We report on the evaluation of lattice thermal conductivity of GeTe/Sb2Te3 superlattice (SL) by using a coherent phonon
spectroscopy at various lattice temperatures. The time-resolved transient reflectivity obtained in amorphous and
crystalline GeTe/Sb2Te3 SL films exhibits the coherent A1 optical modes at terahertz (THz) frequencies with picoseconds
dephasing time. The relaxation time and frequency of the coherent A1 modes are used to compute the lattice thermal
conductivity based on the Debye theory, including scattering by grain boundary and point defect, umklapp process, and
phonon resonant scattering. The results indicate that the thermal conductivity in the amorphous SL film is less
temperature dependent, due to the dominant phonon-defect scattering, while in the crystalline SL it is temperature
dependent because of the main contributions from umklapp and phonon resonant scatterings. We argue the higher
thermal conductivity in the GeTe/Sb2Te3 SL films than that in the Ge2Sb2Te5 alloy films implies that the phase change in
GeTe/Sb2Te3 SL is not purely promoted by thermal process, i.e., lattice heating, but rather by nonthermal process, i.e.,
coherent lattice excitation, because the thermal process generally requires lower thermal conductivity.
Inserting germanium nitride thin films between Sb-Te (for super-resolution readout) and ZnS-SiO2 layers was effective
to improve super-resolution readout durability of a super-RENS disc using a PtOx-SiO2 write-once recording layer.
Waveform of 97-nm (that is below the resolution limit of the optics used) and 340-nm combined marks scarcely
changed even after 50,000 times readout. CNR of 100 nm marks was stable (within 3 dB decrease) after 268,000 times
readout.
KEYWORDS: Diffusion, Germanium antimony tellurium, Super resolution, Laser stabilization, Laser optics, Laser marking, Digital signal processing, Near field, Laser applications, High power lasers
We report the readout stability improvement results of super-resolution near field structure (Super-RENS) write-once
read-many (WORM) disk at a blue laser optical system. (Laser wavelength 405nm, numerical aperture 0.85) By using diffusion barrier structure (GeSbTe sandwiched by GeN) and high transition temperature recording material (BaTiO3), material diffusion of phase change layer and recording mark degradation were greatly improved during high power (Pr=2.0mW) readout process up to 1X105 times.
Selecting a binary compound (Sb-Te, Zn-Sb and Ge-Te) with its composition at around a eutectic point generated a
better super-resolution effect. The thickness optimization of the super-resolution readout layer was also effective for the
property improvement. The temperature estimation indicated thinner the layer thickness is better as far as the readout
laser can easily increase its temperature.
We report the error rate improvement of super-resolution near field structure (Super-RENS) write-once read-many (WORM) disk at a blue laser optical system. (Laser wavelength 405nm, numerical aperture 0.85) We used a disk of which carrier level (CL) of 75nm is improved from -26.3 dBm to -19.0 dBm. We controlled the equalization (EQ) profile characteristics and used the adaptive 5 symbol write strategy and advanced high tap partial-response maximum likelihood (PRML) technique in order to improve the bit error rate (bER) characteristics of the super-RENS random signal. As a result, we obtained bER of 10-4 level with new signal processing techniques and bit error analysis process. This result shows high feasibility of super-RENS technology for practical use in the near future.
The optical reflectance and transmittance of Platinum oxide (PtOx) and palladium oxide (PdOx) mask layer, which are used to super-resolution near-field structure (super-RENS) disk, are investigated using Z-scan technique, under blue laser (442 nm) irradiation. The power thresholds of the PtOx and PdOx decomposition are obtained; the reversible and irreversible features for the two kinds of mask layers are cleared. Deformation in the micro irradiation region on surface of the mask samples, which is formed by decomposition of the PtOx or PdOx driving the Z-scan, is analyzed by means of an atom force microscope (AFM). The deformation analyses agree well with the Z-scan results. The optical features obtained at 442nm wavelength are compared with those at 532-nm wavelength, and the power threshold difference between the two wavelengths is also analyzed in detail based on irradiation power density and absorption spectrum of the mask samples.
We have developed a novel lithography technique, which we refer to here as Volume-Change Thermal-Lithography (VCTL), for application to the mastering process in a manufacture of next-generation ultra-high density optical ROM disks. Using a visible laser beam and conventional optics, we have succeeded in fabricating minute dots with diameters of under 100 nm and with interdot spacing far beyond the optical diffraction limit at a practical fabrication speed of 3 m/s. The combination of the temperature distribution induced by a focused laser beam with a Gaussian profile and a specially designed multilayer consisting of TbFeCo and ZnS-SiO2 was utilized for fabricating the aforementioned nano-structures. A focused laser beam with a Gaussian profile can generate a peak temperature area far smaller than its spot size. TbFeCo and ZnS-SiO2 undergo mutual diffusion when heated, a result of which is that their volume expands. An incident pulsed laser induces mutual diffusion restricted to the highest heated area. As a result, minute convex structures appear as dots on the sample surface. In addition to fabrication of continuous dot patterns, as a demonstration, small letters with dimensions of approximately 1 μm were drawn by specific dot arrangement, confirming the strong possibility of the technique.
KEYWORDS: Super resolution, Polarization, Near field optics, Signal processing, Optical properties, Reflectivity, Near field, Platinum, Signal detection, Silver indium antimony tellurium
KEYWORDS: Eye, Signal detection, Laser stabilization, Laser marking, Laser systems engineering, Laser applications, Disk lasers, Clocks, Laser optics, Signal processing
We report the random pattern signal characteristics of the super resolution near field structure (Super-RENS) disk in a blue laser optical system. (Laser wavelength 405 nm, numerical aperture 0.85) We introduced new structure for blue laser system, which results in 43 dB carrier to noise ratio (CNR) at the 75 nm mark length signal (which is equivalent to 50 GB capacity with 0.32 micrometer track pitch) and much better readout stability were obtained. The relatively clear eye pattern, phase locked loop (PLL) state and data to clock jitter of around 20% for a 50 GB (2T:75 nm) random pattern signal were realized.
KEYWORDS: Reflectivity, Near field optics, Transmittance, Atomic force microscopy, Signal detection, Near field scanning optical microscopy, Particles, Image analysis, Super resolution, Near field
The bubble’s functions in readout process for PdOx and PtOx superresolution near-field structure (super-RENS) disk are studied with the PdOx and PtOx mask sample and with a repetitive Z-scan method. The results indicate that the optical responses on transmittance and reflectance are related to shape and size of the bubble. The deformation of bubbles before and after repetitive scan is observed by an optical microscope, and the sizes of the bubbles corresponding to different repetitive Z-scan order of times are analysed by an atomic force microscope.
KEYWORDS: Nonlinear optics, Near field, Super resolution, Oxides, Optical storage, Near field optics, Metals, Particles, Signal detection, Light scattering
Nonlinear properties and response mechanisms of PtO2 and PdO1.1 mask layers for optical data storage with super-resolution near-field structure were investigated. The results obtained from Z-scan measurement was supported by microscopic observation studies. The 5.1 mW and 6.5 mW, respectively, as the decomposition threshold of the PdO1.1 and PtO2 for leading to metallic nano-particles were confirmed. The scanned PdO1.1 and PtO2 mask samples could be retrieved at less than their own threshold values. It was also found for the PdO1.1 and PtO2 mask samples that the nonlinear optical response not only came from the metallic particles but also from the bubble deformation.
Silver oxide layers were prepared by reactive r.f. magnetron sputtering of a silver target in oxygen containing atmosphere. Spectroscopic analysis of the films revealed a gradually composition change from Ag over Ag2O to AgO with increasing oxygen addition. Raman spectroscopy in combination with optical transmission measurements indicated that the AgOx constituents readily decompose by laser irradiation to optically active silver scattering centers and oxygen. Surface enhanced Raman scattering (SERS) of carbon traces in the silver oxide layers verified the potential to excite local surface plasmons in such silver aggregates. Furthermore, SERS activity of the activated layers is demonstrated by the clear amplification of Raman bands of low concentrated chemicals. The example of the model molecule benzoic acid (BA) applied to AgO films on glass substrates allows to observe the activation process in situ. SER characteristics were found to be dependent on the silver oxide film constitution and the state of intermediate silver cluster formation along with the applied photoactivation-time and power.
A super-resolution near-field structure (super-RENS) has an additional mask layer in the usual phase change optical disk. A thin layer of antimony (Sb) film or a silver oxide (AgOx) layer is used as a mask layer. By focusing a laser beam, a transparent aperture in the Sb layer and a light scattering center in the AgOx layer are formed transitionally, whose diameters are smaller than that of the laser beam spot. The changed portion can generate an intense optical near field and can be used to record and retrieve small marks beyond the diffraction limit. The nonlinear optical properties of Sb and AgOx films with protective layers were examined using a pulse laser. Optical switching, their time response and transient spectroscopic change were investigated. Light scattering property of AgOx film was also examined. A repeated optical switching action can only be realized if the illuminating spot size is confined to very small areas. Time response of Sb film shows first rise-up and then slow exponential decay. Time response of AgOx film shows more complicated decay than Sb film. Transmittance spectra just after the pump irradiation becomes flat over wide spectral range both in Sb and AgOx layers. Scattered light is extremely enhanced by increasing the input light power.
KEYWORDS: Near field scanning optical microscopy, Near field, Super resolution, Near field optics, Silver, Light scattering, Optical discs, Optical storage, Plasmons, Oxides
The principle and recent progress of super-resolution near- field structure (super-RENS) were introduced. Super-RENS is a unique method to retrieve optical near-field without any probes at high speeds approximately millions times faster than that of any other scanning near-field optical microscopes (SNOMs). First super-RENS disks were developed using an optically transparent aperture, and recently we found that a light-scattering-center using nano-explosion of silver oxide (AgOx) is also available in super-RENS.
Optical near-field recording with a super-resolution near- field structure (Super-RENS) records and retrieves small marks with dimensions beyond the diffraction limit. A thin layer of an antimony (Sb) film, added to the usual phase-change optical disk structure, is the key material of this technique. We investigated the nonlinear optical properties of the Sb film, especially the optical switching behavior in the stationary state using a nanosecond pulsed laser. Clear switching was observed under microscopic measurement. Time response of the optical switching properties were examined and found to show fast rise-up and slow decaying times.
KEYWORDS: Near field, Light scattering, Scattering, Near field optics, Super resolution, Finite-difference time-domain method, Optical storage, Diffraction, Signal detection, Optical discs
We describe the angular dependence of near-field signals generated in super-resolution near-field structure (Super- RENS) disks. The carrier-to-noise ratio (CNR) of transmitted signals using different numerical aperture's (NA's) lenses, were mostly the same values, experimentally. While on computer simulation by finite-difference time-domain (FDTD) method, it was found that most scattered signals were trapped around the optical axis.
We report on a novel micromachined sensor head for optical data storage devices. The head is based on a 6 X 6 aperture array with subwavelength dimensions. In comparison to previously introduced flat-aperture heads, a tip-based array is presented which profits from a recently presented reliable fabrication process of apertures in the 100 nm regime and below. This fabrication process exploits inhomogeneous thickness distributions and modified etching rates of thermally generated siliconoxides on <001>-oriented silicon wafers at relatively low temperatures, and makes it possible to open apertures with dimensions below 150 nm in hollow SiO2 pyramidal shaped tips. A metallization process is finally used to provide optical opacity and to further reduce the overall aperture dimensions to less than 100 nm. The fabrication process of these sensors is described in detail and the results are confirmed by SEM photographs.
Organic material with pronounced thermochromism is used as a mask layer in a phase change optical disk. The thermochromic material exhibits significant change in optical properties at increased temperatures as well as fast response upon pulsed laser irradiation. Recording and retrieving signals from marks smaller than that of the diffraction limit is investigated under high speed disk rotation.
KEYWORDS: Silver, Doping, Optical storage, Super resolution, Crystals, Near field optics, Refractive index, Near field, Germanium antimony tellurium, Near field scanning optical microscopy
Scattering-mode super-resolution structure (Super-RENS) is an alternative to realize high-density optical data storage. Ag doped GeSbTe films were investigated to optimize the high- density optical data storage characteristics. It was found that the readout durability could be greatly improved by Ag doped GeSbTe films.
Readout characteristics of light-scattering-mode super- resolution near-field structure (super-RENS) disks are described in detail. Readout intensities in reflected and transmitted signals are compared. Both signals showed mostly the same carrier-to-noise ratios (CNRs) using objective lenses with NA of 0.6. The formation mechanism of light scattering centers in the super-RENS disks ia also described in comparison with several different disks. As increasing oxygen ratio during the deposition of silver oxide (AgOx) layers, two different chemical reactions were identified. It was found that the super-RENS disks with oxygen-rich AgOx films have both characteristics of transparent and light- scattering apertures in one disk. Further study also revealed that the AgOx dynamic nonlinearity is not so high and less than 6% by the film itself; however, it is enhanced to 12% in super-RENS. It is supposed that the imaginary refractive index k of the films is less than 0.1; therefore, it is hard to heat itself to the decomposition temperature without a heat source (GeSbTe film) underneath. This result would be a hint to further increase CNRs in a light- scattering-mode super-RENS disks.
Near-field optics has been applied to optical data storage systems with a high recording density beyond the diffraction limit. However, the actual data storage has not been realized, yet. The difficulty applying a near-field optics to optical data storage is caused by a narrow space between a near-filed probe and a recording medium.
KEYWORDS: Optical storage, Modulation, Finite-difference time-domain method, Near field optics, Optical discs, Antimony, Thermography, Analytical research, Super resolution, Near field
Recently a super-resolution near-field structure (Super-RENS) has been proposed. A storage density of a conventional optical disk has been limited by the diffraction limit of the light. During the readout process of a Super-RENS, a small aperture is formed in a masklayer by a readout laser power. The mask layer is laid very close to a recording layer, and the aperture produced in the mask layer acts as an optical near-field probe. We have retrieved marks as small as 60 nm.
KEYWORDS: Near field, Diffraction, Near field optics, Antimony, Data storage, Nonlinear optics, Optical storage, Scattering, Optical resolution, Signal detection
An approach for high speed optical near-field recording and detection is shortly reviewed, and the basic near-field scattering by super-resolution near-field structure(Super-RENS) is discussed.
KEYWORDS: Antimony, Near field scanning optical microscopy, Dielectrics, Annealing, Optical storage, Near field optics, Crystals, Thin films, Reflection, Raman spectroscopy
The formation and mechanism of nanometer sized apertures of super-resolution nearfield structure (Super-RENS) was thermally, mechanically and optically investigated. An aperture of an antimony film forced in a high compressive stress showed the resolution of less than 100 nm, whereas the film forced in a tensile stress did not. An aperture formation mechanism is proposed by balance between the aperture formation energy and the surface energy including the internal stresses.
KEYWORDS: Near field optics, Near field scanning optical microscopy, Modulation, Near field, Phase shift keying, Refractive index, Objectives, Optical storage, Crystals, Optical microscopes
We have observed near-field optical images of phase change marks and evaluated the readout signals including the optical near-field. Samples were composed of multilayered structures on glass substrates. Crystalline marks were recorded by a focused laser beam with an optical microscope in the as-deposited amorphous films. In readout, the optical and the topographical images of the recorded marks were evaluated at the same time by a collection mode near-field scanning optical microscope (NSOM). The surface profiles showed less than 1 nm dips around the marks. Therefore it means that the NSOM image of phase change marks depends on the refractive index change. The evaluated signal modulation of the optical image showed a sinusoidal curve to the top SiN layer thickness, and the maximum modulation was 60 percent.
KEYWORDS: Near field optics, Antimony, Super resolution, Near field scanning optical microscopy, Near field, Reflection, Optical storage, Nonlinear optics, Crystals, Thin films
A new proposal for near-field data storage and the basic experiment under a high speed disk rotation are described. Using the thermally nonlinear property of an Antimony thin film and an intermediate layer of SiN with a thickness of 20 nm, less than 100 nm sized marks were recorded and retrieved in an optical phase change film beyond the diffraction limit, with a wavelength of 680nm and a lens NA of 0.6.
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