Infrared absorption spectra of amorphous silicon with gold

We determined the optical constants below the absorption edge of amorphous films of Si_1?xAu_x (x between 0.07 and 0.30) prepared by getter sputtering in argon. In the range 0.15 to 0.5 eV we found that the absorption increased with increasing Au content and could be described by Mott's formula for a.c. conductivity σ(ω) ≈ ω²[In (I₀/ω)]⁴ with I₀ ? 4.5 eV at x = 0.11 and 3.8 eV at x = 0.29; in the range 10–100 K it changed only slightly with temperature. The absorption is interpreted as due to direct transitions involving Au atoms.     

Free carrier absorption in quantizing magnetic fields: Nonpolar optical phonon scattering

A quantum theory of free carrier absorption in nondegenerate semiconductors and in strong magnetic fields which was previously developed to treat the case when acoustic phonon scattering dominates the free carrier absorption process [1] is extended to treat the case when nonpolar optical scattering is important. When the electromagnetic radiation field is polarized parallel to the direction of the applied magnetic field, results are obtained which are similar to those when acoustic phonon scattering is dominant. The free carrier absorption is an oscillatory function of the magnetic field which on the average increases in magnitude with the magnetic field. However, more structure in the free carrier absorption occurs when nonpolar optical phonon scattering dominates. This is due to the fact that there are two periods in the oscillatory magnetic field dependence associated with the emission or the absorption of optical phonons during the intraband transitions. When the cyclotron frequency exceeds the sum of the photon and optical phonon frequencies, i.e. ω_c > θ + ω_o, the free carrier absorption is predicted to increase linearly with magnetic field when ?ω_c? k_BT. The magnetic field dependence of the free carrier absorption can be explained in terms of phonon-assisted transitions between the various Landau levels in a band involving the emission and absorption of optical phonons.     

Nonlinear and multiquantum effects in dual-frequency EPR experiment: the intensities of sideband resonances

Two microwave frequencies ω₁ and ω₂ simultaneously exciting a paramagnetic spin probe near the resonance condition ω₁ = γB ₀ effect a pattern of resonances occurring at multiples of the difference frequency δ = ω₂ ? ω₁. Their intensities, measured in the absorption or dispersion mode in the magnetic field-sweep experiment, decrease rapidly as the distance from the centerband increases. Numerically solved Bloch equations are used to discuss the intensities of the transverse components of magnetization up to seven, harmonics of the frequency difference δ. In conclusion, it is suggested that the experimental investigation of these patterns can be used for the purpose of a continuous-wave monitoring the relaxation rates of spin probes.     

Singlet—triplet oscillations of spin-correlated radical pairs due to the Larmor precession in low magnetic fields

Singlet—triplet oscillations in spin-correlated radical pairs have been studied at magnetic field strengths low for one radical and high for the other. Oscillations with frequencies close to the Larmor frequency ω₀ of electron spin precession have been predicted under these conditions. Both numerical and exact analytical solutions in arbitrary magnetic fields are presented for three cases of hyperfine couplings in wide-spectrum radical. For the case of unresolved spectrum, singlet—triplet evolution was found to contain a single oscillating term with frequency ω₀. In the case of one spin-I magnetic nucleus, there are two low frequency oscillating terms with frequencies ω_? = ω₀ ? ω₀/(2I + 1) and ω₊ = ω₀ + ω₀/(2I + 1), the amplitude of the first term being larger than that of the second. The case of a number of equivalent protons also has been analysed as a superposition of one-nucleus oscillations. The predicted oscillations were observed in a time resolved magnetic field effect for several radical ion pairs produced by X-ray irradiation of alkane solutions with charge acceptors. For pairs (p-terphenyl-d ₁₄)^?./(isooctane)^+. and (p-terphenyl-d ₁₄)^?./(2,4-dimethylpentane)^+. the oscillation frequency in a field B ₀ of 0.5–4mT is about 20% lower than ω₀. Oscillations were observed also in pairs with equivalent nuclei: (p-terphenyl-d ₁₄)^+./(C₆F₆)^?. and (p-terphenyl-d ₁₄)^?./(hexamethylethane)^+.     

Red shift of the energy of optical absorption of quantum dot arrays in conditions of dynamic stark effect

The absorption coefficient of quantum dot arrays in conditions of dynamic Stark effect is calculated. It is shown that when ω₁ > ω₀ a red shift of the energy of optical transitions takes place.     

Laser frequency stabilization at 1.5 microns using ultranarrow inhomogeneous absorption profiles in Er:LiYF4

Single-frequency diode lasers have been frequency stabilized to 1.5 kHz Allan deviation over 0.05-50 s integration times, with laser frequency drift reduced to less than 1.4 kHz/min, using the frequency reference provided by an ultranarrow inhomogeneously broadened Er³⁺:⁴I_15/2→⁴I_13/2 optical absorption transition at a vacuum wavelength of 1530.40 nm in a low-strain LiYF₄ crystal. The 130 MHz full-width at half-maximum (FWHM) inhomogeneous line width of this reference transition is the narrowest reported for a solid at 1.5 μm. Strain-induced inhomogeneous broadening was reduced by using the single isotope ⁷Li and by the very similar radii of Er³⁺ and the Y³⁺ ions for which it substitutes. To show the practicability of cryogen-free cooling, this laser stability was obtained with the reference crystal at 5 K; moreover, this performance did not require vibrational isolation of either the laser or crystal frequency reference. Stabilization is feasible up to T=25 K where the Er³⁺ absorption thermally broadens to ∼500 MHz. This stabilized laser system provides a tool for interferometry, high-resolution spectroscopy, real-time optical signal processing based on spatial spectral holography and accumulated photon echoes, secondary frequency standards, and other applications such as quantum information science requiring narrow-band light sources or coherent detection.     

Damping of optical phonons in metals and strongly doped semiconductors

The electron-phonon-induced damping of optical phonons arising in metals and strongly doped semiconductors under laser irradiation is investigated. The damping of both short-wave KV_F > ω₀ and long-wave KV_F < ω₀ optical phonons is calculated; K is the wavevector, ω₀ is the frequency of the optical phonon; V_F is the Fermi velocity. The electron- phonon-induced damping is important if the frequency of the optical phonon is larger than two frequencies of acoustic phonons of all branches in the range of the whole Brillouin zone. The damping of a soft transverse optical phonon in narrow-gap ferroelectric-semiconductors is also defined by the electron-phonon interaction. In other cases the main relaxation process for optical phonons in metals is the decay into two acoustic phonons due to lattice anharmonicity.     

Ultrasonically tunable filter for ψ-rays

A new type of an ultra-narrow frequency transmission filter for ψ-rays is presented. The principle of its operation is based on the Mössbauer-sideband-spectroscopy [1–3]: A resonant absorber onto a thin piezo-electric polymer foil vibrates piston like at an ultrasonic frequency ω. At certain definite amplitudes the zero-order-sideband, i.e. the transition energy ω₀, is fully transmitted without any resonant absorption, whereas simultanously sidebands displaced by energies ±hhΩ from ω₀ are resonantly absorbed. Using several absorbers of this type in series having different frequencies a broadband filter for ψ-rays with a well defined transmission-profile can be designed. Symmetrically, on both sides of the central transmission window there exist resonant absorption bands. The energy width of these different bands is only limited by the possible ultrasonic frequencies and can be adjusted from 10^?o to 10^?5 eV. First experimental results using such an assembly of absorbers are presented. The limit of the efficiency of the filter, caused by non-resonant scattering is discussed.     

Influence of the sum frequency on optically pumped parametric amplifiers

Parametric traveling-wave interactions are calculated with the aid of a plane wave approximation, considering the 4 frequencies ω _s , ω _p , ω _I =ω _p ?ω _s and ω _Σ =ω _p +ω _s . Special attention is paid to the case where ω _p ?ω _s . Competition between parametric amplification and upconversion is studied as a function of phase matching and the results are illustrated by means of numerical examples. It is shown that parametric gain disappears if the linear combination of wave vectors, 2k _p-k_I-k_Σ, vanishes. In this situation upconversion with power gain up to about (ω _Σ /ω _s )² is possible. It is concluded that fork _p?k_s the sum frequency ω _Σ can significantly influence parametric forward amplification but does not affect backward wave amplification.     

Nonlinear Raman scattering of photons by a channeled particle

Channeled particles are characterized by the discrete spectrum of bound transverse motion. The interaction of photons with channeled particles in single crystals can be accompanied by energy transitions between the levels of transverse motion of the channeled particle. The Raman scattering of photons at a quasibound channeled particle leads to the appearance of a combination of frequencies: the incident radiation frequency ω₀ and the frequency Δω_{m, n}, i.e., ω = ω₀ ± Δω_m,n where Δω_m,n = 2Δε_m,nγ²; Δε_{m, n} is the energy of the transition between quantum states (m and n) of the transverse motion of the channeled particle; and γ = E/mc² is the Lorentz factor of the channeled particle. The appearance of a violet satellite (the anti-Stokes component) in the Raman scattering spectrum is analyzed. The three-photon Raman-type transition, which is the process of the simultaneous absorption of two photons with the frequency ω₀ with the emission of a photon with the frequency ωs = 2ω₀ ± 2Δε_m,nγ², is considered. The conditions for resonance observation during the formation of the second harmonic (ω = 2ω₀) are discussed.     

Effect of thermal nonlinearity in high-absorption media on the parameters of the photoacoustic signal detected by the gas microphone method: The fundamental and second harmonics

A perturbation theory is put forward that describes the effect of thermal nonlinearity due to the temperature dependence of the thermophysical parameters of high-absorption systems with a low thermal conductivity on the parameters of the photoacoustic signal detected by the gas microphone technique. It is found that the dependence of the photoacoustic signal amplitude on incident beam intensity I ₀ stems from the dependence of the illuminated surface temperature on I ₀. This dependence is a complicated function instead of being a simple quadratic function as was expected. In the limiting cases (μ_sβ ? 1 and μ_sβ ? 1), this contribution to the photoacoustic signal amplitude is described by simple expressions, which are convenient for determining the thermal coefficients of the thermophysical parameters of the medium. It is found that the thermal nonlinearity significantly affects the photoacoustic signal phase in the frequency region meeting the condition μ_sβ ～ 1. In the above limiting cases, its effect is insignificant. A theory of generation of the photoacoustic signal second harmonic is proposed. The second harmonic is related to the temperature dependence of the thermophysical parameters of the buffer gas and sample. It is shown that the amplitude of the signal is a quadratic function of the incident beam intensity and varies with its frequency as ω^?3/2 for μ_sβ ? 1 and ω^?5/2 for μ_sβ ? 1.     

Change in the shape of a symmetric light pulse passing through a quantum well

A theory for the response of a 2D two-level system to irradiation by a symmetric light pulse is developed. Under certain conditions, such an electron system approximates an ideal solitary quantum well in a zero field or a strong magnetic field H perpendicular to the plane of the well. One of the energy levels is the ground state of the system, while the other is a discrete excited state with energy ?ω₀, which may be an exciton level for H=0 or any level in a strong magnetic field. It is assumed that the effect of other energy levels and the interaction of light with the lattice can be ignored. General formulas are derived for the time dependence of the dimensionless “coefficients” of the reflection ?(t), absorption A(t), and transmission ?(t) for a symmetric light pulse. It is shown that the ?(t), A(t), and ?(t) time dependences have singular points of three types. At points t ₀ of the first type, A(t ₀)=T(t ₀)=0 and total reflection takes place. It is shown that for γ_r?γ, where γ_r and γ are the radiative and nonradiative reciprocal lifetimes, respectively, for the upper energy level of the two-level system, the amplitude and shape of the transmitted pulse can change significantly under the resonance ω_l=ω₀. In the case of a long pulse, when γ_l<γ_r, the pulse is reflected almost completely. (The quantity γ_l characterizes the duration of the exciting pulse.) In the case of an intermediate pulse duration γ_l?γ_r, the reflection, absorption, and transmission are comparable in value and the shape of the transmitted pulse differs considerably from the shape of the exciting pulse: the transmitted pulse has two peaks due to the existence of the point t ₀ of total reflection, at which the transmission is zero. If the carrier frequency ω_l of light differs from the resonance frequency ω₀, the oscillating ?(t), A(t), and ?(t) time dependences are observed at the frequency Δω=ωl?ω₀. Oscillations can be observed most conveniently for Δω?γ_l. The position of the singular points of total absorption, reflection, and transparency is studied for the case when ω_l differs from the resonance frequency.     

Point-contact spectroscopy of electron relaxation mechanisms in semiconductors

The I–V characteristic of a tiny semiconducting channel connecting bulk electrodes is shown to have singularities arising due to phonon emission by hot electrons at energies eV = n?ω₀, where ω₀ is the optical phonon frequency and n = 1, 2, 3,…. The nonlinear part of the I–V curve provides direct information concerning the energy dependence of the elastic-scattering time of charge carriers.     

Dispersion of the linear electrooptic coefficient in ZnS

The measured values and the analysis of the dispersion of the unclamped linear electrooptic coefficient r^T₄₁ in cubic ZnS single crystals operating as optical modulator are presented. The spectral dispersion of the non-linear optical coefficient d₄₁(ω, ω, 0) is also reported and the weak dependence on the light frequency, observed for d₄₁, is discussed by taking into account the opposite sign of ionic and electronic contributions.     

Noninversive partial velocity amplification of radiation by ions due to their rotation in a magnetic field

It is shown that upon the application of an external magnetic field, a gas of ionized particles may experience noninversive partial velocity amplification of radiation by ions due to their Larmor rotation. In this case, virtually all ions may be in the ground state. It may happen that approximately half the number of ions in the medium amplify the incident radiation. The integrated absorption coefficient remains positive due to the enhancement of absorption of radiation by the other half of ions. Noninversive amplification of radiation takes place when the condition ω_c?Γ²/kv _T is satisfied ωw_c is the cyclotron frequency of ions in the magnetic field; Γ is the homogeneous half-width of the absorption line for ions, and kv _T is the Doppler width). In the case of interaction of atomic ions with radiation in the optical range, this corresponds to magnetic fields B?600 G (for the ion mass M～10 amu). Noninversive partial velocity amplification of radiation is a “latent” effect in the sense that it disappears upon averaging over all velocity directions of ions. This effect is associated with the emergence of phase incursion of the induced dipole moment oscillations for ions moving in circular cyclotron orbits, which depends on the ion velocity.     

Scattering of emitted radiation from inhomogeneous and nonisothermal layers

A parametric study is performed for the exiting monochromatic intensities scattered from finite, plane-parallel, inhomogeneous layers that are driven solely by a distribution of thermal sources. Intensities are obtained by invariantly imbedding the standard and thermal scattering functions. The single scattering albedo ω and the Henyey-Greenstein phase-function parameter g are varied independently, and both linear and exponential profiles are considered. Linear temperature profiles are used, including temperature inversions. The resulting intensities I(μ), μ representing the direction cosine of propagation, are discussed from a remote sensing point of view. For an isothermal and homogeneous medium, the gross characteristics of I(μ) represented by its overall slope I(0)/I(1), mean value (magnitude), and an interior maximum value can be related to the total optical depth t₀, ω, and g, respectively. For a homogeneous medium, linearly decreasing (in the line of sight) temperature profiles tend to obscure the g information and decrease the apparent optical depth. On the other hand, linearly increasing temperature profiles tend to retain g information and increase the apparent optical depth. Temperature inversion profiles give intensities very similar to those for purely linear profiles. Linear and exponential variations of both ω and g for constant temperatures give similar intensity fields. Results for a variation in g can be reproduced fairly well with an average g value. This cannot be done, however, for ω profiles.     

Surface effects on optical phonons and on phonon-plasmon modes

The size and shape dependence of the properties of long wave optical phonons in polar crystals is discussed. The main effect occurring due to the presence of surfaces is the appearance of a surface optical phonon band in the frequency region between the frequencies ω_t and ω_l of the long wave transverse and longitudinal bulk modes. The surface modes give rise to strong absorption peaks in the infrared absorption spectra of small samples. For very small crystallites the surface mode absorption is dominant, and as the size of the specimens is increased, the ratio of surface to bulk mode absorption decreases. It is shown that the large spread in particle sizes usually encountered in experimental work and the increased damping of the phonon modes in small samples both tend to obscure the fine structure of the absorption spectrum. The surface phonon-plasmon modes in polar crystals containing free carriers are treated in an analogous manner.     

Collisional damping of transverse waves in a plasma

The effect of collisions on transverse waves in a homogeneous, field free plasma is investigated by means of Gross-Krook collision model. The dispersion relation is calculated by assuming the collision frequency to be small andKλ _D ?1. The damping rate ω _I is obtained as $$\omega _I = \frac{{\nu _{ei} }}{2}\frac{{\omega _p^2 }}{{\omega _0^2 }}\left[ {1 + \frac{{3K^2 \lambda _D^2 \omega _p^2 }}{{\omega _0^2 }} - \frac{{K^2 \lambda _D^2 \omega _p^4 }}{{\omega _0^4 }}} \right] + \frac{{\nu _{ee} }}{2}\frac{{\omega _p^2 }}{{\omega _0^2 }}\left( {\frac{{K^2 \lambda _D^2 \omega _p^2 }}{{\omega _0^2 }}} \right)$$ where ω ₀ ² =c ² K ²+ω ₂ ^p , andv _ei andv _ee are electron-ion and electron-electron collision frequency respectively.     

Dispersive plasma-satellites of a dipole-forbidden transition and the rotating field model

A clear distinction is made between (a) the coupled transitions of optical photons and transverse plasmons in microwave experiments and (b) the dispersive plasma-satellites in a cable-pulsed, double-cathode glow discharge. The former involve a radiation-damped dipole interaction and are not necessarily accompanied by a central component. The latter conserve the sum of the angular momentum projections of spin-one plasmons and atoms in the sense that the ensemble-averaged electron-induced absorption probability at the position of the forbidden component is split into three components, separated by the plasma frequency ω_p. The far and near σ-satellites change the projection quantum number by one unit, while the central π-component does not, because of rotationally transforming the Schrödinger equation at angular velocities of ±ω_p in the atomic (x, y)-plane. The theory is compared with measured optical spectra.     

On the optical absorption due to particle-hole excitations in thin metal films

We present here a qualitative discussion on the optical absorption due to particle-hole excitations in thin metal films. We show that in sufficiently thin films, such excitations yield resonant absorption, when P-polarized light is obliquely incident on the metal surface. For instance, for frequency ω #62; ε_F where ε_F is the Fermi-energy, such resonances occur whenever ω satisfies the condition ω/ε_F = (1 + nπ/dq_F)² - 1, where n = 1,3,5,…,q_F is the Fermi wave-vector and d is the thickness of the film. The experimental observability of this effect is also discussed.