Intensity detection of picosecond ruby laser pulses by two photon absorption

The peak intensity of single picosecond light pulses from a mode locked ruby laser and its second harmonic is detected by two photon transmission measurements. The two photon absorption coefficients α⁽²⁾ of the applied crystals CdS and KI are measured (α⁽²⁾_CdS (14400 cm^-1) = 19 cm/GW, α⁽²⁾_KI (28 800 cm^-1) = 8 cm/GW) and calibration curves are presented.     

Theory of the hot free carrier intraband-absorption coefficient of n-inversion layers

The absorption coefficient K of a quasi two dimensional (2D) hot free electron gas is calculated for the first time as a function of the lattice temperature T, the photon angular frequency w, the carrier density N_s as well as the electron temperature T_e when the carriers are scattered by ionized impurities, acoustic phonons and polar optical phonons. Analytical expressions are derived in the limiting cases of non-degeneracy and degeneracy of the electron system. In the quantum limit ħw/k_BT_e ≳ 1 where the interaction between the electron and the photon is inelastic K sensitively depends on the limiting scattering mechanism showing that the electron motion is completely controlled by the photon field. In the classical limit ħw/k_BT_e ⪡ 1 the absorption decreases proportional to w¹ independent of the limiting scattering mechanism in agreement with the experimental data deduced from far-infrared absorptivity measurements on GaAs heterolayers.     

Total photonuclear absorption cross section for Pb and for heavy nuclei in the Δ-resonance region

The partial sum σ⁽²⁾(E_γ) = Σ_i?2 σ(γ, in…), representing the inclusive cross section for all reaction channels in which at least two neutrons are emitted, has been measured with a quasimonochromatic photon beam obtained by the in-flight annihilation of monoenergetic positrons, and neutron multiplicity counting. These experimental results, taken with photon energies E_γ from 145 up to 440 MeV for Pb and with photon energies E_γ= 235 MeV and 330 MeV for Al, Cu, Zr, Sn, Ho, Ta and U, are subsequently used to determine the total photonuclear absorption cross section σ(tot: E_γ) and to study the dependence upon the mass number A of the normalized cross section σ(tot: E_γ)/A. These results are then compared with other information on the total photonuclear absorption cross section.     

Optical anisotropy of orthorhombic InS

The optical anisotropy of InS single crystals in the range of photon energy from 1.8 to 3.5 eV has been studied by absorption, electroreflectance and wavelength-derivative reflectance measurements. These systematic optical measurements for the polarizations, E//a and E//b, have revealed that the transition at the fundamental absorption edge of InS is allowed for E//b, and there exist three distinct doublet transitions having different selection rules in the photon energy region from 2 to 3.5 eV; Bo and B^'₀doublet allowed only for E//b, A₀ and A^'₀ allowed only for E//a, and E₁ and E^'₁ allowed for both polarizations. The observed results are discussed based on the anisotropic nature of two chemical bonds in InS, cation-cation and cation-anion.     

Measurements of the (γ, n0) reaction on 12C and 16O and the investigation of the reaction mechanism

Differential (γ, n₀) cross sections on ¹²C and ¹⁶O have been measured for photon energies 60 MeV ? E_γ ? 160 MeV. These results combined with the corresponding (γ, p₀) cross sections support an absorption mechanism of the photon by neutron-proton pairs.     

Absolute measurement of an atomic cascade rate using a two photon coincidence technique. Application to the 4p21S0 -4s4p 1P1 -4s21S0 cascade of calcium excited by a two photon absorption

The two photon coincidence technique yields an absolute measurement of a cascade decay rate without knowing the detection efficiency of each detector. This method is applied to calcium atoms in an atomic beam excited to the 4p²¹S₀ state, and decaying via the resonant ¹P₁ state; it yields the excitation rate of the upper 4p²¹S₀ level. Since the excitation process (two photon absorption) is controlled, one can compute the excitation rate of the upper level as a function of the transition probabilities in the cascade. The lower transition is well known and we can therefore deduce the transition probability for the 4p²¹S₀ -4s4p ¹P₁, transition which previously had not been accurately determined. The preliminary measurements yield a line strength s = 0.98 ± 0.3 au in good agreement with a recent measurement of the lifetime of the 4p²¹S₀ state.     

“Negative” absorption and “negative” emission in nonlinear optics

The nonlinear susceptibilities X³ (ω₂ = ω₁ ? ω₁ + ω₂) calculated for stimulated Raman scattering (SRS) and two- photon absorption (TPA) under the condition of one-photon resonance in the case of a multilevel system with broad overlapping absorption bands (corresponding to polyatomic molecules) are discussed. Spectra of Im X³ with changed sign in some spectral regions are shown. The result is connected with a disbalance of the system, due to the large relaxation constants.     

Photoemission of protons from4He in theΔ-resonance region

High resolution proton energy spectra from the⁴He(γ,p) reaction have been measured with tagged photons in the range E_γ = 130?525 MeV using the large Mainz NaI(T1) spectrometer atΘ _p ^lab =37.1°. Three separate reaction channels were identified, viz.⁴He(γ,p)t two-body breakup,⁴He photodisintegration via two-nucleon photon absorption processes and the quasifree pion production channel. Differential cross sections are presented for each of these channels as a function of photon energy. The sum of the two-nucleon photon absorption and the quasifree pion production differential cross sections, in the CM system, resembles that of the corresponding free-nucleon differential cross section when Fermi motion is taken into account.     

Hadron production in 200 GeV μ-copper and μ-carbon deep inelastic interactions

The measurements of the z and p_T² distribution of hadrons produced in the interactions of 200 GeV muons with copper and carbon nuclei are shown in different x_Bj and virtual photon energy intervals. Effects of the jet scattering are seen at the lowest virtual photon energies while for energies above 70 GeV there is no evidence of these effects. Comparison with a theoretical model indicates that at high jet energies the parton fragmentation distance is greater than the nuclear radius and that the parton absorption cross section is less than 10 mb.     

Photoabsorption of Atomic Sodium in the VUV

The absorption spectrum of atomic sodium in the photon energy region from 30 to 150 eV has been investigated. A great number of sharp absorption lines which can be attributed to the excitation of a 2p- or a 2s-electron has been detected. Simultaneous excitation of one 2p- and one 3s-electron gives rise to considerably strong broad and asymmetric absorption structures above the highest series limit (¹ P ₁) for the excitations of a single 2p-electron. Some of the assignments have been confirmed by Hartree-Fock calculations. The relative spectral dependence of the absorption cross-section in this energy range has been determined for the first time. The spectrum of free Na atoms has been compared with theL _{II, III} spectrum of solid sodium.     

Pulsed optoacoustic detection of multiple photon excitation in molecules

Multiple photon excitation, saturation, and linear absorption of SF₆-argon mixtures when irradiated by a high power CO₂ TEA laser is investigated using a pulsed optoacoustic technique. At low intensities the expected linear dependence of the absorption on laser intensity is observed. At intermediate intensities the absorption exhibits a square root dependence on the incident laser intensity, a dependence which is typical for saturation of an inhomogeneously broadened absorption. At even higher intensities, the absorption shows an intensity dependence typical of multiple photon excitation. The laser intensity was varied between 0.016 kW/cm² and 5 MW/cm², values lower than that needed to produce multiple photon dissociation of SF₆. Increasing the collision frequency of the absorbing molecules with an inert buffer gas is observed to quench the multiple photon excitation.     

Optical absorption due to transitions between bands and local levels in CdCr2Se4

The theory of optical absorption due to transitions between a valence band and a hydrogen-like local level associated with a conduction band is modified to permit an arbitrary power-law dependence of energy on the magnitude of the wave-vector of carriers in the valence band. The observed absorption for photon energies below 1.6 eV in the ferromagnetic semiconductor CdCr₂Se₄ is discussed in terms of a combination of two types of terms. The first type of absorption is due to transitions to a local level from a band with two branches, in each of which there is an energy region with a width of 0.28 eV or more beginning 0.10–0.16 eV from the band edge, in which the energy measured from some origin near but not necessarily equal to the band-edge is approximately proportional to (wave-vector)^{($\text{1}\text{3}$)}. The second type of absorption has a dependence on photon energy ?ω of the form (?ω ? E₃)², where E₃ is a threshold energy probably connected with indirect transitions between bands as suggested by Sakai, Sugano and Okabe. After constraints on parameters appearing in the theory are imposed by use of results of these authors and of Shepherd, it is found that curves of Harbeke and Lehmann on optical absorption in CdCr₂Se₄ at 4.2, 78, 130 and 298 K in the photon-energy range 1.14–1.42 eV can be fitted to a mean accuracy of 3%, using an average of 3.75 adjustable parameters for each curve. The strength of the indirect band-to-band absorption does not have the temperature dependence expected for phonon-assisted indirect band-to-band transitions, but can be described by a term independent of temperature plus another term proportional to the square of the deviation of the magnetization from saturation. The fitting of the absorption curves requires that the ratio of the widths of the two branches of the bands varies from about 1.6 at low temperatures to 1.35 at 298 K and that the total width of the bands involved is less than 1 eV.     

Optoacoustic detection of multiple photon molecular absorption in a strong IR field

A new method based on the optoacoustic effect has been proposed to measure multiple photon absorption at vibrational molecular transitions in a strong IR laser field. Comparison measurements of the average absorbed energy have been done by this method for molecules with different dissociation limits in a strong CO₂-laser field, the field intensity changed therewith by four orders. For the poly-atomic molecules C₂H₄ and SF₆ having a comparatively low dissociation limit, the absorption increases monotonously as the power density of laser radiation P increases up to the dissociation limit. For three-atom molecules, such as D₂O and OCS, absorption saturation takes place with P ? 10 MW/cm².     

First-principles calculations of optical properties of Mg2Pb

Based on the density functional theory(DFT),the electronic structures and optical properties of Mg2Pb are calculated by using the local density approximation(LDA)and plane wave pseudo-potential method.The calculation results show that the indirect band gap width of Mg2Pb is 0.02796 eV.The optical properties of Mg2Pb have isotropic characteristics,the static dielectric function of Mg2Pb is1(0)=10.33 and the refractive index is n0=3.5075.The maximum absorption coefficient is 4.8060×105cm 1.The absorption in the photon energy range of 25–40 eV approaches to zero,shows the optical colorless and transparent behaviors.     

Two and four photon absorption and nonlinear refraction in undoped,chromium doped and copper doped ZnS quantum dots

The ZnS quantum dots (QDs) with Cr and Cu doping were synthesized by chemical co-precipitation method. The nanostructures of the prepared undoped and doped ZnS QDs were characterized by UV–vis spectroscopy, Transmission electron microscopy (TEM) and X-ray diffraction (XRD). The sizes of QDs were found to be within 3–5 nm range. The nonlinear parameters viz. Two photon absorption coefficient (β₂), nonlinear refractive index (n₂), third order nonlinear susceptibility (χ³) at wavelength 532 nm and Four photon absorption coefficient (β₄) at wavelength 1064 nm have been calculated by Z-scan technique using nanosecond Nd:YAG laser in undoped, Cr doped and Cu doped ZnS QDs. Higher values of nonlinear parameters for doped ZnS infer that they are potential material for the development of photonics devices and sensor protection applications.     

Absorption of electromagnetic energy by slow electrons under scattering from Coulomb centers

Simple analytical expressions are obtained for the rate of the inverse stimulated bremsstrahlung absorption under electron scattering from a Coulomb center with charge Z in the presence of the electromagnetic field. The initial and final values of electron energy are assumed to be small compared to the Rydberg energy Z ² (atomic units are used throughout). Single-photon processes of absorption and induced radiation of photon by electron are treated. It is assumed that the electromagnetic field frequency ω is rather low, so that the condition Zω/p ³ ? 1, where p is the electron momentum, and the condition ?ω ? p ² are valid. However, this frequency is assumed to be fairly high compared to the electron-Coulomb center collision frequency: ω ? v _nei. The dependences of the rates of photon absorption and induced radiation on the angle θ between the direction of incident electron and the electromagnetic field polarization vector (assumed to be linearly polarized) are obtained. It is demonstrated that, for any angles θ, the rate of photon absorption is higher than the rate of induced radiation and, therefore, the Marcuse effect for slow electrons (electromagnetic field amplification) is absent. It is further demonstrated that a slow electron on the average absorbs double ponderomotive energy per collision with an ion (Coulomb center) in Maxwellian plasma. This agrees both with the known results calculation for fast electrons and with the known results of the calculation based on the classical Boltzmann kinetic equation for plasma.     

Optical absorption in MnIn2S4 single crystals

Optical absorption in MnIn₂S₄ single crystals has been studied. Direct and indirect optical transitions are found to occur at photon energies of 1.90?C2.16 eV in the temperature range of 80?C342 K. The temperature dependence of the band gap is determined; its temperature coefficients E _gd and E _gi are found to be ?4.84 × 10^?4 and ?6.33 × 10^?4 eV/K, respectively. The electron-phonon interaction is the main mechanism of the temperature shift of the intrinsic-absorption edge. MnIn₂S₄ single crystals exhibit anisotropy in polarized light at the absorption edge in the temperature range of 90?C190 K; the nature of this anisotropy is explained.     

Quantum beats in two-photon resonance fluorescence

The two-photon resonance fluorescence spectrum of a three-level atom is shown to consist of the low frequency modes in addition to the high frequency ones in the limit of high photon densities. The spectral function for the low frequency modes consists of two lorentzian lines describing: the peak occuring at the renormalized beat frequency Δ₊ and that of the zero-photon excitation at the frequency Δ_-, where Δ_±=Δ-3Ω²/2ω_a±Ω²u/2ω_a, u²=1+(2Δω_a/Ω ²)². Here, 2Δ is the energy splitting between the two excited states, ω_a is the photon energy of the pump field and Ω is the Rabi frequency. The peak at the renormalized beat frequency Δ₊ occurs provided that the condition (2Δω_a/Ω²)² > 1 is satisfied. The two-photon laser spectroscopy is expected to be a useful tool for the observation of the low frequency modes in question.     

Polarization dependence of the absorption of 14.4 keV γ rays in a single crystal of mercuric chloride

The absorption of polarized 14.4 keV photons in a single crystal of mercuric chloride was measured, for polarization directions parallel to the [$\text{1}$ 0 1] and [0 1 0] axes. A new method was applied where neither the crystal nor the source had to be rotated to alter the photon polarization direction, thus eliminating geometrical effects. The observed absorption difference, Δμ/μ = (1.5 ± 0.7) X 10^-3, was attributed to the polarization dependence of the photoelectric cross section of the 2p electrons of mercury in HgCl₂.     

Oscillations of the photon echo intensity in a pulsed magnetic field: Zeeman splitting in LiLuF<Subscript>4</Subscript>:Er<Superscript>3+</Superscript>

A method of high-resolution time-resolved optical spectroscopy using oscillations of the photon echo intensity in the presence of a perturbation, which splits the optical frequencies of the transitions of two or more ion subgroups, has been proposed and demonstrated. This method has been applied to systems in which the Zee-man effect is manifested. The transition frequencies of ions are switched by a pulsed magnetic field. Oscillations of the photon echo intensity were observed in LiLuF₄:Er³⁺ and LiYF₄:Er³⁺. The first minimum corresponding to the accumulated phase of the electric dipole moment π/2 is reached in the pulsed magnetic field with an amplitude of ～2 G at a duration of 30 ns. The Zeeman splitting in this field is ～10 MHz, which is much less than the laser spectral width (0.15 Å ～ 9 GHz). The g factor of the ⁴ F _9/2(I) excited state of the Er³⁺ ion in the LiLuF₄ matrix has been determined in zero magnetic field. The comparison with the g-factor value found from the measurement of the absorption spectrum in a magnetic field of 8 kG has been performed.