High quality factor photonic crystal nanobeam cavities

Abstract

We investigate the design, fabrication and experimental characterization of high Quality factor photonic crystal nanobeam cavities in silicon. Using a five-hole tapered 1D photonic crystal mirror and precise control of the cavity length, we designed cavities with theoretical Quality factors as high as 1.4 × 10 7 . By detecting the cross-polarized resonantly scattered light from a normally incident laser beam, we measure a Quality factor of nearly 7.5 × 10 5 . The effect of cavity size on mode frequency and Quality factor was simulated and then verified experimentally.For the past decade, there has been a concerted research effort to develop ultra-high Quality (Q) factor electromagnetic cavities with dimensions comparable to the wavelength of light [1,2,3,4,5,6]. By shrinking the modal volume to near the fundamental limit of V = (λ/2n) 3 , these cavities have enabled new applications to emerge in ultrasmall lasers [7,8,9,10], strong light-matter coupling [11,12,13,14,15,16], optical switching [17], and chemical sensing [18,19], among others. Recently, there has been much interest in cavities realized in suspended nanobeams patterned with a onedimensional (1D) lattice of holes [20,21,22,23] due to their exceptional cavity figures of merit (Q and V ), relative ease of design and fabrication, and potential for novel optomechanical effects [24,25]. These apparently simple structures, which resemble very early microcavity prototypes [26], actually have Q/V factors which rival the best 2D planar photonic crystal cavities [3,4]. They also have many inherent advantages, including the possibility of realizing high Q/V cavities in moderate index materials such as SiN x [22] and facilitating coupling to ridge waveguides [21]. In addition, the near-field of the cavity is also highly "accessible", in the sense that there are two dimensions with total-internal-reflection (TIR) interfaces, which should facilitate bio-sensing applications as well as novel techniques for the dynamic control of cavity resonances [27].In this paper we describe the design, fabrication, and experimental characterization of silicon photonic crystal nanobeam (PhCnB) cavities operating near ∼ 1500 nm with measured Q factors of 7.5 ×10 5 . To our knowledge, this represents the highest Q factor ever measured in nanocavities based on photonic crystal nanobeams, and one of the highest Qs ever measured in any photonic crystal cavity. Electromagnetic field confinement in the structure [ Fig. 1(a)] is achieved by index guiding in two directions (y and z), and Bragg scattering from the 1D photonic crystal mirror in the third (x) direction. The mechanism of light confinement has been interpreted in terms of impedance matching [20,22,28] and the mode-gap effect [23]. Conceptually, the cavity can be viewed as a wavelength-scale Fabry-Perot cavity with photonic crystal mirrors which reflect and thus trap the nanobeam waveguide mode. Because the cavity mode penetrates some distance into the mirror, it is crucial that the fields do not abruptly terminate at the mirro...

Silicon photonic devices for mid-infrared applications

“…The devices were undercut using a 7:1 buffered oxide etch for 45 min. A scanning electron micrograph of a completed device is shown in Figure 2 We used the resonant scattering method [26,27] to test our cavities ( Figure 3). Light from a tunable quantum cascade laser (QCL) with emission from 4.315 to 4.615 µm (Daylight Solutions, Inc.) is sent into a ZnSe objective lens with numerical aperture (NA) of 0.22 and focused onto the sample, which is placed so that the cavity mode polarization is oriented at 45° with respect to the E-field of the laser spot [10].…”

Hybrid active photonic crystal structures: III-V based slow light waveguides or nanocavities coupled to SOI wires

“…These high Q factors have even been obtained not only on suspended membranes but also on low index substrates such as SiO2 which makes them suitable for use in devices. So far a lot of attention has been devoted to passive 1-D cavity and nanocavity beams [2]. However, incorporating an active element has not been explored yet though it is an exciting possibility as the strong light confinement promises ultrasmall lasers with low laser thresholds.…”

High-Q silicon photonic crystal cavity for enhanced optical nonlinearities

“…32,33 The present nanocavity thus ranks among those with the highest Q/V ratio ever reported. 1,6,11,15,17,[34][35][36] In the present work, we have additionally investigated an alternate optimal design 13 characterized by a smaller modal volume, that was obtained by introducing a stricter upper bound S 1x < 0.25a in the optimization procedure. The design has an ideal GMEcomputed Q-factor 1.05 Â 10 6 (FDTD: 1.0 Â 10 6 ) and a smaller modal volume V ¼ 0.25(k/n) 3 .…”