Dr. Sankar De

Research

ATOMIC MOLECULAR AND OPTICAL PHYSICS, QUANTUM OPTICS, LIGHT - MATTER INTERACTIONS (1)   Electromagnetically Induced Phenomena with Rubidium atoms           Laboratory of Light - Matter Interactions at SINP performs spectroscopic studies on neutral Rubidium (Rb) atoms using external cavity diode lasers (ECDLs) operating at 780 and 795 nm. Over the years, we had successfully operated and characterized a number of commercially available ECDLs (grating feedback, cat-eye with interference filter, tapered amplifier) and distributed feedback (DFB) lasers. We perform Doppler-free saturation absorption spectroscopy (SAS) with Rb atoms and actively stabilize the ECDL’s frequencies by locking it to particular peaks of the hyperfine Rb-D2 and D1 transitions. We had setup multiple optical beam paths for pump-probe spectroscopy with room temperature, hot and cold atoms.           (a) Study of Electromagnetically Induced Transparency (EIT) and Electromagnetically Induced Absorption (EIA) in room temperature Rb atoms had been our major research area till now. We observed EIT spectrum for V and L-type multi-level systems with 85Rb and 87Rb atoms. To simulate our experimental results theoretically, we have built the density matrix equations for our system from Liouville’s equation. Interaction of coherent electromagnetic field with the atoms induces coherences in the hyperfine levels of the atoms. We have experimentally observed the simultaneous formation of the EIT and the EIA in a multi-level V-type system in 87Rb -D2 transition. We have simulated the observed spectra theoretically using a multi-mode approach for the coherence terms which enable us to study all the frequency contribution of the pump and the probe with clarity. Since we can precisely tune our system and render the medium opaque and transparent simultaneously, it can be useful in optical switching applications.           (b) Laguerre-Gaussian (LG) beams (optical Vortex beam) are characterized by a doughnut-shaped intensity distribution with a phase singularity of the electric field, and hence zero field amplitude, at the center. Spiral phase plates generate LG beams by directly imposing the vortex structure on an incident beam by linearly varying the optical path length around the circumference of the device. In our first attempt, we tried to understand the effects of the LG beam on the Doppler and hyperfine line shapes. We observed narrowing of the line shapes when we used higher orders of the LG beam in comparison to the Gaussian beam. We plan to extend our investigation with the Vortex beam in the transfer of orbital angular momentum (OAM) from the photon to the atom.           (c) We observed the variation of group velocity of light in a coherently prepared atomic medium. The absorption profile of an atomic transition is modified when the upper level is coherently coupled to one of the ground levels with a strong laser field. Under appropriate condition, the atomic medium becomes effectively transparent for a weak probe field giving rise to EIT. The reduced absorption of light gives narrow linewidth and as a result of this we get a steep dispersion. This steep dispersion gives rise to reduction in group velocity of pulsed and continuous wave laser beams. We experimentally investigated the absorptive and dispersive properties of the 87Rb in the D2 transition using room temperature Rb vapour cell by homodyne detection technique.           (d) We further observed subluminal light propagation in a V-type closed system under the EIT condition in the hyperfine levels of 85Rb atoms. The phase coherency between the pump and the probe laser beams was maintained by a single laser. Pulse delay and group velocity dispersion characteristics of the input pulse were studied with the variation of the pump Rabi frequency taking temperature as a parameter. It is observed that pump Rabi frequency or the intensity for which the maximum pulse delay is observed, is a function of temperature. To support our experimental observations, we have formulated an analytical expression for a three level V-type system considering thermal averaging.           (e) Interplay between EIT, EIA and Autler-Townes (AT) splitting have been observed in an N-type closed configuration atomic system. All the phenomena have been observed by tuning the coupling Rabi frequency only. The EIA was observed in the crossover between the EIT and the AT due to the interference contribution. In order to understand the experimental observation, a partial dressed state calculation has been done along with the density matrix calculation. The population dynamics and the coherence contribution in each case have been analyzed by the time dependent solutions. The experimentally observed steady line-shape profiles have been supported by the steady state solution of optical Bloch equations considering the Maxwell-Boltzmann velocity distributions of the atoms.           (f) We studied polarization rotation in EIT (PREIT) in Rb vapour at room temperature in the absence of any magnetic field. PREIT is observed for both 85Rb and 87Rb combining both D1 and D2 transitions with a V-type configuration. For 85Rb, two EIT peaks and for 87Rb, one EIT peak were observed. Nonlinear characteristics of the variations of the angle of rotation corresponding to the EIT position were observed for both the isotopes of Rb. We have modelled a four level system to explain the EIT occurrence and the rotation of plane of polarization. A sharp rotation with a high signal to noise ratio in the vicinity of the EIT can be used as a modulation free and noise insensitive signal for the frequency stabilization.   (2)  Developing methods towards controlled localization and storage of light pulses in a magneto-optical trap (MOT) of cold Rubidium atoms           Optical cooling and trapping of neutral atoms is the most effective and elegant experimental method in today’s atomic physics. We are developing a MOT of Rubidium atoms in SINP, where the combination of six ‘off resonant’ laser beams of opposite helicity polarizations and the splitting of the energy levels due to the anti-Helmholtz magnetic field creates a position dependent force which pushes the atoms towards the center, thereby cooling and trapping a group of Rb atoms both in the velocity space and in the position space to achieve few mK temperatures. We like to study coherent optical interactions with cold Rb atoms for deeper understanding of EIT phenomena in different types of V, Λ and N-type systems.             Moreover, EIT makes the medium highly dispersive to a probe beam, thereby affecting the propagation of the probe pulse through the medium. Our aim is to work towards developing methods for controlled localization (slowing down and stopping) and storage of light pulses in cold atomic vapours. Its potential application will be in all-optical quantum memory devices. One of our future plans is to guide cold atoms from a MOT inside a hollow core photonic crystal fiber (HCPCF). Having the fiber as the atomic guide, information can be stored in the form of stationary light pulses in a tight confinement of atoms and radiation fields over a long interaction length, thereby resulting in an increased probability of single-photon – single-atom type of interactions.   (3) Dissociation of polyatomic molecules due to low energy highly charged ion impact           Determination of the structures and bond rearrangements of multiply charged molecules lead to a wealth of knowledge about the nature of interatomic interactions within molecules. Highly charged ions (HCI)-molecule interaction is a unique probe to study molecular dynamics on time scales in which all internal degrees of freedom remain essentially ‘frozen’. A proposal to dissociate simple polyatomic molecules (C2H2 and C2I2) using low and high energy highly charged ions and study their breakup dynamics by measuring the ion fragments with time-of-flight (TOF) multi-hit imaging setup had been initiated in collaboration with Dr. C. P. Safvan of Inter University Accelerator Centre (IUAC), New Delhi and Prof. Haruo Shiromaru of Tokyo Metropolitan University (TMU), Japan. The first experiments were conducted in TMU under JSPS short-term invitation program and later they were repeated with low energy Ar8+ projectile ions at IUAC.