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Research

Tapered optical fiber

A tapered optical fiber is an optical fiber with a tapered part (diameter less than 1 μm). We fabricate the tapered optical fibers by heating at 1350 degrees with a ceramic heater and pulling the end of the fiber. The typical transmittance is much over 90 % even when the diameter of the tapered region is about 300 nm.

Microsphere laser

The spontaneous emission coupling factor β is essential for realization of quantum information devices. Fiber-coupled microspheres have been attracting attention as ideal cavity systems with ultra-high Q-factors and single spatial mode input-outputs. In order to apply the fiber-coupled microspheres to the quantum information devices, it is necessary to precisely estimate the β factor. For this purpose, we realized fiber-coupled microsphere lasers with an Er³⁺-doped P₂O₅-Al₂O₃-SiO₂thin film using a sol-gel method.

Applied Physics Letters, Vol. 90, pp. 101103/1-3 (2007).

Applied Physics Letters, Vol. 92, pp. 071115/1-3 (2008).

Highly efficient coupling to single metal nanostructure

Localized surface plasmons (LSPs) of metal nanostructures have attracted considerable attention because of the strong enhancement of the exciton-photon coupling. Although the strong-coupling regime can be obtained within the LSP field, the coupling efficiency between a single LSP antenna and propagating light is extremely low.
Therefore, we proposed and realized nearly complete optical coupling method into a single metal nanostructure using a tapered-fiber-coupled microspherical cavity system.

Physical Review A,Vol. 89, pp. 021801(R)/1-5 (2014).

Nanofiber Bragg grating cavity

A Nanofiber Bragg grating cavity (NFBC) is a novel microcavity. The NFBCs are fabricated on an optical tapered fiber using a focused ion beam milling system. This cavity has small mode volume (less than 1 μm³), ultra-wide tuning range (over 20 nm), and loss-less coupling to single mode fiber. By coupling light emitters like quantum dots to the NFBCs, we observed enhanced spontaneous emission.

Sci. Rep. 5, 9619 (2015).

Fabrication of NFBCs using Helium ion microscope

In order to improve Q factors of NFBCs, we used a Helium ion microscope with ultra high resoluton (< 1 nm) instead of Ga FIB machine. As the result, we improved Q factors from about 300 to over 4000.

Optics Express, 27, 6792-6800 (2019).

Nanolight emitters

Diamond, fascinating people from ancient times, is the most excellent materials with outstanding physical properties. It has the highest hardness and thermal conductivity in any materials. Besides, it has the largest optical band-gap, high refractive index, and robust fluorescence in broad optical band from various color centers and impurities as represented by nitrogen vacancies (NVs). The diamond, therefore, has been paid attention as an ideal and novel platform for photonic and quantum information applications. We study about effieict single photon sources and laser devices using the diamond nanocrystals.

Applied Physics Letters, Vol. 105, pp. 011112/1-3 (2014).

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