Ayan Ray

Study of the effect of repump laser on atomic line filter

The improvement in the performance of an atomic line filter (ALF) under the action of a repump laser is reported in the current experimental work. We address the issue of repumping by including an additional laser coupling the 5S1/2(F=1)→5P3/2(F′′=2,1,0) hyperfine levels whereas the optical filter action is exhibited through a pump–probe laser-induced excited state absorption {5S1/2(F=2)→5P3/2(F′=3)→5D3/2(F′′=3)} of the Rb87 atom. It is found that the application of the repump mechanism considerably influences the characterizing parameters (i.e., transmittance and width) of the ALF. To optimize the performance of the ALF, it is required to carefully choose the detuning and intensity of the repump laser for a fixed set of pump–probe combination. For this purpose, the effect of systematic change in detuning and intensity of the repump laser on the ALF signal is studied in detail. For example, it is experimentally found that ALF considerably benefits (∼30%) in transmittance from selective repumping of atoms. It is to be noted that, unlike earlier reports, where the frequency scale of the filter is calibrated with Faby–Perot etalons of comparatively larger free spectral range, the marking is done here with the help of double resonance optical pumping (DROP) signals. The DROP signals, which originate from two-photon coupling within the 5S→5P→5D hyperfine domain, also act as an indicator of the existing “Radiation Trapping” process in the cascade medium. The current study may help in improving the performance of narrow-bandwidth ALF, which is useful for free space optical communication systems and laser spectroscopy.

Coherent pump–probe spectroscopy of a Λ system with a close lying excited level

We investigate coherent pump–probe spectroscopy of a medium of atoms in Λ configuration with a closely placed adjacent excited level. The presence of the additional level results in two simultaneous Λ resonances excited by the same pair of pump and probe fields. A master equation framework is used to discuss the probe absorption spectrum and dispersion in the absence/presence of inhomogeneous broadening, and the results are illustrated by using a model four-level system in the D2 transition of 85Rb. The system is observed to exhibit additional interference effects arising from two competing Λ resonances. These effects manifest in terms of linewidths of the triplet dressed state spectrum and suppression of the subnatural peak for the stationary atoms. The effect on electromagnetically induced transparency is discussed in the light of its shape, position and linewidth. The discussion is further augmented by the study of a six-level model as applicable to the D2 transition of 85Rb. The analysis presented here provides a realistic theoretical description of the pump–probe spectroscopy of hyperfine transitions of alkali atoms.

Coherent spectroscopy of a Λ atomic system and its prospective application to tunable frequency offset locking

We investigate the coherent pump?probe spectroscopy of a three-level ? system, , in the hyperfine manifold of D2 transition (852?nm) of cesium with particular reference to the sub-Doppler linewidth resonance arising from Aulter?Townes (AT) splitting and the possibility of using it for realizing a scheme for tunable atomic frequency offset locking (AFOL). We discuss here the theoretical framework for a ? system interacting with a coherent pump and probe and use it to describe the process of modulation transfer in the AT and electromagnetically induced transparency regimes. We further employ an experimental scheme consisting of a strong pump and a pair of weak probes to resolve the sub-Doppler linewidth (~8?MHz) AT resonance and study its dependence on pump intensity and detuning. In order to explore the possibility of using such a sub-Doppler linewidth resonance for AFOL, we use its first derivative signal as a frequency discriminator to stabilize the probe laser. The frequency stability of the probe is characterized by means of error signal analysis. This study reveals that while the frequency stability of the AT locked laser is limited by the pump laser, the tuning range of the offset frequency lock can cover the entire Doppler profile and its immediate neighbourhood, thereby providing a simple and cost effective alternative to the external modulator. The study described in this paper contributes to the discussion on the subtle link between dressed state spectroscopy and AFOL, which is relevant for developing a master?slave-type laser system in the domain of coherent photon?atom interaction.

Non-contact control of two-photon absorption

In this experimental work, we address the issue of the control of two-photon absorption in a Doppler broadened medium of Rb85 atoms under the ladder level coupling 5S1/2→5P3/2→5D3/2. Here, suppression and enhancement of two-photon absorption are executed by applying an auxiliary laser beam, which is not physically propagating in the same line of the pump-probe combination. Instead, the auxiliary beam, which is in resonance with the Rb85 D2 transition, is at a finite distance from the line of propagation of the pump-probe combination. The resonant auxiliary beam produces an atomic beam in the vapor cell, and when it reaches the pump-probe path, it creates an asymmetry in the population of ground state hyperfine components. This asymmetry is responsible for controlling two-photon absorption. We present here results obtained by us along with a qualitative explanation of the physical principles behind the control mechanism.

Lower Order and Higher Order Entanglement in Four-Wave Mixing Process

Possibilities of generation of lower order and higher order intermodal entanglement in 87Rb 5S − 5P−5D hyperfine manifoldare rigorously investigated using the Sen-Mandal perturba ive technique by showing the equivalence of the system with the four-wave mix ing (FWM) process. The investigation has revealed that for a set of experimentally realizable par ameters we can observe lower order and higher order intermodal entanglement between pump and sign al modes and signal and idler modes in a FWM process associated with the 87Rb 5S − 5P − 5D hyperfine manifold. In addition, trimodal entanglement involving pump, signal and idler modes is also rep rted.

Modulation Transfer Through Coherence and Its Application to Atomic Frequency Offset Locking

We discuss the process of modulation transfer in a coherently prepared three-level atomic medium and its prospective application to atomic frequency offset locking (AFOL). The issue of modulation transfer through coherence is treated in the framework of temporal evolution of dressed atomic system with externally superimposed deterministic flow. This dynamical description of the atom-field system offers distinctive advantage of using a single modulation source to dither passively the coherent phenomenon as probed by an independent laser system under pump-probe configuration. Modulation transfer is demonstrated experimentally using frequency modulation spectroscopy on a subnatural linewidth electromagnetically induced transparency (EIT) and a sub-Doppler linewidth Autler-Townes (AT) resonance in Doppler broadened alkali vapor medium, and AFOL is realized by stabilizing the probe laser on the first/third derivative signals. The stability of AFOL is discussed in terms of the frequency noise power spectral density and Allan variance. Analysis of AFOL schemes is carried out at the backdrop of closed loop active frequency control in a conventional master-slave scheme to point out the contrasting behavior of AFOL schemes based on EIT and AT resonances. This work adds up to the discussion on the subtle link between dressed state spectroscopy and AFOL, which is relevant for developing a master-slave type laser system in the domain of coherent photon-atom interaction.