Nanoscale Infrared Spectroscopy
The mid-infrared (MIR) region of the spectrum (wavelengths of approximately 5-10um) is of particular interest as it coincides with the characteristic vibrational frequencies of many molecules. MIR spectroscopic techniques, such as Fourier-transform (FTIR) spectroscopy, are widely used for the characterisation and analysis of materials in many disciplines and are capable of revealing a wealth of information about the sample ranging from simple chemical composition to more subtle properties such as conformational changes in macromolecules.
While such techniques would provide a valuable tool for determining the properties of individual nanostructures providing, for example, correlation between compositional changes and particular physical structures, the spatial resolution of such optical spectroscopy is restricted by the well-known diffraction limit, corresponding to several micrometers in the infrared. Infrared spectroscopic analysis is therefore generally restricted to bulk measurements averaged over many individual nanoscale features. Scanning Near-field Optical Microscopy (SNOM) makes use of a near-field optical probe, positioned using Atomic Force Microscopy (AFM) techniques, to combine optical measurements with the spatial resolution of AFM. In particular, apertureless or scattering-type SNOM, using the metallized tip of a standard AFM probe as a localised nano-antenna to provide coupling between the near-field of the sample and sources and detectors in the far field, has been shown to be capable of MIR spectroscopic measurements with spatial resolutions of around 10 nm - hundreds of times less than the wavelength of the light used . However, the development of true spectroscopic SNOM in the infrared is somewhat hampered by the lack of suitable infrared lasers able to precisely and rapidly tune their wavelength over ranges sufficient to collect useful spectroscopic data. This project aims to develop novel wide-tuning MIR laser sources and apply these to spectroscopic SNOM allowing broadband MIR spectra with nanoscale localisation to be rapidly acquired from samples.
A series of applications studies of increasing complexity are being carried out focussing on the field of materials the exhibit nanoscale self-assembly. Nanoscale infrared spectroscopy has the potential to offer significant new insights in this area of research due to the close relationships between nanoscale physical structure, chemical composition and molecular structure.
Scattering-type SNOM. A metallized AFM tip acts as a nano-antenna coupling between the near field of a highly localised region of the sample and sources and detectors in the far field. Known parameters such as the permittivity of the tip (εt), and the tip-sample spacing allow the dielectric properties of the sample (εs) to be determined from the measured effective scattering cross section of the tip-sample system (σeff). Detection at the tip modulation frequency (Ω) allows discrimination between the tip-scattered signal (red lines) and general background scattering (black lines).
Laser source development
Laser development is a key component of this project. We have developed a novel laser source capable of tuning over wide MIR ranges in millisecond time scales. This provides the combination of high intensity and wide-range tuning necessary for spectroscopic SNOM and has demonstrated accurately controllable swept operation over ranges as wide as 900cm-1 (see image). This laser source has been combined with a commercial scattering-type SNOM system, and is being used to develop the instrumentation and techniques necessary to acquire near-field spectroscopic information in real time with a fast-tuning laser - a significant and challenging departure from previous techniques demonstrated with fixed-wavelength laser systems.
Broadband spectra recorded using rapidly-tunable MIR laser source. Top: polystyrene (black) and PMMA (green) absorption spectra acquired in a 3.3 ms sweep. Bottom: citric acid spectrum spanning 900 cm-1 recorded in a 3.4ms sweep. All spectra are shown compared to FTIR data from the same samples (dashed lines).
Working in this area
The following people are involved in this research:
- Silva, A, Boller, KJ and Lindsay, ID. ‘Wavelength-swept Yb-fiber master-oscillator-power-amplifier with 70nm rapid tuning range’, Optics Express, 19, (pp. 10511-10517), 2011
- Silva, A, and Lindsay, ID. ‘Wavelength-Swept Optical Parametric Oscillator for Broadband Mid-Infrared Spectroscopy’, Appl. Phys. B. (accepted for publication)