Two different configurations exist for the TERS optical coupling: one in transmission and one in reflection. The reflection configuration itself can be split into “top” and the “side” coupling configurations, each having its own advantages and drawbacks.
The on-axis inverted microscope configuration is ideal for transparent samples: it allows the use of very high numerical aperture (NA) objectives, including oil immersion objectives, for confocal Raman imaging.
The space above the sample is free for SPM experiments, including Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM), and other common SPM modes, in air, liquid or under special environments.
This configuration allows simultaneous SPM and confocal Raman imaging and offers the best spectroscopic performance for Tip-Enhanced Raman Scattering (TERS) of transparent samples by using a radial polarization on the laser beam.
The upright configuration allows simultaneous SPM and spectroscopic Raman measurements (including TERS) when using protruding tips (triangular AFM tips at the extreme end of the cantilever) and high NA objectives (up to 0.7 NA). SPM imaging with a wider selection of probes, and Raman confocal imaging with very high NA objectives (up to 0.95 NA) can also be performed sequentially at the same location. This configuration is optimized for high resolution co-located measurements on opaque samples but also allows TERS measurements with protruding tips.
Oblique illumination is designed for Tip-Enhanced Raman experiments with a large variety of SPM probes on opaque samples.
The setup integrates a high NA long-working distance objective (up to 0.7 NA) at an optimum angle (60°). It brings the laser beam on the tip with ideal polarization orientation for TERS amplification (p-polarization) and ensures maximum collection efficiency by eliminating shadowing from the scanning probe-cantilever.
This configuration is optimized for TERS (near-field imaging) on opaque samples. It also allows simultaneous AFM and confocal Raman imaging; however the geometry inherently limits the far-field spatial resolution which is better achieved with the upright configuration.
Multi-port optical configuration for AFM-Raman and TERS:
HORIBA Scientific AFM/Raman systems offer the versatility of combining these different optical configurations. For example, a dual port configuration with oblique illumination optimized for TERS and upright microscope for high-resolution far-field spectroscopic imaging offers the best performance for both co-localized sequential measurements and TERS on opaque samples.
Another kind of dual portsystem with top-down illumination and inverted capability provides a platform for TERS and colocalized measurements of both opaque and transparent samples.
And finally, a triple port configuration brings all the possible configurations into one unique platform, and offers the most versatile system for optimized capabilities.