4 resultados para P plus Resonant
em CaltechTHESIS
Resumo:
<p>The resonant nuclear reaction
Resumo:
<p>This thesis describes the theoretical solution and experimental verification of
phase conjugation via nondegenerate four-wave mixing in resonant media. The
theoretical work models the resonant medium as a two-level atomic system with the
lower state of the system being the ground state of the atom. Working initially with
an ensemble of stationary atoms, the density matrix equations are solved by third-order
perturbation theory in the presence of the four applied electro-magnetic
fields which are assumed to be nearly resonant with the atomic transition. Two of
the applied fields are assumed to be non-depleted counterpropagating pump waves
while the third wave is an incident signal wave. The fourth wave is the phase conjugate
wave which is generated by the interaction of the three previous waves with the
nonlinear medium. The solution of the density matrix equations gives the local
polarization of the atom. The polarization is used in Maxwell's equations as a source
term to solve for the propagation and generation of the signal wave and phase conjugate
wave through the nonlinear medium. Studying the dependence of the phase
conjugate signal on the various parameters such as frequency, we show how an
ultrahigh-Q isotropically sensitive optical filter can be constructed using the phase
conjugation process.p>
<p>In many cases the pump waves may saturate the resonant medium so we also
present another solution to the density matrix equations which is correct to all orders
in the amplitude of the pump waves since the third-order solution is correct only
to first-order in each of the field amplitudes. In the saturated regime, we predict
several new phenomena associated with degenerate four-wave mixing and also
describe the ac Stark effect and how it modifies the frequency response of the filtering
process. We also show how a narrow bandwidth optical filter with an efficiency
greater than unity can be constructed.p>
<p>In many atomic systems the atoms are moving at significant velocities such that
the Doppler linewidth of the system is larger than the homogeneous linewidth. The
latter linewidth dominates the response of the ensemble of stationary atoms. To
better understand this case the density matrix equations are solved to third-order
by perturbation theory for an atom of velocity v. The solution for the polarization is
then integrated over the velocity distribution of the macroscopic system which is
assumed to be a gaussian distribution of velocities since that is an excellent model
of many real systems. Using the Doppler broadened system, we explain how a tunable
optical filter can be constructed whose bandwidth is limited by the homogeneous
linewidth of the atom while the tuning range of the filter extends over the entire
Doppler profile.p>
<p>Since it is a resonant system, sodium vapor is used as the nonlinear medium in
our experiments. The relevant properties of sodium are discussed in great detail.
In particular, the wavefunctions of the 3S and 3P states are analyzed and a discussion
of how the 3S-3P transition models a two-level system is given.p>
<p>Using sodium as the nonlinear medium we demonstrate an ultrahigh-Q optical
filter using phase conjugation via nondegenerate four-wave mixing as the filtering
process. The filter has a FWHM bandwidth of 41 MHz and a maximum efficiency of
4 x 10
Resumo:
<p>Valence fluctuations of Fe
Resumo:
<p>Since the discovery in 1962 of laser action in semiconductor diodes made from GaAs, the study of spontaneous and stimulated light emission from semiconductors has become an exciting new field of semiconductor physics and quantum electronics combined. Included in the limited number of direct-gap semiconductor materials suitable for laser action are the members of the lead salt family, i.e . PbS, PbSe and PbTe. The material used for the experiments described herein is PbTe . The semiconductor PbTe is a narrow band- gap material (Eg = 0.19 electron volt at a temperature of 4.2°K). Therefore, the radiative recombination of electron-hole pairs between the conduction and valence bands produces photons whose wavelength is in the infrared (λ ≈ 6.5 microns in air).p>
<p>The p-n junction diode is a convenient device in which the spontaneous and stimulated emission of light can be achieved via current flow in the forward-bias direction. Consequently, the experimental devices consist of a group of PbTe p-n junction diodes made from p –type single crystal bulk material. The p - n junctions were formed by an n-type vapor- phase diffusion perpendicular to the (100) plane, with a junction depth of approximately 75 microns. Opposite ends of the diode structure were cleaved to give parallel reflectors, thereby forming the Fabry-Perot cavity needed for a laser oscillator. Since the emission of light originates from the recombination of injected current carriers, the nature of the radiation depends on the injection mechanism.p>
<p>The total intensity of the light emitted from the PbTe diodes was observed over a current range of three to four orders of magnitude. At the low current levels, the light intensity data were correlated with data obtained on the electrical characteristics of the diodes. In the low current region (region A), the light intensity, current-voltage and capacitance-voltage data are consistent with the model for photon-assisted tunneling. As the current is increased, the light intensity data indicate the occurrence of a change in the current injection mechanism from photon-assisted tunneling (region A) to thermionic emission (region B). With the further increase of the injection level, the photon-field due to light emission in the diode builds up to the point where stimulated emission (oscillation) occurs. The threshold current at which oscillation begins marks the beginning of a region (region C) where the total light intensity increases very rapidly with the increase in current. This rapid increase in intensity is accompanied by an increase in the number of narrow-band oscillating modes. As the photon density in the cavity continues to increase with the injection level, the intensity gradually enters a region of linear dependence on current (region D), i.e. a region of constant (differential) quantum efficiency.p>
<p>Data obtained from measurements of the stimulated-mode light-intensity profile and the far-field diffraction pattern (both in the direction perpendicular to the junction-plane) indicate that the active region of high gain (i.e. the region where a population inversion exists) extends to approximately a diffusion length on both sides of the junction. The data also indicate that the confinement of the oscillating modes within the diode cavity is due to a variation in the real part of the dielectric constant, caused by the gain in the medium. A value of τ ≈ 10