Philip L. Hemmer

Homodyne and heterodyne imaging through a scattering medium.

We introduce a novel two-dimensional (2D) homodyne and heterodyne technique for imaging objects through or embedded in a scattering medium. Our imaging approach is based on heterodyning of light with different Doppler broadenings that is scattered from objects of two different textures or from an opaque object and a textured scattering medium. We report on the initial demonstration of pulling signals out of noise for an object hidden behind a scattering medium. Enhancements of signal-to-noise ratio of the order of 50 have been achieved by use of a 2D holographic phase-sensitive detector. We also discuss the experimental feasibility of this approach for objects embedded in a scattering medium.

Porous-matrix holography for nonspatial filtering of lasers

A novel technique is described for laser beam cleanup, the nonspatial filter, which is based on the Bragg selectivity of thick holograms. Unlike pinhole and fiber spatial filters, which employ lenses and apertures in the transform plane, nonspatial filters operate directly on the laser beam. This eliminates the need for laser beam focusing, which is the source of many of the alignment instabilities and laser power limitations of spatial filters. Standard holographic materials are not suitable for this application because differential shrinkage during processing limits the maximum Bragg angle selectivity attainable. This paper describes a new technology which eliminates the problem of differential shrinkage. This technology is based on the use of a rigid porous substrate material, such as porous gas, filled with a light sensitive material, such as holographic photopolymers or dichromated gelatin. We report preliminary results of holographic nonspatial filtering of a laser beam in one dimension, with an angular selectivity of less than 1 milliradian.

Optical processing with feedback using smart- pixel spatial light modulators

A spatial light modulator is often used as the basic element in real-time recognition architectures. However, many of the devices suffer from poor uniformity and/or nonlinearities that affect the overall perfor- mance of any real-time recognition scheme. We propose using optical feedback as a method of improving device performance. We demon- strate both positive and negative feedback separately through a mixture of on-board electronic processors and optical polarization intensity en- coding. Placing the spatial light modulator in a positive feedback system causes the system to act like a memory. Placing the spatial light modu- lator in a negative feedback system makes the spatial light modulator response more analog, which in turn leads to improved uniformity.

First observation of ultraslow group velocity of light in a solid

We report ultraslow group velocities of light in a solid. Light speeds as slow as 45 m/s were observed, corresponding to a group delay of 66 microsecond(s) in a 3-mm thick, optically dense crystal of Pr doped Y2SiO5. Reduction of the group velocity is accomplished by using a sharp spectral feature in absorption and dispersion that is produced by resonance Raman excitation of a ground-state spin coherence. Potential applications of slow and stopped light for the highly efficient storage and recall of optical data are discussed.

Two-dimensional holographic nonspatial filtering for laser beams

In this paper, we discuss our most recent work in 2D nonspatial filtering. A brief introduction describes nonspatial filtering and its relationship to conventional spatial filtering. We then show result from our initial experiments in 2D nonspatial filtering and briefly describe a more advanced design. A variation on the more advanced design demonstrates wavelength-independent operation. We then review our initial findings on adhesives appropriate for implementation of the advanced design and comment on the surface quality requirements of the recording material, a polymer with diffusion amplification. Finally, we compare the robustness and efficiency of nonspatial filters with conventional spatial filters.

Optical memory using resonant Raman pulse excited spin echoes

We report a new technique for high-density, high-speed optical memory based on two-photon coherence excited spin echoes in solids. Unlike conventional echo-based optical memory techniques, a two-photon coherence excited spin echo memory has potential for Tbits/cm2 in memory density and THz in data processing speed at high temperatures.

Raman-excited spin coherences in N-V diamond

Raman excited spin coherences were experimentally observed in nitrogen-vacancy (N-V) diamond color centers via nondegenerate four-wave mixing (NDFWM) and electromagnetically induced transparency (EIT). The maximal EIT-induced absorption suppression was found to be 17%, which corresponds to 70% of what is possible given the four possible geometric orientations of the N-V center in diamond. The properties of these coherences are discussed in the context of potential applications to solid-state quantum computing and high-temperature spectral hole burning memories.

Imaging objects in an opaque scattering medium

In this paper a 2D homodyne and heterodyne technique for imaging objects embedded in an opaque scattering medium is introduced. Our imaging approach is based on heterodyning of light with different Doppler shifts scattered from objects of two different textures or from an opaque object and a textured scattering medium. We report on the initial demonstration of pulling signals out of noise for an object hidden behind a scattering medium. Enhancements of signal- to-noise ratio of the order of 50 have been achieved utilizing a 2D holographic phase-sensitive detector.

Raman-excited spin coherences for high-temperature spectral hole burning memories

The possibility of using Raman excited spin coherences to increase the operating temperature of spectral hole burning memories and processors is being explored experimentally. The approach is to store and/or process data using spin coherences excited by optical Raman transitions. This is motivated by the fact that spin coherence lifetimes are much less sensitive to temperature than optical coherence lifetimes. However, direct microwave excitation of spin coherences does not give a high storage density because of the large microwave wavelength. By using optical Raman fields to excite the spin coherences, full optical spatial resolution can be achieved. Initial experiments in Pr doped YSO demonstrated potential for higher temperature operation. Current experiments are concentrating on further increasing the operating temperature using this and other materials.

Efficient generation of Raman echo and time-domain optical data storage by electromagnetically induced transparency

We have observed excitation of spin echoes and spin free induction decay (FID) by electromagnetically induced transparency (EIT) in an optically dense solid sample. The experiments are done in a double-lambda system of 605.7 nm 3H4 - 1D2 transition of Pr3+:Y2SiO5, where the 10.2 MHz ground state spin coherence is excited by low-power resonant Raman pulses. It has been shown that the spin coherence, including spin echo, is equivalent to the transparent state of EIT, and therefore a high efficiency is expected for such resonant Raman-excited spin echo. The observed efficiency of spin echo is as high as 75% of the FID signal at 5K. A background-free detection scheme is used based on EIT and enhanced nondegenerate four-wave mixing. The technique is applied in the frequency-selective time-domain optical data storage, that utilizes the spin as well as the optical inhomogeneous spectral widths. The data storage scheme is analogous to the stimulated spin echo with resonant Raman excitation of the spin coherence. We verify that the write window is determined by the spin T2 which is much longer than the optical T2, especially at higher temperature. We find that the spin dephasing time T2 is almost constant at approximately 500 microseconds in the range of 2 to approximately 6 K, whereas the optical T2 decreases rapidly, by a factor of approximately 50, above 4 K. These results will be useful in the development of high capacity time-domain optical data storage operating at higher temperature.