Photon scanning microscopy

The operation of a photon scanning tunnelling microscope (PSTM) is analogous to the operation of an electron scanning tunnelling microscope (ESTM), with the primary distinction being that PSTM involves tunnelling of photons instead of electrons from the sample surface to the probe tip. A beam of light is focused on a prism at an angle greater than the critical angle of the refractive medium in order to induce total internal reflection (TIR) within the prism. Although the beam of light is not propagated through the surface of the refractive prism under TIR, an evanescent field of light is still present at the surface.

The evanescent field is a wave which propagates along the surface of the medium and decays exponentially with increasing distance from the surface. The surface wave is modified by the topography of the sample, which is placed on the surface of the prism. By placing a sharpened, optically conducting probe tip very close to the surface (at a distance <λ), photons are able to propagate through the space between the surface and the probe (a space which they would otherwise be unable to occupy) though means of tunnelling, allowing detection of variations in the evanescent field and thus, variations in surface topography of the sample. In this manner, PSTM is able to map the surface topography of a sample in much the same way as in ESTM.

One major advantage of PSTM is that an electrically conductive surface is no longer necessary. This makes imaging of biological samples much simpler and eliminates the need to coat samples in gold or another conductive metal. 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.

Procedure

An evanescent field is attained using a laser beam at an attenuated total reflection geometry for TIR within a triangular prism. The sample is placed on a glass or quartz slide, which is affixed to the prism with an index matching gel. The sample then becomes the surface at which TIR occurs. The probe consists of the sharpened tip of an optical fiber attached to a piezoelectric transducer to control fine motion of the probe tip during scanning. The end of the optical fiber is coupled to a photomultiplier tube, which acts as the detector. The probe tip and piezoelectric transducer are housed within a scanner cartridge mounted above the sample. The position of this assembly is manually adjusted to bring the probe tip within tunnelling distance of the evanescent field.

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 amplitude of the electrical output of the photomultiplier tube is directly proportional to the number of photons collected by the probe, thus allowing measurement of the degree of interaction of the probe with the evanescent field at the sample surface. Since this field exponentially decays with increasing distance from the surface, the degree of intensity of the field corresponds to the height of the probe from the sample surface. 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.

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. The probe must be scanned perpendicular to the sample surface in order to calibrate the instrument and determine the decay constant of the field intensity as a function of probe height. During this scan, a feedback point is set so that the piezoelectric transducer can maintain constant signal intensity during the lateral scan.[1][2][3]

Fiber Probe Tips

The resolution of a PSTM instrument is highly dependent on probe tip geometry and diameter. Probes are typically fabricated via chemical etching of an optical fiber in a solution of HF and can be apertured or apertureless. Using chemical etching, fiber tips with a curvature radius as low as 20 nm have been made. In apertured tips, the sides of the sharpened fiber are sputter coated in a metal or other material. This helps to limit tunnelling of photons into the side of the probe in order to maintain more consistent and accurate evanescent field coupling. Due to the rigidity of the fiber probe, even brief contact with the surface will destroy the probe tip.[4]

Larger probe tips have a greater degree of coupling to the evanescent field and will therefore have greater collection efficiency due to a larger area of the optical fiber interacting with the field. The primary limitation of a large tip is the increased probability of collision with rougher surface features as well as photon tunnelling 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.

In metal coated fiber probes, the diameter and geometry of the aperture, or uncoated area at the tip of the probe, determines the collection efficiency. Wider cone angles result in larger aperture diameters and shorter probe lengths, while narrower cone angles result in smaller aperture diameters and longer probes. Double tapered probe tips have been developed in which a long, narrow region of the probe tapers into a tip with a wider cone angle. 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.[5]

PSTM Coupled Spectroscopy Techniques

Photoluminescence

It has been demonstrated that photoluminescence spectra can be recorded utilizing a modified PSTM instrument. Coupling PL spectroscopy to PSTM allows the observation of emission from local nanoscopic regions of a sample and provides an understanding of how the photoluminescent properties of a material change due to surface morphology or chemical differences in an inhomogeneous sample. In this experiment, a 442 nm He-Cd laser beam under TIR was used as an excitation source. The signal from the optical fiber was first passed through a monochromator before reaching a photomultiplier tube to record the signal. Photoluminescence spectra were recorded from local regions of a ruby crystal sample.[6] 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 technique allows characterization of individual surface features of semiconductor samples whose photoluminescent properties are highly temperature dependent and must be studied at cryogenic temperatures.[7]

Infrared

PSTM has been modified to record spectra in the infrared range. Utilizing both cascade arc and free electron laser CLIO as infrared light sources, infrared absorbance spectra were recorded from a diazoquinone resin. This mode of operation requires a fluoride glass fiber and HgCdTe detector in order to effectively collect and record the infrared wavelengths used. Furthermore, the fiber tip must be metal coated and oscillated during collection in order to sufficiently reduce background noise. The surface must first be imaged using a wavelength that will not be absorbed by the sample. Next, the light source is stepped through the infrared wavelengths of interest at each point during collection. The spectrum is acquired by analysis of the differences in the images recorded at different wavelengths.[8]

Atomic Force Microscopy

A silicon nitride cantilever can be used as the optical probe tip in order to simultaneously perform atomic force microscopy (AFM) and PSTM. This allows comparison of the recorded optical signal with the higher resolution topography data obtained by AFM. Silicon nitride is a suitable material for an optical probe tip as it is optically transparent down to 300 nm. However, since it is not optically conducting, the photons collected by the probe tip must be focused through a lens to the detector instead of traveling through an optical fiber. The instrument can be operated in constant height or constant force mode and resolution is limited to 10-50 nm due to tip convolution. Since the optical signal obtained in PSTM is affected by the optical properties of the sample as well as topography, comparison of the PSTM data with AFM data allows determination of the absorbance of the sample. In one study, the 514 nm absorbance of a Langmuir-Blodgett film of 10,12-pentacosadiynoic acid (PCA) was recorded using this method.[9]

References

  1. ↑ Reddick, R.C.; Warmack, R.J.; Ferrell, T.L. (1989). "New Form of Scanning Optical Microscopy". Physical Review 39 (1): 767–770.
  2. ↑ Reddick, R.; Warmack, R.; Chilcott, D.; Sharp, S.; Ferrell, T. (1990). "Photon Scanning Tunneling Microscopy". Review of Scientific Instruments 61: 3669. doi:10.1063/1.1141534
  3. ↑ Courjon, D.; Sarayeddine, K.; Spajer, M. "Scanning Tunneling Optical Microscopy". (1989). Optics Communications 71 (1,2): 23–28.
  4. ↑ Takahashi, S.; Fujimoto, T.; Kato; Kojima. “High Resolution Photon Scanning Tunneling Microscope”. (1997). Nanotechnology 8: A54-A57
  5. ↑ Saiki, T.; Mononobe, S.; Ohtsu, M. “Tailoring a High-Transmission Fiber Probe for Photon Scanning Tunneling Microscope”. (1996). Appl. Phys. Lett. 68 (19) 2612-2614.
  6. ↑ Moyer, P.J.; Jahnck, C.L.; Paesler, M.A. “Spectroscopy in the Evanescent Field with an Analytical Photon Scanning Tunneling Microscope”. (1990). Physics Letters A 145 (6,7): 343-347.
  7. ↑ Jahncke, C.L.; Paesler, M.A. “Low Temperature Photon Scanning Tunneling Microscope”. (1993). Near Field Optics 242: 115-120.
  8. ↑ A. “Spectroscopy in the Evanescent Field with an Analytical Photon Scanning Tunneling Microscope”. (1990). Physics Letters A 145 (6,7): 343-347.Piednoir, A.; Licoppe, C.; Creuzet, F. “Imaging and Local Infrared Spectroscopy with a Near Field Optical Microscope”. (1996). Optics Communications 129 414-422.
  9. ↑ Moers, M.; Tack, R.; van Hulst, N.; Bolger, B. “Photon Scanning Tunneling Microscope in Combination with a Force Microscope”. (1994). J. Appl. Phys. 75 (3); 1254-1257.
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