We observe coherent resonant coupling of optical whispering-gallery modes in fluorescence from dye doped polymer bispheres with diameters ranging from 2 to 5 mm. By monitoring the frequencies of fluorescence peaks of individual spheres, we sort out two spheres with appropriate size matching and bring them into contact. Wave optics calculation also gives good agreement with the experiment. By taking into account harmonic coupling of the whispering-gallery modes, the obtained features of normal mode splitting are well explained by the tight-binding photon picture. [S0031-9007(99)09349-7] PACS numbers: 42.60.DaManipulation of light path in micrometer length scale has recently attracted considerable attention from both fundamental and application points of view. Conventionally, the manipulation is based on the photonic crystal concept [1][2][3]. In photonic crystals, which have periodic modulation of the refractive index, propagation of the light wave is governed by a weak potential. Correspondingly, such an approach can be referred to as a nearly free photon approach analogous to the nearly free electron approach in band theory. Alternatively the micromanipulation of light can be achieved by exploring the possibility of confining the light in a small unit of the wavelength size. Light propagates through the system of such units due to the coupling between the nearest neighbors. This approach is referred to as the tight-binding photon approach [4]. Within the tight-binding photon approach we can guide the optical waves by connecting the units in the arbitrarily shaped microstructures.The microspheres are the most natural choice of the unit to be employed in the tight-binding photon device. It is known that a dielectric sphere acts as a unique optical microcavity which has very long photon storage time within a small mode volume [5][6][7][8]. In particular, Q factors of the order of 10 10 have been observed for whispering gallery modes (WGM's) in quartz spheres with a diameter of several tens of micrometers [9-13], and the mode structure of a pair of these large spheres in contact has been studied [14]. However, in order to explore the feasibility of micromanipulation of light, one has to confirm the existence of the coherent coupling between spheres of the size of a few times of optical wavelength. Lorenz-Mie theory predicts long photon lifetime even for small spheres, giving, for example, nearly 30 ps for a 4 mm sphere with a refractive index of 1.59. This has allowed one to propose such relatively small spheres to be employed as "photonic atoms" [15] for the tight-binding scheme. However, the coherent coupling between two adjacent microspheres of such size range have not been realized until now. The coherent coupling results in the splitting of the corresponding WGM's and is a manifestation of the well-known phenomena of the normal mode splitting (NMS) in coupled harmonic oscillators. However, although some attempts have been made [16], NMS has not yet been observed because of the difficulty in the precise size con...

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Optical patch antennas sandwiching dielectrics between metal layers have been used as deep subwavelength building blocks of metasurfaces for perfect absorbers and thermal emitters. However, for applications of these metasurfaces for optoelectronic devices, wiring to each electrically isolated antenna is indispensable for biasing and current flow. Here we show that geometrically engineered metallic wires interconnecting the antennas can function to synchronize the optical phases for promoting coherent resonance, not only as electrical conductors. Antennas connected with optimally folded wires are applied to intersubband infrared photodetectors with a single 4-nm-thick quantum well, and a polarization-independent external quantum efficiency as high as 61% (responsivity 3.3 A W −1 , peak wavelength 6.7 μm) at 78 K, even extending to room temperature, is demonstrated. Applications of synchronously wired antennas are not limited to photodetectors, but are expected to serve as a fundamental architecture of arrayed subwavelength resonators for optoelectronic devices such as emitters and modulators.

Theoretical studies have revealed that coupling with a dielectric substrate significantly affects the transmission spectrum of a 2D photonic crystal of monolayer dielectric spheres. The dielectric constant of a semiinfinite substrate has been found to have a threshold, above which dips in the transmission spectrum broaden drastically. A substrate of finite thickness yields additional dips in the spectrum corresponding to localized eigenstates within the substrate. The transmission spectrum is well explained by the anticrossing of the eigenstates of a monolayer and a substrate.Research on photonic crystals 1 ͑PC's͒ has been focused for many years on the realization of a complete-gap PC, i.e., a PC having a common photonic gap in all directions. A complete-gap PC could be used, for example, in fundamental QED experiments to control the atomic lifetime by the suppression of spontaneous emission. 2 A variety of possible complete-gap PC's have been found in theoretical studies. 3 Since gaps appear as a result of interference, long-range order in all directions is indispensable for three-dimensional ͑3D͒ PC's. To realize complete-gap PC's, therefore, highly sophisticated methods such as self-assembly or autocloning 4 have been developed in addition to the advanced technology of lithography. 5 At present, however, it is still difficult to obtain 3D PC's of high quality.In contrast, 2D slablike PC's with long-range order, 6 a typical example being a monolayer of periodically arrayed dielectric spheres on a substrate, can easily be fabricated. Many theoretical 7 and experimental 8-10 studies on the transmission spectra and photonic band structures of such PC's are currently being carried out. A slablike PC is different from ordinary 2D PC's composed of infinitely long parallel cylinders, in that it has a finite thickness in one direction. Consequently, there is an energy dissipation in that direction, which gives rise to a finite lifetime of its eigenstates. There is also a significant enhancement of the electric field near the surface ͑near field͒ by diffracted evanescent waves due to the 2D periodicity. 7 This enhancement can be observed by the use of a scanning near-field optical microscope. 11 There is, in fact, a strong demand for the development of new photon technology by utilizing enhanced near fields such as laser manipulation of atoms. 12 The relevant wavelength of a slab-type PC at present is comparable with the 2D periodicity. For example, it lies in the lower Mie resonance region (lХ3,4) for monolayer spheres. 13 An electric field of such lower resonances is not strongly localized within the spheres and leaks out of the monolayer. This extended nature of the electric field is expected to induce strong coupling with the substrate. However, this coupling was not taken into account in our previous study. 7 A few theoretical attempts 14 have been made to ana-lyze the optical properties of a 2D periodic system on a semiinfinite substrate, but all of them lack the viewpoint of PC's such as band structure. There has...

Theoretical analysis is given for the internal electric-field distribution of two-dimensional periodic dielectric spheres by extending the vector Korringa-Kohn-Rostoker method. It is shown that the internal electric-field intensity distribution of the monolayer with high dielectric constant is in good agreement with that of an isolated sphere at the Mie resonance. In contrast, the field distribution of the monolayer with low dielectric constant is complicated and strongly influenced by the interaction between spheres. Our results show that the internal electric field of the monolayer presents important information about the origin of each eigenstate.Photonic crystals (PhC's) are expected to play an important role for the development of optoelectronic technology, because PhC's have attractive properties for application. These properties are brought about by the photonic band structure of PhC's due to the periodicity of dielectric constant. PhC's with large photonic band gaps (PBG's) have particularly attracted much interest, 1-3 since they can be used for control of the spontaneous emission of atomic systems and waveguides. In addition, it is reported that the photonic band dispersion also exhibits interesting optical properties, for example, the superprism effect. 4 To fully utilize these properties, we need to understand the origin of the photonic band structure.A typical example of PhC's is the two-dimensional (2D) periodic dielectric spheres. Since the monolayer is an important system for the physical understanding of the origin of the photonic band structure, those with high or low dielectric constant have been widely investigated. [5][6][7][8][9][10][11] There are two widely used concepts for the understanding of the band structure. 12 One is the tight-binding description, and the other is the nearly-free-photon approximation. In the case of dielectric spheres, the tight-binding approximation is suitable for describing the eigenstate due to the Mie resonance states. 13 In contrast, the nearly-free-photon approximation is based on the folding of the dispersion of free photon in the Brillouin zone. 14 We can discuss the origin of the eigenstate of each band by the electric-field intensity distribution near the crystal, because we can obtain the characteristic field intensity distribution unique to each eigenstate when the eigenstate is excited by the incident light. The near-field intensity, i.e., the electric-field intensity distribution just outside the monolayer, has been studied 6 by the vector Korringa-KohnRostoker (KKR) method 15,16 with very high accuracy and faster convergence. We expect that the internal electric-field intensity distribution of the monolayer gives more detailed information than the near-field intensity distribution. So far, however, the field distribution inside the monolayer has not yet been studied.In this paper, we extend the vector KKR method to calculate the internal electric field of the monolayer. We examine numerically the internal electric-field intensity distribution of the ...