Research Areas

Project #1 : Subwavelenth Imaging via Coherent Population Trapping (CPT)

Optical imaging of arbitrary images beyond the diffraction limit in an atomic waveguide

We have theoretically investigated the possibility of cloning of an arbitrary image carried on control field to another probe beam. Both the control and probe beams are coupled to a three-level atomic lambda system to form a CPT configuration. We assume the two laser fields to be of comparable strength, such that perturbation theory for the probe field is not valid any more to describe the effect of the atomic medium on both the fields. We start by calculating the susceptibilities including linear and nonlinear effects for both fields by solving the related density-matrix equations. We found that the refractive index for the probe field is modulated transversely by the spatial intensity profile of the control beam. In particular, the generated structures enable one to transfer the transverse distribution of the control field onto the transmission profile of the probe field. We begin our analysis with a Gaussian control and a super-Gaussian probe field and observe the gradual mapping of the control field onto the probe field throughout the propagation. We find in particular that in the case of a strong probe field, the transmitted probe beam is focused more tightly by a factor of two compared to the weak probe field case. Next, we consider a control field with a spatial two-peaked Hermite-Gaussian profile, and demonstrated cloning of the profile onto the probe beam with feature size reduced by a factor of about 2.5. In order to verify that our method can serve as an universal tool for cloning of arbitrary image, we finally simulate the three-dimensional light propagation for both fields, in which the spatial profile of the control field carries the three letters “CPT”. We show that also this structure can be cloned onto the probe beam which initially has a simple plane-wave profile, even though the control field is severely distorted throughout the propagation due to diffraction. The present work might be useful in optical imaging and quantum optical lithography techniques.

Project #3 : Active Raman Gain (ARG) Medium

Optical laser beam steering and splitting via antiwaveguiding in active Raman gain medium

we have proposed an efficient scheme for optical cloning, steering, and splitting of a laser beam in an antiwaveguiding configuration. We exploit an inhomogeneously broadened coherent Raman gain medium consisted of an N -type four-level 87Rb atoms. We include the effect of Doppler broadening in susceptibility of medium. As a result, the dispersion and gain spectrum becomes more steeper and narrower, respectively, than when compared with cold atomic system. This gives considerable increment in spatial resolution. The optical cloning, steering, and splitting of a beam are achieved via a strong nonlinear effect of spatial cross-phase modulation (XPM). In the process of XPM, the probe’s refractive index, n = 1 + 2πRe[χ], where Re[χ] is the real part of susceptibility of probe beam, increases with pump intensity for blue detuned probe and thereby modulates the phase of a weak probe beam during its propagation. First, we demonstrate optical cloning of a doughnut-shaped (Laguerre-Gaussian charge 3) of pump beam profile by a single Gaussian-shaped of probe beam centered initially at the core of the pump. The doughnut-shaped pump creates spatial modulation in probe’s refractive index which increases in annulus region and decreases in center region of pump. This structure resembles an antiwaveguide where core has lower refractive index than cladding in contrast to waveguide. Therefore, the probe beam is slowly guided out from core region to cladding region in course of propagation and transmitted with doughnut-shaped profile which is identical with pump beam. We next show that the gradient of index induced by a single Gaussian mode of pump beam can steer copropagating probe beam which has its intensity peak at a different position to that of pump beam. Finally, we show that a Gaussian mode of probe beam can be split into two Gaussian modes. The splitting occurs when probe is positioned initially at the center between two Gaussian modes of pump beam. The proposed scheme can be used in optical imaging, beam steering, and beam breaking techniques.

Project #4 : Saturation Absorption Spectroscopy (SAS)

All-Optical Imaging of Multimode Profiles in a Saturated Absorption Medium

We proposed an all-optical imaging technique based on saturated absorption spectroscopy in an atomic vapor. The atoms are driven by a counter-propagating strong pump laser on a transition which is probed by a comparatively weak probe laser. We show that the different orders of Hermite–Gaussian beam carried by pump laser can be efficiently imaged onto a Gaussian-shaped probe laser within a Rayleigh length. The feature size of transmitted probe beam is reduced by a factor 4 when compared to the original pump beam. To generalize the scheme, an arbitrary image imprinted initially on pump beam is successfully transferred onto the output probe beam. Such a method of high resolution multimode imaging may find potential applications in all-optical imaging and lithography technologies.