Optical recording medium having a liquid crystal layer

[54] OPTICAL RECORDING MEDIUM HAVING A LIQUID CRYSTAL LAYER

[75] Inventors: Akihiko Kuroiwa; Shigeru Asami;

Toshiki Aoi; Kazuo Takahashi;
Kenryo Namba, all of Tokyo, Japan

[73] Assignee: TDK Corporation, Tokyo, Japan

[21] Appl. No.: 706,447

[22] Filed: Feb. 27,1985

[30] Foreign Application Priority Data

Feb. 27, 1984 [JP] Japan 59-035686

Feb. 15, 1985 [JP] Japan 60-027988

[51] Int. Q.* G11C 13/04; G02F 1/13;

G01D 9/00

[52] U.S. a 365/108; 346/135.1;

350/351

[58] Field of Search 365/108; 346/135.1;

350/330, 351

[56] References Cited

U.S. PATENT DOCUMENTS

3,894,794 7/1975 Mitchell, Jr 365/108

4,514,045 4/1985 Huffman et al 350/351

4,581,317 4/1986 Simmons, III 346/135.1

4,622,654 11/1986 Yaniv et al 365/108

Primary Examiner—Terrell W. Fears
Assistant Examiner—Melissa J. Koval
Attorney, Agent, or Firm—Obion, Fisher, Spivak,
McClelland & Maier

[57] ABSTRACT

An optical recording medium comprising a pair of substrates each having an electrode, the space between the substrate is filled with a high-dielectric liquid crystal to form a rewritable recording layer. The recording medium comprises radiation absorbing means to provide a change of directional orientation of the liquid crystal upon exposure to a laser beam.

13 Claims, 1 Drawing Sheet

1

OPTICAL RECORDING MEDIUM HAVING A
LIQUID CRYSTAL LAYER

BACKGROUND OF THE INVENTION 5

This invention relates to optical recording media, and more particularly, to optical recording media having a liquid crystal layer.

In these years, great activities are concentrated on the development of erasable, rewritable optical recording 10 media. One known example of such optical recording media utilizes the phase transition of liquid crystal [see SPIE, 420, 194-199 (1983)]. In this typical example, liquid crystal, typically smectic liquid crystal is sandwiched between a light absorbing electrode layer on a 15 substrate and a transparent electrode layer on a transparent substrate. A laser beam is directed to the medium from above the transparent substrate under a certain electric field across the electrodes and the light absorbing electrode layer then absorbs radiation to produce 20 heat. Phase transition, for example, for smectic to nematic takes place in the exposed spot at an elevated temperature. Upon removal of the beam, the heated spot is quenched so that the phase transition is kept unchanged. A recorded spot is formed in this way to 25 complete writing. Read-out may be carried out by directing a reading laser beam to the recorded medium where light scattering occurs at recorded spots.

Erasing may be carried out by applying an electric field across the electorodes of the recorded medium or 30 heating and gradual cooling, thereby inducing phase transition back to the original phase, for example, smectic phase. Alternatively, erasing operation may be carried out by applying a laser beam under a certain voltage applied across the electrodes or by heating without 35 any voltage. The medium is ready for writing, that is, rewritalble.

Preferably, the liquid crystal layer contains a dichroic dye. A layer of aligner may be provided between the electrode layer and the liquid crystal layer to assist in 40 the alignment of liquid crystal molecules.

These media generally use a light-absorbing electrode layer formed of metal, for example, Cr, Cr alloy, Au, Al, etc. which is rather less sensitive to light, resulting in some disadvantages including low recording sensitiv- 45 ity and low S/N ratio upon reading out.

The inventors have found that nematic and smectic liquid crystals are not fully satisfactory in directional orientation or sensitivity and S/N ratio. There is a need for further improving the sensitivity, S/N ratio, storage 50 life, and other properties of the liquid crystal layer of optical recording media.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to 55 provide an improved rewritable optical recording medium of the heat mode having a liquid crystal layer.

Another object of the present invention is to provide such an optical recording medium having improved recording sensitivity and reading S/N ratio. 60

According to the present invention, there is provided an optical recording medium comprising a substrate having an electrode layer thereon; a transparent substrate having a transparent electrode layer thereon; an optical recording layer disposed between the electrode 65 layers and containing a liquid crystal; and means capable of absorbing radiation to produce sufficient heat to provide a change of directional orientation of the liquid

crystal. According to the feature of the present invention, the liquid crystal is a high-dielectric liquid crystal.

In preferred embodiments of the invention, the radiation absorbing means comprises (1) a radiation absorbing layer applied onto the electrode layer on the substrate, (2) a radiation absorbing, electroconductive layer which also serves as the electrode layer, or (3) a dye, typically dichroic dye which is contained in the recording layer along with the liquid crystal. In the most preferred embodiment, a phthalocyanine is applied onto the substrate to form a radiation-absorbing electrode layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the present invention will be more fully understood by reading the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of an optical recording medium according to the present invention;

FIG. 2 is a schematic enlarged view of a portion of the medium shown in FIG. 1; and

FIGS. 3A and 3B are schematic plan views of the track shown in FIG. 2 before and after writing by beam incidence, respectively.

It should be noted that the drawings are not drawn on actual scale or proportion because they are intended to facilitate the understanding of the construction of the recording medium of the invention.

DESCRIPTION OF THE PREFERRED
EMBODIMENTS

Referring to FIG. 1 there is schematically illustrated in cross section one embodiment of the optical recording medium according to the present invention. The recording medium is illustrated as comprising a pair of substrates and electrodes, that is, a first substrate 11 having thereon an electrode layer 12, preferably a light absorbing electrode layer, and a second, preferably transparent substrate 21 having thereon a transparent electrode layer 22. A recording layer 31 of liquid crystal is sandwiched between the electrode layer 12 on the substrate 11 and the transparent electrode layer 22 on the transparent substrate 21. In the illustrated embodiment, layers 13 and 23 of an aligner are applied to the electrode layers 12 and 22, respectively, to assist in the directional orientation of liquid crystal molecules.

The first substrate 11 may be formed of any insulating material, for example, glass, resins such as polycarbonate and epoxy resins, photosensitive resins, and metals and is provided on one of its major surfaces with the electrode layer 12.

Preferably, the electrode layer itself may also serve as a light absorbing layer. The preferred light absorbing agent is a phthalocyanine. Then the electrode layer may preferably be formed of a phthalocyanine alone. Other examples of the radiation-absorbing, electroconductive materials from which the light-absorbing electrode layers can be formed include CdTe, Cd, CdS, GaAs, GaSe, SbSe, and various organometallic compounds. In many cases, the light absorbing layer is separately formed on the electrode layer. In general, the electrode layer is formed from metal oxides such as indium tin oxide (ITO), indium oxide, tin oxide, etc.; single metals such as Al, Au, Cr, Pb, etc.; alloys such as chromium alloys;