Dynamic High-Q Cavity Formation and Photon Pinning by Index Modulation
Masaya Notomi and Hideaki Taniyama
Optical Science Laboratory
Recently, ultrahigh-Q nanocavities have been realized by small local structural modulation of (typically, several nm shift of the air hole position) line-defect waveguides in two-dimensional photonic crystals  [Fig. 1(a)]. This confinement mechanism has been applied to various systems. Periodic structural modulation has realized large-scale ultrahigh-Q coupled nanocavities , and ultrahigh-Q nanocavity modes have been also found in one-dimensional systems for the first time by the structural modulation scheme .
Here we report that we have found similar ultrahigh-Q cavity modes can be formed only by refractive-index modulation of line-defect waveguides without any structure modulation . The most amazing feature of this finding is that extremely small index modulation can realize ultra-strong light confinement. For example, a Q value higher than 107 is possible with index modulation as small as Δn/n =4×10-4. In the case of Δn/n =3×10-3, the calculated Q reaches a surprisingly high value of 5 billions.
Such small index modulation can be achieved by various ultrafast optical nonlinearity. Thus, it may become possible to dynamically form an ultrahigh-Q cavity by local optical pumping. If so, we can pin (in other words, freeze) a part of an optical pulse traveling in a line-defect waveguide by shining a focused light pulse from the top surface. Figure 2 shows a numerical simulation of this process, which indeed shows that the light intensity inside the tuned area marked by a circle is pinned in a dynamically-formed cavity mode . Detailed analysis has showed that this process is driven by adiabatic wavelength conversion recently experimentally observed in our group .
These results indicate that we can freely control ultrastrong light confinement by very small modulation, which may open up novel functionalities in future optical devices.
 T. Tanabe et al., Nature Photon. 1 (2007) 49.
 M. Notomi, E. Kuramochi et al., Nature Photon. 2 (2008) 741.
 M. Notomi et al., Opt. Express 16 (2008) 11095.
 M. Notomi and H. Taniyama, Opt. Express 16 (2008) 18657.
 T. Tanabe et al., Phys. Rev Lett. 102 (2009) 043907.
Fig. 1. Ultrahigh-Q cavities formed by local modulation of line-defect waveguide.
Fig. 2. Numerical simulation of photon pinning induced by dynamic index modulation of a line-defect waveguide.
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