Techniques
The Near-Field Optical Spectroscopy Centre combines a range of lasers with an atomic force microscope to perform a variety of s-SNOM and other tip-enhanced optical measurements.
Our illumination range covers the entire visible spectrum, and a significant fraction of the near and mid-infrared regions up to a wavelength of 15 µm (down to a wavenumber of 650 cm-1). Any material that is suitable for atomic force microscopy (AFM) measurements and that have an optical response within our illumination range can be probed with our equipment.
A range of spectroscopy tools can also be utilised from our other facilities at the University of Sheffield to investigate the sample further.
Main measurement techniques
Atomic force microscopy
One of the cornerstones of s-SNOM, atomic force microscopy (AFM) measures the interaction between the measurement sample and an extremely sharp needle to gain nanometre-resolved topographical and mechanical data from the sample material.
Pseudo-heterodyne s-SNOM
By directing monochromatic light onto the apex of a conductive AFM tip, extremely strong, short-ranged electric fields are produced that can be used to probe the optical properties of a sample on the scale of nanometres, beating the diffraction limit. Our combination of lasers allows for s-SNOM to be performed at illumination wavelengths from 450 nm up to 1600 nm (6,300 cm-1 to 22,000 cm-1), and from 5.2 µm up to 11 µm (910 cm-1 to 1,900 cm-1), with a small gap around 7 µm (1,400 cm-1).
Nano-FTIR
By utilising both s-SNOM techniques and Fourier-transform infrared (FTIR) spectroscopy, a broadband incident light source is used to rapidly measure the optical properties of a sample both on the nanoscale and over a range of illumination wavelengths. Our broadband illumination wavelengths include from 4.2 µm to 16 µm (630 cm-1 to 2400 cm-1).
Tapping AFM-IR
A monochromatic, tuneable infrared source is used to illuminate a sample which, depending on the absorbance of the material, causes regions of the sample to heat up and expand. This expansion can be measured with an AFM tip, allowing for the optical absorption of the sample to be measured on the scale of nanometres, and across a range of wavelengths. Our illumination wavelengths cover from 5.2 µm up to 11 µm (910 cm-1 to 1,900 cm-1), with a small gap around 7 µm (1,400 cm-1).
Measurement techniques in development
Tip-enhanced PL and time-resolved PL
By measuring the wavelength of the photoluminescence (PL) emitted by excited charge carriers, information about the energy levels of a material can be revealed. By selectively enhancing the rate of PL emission from a small sample region by introducing a strong, short-ranged electric field, the energy states of the material can be resolved with nanometre resolution.
Tip-enhanced Raman
Raman spectroscopy utilises the inelastic scattering of photons to deduce information about vibrational and other low-frequency modes within a sample material. By utilising the short-range electric fields generated by an s-SNOM probe to enhance the Raman scattering strength, these low-frequency modes can be resolved with nanometre resolution.
Nanoscale pump-probe spectroscopy
Pump-probe spectroscopy allows for a huge range of information to be gained from a sample, in particular regarding the lifetimes and nature of excited energy states. Our setup will allow for a monochromatic pump wavelength anywhere from 350 nm through to 1600 nm, and a broadband probe spectrum in the range 4.2 µm to 16 µm (630 cm-1 to 2400 cm-1).
