Nanospectroscopy, using the apertureless scattering method, has become a standard method for achieving spectral analysis of surfaces with spatial resolution approaching 10 nm. The method involves light scattered from the tip of an atomic force microscope (AFM) in combination with a Michelson-type interferometer and infrared detector. A high spectral radiance source (such as a laser) is necessary for overcoming the extremely small scattering efficiency. Examples include the quantum cascade laser (QCL), although having somewhat limited spectral coverage. Here we describe our capabilities at the National Synchrotron Light Source II (NSLS II) using a truly broadband (continuum) source of synchrotron infrared to perform FTIR spectroscopy into the far-IR. A key component for achieving this is a long-wavelength MCT photoconductor operating at T=4.2K and reaching wavelengths beyond 50 microns (i.e. below 200 cm-1). Results showing polaritons in 2D materials as well as semiconductor transport characterization will be included.
*This work supported by the U.S. DOE under contract DE-SC0012704 at NSLS-II and BNL.
The Neaspec SNOM used in this work provided courtesy of Attocube GmbH.
We describe the performance of long-wavelength MCT operating at 4.2K as a fast photoconductive detector for far-infrared nanospectroscopy. The technique employs scattering from the tip of an atomic force microscope (AFM) engaged with a sample surface while in "tapping mode" at a frequency f, with the scattered infrared sensed at a higher harmonic, e.g. 2f, 3f or even 4f to improve spatial discrimination. With typical tapping frequencies >100 kHz, the infrared detector requires a bandwidth of 1 MHz or higher, for which thermal-type IR detectors are not sufficiently fast. MCT detectors are usually limited to wavelengths shorter than 25µm when operating at T=77K, but this can be overcome by cooling to 4.2K, in which case the detection threshold wavelength extends to beyond 50 microns. An additional benefit is an overall 5X improvement in S/N.
*This work supported by the U.S. Department of Energy under contract DE-SC0012704 at NSLS-II and BNL.
See ACS Photonics, 10, 4329-4339 (2023), (https://doi.org/10.1021/acsphotonics.3c01148)
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