Ryuichi Katayama

Aberration multiplexing in microholographic recording

The feasibility of aberration multiplexing in microholographic recording was investigated through a numerical simulation. The multiplexing is performed by forming multiple microholograms at the same position by changing the aberration of recording beams and selectively reproducing each of multiple microholograms by changing the aberration of a readout beam. The simulation showed that multiplexing of more than 10 bits would be possible by combining the spherical, coma, and astigmatic aberrations.

Influence of aberrations in microholographic recording

Abstract. The influence of various types of aberrations (spherical, coma, and astigmatic) of recording and readout beams on the readout signal in a microholographic recording was investigated through a numerical simulation. The simulation conditions were that the wavelength of the laser was 405 nm and the numerical aperture of the objective lenses was 0.85. The tolerance of the root-mean-square (RMS) wavefront aberrations was defined as the aberration when the normalized signal level decreased to 0.8. Among the three types of aberrations, the influence of the spherical aberration was the most significant. When both the recording and readout beams were aberrated and the signs of the aberrations were in the worst case, the tolerance of the RMS wavefront aberrations was less than half of the Maréchal’s criterion. Moreover, when the RMS wavefront aberrations of the recording and readout beams were within the above tolerance, the bit intervals of 0.13 and 0.65  μm in the inplane and vertical directions, respectively, which correspond to the recording density of 91  bit/μm3 (recording capacity of 16 TB for a 120-mm-diameter optical disk having a 300-μm-thick recording layer), were shown to be feasible for confocal detection with an allowable signal-to-noise ratio.

SIL-STED microscopy technique enhancing super-resolution of fluorescence microscopy

We have characterized a new type STED microscope which combines a high numerical aperture (NA) optical head with a solid immersion lens (SIL), and we call it as SIL-STED microscope. The advantage of a SIL-STED microscope is that its high NA of the SIL makes it superior to a general STED microscope in lateral resolution, thus overcoming the optical diffraction limit at the macromolecular level and enabling advanced super-resolution imaging of cell surface or cell membrane structure and function Do. This study presents the first implementation of higher NA illumination in a STED microscope limiting the fluorescence lateral resolution to about 40 nm. The refractive index of the SIL which is made of material KTaO3 is about 2.23 and 2.20 at a wavelength of 633 nm and 780 nm which are used for excitation and depletion in STED imaging, respectively. Based on the vector diffraction theory, the electric field focused by the SILSTED microscope is numerically calculated so that the numerical results of the point dispersion function of the microscope and the expected resolution could be analyzed. For further investigation, fluorescence imaging of nano size fluorescent beads is fulfilled to show improved performance of the technique.

Possibility of the market expansion of large capacity optical cold archive

The field, IoT and Big data, which is activated by the revolution of ICT, has caused rapid increase of distribution data of various business application. As a result, data with low access frequency has been rapidly increasing into a huge scale that human has never experienced before. This data with low access frequency is called “cold data”, and the storage for cold data is called “cold storage”. In this situation, the specifications of storage including access frequency, response speed and cost is determined by the applications request.

Effect of temperature change in microholographic recording

In microholographic recording, temperature change causes the thickness and refractive index change of the recording medium, which lead to the decrease in the diffraction efficiency of a microhologram due to the Bragg mismatch. Therefore, the effect of the temperature change in the microholographic recording and its compensation method were investigated through a numerical simulation. The wavelength of the laser was 405 nm and the numerical aperture of the objective lenses was 0.85. The thickness change ratio and refractive index change of the recording medium due to the temperature change were 5.0 × 10-4 / deg. and −3.0 × 10-4 / deg., respectively. The diffraction efficiency of the microhologram was calculated using the coupled wave theory. The tolerance of the temperature change increased from ±1.8 deg. to ±12 deg. with the compensation by the wavelength change of the laser. However, the width of the readout signal after the compensation increased with the temperature change in both the in-plane and vertical directions. In the microholographic recording, the beam consists of multiple plane waves and the microhologram consists of multiple diffraction gratings. At the center of the beam, the corresponding wave vector of the plane wave and grating vector of the diffraction grating are perpendicular to the recording medium. On the other hand, at the periphery of the beam, they are slanted to the recording medium. Therefore, the Bragg matching conditions at the center and periphery of the beam are different from each other. The above results are attributed to this fact.

Mastering multi-depth bio-chip patterns with DVD LBRs

Bio chip and bio disc are rapidly growing technologies used in medical, health and other industries. While there are numerous unique designs and features, these products all rely on precise three-dimensional micro-fluidic channels or arrays to move, separate and combine samples under test. These bio chip and bio disc consumables are typically manufactured by molding these parts to a precise three-dimensional pattern on a negative metal stamper, or they can be made in smaller quantities using an appropriate curable resin and a negative mold/stamper. Stampers required for bio chips have been traditionally made using either micro machining or XY stepping lithography. Both of these technologies have their advantages as well as limitations when it comes to creating micro-fluidic patterns. Significant breakthroughs in continuous maskless lithography have enabled accurate and efficient manufacturing of micro-fluidic masters using LBRs (Laser Beam Recorders) and DRIE (Deep Reactive Ion Etching). The important advantages of LBR continuous lithography vs. XY stepping lithography and micro machining are speed and cost. LBR based continuous lithography is >100x faster than XY stepping lithography and more accurate than micro machining. Several innovations were required in order to create multi-depth patterns with sub micron accuracy. By combining proven industrial LBRs with DCA’s G3-VIA pattern generator and DRIE, three-dimensional bio chip masters and stampers are being manufactured efficiently and accurately.

An application of OFDM method to optical disc recording

An application of Orthogonal Frequency Division Multiplexing (OFDM) method to optical disc recording/readout is presented. OFDM has been widely used in the field of telecommunication owing to its highly efficient frequency usage. However OFDM has not been applied to optical disc recording because it is a multiple data transfer method and needs to record analog signals. Partial Response Maximum Likelihood (PRML) used in the current optical disc systems requires a certain kind of analog recording. Although optical recording usually creates binary marks, it is possible to obtain arbitrary analog readout signals by using PWM method. Another technique to generate analog signals using the oversampled binary recording is described and applied to multiple level recording. In addition, it is found that the level adjustment of multiple carriers for OFDM leads to the advantage when it is applied to the optical disc system. Using the simple transfer function model of the optical disc system, two types of readout signals using PRML and OFDM are calculated and then their qualities are compared. Since Quadrature Amplitude Modulation (QAM) method can be combined with OFDM, it is possible to increase the recoding density of optical disc systems. A method employing OFDM with 64-QAM and the pre-enhance method to the high frequency carrier shows an ability of 1.5 times recording density of the conventional Bru-ray Disc (BD).

The reduction of graphene oxide induced by rare-earth doped nanocrystals towards super-resolution optical data storage (Conference Presentation)

The growing amount of data generated every year creates an urgent need for improved methods and new storage media. Far-field super-resolution techniques have provided the foundation for nanoscale 3D optical data storage towards a petabyte-level capacity on DVD-sized discs. However, a suitable recording medium for high-density information storage over long-term periods and with low energy consumption is still lacking. Rare-earth doped nanocrystals, which feature ladder-like-arranged energy levels enabling emission from ultraviolet to near-infrared, have a fluorescence lifetime two to three orders of magnitude longer than that of other fluorophores and offer the potential for low-power super-resolution data reading. Moreover, the reduction of graphene oxide to reduced graphene oxide induces permanent changes in its chemical and optical properties which can be used for data recording. Here, we demonstrate the reduction of graphene oxide induced by rare-earth doped nanocrystals via FRET towards super-resolution optical data storage with ultra-high capacity, ultra-long lifetime and ultra-low energy consumption. Yb3+/Tm3+-doped core-shell nanoparticles were synthesized via co-precipitation method and their fluorescence spectrum was obtained using a home-built microscope. A solution of rare-earth doped nanocrystals and graphene oxide nanoflakes was spin-coated on coverslip glass and fluorescence lifetime measurements were conducted to confirm efficient FRET. The reduction of graphene oxide was attributed to the transfer of energy quanta from up-converting rare-earth doped nanocrystals under 980-nm laser excitation. High-contrast images of the data bits were generated by super-resolution optical microscopy based on rare-earth doped nanocrystals due to the different degree of fluorescence quenching between graphene oxide and reduced graphene oxide.

Temperature tolerance analysis on the volume holographic data storage (Conference Presentation)

Publishers Note: This conference presentation was withdrawn 15 January 2018 per author request.

Research of circular polarized holography with a large crossing angle under a common condition (Conference Presentation)

Polarization holography is the coherent interference of the beams that can have the different polarized states. The early-stage theory of polarization holography is based on Jones matrix, where the paraxial approximation is assumed, while the theory of polarization holography represented by dielectric tensor can describe the case with a large crossing angle. And it also depicts the relationship between diffraction light and interference light. During the research people find some extraordinary phenomenon, such as null reconstruction and inverse polarizing effect. But there is a disadvantage in this new polarization holography theory, where only under a peculiar circumstance can we get a faithful reconstruction. The circumstance can be expressed as “A+B=0”, where A and B refer to the coefficients for intensity and polarization holograms respectively. In this research, we calculate the formula of diffraction light’s polarization, and extract the A+B factor in it. Then we establish a series of equations which can let the diffraction light faithfully reconstruct, no matter what value of A plus B is. From the result, we can use an artificial reference beam which is corresponding to the signal beam to generate the hologram. Under this condition, the polarization of the diffraction light is similar to the signal. For simplification, we only discuss the signal wave with circular polarization and experimentally verify the result.