Optical Micromachine by Ultrahigh-Q Nanocavities


Masaya Notomi and Hideaki Taniyama
Optical Science Laboratory

 Recently, ultrahigh-Q and simultaneously ultra-small optical cavities have been realized by photonic crystals [1]. In such ultrahigh-Q nanocavities, various light-matter interactions are expected to be greatly enhanced. We have theoretically found that we can convert optical energies into mechanical energies extremely efficiently by employing specially-designed photonic-crystal nanocavities [2]. Our result demonstrates that one can introduce mechanical displacement by extremely weak light, and indicates possibilities towards ultra-efficient optical micro-machines in future.
 Figure 1 shows our sample structure, which is a point defect cavity of a double-layer photonic crystal. It is practically the same as our ultrahigh-Q photonic-crystal cavity design except an air slit is inserted in the center of the slab. This cavity is special because one can largely change the resonant wavelength by slight change of the slab spacing (slit width) without deteriorating cavity Q. Owing to this feature, an optical pulse captured in this cavity can generate extraordinary large radiation force (〜 1µN per 1 pJ), because this force is determined by the spatial derivative of the electromagnetic energy in the cavity. Furthermore, this large force can do a large mechanical work due to the long cavity photon lifetime. Consequently, the optical energy of the optical pulse is converted to the mechanical energy very efficiently. We calculated this efficiency assuming realistic parameters, as shown in Fig. 2. Generally, such opto-mechanical energy conversion is intrinsically very inefficient (〜10-12) because of the mass-less nature of light (except photon rockets having relativistic speed). Figure 2 shows that the efficiency can reach up to 10%. Such extremely-high efficiency is only possible for mechanical displacement incorporated to ultrahigh-Q and ultrasmall cavities.
 When the optical energy is converted to mechanical, the frequency of light in the cavity is lowered. In other words, one can realize wavelength conversion of light using the reverse process. Our numerical calculation shows that large wavelength conversion (larger than 20% of the original wavelength) is indeed possible. Last year, we have reported that adiabatic wavelength conversion is possibly by dynamically tuning the resonance frequency of a cavity [3]. In fact, the present opto-mechanical process is physically identical. That is, very efficient optical micromachines are intrinsically very efficient opto-mechanical wavelength converters.

[1] E. Kuramochi, et al., Appl. Phys. Lett. 88 (2006) 041112.
[2] M. Notomi, et al., Phys. Rev. Lett. 97 (2006) 023903.
[3] M. Notomi, et al., Phys. Rev. A73 (2006) 051803(R).

Fig. 1. Double-layer photonic crystal cavity.
Fig. 2. Energy conversion efficiency.

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