Dynamically Reconfigurable Photonic Crystal Nanobeam Cavities

Wavelength-scale, high Q-factor photonic crystal cavities [1, 2] have emerged as a platform of choice for on-chip manipulation of optical signals, with applications ranging from low-power optical signal processing [3] and cavity quantum electrodynamics [4, 5] to biochemical sensing. Many of these applications, however, are limited by the fabrication tolerances and the inability to precisely control the resonant wavelength of fabricated structures. Various techniques for postfabrication wavelength trimming [6, 7] and dynamical wavelength control – using, for example, thermal effects [8, 9, 10], free carrier injection [11], low temperature gas condensation [12], and immersion in fluids [13] – have been explored. However, these methods are often limited by small tuning ranges, high power consumption, or the inability to tune continuously or reversibly. In this letter, by combining nano-electro-mechanical systems (NEMS) and nanophotonics, we demonstrate reconfigurable photonic crystal nanobeam cavities that can be continuously and dynamically tuned using electrostatic forces. A tuning of ∼ 10 nm has been demonstrated with less than 6 V of external bias and negligible steady-state power consumption. Recently, it has been theoretically predicted [14, 15, 16] and experimentally verified [2, 17, 18, 19] that photonic crystal nanobeam cavities (PhCNB) can have ultra-high quality factors, on-par with those demonstrated in conventional photonic crystal cavities based on a two-dimensional lattice of holes. PhCNB cavities can be viewed as a doubly clamped nanobeam, the simplest NEMS device, perforated with a one-dimensional lattice of 1 ar X iv :0 90 9. 22 78 v1 [ ph ys ic s. op tic s] 1 1 Se p 20 09 holes, a textbook example of an optical grating. By introducing an appropriate chirp in the grating, ultra-high Q factors and small mode volume optical resonators can be realized [2]. When two PhCNB cavities are placed in each other’s near field, as shown in Fig. 1, their resonant modes couple, resulting in two supermodes with resonant frequencies that are highly dependent on the spacing between the nanobeams [20]. This can be attributed to two major factors. First, the coupling between the two resonators increases with the reduction in the lateral separation between the nanobeams, which results in greater splitting between the two supermodes. At the same time, as the nanobeams are drawn closer together, the higher order effect of the coupling induced frequency shift [21] becomes significant (especially for separations < 100 nm) and red shifting of both of the supermodes occurs. The net effect of these two factors is that the even supermode experiences a considerable red shift as the separation is reduced, while the wavelength of the odd supermode stays relatively constant (the two effects cancel out) [20]. The strong dependence of the wavelength of the even supermode on the separation between two nanobeams renders coupled-PhCNB cavities highly suited for applications in motion and mass sensing. In addition, the strong optical fields that exist in the air region between the coupled-PhCNB cavities makes these devices excellent candidates for biochemical sensing applications. Finally, by simultaneously taking advantage of both the optical and mechanical degrees of freedom of such these cavities, a plethora of exciting optomechanical phenomena can be realized [18, 22]. In this work, we take advantage of the mechanical flexibility of coupled PhCNBs to realize reconfigurable optomechanical devices that can be electrostatically actuated [23]. By applying a potential difference directly across the nanobeams, an attractive electrostatic force can be induced between the two nanobeams, resulting in a decrease of the gap between the nanobeams, as can be seen in Fig. 1 b and c. This, in turn, results in the change of the resonant wavelength of the two supermodes. Self-consistent optical and mechanical finite-element simulations were used to model the deflection of the nanobeams due to the electrostatic forces, and its influence on the optical eigenfrequencies (Fig. 1c). Fig. 1d shows the dependence of the nanobeam separation (red curve) on the applied voltage, as well as the differential separation change for different bias voltages, in the case of a device with 77 nm initial separation between nanobeams. It can be seen that nanobeam separation, measured at the middle of the nanobeams, can be reduced to 50nm with ≈ 5 V of external bias. Moreover, the change in the separation per unit of applied voltage strongly depends on the applied bias, and is on the order of 25 nm/V for V ≈ 5 V. The influence of the