Optical Pumping Conditions Research Articles

Difference in the IR absorption of thermal donors in n-Si and p-Si

The low-temperature thermal donors in silicon are currently an object of investigation; however, their nature has not been reliably established. The presence of such thermal donors in silicon with concentrations N /SUB d/ 10 U cm T results in the appearance of specific bands in the IR absorption spectrum. These bands are attributed to the photoexcitation of electrons found in the levels of the thermal donors. However, according to current theories, such absorption is possible in samples of p-Si only under the conditions of additional optical pumping, which provides for the nonequilibrium filling of the energy levels of the thermal donors. This paper is devoted to a systematic investigation of the absorption by thermal donors in n-Si and p-Si subjected to isothermal heat treatment at a temperature of 450C for various time periods. In particular an attempt is made to evaluate the validity of the interpretation of the absorption by thermal donors in p-Si as proposed by Pajot et. al.

Conditions for excited-state Raman and absorption processes during optical pumping

A theoretical analysis is presented for conditions of optical pumping of an electronic excited state and the subsequent induction of Raman scattering thereof, using the radiation of a tunable laser with a specific λ for pumping and Raman probing.The analysis shows that the number of excited states created by the optical pump can be estimated as a function of the photon flux from the (non-linear) behaviour of the transmission of the sample at the selected laser wavelength. It is shown that such transmission studies are a prerequisite before attempts are made to record any excited level Raman spectrum.

Carrier diffusion in semiconductors subject to large gradients of excited carrier density

Under conditions of intense optical pumping or electrical injection it is possible to excite such a dense plasma of electrons and holes that these carriers interact among themselves much more rapidly than they interact with the lattice phonons. In this circumstance there will be established a temperature of excited carriers, ${T}_{e}$, much larger than the temperature of the lattice, ${T}_{L}$, for periods of time sufficient for many effects to be observed. In the accompanying paper it is shown that the temperature dependence of the forbidden band gap is then described by ${\ensuremath{\Delta}E}_{\mathrm{cv}}({T}_{e},{T}_{L})={\ensuremath{\Delta}H}_{\mathrm{cv}}({T}_{L})\ensuremath{-}{T}_{e}{\ensuremath{\Delta}S}_{\mathrm{cv}}({T}_{L})$. With ${T}_{e}\ensuremath{\gg}{T}_{L}$, this means that anomalously large changes in the band gap will occur and, if also there are large gradients of ${T}_{L}$ present in the sample, there will be very large gradients in the band gap. This means there will be very large gradients in the chemical potentials of the excited electrons and holes. As the band gap tends to be least where the carriers are most dense and ${T}_{L}$ the greatest, these chemical potential gradients oppose the normal outward diffusion of the excited carriers. We find that, under certain conditions, this effect may be sufficient to produce negative diffusivity or carrier self-confinement.

Variation of semiconductor band gaps with lattice temperature and with carrier temperature when these are not equal

Under conditions of intense optical pumping or electrical injection it is possible to establish a temperature of excited carriers, ${T}_{e}$, larger than the temperature of the lattice, ${T}_{L}$, for periods of time sufficient for many effects to be observed. It is well known that semiconductor band gaps are a function of temperature, but the variation with the two temperatures, ${T}_{e}$ and ${T}_{L}$, when these are different seems not to have been discussed previously. Simple thermodynamic arguments may be applied when it is recognized that a band gap is a chemical potential. The simple formula, ${\ensuremath{\Delta}E}_{\mathrm{cv}}({T}_{e},{T}_{L})={\ensuremath{\Delta}H}_{\mathrm{cv}}({T}_{L})\ensuremath{-}{T}_{e}{\ensuremath{\Delta}S}_{\mathrm{cv}}({T}_{L})$, is deduced. Physically this formula states that the vibronic degeneracy of the electronic states (valence and conduction band or bonding and antibonding) among which the carriers are distributed with characteristic temperature ${T}_{e}$ is determined by the lattice temperature, ${T}_{L}$. Thus when ${T}_{e}>>{T}_{L}$, anamalously large variations in the gap will occur. It is found that under certain conditions loss of energy from the carrier system to the lattice will cause the density of excited carriers to increase, rather than decrease.

Possibility of obtaining efficient cw stimulated emission from optically pumped lasers utilizing self-terminating transitions

The possibility of obtaining cw stimulated emission from lasers utilizing self-terminating transitions (for example, Cu and Mn vapor lasers) under conditions of optical pumping of the resonance levels of the active element is demonstrated theoretically. It is shown that optical pumping permits the familiar methods of depopulating the metastable levels of the active gas to be utilized more efficiently. The pumping efficiency of this laser system is close to the quantum efficiency and may reach several tens of percent. It is shown that a gain higher than ~10–3 cm–1 and a laser radiation output power of the order of 100 W/m of the active medium may be achieved in these laser systems at a pump power density of ~0.3 W/cm2.

Optical detection of electron spin resonance in CdTe

We report the optical detection of electron spin resonance in p-type CdTe at 1.7 K in optical pumping conditions. The Overhauser shift of the electronic resonance, of the order of 45 G, is related to the sign of the electron g-factor g ∗. We measure g ∗ = -1.59±0.02 . Using this g ∗ value and the previous results on the Knight shift, we deduce the value of the electron wavefunction on Cd in CdTe, which is consistent with the value in CdS.

Saturation of absorption of the powerful radiation of a system of anharmonic oscillators

Recently, the theory of nonequilibrium systems simulated by a set of anharmonic oscillators has received significant development. The investigation of such kinds of systems is especially important in the study of problems associated with the stimulation of chemical reactions and the development of effective molecular lasers. The systematic analysis of the kinetics of anharmonic oscillators assumes the simultaneous solution of a large number of nonlinear equations describing the population balance of the vibrational levels. Realization of this approach is associated with cumbersome numerical calculations and does not permit obtaining a qualitative picture of the behavior of the system as a function of the different parameters (pressure, temperature, etc.). An approximate analytical theory has been formulated in [1, 2] which permits finding the distribution function over the vibrational states with the effects of anharmonicity taken into account. We will employ the approach developed in these papers to describe a system of anharmonic oscillators under conditions of powerful optical pumping. This problem was discussed in [3], where it was found that such a system changes into a saturation mode in the case of high pumping levels. The existence of this mode is explained by the fact that the maximum rate of energy input into a vibrational degree of freedom is determined by the rate of distribution of this energy over all the vibrational levels, i.e., by the constant of V—V-exchange. For sufficiently large pumpings the approximation of the Boltzmann distribution function adopted in [3] in connection with the calculation of the saturation parameters is too crude. The goal of this paper is to derive in explicit form expressions for the vibrational energy supply, the absorbed power, and so on, under saturation conditions without the use of the approximation indicated above [3].

Distributed-feedback semiconductor lasers operating in higher Bragg interaction orders

An investigation was made of the threshold and spatial characteristics of the radiation emitted from distributed-feedback semiconductor lasers operating in high even orders of the interaction under optical and electron pumping conditions. Distributed-feedback lasers with low thresholds, operating in high interaction orders, were consturcted.

Nonlinear and parametric effects in atomic rf spectroscopy

Nonlinear phenomena occurring during interaction between an atom and a strong rf of resonant or nonresonant frequency are reviewed. The account is given in both semiclassical and consistently quantummechanical languages. The concept of an atom dressed by the radiation field in which not only the optical-frequency but also the radio-frequency is quantized is presented in detail. The results are summarized of theoretical and experimental studies of multiphoton transitions, radiative shifts of normal magnetic resonance lines, parametric resonance, and coherent resonances under the conditions of optical pumping. The influence of nonresonance rf fields on magnetic properties and the rate of magnetic relaxation of atoms is discussed.

Dielectric response of saturated semiconductors

AbstractThe optical properties of a direct‐gap semiconductor are investigated under conditions of a strong optical pumping by an intense laser source of frequency slightly larger than the energy band gap. By taking into account the Coulomb interaction between electrons and holes we found that the energy gap in the quasi‐particle spectrum diminishes. The dielectric constant of the system is calculated in the generalized Hartree‐Fock approximation. The Raman cross‐section is found to vanish for energies below 2Δ, where Δ is the quasi‐particle energy gap.

Low field electron-nuclear spin coupling in gallium arsenide under optical pumping conditions

In a semiconductor, absorption of circularly polarized light (optical pumping) leads to spin-oriented photoelectrons. In this situation, the nuclei of the crystal are dynamically polarized through their hyperfine interaction with the electronic spins. Consequently the electrons experience a hyperfine magnetic field due to the oriented nuclei which may reach several kilogauss. When this large nuclear field is driven obliquely with respect to the direction of the exciting light, the precession of the electronic spins around the nuclear field leads to a decrease of the electronic polarization along the light excitation: it is a nuclear Hanle effect. This work is an experimental and theoretical study of these effects in weak external magnetic fields, of the order of the local field which characterizes the nuclear spin-spin interactions (a few gauss). Large nuclear fields are obtained at 77\ifmmode^\circ\else\textdegree\fi{}K in strongly doped and compensated $p$-type GaAs samples. We present a model which includes the different effects of the hyperfine coupling when there is a nuclear spin temperature among all the nuclei of the sample: Dynamic polarization, nuclear field, but also, existence of an electronic field acting on the nuclei. We show that a small external field is able to drive the large nuclear field acting on the electrons; consequently the electronic polarization is very sensitive to external fields which are too small to have a direct effect on the electronic spin motion. We study experimentally the variation of the electronic polarization with the direction and magnitude of a small external magnetic field, by measuring the circular polarization of the luminescence light. The experimental results are in quantitative agreement with the theoretical predictions. The usual Hanle Lorentzian depolarization curve is strongly modified in low fields and $W$-like singularities appear around zero field. The experimental values of the average electronic and nuclear fields are in reasonable agreement with theoretical evaluations. These nuclear effects may strongly alter the measurement of the Hanle linewidth in standard optical pumping experiments.

Laser emission from the XeO molecule under optical pumping conditions

Laser emission was obtained from a mixture of xenon with N2O pumped with radiation of a high-current discharge. The laser action was attributed to the XeO(1S)≈XeO(1D) transition at λ≈0.54 μ. The gain achieved was ~5×10–4 cm–1 and the specific output energy was ~10–5 J/cm3.

Microwave Optical Double Resonance Spectroscopy of Metal Oxides

AbstractContinuous wave lasers have been employed in gas phase microwave optical double resonance (MODR) experiments. MODR spectroscopy combines the high resolution of microwave spectroscopy with the high sensitivity to transient molecules of optical spectroscopy. Detectability of MODR effects under weak (linear) and strong (nonlinear) optical pumping conditions is discussed. Experiments are described in which the 496.5 nm line of an Ar ion laser and a tunable, cw dye laser are used to observe fourteen microwave rotational transitions in the X1Σ (ν = 0,1) and A1Σ (ν = 0‐5) states of 138Ba16O and one transition in the A1Σ (ν = 1) state of 137Ba16O. Partially deperturbed rotational constants for BaO A1Σ are B (ν) = 0.25832(2)‐0.001070(5) (ν + 1/2) cm−1. A description is given of the cw dye laser and of the procedure for tuning the laser to coincide with any desired rotational line of the BaO A1Σ‐X1Σ transition. A wide range of metal oxides, halides and hydrides have been produced under conditions suitable for MODR spectroscopy.

Thermally Induced Focusing in a Nd:YAG Laser Rod at Low Input Powers

Thermally induced focusing in a neodymium-doped yttrium aluminum garnet rod was measured for pulsed optical pumping conditions with low average pump powers (400 W or less). A transient defocusing and a quasi-steady-state equilibrium with positive focusing are identified. The equilibrium induced focusing is shown to depend only on the average pump power for pulse rates above about 5 Hz. The quantitative dependence of the equilibrium induced focusing power of the rod on average pump power was measured, and effects of this induced focusing on the parameters of a laser resonant cavity containing the rod were investigated. A technique for accurately measuring focal powers of the order of 0.02 diopter in systems with f-numbers as high as 8000 is described.

Verification of the Kramers–Kronig relations in optically pumped ruby

The real and imaginary parts of the complex index of refraction have been measured in the R1 line of optically pumped ruby. The experiments, which employed a temperature-controlled, single-mode ruby laser as a variable-frequency monochromatic source, were performed near liquid-nitrogen temperature, where the R1 doublet structure is well resolved. R1 absorption profiles were measured for several optical-pumping conditions, ranging from zero up to a pump power that excited a population in the 2E state equal to about 60% of the population of the 4A2 ground state. For each pumping condition, the anomalous-dispersion curve was measured interferometrically. The spectral profiles of the absorption coefficient and the anomalous dispersion display a dependence on the optical-pumping conditions that is more complicated than simple saturation of the R1 transitions, but for given pumping conditions the two quantities are correlated as predicted by the Kramers–Kronig relations.

Optical Pumping of Molecules

A technique is described for the optical pumping of gas-phase molecules based on the orientation dependence of their absorption cross section. It is shown that when the rate of excitation exceeds the relaxation rate of a ground-state (υ″, J″) level, the magnetic sublevels are preferentially “burned away” and the steady-state M-level population is altered appreciably. An experiment to demonstrate molecular alignment has proved successful using the 4880-Å line of a cw argon-ion laser to cause the transition (υ″ = 3, J″ = 43)→(υ′ = 6, J′ = 43) in the B 1Σu − X 1Σg+ blue-green band system of the Na2 molecule. The establishment of an unequal magnetic sublevel population is detected by monitoring the degree of polarization P of the fluorescence as a function of laser intensity. It is found that P first remains constant then decreases by about 50% as the laser-beam power increases from 0.1 to 150 mW. In addition, the absorption becomes nonlinear with a power threshold corresponding to the onset of the decrease in P. This optical pumping technique is applicable to a wide class of molecules, and both simple classical and quantum mechanical theories of the pumping process have been formulated. These theories relate the degree of polarization to both the pumping rate and the relaxation time of the pumped level. The time-dependent as well as steady-state behavior of P is examined. Under conditions of strong optical pumping it is predicted that the degree of polarization as a function of time will overshoot its steady-state value, and as the pumping rate increases large negative transients in P will appear.

Should resonance curves in optical pumping be Doppler-broadened?

Existing theories of optical pumping do not take into account the relative phase of the optical field which monitors the resonances. In experiments on excited atoms the optical phase relations have a profound effect on magnetic resonance curves. Signals in the forward-scattered light show Doppler broadening, modified by strong coherence narrowing; a result quite different from that for the laterally scattered light. The question is raised as to whether similar effects are to be expected for resonances in the ground states. The problem is analysed in terms of the classical theory of the propagation of light in polarizable media. The (complex) polarizability of the sample under conditions of optical pumping and magnetic resonance is calculated to first order in perturbation theory. An expression is obtained similar to that found for resonance in the excited states, but different in so far as the factor which describes the magnetic resonance (the density matrix for the ground states) is independent of the factor which describes the Doppler effect (the plasma dispersion function). Since the intensity of the transmitted light is a function of the imaginary part of the polarizability, the conclusion is that the ground-state resonance curves should not be Doppler-broadened, which is in agreement with the known facts.