NEAR FIELD SCANNING OPTICAL MICROSCOPY(NSOM)

Dhanya M SIndian Institute of Space Science &

Technology

PHOTON INDUCED NEAR FIELD ELECTRON MICROSCOPY(PINEM)

&

WHAT IS NSOM? NSOM/SNOM is a form of scanning probe

microscopy. NSOM offers higher resolution around 50

nm . Breaks the far field resolution limit by

exploiting the properties of evanescent waves.

These fields carry the high frequency spatial information about the object and have intensities that drop off exponentially with distance from the object. Because of this, the detector must be placed very close to the sample in the near field zone, typically a few nanometers.

As a result, near field microscopy remains primarily a surface inspection technique

WHAT IS NEAR-FIELD?

For high spatial resolution, the probe must be close to the sample

λ/2 ~ 250 nm

~50nm

d=0.61*λ/NA

WHY HIGH RESOLUTION IN NSOM?

• Resolution is controlled by the diffraction of light.

• When a point object is magnified, its image is a central spot (Airy disk ) surrounded by a series of diffraction rings, not a single spot.

• To distinguish between the point objects, the Airy disks should not overlap each other.

THE BASIC PRINCIPLE OF SNOM• Light passes through a sub-wavelength

diameter aperture and illuminates a sample that is placed within its near field, at a distance much less than the wavelength of the light.

• Light is localized in a spot of nanometer dimension with a diameter smaller than the wavelength of light.

• In NSOM, the image is a central spot only, no other diffraction rings. Hence appear as a single spot and has high resolution.

BASIC COMPONENTS OF NSOM

The primary components of an NSOM setup are theLight sourceScanning tipDetector Feedback mechanism

• The light source is usually a laser focused into an optical fiber through a polarizer, a beam splitter and a coupler.

• The scanning tip is usually a pulled or stretched optical fiber coated with metal except at the tip or just a standard AFM cantilever with a hole in the center of the pyramidal tip

• Standard optical detectors, such as avalanche photodiode, photomultiplier tube (PMT) or CCD, can be used

The distance between the point light source and the sample surface is usually controlled through a feedback mechanism. Currently, most instruments use one of the following two types of feedback:

Constant force feedback: This mode is very similar to the feedback mechanism used in atomic force microscopy . Experiments can be performed in contact, intermittent contact, and non-contact modes.

Shear force feedback: In this mode, a tuning fork is mounted alongside the tip and made to oscillate at its resonance frequency. The amplitude is closely related to the tip-surface distance, and thus used as a feedback mechanism.

FEED BACK MECHANISM

Modes of Operation

Aperture Mode It is very popular and could provide very

highly resolution images. Aperture modes include five operational

mode: • Illumination• Collection• Illumination-collection• Reflection • Reflection collection

the schematic illustration of (a) aperture SNOM and (b) apertureless (scattering) SNOM

Aperture Operational modes

Illumination collection Illumination/collection

reflection Reflection/ collection

Aperture less mode

The aperture less mode is another mode which needs

more complicated instrument.

The tips are very sharp non-metal tips.

The sample is illuminated by external illumination. The

light that is scattered by oscillating tip and sample.

THE FINAL IMAGE PRODUCED BY SNOM CAN BE USED TO ANALYZE:

Changes in the index of refraction Changes in the reflectivity Changes in the transparency Changes in polarization Stress at certain points of the sample that changes its optical

properties Magnetic properties, which can change the optical properties Fluorescent molecules Molecules excited through a Raman shift or other effects Changes in the material

ADVANTAGES OF SNOM

High resolution (up to 25 nm). Analysis of various properties made

possible through contrast. No special sample preparation needed.

Can be used for different kind of samples (conductive, non-conductive & transparent).

LIMITATIONS OF SNOM Very low working distance and

extremely shallow depth of field. Not conducive for studying soft

materials, especially under shear force mode.

Long scan times for large sample areas for high resolution imaging.

APPLICATIONS OF SNOM

Ideally suited to quickly and effortlessly image the optical properties of a sample with resolution below the diffraction limit.

In Nanotechnology research. In Nano-Photonics and Nano-Optics. Life Science and materials research - optical

detection of the most miniscule surface Single molecule detection is easily

achievable. Dynamic properties can also be studied at a

sub-wavelength scale.

NSOM topography of fibroblast cell

AFM topography of fibroblast cell

the blue arrow denotes differentiation within the cluster of particles in the NSOM image whereas the AFM image just shows one large domain.

Simultaneous noncontact shear-force AFM (a) and SNOM contrast (b) of nanoparticles with 100-200 nm diameter using an apparently axially symmetric broken tip showing topographic and optical artifacts; cross sections as indicated with the lines.

a) b)

Topographic (left) and fluorescent (right) SNOM images of human epithelial cells initiating cell–cell contact. Protein (E-cadherin) clusters location is revealed

PHOTON INDUCED NEAR FIELD ELECTRON MICROSCOPY(PINEM)

PHOTON INDUCED NEAR FIELD ELECTRON MICROSCOPY(PINEM)

• Used for capturing dynamic images of nanometre sized structures.

• electrons can directly interact with photons via the near field component of light scattering by nanostructures, and either gain or lose light quanta discretely in energy.

• By energetically selecting those electrons that exchanged photon energies, we can map this photon-electron interaction

In PINEM, an ultra short optical pulse is used to excite evanescent electromagnetic fields near a nanostructure or at an interface.

The electrons can absorb/emit one or more scattered photons and then be detected by their contributions to displaced energy peaks in the electron energy spectrum

When using energy filtering to select for imaging only those electrons gaining energy, the resulting PINEM image reflects the strength and topology of the excited near field around the nanostructure or interface.

Photon-induced near-field EM (PINEM) imaging results in the mapping of nanostructure plasmonics with high spatial–temporal resolution.

The PINEM technique has been used to detect the evanescent near field surrounding a variety of structures with different materialsproperties and different geometries, such as

Carbon nanotubes Silver nanowires Nanoparticles Cells and protein vesicles and Several-atoms-thick graphene-layers.

THANK YOU