Introduction to Optoelectronics Optical storage (2) Prof. Katsuaki Sato

What we learn today. Optical storage is a storage using light for read-out of recorded information Record density is determined by the spot size of the light beam, which is limited by the wavelength of the light and the NA (numerical aperture) of lens. There are three categories of optical storage, i.e., read-only type, write-once type and rewritable type. Different physical phenomena are used for recording of the signal on optical disks.

Spot size at the focal point spot size　d α Numerical aperture of lens NA=nsinα d=0.6λ/NA CD-ROM: NA=0.6 λ=780nm→d=780nm DVD: λ=650nm→d=650nm BD: NA=0.85 　　　　λ=405nm→d=285nm HD-DVD: NA=0.6 　　　　λ=405nm→d=405nm

Classification of optical storages Optical disk Read only type CD, CD-ROM, DVD-ROM Recordable type Direct read after write (Write once type) CD-R, DVD-R Rewritable (recording and erasing) Phase change CD-RW, DVD-RAM, DVD-RW, DVD+RW, BD, HD-DVD Magneto-optical: MO, GIGAMO, MD, Hi-MD, AS-MO, iD-Photo Holographic memory, Hole-burning memory

Physical phenomena used in optical disk technology CD-ROM, DVD-ROM: pit formation CD-R, DVD-R: Chemical decomposition of organic dye CD-RW, DVD-RAM, DVD-RW, DVD+RW : Phase change between ordered and disordered states MO, MD, GIGAMO, iD-Photo, HD-MD: Magnetic phase change between ferromagnetic and paramagnetic states Holographic memory： Photorefractive effect Hole-burning memory: Local structure change

Characteristics of optical disk Removable Large capacity, high density 10Gb/in2 (far less than HD(100 Gb/in2)) Aiming at 100 Gb/in2 using near-field technique Random accessibility Cassette  MD, VTR  DVD Shorter access than magnetic tape Longer seek time than HD High reliability Higher head clearance than HD

Increase of Areal Density in Optical Disks T. Suzuki:113th Topical Meeting of Magn. Soc. Jpn. (2000.1) p.11 Hard disk Optical disk MO

Different Disks

CD-ROM Polycarbonate substrates：n=1.55 λ=780nm →λ’=503nm (wavelength in the substrate) Pit depth:110nm ~ ¼wavelength Phase difference in reflectionπ：Destructive addition of reflected beams http://www.infonet.co.jp/ueyama/ip/multimedia/cd.html

CD-ROM Drive Focusing servo Tracking servo Optical pickup Objective lens Tracking Servo Focusing Servo Quarter wave-plate Collimating lens Grating Polarization Beam Splitter Cylindrical lens Optical detector http://www.infonet.co.jp/ueyama/ip/multimedia/cd.html

CD-RW Phase change Crystalline and amorphous Substrate Protective layers UV coat Land Recording layer Reflection layer Printed surface Phase change Crystalline and amorphous http://www.cds21solutions.org/main/osj/j/cdrw/rw_phase.html

Phase change recording Phase change between different phases Rewritable:　As grown amorphous state is initialized to crystalline state by annealing. Recording is performed by heating above the melting point Tm (600C) followed by quenching to amorphous state. Erasing is done by heating to Tcr(400 C) to crystallize. High level ：Heating above Tm→rapid cool→amorphous Low level：Heating above Tcr→slow cool→crystalline DVD-RAM: GeSbTe based alloy DVD±RW: Ag-InSbTe based alloy

Recording and erasing Rapid cooling： amorphous →low reflectivity melting point crystalli-zation crystalline amorphous Energy low reflectivity high reflectivity activation energy temperature Rapid cool Slow cool time Rapid cooling： amorphous →low reflectivity Slow cooling below Tm crystalline →high reflectivity http://www.cds21solutions.org/main/osj/j/cdrw/rw_phase.html

Crystalline and amorphous Initial：crystalline recorded: amorphous R: high Record R: low Erase laser spot recorded mark

What is amorphous? Amorphous non crystalline (disordered) state without LRO (long range order) but with SRO (short range order) Atomic arrangement of liquid is frozen Metastable state introduced by rapid cooling of liquid Random metallic alloy, chalcogenide glass, tetrahedral system, oxide glass DRPHS (dense random packing of hard spheres) can explain RDF (radial distribution function)

Radial distribution function (RDF) G(r): Probability to find a neighboring atom at a distance of r. Calculated experiment http://cmt.dur.ac.uk/sjc/thesis/thesis/node79.html

CD-R Organic dye is used Thermal decomposition PC substrate Organic dye layer Protective layer Reflecting layer Pre-groove Recorded mark CD-R CD Pit Organic dye is used Thermal decomposition Deformation of substrate by heat Work as a pit laser beam deformation of substrate protective layer PC substrate Dye layer

DVD Family 105 103-104 0.6 0.65 DVD-ROM DVD-R DVD-RAM DVD-RW DVD+RW   DVD-ROM DVD-R DVD-RAM DVD-RW DVD+RW capacity (GB) 4.7 / 9.4 2層8.54 3.95 / 7.9 4.7/9.4 Form disk cartridge Mark formation/ Material/ reflectivity pit formation 1L R=45-85 2L R=18-30 thermal deform organic dye R=45-85% phase change GeSbTe alloy R=18-30% AgInSbTe alloy wavelength nm lens NA 650/635 0.6 650 638/650 0.65 shortest mark size １層:0.4 2層：0.44 0.4 0.41-0.43  0.4 track width 0.74 0.8 Wobbled Land pre-bit 0.74 Wobbled L/G 0.74 HF Wobbled groove Cyclability － 105 103-104

MO（magneto-optical）Recording Recording： Thermomagnetic (Curie point）recording Heat-assisted magnetic recording Playback：　Magneto-optical effect Rotation of linear polarization is converted to the electrical signal Employed in MO, MD disks Compatibility High repeatability：10,000,000 times Complicated optical head (Polarization detection） Novel inventions such as MSR, MAMMOS, DWDD are realized as commercial products

Magneto-optical (MO) Recording Recording:Thermomagnetic recording Magnetic recording using laser irradiation Reading out: Magneto-optical effect Magnetically induced polarization state MO disk, MD(Minidisk) High rewritability：more than 107 times Complex polarization optics New magnetic concepts: MSR, MAMMOS and DWDD

History of MO recording 1962 Conger,Tomlinson Proposal for MO memory 1967 Mee Fan Proposal of beam-addressable MO recording 1971 Argard (Honeywel) MO disk using MnBi films 1972 Suits(IBM) MO disk using EuO films 1973 Chaudhari(IBM) Compensation point recording to a-GdCo film 1976 Sakurai(Osaka U) Curie point recording on a-TbFe films1980 Imamura(KDD) Code-file MO memory using a-TbFe films 1981 Togami(NHK) TV picture recording using a-GdCo MO disk 1988 Commercial appearance of 5”MO disk (650MB) 1889 Commercial appearance of 3.5 ”MO disk(128MB) 1991 Aratani(Sony) MSR 1992 Sony MD 1997 Sanyo ASMO(5” 6GB：L/G, MFM/MSR) standard 1998 Fujitsu GIGAMO(3.5” 1.3GB) 2000 Sanyo, Maxell iD-Photo(5cmφ730MB) 2004 Sony Hi-MD

Structure of MO disk media MO disk structure Polycarbonate substrate SiNx layer for protection and MO-enhancement Al reflection layer MO-recording layer (amorphous TbFeCo) Groove Land Resin

MO recording How to record(1) Temperature increase by focused laser beam Magnetization is reduced when T exceeds Tc Record bits by external field when cooling M Tc Temp Tc Laser spot MO media Coil External field

MO recording How to record(2) Use of compensation point writing Amorphous TbFeCo: Ferrimagnet with Tcomp HC takes maximum at Tcomp Stability of small recorded marks Hc M Tb FeCo Mtotal Fe,Co Tb Tcomp Tc T RT

Amorphous TbFeCo Film TM (Fe,Co) R (Tb)

Two recording modes Light intensity modulation (LIM)： present MO Laser light is modulated by electrical signal Constant magnetic field Elliptical marks Magnetic field modulation (MFM)：MD, ASMO Field modulation by electrical signal Constant laser intensity Crescent-shaped marks Modulated laser beam Constant Constant field Modulated field Magnetic head (a) LIM (b) MFM

Shape of Recorded Marks (a) LIM (light intensity modulation) (b) MFM (magnetic field modulation)

MO recording How to read Magneto-optical conversion of magnetic signal to electric signal D1 + - LD D2 Differential detection Polarized Beam Splitter N S S N N S

Structure of MO Head Focusing lens Rotation of polarization LD Bias field coil Recorded marks Track pitch Focusing lens Rotation of polarization MO film Beam splitter mirror Half wave-plate lens PBS (polarizing beam splitter) LD Laser diode PD=photodiode Photo-detector

Advances in MO recording Super resolution MSR MAMMOS/DWDD Use of Blue Lasers Near field SIL Super-RENS (AgOx)

MSR (Magnetically induced super-resolution) Resolution is determined by diffraction limit d=0.6λ/NA, where NA=n sin α Marks smaller than wavelength cannot be resolved Separation of recording and reading layers Light intensity distribution is utilized Magnetization is transferred only at the heated region α d

Illustration of 3 kinds of MSR

AS-MO standard

iD-Photo specification

MAMMOS (magnetic amplification MO system)

Super-RENS super-resolution near-field system AgOx film：decomposition and precipitation of Ag Scattering center→near field Ag plasmon→enhancement reversible Applicable to both phase-change and MO recording 高温スポット 近接場散乱

To shorter wavelengths DVD-ROM: Using 405nm laser, successful play back of marks was attained with track pitch =0.26m、mark length =213m (capacity 25GB) using NA=0.85 lens [i]。 [i] M. Katsumura, et al.: Digest ISOM2000, Sept. 5-9, 2000, Chitose, p. 18. DVD-RW: Using 405nm laser, read / write of recorded marks of track pitch=0.34m and mark length=0.29m in 35m two-layered disk(capacity:27GB) was succeeded using NA=0.65 lens, achieving 33Mbps transfer rate [ii] 。 [ii] T. Akiyama, M. Uno, H. Kitaura, K. Narumi, K. Nishiuchi and N. Yamada: Digest ISOM2000, Sept. 5-9, 2000, Chitose, p. 116.

Read/Write using Blue-violet LD and SIL (solid immersion lens) NA=1.5 405nm 80nm mark 40GB ＳＩＬhead 405nm LD I. Ichimura et. al. (Sony), ISOM2000 FrM01

SIL (solid immersion lens)

Optical recording using SIL

Hybrid Recording 405nm LD Recording head (SIL) Readout MR head Achieved 60Gbit/in2 H. Saga et al. Digest MORIS/APDSC2000, TuE-05, p.92. TbFeCo disk