density and amplification measurements. The net gain coefficient is reduced however to 0.5–1 cm^–1 by transient absorption of excited centers and scattering by irradiation induced defects. The results are analysed by a system of rate equations for the excitation, relaxation, quenching, and amplification processes. A peculiar temperature dependence of the quantum efficiencies and time courses is attributed to electron trapping at grain boundaries.     

Collisional excitation X-ray laser experiments in plasmas produced by 0.53-μm and 0.35-μm laser light

Recent Ne- and Ni-like X-ray laser experiments carried out at the Centre d'Etudes de Limeil-Valenton (CEL-V) are reviewed. A variety of experiments in Ne-like X-ray lasers were performed; here we discuss measurements of soft X-ray amplification in Ge (Z=32) and Sr (Z=38) plasmas. In Ge plasmas produced by 0.53-m laser light at an irradiance of 6.0×10¹³ W/cm², gains between 2.2–2.5 cm^–1 on the 232.2 and 236.2 Å J=2–1 lines and a gain of 1.0 cm^–1 on the 196.1 Å J=0–1 line were measured. In addition, gains of 4.4 cm^–1 and 4.0 cm^–1 have been demonstrated on the J=2–1 transitions at 164.1 and 166.5 Å in Nelike Sr at laser intensities of 1.3×10¹⁴ W/cm². The effects of pumping the Ne-like Se X-ray laser with 0.35-m laser light have also been investigated; the Se lasing spectra is similar to that obtained with 0.53-m light. Experiments have also been carried out to optimize the gain of the 50.3 Å Ni-like Yb (Z=70) J=0–1 line. For Yb, no significant increase in gain over that previously reported was seen, but the time history of the Ni-like Yb X-ray laser was measured for the first time. Finally, attempts to extrapolate the Ni-like results to shorter wavelength were made using Ta (Z=73), W (Z=74), and Re (Z=75). No definitive observation of the Ni-like J=0–1 lasing lines was made in these experiments.     

Prepulse dependence ofJ=0−1 lasing at 32.6 nm in neon-like titanium

The dependence of the intensity of the 3p—3s,J = 0–1 lasing line at 32.6 nm in neon-like titanium on the prepulse level has been investigated experimentally. Titanium slabs were irradiated with 1.315 µm/450 ps pulses from the Asterix IV iodine laser using a defined prepulse of 5.2 ns before the main pulse. It is found that for pump energies close to the minimum energy for which lasing is observed, a prepulse level of order 0.5% gives the highest XUV laser intensity, whereas a higher prepulse level, of order 10% and more, is required for optimum XUV lasing far above the threshold. For a 2.7 cm long titanium target lasing was observed down to a pump irradiance of 2.5 TW/cm² (50 J/450 ps) for the 0.5% prepulse.     

Lasing characteristics of Rhodamine B and Rhodamine 6G as a sensitizer in sol–gel silica

We report lasing characteristics of Rhodamine B (Rh. B) in sol–gel silica under excitation with frequency-doubled Nd:YAG laser and sensitization with Rhodamine 6G (Rh. 6G). The principle of radiative energy transfer (from Rh. 6G to Rh. B) has been utilized as a longitudinally Rh. 6G laser (at 585 nm)-pumped Rh. B laser process in the same sample. Rh. B offers a high photostable and efficient laser dye in sol–gel silica sensitized with Rh. 6G; 75,000 shots as a laser half-lifetime of the sample and 24% efficiency at pumping intensity 0.1 J/cm² of 532 nm. Wavelength shift occurs from 606 to 630 nm in the Rh. B laser with increasing its concentration from 1×10⁻⁴ to 8×10⁻⁴ M. The measured optical gain for Rh. B sensitized with Rh. 6G in sol–gel silica is higher than that in ethanol. A new effect has been observed; at 1×10⁻⁴ M of Rh. B and 0.5×10⁻⁴ M of Rh. 6G mixture, the emitted color of laser is changed by changing the pump intensity of frequency-doubled Nd:YAG laser.     

Electron states of manganese luminescence centres in A1N

The energy spectrum of manganese luminescence centres in AlN was studied by means of excitation and emission spectra. Two regions of excitation were found: in the first region (from 40 000 to 31 000 cm^–1) excitation occurs as a result of transfer of energy from other impurity centres to the manganese centres; in the second region (from 26 000 to 17 000 cm^–1) the manganese centres are excited directly. In the excitation band with a peak at about 19 200 cm^–1 a phonon structure was observed having a temperature dependence analogous to that of the phonon structure of the emission band. The phonon energy at excitation differs from that at emission (_exc260 cm^–1, _em160 cm^–1). A scheme of electron-phonon levels of manganese centres in AlN is presented, and problems relating to the structure of these centres are discussed.Na Slovance 2, Praha 8, Czechoslovakia.The authors would like to thank Dr. L. Kratina and Dr. F. Kubec (Institute of Radio Engineering and Electronics, Czechoslovak Academy of Sciences), who kindly performed the EPR measurements, and to Professor M. Trlifaj and Dr. J. Pastrák for valuable discussions.     

Drift mobility and kinetics of transport of excess charge carriers in glassy CdGexAs2

Conclusions We tried to measure transient conductivity response to pulse strongly absorbed excitation (light, accelerated electrons) in sandwich type samples of glassy CdGe _x As₂ compounds. We observed the signal due to transport of free excess carriers. From analysis of experimental results we conclude that in our materials strong trapping effects are present, so the range of excited carriers is very short (10^–4-10^–3 cm) even in the highest electrical fields used (to 10⁴ V. cm^–1). Estimates of upper limit of drift mobility give the values 10^–1- 1 cm² V^–1 sec^–1. We did not succeed in determining the type of carriers which are responsible for the observed effects.     

Nitrogen atom detection in low-pressure flames by two-photon laser-excited fluorescence

Nitrogen atoms have been detected in stoichiometric flat premixed H₂/O₂/N₂ flames at 33 and 96 mbar doped with small amounts of NH₃, HCN, and (CN)₂ using two-photon laser excitation at 211 nm and fluorescence detection around 870 nm. The shape of the fluorescence intensity profiles versus height above the burner surface is markedly different for the different additives. Using measured quenching rate coefficients and calibrating with the aid of known N-atom concentrations in a discharge flow reactor, peak N-atom concentrations in these flames are estimated to be on the order of 10¹²–5×10¹³ cm^–3; the detection limit is about 1×10¹¹ cm^–3.     

Laser-induced breakdown of krypton near five-photon ionization threshold

Selective excitation of laser-induced breakdown in krypton in the pressure range 1–4 bar and the spectral range 420–620 nm at a laser intensity of 10¹¹ W/cm² has been studied. It is shown that the breakdown of krypton can be induced selectively due to resonantly enhanced multiphoton ionization via excited atomic states. A number of four- and five-photon atomic resonances have been identified in breakdown excitation spectra. The role of different factors determining the selectivity of the breakdown has been studied. Possible analytical applications of a selective breakdown are discussed.     

Torsion-rotation far-infrared spectrum of O-18 methanol

High-resolution Fourier transform spectra have been recorded from 15–470 cm^–1 for the far-infrared trosion-rotation band of O-18 methanol in the vibrational ground state. So far, 57 subbands have been assigned in the 15–220 cm^–1 region for a wide range of rotational and torsional states, and their J-independent origins have been determined to an estimated accuracy of ±0.01 cm^–1. The observed origins were found to deviate in many cases by several tenths of a cm^–1 from the values calculated with the previous molecular parameters. Together with 4 known microwave origins, the new data have been fitted to a model torsion-rotation Hamiltonian in order to refine the set ofb-type molecular constants for the ground state. With the new parameter set, the experimental subband origins are reproduced with an rms error of ±0.02 cm^–1, representing a substantial improvement over the earlier situation. The spectroscopic results have also been of great assistance with our assignments of optically-pumped FIR laser emission in CH₃ ¹⁸OH, in providing FIR data for checking the identification of the IR-pump/FIR-laser transition systems through combination loop relations.     

Tunable far-infrared solid-state lasers based on hot holes in germanium

The basic principles for achieving population inversion and stimulated emission in the far-infrared fromp-Ge are discussed. In the heavy-light hole lasing mode a broad gain region is found resulting in a broad multimode spectrum due to intracavity modes. By attaching external plates of germanium a single mode operation is realized. The obtained powers are in the W range with linewidths of 0.2 cm^–1. A single mode magnetically tunable coherent source is achieved with the light hole cyclotron resonance laser. With external mirrors a tuning range from 20 to 120cm^–1 with magnetic fields between 1 and 6 T is achieved. The intensity of the single mode is in the order of W, the linewidth below 0.2 cm^–1.     

Raman studies of nonlinear phenomena in fullerene crystallites

The resonant Raman scattering of C₆₀ crystallites is monitored as a function of excitation intensity at 514.5 nm. At low intensities, a strong line at 1468 cm^–1 is observed. No feature at 1459 cm^–1 is observable. With increasing intensities, the 1468 cm^–1 line shifts continuously and reversibly to lower frequencies. The mode softening is nonlinearly dependent on the input intensity and is accompanied by a nonlinear increase in the Raman intensity. The spectral changes are discussed in terms of a nonlinear reduction in force constant and increase in bond polarisability as a result of an increased intermolecular delocalisation of the -electron cloud. The nonlinear changes are associated with the nonlinear luminescence and photoconductive response observed in fullerenes and are contrasted to the irreversible phototransformation recently discussed.     

Nonlinear optical properties of a new two-photon-absorption organic dye: HEASPS

Trans-4-[p-(N-hydroxyethyl-N-ethylamino)styryl]-N-methylpyridinium p-toluene sulfonate (abbreviated as HEASPS) is a two-photon-absorption (TPA) dye newly synthesized by our research group. It possesses a much larger TPA cross-section and much stronger upconversion fluorescence emission than those of common organic dyes (such as rhodamine) when excited with near-infrared (IR) radiation. TPA spectrum and upconversion efficiency spectrum of HEASPS solution at different wavelengths have been measured. The largest molecular TPA cross-section σ₂ ^′ is measured to be 2.06×10^-47 cm⁴ s/photon at 930 nm. At 1064 nm, σ₂ ^′ is 2.71×10^-48 cm⁴ s/photon, which is only one-ninth of that at 930 nm. The upconverted lasing efficiency spectrum has been measured at different wavelengths. The highest efficiency is 5.1% at 1020 nm, whereas it is 3.5% at 1064 nm. Its optical-power-limiting properties at 930 nm have also been illustrated. Received: 30 November 2000 / Published online: 27 April 2001     

Population densities and optical gain of VUV transitions of Cu II in Cu−He hollow cathode discharges

Population densities of excited Cu II-levels between 16 and 25 eV in Cu–He hollow cathode discharges were determined by emission spectroscopy. Population inversion was detected for several 6s-4p transitions. Investigation of the enhancement of Cu II vuv lines in He compared to Ne discharges showed that the excitation of the 6s levels by charge transfer is up to 100 times higher in He than in pure Ne discharges. Using the population densities and known transition probabilities, a single pass gain of 55% m^–1 at 780.8 nm and 1.2% m^–1 at 154.17 nm at a current density of 0.4 A cm^–2 was calculated.This paper is dedicated to Prof. Dr. H. Gobrecht on the occasion of his 75th birthday     

Carbon suboxide as a semirigid bender

The semirigid bender Hamiltonian [Bunker and Landsberg, J. Mol. Spectrosc. 67, 374–385 (1977)] is used to fit the rotation-vibration energy level separations in the carbon suboxide molecule C₃O₂. We allow the CC bond lengths and CCO bond angles to change with the CCC bending angle ρ. A very good fit to the energy levels is obtained and, in particular, the B values are systematically fitted better than when the rigid bender is used. The dependence of the effective CCC bending potential function on the vibrations ν₂, ν₃, and ν₄ is determined, and we find that excitation of ν₃ or ν₄ raises the barrier to linearity whereas excitation of ν₂ lowers it. These results can be understood by considering the ρ dependence of the G-matrix elements. We determine that the barrier to CCC linearity in the zero-point vibrational state is 28 cm⁻¹ but until more data are available for the ν₁, ν₅, and ν₆ vibrations we cannot precisely determine the true barrier. However, it has been previously shown that the barrier is little affected by excitation of ν₁ or ν₅, and that it is reduced by 10–15 cm⁻¹ by excitation of ν₆. From these results we deduce that the barrier to CCC linearity in the true bending potential function is 33 cm⁻¹ with an uncertainty of about 5 cm⁻¹. Thus the equilibrium structure is bent at the central carbon atom; the equilibrium CCC angle is 157°.     

Multi-photon-pumped stimulated emission from ZnO nanowires: A time-resolved study

In this work, we study temporal evolution of multi-photon-pumped stimulated emission from ZnO nanowires. In addition to second harmonic generation, ultraviolet stimulated emission is observed in ZnO nanowires under femtosecond pulse excitation at 800 nm. Sharp emission peaks appear when excitation flux reaches a threshold of 80 mJ/cm², which can be interpreted as lasing action in self-formed nanowire microcavities. Temporal evolution of the emission captured by Kerr shutter technique shows strong excitation-power dependence. The dynamic trace of stimulated emission exhibits a fast decay with a lifetime about 4.5 ps at intermediate excitation (∼100 mJ/cm²) and a lifetime about 2 ps at high excitation (>160 mJ/cm²). The difference in the lifetime can be attributed to different gain mechanisms related to excitonic interaction and electron-hole plasma, respectively.     

Optical properties of low-temperature magnesium laser plasma

Results are reported of an experimental study of ehe optical properties of the plasma produced when magnesium vapor is irradiated by resonant laser emission ( = 285.2 nm). The spectral and temporal characteristics of the vapor luminescence were measured. An additional laser source was used to study the kinetics of the plasma formation and decay. The conditions for obtaining a plasma with a large ion content were determined. At 1 kW resonance radiation and at a magnesium-atom concentration 4·10¹⁵ cm^-3 the ion concentration was 2·10¹⁴ cm^-3. Lasing on the 4³S₁–3³P₂ transition of the magnesium atom ( = 518 nm) was obtained for the first time and was the result of rapid transfer of the excitation from the singlet system of levels to the unperturbed triplet system via secondary processes accompanying the particle collisions.Translated from Lazernye Sistemy, pp. 35–45, 1982.     

Investigation of the active media in pulsed gas discharge lasers by the Rozhdestvenskii hook method

In this review we generalize the results of our experimental investigations in which the Rozhdestvenskii hook method was first used to look for an inverted population in new spectral transitions. An inverted population in new 3p–3s transitions of the neon atom with =588.1, 594.4, 650.6, and 653.2 nm was discovered in the active medium of a Penning plasma laser emitting the =585.3 nm neon line in Ne-H₂ and Ne-H₂-Ar mixtures. Results of investigations of a number of active media are given. An analysis is made of the spatial and temporal distributions of copper atoms in the ground state and of their influence on the lasing efficiency of a copper-vapor laser. Using the rhenium atom as an example, it was shown to be possible to obtain a sufficiently high concentration of refractory metal atoms (>10¹⁴ cm^–3) in an active medium to form active media of lasers utilizing self-terminating transitions and also plasma lasers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–27, December, 1994.     

Studies of vibrational relaxation in CDF3 following intense laser excitation

The V-T/R relaxation time of CDF₃ was measured studying the laser-induced infrared fluorescence emitted by vibrationally excited CDF₃. Following excitation by the 10R(12) line of a TEA CO₂ laser infrared fluorescence has been detected without spectral resolution in the 1100–700 cm^–1 range. A decay rate of 28.8 ms^–1 Torr^–1 was obtained for pure CDF₃ when it is excited with a fluence of 390 mJ/cm². Measurements have also been made in the presence of different bath gases (He, Ne, Ar, Xe, and CHF₃).     

High-Resolution Laser Spectroscopy of YbCl: The AΠ–XΣ Transition

The A²Π–X²Σ⁺ transition of ¹⁷⁴Yb³⁵Cl and ¹⁷²Yb³⁵Cl has been rotationally analyzed for the first time. Doppler-limited laser excitation spectroscopy with selective detection of fluorescence was used to obtain spectra of the 0–0 and 1–0 bands with a measurement accuracy of approximately 0.0035 cm⁻¹. Resolved fluorescence was used to record the 0–1, 0–2, and 0–3 bands and to unequivocally assign the rotational numbering, N, to the laser excitation spectra. In total, over 1300 line positions have been measured and assigned for each of the two isotopomers and employed in least-squares fits of molecular parameters. The principal results for the A²Π state are A_e = 1491.494(2) cm⁻¹ and R_e = 2.4433(1) Å, and for the X²Σ⁺ state, R_e = 2.4883(2) Å and γ_e = 4.59(2) × 10⁻³ cm⁻¹. The interaction between the X²Σ⁺ and A²Π states has been investigated and is shown to be the main contributor to the spin–rotation splitting in the ground state.     

Injection locking of ArF excimer lasers

Injection-locking characteristics of an ArF excimer oscillator-amplifier laser are described including the use of stable-unstable optical cavities. Output intensities of 1 MW cm^–2 have been produced with 3 mJ output energy, a spectral linewidth better than 5×10^–3 nm and an injection locking efficiency of 0.9.