“This work is currently indeed accelerating studies of fields such as nanolasers, photonic chips, nonlinear optics, and quantum communications and computing.”

A photonic cavity able to strongly confine photons is required in broad areas of physics and engineering, including coherent electron-photon interactions, ultra-low threshold nanolasers, photonic chips, nonlinear optics, and quantum information processing. For these applications it is important to realize a cavity with both high-Q and very small modal volume V. Q/V determines the strength of various cavity interactions; an ultra-small cavity enables large-scale integration and single-mode operation for a broad range of wavelengths. However, a high-Q nanocavity of optical wavelength size had been difficult to build, since radiation loss increases in inverse proportion to cavity size. In this paper, we reported an important concept, i.e., that "light should be confined gently to be confined strongly." Based on this concept, we demonstrated a nanocavity with Q=45,000 and V=7.0x10^-14cm³, or Q/V=6.4x10¹⁷cm^-3, a factor 10-100 times larger than in previous studies. Currently, the Q of nanocavities has been increased up to the order of 1,000,000 by extending this concept (also see our paper, Nature Materials 4: 207-210, 2005). Fortunately, these works have impacted researchers in the aforementioned various fields, which has led to its high rate of citation.

The confinement of photons in an ultrasmall cage (nanocavity) the size of an optical wavelength has been realized in this work. Interactions between photon and matter can be dramatically increased inside such a small cavity, but it had been very difficult to confine photons in an ultrasmall cavity because leakage becomes a more important factor. In this paper, we reported on an important concept which addresses this issue: "light should be confined gently to be confined strongly." Using this concept, we succeeded in building a nanocavity having 10-100 times better confinement of photons than any previously constructed. This work is currently causing an acceleration of studies within several different fields; such as nanolasers, photonic chips, nonlinear optics, quantum communications, and computing.

In early 1990s, we began a study of photonic crystals in which the refractive index is changed periodically. A photonic bandgap, which inhibits the existence of photons for certain wavelengths, can be formed, and by introducing artificial point- and/or line-defects into the photonic crystal, a manipulation of photons becomes possible. In 2000, we demonstrated that an artificial point-defect cavity (nanocavity), formed in a two-dimensional photonic crystal slab, can trap and emit photons which propagate through a line-defect waveguide formed at the vicinity of the nanocavity (Nature 407: 608-610, 2000). This demonstration was an important step for the realization of a full photonic-crystal network with waveguides, nanocavities, etc., which is one of the holy grails in nano-optics. At that time, however, the cavity Q was limited to ~400. To increase the cavity Q was, therefore, all in due course for the direction taken by such research. As described above, we finally found an important concept that photons should be confined gently in order to be confined strongly. More precisely, the form of the cavity electric field distribution should vary slowly, ideally as described by a Gaussian function, in order to suppress out-of-slab photon leakage. Tuning of air holes at the cavity edge and/or the formation of a photonic double-heterostructure were found to be very effective in order to satisfy the concept and to realize an ultrahigh-Q nanocavity. As described above, the cavity Q on the order of ~1,000,000 has been realized successfully.