Development of photothermal scanning near-field optical microscope

扫描近场光学显微镜(SNOM)突破了衍射极限，达到纳米尺寸，近十年来备受关注。光热光谱是一种灵敏度高、用途广的技术，适用于非荧光分子，因此光热(PT)-SNOM可以作为一种很有前途的薄膜分析工具。在本研究中，我们开发了PT-SNOM并对其性能进行了研究。分子由光纤探头小孔径发射的逝去波激发，用于探测诱导折射率的探测光束与激发光束同轴引入。PT-SNOM基于倒置光学显微镜。用He-Cd激光器作为422 nm的激发源，用He-Ne激光器作为633 nm的探测光。将两束激光同轴引入一个顶端直径约为100 nm的镀金属光纤探头中。剪切力反馈使光纤探头-样品的距离保持在10 nm。通过物镜采集探测光，通过全息陷波滤光片去除422 nm的激发光，用镀金薄膜(80 Nm)制作样品，在PT-SNOM中获得光热信号图像。激发激光或探测激光的照射都不会产生任何信号。这是首次在SNOM中探测到光热信号和图像的证据。空间分辨率估计为1μm，需要激励激光的快速调制频率和锁相放大器的短时间常数才能获得光热信号图像，因此正在进行S/N比的改进。

Much attention to a scanning near-field optical microscope(SNOM) has been paid in this decade by overcoming the diffraction limit to reach nanometeric size. Photothermal spectroscopy is highly sensitive and versatile technique, which is applicable for non-fluorescent molecules, so that photothermal (PT) -SNOM can be used as a promising tool in thin-film analysis. In this study we have developed the PT-SNOM and investigated the performance. The molecules are excited by the evanescent wave emitted from the small aperture of the optical fiber probe and the probe laser beam for detection of the induced refractive index is coaxially introduced with the excited beam.The PT-SNOM was based on inverted optical microscope. A He-Cd laser was used as an excitation source at 422 nm, while a He-Ne laser as a probe beam at 633 nm. Both the laser beams were coaxially introduced into a metal-coated optical fiber probe with an apex diameter of around 100 nm. A shear-force feedback kept the fiber probe-sample distance of 10 nm. The transmitted probe beam was collected by the objective lens and passed through a holographic notch filter to remove excitation light of 422 nm.The thin film of gold (80 nm) on a glass is used for the sample and the photothermal signal image can be obtained in PT-SNOM. The irradiation either of excitation laser or of probe laser does not results in any signals. This is the first evidence of detection of photothermal signal and image in SNOM. The space resolution is estimated to be 1 μm. Fast modulation frequency for excitation laser and Short time constant in lock-in amplifier are required in order to get the photothermal signal image, so the improvement of S/N ratio is in progress.