図1. THz波のパルス
THz波は、通信やセンシング(物を検知する技術)の用途で使用されることが多い波長です。近年は、高出力のTHz波を作る技術が進歩しつつあり、強いTHz波を物質に当ててその反応を調べる研究が盛んになっています。この研究の進展により、新しい材料の特性を明らかにしたり、新しい技術を開発したりすることが期待されています。
強いTHz波と物質の反応を調べる際には、THz波パルス内部の電場の違いを見分けることが非常に重要です(図1)。
図2. THz位相シフタ
THz波の高出力応用において、重要な役割を果たすのがTHz位相シフタ(図2)です。
THz位相シフタは複数の当社製テラヘルツ波長板を組み合わせて構成され、THz波パルスのCEP制御を実現します。テラヘルツ波長板はTHz波の偏光を制御できる光学素子で、Si(シリコン)製のプリズムです。
テラヘルツ波長板①(1/4波長板:A16394-04)を通ったTHz波パルスは円偏光になり、テラヘルツ波長板②(1/2波長板:A16394-02)を通過後にテラヘルツ波長板③(1/4波長板:A16394-04)により元と同じ直線偏光に戻されます。1/2波長板を回転させることで、その回転角度(α)により所望のCEPシフトが達成できます (テラヘルツ波長板③は偏光子に置き換えることも可能です)。
1.超短パルスのCEP制御にはすべてのスペクトルで同じ量の位相変化が必要
THz波に限らず、超短パルスのCEPを制御するには、パルスに含まれるすべての光の波長で同じ量の位相変化をさせる必要があります。しかし、THz波パルスには異なる波長の光が含まれているため、これを実現するのは容易ではありません。例えば、THz波パルスには波長が5倍も違う波長100 µmの光や波長500 µmの光も含まれます。ガラス板を光が通過すると屈折率の影響で光はある時間だけ遅れて出てきますが、位相で考えると100 µmの光は500 µmの光の5倍も位相が変化してしまいます。
2.偏光の制御で位相をコントロールできる
THz位相シフタでは、偏光を変えることで位相を制御します。このような偏光の変化により制御できる位相を「ベリー位相」と呼びます。ベリー位相を利用してTHz波パルスのCEPを制御するためには、すべての波長で同じ偏光制御が必要です。
3.全反射を用いた超広帯域偏光制御
すべての波長で同じ偏光制御を実現するために、テラヘルツ波長板を使用します。この波長板は、プリズムの内部全反射を利用しており、材質としてSiを使用しています。これにより、ほぼすべてのTHz波スペクトルで同じ偏光制御が可能です。また、入射するTHz波と出射するTHz波が一直線上になるように設計されているため、通常の波長板と同じように回転させながら使用できます。
STM(Scanning Tunneling Microscope:走査型トンネル顕微鏡)は、物質の“原子”を見ることができる顕微鏡です。原子と極細の探針の間に電池をつなぐと、近接した距離で電流(トンネル電流)が流れ、これを利用して原子サイズの凹凸を調べます。
高強度のTHz波パルスは、例えばそのパルス幅(ピコ秒:10-12秒)の間だけ電池の役割を果たします。これにより、ピコ秒という非常に短い時間での変化を観測することができます。
THz位相シフタは、この電池のつなぐ向きを変えるために使用されます。電池のつなぐ向きを変えると、観測する原子の情報が変わります(分子軌道のLUMOとHOMOが変化)。THz波のCEPを精密に制御することで、電荷移動の中間状態など、極限の時空間分解能で分子の状態を調べることが可能になります。
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