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原理 | テラヘルツ（THz）センサ
メタサーフェスを用いたTHz検出器の開発
光電子増倍管（PMT）やイメージインテンシファイア（I.I.）は光電効果を原理とした光電面を利用しています。
しかし、THz波は光波とは異なり光子エネルギーが低いため、直接光電効果を起こすことが困難です。
そこで、私たちはTHz波を電子に変換するメタサーフェスを開発しました。ここでは、メタサーフェスとは何か、またTHz波-電子変換の原理をご紹介します。
メタサーフェスとは
メタサーフェスは、微細な構造を持つ人工的に設計された表面構造で、さまざまな応用技術に活用されています。周期的な微細構造（メタ）が並んだ面（サーフェス）であることから、メタサーフェスと呼ばれます。
浜松ホトニクスは、このメタサーフェスをTHz検出に応用しています。そのため、ここでのメタサーフェスはTHz波-電子変換素子のことを指しています。
メタサーフェスは石英・シリコンなどの基板上に金のアンテナパターンが配列された構造を有しており、THz波が入射すると電界電子放出の原理によって電子が放出されます。この電子を増幅し、出力することでTHz波を検出しています。
電界電子放出のイメージ
メタサーフェス微細構造の拡大イメージ
電界電子放出の原理
THz波-電子変換には電界電子放出の原理を利用しています。ここでは、メタサーフェスとTHz波電界による電子放出の原理を、通常の光電子増倍管で用いられる光電変換と比較してご紹介します。
メタサーフェスを用いた電界電子放出は、電界強度に依存した非常に非線形な現象として知られています。
また、研究を通じてアルカリ金属をコーティングすることによってアンテナ金属（金）表面の仕事関数が下がり、THz波電界に対する感度が向上することが分かってきています。この性質を応用し、浜松ホトニクスのメタサーフェスは、高感度化のためにアンテナの表面にアルカリ金属をコーティングしています。
また、アンテナのデザインによってTHz波がアンテナと共振する周波数帯域を自由に設計することができるため、サブTHz領域から中赤外領域にわたる幅広い帯域において対応が可能です。
メタサーフェス（THz光電子増倍管）による電界電子放出
メタサーフェスのバンドモデル
メタサーフェスを用いた電子放出では、THz波の電界をアンテナで増強してエミッタ部に集中させます。
エミッタ部に集中した電界は、高電界状態となります。THz波によって生じた高電界は金アンテナ表面の真空準位を傾斜させ、金のフェルミ準位の電子を真空側にトンネリングさせます。これがいわゆる電界電子放出として知られる現象です。
電界電子放出によって放出された電子は、周囲の電界に従って真空中を移動します。
1. THz波がアンテナへ入射し電界が集中する
2. 高電界によって金アンテナ表面の真空レベルが傾斜する
3. 金のフェルミレベルの電子が真空側へトンネリングする
アルカリ光電面（通常の光電子増倍管）による光電効果
光電面のバンドモデル
半導体には、電子が存在することができない禁止帯の幅である「エネルギーギャップ」と伝導帯と真空レベルの間隔である「電子親和力」、そしてフェルミレベルと真空レベルとの間のエネルギー差である「仕事関数」が存在します。
光電面に光子が入射すると、価電子帯の電子が光子エネルギーを吸収して励起され、表面に拡散していきます。拡散した電子は真空レベルを超え、光電子として真空中に放出されます。
1. 光子が光電面に入射し、価電子帯の電子が励起される
2. 励起された光電子が表面に拡散される
3. 真空レベルを超えた光電子が真空中へ放出される
電界電子放出による非線形な出力
電界電子放出の原理はFowler-Nordheimの関係式に従います。
ここで、a,bはFN定数、Fは仕事関数、βは電界集中係数、EはTHz波電界強度となります。電子放出量はTHz波の電界強度Eによって決まります。そのため、この原理を用いたメタサーフェスはTHz波電界強度を直接計測できる可能性があります。
また、この電子放出量と電界強度Eは非常に強い非線形性の関係にあるため、わずかな電界強度の変化に対して電子放出量は大幅に変化します。グラフのように線形デバイスと比較すると非線形な応答特性がよく分かります。この非線形な応答特性は、微小変化計測に応用することができます。
Fowler-Nordheim relationの関係式
応答特性比較
関連論文
| 分類 | タイトル | 著者 | リンク |
|---|---|---|---|
| 基本原理 | Lightwave-Driven Long-Wavelength Photomultipliers | Simon J. Lange, et al., | https://doi.org/10.1002/lpor.202470002 |
| 応用 | IEEE Transactions on Terahertz Science and Technology | T. O. Buchmann et al., | https://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=5503871 |
| 応用 | Fourier-transform THz spectroscopy based on electric-field interferometry using THz-PMT | H. Takahashi et al., | https://opg.optica.org/oe/fulltext.cfm?uri=oe-32-7-12774&id=548237 |
- Confirmation
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