JP3991551B2 - Optical recording method and optical recording apparatus - Google Patents

Description

【０００９】
構造式中、Ｘはシアノ基、メチル基、メトキシ基またはニトロ基を示し、Ｙはエーテル結合またはケトン結合を示す。ｌおよびｍはそれぞれ２から１８の整数を、ｎは５ないし５００の整数を示す。
（１２）光異性化基を有する高分子が、上記構造式中、ｌおよびｍがそれぞれ４ないし１０の整数であり、ｎが１０ないし１００の整数で示されるポリエステル樹脂であることを特徴とする、前記（１）ないし（１１）のいずれか１に記載の光記録方法。
【００１０】
（１３）前記高分子膜の厚みｄを次式１のように調整することを特徴とする、前記（１）ないし（１２）のいずれか１に記載の光記録方法。
式１
ｄ＝ｍ・λ／(4・Δn)
式中、ｄは高分子膜の厚さ、ｍは１以上の整数、λは読出光の波長、Δｎは飽和屈折率変化量を表す。
（１４）光源から出射された記録光の偏光角を制御する工程と、前記記録光を光記録媒体に照射する工程を有し、前記偏光角に対応した方位の波長板を前記光記録媒体に形成する光記録方法であって、前記光記録媒体が分子中に光異性化基を有する高分子膜または光異性化分子を含む高分子膜を有し、かつ光記録媒体の高分子膜に記録光を照射する工程に続いて、記録光を遮断し放置する工程が行われ、前記記録光照射工程と放置工程の両方の工程において、前記高分子膜の温度を、高分子のガラス転移温度Ｔ _g の±７℃以内になるように温度制御する光記録方法に用いる光記録装置であり、記録光を発生する光源と、前記記録光の偏光角を制御する偏光素子と、前記偏光子を介して得られた記録光を光記録媒体に照射する結像光学系を有し、前記記録光照射工程と放置工程の両方の工程において、前記高分子膜の温度を、高分子のガラス転移温度Ｔ _g の±７℃以内になるように温度制御するための温度制御手段を備えることを特徴とする光記録装置。
【００１１】
【発明の実施の形態】
本発明の光記録方法は、以下で説明する光配向方法を利用することを特徴とする。この光配向方法を使用することにより、記録時に高い複屈折（Δｎ）を達成することができる。したがって、光記録媒体の光記録層を薄くすることができ、たとえばスピンコート法による成膜などにより、均一な膜厚の光記録層の形成が可能で、読み取り時のエラーが少なくなる。（前記光配向方法は特願２０００−１２１４０号に詳しく説明されている。）
［光配向方法］
本発明において使用する光配向方法は、分子中に光異性化基を含む高分子膜または光異性化分子を含む高分子膜を使用する。
このような高分子膜に偏光照射を行った場合、偏光照射時および偏光遮断後にに生ずる現象を図１に模式的に示す。（光異性化分子としてアゾベンゼンを例にとって説明する。）
等方的なコンフォメーションを有するアゾベンゼン（図１のａ）参照）に感度のある波長の直線偏光を照射すると、分子が偏光方向と同方向に並んでいるアゾベンゼンのみが光を吸収しシス体へと異性化し、異性化したシス体は熱戻り反応により再びトランス体へ異性化する。このように偏光照射時には、偏光方向に規定されたトランス−シス−トランスの異性化サイクルが繰り返され、励起偏光に対して吸収の小さい方向（偏光軸に対して垂直方向）へ配向変化が生ずる（図１のｂ）参照）。またこの際、アゾベンゼンの配向変化に加えて、アゾベンゼンの異性化と配向変化によって高分子の配向変化も誘起される。
【００１２】
偏光照射を遮断した時点においてはシス体が残っており（図１のｂ）参照）、これは光学異方性を小さくする要因となる。このシス体が熱戻り反応によりトランス体に戻ることを促進すると、偏光照射の工程においてかなりな程度で優位な方向に配向されている、すなわち、偏光軸に対して垂直方向に配向されているトランス体と同方向に配向が促進され、光学異方性（複屈折）が増大する（図１のｃ）を参照）。また、偏光照射時においても前記配向を促進することにより光学異方性が増大する。さらに光異性化基あるいは光異性化分子の配向が増大する結果、上記の高分子主鎖の配向も一層促進される。
【００１３】
前記光配向方法は、上記の高分子膜に対して偏光を照射する工程、および偏光を遮断した後に放置する工程の両方の工程において、高分子膜の温度を、高分子のガラス転移温度Ｔ _g の±７℃以内になるように温度制御することを特徴とする。そうすると、光異性化基あるいは光異性化分子が熱戻り反応により異性化する際に上記のような配向を促進して高分子膜に高い光学異方性を付与することができる。（したがって、前記温度制御は前記のごとき配向を促進して高分子膜に高い光学異方性を付与するための温度制御である。）
すなわち、前記温度制御により、シス体からトランス体への熱戻り反応を促進させ、同時に高分子セグメント（および分子）の運動性を高め、さらにガラス転移温度近傍において増大するセグメント同士の協同運動性を積極的に配向変化に作用させることが可能となる。
【００１４】
次に、前記光配向方法において使用する、分子中に光異性化基を含む高分子膜または光異性化分子を含む高分子膜、特に、該光異性化基あるいは光異性化分子が、光によって異性化した後熱戻り反応によって異性化しうるT（thermal）型フォトクロミック性を有する高分子膜について説明する。
最初に分子中に光異性化基を含む高分子について説明する。光異性化基は、光によって異性化した後熱戻り反応によって異性化しうるT（thermal）型フォトクロミック性を有する基であって、このような異性化によって光異性化基の配向が生じ、光学異方性（複屈折や二色性等）を誘起させる基であれば特に制限はないが、偏光照射により選択的な光異性化反応、たとえば偏光軸に平行な分子のみが光を吸収して光幾何異性化反応を起こすものが好ましい。異性化としてはシス−トランス異性化だけでなく、シン−アンチ異性化も含まれる。
【００１５】
前記のごとき光異性化基としてはシアノ基、メチル基、ｎ−ブチル基等のアルキル基（炭素数１〜８）、トリフルオロメチル基、フェニル基、シクロヘキシル基等のシクロアルキル基、メトキシ基、トリフルオロメトキシ基等のアルコキシ基（炭素数１〜６）、ニトロ基、エトキシカルボニル基、フッ素原子等のハロゲン原子等で置換したアゾベンゼン構造を有する基の他、スチルベン、アゾメチン、インジゴ、チオインジゴなどの構造を有する基が挙げられる。中でもアゾ基を含む光異性化基が好ましく、特に、シアノ基、メチル基、メトキシ基またはニトロ基で置換したアゾベンゼンを含む基が好ましい。
【００１６】
また、前記の光異性化基は高分子の側鎖にあることが好ましい。高分子の主鎖は付加系ポリマーでも縮合系ポリマーのいずれでもよく、付加系ポリマーとしてはアクリレート系ポリマー、メタクリレート系ポリマー等が挙げられる。アクリレート系ポリマー、メタクリレート系ポリマーの場合、アゾベンゼンは、−ＣＯＯ−（ＣＨ）ｎ−Ｏ−等によってポリマーと連結される。また、縮合系ポリマーとしてはポリエステル系、ポリイミド系、ポリシロキサン系、ポリウレタン系、ポリウレア系などを挙げることができるが、中でもポリエステル系が好ましい。
また、光異性化基の熱異性化と配向変化によって高分子の配向変化が誘起されるような高分子を使用すると、複屈折増大効果が効率よく実現する。このため、側鎖あるいは主鎖におけるセグメント間の相互作用が適度に強いことが望まれる。たとえば、高分子の主鎖部に芳香族炭化水素環により構成された剛直な部分を含むことが好ましい。また、芳香族炭化水素環は分極率異方性が大きいので、配向変化による複屈折への寄与も大きい。芳香族炭化水素環としては連結基により連結された２つ以上のベンゼン環が挙げられる。前記連結基としては、エーテル結合、ケトン結合等が挙げられる。
また、アゾベンゼン誘導体が主鎖の芳香族環に連結しているものが好ましい。
さらに、光異性化基は、高分子の主鎖の繰り返し単位の中に少なくとも１つ含まれることが好ましいが、側鎖部に光異性化基を持たない構成単位との共重合体としてもよい。
【００１７】
中でも側鎖に光異性化基を有するポリエステルとして以下の構造式で示されるポリエステルが好ましい。
【００１８】
【化３】

【００１９】
式中、Xはシアノ基、メチル基、メトキシ基、またはニトロ基を示し、Yはエーテル結合、またはケトン結合を示す。lおよびmは、2から18の整数、より好ましくは4から10の整数を示し、nは5から500の整数、より好ましくは10から100の整数を示す。
前記構造式で示されるポリエステルの具体例を以下に挙げるがこれらに限定されるものではない。
【００２０】
【化４】

【００２１】
【化５】

【００２２】
【化６】

【００２３】
【化７】

【００２４】
【化８】

【００２５】
【化９】

【００２６】
【化１０】

【００２７】
ここで示した各高分子については、以下の具体的な実験で示されるような複屈折増大効果を確認することができた。
【００２８】
次に、本発明で使用する光異性化分子について説明する。光異性化分子としては、メチルレッド、メチルオレンジ、ディスパーズレッド１、ディスパーズレッド１３、ディスパーズオレンジ３などのアゾ基を含む色素分子が挙げられる。
また、光異性化分子が添加される高分子としては、ポリメチルメタクリレート、ポリビニルアルコール、ポリビニルアセテート、ポリアリルアミン等が挙げられる。
【００２９】
前記光配向方法によれば、光異性化基あるいは光異性化分子が熱異性化（熱戻り）する際に、それを含む高分子膜を、熱戻り反応によって誘起される前記光異性化基あるいは光異性化分子の配向を増強するための温度制御を行うため、シス体からトランス体への熱戻り異性化・配向を効果的に誘起させることができ、偏光遮断後の複屈折増大効果を大幅に改善することができる。またこのことに加え、側鎖部と主鎖部のセグメント間に働く相互作用によって、主鎖部の配向も誘発されて複屈折増大に寄与する。これにより、単に配向させる時間を短縮させるだけではなく、従来技術では不可能であった高い複屈折を得ることができる。
さらに、前記光配向方法における特徴的な異性化反応は、その運動を誘起させるために高い温度は必要としない。したがって、任意の配向状態を増強するうえで、従来技術よりも高い分解能を有する。後記の具体例において、複屈折増大効果は少なくとも1μmの分解能を示した。一方、従来の光配向方法においては、配向の駆動源であるセグメントの運動性を高める必要から熱処理を行っているが、セグメントの運動性を高めることは、反面、任意の配向分布を増強させるための分解能低下を招く。
【００３０】
次に、前記光配向方法について、実験例を示して、具体的に詳しく説明する。
１．まず、以下の実験において使用した側鎖に光異性化基であるシアノアゾベンゼンを有するポリエステル（以下においてこれを「光配向層材料」ということがある。）の製造、およびこのポリエステルを用いた光配向媒体の作製について説明する。ここでは、上記［化４］で示すポリエステルの製造例を示すが、上記構造式で示されるポリエステルは、下記の製造方法に準じた方法で同様に製造することができる。
以下において使用する光配向層材料は、M.Satoらの"Synthesis and properties of polyesters having cyanoazobenzene units in the side chain", Macromol. Rapid Commun. 15, 21-29 (1994)に報告されている。発明者は、上記文献に記載されている方法によって、本高分子を合成した。実施した合成手順を次に記す。
【００３１】
4- ヒドロキシ -4'- シアノアゾベンゼンの合成
4-アミノベンゾニトリル 2mol（236.3g），12N HCl 600ml，純水 600mlを氷浴中で攪拌しながら、NaNO₂ 水溶液（NaNO₂ 150g，純水 750ml）を滴下した。（Step1） フェノール 2mol（191.8g）とKOH 2mol（112.3g）を約2lの純水にすばやく溶かし、Step1の生成物を滴下した。吸引ろ過後、純水で洗浄して減圧乾燥させた。生成物をメタノールで再結晶し、4-ヒドロキシ-4'-シアノアゾベンゼン 1.3mol （292.3g，収率65.5%）を得た。
【００３２】
4-(6- ブロモヘキシルオキシ )-4'- シアノアゾベンゼンの合成
4-ヒドロキシ-4'-シアノアゾベンゼン 0.2mol（44.6g），1,6-ジブロモヘキサン 2mol（488.1g），K₂CO₃ 1.45mol（200.4g），アセトン 800mlを2lの三つ口フラスコに入れ、ウォータバスを用いて20時間還流し反応させた。室温まで冷やした後、副生成物と過剰のK₂CO₃をろ過して取り除いた。ロータリエバポレータを用いて約1/2程度まで濃縮した後に冷凍庫に放置して結晶化させた。吸引ろ過後、n-ヘキサンで洗浄して減圧乾燥させた。（0.117mol（45.3g，収率58.6%））さらに、エタノールで再結晶し4-(6-ブロモヘキシルオキシ)-4'-シアノアゾベンゼン 0.094mol（36.3g，収率47.0%）を得た。
【００３３】
5- ヒドロキシイソフタル酸ジエチルエステルの合成
5-ヒドロキロキシイソフタル酸 1mol（182.4g），エタノール 1500ml，濃硫酸 10mlを2lの三つ口フラスコに入れ、ウォータバスを用いて24時間還流し反応させた。ロータリエバポレータを用いて濃縮させNaHCO₃水溶液に注いだ後、ろ過・減圧乾燥させ5-ヒドロキシイソフタル酸ジエチルエステル0.096mol（228.8g，収率96.0%）を得た。さらに、エタノールで再結晶させた後に加熱（50〜60℃）減圧乾燥させた。
【００３４】
側鎖部モノマー： 5-(4- シアノベンゼンアゾフェノキシヘキシルオキシ )- イソフタル酸エチルエステルの合成
4-(6-ブロモヘキシルオキシ)-4'-シアノアゾベンゼン 0.08mol（30.9g），5-ヒドロキシイソフタル酸ジエチルエステル0.08mol，K₂CO₃ 0.12mol（16.58g），アセトン 400mlを1lの三つ口フラスコに入れ、ウォータバスを用いて24時間還流し反応させた。放冷後、約4lの純水に注ぎ沈殿物をろ過して取り出し減圧乾燥させた。（0.071mol（38.8g，収率89.2%））その後アセトンで再結晶させ、側鎖部モノマーとなる5-(4-シアノベンゼンアゾフェノキシヘキシルオキシ)-イソフタル酸エチルエステル 0.058mol（31.4g，収率72.2%）を得た。融点：99.0℃，吸収ピーク：364.2nm
側鎖部モノマーの同定は、FTIRスペクトルと¹H NMRによって行った。FTIRの結果を次に示し、［化１２］に¹H NMR分析スペクトル結果を示す。
FTIR（KBr）：2947.7cm^-1（CH伸縮），2227.4 cm^-1（CN），1713.4 cm^-1（エステルC＝O），1599.7 cm^-1（C＝C），1580 cm^-1（N＝N），1244.8 cm^-1（C−O−C）
【００３５】
【化１１】

【００３６】
主鎖部モノマー：ビス (4- ヒドロヘキシルオキシフェニル ) エーテルの合成
4,4'-ジヒドロキシジフェニルエーテル 0.3mol（60.66g），6-クロロ-1-ヘキサノール 0.66mol（90.16g），K₂CO₃ 0.7mol（97g），N,N-ジメチルホルムアミド 250mlを混合し、オイルバスを用いて160℃に加熱し24時間反応させた。その後、少量の塩酸を含む水に注ぎ生成物をろ過・減圧乾燥させ、ビス(4-ヒドロヘキシルオキシフェニル)エーテルを得た。さらに、水−N,N-ジメチルホルムアミド系より再結晶させた。（ほぼ定量的）融点：119℃
側鎖部モノマーの同定は、FTIRスペクトルと¹H NMRによって行った。FTIRの結果を次に示し、［化１３］に¹H NMR分析スペクトル結果を示す。
IR（KBr）（JASCO FT/IR-230）：3312.1cm^-1（OH），2936.1cm^-1（CH伸縮），1505.2 cm^-1（芳香族），1241.9 cm^-1（C−O−C），
【００３７】
【化１２】

【００３８】
アゾベンゼンを側鎖に持つポリエステルの溶融重縮合
側鎖部モノマー：5-(4-シアノベンゼンアゾフェノキシヘキシノキシ)-イソフタル酸エチルエステル 0.01mol（5.4361g），主鎖部モノマー：ビス(4-ヒドロヘキシルオキシフェニル)エーテル 0.01mol（4.0253g），無水酢酸亜鉛 0.1gを300mlの三つ口フラスコに入れ、窒素置換後に次のステップで反応させた。
1)約160℃で2時間反応
2)10Torrに減圧させて20分反応
3)30分かけて180℃，2〜5Torrに昇温・減圧
4)180℃，2〜5Torrで2時間反応
上記反応後、クロロホルムに溶解させてメタノール中に注いだ。沈殿物をろ過して取り出し純水→メタノールで加熱洗浄させた後に減圧乾燥させ、シアノアゾベンゼンを側鎖に持つポリエステルを得た。（ほぼ定量的）
【００３９】
図２に上記方法で合成した高分子のDSC曲線を示す。ガラス転移温度Tgは38℃、融点Tmは65℃であった。また平均分子量は約１１，９００であった。したがって平均繰り返し単位数ｎ＝１４である。偏光顕微鏡観察によると、液晶相は存在せず固体状態において複屈折を有する高分子であることがわかった。
光配向媒体の作製
上述した方法によって合成した高分子をクロロホルムに0.8g/mlの濃度で溶かし、洗浄したガラス基板上にスピンコートした（1000rpm，20sec）。乾燥させた後、等方相まで加熱し急冷した。偏光顕微鏡観察により、作製された膜（光配向層）は等方的なアモルファス膜であることを確認した。また、触針式の表面あらさ計を用いて膜厚を測定したところ1.5μmであった。
【００４０】
２．次に、前記光配向層を光配向する方法、および該方法に使用するための装置（以下において、この装置を「光配向装置」ということがある。）について説明する。
（１） 最初に、使用した光配向装置について説明する。３は光配向層に光学異方性を誘起し、かつ誘起された光学異方性を確認するための装置の概要を示す。図３中、１０は光配向媒体、２０はヒーター、３０はヒーター２０によって加熱される光配向層の温度を制御するための温度制御装置をそれぞれ示し、また、１は光配向層に光学異方性（複屈折）を誘起させるためのポンプ光を示す。ポンプ光１として、シアノアゾベンゼンを側鎖に持つポリエステルに感度のある、アルゴンイオンレーザの発振線５１５ｎｍを用いた。２はプローブ光、３は偏光子、４は検光子をそれぞれ示す。これらは、ポンプ光により誘起される光学異方性（複屈折）を調べるためのものである。プローブ光２としては、光配向媒体に感度のないヘリウムネオンレーザの発振線６３３ｎｍを用いた。
ヘリウムネオンレーザの光路中に光配向媒体１０を置き、その前後を偏光子３と検光子４で挟む。ここで、偏光子３の方位を０°偏光、検光子４の方位を９０°偏光とし、互いに垂直な方位とした。この条件では、光配向媒体１０に異方性がなければ、プローブ光２の偏光方向は変化せず、検光子４を通過できない。
ポンプ光１の偏光方向を、偏光子３と検光子４の方位のちょうど中間の４５°にして光配向媒体１０に照射し、複屈折を誘起した。このとき、プローブ光２は光配向媒体の誘起された複屈折により偏光が回転され、検光子４を通過する。検光子４を通過した光パワーＩは、以下の数式２で表される。この式を用いて、Δｎを計算することができる。
【００４１】
【数１】

【００４２】
式中、Ｉ₀は光配向媒体を通過する前のプローブ光の強度、Ｉは検光子４を通過した光強度を示し、λはプローブ光の波長を、ｄは光配向層の膜厚を示す。
上述した方法と同様の方法を用いたシアノアゾベンゼンを側鎖に持つポリエステル膜の光誘起複屈折は、例えばK.Nakagawa et al. "Holographic recording in polyesters having cyanoazobenzene units in the side chain", Proc. SPIE 2778, 571-572 (1996)、山田サユ他 "シアノアゾベンゼンを側鎖に持つポリエステルの光誘起異方性記録", 第58回応用物理学会学術講演会 講演予稿集, 952 (1997)に報告されている。それらによると、ポンプ光で誘起される複屈折は時間とともに増加して飽和し、ポンプ光を遮断した後はわずかに増加するものの、ほぼ等しい値で保存されることが示されている。
ポンプ光遮断後に同様の挙動を示す材料としては、例えばA.Natansohnらの"Azo polymers for reversible optical storage. 4. Cooprative motion of rigid groups in semicrystalline polymers", Macromolecules 27, 2580-2585 (1994)に報告されている。この材料は、主鎖に芳香族炭化水素を有する半結晶性高分子であり、ポンプ光遮断後に複屈折がわずかに増加して保存される。A.Natansohnらは、この現象に対し、ポンプ光照射による熱の影響を取り上げ、ガラス状態から緩和した半結晶性による効果と考察している。
【００４３】
（２）光配向層の温度制御
図３で示される装置を用い、光配向層に対して偏光を照射し、その後偏光を遮断し放置する２つの連続する処理を施し、その際光配向層の温度制御を行い、それぞれの温度で得られる光学異方性を調べた。この例では、偏光照射工程と偏光遮断放置工程の両方において温度制御を行った。
前記１．で作製した光配向媒体を用いて、温度制御装置30によって室温の25℃から47℃まで媒体の温度を変化させ、それぞれの温度で誘起される複屈折を測定した。温度制御にはヒーターと熱電対を備え、温度制御装置により設定温度を保持可能な顕微鏡用加熱装置（リンカム社製ＴＨ−１５００ＳＭ）を用いた。ポンプ光強度は、光照射による温度変化を小さくするために1Ｗ/cm²とした。（なお、仮に光照射により媒体の温度が上昇したとしても、温度制御装置30によって設定温度に補正される。）
ここでの制御温度の精度は±1℃であった。また、偏光照射領域を1mmφ、照射時間を1800secとした。
複屈折Δｎは前記式２に基づき、プローブ光強度Ｉ₀と検光子を通過した光強度Ｉ、λ、ｄから求めた。ここでλ＝６３３ｎｍ、ｄ=１．５μmである。結果を図４に示す。
【００４４】
図４は、光配向層の温度を25℃、39℃、44℃、47℃と変化させたときの実験結果である。どの温度においても、ポンプ光照射によって複屈折が誘起され時間とともに成長した。ここで、これら複屈折の大きさは媒体温度によって異なる値となった。さらに興味深いことに、ポンプ光遮断後に複屈折が非常に大きく増加することがわかった。この増大効果も、媒体温度に依存した。ヒーター20をオフにして室温まで放冷させると（7200sec）、その挙動は媒体温度により異なるが、わずかに上昇するか等しい値で保存されることがわかった。
【００４５】
図５は、偏光を照射する工程（偏光照射時間1800sec）において誘起された複屈折（図中、四角で示す。）と、偏光を遮断して放置した工程（放置時間7200sec）を経た複屈折（図中、丸で示す。）を媒体温度に対してプロットしたものである。偏光を照射する工程で誘起された複屈折と、偏光を遮断して放置した工程において誘起された複屈折（図５中、両曲線の差）は、ともに高分子のガラス転移温度（Tg＝38℃）付近で最大となることがわかった。本結果より、各工程の制御温度は光配向層材料のTg±7℃程度が好適である。
【００４６】
以上の結果より、光配向媒体の光配向層をそのガラス転移温度T_g付近とすれば、高い複屈折が誘起できることに加え、ポンプ光遮断後も配向増強が起こり高い複屈折が得られることがわかった。シアノアゾベンゼンを側鎖に持つポリエステル膜（［化４］参照）は、光誘起複屈折が約0.04との報告がある。（K.Nakagawa et al. "Holographic recording in polyesters having cyanoazobenzene units in the side chain", Proc. SPIE 2778, 571-572 (1996)）
上で示した光配向装置および光配向方法を用いた場合、誘起される最大の複屈折は約3倍の0.12を越え、従来の技術に比べて非常に高い配向を実現することができる。
（４）偏光の照射による光配向層の温度制御
ここでは、偏光の照射強度あるいは照射パワーを調整することによって、前記高分子膜の温度を偏光照射によって制御する方法について説明する。したがって、ポンプ光1はアゾ基を光異性化させるのに加えて、媒体温度を制御するように作用する。使用する装置としては、図３に示す光配向装置の光学系を用いることができる。媒体に感度のあるアルゴンイオンレーザの発振線515nmの波長は、アゾベンゼンのトランス−シス異性化を誘起するとともに、媒体の温度を上昇させることができる。この媒体温度の制御は、ポンプ光の強度またはパワーを変化させることによって実現可能である。
偏光の照射強度やパワーによって温度制御する際、媒体温度は光配向媒体の構成や膜厚など、媒体の畜熱特性に注意する必要がある。
【００４７】
次に、本発明の光記録方法および光記録装置について説明する。本発明の光記録方法は前記の光配向方法を利用するものであり、光記録媒体の、分子中に光異性化基を有する高分子膜または光異性化分子を含む高分子膜（以下において「光記録層」ということがある。）に記録光を照射する工程の後、引き続いて記録光を遮断し放置する工程を行い、前記記録光照射工程および放置工程のいずれかまたは両方の工程において、前記光配向方法において説明した温度制御を行うことを特徴とする。
［光記録媒体］
次に本発明の光記録方法において使用する光記録媒体について説明する。光記録媒体の一例を図６（Ａ）および図６（Ｂ）に示す。図６（Ａ）で示される光記録媒体５０は、光透過性基板５１の一面側に光記録層５２と反射層５４からなる反射型光記録媒体であり、また図６（Ｂ）で示される光記録媒体５０は、前記と同様な基板５１と光記録層５２のみからなる透過型光記録媒体である。
光透過性基板としては、ガラス基板などが用いられ、また反射層としてはアルミニウムなどの金属蒸着膜が用いられる。
【００４８】
また、光記録層１２に用いられる材料分子中に光異性化基を有する高分子材料または光異性化分子を含む高分子材料であって、偏光照射により光異性化を引き起こし、これが高分子に伝達して高い光学異方性（複屈折や二色性）を示すものであればどのようなものであってもよく、前述の光配向方法の箇所で説明した光異性化基を有する高分子材料または光異性化分子を含む高分子材料を用いることができる。
【００４９】
［光記録方法および光記録装置］
本発明の光記録方法を用いて波長板が形成された光記録媒体を作製する場合、光記録層の厚みｄを次式１のようにして調整することができる。
式１
ｄ＝ｍ・λ／(4・Δn)
式１において、ｄは光記録層の厚み、ｍは整数、λは読出光の波長、Δｎは飽和屈折率変化量をそれぞれ示す。
ｍが１の場合は、λ／４波長板、２の場合はλ／２波長板とすることができる。
前記［化４］で示されるポリエステル重合体の層を光記録層（膜厚１．５μｍ）とする光記録媒体を用い、図４で示すように、記録光照射工程（アルゴンイオンレーザ（５１５ｎｍ）、１Ｗ／ｃｍ²）およびその後の記録光遮断後放置工程において、光記録層の温度を、前記ポリエステル重合体のガラス転移温度（３８℃）に相当するように温度制御を行うと、０．１２の飽和屈折率変化を得ることができる。したがって、この光記録媒体の読出光を波長λ＝６３３ｎｍとすると、λ／４波長板として用いる場合には、式１より厚みｄは1.3μｍ、λ／２波長板として用いる場合には厚みｄが2.6μｍとなる。
一方、同様の光記録媒体を用い、特に温度制御を行わずに室温でポンプ光を照射した場合の飽和屈折率変化は０．０６１である（図４の２５℃における飽和複屈折を参照）ので、λ／４波長板として用いる場合には、前記式１より厚みｄは２．６μｍ、λ／２波長板では厚みｄが５．２μｍとなる。
したがって、ガラス転移温度付近の温度に温度制御を行う場合は、温度制御を行わない（室温）場合に比較して、光記録層の厚みをおよそ２分の１に薄くすることができる。また、このことは、たとえばスピンコート法による成膜などにより、均一な膜厚の光記録層の形成が可能なことを意味し、読み取り時のエラーが少なくなる。また、ディスク化が容易で、記録ピットの微小化、読取のS/Nなども実現できる。
【００５０】
次に、本発明の光記録方法において使用する光記録・読取装置の一例を説明する。上述の側鎖にシアノアゾベンゼンを持つポリエステルを光記録媒体に用いた例を説明する。図７は、図６(Ａ)に示した反射層を含む光記録媒体10を用いるときの光記録・読取装置である。光記録装置は、記録光源及び読み出し光源100、光分離素子110、光分離素子120、レンズ130、ヒーター20、温度制御装置30、偏光素子140、光検出器150から構成される。
記録光源及び読み出し光源としてはたとえばアルゴンイオンレーザの５１５ｎｍの発振線を用いることができる。記録時には、光源から出た光は偏光素子110によって直線偏光の方位が調整され、光分離素子120を透過後、レンズ130によって光記録媒体10に集光される。この時、ヒーター20及び温度制御装置30によって、光記録媒体１０の光記録媒層の温度を制御（たとえば記録層の高分子のガラス転移温度付近）にする。この状態で光誘起複屈折が記録される。
図４に示したように、光記録媒体10は記録光遮断後もその温度制御を続ければ、屈折率変化が成長し飽和する。
したがって、たとえば前記［化４］で示されるポリエステルを光記録層として用い、温度制御をそのガラス転移温度（３８℃）で行う場合は、記録光遮断後も飽和複屈折が0.12の値となるまで温度制御を継続する。屈折率変化が飽和した後は、温度を室温まで戻しても記録は維持されている。またこの際、光記録層の厚さは、読み出し光源110として前記のようにアルゴンイオンレーザの515nmの発振線を用いる場合、光記録媒体をλ／4波長板として機能させるためには、先の式１に基づき、光記録層の厚みを1.07μmとする。
【００５１】
このようにして光記録を行った光記録媒体を読み取るためには、図７において、読出光を光源100から出射させる。光源からの光は偏光素子110によって方位を持つ直線偏光に調節され、光分離素子120を透過し、レンズ130によって光記録媒体10に集光され、反射層で反射する。反射した光は上記のようにして記録された記録ピットを二度通過する。λ／4板が記録されたピットを読出光の直線偏光が２度通過することで、λ／2波長板を通過したのと同じ作用を受けるため、読出光の偏光方位は記録された複屈折方位に対して対称に回転する。この回転された偏光は来た光路を逆進し、レンズ130によってコリメートされた後、光分離素子120で反射され、偏光素子140と光検出器150によってその偏光方位が検出される。
【００５２】
図８は記録光と読み出された反射光の偏光方位を示す図である。記録光の偏光方位を45°とし、読出光の偏光方位を0°とした場合、検出された反射光の偏光方位は90°となり、偏光角による記録が行えていることを示す。例えば、記録光の偏光を1°刻みで45°まで変化させることを考えると、45値の記録が行え、反射光の偏光は、2°刻みで0°から90°の範囲で読み出せる。
【００５３】
図９は、図６(Ｂ)に示した反射層を含まない光記録媒体10を用いるときの光記録・読取装置である。この光記録・読取装置は、記録光源及び再生光源100、光分離素子110、レンズ130、ヒーター20、温度制御装置30、偏光素子140、光検出器150から構成される。記録時には、光源から出た光は偏光素子110によって直線偏光の方位が調整され、レンズ130によって光記録媒体10に集光される。この時、ヒーター20及び温度制御装置30によって、光記録媒体10の光記録層の温度を制御（たとえば記録層のガラス転移温度付近）する。この装置を用いた場合も、光記録層は図４に示されるような複屈折の変化を示す。また、読み出し光源110としてアルゴンイオンレーザの515nmの発振線を用いる場合、光記録媒体をλ／2波長板として機能させるためには、先の式１に基づき、光記録層の厚みを2.1μmとする。
光記録された光記録媒体の読み取りも図７におけるものと同様である。光源100から出射された読出光は、偏光素子110によって偏光角が調整され、レンズ130によって光記録媒体10に集光され、記録された記録ピットを通過する。このピットはλ／2波長板として機能するので、読出光の直線偏光が通過することで、読出光の偏光方位は記録された複屈折方位に対して対称に回転する。この回転された偏光はレンズ160によってコリメートされた後、偏光素子140と光検出器150によってその偏光方位が検出される。
反射型のときと同様に、記録光の偏光方位を45°とし、読出光の偏光方位を0°とした場合、検出された反射光の偏光方位は90°となる。
【００５４】
［温度制御を光照射強度の調節により行う方法］
本発明の光記録方法においては、温度制御を光照射強度の調節により行うことも可能である。すなわち、光記録及び温度制御を単一光源で行う光記録方法である。この方法では、記録光は光異性化基あるいは光異性化分子を光異性化させるのに加えて、光記録媒体の温度を制御するように作用される。したがって、図に示すようなヒーター20及び温度制御機構30は不要である。この場合たとえば、光記録層に感度をもつ発振線515nmのアルゴンイオンレーザは、光異性化基あるいは光異性化分子のトランス−シス異性化を誘起するとともに、光記録層の温度を上昇させることができる。この光記録層の温度の制御は、記録光の強度を変化させることによって実現可能である。
図１０は、［００３９］に記載の光配向媒体を光記録媒体として用いた場合に、記録光の強度を変えて複屈折を測定した結果である。横軸にはその時の光記録層温度を同時に示した。図より記録光強度が2Ｗ/cm²と5Ｗ/cm²のときに高い複屈折が誘起されていることがわかる。この時の光記録層の温度は、それぞれ32℃と39℃付近であり、この温度は、ガラス転移温度T_g±7℃に相当する。したがって、特にこの温度範囲内で高い複屈折が誘起できることがわかる。このように本方式では、特別な温度制御機構を必要とせず、記録光だけで効率よく光配向ができ高い複屈折を得ることがきる。
【００５５】
図７及び図９の光記録装置を用いて、光記録を行った。用いた光記録媒体の光記録層は前記の［化４］のポリエステル重合体を用いた。光源は発振線515nmのアルゴンイオンレーザを用い、光記録層の温度をガラス転移温度である38℃に上昇させるため、光強度を5Ｗ/cm²とした。（ヒーター20及び温度制御装置30は取り外した。）
光記録装置として図７のものを使用する場合は、光記録媒体として図６（Ａ）に示した反射型記録媒体を用い、光記録層の厚さは、光誘起可能な屈折率差0.04を考慮して3.2μｍとした。また、光記録装置として９のものを使用する場合は、光記録媒体として図６（Ｂ）に示した透過型記録媒体を用い、光記録層の厚さは6.4μmとした。図８に示したように、記録光の偏光方位を45°とし、読出光の偏光方位を0°とした場合、検出された反射光の偏光方位は90°となった。
【００５６】
【発明の効果】
本発明によれば、光記録媒体に偏光により任意の方位に複屈折を誘起し、この光照射ピットをλ／４波長板あるいはλ／２波長板として機能させ、その光学軸によって多値記録する方法及び装置において、効率よく分子を配向させ非常に高い複屈折を得ることができる。したがって、光記録層を薄膜化することが可能で、このことは、たとえばスピンコート法による成膜などにより、均一な膜厚の光記録層の形成が可能なことを意味し、読み取り時のエラーが少なくなる。また、ディスク化が容易で、記録ピットの微小化、読取のS/Nなども実現できる。
【図面の簡単な説明】
【図１】 アゾベンゼンが偏光を選択吸収して光異性化し、熱戻り反応により異性化・配向する状態を説明する概念図である。
【図２】 化４で示すポリエステルのDSC曲線を示す図である。
【図３】 本発明が用いる光配向方法に使用するための装置の概念図である。
【図４】 本発明が用いる光配向方法において、光配向層温度を種々変えた場合の複屈折の時間的変化を示す図である。
【図５】 本発明が用いる光配向方法において、光配向層温度の影響を説明する図である。
【図６】 本発明において使用する光記録媒体の一例を示す模式的断面図で、図６（Ａ）は反射型の、図６（Ｂ）は透過型のものを示す。
【図７】 本発明の光記録装置の一例を示す概念図である。
【図８】 図７の装置を用いて光記録した場合の記録光と再生光の偏光方位を示す図である。
【図９】 本発明の光記録装置の他の一例を示す概念図である。
【図１０】 本発明の光記録方法において、温度制御をポンプ光照射により制御した場合の複屈折の飽和値を示す図である。
【符号の説明】
１ ポンプ光
２ プローブ光
３ 偏光子
４ 検光子
１０ 光配向媒体
２０ ヒーター
２２ 熱電対
３０ 温度制御装置
５０ 光記録媒体
５１ 基板
５２ 光記録層
５４ 反射層
１００ 光源
１１０、１４０ 偏光素子
１２０ 光分離素子
１３０、１６０ レンズ
１５０ 光検出器[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical recording / reproducing method and apparatus capable of recording data at high density and capable of high-speed data transfer.
[0002]
[Prior art]
In Japanese Patent Laid-Open No. 11-238251, birefringence is induced in an arbitrary direction by polarization on an optical recording medium, and this light irradiation pit functions as a λ / 4 wavelength plate or a λ / 2 wavelength plate, and multi-value recording is performed by using the optical axis. A method and apparatus are disclosed. The prior invention will be described in detail below.
The optical recording medium is manufactured so as to include at least one optical recording layer made of an optical recording material having light-induced birefringence, and the optical recording layer functions as a half-wave plate or a quarter-wave plate. . Specifically, the optical recording layer is composed of a polymer compound or polymer liquid crystal having a photoisomerizable group in the side chain. In the optical recording method of the prior invention, birefringence is induced by irradiating an optical recording medium with linearly polarized recording light to form a half-wave plate or a quarter-wave plate in the polarization plane direction. By rotating the orientation of the polarization plane of the recording light (referred to as the polarization angle), the orientation of the half-wave plate or quarter-wave plate is rotated. The medium thickness and the light-induced birefringence are adjusted so that the reading method is a half-wave plate when transmitted light is used, and the quarter-wave plate is used when reflected light is used. By making the polarization angle of the recording light multi-valued, multi-value recording can be performed using this optical recording medium. In the optical reproduction method of the prior invention, the reproduction light having an arbitrary polarization angle is irradiated on the half-wave plate or the quarter-wave plate recorded in a specific direction by the recording light, and the transmitted light or reflection is applied. Changes in the polarization angle of light are detected. This change in the polarization angle corresponds to twice the difference between the polarization angle of the reproduction light and the recording light polarization angle. Therefore, the multi-value recording can be reproduced by detecting this angle.
[0003]
However, the optical recording medium described in the prior invention has a light-induced refractive index change of at most about 0.05, and when used as a λ / 4 wavelength plate, the medium thickness d needs to be 3 μm or more. Furthermore, when considering use as a λ / 2 wavelength plate, a film thickness of 6 μm or more is required. This optical recording layer is produced on a transparent substrate such as glass by spin coating, but it is very difficult to produce such a thick film. In addition, in an optical disc represented by a CD or DVD, it is necessary to reduce the medium thickness and improve the recording light condensing property in order to increase the density. More problematically, when the thickness of the medium changes, the substantial optical path (that is, the phase difference) changes and the reading light becomes elliptically polarized light. This causes an error when reading multi-value data based on the polarization angle as in the prior invention. Therefore, in order to put the prior invention to practical use, it is essential to reduce the thickness of the medium. In order to function as a wave plate as described above even if the medium is thin, a method for inducing a high refractive index change is necessary. The present invention achieves such an object.
[0004]
[Problems to be solved by the invention]
The present invention has been made in view of the above problems, and its purpose is to obtain high birefringence during recording, to reduce the thickness of the optical recording layer, and to error during reading. Is to provide an optical recording method, an optical recording medium and an optical recording apparatus used in this method.
[0005]
[Means for Solving the Problems]
  The above object can be solved by providing the following optical recording method, optical recording medium, and optical recording apparatus.
(1) A step of controlling a polarization angle of recording light emitted from a light source and a step of irradiating the optical recording medium with the recording light, and a wave plate having an orientation corresponding to the polarization angle is applied to the optical recording medium In the optical recording method to be formed, the optical recording medium has a polymer film having a photoisomerizable group in the molecule or a polymer film containing a photoisomerized molecule, and recording light is applied to the polymer film of the optical recording medium. Subsequent to the irradiation step, a step of blocking and leaving the recording light is performed, and the recording light irradiation step and the leaving step are performed.BothIn the process, the temperature of the polymer filmThe glass transition temperature T of the polymer _g Within ± 7 ℃Temperature controlYouAn optical recording method.
  By the optical recording method of the present invention, the thickness of the optical recording layer can be made thinner than the conventional one. This means that an optical recording layer having a uniform film thickness can be formed by, for example, film formation by spin coating, and errors in reading are reduced. Further, it has the characteristics that it can be easily made into a disk, and the recording pits can be miniaturized and reading S / N can be realized.
[0006]
(2) The optical recording method according to (1), wherein the wave plate is a λ / 4 wave plate or a λ / 2 wave plate.
(3) In at least one of the two steps, the temperature of the polymer film is a glass transition temperature T of the polymer._gThe temperature is controlled so as to become (1)Or(2) Optical recording method.
(4) In the step of blocking and leaving the recording light, the temperature of the polymer film is controlled to be equal to or higher than the temperature of the polymer film in the step of irradiating the recording light. (1) to (1)3The optical recording method according to any one of 1).
(5) The temperature control of the polymer film is performed by adjusting the irradiation intensity or irradiation power of the recording light.4The optical recording method according to any one of 1).
[0007]
(6The main chain of the polymer contains an aromatic hydrocarbon ring,5The optical recording method according to any one of 1).
(7) The aromatic hydrocarbon ring is two or more benzene rings linked by a linking group, wherein (1) to (1)6The optical recording method according to any one of 1).
(8) The photoisomerization group or photoisomerization molecule contains an azo group,7The optical recording method according to any one of 1).
(9The polymer having a photoisomerizable group is a polymer having an azobenzene derivative introduced into the side chain,8The optical recording method according to any one of 1).
(10The polymer having a photoisomerizable group is characterized in that an azobenzene derivative is introduced into a side chain, and the azobenzene derivative is linked to an aromatic ring of the main chain.9The optical recording method according to any one of 1).
(11) The polymer having a photoisomerizable group is a polyester resin represented by the following structural formula:10The optical recording method according to any one of 1).
[0008]
[Chemical 2]

[0009]
  In the structural formula, X represents a cyano group, a methyl group, a methoxy group, or a nitro group, and Y represents an ether bond or a ketone bond. l and m each represent an integer of 2 to 18, and n represents an integer of 5 to 500.
(12) The polymer having a photoisomerizable group is a polyester resin in which l and m are each an integer of 4 to 10 and n is an integer of 10 to 100 in the above structural formula, (1) to (11The optical recording method according to any one of 1).
[0010]
(13) The thickness d of the polymer film is adjusted as shown in the following formula 1.12The optical recording method according to any one of 1).
    Formula 1
d = m · λ / (4 · Δn)
  In the formula, d is the thickness of the polymer film, m is an integer of 1 or more, λ is the wavelength of the readout light, and Δn is the saturation refractive index change.
(14)Light for controlling the polarization angle of recording light emitted from a light source and irradiating the optical recording medium with the recording light, and forming a wave plate having an orientation corresponding to the polarization angle on the optical recording medium A recording method, wherein the optical recording medium has a polymer film having a photoisomerizable group or a polymer film containing a photoisomerized molecule in the molecule, and the polymer film of the optical recording medium is irradiated with recording light Subsequent to this step, a step of blocking and leaving the recording light is performed. In both the recording light irradiation step and the leaving step, the temperature of the polymer film is set to the glass transition temperature T of the polymer. _g Is an optical recording apparatus used in an optical recording method for controlling the temperature to be within ± 7 ° C.A light source that generates recording light; a polarizing element that controls a polarization angle of the recording light; and an imaging optical system that irradiates the optical recording medium with the recording light obtained through the polarizer.In both the recording light irradiation step and the leaving step,Polymer membrane temperatureThe glass transition temperature T of the polymer _g Temperature within ± 7 ℃An optical recording apparatus comprising temperature control means for controlling.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The optical recording method of the present invention is characterized by utilizing the optical alignment method described below. By using this photo-alignment method, high birefringence (Δn) can be achieved during recording. Therefore, the optical recording layer of the optical recording medium can be thinned, and an optical recording layer having a uniform film thickness can be formed by, for example, film formation by a spin coating method, and errors in reading are reduced. (The photo-alignment method is described in detail in Japanese Patent Application No. 2000-12140.)
[Photo-alignment method]
The photo-alignment method used in the present invention uses a polymer film containing a photoisomerizable group in the molecule or a polymer film containing a photoisomerized molecule.
When such a polymer film is irradiated with polarized light, the phenomenon that occurs during polarized light irradiation and after polarization blocking is schematically shown in FIG. (This will be explained using azobenzene as an example of a photoisomerization molecule.)
When azobenzene having an isotropic conformation (see Fig. 1a) is irradiated with linearly polarized light with a sensitive wavelength, only azobenzene whose molecules are aligned in the same direction as the polarization direction absorbs light and enters the cis-form. And the isomerized cis form is isomerized again to the trans form by a heat return reaction. Thus, during polarized light irradiation, the trans-cis-trans isomerization cycle defined in the polarization direction is repeated, and the orientation change occurs in the direction of small absorption (perpendicular to the polarization axis) with respect to the excitation polarization ( See b) of FIG. At this time, in addition to the change in orientation of azobenzene, a change in orientation of the polymer is also induced by isomerization and orientation change of azobenzene.
[0012]
At the time when the polarized light irradiation is interrupted, a cis isomer remains (see b in FIG. 1), which becomes a factor for reducing the optical anisotropy. When this cis isomer is promoted to return to the trans isomer by the heat return reaction, the cis isomer is oriented in a dominant direction to a considerable extent in the polarization irradiation process, that is, the transformer is oriented perpendicular to the polarization axis. Orientation is promoted in the same direction as the body, and optical anisotropy (birefringence) increases (see c in FIG. 1). In addition, the optical anisotropy is increased by promoting the orientation even during irradiation with polarized light. Furthermore, as a result of the increased orientation of the photoisomerizable group or photoisomerized molecule, the orientation of the polymer main chain is further promoted.
[0013]
  The photo-alignment method includes a step of irradiating polarized light to the polymer film and a step of leaving the polarized light after blocking the polarized light.BothIn the process, the temperature of the polymer film, Polymer glass transition temperature T _g Within ± 7 ℃It is characterized by temperature control. Then, when the photoisomerization group or photoisomerization molecule is isomerized by a heat return reaction, the above-described orientation can be promoted to impart high optical anisotropy to the polymer film. (Therefore, the temperature control is a temperature control for promoting the alignment as described above to impart high optical anisotropy to the polymer film.)
  That is, the temperature control promotes the heat return reaction from the cis isomer to the trans isomer, and at the same time, enhances the mobility of the polymer segment (and molecule), and further increases the cooperative motility of the segments that increase near the glass transition temperature. It is possible to positively affect the orientation change.
[0014]
Next, a polymer film containing a photoisomerizable group in a molecule or a polymer film containing a photoisomerized molecule used in the photo-alignment method, in particular, the photoisomerizable group or photoisomerized molecule is A polymer film having T (thermal) type photochromic properties that can be isomerized by a heat return reaction after isomerization will be described.
First, a polymer containing a photoisomerizable group in the molecule will be described. A photoisomerization group is a group having a T (thermal) photochromic property that can be isomerized by a heat return reaction after being isomerized by light. Such isomerization causes orientation of the photoisomerization group, resulting in an optical heterogeneity. There is no particular limitation as long as it is a group that induces anisotropy (birefringence, dichroism, etc.), but selective photoisomerization reaction by polarized light irradiation, for example, only molecules parallel to the polarization axis absorb light and light. Those that undergo geometric isomerization are preferred. Isomerization includes not only cis-trans isomerization but also syn-anti isomerization.
[0015]
Examples of the photoisomerization group as described above include alkyl groups (1 to 8 carbon atoms) such as cyano group, methyl group and n-butyl group, cycloalkyl groups such as trifluoromethyl group, phenyl group and cyclohexyl group, methoxy group, In addition to alkoxy groups (1 to 6 carbon atoms) such as trifluoromethoxy groups, nitro groups, ethoxycarbonyl groups, groups having an azobenzene structure substituted with halogen atoms such as fluorine atoms, stilbene, azomethine, indigo, thioindigo, etc. Examples include groups having a structure. Among them, a photoisomerization group containing an azo group is preferable, and a group containing azobenzene substituted with a cyano group, a methyl group, a methoxy group or a nitro group is particularly preferable.
[0016]
The photoisomerization group is preferably in the side chain of the polymer. The main chain of the polymer may be either an addition polymer or a condensation polymer, and examples of the addition polymer include an acrylate polymer and a methacrylate polymer. In the case of an acrylate polymer or a methacrylate polymer, azobenzene is linked to the polymer by —COO— (CH) n—O— or the like. Examples of the condensation polymer include polyesters, polyimides, polysiloxanes, polyurethanes, and polyureas. Among them, polyesters are preferable.
In addition, when a polymer in which the orientation change of the polymer is induced by thermal isomerization of the photoisomerization group and the orientation change, the birefringence increasing effect is efficiently realized. For this reason, it is desired that the interaction between segments in the side chain or main chain is moderately strong. For example, it is preferable that the main chain portion of the polymer includes a rigid portion constituted by an aromatic hydrocarbon ring. In addition, since the aromatic hydrocarbon ring has a large polarizability anisotropy, the contribution to the birefringence due to the orientation change is large. Examples of the aromatic hydrocarbon ring include two or more benzene rings linked by a linking group. Examples of the linking group include an ether bond and a ketone bond.
Further, those in which the azobenzene derivative is linked to the aromatic ring of the main chain are preferred.
Furthermore, the photoisomerization group is preferably contained in at least one repeating unit of the main chain of the polymer, but may be a copolymer with a structural unit having no photoisomerization group in the side chain portion. .
[0017]
Of these, polyesters represented by the following structural formula are preferred as the polyester having a photoisomerizable group in the side chain.
[0018]
[Chemical 3]

[0019]
In the formula, X represents a cyano group, a methyl group, a methoxy group, or a nitro group, and Y represents an ether bond or a ketone bond. l and m represent an integer of 2 to 18, more preferably an integer of 4 to 10, and n represents an integer of 5 to 500, more preferably an integer of 10 to 100.
Specific examples of the polyester represented by the structural formula are listed below, but are not limited thereto.
[0020]
[Formula 4]

[0021]
[Chemical formula 5]

[0022]
[Chemical 6]

[0023]
[Chemical 7]

[0024]
[Chemical 8]

[0025]
[Chemical 9]

[0026]
[Chemical Formula 10]

[0027]
For each polymer shown here, the birefringence increasing effect as shown in the following specific experiment could be confirmed.
[0028]
Next, the photoisomerized molecule used in the present invention will be described. Examples of the photoisomerized molecule include dye molecules containing an azo group such as methyl red, methyl orange, disperse red 1, disperse red 13, and disperse orange 3.
Examples of the polymer to which the photoisomerized molecule is added include polymethyl methacrylate, polyvinyl alcohol, polyvinyl acetate, and polyallylamine.
[0029]
According to the photo-alignment method, when the photoisomerization group or photoisomerization molecule undergoes thermal isomerization (thermal return), the polymer film containing the photoisomerization group or photoisomerization group is induced by the thermal return reaction or Since temperature control is performed to enhance the orientation of photoisomerized molecules, it is possible to effectively induce heat-return isomerization and orientation from the cis form to the trans form, greatly increasing the birefringence increase effect after polarization blocking. Can be improved. In addition to this, the interaction between the side chain portion and the main chain segment also induces the orientation of the main chain portion and contributes to an increase in birefringence. This not only shortens the alignment time, but also provides high birefringence that was impossible with the prior art.
Furthermore, the characteristic isomerization reaction in the photo-alignment method does not require a high temperature to induce the motion. Therefore, it has a higher resolution than the prior art in enhancing an arbitrary alignment state. In the specific examples described later, the birefringence increasing effect showed a resolution of at least 1 μm. On the other hand, in the conventional photo-alignment method, heat treatment is performed because it is necessary to increase the mobility of the segment which is the driving source of the alignment. On the other hand, increasing the mobility of the segment is to enhance an arbitrary orientation distribution. Lowers the resolution.
[0030]
Next, the optical alignment method will be described in detail with reference to experimental examples.
1. First, the production of a polyester having a photoisomerizable group cyanoazobenzene in the side chain used in the following experiment (hereinafter sometimes referred to as “photo-alignment layer material”), and photo-alignment using this polyester Production of the medium will be described. Here, although the example of manufacture of the polyester shown by said [Chemical Formula 4] is shown, the polyester shown by the said structural formula can be manufactured similarly by the method according to the following manufacturing method.
The photo-alignment layer material used in the following is reported in “Synthesis and properties of polyesters having cyanoazobenzene units in the side chain”, Macromol. Rapid Commun. 15, 21-29 (1994). The inventor synthesized the present polymer by the method described in the above literature. The synthetic procedure performed is described below.
[0031]
Four- Hydroxy -Four'- Synthesis of cyanoazobenzene
While stirring 2-aminobenzonitrile 2mol (236.3g), 12N HCl 600ml, pure water 600ml in an ice bath, NaNO₂Aqueous solution (NaNO₂ 150 g, pure water 750 ml) was added dropwise. (Step 1) 2 mol of phenol (191.8 g) and 2 mol of KOH (112.3 g) were quickly dissolved in about 2 l of pure water, and the product of Step 1 was added dropwise. After suction filtration, it was washed with pure water and dried under reduced pressure. The product was recrystallized from methanol to obtain 1.3 mol (292.3 g, yield 65.5%) of 4-hydroxy-4′-cyanoazobenzene.
[0032]
4- (6- Bromohexyloxy )-Four'- Synthesis of cyanoazobenzene
4-hydroxy-4'-cyanoazobenzene 0.2mol (44.6g), 1,6-dibromohexane 2mol (488.1g), K₂CO_Three 1.45 mol (200.4 g) and 800 ml of acetone were placed in a 2 l three-necked flask and refluxed for 20 hours using a water bath. By-products and excess K after cooling to room temperature₂CO_ThreeWas removed by filtration. After concentration to about 1/2 using a rotary evaporator, it was allowed to crystallize in a freezer. After suction filtration, it was washed with n-hexane and dried under reduced pressure. (0.117 mol (45.3 g, yield 58.6%)) Further, recrystallization from ethanol gave 0.094 mol (36.3 g, yield 47.0%) of 4- (6-bromohexyloxy) -4′-cyanoazobenzene.
[0033]
Five- Synthesis of diethyl ester of hydroxyisophthalic acid
5-hydroxyl-isophthalic acid 1 mol (182.4 g), ethanol 1500 ml, concentrated sulfuric acid 10 ml were placed in a 2 l three-necked flask and refluxed for 24 hours using a water bath. Concentrate using a rotary evaporator_ThreeAfter pouring into an aqueous solution, filtration and drying under reduced pressure yielded 0.096 mol (228.8 g, yield 96.0%) of 5-hydroxyisophthalic acid diethyl ester. Furthermore, after recrystallization with ethanol, heating (50 to 60 ° C.) and drying under reduced pressure were performed.
[0034]
Side chain monomer: 5- (4- Cyanobenzeneazophenoxyhexyloxy )- Synthesis of isophthalic acid ethyl ester
4- (6-Bromohexyloxy) -4'-cyanoazobenzene 0.08mol (30.9g), 5-hydroxyisophthalic acid diethyl ester 0.08mol, K₂CO_Three 0.12 mol (16.58 g) and acetone (400 ml) were placed in a 1 l three-necked flask and refluxed for 24 hours using a water bath. After allowing to cool, the mixture was poured into about 4 l of pure water, filtered out, and dried under reduced pressure. (0.071 mol (38.8 g, yield 89.2%)) Then recrystallized with acetone, and 0.058 mol (31.4 g, yield) of 5- (4-cyanobenzeneazophenoxyhexyloxy) -isophthalic acid ethyl ester as the side chain monomer. 72.2%). Melting point: 99.0 ° C, absorption peak: 364.2nm
The identification of the side chain monomer is based on the FTIR spectrum.¹Performed by 1 H NMR. The results of FTIR are shown below.¹H NMR analysis spectrum results are shown.
FTIR (KBr): 2947.7cm^-1(CH expansion and contraction), 2227.4 cm^-1(CN), 1713.4 cm^-1(Ester C = O), 1599.7 cm^-1(C = C), 1580 cm^-1(N = N), 1244.8 cm^-1(C-O-C)
[0035]
Embedded image

[0036]
Main chain monomer: Screw (Four- Hydrohexyloxyphenyl ) Synthesis of ether
4,4'-dihydroxydiphenyl ether 0.3 mol (60.66 g), 6-chloro-1-hexanol 0.66 mol (90.16 g), K₂CO_Three 0.7 mol (97 g) and 250 ml of N, N-dimethylformamide were mixed, heated to 160 ° C. using an oil bath, and reacted for 24 hours. Thereafter, the product was poured into water containing a small amount of hydrochloric acid, and the product was filtered and dried under reduced pressure to obtain bis (4-hydroxyhexyloxyphenyl) ether. Furthermore, recrystallization was performed from a water-N, N-dimethylformamide system. Melting point: 119 ° C (almost quantitative)
The identification of the side chain monomer is based on the FTIR spectrum.¹Performed by 1 H NMR. The results of FTIR are shown below.¹H NMR analysis spectrum results are shown.
IR (KBr) (JASCO FT / IR-230): 3312.1cm^-1(OH), 2936.1cm^-1(CH expansion and contraction), 1505.2 cm^-1(Aromatic), 1241.9 cm^-1(C-O-C),
[0037]
Embedded image

[0038]
Melt polycondensation of polyesters with azobenzene in the side chain
Side chain monomer: 5- (4-cyanobenzeneazophenoxyhexoxy) -isophthalic acid ethyl ester 0.01 mol (5.4361 g), main chain monomer: bis (4-hydroxyhexyloxyphenyl) ether 0.01 mol (4.0253 g) ), 0.1 g of anhydrous zinc acetate was placed in a 300 ml three-necked flask and reacted in the next step after purging with nitrogen.
1) Reaction at about 160 ° C for 2 hours
2) Depressurize to 10 Torr and react for 20 minutes
3) Heating / depressurizing at 180 ° C and 2-5 Torr over 30 minutes
4) Reaction for 2 hours at 180 ℃, 2-5Torr
After the reaction, it was dissolved in chloroform and poured into methanol. The precipitate was filtered out and washed with pure water → methanol and dried under reduced pressure to obtain a polyester having cyanoazobenzene in the side chain. (Almost quantitative)
[0039]
FIG. 2 shows a DSC curve of the polymer synthesized by the above method. The glass transition temperature Tg was 38 ° C. and the melting point Tm was 65 ° C. The average molecular weight was about 11,900. Therefore, the average number of repeating units n = 14. According to observation with a polarizing microscope, it was found that the polymer had no liquid crystal phase and had a birefringence in the solid state.
Fabrication of photo-alignment media
The polymer synthesized by the above-described method was dissolved in chloroform at a concentration of 0.8 g / ml and spin-coated on a cleaned glass substrate (1000 rpm, 20 sec). After drying, it was heated to an isotropic phase and quenched. It was confirmed by polarizing microscope observation that the produced film (photo-alignment layer) was an isotropic amorphous film. Further, the film thickness was measured by using a stylus type surface roughness tester and found to be 1.5 μm.
[0040]
2. Next, a method for photo-aligning the photo-alignment layer and an apparatus for use in the method (hereinafter, this apparatus may be referred to as “photo-alignment apparatus”) will be described.
(1) First, the used photo-alignment apparatus will be described. 3 shows an outline of an apparatus for inducing optical anisotropy in the photo-alignment layer and confirming the induced optical anisotropy. In FIG. 3, 10 is a photo-alignment medium, 20 is a heater, 30 is a temperature control device for controlling the temperature of the photo-alignment layer heated by the heater 20, and 1 is optically anisotropic to the photo-alignment layer. The pump light for inducing property (birefringence) is shown. As pump light 1, an oscillation line of 515 nm of an argon ion laser sensitive to polyester having cyanoazobenzene in the side chain was used. 2 is a probe beam, 3 is a polarizer, and 4 is an analyzer. These are for examining the optical anisotropy (birefringence) induced by the pump light. As the probe light 2, an oscillation line of 633 nm of a helium neon laser having no sensitivity to the photo-alignment medium was used.
The photo-alignment medium 10 is placed in the optical path of the helium neon laser, and the front and rear thereof are sandwiched between the polarizer 3 and the analyzer 4. Here, the orientation of the polarizer 3 was 0 ° polarization, the orientation of the analyzer 4 was 90 ° polarization, and the orientations were perpendicular to each other. Under this condition, if the photo-alignment medium 10 has no anisotropy, the polarization direction of the probe light 2 does not change and cannot pass through the analyzer 4.
The polarization direction of the pump light 1 was set to 45 °, which is exactly the middle between the polarizers 3 and 4, and the photo-alignment medium 10 was irradiated to induce birefringence. At this time, the probe light 2 is rotated in polarization by the birefringence induced by the photo-alignment medium, and passes through the analyzer 4. The optical power I that has passed through the analyzer 4 is expressed by the following formula 2. Using this equation, Δn can be calculated.
[0041]
[Expression 1]

[0042]
Where I₀Is the intensity of the probe light before passing through the photo-alignment medium, I is the intensity of light that has passed through the analyzer 4, λ is the wavelength of the probe light, and d is the film thickness of the photo-alignment layer.
For example, K. Nakagawa et al. “Holographic recording in polyesters having cyanoazobenzene units in the side chain”, Proc. SPIE 2778, 571-572 (1996), Sayu Yamada et al. "Photo-induced anisotropy recording of polyesters with cyanoazobenzene in the side chain", Proceedings of the 58th JSAP Conference, 952 (1997) ing. According to them, it is shown that the birefringence induced by the pump light increases and saturates with time, increases slightly after the pump light is cut off, but is preserved at an approximately equal value.
For example, A.Natansohn et al., “Azo polymers for reversible optical storage. 4. Cooprative motion of rigid groups in semicrystalline polymers”, Macromolecules 27, 2580-2585 (1994). Has been. This material is a semi-crystalline polymer having an aromatic hydrocarbon in the main chain, and is stored with a slight increase in birefringence after blocking the pump light. A. Natansohn and colleagues consider the effect of heat due to pump light irradiation on this phenomenon and consider it as an effect of semi-crystallinity relaxed from the glass state.
[0043]
(2) Temperature control of photo-alignment layer
Using the apparatus shown in FIG. 3, two continuous treatments are performed in which the photo-alignment layer is irradiated with polarized light, and then the polarized light is blocked and allowed to stand, and the temperature of the photo-alignment layer is controlled at each temperature. The obtained optical anisotropy was examined. In this example, temperature control was performed in both the polarized light irradiation step and the polarized light blocking step.
1 above. The temperature of the medium was changed from 25 ° C. at room temperature to 47 ° C. by the temperature control device 30 using the photo-alignment medium produced in the above, and the birefringence induced at each temperature was measured. For the temperature control, a heating device for a microscope (TH-1500SM manufactured by Linkham Co., Ltd.) equipped with a heater and a thermocouple and capable of holding a set temperature by the temperature control device was used. The pump light intensity is 1 W / cm to reduce the temperature change due to light irradiation.²It was. (Note that even if the temperature of the medium rises due to light irradiation, the temperature controller 30 corrects it to the set temperature.)
The accuracy of the control temperature here was ± 1 ° C. The polarized light irradiation area was 1 mmφ, and the irradiation time was 1800 sec.
The birefringence Δn is based on the above formula 2 and the probe light intensity I₀And the light intensities I, λ, and d that passed through the analyzer. Here, λ = 633 nm and d = 1.5 μm. The results are shown in FIG.
[0044]
FIG. 4 shows experimental results when the temperature of the photo-alignment layer is changed to 25 ° C., 39 ° C., 44 ° C., and 47 ° C. At any temperature, birefringence was induced by irradiation with pump light and grew with time. Here, the magnitudes of these birefringences differed depending on the medium temperature. More interestingly, it was found that the birefringence increases greatly after the pump light is cut off. This increase effect also depended on the medium temperature. It was found that when the heater 20 was turned off and allowed to cool to room temperature (7200 sec), the behavior varied depending on the medium temperature, but increased slightly or stored at an equal value.
[0045]
FIG. 5 shows the birefringence induced by the step of irradiating polarized light (polarized light irradiation time: 1800 sec) (indicated by a square in the figure) and the birefringence after passing through the step of blocking polarized light (leaving time: 7200 sec) ( In the figure, these are plotted with respect to the medium temperature. Both the birefringence induced in the process of irradiating polarized light and the birefringence induced in the process of leaving the polarized light blocked (the difference between the two curves in FIG. 5) are both the glass transition temperature of the polymer (Tg = 38). It was found that the maximum was in the vicinity of ° C. From this result, the control temperature of each step is preferably about Tg ± 7 ° C. of the photo-alignment layer material.
[0046]
From the above results, the photo-alignment layer of the photo-alignment medium has a glass transition temperature T_gIn the vicinity, in addition to being able to induce high birefringence, it was found that alignment enhancement occurs even after the pump light is cut off and high birefringence can be obtained. A polyester film having cyanoazobenzene in the side chain (see [Chemical Formula 4]) has been reported to have a light-induced birefringence of about 0.04. (K. Nakagawa et al. "Holographic recording in polyesters having cyanoazobenzene units in the side chain", Proc. SPIE 2778, 571-572 (1996))
When the photo-alignment apparatus and the photo-alignment method shown above are used, the maximum birefringence induced exceeds 0.12, which is about three times, and a very high orientation can be realized as compared with the conventional technique.
(4) Temperature control of photo-alignment layer by irradiation of polarized light
Here, a method of controlling the temperature of the polymer film by applying polarized light by adjusting the irradiation intensity or irradiation power of polarized light will be described. Therefore, the pump light 1 acts to control the medium temperature in addition to photoisomerizing the azo group. As an apparatus to be used, the optical system of the optical alignment apparatus shown in FIG. 3 can be used. The wavelength of the 515 nm oscillation line of an argon ion laser sensitive to the medium can induce trans-cis isomerization of azobenzene and raise the temperature of the medium. This control of the medium temperature can be realized by changing the intensity or power of the pump light.
When the temperature is controlled by the irradiation intensity and power of polarized light, it is necessary to pay attention to the storage heat characteristics of the medium, such as the structure and film thickness of the photo-alignment medium.
[0047]
Next, the optical recording method and the optical recording apparatus of the present invention will be described. The optical recording method of the present invention utilizes the above-described photo-alignment method, and is a polymer film having a photoisomerizable group in a molecule or a polymer film containing a photoisomerizable molecule (hereinafter referred to as “ After the step of irradiating the recording light on the optical recording layer)), the recording light is subsequently blocked and left standing, and in either or both of the recording light irradiation step and the leaving step, The temperature control described in the photo-alignment method is performed.
[Optical recording medium]
Next, the optical recording medium used in the optical recording method of the present invention will be described. An example of the optical recording medium is shown in FIGS. 6 (A) and 6 (B). An optical recording medium 50 shown in FIG. 6A is a reflective optical recording medium including an optical recording layer 52 and a reflective layer 54 on one surface side of a light-transmitting substrate 51, and is also shown in FIG. 6B. The optical recording medium 50 is a transmission type optical recording medium composed of only the substrate 51 and the optical recording layer 52 similar to the above.
A glass substrate or the like is used as the light transmissive substrate, and a metal vapor deposition film such as aluminum is used as the reflective layer.
[0048]
In addition, a polymer material having a photoisomerization group in a material molecule used for the optical recording layer 12 or a polymer material containing a photoisomerization molecule, which causes photoisomerization by irradiation with polarized light, which is transmitted to the polymer. Any polymer that has high optical anisotropy (birefringence or dichroism) can be used, and the polymer material having the photoisomerization group described in the above-mentioned photo-alignment method section. Alternatively, a polymer material containing a photoisomerized molecule can be used.
[0049]
[Optical recording method and optical recording apparatus]
When an optical recording medium on which a wave plate is formed using the optical recording method of the present invention, the thickness d of the optical recording layer can be adjusted as shown in the following formula 1.
Formula 1
d = m · λ / (4 · Δn)
In Equation 1, d is the thickness of the optical recording layer, m is an integer, λ is the wavelength of the readout light, and Δn is the amount of change in saturated refractive index.
When m is 1, it can be a λ / 4 wavelength plate, and when m is 2, it can be a λ / 2 wavelength plate.
Using an optical recording medium in which the polyester polymer layer represented by [Chemical Formula 4] is an optical recording layer (film thickness: 1.5 μm), as shown in FIG. 4, a recording light irradiation step (argon ion laser (515 nm)) 1W / cm²) And the subsequent recording light blocking step, the temperature of the optical recording layer is controlled so as to correspond to the glass transition temperature (38 ° C.) of the polyester polymer. Can be obtained. Therefore, assuming that the reading light of this optical recording medium has a wavelength λ = 633 nm, when used as a λ / 4 wavelength plate, the thickness d is 1.3 μm from Equation 1, and when used as a λ / 2 wavelength plate, the thickness d is 2.6 μm.
On the other hand, when the same optical recording medium is used and pump light is irradiated at room temperature without performing temperature control, the saturation refractive index change is 0.061 (see the saturated birefringence at 25 ° C. in FIG. 4). In the case of using as a λ / 4 wavelength plate, the thickness d is 2.6 μm from the formula 1, and the thickness d is 5.2 μm in the λ / 2 wavelength plate.
Therefore, when temperature control is performed at a temperature near the glass transition temperature, the thickness of the optical recording layer can be reduced to about one half as compared with the case where temperature control is not performed (room temperature). This also means that an optical recording layer having a uniform film thickness can be formed by, for example, film formation by spin coating, and errors in reading are reduced. In addition, it is easy to make a disc, and the recording pits can be miniaturized and read S / N can be realized.
[0050]
Next, an example of an optical recording / reading apparatus used in the optical recording method of the present invention will be described. An example in which the above polyester having cyanoazobenzene in the side chain is used for an optical recording medium will be described. FIG. 7 shows an optical recording / reading apparatus using the optical recording medium 10 including the reflective layer shown in FIG. The optical recording apparatus includes a recording light source and readout light source 100, a light separation element 110, a light separation element 120, a lens 130, a heater 20, a temperature control device 30, a polarization element 140, and a photodetector 150.
As a recording light source and a reading light source, for example, an 515 nm oscillation line of an argon ion laser can be used. At the time of recording, the light emitted from the light source is adjusted in the direction of linearly polarized light by the polarizing element 110, transmitted through the light separating element 120, and then condensed on the optical recording medium 10 by the lens 130. At this time, the temperature of the optical recording medium layer of the optical recording medium 10 is controlled by the heater 20 and the temperature control device 30 (for example, near the glass transition temperature of the polymer of the recording layer). In this state, light-induced birefringence is recorded.
As shown in FIG. 4, if the temperature control of the optical recording medium 10 continues even after the recording light is cut off, the refractive index change grows and becomes saturated.
Therefore, for example, when the polyester represented by the above [Chemical Formula 4] is used as an optical recording layer and the temperature control is performed at the glass transition temperature (38 ° C.), the saturation birefringence becomes 0.12 even after the recording light is cut off. Continue temperature control. After the refractive index change is saturated, the recording is maintained even if the temperature is returned to room temperature. At this time, the thickness of the optical recording layer is set so that the optical recording medium functions as a λ / 4 wavelength plate when the 515 nm oscillation line of the argon ion laser is used as the reading light source 110 as described above. Based on Equation 1, the thickness of the optical recording layer is 1.07 μm.
[0051]
In order to read the optical recording medium on which optical recording has been performed in this manner, reading light is emitted from the light source 100 in FIG. Light from the light source is adjusted to linearly polarized light having an orientation by the polarizing element 110, passes through the light separating element 120, is condensed on the optical recording medium 10 by the lens 130, and is reflected by the reflective layer. The reflected light passes twice through the recording pits recorded as described above. Since the linearly polarized light of the readout light passes through the pit where the λ / 4 plate is recorded twice, it undergoes the same action as that passed through the λ / 2 wavelength plate, so the polarization direction of the readout light is the recorded birefringence Rotate symmetrically with respect to orientation. The rotated polarized light travels backward in the optical path, collimated by the lens 130, then reflected by the light separating element 120, and the polarization direction is detected by the polarizing element 140 and the photodetector 150.
[0052]
FIG. 8 is a diagram showing the polarization directions of the recording light and the read reflected light. When the polarization direction of the recording light is 45 ° and the polarization direction of the readout light is 0 °, the polarization direction of the detected reflected light is 90 °, which indicates that the recording can be performed with the polarization angle. For example, considering that the polarization of the recording light is changed to 45 ° in 1 ° increments, 45 values can be recorded, and the polarization of the reflected light can be read in the range of 0 ° to 90 ° in 2 ° increments.
[0053]
FIG. 9 shows an optical recording / reading apparatus when the optical recording medium 10 not including the reflective layer shown in FIG. 6B is used. The optical recording / reading apparatus includes a recording light source / reproducing light source 100, a light separation element 110, a lens 130, a heater 20, a temperature control device 30, a polarizing element 140, and a photodetector 150. At the time of recording, the light emitted from the light source is adjusted in the direction of linearly polarized light by the polarizing element 110 and condensed on the optical recording medium 10 by the lens 130. At this time, the temperature of the optical recording layer of the optical recording medium 10 is controlled by the heater 20 and the temperature control device 30 (for example, near the glass transition temperature of the recording layer). Even when this apparatus is used, the optical recording layer exhibits a change in birefringence as shown in FIG. In addition, when the 515 nm oscillation line of an argon ion laser is used as the reading light source 110, in order to make the optical recording medium function as a λ / 2 wavelength plate, the thickness of the optical recording layer is 2.1 μm based on the above equation 1. To do.
Reading of the optically recorded optical recording medium is the same as that in FIG. The reading light emitted from the light source 100 is adjusted in polarization angle by the polarizing element 110, condensed on the optical recording medium 10 by the lens 130, and passes through the recorded recording pits. Since this pit functions as a λ / 2 wavelength plate, the polarization direction of the readout light rotates symmetrically with respect to the recorded birefringence direction when the linearly polarized light of the readout light passes through. The rotated polarized light is collimated by the lens 160, and the polarization direction is detected by the polarizing element 140 and the photodetector 150.
Similarly to the reflection type, when the polarization direction of the recording light is 45 ° and the polarization direction of the reading light is 0 °, the polarization direction of the detected reflected light is 90 °.
[0054]
[Method of controlling temperature by adjusting light irradiation intensity]
In the optical recording method of the present invention, temperature control can also be performed by adjusting the light irradiation intensity. That is, this is an optical recording method in which optical recording and temperature control are performed with a single light source. In this method, the recording light acts to control the temperature of the optical recording medium in addition to photoisomerizing the photoisomerizable group or photoisomerized molecule. Therefore, the heater 20 and the temperature control mechanism 30 as shown in the figure are unnecessary. In this case, for example, an argon ion laser with an oscillation line of 515 nm sensitive to the optical recording layer can induce trans-cis isomerization of the photoisomerization group or photoisomerization molecule and raise the temperature of the optical recording layer. it can. This temperature control of the optical recording layer can be realized by changing the intensity of the recording light.
FIG. 10 shows the result of measuring the birefringence while changing the intensity of the recording light when the photo-alignment medium described in [0039] is used as the optical recording medium. The horizontal axis shows the temperature of the optical recording layer at that time. From the figure, the recording light intensity is 2 W / cm²And 5W / cm²It can be seen that high birefringence is induced at this time. The temperature of the optical recording layer at this time is around 32 ° C and 39 ° C, respectively, and this temperature is the glass transition temperature T_gCorresponds to ± 7 ° C. Therefore, it can be seen that high birefringence can be induced particularly within this temperature range. As described above, in this method, a special temperature control mechanism is not required, and the photo-alignment can be efficiently performed only with the recording light, and high birefringence can be obtained.
[0055]
Optical recording was performed using the optical recording apparatus shown in FIGS. The optical recording layer of the optical recording medium used was the polyester polymer of the above [Chemical Formula 4]. The light source is an argon ion laser with an oscillation line of 515 nm, and the light intensity is increased to 5 W / cm to raise the temperature of the optical recording layer to the glass transition temperature of 38 ° C.²It was. (The heater 20 and the temperature control device 30 were removed.)
When the optical recording apparatus shown in FIG. 7 is used, the reflective recording medium shown in FIG. 6A is used as the optical recording medium, and the optical recording layer has a refractive index difference of 0.04 that can be photoinduced. The thickness is set to 3.2 μm. When the optical recording apparatus of 9 is used, the transmissive recording medium shown in FIG. 6B is used as the optical recording medium, and the thickness of the optical recording layer is 6.4 μm. As shown in FIG. 8, when the polarization direction of the recording light was 45 ° and the polarization direction of the reading light was 0 °, the polarization direction of the detected reflected light was 90 °.
[0056]
【The invention's effect】
According to the present invention, birefringence is induced in an arbitrary direction by polarization on an optical recording medium, and this light irradiation pit is caused to function as a λ / 4 wavelength plate or a λ / 2 wavelength plate, and multilevel recording is performed by the optical axis. In the method and apparatus, molecules can be oriented efficiently and very high birefringence can be obtained. Therefore, it is possible to reduce the thickness of the optical recording layer, which means that it is possible to form an optical recording layer with a uniform film thickness by, for example, film formation by a spin coating method. Less. In addition, it is easy to make a disc, and the recording pits can be miniaturized and read S / N can be realized.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram illustrating a state in which azobenzene selectively absorbs polarized light and photoisomerizes and is isomerized and oriented by a heat return reaction.
FIG. 2 is a diagram showing a DSC curve of a polyester represented by Chemical Formula 4;
FIG. 3 is a conceptual diagram of an apparatus for use in the photo-alignment method used by the present invention.
FIG. 4 is a diagram showing temporal changes in birefringence when the photo-alignment layer temperature is variously changed in the photo-alignment method used in the present invention.
FIG. 5 is a diagram for explaining the influence of the temperature of the photo-alignment layer in the photo-alignment method used in the present invention.
6A and 6B are schematic cross-sectional views showing an example of an optical recording medium used in the present invention. FIG. 6A shows a reflective type and FIG. 6B shows a transmissive type.
FIG. 7 is a conceptual diagram showing an example of an optical recording apparatus of the present invention.
FIG. 8 is a diagram showing polarization directions of recording light and reproducing light when optical recording is performed using the apparatus of FIG.
FIG. 9 is a conceptual diagram showing another example of the optical recording apparatus of the present invention.
FIG. 10 is a diagram showing a saturation value of birefringence when temperature control is controlled by pump light irradiation in the optical recording method of the present invention.
[Explanation of symbols]
1 Pump light
2 Probe light
3 Polarizer
4 Analyzer
10 Photo-alignment medium
20 Heater
22 Thermocouple
30 Temperature controller
50 Optical recording media
51 substrates
52 Optical recording layer
54 Reflective layer
100 light sources
110, 140 Polarizing element
120 light separation element
130, 160 lenses
150 photodetectors

Claims (14)

Light for controlling the polarization angle of recording light emitted from a light source and irradiating the optical recording medium with the recording light, and forming a wave plate having an orientation corresponding to the polarization angle on the optical recording medium In the recording method, the optical recording medium has a polymer film having a photoisomerizable group in the molecule or a polymer film containing a photoisomerized molecule, and the polymer film of the optical recording medium is irradiated with recording light Subsequently, a step of blocking and leaving the recording light is performed, and in both the recording light irradiation step and the leaving step, the temperature of the polymer film is set to ± 7 ° C. of the glass transition temperature T _g of the polymer. the optical recording method characterized that you temperature controlled to be within.   2. The optical recording method according to claim 1, wherein the wave plate is a λ / 4 wave plate or a λ / 2 wave plate. Oite to as the 2 Tsunoko, the temperature of the polymer film, characterized in that the temperature controlled to be the glass transition temperature T _g of the polymer, the optical recording according to claim 1 or claim 2 Method. In the step of blocking and leaving the recording light, the temperature of the polymer film is controlled to be equal to or higher than the temperature of the polymer film in the step of irradiating the recording light. The optical recording method according to any one of claims 1 to 3 . The temperature control of the polymer film, and performing by adjusting the irradiation intensity or irradiation power of the recording light, an optical recording method according to any one of claims 1 to 4. The optical recording method according to any one of claims 1 to 5 , wherein the main chain of the polymer includes an aromatic hydrocarbon ring. The optical recording method according to any one of claims 1 to 6 , wherein the aromatic hydrocarbon ring is two or more benzene rings connected by a linking group. The optical isomers groups or photoisomerization molecules, characterized in that it comprises an azo group, an optical recording method according to any one of claims 1 to 7. Polymer having a photoisomerization group, can be a polymer obtained by introducing an azobenzene derivative in the side chain, an optical recording method according to any one of claims 1 to 8. Polymer having a photoisomerization group, azobenzene derivatives are introduced into the side chains, characterized in that said azobenzene derivative is linked to the aromatic ring in the main chain, any one of claims 1 to 9 2. The optical recording method according to item 1. The optical recording method according to any one of claims 1 to 10 , wherein the polymer having a photoisomerizable group is a polyester resin represented by the following structural formula.

In the structural formula, X represents a cyano group, a methyl group, a methoxy group, or a nitro group, and Y represents an ether bond or a ketone bond. l and m each represent an integer of 2 to 18, and n represents an integer of 5 to 500. The polymer having a photoisomerizable group is a polyester resin in which l and m are each an integer of 4 to 10 and n is an integer of 10 to 100 in the above structural formula. the optical recording method according to any one of 1 to claim 11. And adjusting as following equation 1 the thickness d of the polymer film, an optical recording method according to any one of claims 1 to 12.
Formula 1
d = m · λ / (4 · Δn)
In the formula, d is the thickness of the polymer film, m is an integer of 1 or more, λ is the wavelength of the readout light, and Δn is the saturation refractive index change. Light for controlling the polarization angle of recording light emitted from a light source and irradiating the optical recording medium with the recording light, and forming a wave plate having an orientation corresponding to the polarization angle on the optical recording medium A recording method, wherein the optical recording medium has a polymer film having a photoisomerizable group or a polymer film containing a photoisomerized molecule in the molecule, and the polymer film of the optical recording medium is irradiated with recording light Following the steps of a step of standing block a recording light is performed, in both steps of the recording light irradiation step and left step, the temperature of the polymer film, ± glass transition temperature T _g of the polymer An optical recording apparatus for use in an optical recording method for controlling the temperature so as to be within 7 ° C., which is obtained via a light source that generates recording light, a polarizing element that controls a polarization angle of the recording light, and the polarizer. the recording light possess an imaging optical system for irradiating the optical recording medium, the recording And a temperature control means for controlling the temperature of the polymer film so as to be within ± 7 ° C. of the glass transition temperature T _g of the polymer in both the light irradiation step and the standing step. An optical recording apparatus.

Images