Photon scanning microscopy
Furthermore, PSTM can be used to measure the optical properties of a sample and can be coupled with techniques such as photoluminescence, absorption, and Raman spectroscopy.
Conventional optical microscopy utilizing far-field illumination achieves resolution that is restricted by the Abbe diffraction limit.
Researchers have long sought to break the diffraction limit of conventional optical microscopy in order to achieve super-resolution microscopes.
[1] SOM involves scanning individual regions of the sample with a very small field of light illuminated through a diffraction limited aperture.
The resolution of these devices was extended beyond the diffraction limit in 1972 by Ash and Nicholls,[2] who first demonstrated the concept of near-field scanning optical microscopy.
This occurs when a third medium (in this case the sharpened fiber probe) of refractive index n3 (with n3>n2) is brought near the interface at a distance <λ.
In PSTM, the tunneled photons are conducted through the fiber probe into a detector where a detailed image of the evanescent field can then be reconstructed.
In practice, this is usually determined during instrument calibration by scanning the probe perpendicular to the surface and monitoring the detector signal as a function of tip height.
Theoretical models have also been developed by Reddick to account for modulation of the evanescent field by secondary effects such as scattering and absorbance at the sample surface.
The position of this assembly is manually adjusted to bring the probe tip within tunneling distance of the evanescent field.
[5][11] As photons tunnel from the evanescent field into the probe tip, they are conducted along the optical fiber to the photomultiplier tube, where they are converted into an electrical signal.
The electrical signals are sent to a computer where the topography of the surface is mapped based on the corresponding changes in the detected evanescent field intensity.
[7][11] The electrical output from the photomultiplier tube is used as constant feedback to the piezoelectric transducer to adjust the height of the tip according to variations in surface topography.
This helps to limit tunneling of photons into the side of the probe in order to maintain more consistent and accurate evanescent field coupling.
The primary limitation of a large tip is the increased probability of collision with rougher surface features as well as photon tunneling into the side of the probe.
A narrower probe tip is necessary to resolve more abrupt surface features without collision, however the collection efficiency will be reduced.
This provides a wider aperture for greater collection efficiency while still maintaining a long narrow probe tip capable of resolving abrupt surface features with low risk of collision.
[14] A subsequent publication successfully demonstrated the use of PSTM to record the fluorescence spectrum of a Cr3+ ion implanted sapphire cryogenically cooled under liquid nitrogen.
This mode of operation requires a fluoride glass fiber and HgCdTe detector in order to effectively collect and record the infrared wavelengths used.
[17] PSTM can be combined with both electron scanning tunneling microscope and AFM in order to simultaneously record optical, conductive, and topological information of a sample.
This apparatus was used to characterize copper phthalocyanine deposited over an array of gold squares patterned on an ITO substrate affixed to a prism.
