Development of photothermal scanning near-field optical microscope

近十年来，扫描近场光学显微镜（SNOM）由于克服了衍射极限而达到纳米尺寸，引起了人们的极大关注。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可以作为一种很有前途的工具用于薄膜分析。在这项研究中，我们已经开发了PT-SNOM和调查的性能。该方法利用光纤探针的小孔径发射的倏逝波激发分子，并将探测激光束与激发光同轴引入，测量分子的诱导折射率。使用He-Cd激光器作为422 nm的激发源，而He-Ne激光器作为633 nm的探测光束。将两个激光束同轴地引入到具有约100 nm的顶点直径的金属涂覆的光纤探针中。剪切力反馈使光纤探针与样品的距离保持在10 nm。透射的探测光被物镜透镜收集并通过全息陷波滤波器以去除422 nm的激发光。在玻璃上的金薄膜（80 nm）被用作样品，并且光热信号图像可以在PT-SNOM中获得。无论是激发激光还是探测激光的照射都不会产生任何信号。这是SNOM探测到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，要求激发激光的调制频率快，锁定放大器的时间常数短，因此S/N比正在提高。

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.