Scattering theory of Bardeen's formalism for tunneling: new approach to near-field microscopy

Probing vectorial near field of light: imaging theory and design principles of nanoprobes

Near-field microscopy is widely used for characterizing electromagnetic fields at nanoscale, where nanoprobes afford the opportunity to extract subwavelength optical quantities, including the amplitude, phase, polarization, chirality, etc. However, owing to the complexity of various nanoprobes, a general and intuitive theory is highly desired to assess the vectorial responses of nanoprobes and interpret the mechanism of the probe-field interaction. Here, we develop a general imaging theory based on the reciprocity of electromagnetism and multipole expansion analysis. The proposed theory closely resembles the multipolar Hamiltonian for light-matter interaction energy, revealing the coupling mechanism of the probe-field interaction. Based on this theory, we introduce a new paradigm for the design of functional nanoprobes by analyzing the reciprocal dipole moments, and establish effective design principles for the imaging of vectorial near fields. As application examples of the proposed theory, we numerically analyze the responses of two typical probes, a split-ring probe and a nanoparticle probe, which can quantitatively reproduce and well explain the experimental results of previously reported measurements of the optical magnetism and the transverse spin angular momentum. Our work provides a powerful tool for the design and analysis of new functional probes that may enable the probing of various physical quantities of the vectorial near field.

Probing the Hydrogen Enhanced Near-Field Emission of ITO without a Vacuum-Gap

Electromagnetic fields produced by thermal fluctuation can excite the near-field optical states, creating the potential for thermal radiation orders of magnitude greater than what is predicted by Plank's blackbody theory. The typical schemes employed to probe the trapped electromagnetic energy of the near-field are with considerable technical challenges, suffering from scalability and high costs, hindering widespread use. A waveguide-based scheme relying on photon tunneling is presented as an alternate approach, as waveguides inherently provide a large density of channels for photons to tunnel to with the required k-vector matching and probability density overlap. The conducted experiments with a 10 nm indium tin oxide film, having plasmonic resonance in the 1500 nm wavelength range, show that the near-field EM radiation can be extracted to the far-field by establishing the mode of de-excitation to be that of photon tunneling to a nearby waveguide. Furthermore, it is also demonstrated that the thermally emitted energy is very sensitive to changes in the surface free electron density, a property that is unique to the near-field. In addition to the ease of implementation and scalability, the proposed waveguide-based extraction method does not require a vacuum-gap, which is a significant reduction in the required complexity.

Infrared Vibrational Nanospectroscopy by Self-Referenced Interferometry

Infrared vibrational scattering scanning near-field optical microscopy (s-SNOM) has emerged as a new frontier in imaging science due to its potential to provide nanoscale spatially resolved chemical spectroscopy for the investigation of molecular, soft-matter, and biological materials. As a phase-sensitive technique able to yield the full complex dielectric function of materials, different interferometric schemes have been developed involving asymmetric interferometry between sample and reference arms. In this work, we take advantage of a greatly simplified symmetric geometry that uses the spatially coherent background scattered light from within the confocal sample volume as a reference field for signal amplification in both self-homodyne and self-heterodyne interferometry. On the basis of a simple model for tip-sample scattering and interferometric detection, we demonstrate the measurement of the vibrational response of molecular materials in good agreement with established values. In addition to a compact design, enhanced signal levels, and a reduced sensitivity to fluctuations and drift, including those from the light source, self-referenced interferometry brings benefits for routine s-SNOM chemical spectroscopy, remaining robust even under a wide range of challenging experimental environments.

Reciprocity theory of apertureless scanning near-field optical microscopy with point-dipole probes

Near-field microscopy offers the opportunity to reveal optical contrast at deep subwavelength scales. In scanning near-field optical microscopy (SNOM), the diffraction limit is overcome by a nanoscopic probe in close proximity to the sample. The interaction of the probe with the sample fields necessarily perturbs the bare sample response, and a critical issue is the interpretation of recorded signals. For a few specific SNOM configurations, individual descriptions have been modeled, but a general and intuitive framework is still lacking. Here, we give an exact formulation of the measurable signals in SNOM which is easily applicable to experimental configurations. Our results are in close analogy with the description Tersoff and Hamann have derived for the tunneling currents in scanning tunneling microscopy. For point-like scattering probe tips, such as used in apertureless SNOM, the theory simplifies dramatically to a single scalar relation. We find that the measured signal is directly proportional to the field of the coupled tip-sample system at the position of the tip. For weakly interacting probes, the model thus verifies the empirical findings that the recorded signal is proportional to the unperturbed field of the bare sample. In the more general case, it provides guidance to an intuitive and faithful interpretation of recorded images, facilitating the characterization of tip-related distortions and the evaluation of novel SNOM configurations, both for aperture-based and apertureless SNOM.

Quantum Zeno effect explains magnetic-sensitive radical-ion-pair reactions

Chemical reactions involving radical-ion pairs are ubiquitous in biology, since not only are they at the basis of the photosynthetic reaction chain, but are also assumed to underlie the biochemical magnetic compass used by avian species for navigation. Recent experiments with magnetic-sensitive radical-ion-pair reactions provided strong evidence for the radical-ion-pair magnetoreception mechanism, verifying the expected magnetic sensitivities and chemical product yield changes. It is here shown that the theoretical description of radical-ion-pair reactions used since the 70s cannot explain the observed data, because it is based on phenomenological equations masking quantum coherence effects. The fundamental density-matrix equation derived here from basic quantum measurement theory considerations naturally incorporates the quantum Zeno effect and readily explains recent experimental observations on low- and high magnetic-field radical-ion-pair reactions.

Thermal radiation scanning tunnelling microscopy

In standard near-field scanning optical microscopy (NSOM), a subwavelength probe acts as an optical 'stethoscope' to map the near field produced at the sample surface by external illumination. This technique has been applied using visible, infrared, terahertz and gigahertz radiation to illuminate the sample, providing a resolution well beyond the diffraction limit. NSOM is well suited to study surface waves such as surface plasmons or surface-phonon polaritons. Using an aperture NSOM with visible laser illumination, a near-field interference pattern around a corral structure has been observed, whose features were similar to the scanning tunnelling microscope image of the electronic waves in a quantum corral. Here we describe an infrared NSOM that operates without any external illumination: it is a near-field analogue of a night-vision camera, making use of the thermal infrared evanescent fields emitted by the surface, and behaves as an optical scanning tunnelling microscope. We therefore term this instrument a 'thermal radiation scanning tunnelling microscope' (TRSTM). We show the first TRSTM images of thermally excited surface plasmons, and demonstrate spatial coherence effects in near-field thermal emission.

Energy transfer in near-field optics

When the probe tip of a near-field optical microscope illuminates nanoparticles with marked absorption bands, a large number of photons are absorbed before reaching the detector. These energy losses enhance the dark contrast usually observed in the vicinity of metallic nanoparticles. We demonstrate theoretically that this phenomenon can be exploited to image, in the optical frequency range, dissipative domains with a nanometer scale resolution. Simulations performed with noble-metal particles indicate that the detected signal significantly drops down when the excitation frequency is approaching the plasmon resonance of the particles.

Near-field optical investigation of three-dimensional photonic crystals

We show that the coupling of light from an external pointlike light source into a three-dimensional photonic crystal depends on the relative launching position with respect to the crystal lattice as well as on the frequency of light. The results are obtained with a near-field technique which is used to acquire optical information beyond the diffraction limit and to access optical details within the unit cell of the crystal. The experiments are performed at frequencies near the second-order L-gap. As a result, the changes in the shape of the near-field pattern are explained by the photonic properties of the crystal.

Imaging the local density of states of optical corrals

This paper reports the experimental observation, at optical frequencies, of the electromagnetic local density of states established by nanostructures corresponding to the recently introduced concept of optical corral [G. Colas des Francs et al., Phys. Rev. Lett. 86, 4950 (2001)]. The images obtained by a scanning near-field optical microscope under specific operational conditions are found in agreement with the theoretical maps of the optical local density of states. A clear functionality of detection by the scanning near-field optical microscope is thereby identified since the theoretical maps are computed without including any specific tip model.