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 [1] [Fig. 1(a)]. This confinement mechanism has been applied to various systems. Periodic structural modulation has realized large-scale ultrahigh-Q coupled nanocavities [2], and ultrahigh-Q nanocavity modes have been also found in one-dimensional systems for the first time by the structural modulation scheme [3].
 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 [4]. 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 [4]. Detailed analysis has showed that this process is driven by adiabatic wavelength conversion recently experimentally observed in our group [5].
 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.

[1] T. Tanabe et al., Nature Photon. 1 (2007) 49.
[2] M. Notomi, E. Kuramochi et al., Nature Photon. 2 (2008) 741.
[3] M. Notomi et al., Opt. Express 16 (2008) 11095.
[4] M. Notomi and H. Taniyama, Opt. Express 16 (2008) 18657.
[5] 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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