Pramod K. Khulbe

Crystallization behavior of as-deposited, melt quenched, and primed amorphous states of Ge2Sb2.3Te5 films

We compare the crystallization behavior of thin films of Ge2Sb2.3Te5 in various amorphous states, namely, as-deposited, melt-quenched, and primed. These films are embedded in a quadrilayer stack similar in structure to the commercially available phase-change optical disks. This study shows that the melt-quenched amorphous film has a shorter crystallization onset time and a higher crystallization rate in comparison to the as-deposited amorphous film. We also observed that variable priming leads to crystallization behavior falling between that of the as-deposited and melt-quenched states. A qualitative model of the modification in crystallization behavior due to priming is given based on the notion that priming produces crystalline embryos which hastens crystallization process.

Femtosecond pulsed laser micromachining of glass substrates with application to microfluidic devices.

We describe a technique for surface and subsurface micromachining of glass substrates by using tightly focused femtosecond laser pulses at a wavelength of 1660 nm. A salient feature of pulsed laser micromachining is its ability to drill subsurface tunnels into glass substrates. To demonstrate a potential application of this micromachining technique, we fabricate simple microfluidic structures on a glass plate. The use of a cover plate that seals the device by making point-to-point contact with the flat surface of the substrate is necessary to prevent the evaporation of liquids in open channels and chambers. Methods for protecting and sealing the micromachined structures for microfluidic applications are discussed.

Crystallization behavior of Ge-doped eutectic Sb70Te30 films in optical disks.

We report laser-induced crystallization behavior of binary Sb-Te and ternary Ge-doped eutectic Sb70Te30 thin film samples in a typical quadrilayer stack as used in phase-change optical disk data storage. Several experiments have been conducted on a two-laser static tester in which one laser operating in pulse mode writes crystalline marks on amorphous film or amorphous marks on crystalline film, while the second laser operating at low-power cw mode simultaneously monitors the progress of the crystalline or amorphous mark formation in real time in terms of the reflectivity variation. The results of this study show that the crystallization kinetics of this class of film is strongly growth dominant, which is significantly different from the crystallization kinetics of stochiometric Ge-Sb-Te compositions. In Sb-Te and Ge-doped eutectic Sb70Te30 thin-film samples, the crystallization behavior of the two forms of amorphous states, namely, as-deposited amorphous state and melt-quenched amorphous state, remains approximately same. We have also presented experiments showing the effect of the variation of the Sb/Te ratio and Ge doping on the crystallization behavior of these films.

Information storage and retrieval using macromolecules as storage media

No abstract available.

Static tester for characterization of phase-change, dye-polymer, and magneto-optical media for optical data storage.

We have designed and built a static tester around a commercially available polarized light microscope. This device employs two semiconductor laser diodes (at 643- and 680-nm wavelengths) for the purpose of recording small marks on various media for optical data storage and for the simultaneous monitoring of the recording process. We use one of the lasers in the single-pulse mode to write a mark on the sample and operate the other laser in the cw mode to monitor the recording process. The two laser beams are brought to coincident focus on the sample through the objective lens of the microscope. The reflected beams are sent through a polarizing beam splitter and thus divided into two branches, depending on whether they are p or s polarized. In each branch the beam is further divided into two according to the wavelength. The four beams thus produced are sent to four high-speed photodetectors, and the resulting signals are used to monitor the reflectance as well as the polarization state of the beam on reflection from the sample. We provide a comprehensive description of the testers design and operating principles. We also report preliminary results of measurements of phase-change, dye-polymer, and magneto-optical samples, which are currently of interest in the areas of writable and rewritable optical data storage.

DNA translocation through α-hemolysin nanopores with potential application to macromolecular data storage

Digital information can be encoded in the building-block sequence of macromolecules, such as RNA and single-stranded DNA. Methods of “writing” and “reading” macromolecular strands are currently available, but they are slow and expensive. In an ideal molecular data storage system, routine operations such as write, read, erase, store, and transfer must be done reliably and at high speed within an integrated chip. As a first step toward demonstrating the feasibility of this concept, we report preliminary results of DNA readout experiments conducted in miniaturized chambers that are scalable to even smaller dimensions. We show that translocation of a single-stranded DNA molecule (consisting of 50 adenosine bases followed by 100 cytosine bases) through an ion channel yields a characteristic signal that is attributable to the two-segment structure of the molecule. We also examine the dependence of the translocation rate and speed on the adjustable parameters of the experiment.

Dynamic theory of crystallization in Ge2Sb2.3Te5 phase-change optical recording media.

We develop a theory of the crystallization dynamics of Ge(2)Sb(2.3)Te(5) thin films that shows good qualitative agreement with experimental reflectivity results from a two-laser static tester. The theory is adapted from the nucleation theory of liquid droplets from supersaturated vapor and elucidates the physics underlying the amorphous-to-crystalline phase transformation under short-pulse excitation. In particular, the theory provides a physical picture in which crystalline islands, or basic embryos, are thermally activated in the amorphous material and subsequently grow as stable nuclei are formed.

Crystallization and amorphization studies of a Ge 2 Sb 2.3 Te 5 thin-film sample under pulsed laser irradiation

We present the results of crystallization and amorphization studies on a thin-film sample of Ge(2)Sb(2.3)Te(5), encapsulated in a quadrilayer stack as in the media of phase-change optical disk data storage. The study was conducted on a two-laser static tester in which one laser, operating in pulsed mode, writes either amorphous marks on a crystalline film or crystalline marks on an amorphous film. The second laser, operating at low power in the cw mode, simultaneously monitors the progress of mark formation in terms of the variations of reflectivity both during the write pulse and in the subsequent cooling period. In addition to investigating some of the expected features associated with crystallization and amorphization, we noted certain curious phenomena during the mark-formation process. For example, at low-power pulsed illumination, which is insufficient to trigger the phase transition, there is a slight change in the reflectivity of the sample. This is believed to be caused by a reversible change in the complex refractive index of the Ge(2)Sb(2.3)Te(5) film in the course of heating above the ambient temperature. We also observed that the mark-formation process may continue for as long as 1 mus beyond the end of the write laser pulse. This effect is especially pronounced during amorphous mark formation under high-power, long-pulse illumination.

Polarization holograms for source-mask optimization

A new technique is introduced to replace DOEs that are used for illumination in lithographic projectors with polarization computer generated holograms (PCGHs) that produce both arbitrary intensity and arbitrary polarization state in the illumination pupil. The additional capability of arbitrary polarization state adds an additional degree of freedom for source-mask optimization. The PCGHs are similar in design and construction to DOEs, but they incorporate polarizationsensitive elements. Three experiments are described that demonstrate different configurations of PCGHs deigned to produce a tangentially polarized ring. Measurements of ratio of polarization and polarization orientation indicate that all three configurations performed well. Experimetns are performed with visible (λ = 632.8nm) light.

Temperature-dependence of optical constants in phase-change media

The reflectivity of the phase-change media of optical recording changes slightly upon the application of a weak laser pulse (i.e., a pulse of insufficient power to write/erase a mark), returning to its initial value at the end of the pulse. This behavior is shown to be due to the temperature-dependence of the optical constants of the phase-change layer.