B. N. Jagatap

Engineering disorder in three-dimensional photonic crystals

Abstract We demonstrate the effect of introducing controlled disorder in self-assembled three-dimensional photonic crystals. Disorders are induced through controlling the self-assembling process using an electrolyte of specific concentrations. Structural characterization reveals increase in disorder with increase in concentrations of the electrolyte. Reflectivity and transmittance spectra are measured to probe the photonic stop gap at different levels of controlled disorder. With increase in disorder the stop gap is vanished and that results in a fully random photonic nanostructure where the diffuse scattered intensity reaches up to 100%. The estimated scattering mean free path shows significant reduction for photonic crystals with 100% controlled disorder as compared to those with 0% controlled disorder. Our random photonic nanostructure is unique in which all scatters have the same size and shape. Therefore, we observe the resonant characteristics in the multiple scattering of light.

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.

Magnetoassisted pump–probe spectroscopy of cesium atoms

We studied the spectral behavior of a probe transmission in the presence of a counterpropagating pump beam for various laser polarizations and magnetic fields with and without the assistance of an additional repopulation laser beam. These parametric studies enabled us to distinctively extricate the influence of saturation, optical pumping, and resonant light pressure. The enhanced probe absorption resulting from the pump-beam-induced Zeeman substate dressing was maximized when the change in the resonance frequency was suitably compensated by the change in the Doppler shift arising from resonant light pressure. The possibility of tuning the laser frequency locked to saturation absorption spectroscopy was investigated and implemented to a laser cooling and trapping setup.

Multiple Bragg diffraction at W point in the face centered cubic photonic crystals

Abstract. We report the experimental observation of multiple Bragg diffraction that occurs when the tip of the incident wave vector lies on a line joining the L and W points in the Brillouin zone of a face-centered cubic (FCC) photonic crystal. The multiple Bragg diffraction is analyzed for photonic crystals with different lattice constants and refractive index contrasts. Angle-dependent reflectance spectroscopy indicates strong hybridization of diffraction resonances when the tip of the incident wave vector crosses the W point and the multiple Bragg diffraction is seen to be extended over an angular range of 8 deg around the W point. We also observe a new diffraction resonance in the short-wavelength region for wave vectors shifting toward the W point in the hexagonal facet of the FCC Brillouin zone. Each diffraction resonance is fitted using the Bragg’s law for different planes in the FCC photonic crystal taking into account the internal angle between the planes. The diffraction resonances in the multiple Bragg diffraction regime are assigned to FCC crystal planes with Miller indices (111), (200), (1¯11), and (220). Our results have implications for diverse kinds of wave propagations in periodic structures and applications in light emission, sensing, and structural color pigments.

Interaction between dual cavity modes in a planar photonic microcavity

Abstract We theoretically study the interaction between dual cavity modes in a planar photonic microcavity structure in the optical communication wavelength range. The merging and splitting of cavity mode is analysed with realistic microcavity structures. The merging of dual cavity resonance into a single cavity resonance is achieved by changing the number of layers between the two cavities. The splitting of single cavity resonance into dual cavity resonance is obtained with an increase in the reflectivity of mirrors in the front and rear side of the microcavity structure. The threshold condition for the merging and splitting of cavity mode is established in terms of structural parameters. The physical origin of the merging of dual cavity modes into a single cavity resonance is discussed in terms of the electric field intensity distribution in the microcavity structure. The microcavity structure with dual cavity modes is useful for the generation of entangled photon pairs, for achieving the strong-coupling regime between exciton and photon and for high-resolution multi-wavelength filters in optical communication.

Manipulating light propagation and emission using photonic crystals

We discuss the synthesis and characterization of self-assembled photonic crystals using polymer colloids having sub-micron diameters. The angle resolved optical reflectivity measurements indicate the hybridization between stop gaps in the multiple Bragg diffraction regimes. Each diffraction resonances in the multiple Bragg diffraction regimes are assigned to respective crystal planes. We also discuss laser-induced studies of spontaneous emission in self-assembled photonic crystals having Rhodamine-B dye doped colloids. Our experimental results reveal more than 51% inhibition in emission intensity within the stop gap as compared to a proper reference sample.

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.

Signature of band-edge-induced lasing observed in self-assembled photonic crystals

We report experimental demonstration of band-edge-induced lasing from Rhodamine B-dyed self-assembled all-solid photonic crystals. We discuss the origin of lasing as due to the enhancement of density of states and field distribution at band edge frequencies.

Electromagnetically induced transparency in a Λ-type molecular system with permanent dipole moments revisited

Electromagnetically induced transparency (EIT) in a molecular three-level Λ system with permanent dipole moments and undergoing m- and n-photon transitions by pump and probe lasers is investigated. Analytical expressions are derived for probe absorption spectrum and dispersion for a medium of stationary as well as thermal molecules. Contrary to the earlier study by Zhou et al. [J. Chem. Phys. 131, 034105 (2009)], we observe no amplification in 2 + 2 photon process when the sign of the difference of the permanent moments of the excited and the ground levels is reversed. Reasons for these contrasting observations are discussed. Our study shows that the permanent moments essentially damp the laser-molecule Rabi frequency to result in narrower EIT line width and larger group velocity index. These effects are further enhanced when the order of the multi-photon process is increased. The importance of the virtual mechanism is discussed by considering the special case of 2 + 1 photon EIT.

Tunable Photonic Stop Gaps in Visible Wavelength Region

We report the experimental demonstration of tunable stop gap in the visible wavelength region by changing the angle of incidence of light. The width and peak reflectance of the stop gap do not change till an angle of incidence of 45°. For angle of incidence greater than 45°, new diffraction resonances appear in the reflectance spectra. At 55° incidence, the stop band split into to two peaks with equal reflected intensity. This multiple Bragg diffraction is in good agreement with the theoretical calculations.