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Journal of Applied Physics

Optical storage using refractive index changes induced by two-step excitation

Dietrich von der Linde, Alexander M. Glass, K. F. Rodgers

Published Jan 1, 1976• 81 citations
Publisher

Abstract

Multiphoton excitation allows reversible photorefractive recording without the disadvantage of erasure during readout. The light intensity required for two-photon storage can be drastically reduced if two-step excitation via long†lived real intermediate states is used. Experimental results for LiNbO3:Cr3+ and LiTaO3:Cr3+ demonstrate the advantages of the two-step process. The energy and intensity requirements for optical two-step recording are within reach of presently available repetitively pulsed lasers.

DOI
10.1063/1.322348

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Multiphoton excitation allows reversible photorefractive recording without the disadvantage of erasure during readout. The light intensity required for two-photon storage can be drastically reduced if two-step excitation via long†lived real intermediate states is used. Experimental results for LiNbO3:Cr3+ and LiTaO3:Cr3+ demonstrate the advantages of the two-step process. The energy and intensity requirements for optical two-step recording are within reach of presently available repetitively pulsed lasers.
Summary Holographic data storage systems [1] require sensitive optical materials that provide some means of nondestrective readout [2]. Among the number of techniques for nonvolatile readout available [2]. the sensitazation of a photorefractive medium by illumination at one wavelength, called gating, for writing at a second using a resonant two-photon process, constitutes the most versatile all-optical nondestructive method. Typically, this process consists of absorption of a photon at frequency ω1 to generate an intermediate excited state, followed by absorption of a second at ω2 to generate a free carrier [3, 4, 5]. Gating occurs for absorption of two photons at two different wavelengths. In the present work, we identify the critical physical characteristics of the processes that provide the simultaneous realization of true gating and writing sensitivity at a variety of frequencies, evaluate the refractive index perturbation and the response rate, and investigate the significance of the the intermediate state occupancy and the arisining nonlinear photorefractive behavior at its saturation regime.
Recently, several works have shown that the light group velocity can be reduced using the recording of dynamic refractive index gratings in photorefractive (PR) crystals [1] , [2] , [3] . The slowdown of light is performed using optical beams wave mixing: it yields the output pulse amplification and increases the light dispersion inside the PR crystal. For example, the TWM method demonstrated that the delay time ∆ τ can be varied as a function of the TWM gain values Γ and the input pulse widths T 0 . However, several works have shown that the results achieved by this method are limited in terms of pulse duration. Indeed, only long pulses with time duration of ms−s can be slowdown in a PR crystal both at the visible and the infrared wavelengths [4] .
We present a method for optical encryption of information, based on the time-dependent dynamics of writing and erasure of refractive index changes in a bulk lithium niobate medium. Information is written into the photorefractive crystal with a spatially amplitude-modulated laser beam which when overexposed significantly degrades the stored data making it unrecognizable. We show that the degradation can be reversed and that a one-to-one relationship exists between the degradation and recovery rates. It is shown that this simple relationship can be used to determine the erasure time required for decrypting the scrambled index patterns. In addition, this method could be used as a straightforward general technique for determining characteristic writing and erasure rates in photorefractive media.
Physics Photons are ideal carriers of information. They are fast and coherent, and because they don't interact much with each other they can have very long lifetimes. However, stopping, processing, and manipulating the photons requires them to be stored in a memory, where doing so can damage their fragile quantum properties, which leads to a loss of coherence. Although photons have been coherently stored in and retrieved from ensembles of atom gases, both warm and cold atom clouds, for practical applications a solid-state memory would be desired. Heinze et al. have developed a method to enhance the storage time of photons in a rare-earth–ion—doped crystal. They use a series of laser pulses to initialize the crystal for photon storage and then load it with their “information photons.” By iteratively optimizing the experimental parameters, they show that they can store light pulses and even entire images in the crystal and then retrieve them up to a minute later with their coherent properties intact. The results present an important step toward building a viable quantum network. Phys. Rev. Lett. 111 , 033601(2013).