5. Scanning Near-Field Optical Microscopy 5.1. Resolution of conventional optical microscopy Resolution of optical microscope is limited by diffraction. Light going through an aperture makes diffraction pattern. The minimum distance between resolved point objects x of equal intensity approximately amounts to the Airy disk radius r. sin Θ = 1.22 λ D sinθ = For microscopes E.K. Abbe s theory gives r F r F = 1.22λ D D F λ x = 1.22 2N. A. numerical aperture N. A. = n sinθ λ x 2n Airy disk SPM Chapter 5 1
5.2. Near-field optics Classical optics deals with far-field regime (λ >> R) of propagating waves (radiation is far from the source). Near-field optics considers optical interaction of light (electromagnetic radiation) emerging from a sub-wavelength aperture or scattered by a sub-wavelength metallic tip or nanoparticle with an object in the immediate vicinity (λ << R). It deals with the evanescent waves which are confined to sub-wavelength distance. Evanescent waves decay exponentially in the far-field region but in the near-field region they allow the surface inspection with high spatial, spectral and temporal resolving power. For visible light (400-800 nm) and the aperture or tip radius lying in sub-wavelength region one can consider near-field regime to be valid for distances below 50-100 nm. SPM Chapter 5 2
5.3. Scanning near-field optical microscopy 5.3.1. Various modes SNOM (or NSOM) utilizes near-field interactions. The idea was suggested in [E.H. Synge (1928). "A suggested method for extending the microscopic resolution into the ultramicroscopic region". Phil. Mag. 6: 356] and realised for the first time in 1984 (see Chapter 1). Illumination mode Collection mode Transmission mode Reflection mode SPM Chapter 5 3
5.3.2. Share-force approach To control tip-surface separation a so-called share-force feedback technique is typically used. Optical probe is attached to a tuning fork (quartz-crystal resonator). Its oscillations are induced by piezoelectric vibrator and transferred to a probe which oscillated in lateral direction with a few nm amplitude. As the probe approaches the surface, the probe-sample interaction dampens the amplitude and shifts the phase of the resonance (similar as in NC AFM). Change in amplitude and phase are used by the feed back to monitor the distance. SPM Chapter 5 4
5.3.3. Near-field optical probes and resolution Requirements: - the spot size determined by the aperture should be as small as possible; - the light intensity at the aperture should be as high as possible. These requirements are well met by optical fibers coated by metal (most often aluminium). Size of a probe apex is typically < 100 nm optical fiber protective layer metal coating Routinely a lateral resolution of < λ / 20 can be achieved For special configuration a resolution as good as < λ / 43 was reported [E.Betzig et al. Science 251 (1991) 1468] SPM Chapter 5 5
5.3.4. SNOM applications There is a number of areas where SNOM can contribute. For instance: - study of quantum dots (probing of single Q-dot); - single-molecule spectroscopy; - imaging of biological samples with fluorescent labels; - study of nonlinear optical properties (second and third harmonic generation) Share-force and SNOM images of InAs Q-dots www.ntmdt.com SNOM of organic crystals for two orthogonal light polarisations (0 o and 90 o ) Y. Shen et al. Opt. Lett. 26 (2001) 725. SPM Chapter 5 6
5.4. Scanning plasmon near-field optical microscopy This method utilizes surface plasmon excitation in thin metal layer which is ignited by laser radiation at frequency of plasmon resonance. Lateral resolution of 3 nm (λ / 200) was obtained on thin (50 nm) Ag films. STM SPNOM area size is 600x600 nm SPM Chapter 5 7
5.5. Use of SNOM for plasmon propagation If there is an array of metal nanoparticles on surface SNOM can be used for their excitation and detection. These arrays can be used as waveguides or more complex devices: switches, couplers, interferometers etc. Plasmon resonance in the NP creates a considerably enhanced external local electric field. For instance, the enhancement can reach factor of 200 for spherical Ag NP with radius less than 20 nm near the plasmon wavelength (410 nm). SPM Chapter 5 8
Key Points Resolution of conventional optical microscopy is limited by diffraction to about λ / 2n. Classical optics deals with propagating waves or far-field regime. Near-field optics deals with confined to sub-wavelength distance evanescent waves. In near-field region spatial resolution can be significantly improved. SNOM utilizes near-field interactions using probes with sub-wavelength dimensions. Routine lateral resolution of below λ/20 (or better) can be achieved. SNON is a powerful method to study Q-dots, single molecules, biological objects and non-linear optical properties of nanostructures. Optical resolution on thin metal layers or nanoparticulate materials can be improved up to λ/200. SPM Chapter 5 9