As stated in the text, Figs. 3 and 4 (transmission versus probe laser detuning) were taken at an energy of 10 nJ in a 0.5-mm-diam beam. We should have noted that when the probe energy is increased to 150 n3 (— 10" W/cm ) the absorption cross section at the transmission maximum increases by a factor of approximately 2. At this energy density, with or without the coupling laser present, transmitted probe energy is no longer linear with incident probe energy.

Observation of Electromagnetically Induced Transparency in Collisionally Broadened Lead Vapor

Summary form only given. It is well-known that nonlinear interference effects in the resonant atom-light interaction can lead to interesting and important phenomena, such as electromagnetically induced transparency (EIT) of atomic medium, coherent population trapping, lasing without inversion, and others. Common to all these phenomena is the appearance of light-induced coherence between atomic levels, which are not coupled by dipole transitions. Recently Akulshin and co-workers (1998-9) have observed subnatural-width resonances in the absorption on the D/sub 2/ line of rubidium vapor under excitation by two copropagating optical waves with variable frequency offset. It is remarkable that, apart from expected EIT-resonances with negative sign, authors detected positive resonances termed as electromagnetically induced absorption (EIA). More recently, similar effects in a (3+5)-state system have been described as a result of constructive interference of the dipole excitation channels. We propose a simple theoretical model for EIA - a four-state N-atom. Remember that three-state /spl Lambda/- and V-systems give the EIT-resonances only.

Electromagnetically induced absorption in a four-state system

Many atmospheres (cool stars, brown dwarfs, giant planets, extrasolar planets) are predominately composed of molecular hydrogen and helium H216O is one of the best measured molecules in extrasolar planetary atmospheres to date and a major compound in the atmospheres of brown-dwarfs and oxygen-rich cool stars, yet the scope of experimental and theoretical studies on the pressure broadening of water vapour lines by collision with hydrogen and helium remains limited Theoretical H2- and He-broadening parameters of water vapour lines (rotational quantum number J up to 50) are obtained for temperatures in the range 300–2000 K Two approaches for calculation of line widths were used: (i) the averaged energy difference method and (ii) the empirical expression for J ′ J ″ -dependence Voigt profiles based on these widths and the BT2 line list are used to generate high resolution ( Δ ν ˜ = 001 cm − 1 ) pressure broadened cross sections for a fixed range of temperatures and pressures between 300 and 2000 K and 0001–10 bar An interpolation procedure which can be used to determine cross sections at intermediate temperature and pressure is described Pressure broadening parameters and cross sections are presented in new ExoMol format

Pressure-dependent water absorption cross sections for exoplanets and other atmospheres

The intermolecular interaction potential, taken as the sum of pair potentials which, in turn, were modeled by the Lennard–Jones potential has been determined by means of nonlinear least squares fitting its parameters to the experimental data on the lines broadening ( γ ) and shift ( δ ) coefficients. The data on γ and δ for nine vibration bands ν 1 , ν 3 , 2 ν 2 , ν 1 + ν 2 , ν 2 + ν 3 , 2 ν 1 , ν 1 + ν 3 , 2 ν 2 + ν 3 and ν 1 + ν 2 + ν 3 have been obtained from the analysis of the H 2 O–He absorption spectra, recorded from 3000 to 9000 cm −1 with help of IFS 125HR Fourier spectrometer at room temperature, spectral resolution of 0.01 cm −1 and in wide pressure range of He. Additionally we use literature data for rotational band (∼180 GHz) and ν 2 band (1850–2140 cm −1 ). The vibrational and rotational dependence of the potential parameters as well as the temperature dependence of the calculated lines broadening coefficients have been demonstrated.

Vibrational dependence of an intermolecular potential for H2O–He system

In a variety of astronomical objects including gas giant (exo-)planets, brown dwarfs and cool stars, molecular hydrogen and helium are the major line broadeners. However, there is currently no systematic source for these parameters, particularly at the elevated temperatures encountered in many of these objects. The ExoMol project provides comprehensive molecular line lists for exoplanet and other hot atmospheres. The ExoMol database has recently been extended to provide additional data including temperature-dependent, pressure-broadening parameters. Here we assemble H 2 and He pressure-broadening datasets for the molecules H 2 O, NH 3 , SO 2 , CH 4 , PH 3 , HCN and H 2 CO using available experimental and theoretical studies.

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The ExoMol pressure broadening diet: H 2 and He line-broadening parameters

On line shape of electromagnetically induced transparency in a multilevel system

Variations of the coherent resonance, i.e. an electromagnetically induced transparency (EIT) peak with the cell temperature in Λ- and V-type systems are observed for the 85Rb-D2 transition. In a Λ-type system, the amplitude of the EIT peak gradually decreases and it is finally masked by the broadened optical pumping dips with the increase of the cell temperature. But for the V-type system, the EIT and the saturation peaks are greatly enhanced with the increase of the cell temperature. The effect of the external magnetic field on the EIT resonance is also investigated considering both types of systems. For the Λ-type system in place of a single EIT resonance, five transparency windows with broadened width and reduced contrast are obtained depending on the Zeeman levels formed by the applied magnetic field. In the V-type system, the coherent resonance peak could not be resolved in the presence of the external magnetic field, but a greater number of saturation peaks with broadened width appear in the probe transmission spectrum. A theoretical model is adopted to represent the experimental results. Good agreement is found between the experimental and numerically simulated results.

Temperature and magnetic field effects on the coherent and saturating resonances in Λ- and V-type systems for the 85Rb-D2 transition

The line shape parameters of rovibrational transitions of water vapour belonging to the (2ν1 + ν2 + ν3) overtone band due to collisions between absorber molecules and noble gas helium have been measured in the spectral range between 11988.494 cm−1 and 12218.829 cm−1 using NIR diode laser spectrometer. In addition nitrogen and air broadening effects on some water vapour transitions belonging to the same band have also been studied. Wavelength modulation spectroscopy along with phase sensitive detection technique are used to record first derivative (1f) signal of buffer gas broadened water vapour transitions. Observed line shapes are fitted to standard Voigt profiles by non-linear least squares fitting program to extract the line shape parameters, like line strength and pressure broadening coefficients. The broadening effects induced by different types of buffer gases on water vapour line shapes are compared. Rotational quantum number (J) dependence of broadening coefficients of water vapour transitions is ...

Diode laser spectroscopy of He, N2 and air broadened water vapour transitions belonging to the (2ν1 + ν2 + ν3) overtone band

Atomic coherence resonances such as electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) signals are observed in a Doppler broadened multi-level system for the D2 line of 85Rb atoms in a vapour cell considering different types of atom–laser coupling schemes. Besides the coherence resonances, the nonlinear resonances like velocity selective optical pumping (VSOP) dips and velocity selective resonances (VSR) are also observed for the non-zero velocity selective groups of atoms. In the presence of pump, control and probe laser fields, we observe two EIT signals in a double Λ-type system and two EIA signals in a double V-type system. We are able to see the EIA signal for the non-zero velocity group of atoms when the pump laser is locked with an open transition by a small red detuning. We also report the experimental observation of simultaneous EIT and EIA signals in a (Λ+V)-type system. The effect of frequency tuning of the control laser is also studied in the presence of the frequency locked pump laser. A simple theoretical analysis explains the experimental results.

http://iopscience.iop.org/article/10.1088/0953-4075/44/11/115501/pdf

Nonlinear resonances caused by coherent, optical pumping and saturating effects in the presence of three laser fields for the 85Rb-D2 line

We report the experimental observation of electromagnetically induced transparency (EIT) in a Doppler broadened rubidium vapour at room temperature for different probe intensities at a fixed pump intensity in a five-level Λ-type system formed by the D2 transition of 85Rb. For a constant pump intensity, we find that the EIT width and height change with the variation of probe intensity. We observe a nonlinear variation of the height of the EIT peak and a linear variation of the width (FWHM) of the EIT signal with probe intensity. In the Doppler broadened multilevel system, we also observe the velocity selective dips along with the EIT signal. A numerical simulation of the probe response signal based on density matrix representation in a five-level system is carried out to reproduce the experimentally observed spectra.

M. M. Hossain

Papers

On line shape of electromagnetically induced transparency in a multilevel system

Temperature and magnetic field effects on the coherent and saturating resonances in Λ- and V-type systems for the 85Rb-D2 transition

Diode laser spectroscopy of He, N2 and air broadened water vapour transitions belonging to the (2ν1 + ν2 + ν3) overtone band

Nonlinear resonances caused by coherent, optical pumping and saturating effects in the presence of three laser fields for the 85Rb-D2 line

Study of width and height of EIT resonance in a Doppler broadened five-level system with varying probe power