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チップ増強ラマン散乱 (TERS) におけるqCMOSカメラ評価
qCMOSカメラは、その低ノイズ性能により微弱信号の検出能力に優れています。本実験では、qCMOSカメラと分光器を組み合わせ、両者を同時に制御してチップ増強ラマン散乱 (TERS: Tip-Enhanced Raman Scattering) を検出するためのソフトウェアを新たに開発しました。この目的のために、ラマンや微弱信号のラマンに対するqCMOSカメラの検出能力を検証し、より多くの応用可能性を探りました。
チップ増強ラマン散乱 (TERS) について
走査型トンネル顕微鏡 (STM：Scanning Tunneling Microscope) は、原子レベルの分解能でサンプルを観察・操作できるだけでなく、そのトンネル電流を局所的な励起源として用いて接合部から光を発生させる、いわゆるSTM誘導ルミネセンス (STML) が可能です。これにより様々な励起の減衰に係る局所電磁気特性に関する追加の情報を得ることができます。一方、金属チップの先端で発生する強い信号増幅を利用して、チップ増強ラマン散乱 (TERS) により空間分解ラマン分光を実現し、高い空間分解能で実空間の化学的認識能力を提供することができます。さらに、チップ増強フォトルミネッセンス (TEPL) により、分子光物理学や光と物質の相互作用、サブ分子スケールでのエネルギー移動の探索が可能です。
図1：チップ増強ラマン散乱 (TERS) の実験系模式図
本実験では、試料 (MoS2) をSTMに入れ、532 nmのDPSSレーザでスペクトルを励起し、図1に示すような光路を持つ自作キューブでラマン信号を収集しました。焦点距離550 mmの分光器にqCMOSカメラORCA-Questを搭載し、ラマンスペクトルを収集しました。
実験系のセットアップ
上述したように、試料 (MoS2) をSTMに入れ、532 nmのDPSSレーザで励起しました。図2に示すように、ファイバと集光キューブを用いてMoS2のラマンスペクトルを収集しました。弱いラマンスペクトルは、ORCA-Questの露光時間を180秒に設定し検出しました。ORCA-Questは読み出しノイズが非常に小さいため、SNRは液体窒素冷却CCDカメラよりも優れています。また、画素サイズが4.6 μm×4.6 μmのため、通常のCCDカメラの画素サイズ (13 μm、20 μｍ、26 μm等) よりも小さく、優れた分解能 (FWHM) を有しています。
図2：実験系のセットアップ画像
液体窒素冷却CCDとqCMOSの比較
比較のため、別のシステムである液体窒素冷却CCD＋焦点距離300 mmの分光器の組み合わせと、ORCA-Quest＋焦点距離550 mmの分光器の組み合わせそれぞれで実験を行いました。システムセットアップは、集光ファイバより前の構成は全て同じで、ファイバと検出器のみを交換しています。
図3(a)：qCMOSカメラによるMoS2ラマン信号。ORCA-Quest + 焦点距離550 mmの分光器 (f/6.4), 200 μm光ファイバ, 300 g/mm回折格子, 露光時間180秒
図3(b)：液体窒素冷却CCDによるMoS2ラマン信号。液体窒素冷却CCD＋焦点距離300 mmの分光器（f/3.9）、400 μmファイバ、600 g/mm回折格子、露光時間180秒
比較の結果、ORCA-Quest＋焦点距離550 mmの分光器の組み合わせには、以下の3つの特徴があることがわかりました。
1） 一般的に短時間露光で使用される従来のsCMOSカメラと異なり、qCMOSカメラは露光時間を180秒にしても液体窒素冷却CCDに匹敵する低いノイズレベル実現しており、優れたラマンSNRを得ることができます。また、ファイバや分光器の集光効率への影響を取り除いたとしても、qCMOSカメラのS/N比は液体窒素冷却CCDと同程度であることがわかります。
2） CCD検出器では、長時間露光時の宇宙線干渉を除去することが困難である一方、長時間の積分でもは、qCMOSは液体窒素冷却CCDよりも優れた宇宙線干渉防止能力を発揮します。
3） qCMOSのダークノイズ抑制能力も長時間の積分時間で検証した結果、液体窒素冷却CCDと同レベルであることが確認されました。
Han Tao博士について
2005年、上海の復旦大学 (Fudan university) を卒業。堀場製作所での長年の技術職を経て、Shanghai UPU Optoelectronic Technology Co. (http://www.upu-opt.com/)を2018年に共同創業者として設立し、分光学とスペクトル測定の分野で20年近い経験を有しています。
Shanghai UPU Optoelectronic Technology Co.は、光電技術分野に特化したハイテク企業です。主にイメージングとスペクトルのビジネスに注力しており、近年は、共焦点ラマン分光器と蛍光分光器の開発・生産に力を入れています。科学研究市場の需要に応じ、ユーザに様々な高性能の標準化された製品モジュールやカスタマイズされたシステムを提供しています。
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