Nonlinear behaviour of atomic fluorescence in mercury vapours following double-resonance laser excitation

When mercury atoms, present with a buffer gas (N₂ or Ar) in a quartz cell or in a graphite furnace are excited into the 7³ S ₁ level by means of two pulsed dye lasers tuned at 253.652 nm (6¹ S _o 6³ P ₁) and 435.835 nm (6³ P ₁ 7³ S ₁), a laser-like, collimated emission is observed along the axis of the cell at the green mercury line (7³ S ₁ 6³ P ₂), 546.074 nm). This radiation exhibits a pronounced non-linear dependence upon the number density of the mercury atoms in the cell. This behaviour is interpreted as being due to amplified spontaneous emission (ASE), which occurs as a result of the transient population inversion between the 7s ³ S ₁ and 6p ³ P ₂ ^o levels. The parameters governing the population inversion can be deduced by applying a rate equation analysis to the atomic system. A time-resolved observation of both ASE and spontaneous fluorescence signals confirms the totally different characteristics of the two emission processes.     

Relativistic resonances in combined magnetic and laser fields

We propose a mechanism for experimental investigation of relativistic effects in the laser-atom interaction with moderate (nonrelativistic) laser intensities that involves placing the system in a static magnetic field parallel to the laser's magnetic-field component. The resonantly induced relativistic motion of the atomic electron leads to a variety of novel phenomena: a relativistic dephasing leading to a ringlike spatial probability density, a counterintuitive window of relativistically enhanced motion, and a sequence of sawtooth-shaped resonances that may increase the harmonic generation.     

Two-color laser modulation of magnetic Feshbach resonances

We investigate the two-color laser modulation of the magnetically induced Feshbach resonance. The two-color laser is nearly resonant with an optical bound-to-bound transition at the resonance position. The analytical formula of scattering length is obtained by solving the Heisenberg equation. The scattering length can be modified by changing the Rabi frequencies or optical field frequency. By choosing the suitable optical parameters, the two-body loss coefficient K2 can be greatly reduced compared to the usual single optical scheme.     

Optical control of magnetic Feshbach resonances in Bose gases

We theoretically investigate optical control of magnetic Feshbach resonance in Bose gases with two optical fields. The two optical fields couple two ground states through an excited state. Compared with the usual single-optical scheme, two optical fields can greatly suppress the inelastic loss resulting from spontaneous emission by the destructive quantum interference. Using the mean field theory, the analytical formula of the scattering length is obtained. The results show that the scattering length can be modified in a large range by changing the Rabi frequency or the optical field frequency. The strong atom–molecule interaction has obvious effect on the scattering length.     

Level mixing resonances on oriented nuclei

The level crossing and level mixing resonance methods have been developed in order to enable the measurement of quadrupole interaction frequencies of nuclei with lifetimes between microseconds and minutes in solids. Both methods are shown to be better suited for application 1n this lifetime region than the existing ones. Only the level mixing resonance method can be applied to nuclei with lifetimes up to several minutes. A review of our level crossing and level mixing experiments is also given in this paper.     

Optical relaxation properties of magnetic fluids under externally magnetic fields

The relaxation behavior (including the rising and falling relaxation processes) of the transmitted light after the magnetic fluid thin films under longitudinal and transverse magnetic fields is investigated, respectively. The physical mechanisms of the two different relaxation processes are discussed. The experimental data of the rising and falling relaxation processes are fitted by using two exponential functions to achieve the rising and falling response times. The relationship between the response time and the strength of applied magnetic field, the concentration of magnetic fluid is studied experimentally. The modulation depth of the transmitted light is researched quantificationally and the deepest modulation depth is obtained with Sample 3 (with volume fraction of 5.62%) in our experiments.     

Current effects on high magnetic field resonances in a dot array

In recent studies of disordered quantum dot arrays, it was found that a re-entrant metal–insulator transition occurs in these structures as the gate voltage is varied, even though there is no change in carrier density or disorder. Here, we study resonances that appear with the quantum Hall effect in these dot arrays. Several resonances are observed on the side of the plateaus, which may be due either to charging of isolated islands or to transmission resonances in the quantum point contacts. Heating of the carriers causes these peaks to decrease with an energy relaxation time comparable to that of the dots themselves.     

Studies of vibrational relaxation in CDF3 following intense laser excitation

The V-T/R relaxation time of CDF₃ was measured studying the laser-induced infrared fluorescence emitted by vibrationally excited CDF₃. Following excitation by the 10R(12) line of a TEA CO₂ laser infrared fluorescence has been detected without spectral resolution in the 1100–700 cm^–1 range. A decay rate of 28.8 ms^–1 Torr^–1 was obtained for pure CDF₃ when it is excited with a fluence of 390 mJ/cm². Measurements have also been made in the presence of different bath gases (He, Ne, Ar, Xe, and CHF₃).     

The excitation of acoustic resonances by vortex shedding

A single acoustic resonance, excited by vortex shedding from the trailing edge of a flat plate, has been studied in detail. The resonance was generated in the working section of a low-speed wind tunnel and the scale was large enough for accurate measurements to be made. The trailing edge was designed so that it could be made to oscillate slightly to simulate the correlating effect of the resonance on the vortex shedding.     

Optical nonlinearities in GaSe and InSe crystals upon laser excitation

The nonlinear absorption of light and its temporal evolution in the vicinity of exciton resonance in layered GaSe and InSe crystals under high optical excitation have been experimentally investigated. The decisive factor for the observed temporal dependence of the absorption coefficient and its dependence on the excitation intensity is screening excitons by nonequilibrium-carrier plasma. It is shown that the increase in the transmittance in the absorption-band edge in GaSe with a simultaneous blue shift of the band edge is caused by filling the energy bands under high optical excitation.     

Spin wave excitation in magnetic semiconductors in two laser fields

The magnon scattering by free carriers in the simultaneous presence of two laser fields is considered. A kinetic equation for the magnon population is derived. It is foubd that spin waves propagating parallel to the direction of polarization of the fields may be amplified over a relatively narrow band of magnon wavenumbers.     

Intermolecular vibrational relaxation upon multiphoton excitation of triplet molecules by a CO2 laser

Intermolecular vibrational relaxation is studied in mixtures of polyatomic molecules (benzophenone and fluorene) with bath gases after multiphoton excitation of the triplet molecules by CO₂ laser radiation. The dependences of the decay rate and the intensity of laser-induced delayed fluorescence on the laser energy density E _CO2 and pressure P _fg of bath gases are analyzed. They are found to be different for the fast and slow components of delayed fluorescence, which decays nonexponentially. It is shown that a change in the decay rate of the fast fluorescence component with increasing pressure P _fg is governed by the properties of vibration-translation relaxation. The efficiency β of this process is estimated in a broad range of vibrational energies. It is found that β weakly changes with increasing E _vib upon excitation of molecules to high vibrational levels. The features of intermolecular vibrational relaxation at high densities of anharmonically coupled vibrational states are discussed.     

Quadrupole relaxation in nitrogen nuclear resonances

The ¹⁴N resonance line-widths have been measured for a wide range of nitrogen compounds. The variation of line-width with viscosity of MeCn, EtCN, PrCN and Me₂NCHO in various solvents is discussed in terms of the theories of molecular correlation times. The values of the line-widths in substituted nitrobenzenes are discussed in terms of the quadrupole coupling constants. The relative line-widths in the azide ion are discussed in terms of its electronic structure.     

Phase-matched excitation of whispering-gallery-mode resonances by a fiber taper

We show that high-Q whispering-gallery modes in fused-silica microspheres can be efficiently excited by an optical fiber taper. By adjusting the taper diameter to match the ropagation constant of the mode in the taper with that of the resonant mode of interest, one can couple more than 90% of the light into the sphere. This represents a significant improvement in excitation efficiency compared with other methods and is, we believe, the most efficient excitation of a high- Q microcavity resonance by a monomode optical fiber yet demonstrated.     

Higher order level mixing resonances on oriented nuclei

Higher order level mixing resonance have been measured on the 40 s half-life ^109mAg in Zn. It is shown that the line-width is tunable with the misalignment angle of the electric to magnetic axes, and enables precise quadruple coupling determinations. For ${}^{\mathrm{<mtext>109</mtext><mtext>m</mtext>}}\text{Ag}\text{Zn}\text{eQV}{}_{\mathrm{zz}}\text{/μ = 3.08(2)}$ T has established.     

Optical size resonances in nanostructures

The existence of optical size resonances in atomic nanostructures is proved. The properties of optical size resonances strongly depend on the interatomic distances and on the polarization of an external radiation field. The properties of linear and nonlinear size resonances are considered in the case of two-dimensional nanostructures. The linear optical size resonances are described based on a closed system of equations for dipole oscillators and nonlocal field equations taking into account the dipole-dipole interactions of atoms in the radiation field. Using a stationary solution to these equations, it is demonstrated that two isotropic atoms with definite intrinsic frequencies form an anisotropic system in the radiation field, possessing two or four size resonances depending on whether the component atoms are identical or different. The nanostructure composed of two different atoms possesses two size resonances with positive dispersion and two other resonances with negative dispersion. The frequencies of the size resonances significantly differ from the intrinsic frequencies of isolated atoms entering into the nanostructure. By changing the angle of incidence of the external wave, it is possible to excite various size resonances. The properties of nonlinear optical size resonances excited by an intense radiation field were theoretically and numerically studied using the modified Bloch equations and nonlocal field equations. Dispersion relationships for the nonlinear resonances were derived and the inversion properties of atoms in the nanostructure were studied for various polarizations of the external optical wave.     

Optical transmission resonances tuned by external static magnetic field in an n-doped semiconductor grating with subwavelength slits

Transmission behavior of light through a grating consisting of n-doped semiconductor with subwavelength slits under the application of external static magnetic fields is investigated. As dielectric constant of n-doped semiconductor can be substantially altered by applied magnetic field, in the Voigt configuration and for TM-polarized illumination, two transmission resonance peaks associated with localized waveguide modes of slits are significantly shifted toward the lower frequency regime with the increase of the applied magnetic field. These characteristics can be assigned to a reduction of effective plasma frequency of n-doped semiconductor under the applied magnetic field. Our findings may provide possibility for achieving tunable transmission resonance spectrum.     

Pionic excitation of giant resonances in Y

Inelastic scattering of π⁺ and π⁻ exhibits strongly excited giant resonance structures. Besides the giant quadrupole resonance centered around 14 MeV, structures are observed at 16.8 MeV which can be fitted with a mixing of L=0 and L=1, and around 25 MeV (E_x = 110A^−1/3).