Kiyoshi Kobayashi

Lower bound of energy dissipation in optical excitation transfer via optical near-field interactions.

We theoretically analyzed the lower bound of energy dissipation required for optical excitation transfer from smaller quantum dots to larger ones via optical near-field interactions. The coherent interaction between two quantum dots via optical near-fields results in unidirectional excitation transfer by an energy dissipation process occurring in the larger dot. We investigated the lower bound of this energy dissipation, or the intersublevel energy difference at the larger dot, when the excitation appearing in the larger dot originated from the excitation transfer via optical near-field interactions. We demonstrate that the energy dissipation could be as low as 25 μeV. Compared with the bit flip energy of an electrically wired device, this is about 10⁴ times more energy efficient. The achievable integration density of nanophotonic devices is also analyzed based on the energy dissipation and the error ratio while assuming a Yukawa-type potential for the optical near-field interactions.

Information theoretical analysis of hierarchical nano-optical systems in the subwavelength regime

Optical near-field interactions exhibit a hierarchical response, which is one of the most unique attributes of light-matter interactions occurring locally on the nanometer scale. It allows hierarchical nano-optical systems that break through the integration restrictions posed by the diffraction limit of conventional propagating light and offers multiple hierarchical functionalities at different physical scales in the subwavelength regime. Here we demonstrate an information theoretical approach to such nano-optical systems while assessing their electromagnetic and logical aspects via angular-spectrum analysis. Mutual information at each level of the hierarchy reveals quantitatively the relation between the physical effects associated with the hierarchy in the optical near-fields, as well as possible environmental disturbances affecting the system locally or globally, and the systems capabilities for information processing and communication.

Quantum theory and virtual photon model of near-field optics

We point out that recent experimental results exemplify the need for quantum theoretical treatment of optical near-field problems, as well as the need for an intuitive model that provides clear insights into near-field optical systems. In this context, the virtual photon model as an intuitive model is discussed, and a quantum theoretical formulation of an optical near-field system is proposed on the basis of the projection-operator method. Special attention is paid to nanometric probe tip and quantum-mechanical sample systems such as atoms, molecules, and quantum dots. The effective probe tip-sample interaction is derived from the microscopic viewpoint; this interaction is essential for describing such phenomena as atom guidance and manipulation, or local excitation of a single quantum dot. The relationship to the virtual photon model is also discussed by focusing on the latters empirical assumption of Yukawa-type interaction between the probe tip and sample. The key points are that a probe tip exists near the sample, and that the electron energies in the probe tip or sample are inversely proportional to the square of its size, owing to the confinement effect. Several applications and the future prospects of our theory are also briefly outlined.

Quantum Coherent Dynamics Enhanced by Synchronization with Nonequilibrium Environments

We report the discovery of the anomalous enhancement of quantum coherent dynamics (CD) due to a non-Markovian mechanism originating from not thermal-equilibrium phonon baths but nonequilibrium cohe...

Picture of Optical Near Field as a Virtual Cloud Around a Nanometric System Surrounded by a Macroscopic System

Previous chapters used classical electromagnetism to describe a nanometric system composed of a sample, a probe, and an optical near field. This chapter presents a quantum mechanical model based on a projection operator method to describe the interaction between nanometric material systems via an optical near field surrounded by a macroscopic system. This model can also be used to describe the interaction between an atom and a probe, and its application to atom photonics is discussed in Chap. 9. Appendices C and D provide supplementary explanations of the concepts to be used in this chapter. An outstanding advantage of this model is its ability to systematically describe the light-matter interactions in nanometric material and atomic systems. This is because the model is based on concepts developed in the fields of elementary particle physics, statistical mechanics, quantum chemistry, and quantum optics. Furthermore, the model provides an intuitive physical picture in which the localized optical near field can be described in the same way as an electron cloud localized around an atomic nucleus.

Self-Consistent Method Using a Propagator

In Chap. 4, we derived the change in the polarizability of the sphere P, which was induced by the electric field from an electric dipole in the sphere S. In this derivation, the effect of multiple scattering was neglected, i.e., we neglected the changes in the polarizability of the sphere S induced by the above-mentioned change in the polarizability of the sphere P. The present chapter discusses the effect of multiple scattering for the more precise investigation of an optical near field. A propagator, i.e., the transfer function, is derived in Sect. 6. 1, in order to evaluate the electric field at an arbitrary position generated by a light source at another position. The result of this derivation is applied to collection-mode near-field optical microscopy in Sect. 6.2. It should be noted that these results can be applied, not only to the two spheres S and P, but also to arbitrarily shaped material objects. However, a long computation time is required to derive quantitative results in such numerical analysis.

Nano-imaging for glia-synapse fine structures with a homemade near-field optical microscope

To investigate basic processes responsible for brain functions by nano-imaging, we have developed a near-field optical microscope and successfully visualized fine structures and snapshot of the states of astrocytes and neurons.

Optical pulsation based on near-field interactions at the nanoscale by continuous-wave light excitation

We theoretically demonstrate optical pulsation based on optical near-field interactions at the nanoscale. We observe pulsation in populations pumped by continuous-wave light excitation based on optical energy transfer from smaller to larger quantum dots. This will provide critical insights toward the design and implementation of experimental nanophotonic pulse generating devices.

Theory of superradiance by nano-array of quantum-well dots

The superradiance had been predicted by Dicke in 1954 theoretically [1] and observed in atomic and molecular systems experimentally [2]. In the recent experiments, the superradiance and superfluorescence by nano-structures of solid-state materials such as an ensemble of quantum dots were observed [3, 4]. The superradiance by solid-state materials has advantage for the application of devices as compared with that by other materials such as atoms and molecules. Our group also observed the superradiant behaviour of emissions from a semiconductor nano-rod as shown in Fig. 1 (a). This sample is a nano-array of quantum-well dots (QWDs) with the width of 3.25 nm and barrier layers with the width of 9 nm. The diameter of the rod is 80 nm. The number of QWDs is about ten. In order to clarify the mechanism of the new type of superradiance by solid-state materials, we constructed the full-quantum-mechanical theory of superradiance [5], in which theory, the fundamental equations of the superradiant photoluminescence were derived by the scheme of the semiconductor luminescence equations [6]. The important development of our theory is an introduction of a correlation between polarizations via a radiation field which is the origin of superradiance. Here we will clarify the characteristics of superradiant emissions from the nano-array of QWDs and prove that the superradiance by our sample of the nano-rod can be realized theoretically.

Minimum energy dissipation in signal transfer via optical near-field interactions in the subwavelength regime

We theoretically demonstrate that the minimum energy dissipation required for optical excitation transfer in quantum dot systems on the nanometer-scale via optical near-field interactions could be about 25 μeV, nearly 104 times more energy-efficient than conventional electrically wired devices.