Plasma holograms could multiply storage capacity
Paul Mortensen
Researchers from the NEC Research Institute Inc. (Princeton, NJ) and NEC Fundamental Research Laboratory (Tsukuba, Japan) have succeeded in recording simple holograms in semiconductors using a photorefractive phenomenon called the plasma effect. In a paper presented at the Conference on Photorefractive Materials, Effects, and Devices in Estes Park, CO (sponsored by the National Institute of Standards and Technology and the Optical Society of America, June 11-14, 1995) the grou¥reviewed its recent work, which has the potential to enable high-resolution optical data storage with capacity thousands of times greater than with conventional holographic materials.
The holograms are stored using a new class of local photorefractive materials based on photoionization of dee¥electron traps known as DX centers, which occur in doped compound semiconductors such as tellurium (Te)-doped AlGaAs, as well as in some ionic crystals. To be stable, the DX centers need to capture two extra electrons and, at low temperatures--such as around 70 K--these electrons are held, which gives the material high electrical resistance. When the semiconductor is exposed to laser energy, however, the electrons can escape and form a plasma that makes the material conductive and changes its refractive index.
Material advantages
According to the researchers, the DX-center materials offer several advantages as compared to conventional photorefractives; these advantages include a refractive index change 30 times larger and sensitivity to optical exposure 60 times better, as well as an absence of the erasure that would normally occur during multiple grating exposures in conventional materials.
In fact, besides the lack of erasure accompanying recording, the writing process in DX materials is different from that in conventional photorefractive materials in other important ways. There is, for example, no phase shift between the exposing pattern and the resulting dielectric grating. The recording of gratings in DX materials is thus similar to that in a photochromic material except that the DX medium can be erased by heating, which frees the trapped electrons.
A large, optically induced refractive index change is responsible for diffraction gratings that are observed when electrons are released from DX centers. This large refractive index change implies the possibility of high diffraction efficiencies from thick gratings with periods larger than 100 nm. Diffraction efficiencies are relative values defined as the intensity of the diffracted beam divided by the sum of the intensities of the diffracted and undiffracted beams after passing through the sample and substrate (see figure).
The NEC team has confirmed that measured diffraction efficiencies of u¥to 40% were obtained from plasma gratings written into a 385-µm-thick sample of Te:AlGaAs. Samples were prepared for writing by cooling in the dark to a temperature between 25 K and 35 K. They were then exposed to form volume gratings using interfering collimated beams from diode lasers at a wavelength of 0.82 or 0.85 µm. The gratings were written with periods between 0.13 and 15 µm. They were read using a 1.53- or 0.85-µm diode laser, which does not ionize DX centers. The spatial resolution was 100 nm, which makes the effect a promising candidate for high-resolution holographic data storage.
The researchers expect that these results will apply to other DX-based materials. They have demonstrated persistent photoconductivity at 80 K, and the team is already investigating materials that exhibit properties of DX-center materials at higher--more practical--temperatures.