D. Slavov

Sub-Doppler spectroscopy of cesium vapor layers with nanometric and micrometric thickness

The saturation behavior of absorption and fluorescence spectra on the D2 line of Cs is presented, demonstrating a significant difference between open and closed transitions. Cs vapor is confined in an extremely thin cell (ETC) with widely tunable thickness L=(0.5-3)λ, where λ is the light wavelength. In the saturation regime, the closed transition demonstrates enhanced absorption in a narrow spectral interval due to the Dicke effect, while the open one demonstrates only a velocity-selective dip in the absorption. The fluorescence of open transitions shows reduced fluorescence dips, enhancing their contrast with ETC thickness. The closed transition exhibits only a small plateau around the optical transition center. Applying two-level theoretical modeling based on optical Bloch equations, a qualitative agreement with experimental observations is achieved. The rate of contrast enhancement with cell thickness is larger for the theoretical than for the experimental dips. In addition, for the closed transition a tiny peak in the fluorescence is theoretically predicted, with the first experimental confirmation presented. The sub-Doppler spectra of vapor layers with a thickness of several light wavelengths show potential for realization of precise frequency references and photonics sensors.

Improved passive self-injection locking method for spectral control of dye and Ti:Al2O3 lasers using two-step pulse pumping

Abstract An improved passive self-injection locking (PSIL) method for the spectral control of dye and Ti:Al 2 O 3 lasers using two-step pulse laser pumping is reported (theoretically, experimentally). By this pump technique, the typical unwanted free lasing which occurs in the non-selective cavity at high pump level is reduced by a few orders of magnitude compared with the standard PSIL application. This combined approach ensures high purity tunable emission at laser efficiency nearly equal to the maximum obtained in an optimized non-selective cavity and avoids optical damage problems. This makes the PSIL method competitive or preferable to other frequently used spectrally selective methods. Analysis and experimental check for two-wavelength lasers are included.

Ground-state magneto-optical resonances in cesium vapor confined in an extremely thin cell

Experimental and theoretical studies are presented related to the ground-state magneto-optical resonance observed in cesium vapor confined in an extremely thin cell (ETC), with thickness equal to the wavelength of the irradiating light. It is shown that utilization of the ETC allows one to examine the formation of a magneto-optical resonance on the individual hyperfine transitions, thus distinguishing processes resulting in dark (reduced absorption) or bright (enhanced absorption) resonance formation. We report experimental evidence of bright magneto-optical resonance sign reversal in Cs atoms confined in an ETC. A theoretical model is proposed based on the optical Bloch equations that involves the elastic interaction processes of atoms in the ETC with its walls, resulting in depolarization of the Cs excited state, which is polarized by the exciting radiation. This depolarization leads to the sign reversal of the bright resonance. Using the proposed model, the magneto-optical resonance amplitude and width as a function of laser power are calculated and compared with the experimental ones. The numerical results are in good agreement with those of experiment.

Sub-natural width N-type resonance in cesium atomic vapour: splitting in magnetic fields

The sub-natural width N-type resonance in a Λ-system, on the D2 line of Cs atoms is studied for the first time in the presence of a buffer gas (neon) and the radiations of two continuous narrow-band diode lasers. A L = 1 cm long cell is used to investigate the N-type process. The N-type resonance in a magnetic field for 133Cs atoms is shown to split into seven or eight components, depending on the magnetic field and laser radiation directions. The results obtained indicate that the levels Fg = 3, 4 are the initial and final ones in the N-resonance formation. The experimental results with magnetic field agree well with theoretical descriptions.

Output control of a ring laser using bi-directional injection : a new approach for unidirectional pulse generation at a reference atomic absorption line

Spatial and spectral control of a ring laser with a homogeneously broadened active medium has been achieved using a bi-directional injection scheme. Such a type of injection provides a new technique to obtain a controlled pulsed laser emission spectrally locked to a selected atomic absorption line. The proposed technique makes use of the strong competition between counterpropagating waves in the ring cavity. The output control relies on the unbalance between the two injected beam intensities produced by the absorption. When the injected light is scanned around the absorption line, an unidirectional light generation occurs only at the wavelength of the desired absorption line. Due to the non-linear character of the process, the emitted spectrum can be an order of magnitude narrower than the absorption line. Both computer simulation and experimental study of the phenomena are reported for a pulsed dye laser pumped with the second harmonic of a Q-switched Nd:YAG laser.

Magneto-optical harmonic susceptometry of superparamagnetic materials

We describe a technique to optically characterize superparamagnetism. Faraday rotation measurements are performed on a superparamagnetic nanocomposite using small alternating current magnetic fields. The superparamagnetism of the iron oxide nanoparticles causes signals at the uneven harmonics of the magnetic field frequency. These signals provide information on the magnetic moment of the superparamagnetic nanoparticles. Dia- and paramagnetism do not cause signals at higher harmonics, resulting in a high sensitivity to superparamagnetism, even in samples with large dia- or paramagnetic contributions. This technique provides a rapid, economical method to characterize superparamagnetism in composite samples not easily accessible by other techniques.

Observation and theoretical simulation of electromagnetically induced transparency and enhanced velocity selective optical pumping in cesium vapour in a micrometric thickness optical cell

Observation of electromagnetically induced transparency (EIT) and enhanced velocity selective optical pumping (VSOP) signals in a micrometric cell with cesium is reported. The line shape and non-linear features observed in the case of fluorescence in the direction parallel to the cell windows and the transmission spectra observed along the propagation direction of the probe beam show considerable differences in the spectral profile. A theoretical model based on five level optical Bloch equations is used to simulate the spectra. The Doppler convolution includes all possible orientations of atomic velocities with respect to the laser beam direction. Atoms moving nearly parallel to the windows and perpendicular to the collinear pump and probe beams have much lower Doppler shift and hence produce considerable narrowing of the Doppler background in the fluorescence spectra. The coherence decay rate is also low for such atoms as they do not meet with the cell walls. The simulated curves reproduce the observed sharp EIT peaks and enhanced broad VSOP signals for the closed probe transition in the fluorescence and absorption spectra. The observed effect of detuning of the pump frequency on the non-linear features is also reproduced by the simulation.

Frequency stability comparison of diode lasers locked to Doppler and sub-Doppler resonances

We present a systematic quantitative comparison of the performance parameters of two Extended Cavity Diode Lasers (ECDL), stabilized to Doppler and sub-Doppler profiles. The experimental study is carried out on Rb atomic vapour cells. The frequency shift of the resonance under investigation with respect to a reference resonance is measured and analyzed, in dependence on the modulation amplitude, cell temperature, laser power, applied magnetic field and misalignment of the pump-probe beams angle. The Allan variance is measured for both locking methods. It shows that the stabilization to the Doppler profile results in about one order of magnitude lower stability than the one to SA resonances for the short-term measurement. However, the stability achieved using both methods is sufficient for many applications and for some of them the simplicity of the Doppler locking method can be advantageous.

Reducing light-shift effects in optically-pumped gas-cell atomic frequency standards

We investigate a novel method to suppress the light-shift effect in laser-pumped passive rubidium frequency standards. The reduction of the light-shift due to additional frequency components in the pumping light is studied theoretically and compared to experimental results obtained with a frequency-modulated DBR laser diode as a pumping light source. In this way, a reduction of the light shift by a factor of 55 was successfully demonstrated. Complete cancellation of the light shift can be reached with a suitably adjusted modulation index and frequency.

Self-alignment and conductivity of a glow discharge

A magnetically induced resonance of the conductivity is analysed within the frames of self-aligned atomic states in a hollow cathode discharge. The resonance ΔU(B0) arises when the magnetic induction applied B0 destroys the self-alignment. The value of ΔU(B0) characterizes the difference in the conductivity of both self-aligned and non-aligned atomic ensembles. Generally, all self-aligned atomic states contribute to the resonance measured. This study is an attempt to specify the contribution of NeI levels 1s5 and 1s4 to the resonance ΔU(B0). Selective light depopulation of these levels is accomplished by using irradiating λ = 640.2 nm and λ = 638.3 nm.