In this talk, I will present the results of two of our research projects. I will start with a simple technique for nanoscale mid-infrared spectroscopy that we have developed recently. Subwavelength resolution is achieved by detecting optical absorption through measuring local photothermal expansion with an atomic force microscope (AFM). Spatial resolution is determined by thermal diffusion length, which is smaller than 50 nm in typical chem/bio samples excited with nanosecond laser pulses. Tunable quantum cascade lasers are used as light sources. To detect minute sample expansions, we moved the repetition rate of the laser pulses in resonance with the AFM cantilever bending frequency and benefited from the resultant resonant enhancement. Plasmonic tip enhancement of light intensity is used to further improve spatial resolution and sensitivity of the technique. We were able to take mid-IR images and vibrational spectra of polymer films as thin as 10 nm with l/170 spatial resolution. The extension of this method to THz spectral range and possible improvements to achieve monolayer sensitivity will also be discussed. In the second part of the talk, I will present the results of our project aimed to develop broadly-tunable monolithic bandpass filters based on unique properties of long-range surface plasmon-polaritons. A small change of the refractive index of the cladding material in these filters may be translated into a large bandpass wavelength shift. We present experimental results with proof-of-principle devices operating at telecom wavelengths in which 0.004 change of the refractive index of the cladding material is translated into 210 nm bandpass tuning.
Physics, Room 203, Purdue University, West Lafayette, IN