Towards ultrahigh-density multilayer disk recording by two-photon absorption

Abstract

Advances in healthcare imaging products and multimedia entertainment systems are demanding higher media capacity (TB/disk) and faster data transfer speeds (lGb/s) from future removable disk based data storage systems. One promising direction to satisfy these requirements with low cost systems is the use of optical volumetric multi-layer disk media. At Call/Recall we have demonstrated the scalability of two-photon recordable photochromic doped polymeric Write Once Read Many (WORM) disk media to more than 1 00 layers with negligible interlayer crosstalk and excellent stability of the written bits2'3'4'5'6. The media employed in our WORM system, consist ofphotochromic organic molecules designed and synthesized so that they change their structure, upon excitation at the absorption band of the molecule5'7'8'9'. A spot is written in the volume of the medium only at points of temporal and spatial intersection of two photons with sufficient energy to record by altering the structure of the photochromic molecule. A high power short pulse laser beam is tightly focused for recording. Recording occurs only within a small volume around the focus of the laser beam due to twophoton absorption. The recording response ofthe material follows the square ofthe optical system point spread ftinction (PSF) resulting in a recorded bit size that is 30% less than the Rayleigh criterion PSF. The recorded bits are read by fluorescence when excited by suitable optical radiation absorbed within the written spot volume. The doped polymer media is low cost, flexibly shaped and molded, and its properties may be customized (by changing the dopant molecules) to match evolving application and technology requirements. We had previously reported bit dimensions of 0.5 x0.5x4.5jim 2 withNA 0.75 and 532nm wavelength exhibiting raw bit-error rates (BER) of iO'. Here we show a ftjrther decrease in the bit size to 0.4 x 0.4 x 2jtm with a higher NA1 .4 objective lens at 532nm wavelength, resulting in ultra high density volumetric disk recording using up to 100 layers and providing a potential for 400bits4im2 effective area! density. This paper discusses the theory and approach as well as experimental results we used and obtained to demonstrate the feasibility of high NA recording of volumetric multilayer disks by two photon absorption.