Pump Photon Energy Research Articles

Room temperature photopumped ZnSe lasers

Room-temperature laser oscillation at 469 nm observed in photopumped bulk ZnSe is discussed. The result was obtained by using a pump photon energy very close to the bandgap energy and by enhancing the resonator Q with high-reflectivity coatings. Threshold pump power densities as low as 370 kW/cm/sup 2/ were measured.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nonequilibrium spin polarization of F centers in KBr and KI under saturated optical pumping with modulated circular polarization.

It has been known that, under the saturated optical pumping of an F center in the presence of an applied magnetic field (H), the nonequilibrium spin polarization (NESP) increases in the relaxed excited state (RES) of the F center. The NESP is relevant to a spin-lattice relaxation time ${T}_{1}^{\mathrm{*}}$ in the spin system in the RES. Under conditions in which the circular polarization of the pumping light is modulated between right and left circular polarization with an angular frequency \ensuremath{\omega}, we postulate that the NESP oscillates sinusoidally with \ensuremath{\omega} and gives rise to the magnetic circular dichroism in the absorption, ${\mathrm{scrS}}_{\mathrm{MCD}(\mathrm{\ensuremath{\omega}}}$), and the magnetic circular polarization in the emission, ${\ensuremath{\Delta}}_{\ensuremath{\eta}}$(\ensuremath{\omega}), where \ensuremath{\eta} shows the polarization of the analyzer, either circular or linear. Theoretical forms of both quantities have been derived from the first-order approximation to successive solutions of rate equations. ${\ensuremath{\Delta}}_{\ensuremath{\eta}}$(\ensuremath{\omega}) for KBr and KI have been measured at 2 K as a function of H below 40 kOe and as a function of pumping photon energy. Our present data for ${\ensuremath{\Delta}}_{\ensuremath{\eta}}$(\ensuremath{\omega}) and ${\mathrm{scrS}}_{\mathrm{MCD}(\mathrm{\ensuremath{\omega}}}$), observed by Baldacchini et al. [Phys. Rev. B 19, 1283 (1979)], are both explained in the same theoretical framework by introducing a common empirical form of H dependence of (${T}_{1}^{\mathrm{*}}$${)}^{\mathrm{\ensuremath{-}}1}$, which is presented as proportional to ${H}^{\ensuremath{\alpha}}$, where \ensuremath{\alpha} is a negative constant. This implies that the spin-lattice relaxation is caused by the exchange effects between two F centers in the RES. We have optically detected the electron-spin resonance (ODESR) of F centers in the RES from the changes observed in ${\ensuremath{\Delta}}_{\ensuremath{\eta}}$(\ensuremath{\omega}) by the resonant microwave at 50 GHz. The ODESR has been accounted for within the same theoretical framework.

The Spin-Mixing Parameters in the Optical Pumping Cycle of the F Centers in KBr and KI

The spin-mixing parameter e is the probability that a spin is flipped into the opposite spin level in the optical processes in the presence of a magnetic field. In the Franck-Condon state, the mean and anisotropic parameters e 0 and δe have been calculated as a function of pumping photon energy E ph , on a theoretical framework in which the spin-orbit and electron-lattice interactions with Γ 1 + -, Γ 3 + -, and Γ 5 + -modes are included. The effect of the Γ 5 + -mode is predominant. By using e 0 and δe calculated, and introducing a mixing parameter e r1 in the non-radiative relaxation process, the experimentally-determined E ph -dependence of the net spin-mixing parameter e t and that of δe(1-2e r1 ) have been explained consistently. The e ph -dependence of fractional dichroic differential pumping rate has been calculated.

Spin mixing processes of the optically pumped F center via the study of magnetic circular polarization of luminescence

The isotropic and anisotropic spin-mixing parameters of ∈ o and δ∈ in the Franck-Condon state of an F center are theoretically calculated as a function of pumping photon energy E ph. On the other hand, the E ph dependence of the net spin-mixing parameter ∈ t and δ∈(1–2∈ rl) for KI, where ∈ rl is the spin-mixing parameter in the nonradiative relaxation process of pumping F center, are determined from the analysis of the magnetic circular polarization of F center emission observed. These experimental dependences are explained using the theoretical plots of ∈ o and δ∈ as well as assuming the empirical E ph dependence of ∈ rl.

Threshold analysis of cleaved-coupled-cavity lasers

We present results of a cleaved-coupled-cavity laser analysis which, in contrast to previous studies, incorporates all the following: (1) a model of the gain variation with pumping current and photon energy; (2) the refractive index dependence on the active region injection current; and (3) traveling wave equations for each laser segment. Based on this realistic formulation, we solve for the precise current required by one segment to attain threshold when the current in the other segment is specified for various gap spacings between the segments, and show that device behavior is very sensitive to that dimension. In addition, we calculate quantitatively the spectral tuning and previously unreported jumps from one extreme of the gain spectrum to the opposite extreme in particular current ranges.

Theory and operation of the IBr-photodissociation Br laser

IBr as a potential candidate for solar-powered lasers is discussed. Theory and operation of a Br-atom laser obtained by photodissociation of IBr are described. Computed Br* : Br ratios are predicted to increase with pump-photon energy — in good agreement with experiment. Lasing peak power of up to 50 kW at an efficiency of 0.03% with input electrical energy as low as 90 J was demonstrated, and effects of pressure and input energy, on the laser pulse and onset were investigated.

Relaxation Process of Excitonic Molecules in CuCl under the Two-Photon Resonant Excitation. II. Transverse Relaxation

Under the off-resonant excitation at the slightly higher energy side of the giant two-photon absorption band (GTA) for the direct generation of excitonic molecules (EM) in CuCl, new narrow emission bands designated as X T and X L have been found in the energy region of the M T and M L broad bands, respectively. Their photon energies increase with the decrease of the pump photon energy from the higher-energy side of the resonance and finally, at the on-resonant excitation, their bands merge into M T 0 and M L 0 bands, respectively, previously reported by Mita et al. Based on the detailed studies on these bands, it is found that there exists a certain kind of transverse relaxation process which acts on the EM just after their generation by the GTA and brings about the X emission as a hot luminescence.

Optical detection of electron spin resonances via magnetic circular polarization of F-center emission

Abstract A new method for optical detection of electron spin resonance ESR for both ground and relaxed excited states of F center has been tested for KBr, KI and RbCl using microwave at 49.8 GHz via detecting the microwave-induced change in magnetic circular polarization of F center emission. The g-values and half-widths are tabulated. The dependence of ESR signal intensities for KBr on microwave power, laser power, and pumping photon energy are measured and explained in terms of forms derived from the solutions of rate equations for optical pumping cycle proposed.

Magnetic Nature of the Relaxed Excited State of F Centers and Spin Flipping Processes in the Optical Pumping Cycle

The magnetic nature of the relaxed excited state (RES) of F centers and spin-flipping processes in the optical pumping cycle have been clarified by studying the magnetic circular polarization (MCP) of emission when the pumping light is modulated between right and left circular polarization at the frequency of (ω/2π). Paramagnetic component Δ p (ω) of MCP is found to be proportional to the spin polarization P * (ω) in the RES with a proportionality constant F p . From the analysis of the dependence of Δ p (ω) at particular frequencies on the pumping photon energy E p h , F p is estimated. Thus, the E p h -dependence of the net spin-mixing parameter e t is determined. The values of F p and e t are found to be smaller than those determined previously. The source of discrepancy is discussed. It is suggested that the spin-mixing in the non-radiative transition process is not neglected.

Magnetic Circular Polarization of F Center Emission and the Spin Mixing Processes in Optical Cycle

Magnetic circular polarization (MCP) of F centar emission has been measured for KCl, KBr, KI, RbCl, and RbBr. The optical pumping cycle is proposed to include three spin mixing parameters in absoption, radiationless decay ( e r1 ), and emission processes. From the analysis of experimental data using formulae derived from rate equations for the cycle, para- and dia-magnetic components ( Δ p and Δ d ) of MCP, and the spin mixing parameters are determined. Pumping photon energy dependence of Δ p are observed. It shows that e r1 for KBr depends on photon energy and predominates the net spin mixing parameter ( e t ). Temperature dependence of Δ d are shown to be relevant to the spin orbit interaction in the relaxed excited state of F center. The source of the discrepancy found between our determined values of e t and those by other schools is clarified.

Cooperative fluorescence by excitonic resonant pumping in PbI 2 and HgI 2

The behaviour of the cooperative emission from PbI 2 and HgI 2 at 80 K has been studied at high exciton density by means of a tunable high intensity dye laser pumping. From a comparative analysis of the observed M and S stimulated emission lines from PbI 2 and HgI 2 we find that they strongly depend in two different ways on the pumping photon energy. Then, it is also possible to conclude that exciton-exciton and exciton-electron inelastic scattering are responsible for stimulated emission in PbI 2 and HgI 2 respectively.

Stimulated two-photon emission from excitonic molecules in ZnO

The M-band emission in ZnO at 1.7 K is investigated by tuning the excitation light through the A-B exciton region. Externally stimulated two-photon emission from excitonic molecules is observed when the pump photon energy is resonant with the upper B-polariton. The experiments suggest two excitonic molecule levels separated by 4.6 meV and with a ground state energy E M = 6.7394 eV.

Pump wavelength dependence of hot electron temperature in GaAs

We have measured the variation of the temperature of the photoexcited hot electrons in GaAs as the pump photon energy is varies from 1.54 to 2.83 eV using various dyes in N 2 laser pumped dye laser. We find that the photon flux required to obtain a given temperature (45°K) increases by a factor of 15 as the photon energy decreases from 2.83 to 1.54 eV. The results are in agreement with a simple theory, and allow us to draw interesting conclusions about energy relaxation processes in semiconductors.

Method of virtual sources in the underwater-acoustical description of high-frequency sound fields in a wedge : Kuznetsov, V.K. 18 (October–December 1972) 223

The results of transient transmission studies utilizing femtosecond laser pulses on hydrogenated amorphous silicon alloys are presented. In these studies, both the pump and probe photon energies are tuned through the exponential band tail region. The responses of the different alloys are similar, but the technique is able to clearly distinguish between them based on their alloy composition and bandgap. It is shown that the results can be modeled as a free-carrier absorption spike followed by a residual plateau caused by both recombination and intraband heating. This model is adapted to work in the band tail region.

Optically pumped room-temperature GaAs lasers

Room-temperature pulsed operation of GaAs cleaved platelet lasers bulk optically pumped near the band gap by a tunable parametric oscillator has been studied as a function of several variables. Both n-type high-purity GaAs ( 1.6 \times 10^{14} cm-3 \leq N_{d} + N_{a} \leq 1.6 \times 10^{16} cm-3) and n-type Si-doped GaAs ( N_{d} - N_{a} \simeq 2-5 \times 10^{18} cm-3) have been investigated. The peak of the laser emission at threshold for all samples occurred in the range from 0.88 to 0.89 μm, but the total width of laser emission in the Si-doped samples was significantly broader than in the pure samples. As the pump photon energy E p decreased, the threshold power necessary for lasing increased. This is ascribed to the increasing penetration of the pump radiation and the consequent decrease in excess carrier density, and to the lower degree of population inversion as the pump photon energy approaches the threshold quasi-Fermi level separation necessary for stimulated emission. For the high-purity samples, the lasing behavior did not depend heavily on impurity concentration and was more strongly a function of surface and cleaved-facet quality of individual samples. Typical threshold power densities of 1.75 W/mil (corresponding to approximately 7 \times 10^{5} W/cm2) were measured and at an incident power level of twice threshold, peak power conversion efficiencies up to 4.5 percent were obtained. Measurement of a typical far-field angular mode pattern showed a full-width half-maximum power (FWHM) transverse angle of about 10°, corresponding to an emitting depth perpendicular to the pump face \sim5 \mu m. At higher excitation, stimulated emission was observed over a width of 150 A. Measurements on the Si-doped material showed marked differences from the high-purity n-type samples. At the lower pump energies used on the high-purity samples, the GaAs:Si samples were more transparent, and consequently, higher pump energies had to be used to obtain lasing. Thresholds comparable to those in the pure GaAs were measured, and lasing occurred at roughly the same wavelength as before. However, at a pump power twice threshold, stimulated emission occurred over an increased range from 0.88 to 0.91 μm, a width of 300 A, and the measured FWHM angle decreased to 6°. Peak power conversion efficiencies up to 11 percent have been obtained. This material seems to be quite promising for obtaining efficient bulk optical pumping, with the advantages of lower optical flux densities at the resonator faces and better spatial mode quality.