R. Krishna Mohan

Transient phase conjugation in dye-doped polymer saturable absorbers

Abstract Dye-doped polymer saturable absorbers are an important class of nonlinear optical materials where nonlinear interactions like four-wave mixing and two-wave mixing can be observed even at low laser intensities. The modulation of the nonlinear absorption coefficient of the dye medium by an appropriate interaction of laser radiation can be utilized to generate a large phase conjugate (PC) signal in degenerate four-wave mixing geometries. By probing this population grating at time scales less than the dye relaxation times very high transient PC signals can be observed. Here we report on the generation of transient PC signals using azo dye-doped polymer films, by delaying the arrival time of the backward propagating pump beam with respect to the grating writing beams. A theoretical analysis of this delayed four-wave interaction is given to explain the large transient gain in the phase conjugate intensity. Experimental observations on the magnitude of the transient gain along with its dependence on various parameters like the intensity of the interacting beams, the number density of the saturable absorbers, the read beam time delay, etc. are also presented.

Optical phase conjugation by two-wave mixing in iron-doped lithium niobate

A novel technique to generate forward phase conjugate wave by two-wave mixing (TWM) in photorefractive iron-doped lithium niobate crystal has been demonstrated. An optical beam from a positive transparency was forward phase conjugated by TWM technique. The experimental scheme was then extended to a specific interferometric application.

Experimental observations of transient phase conjugate waves in photorefractive bismuth silicon oxide crystals

The formation of transmission or reflection gratings is experimentally identified in the

Interferometric schemes using forward phase conjugation in a ferroelectric photorefractive crystal

Bi_{12}SiO_{20}

Self-pumped phase-conjugate interferometer with a photorefractive iron-doped lithium-niobate crystal

crystal. Transient phase conjugate waves with intensities larger than the phase conjugate waves have been observed. The influence of optical bias on the generation of the phase conjugate waves is studied.

Phase conjugate interferometry using a photorefractive crystal-dye film composite

Abstract Forward Phase Conjugate waves have been generated by the 180 degree rotation of the holograms stored in photorefractive iron doped Lithium Niobate crystal. The holograms are formed by the Two-Wave Mixing scheme either in transmission or in reflection geometry. Such a scheme is then extended to demonstrate the in-plane and out-of-plane displacement of the object wave.

Phase conjugate interferometry with a novel ferroelectic crystal-dye film composite

A single object wave is amplitude divided by a beam splitter into two waves of equal intensity that are made to interfere at the back surface of an iron-doped lithium-niobate crystal so that the normal to the back surface is the angular bisector of the input waves. The interference results in the formation of a phase grating (Bragg grating) in the volume of the crystal. These waves are diffracted at the Bragg grating on both the front focal plane and the back focal plane of the crystal. The wave diffracted in the back focal plane from the Bragg grating and counterpropagating to the incident wave is observed to be the phase conjugate of the input object wave. The wave diffracted in the front focal plane of the Bragg grating is incorporated into the design of an interferometer to measure a specific in-plane displacement of the object wave. It is theoretically evaluated and experimentally demonstrated that interferometers such as those that incorporate conjugate-wave pairs are highly sensitive.

Wavelength dependence of phase conjugate reflectivity in absorbing media and thermal grating studies by four wave mixing

A novel approach for phase conjugate interferometry using a composite material which contains both a photorefractive crystal and a dye-doped polymer thin film, in a degenerate four-wave mixing geometry, is reported. The preparation of this composite material, the time scales of the mechanisms involved and the working principle of this interferometer are discussed. Both double exposure and real-time interferometry are demonstrated by considering specific cases of interest. A comparative analysis with respect to the other interferometric schemes is made.

Raman induced phase conjugation spectroscopy

Abstract A new approach to integrate the large holographic storage times of the photorefractive crystals and the fast response times of dye-doped thin films by forming a new composite material is discussed. The potential of this new composite to realize Phase Conjugate Interferometry in a Degenerate Four Wave Mixing geometry is investigated. This is done by studying the difference in the temporal evolution of the two kinds of phase conjugate signals that are generated by the composite. An application of this technique to a qualitative study of thermal currents in a liquid is also described.

Holographic and phase conjugate interferometry with nonlinear optical media

The development of optical phase conjugation using thermally formed gratings in absorbing media, especially the dye solutions, has besides the high efficiencies, many other characteristics which are worthy of study. Here the wavelength dependence of phase conjugate reflectivity in relation to absorption spectrum of rhodamine-6G and rhodamine-B in water, methanol and ethanol is reported. Further the Bragg condition has been verified for the dominant diffracted beam which shows that the thermal grating behaves like an optically thick grating.