Development of photothermal scanning near-field optical microscope

近十年来，扫描型近场光学显微镜（SNOM）克服了衍射极限，达到了纳米级尺寸，受到了广泛的关注。光热光谱是一种灵敏度高、用途广泛的技术，适用于非荧光分子，因此光热(PT) -SNOM可以作为一种有前途的薄膜分析工具。在这项研究中，我们开发了PT-SNOM并研究了其性能。分子由光纤探针小孔发射的倏逝波激发，用于探测诱导折射率的探针激光束与激发光束同轴引入。PT-SNOM基于倒置光学显微镜。在422 nm处用He-Cd激光器作为激发源，在633 nm处用He-Ne激光器作为探测光束。两束激光被同轴地引入一个尖端直径约为100nm的金属涂层光纤探针中。剪切力反馈使光纤探针与样品的距离保持在10 nm。透射探头光束经物镜采集，通过全息陷波滤光片去除422 nm的激发光。样品采用玻璃上80 nm的金薄膜，在PT-SNOM中可获得光热信号图像。激发激光器和探测激光器的辐照都不产生任何信号。这是在SNOM中检测光热信号和图像的第一个证据。空间分辨率估计为1 μm。为了获得光热信号图像，对激发激光器的调制频率要求快，对锁相放大器的时间常数要求短，因此信噪比的提高仍在进行中。

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.