We report a method of taking mid-infrared and terahertz spectra on nanoscale using compact mW-level sources, such as quantum cascade lasers, and a standard atomic force microscope (AFM). Light absorption is detected via deflection of an AFM cantilever due to local sample thermal expansion. The spatial resolution is principally determined by the diameter of the high-intensity spot in the vicinity of a sharp metalized AFM tip, and is below 50nm. To enable detection of minute sample expansion, the repetition rate of the laser pulses is moved in resonance with the cantilever mechanical frequency. The technique requires no optical detectors.
Mid-IR photoexpansion nano-spectroscopy measures spectra of samples on nanoscale by detecting local thermal expansion associated with light absorption using a standard atomic force microscope (AFM). Cantilever deflection is directly proportional to sample absorption. This method results in a simple experimental setup with no optical detectors. We have recently demonstrated that the sensitivity of photoexpansion nano-spectroscopy can be dramatically enhanced by moving the laser pulses repetition frequency in resonance with the mechanical frequency of the AFM cantilever. We were able to produce spectra from ~100 nm thin films using low energy (4 nJ) pulses from a tunable quantum cascade laser (QCL). The spatial resolution, which is determined by thermal diffusion length, has been demonstrated to be better than 50 nm. Sample heating is limited to ~10 mK. Here we present a novel approach to increase both the sensitivity and spatial resolution of photoexpansion nano-spectroscopy. We utilize the plasmonic local-intensity enhancement below a gold-coated AFM tip. We successfully produced high quality vibrational absorption spectra from samples as thin as 10 nm positioned on top of gold-coated silicon substrates. In addition to higher photoexpansion signal, our technique features higher spatial resolution, which is no longer limited by thermal diffusion length but is instead determined by the dimensions of the high-intensity field region below the metal tip, which can be 10 nm or smaller.
We report that unique properties of long-range surface plasmon polaritons (LR SPP) allow one to produce optical
components with very wide tuning range using small variations in the refractive index of the dielectric layer. Our filter is
based on integration of a thin metal film between two dielectrics with dissimilar refractive index dispersion. In this
configuration, the filter only has low insertion loss at a wavelength for which the refractive indices of the top and bottom
dielectrics are the same, leading to a bandpass filter. As a proof-of-principle demonstration, we present operation of LR-SPP-
based bandpass optical filters with refractive index matching fluids on an Au/SiO<sub>2</sub> surface in which a 0.004
variation in the refractive index of the top dielectric translates into 210nm of bandpass tuning at telecom wavelengths.
To make a more practical solid-state device, thermo-optic polymer can be used as a top dielectric and we expect that
only 8°C of temperature variation translates into 200nm. The tuning mechanism proposed here may be used to create
monolithic filters with tuning range spanning over more than an optical octave, compact and widely-tunable laser
systems, multi-spectral imagers, and other plasmonic components with broadly-tunable optical response.