Nonlinear absorption of monochromatic radiation in gases

The absorption coefficient as a function of the density of monochromatic collimated radiation is discussed for an arbitrary ratio of natural to Doppler width, with allowance for the effects of collisions between molecules.We are indebted to M.A. El'yashevich for a discussion and to Ya. S. Vasilevskaya and UP. Pesotskaya in preparing the paper.     

Nonlinear absorption of non-chain HF laser radiation in germanium

The transmission of high-power HF(DF) laser radiation through Ge single crystals of different quality has been experimentally and numerically investigated. Based on the experimental data for the lasing spectrum, the two-photon absorption coefficient in Ge is estimated to be K₂ =55±10 cmGW?1 at λ=2.8μm. The transmission dynamics of high-power radiation with λ=2.6?3 μm through germanium crystals is numerically simulated.     

Nonlinear absorption of CO2 laser radiation by nonequilibrium carriers in germanium

Nonlinear absorption of CO₂ laser pulses at the multi-MW level has been studied in n-type germanium at room temperature. This nonlinear absorption limits the level of CO₂ laser intensity that can be transmitted through an optical grade germanium crystal. The observed nonlinearity may be interpreted in terms of nonequilibrium electron-hole pairs generated by hot electrons created by the intense laser field. The number of nonequilibrium carriers generated is measured by photoconductivity experiments, and the results are correlated to the absorption measurements.     

The 275-nm absorption system of anisole

The 275-nm absorption systems of anisole (methoxybenzene) and three deuterated derivatives have been photographed at medium to high resolution. The origin bands lie at 36 386.4, 36 389.6, 36 557.2, and 36 560.4 cm^?1 for C₆H₅OCH₃, C₆H₅OCD₃, C₆D₅OCH₃, and C₆D₅OCD₃. The vibrational structure, which was analyzed in some detail, was found to be very similar to that of the analogous system of phenol. The spectrum consists of both allowed and forbidden components although the forbidden component, principally evident through activity of vibrations 18b and 6b, is relatively weak.     

Nonlinear waves in zinc

A theoretical analysis is made of Doppler-shifted cyclotron resonance in zinc in linear and nonlinear regimes. It is shown that the absence of a threshold for cyclotron absorption by holes makes the doppleron strongly damped or eliminates it for low-amplitude wave-fields. At high amplitudes capture of holes by the magnetic field of the wave suppresses collisionless absorption and the doppleron can propagate. As a result, the impedance of the plate is an oscillating function of the magnetic field. It is shown that the effect should be observed at frequencies of the order of a few tens of kilohertz, in magnetic fields of the order of a few kilogauss, and for exciting field amplitudes of the order of a few tens of gauss. Zh. éksp. Teor. Fiz. 114, 725–734 (August 1998)     

The 280-nm absorption system of benzoyl fluoride

Benzoyl fluoride exhibits a weak, discrete absorption system in the region 290 – 260 nm. Vibrational structure associated with the C₆H₅COF and C₆D₅COF isotopes has been analyzed in detail. The C₆H₅COF and C₆D₅COF origin bands lie at 35 685 and 35 829 cm^?1, respectively. The intensity in the spectrum is principally associated with two progressions, ν′₁₉, the breathing vibration of the benzene ring and ν′₂₄, a COF in-plane bend, and sequence bands involving the COF torsion motion ν₃₆. The $\text{C}{}_6\text{H}{}_5\text{COF}\text{C}{}_6\text{D}{}_5\text{COF}$ν₁₉, ν₂₄, and ν₃₆ vibrational frequencies are respectively 1005/-, $\text{376}\text{357}\text{,}\text{and}\text{57}\text{55}\text{cm}{}^{\mathrm{?1}}$ in the ground state and $\text{950}\text{919}\text{,}\text{348}\text{331}\text{,}\text{and}\text{89}\text{85}\text{cm}{}^{\mathrm{?1}}$ in the excited state. The barriers to rotation of the COF group are estimated from the torsional frequency data to be 1450 and 3450 cm^?1 in the ground and excited electronic states, respectively.     

The 290-nm absorption system of propiolyl fluoride

The vapor-phase absorption spectrum of propiolyl fluoride HCCCOF and deuterio-propiolyl fluoride has been examined under medium resolution in the region from 303 to 260 nm. While considerable vibrational structure is evident, all bands are found to be rotationally diffuse. The electronic transition is identified as the singlet-singlet $\text{π}{}^1\text{← n}$ transition analogous to the 382-nm system of propynal. The evidence for a nonplanar excited-state geometry is discussed. The principal excited-state vibrational frequencies identified are 158 (ν′₉), 396 (ν′₈), and 1222 cm^?1 (ν′₃) for HCCCOF. The corresponding values for DCCCOF are 150, 390, and 1217 cm^?1, respectively. The triplet ← singlet system involving the same electron promotion has not been found.     

Switching from "absorption within transparency" to "transparency within transparency" in an electromagnetically induced absorption dominated transition

The absorption of a resonant coupling laser driving a closed degenerate two-level system in an atomic cesium beam was investigated as a function of the detuning of a second laser probing the same transition. The measurements were performed for four different polarization combinations of the two laser beams. Except for the beams of counterrotating polarizations all coupling-laser absorption profiles showed "absorption within transparency," i.e., the absorption in the region around the two-photon resonance was smaller than the absorption corresponding to the one-photon transition induced by the coupling laser, and an extra absorption peak was observed on this curve at the two-photon resonance. With regard to the beams of counterrotating polarizations we observed a switch from absorption within transparency to "transparency within transparency" when the probe-laser power exceeded the constant coupling-laser power. In other words, the cesium ensemble became mostly transparent to the coupling-laser beam at the two-photon resonance.     

Efficient 355-nm generation in CsB3O5 crystal

We demonstrate UV generation in CSB3O5 (CBO) crystals grown by the top-seeded solution growth technique. 355-nm UV light was generated by use of a type II CBO crystal as a sum frequency of the fundamental light and the second harmonic of a nanosecond Nd:YVO4 laser. A 3.0-W output of the third-harmonic was obtained at a repetition rate of 31 kHz. The conversion efficiency from the fundamental light to the third harmonic reached 30%, which was 1.5 times higher than that obtained with a type II LiB3O5 crystal under the same experimental conditions.     

Parametric generation and absorption of tunable vacuum-ultraviolet radiation controlled by laser-induced autoionizing-like resonances in the continuum

The effect of a strong additional laser field on the absorption and parametric generation at the atomic transition between ground state and continuum is analyzed. The possibility is shown of inducing the resonant decrease of absorption and enhancement of the nonlinear susceptibility at arbitrary range of the continuum.     

Self-induced generation of an off-axis frequency shifted radiation from atoms

The transient resonant linear response at wavelength λ_a of an N two-level atom vapor driven by a strong pulse with wavelength λ_f = λ_a - |Δλ| is shown to promote an emission of radiation peaked at wavelength λ_c = λ_a + |Δλ| in a conical shell around the propagation axis of the incident beam. In the limit of weak excitation, i.e. for an incident Rabi frequency much smaller than the detuning, the cone angle is found to be equal to 2λμ(2N/ch|Δλ|)^{$\text{1}\text{2}$} where μ is the transition dipole moment.     

The effect of two-photon absorption and optical excitation area on the generation of THz radiation

It was found experimentally that the THz radiation throughput based on optical rectification is strongly related to the optical excitation size in a ZeTe emitter. The factors affecting the THz radiation throughput are investigated in detail both experimentally and theoretically. By taking into account optical rectification, diffraction and two-photon absorption effects, a theoretical model is established to describe the emitting field intensity of THz radiation. There is excellent agreement between the theoretical results and the experimental data under high excitation power. Both theoretical and experimental results demonstrate that there is a trade-off between these three effects for THz radiation, and that in order to increase the THz radiation throughput based on optical rectification, one should choose suitable excitation size under a given pump power.     

Generation of continuous-wave 243-nm radiation by sum-frequency mixing

We have generated tunable cw radiation near 243 nm with a linewidth of less than 4 MHz by sum-frequency mixing the 351 nm radiation from an argon-ion laser with the 789 nm radiation from a ring dye laser in a crystal of ammonium dihydrogen phosphate held at moderate temperature. An external ring cavity, resonant with the dye laser, gives a power enhancement of about 12 in the sum-frequency generated radiation. Thermal lensing due to laser heating of the nonlinear crystal, distorded the 351 nm mode structure. This effect could limit the efficiency of the sum frequency mixing process.     

Four-photon pressure-induced ionization of Xe by 351-nm radiation

The four-photon ionization of xenon gas at 351 nm is observed to be primarily pressure-induced. These observations indicate that the absorption occurs mainly through interaction with bound Xe₂ molecules. A three-photon near-resonance was found to enhance appreciably the cross-section for multiquantum ionization. The radiative lifetime of the resulting Xe¹₂ was determined to be τ_rad = 51 ns, implying only a partial mixing of the 0⁺_u and 1_u, 0^-_u states.     

Nonlinear optical absorption studies of an organo-metallic complex by Z-scan technique

An organo-metallic complex, [(CH₃)₄N][Ni(dmit)₂] (dmit²⁻ = (1,3-dithiole-2-thione-4,5-dithiolate), abbreviated as MeNi, is synthesized. The nonlinear optical absorption properties of MeNi dissolved in acetone have been studied using the open-aperture Z-scan technique with 40 ps pulse width at 1064 nm and 1 ns, 15 ns pulse width at 1053 nm, respectively. Strong saturable absorption has been found when the sample solution is irradiated by 40 ps and 1 ns laser pulses. While irradiated with 15 ns laser pulse, a stronger reverse saturable absorption has been found. The nonlinear optical absorption coefficients are −1.03 × 10⁻¹¹ m/W, −1.85 × 10⁻¹¹ m/W and 3.84 × 10⁻¹⁰ m/W, respectively. The mechanism responsible for the difference between the results is analyzed. All the results suggest that this material may be a promising candidate for the application to laser pulse compression in the near-infrared waveband.     

Excitation-density-dependent generation of broadband terahertz radiation in an asymmetrically excited photoconductive antenna

The generation of terahertz (THz) transients in photoconductive emitters has been studied by varying the spatial extent and density of the optically excited photocarriers in asymmetrically excited, biased low-temperature-grown GaAs antenna structures. We find a pronounced dependence of the THz pulse intensity and broadband (>6.0 THz) spectral distribution on the pump excitation density and simulate this with a three-dimensional carrier dynamics model. We attribute the observed variation in THz emission to changes in the strength of the screening field.     

Photoinduced transient absorption within zinc sulphide phosphors in powder form detected by diffuse reflectance laser flash photolysis

Transient absorptions have been obtained in powdered samples of ZnS : Ag, ZnS : Cu and undoped ZnS at room temperature by the technique of diffuse reflectance laser flash photolysis. For the undoped material an absorption was detected that maximises at 400 nm and decays non-exponentially with an initial half-life of (6.8±1.0)×10^?6s. The absorption is assigned to a transition within a donor impurity. In the cases of ZnS : Ag and ZnS : Cu, broad featureless absorptions in the 500–850 nm region were recorded. These decays are non-exponential and have initial half-lives of (3.1±0.2) ms and (0.48±0.05) ms for the Ag- and Cu-doped phosphors, respectively. These transient absorptions are considered to occur from electron traps introduced by the metal dopant. Laser-induced emission was also studied, and only in the case of undoped ZnS was a correlation observed between the decay of the weak emission at 520 nm and the decay of the transient absorption.