Nanocrystalline silicon optomechanical cavities

Abstract

Silicon on insulator photonics has offered a versatile platform for the recent development of integrated optomechanical circuits. However, there are some constraints such as the high cost of the wafers and limitation to a single physical device level. In the present work we investigate nanocrystalline silicon as an alternative material for optomechanical devices. In particular, we demonstrate that optomechanical crystal cavities fabricated of nanocrystalline silicon have optical and mechanical properties enabling non-linear dynamical behaviour and effects such as thermo-optic/free-carrier-dispersion self-pulsing, phonon lasing and chaos, all at low input laser power and with typical frequencies as high as 0.3 GHz.

Properties of nanocrystalline silicon probed by optomechanics

“…This is not surprising since the nc-Si layers in all samples have the same thickness and refractive index as measured by spectroscopic reflectometry (see Table 1). However, the intrinsic optical decay rate of the fundamental mode κ i,1 at the resonant wavelength λ r,1 ≈ 1.54 μm, extracted indirectly by measuring the overall decay rate (κ 1 ) and the coupled power fraction in resonance [19], decreases in samples annealed at higher temperatures, as shown in Figure 4B. The lowest κ i,1 value is still a factor of 1.5 higher than that of a c-Si OMC with the same geometry and leads to an intrinsic optical Q-factor of Q i,1 = 1.3 × 10 4 at Room temperature (RT).…”

“…Considering that the heat dissipation has a direct impact on the frequency bandwidth of the self-pulsing dynamics occurring in the OMCs [19], the behavior of heat dissipation rates of the OMCs was studied as a function of T a . A single laser configuration was used to excite the fundamental optical mode λ r,1 and adjusted P in to obtain n o,max ≈ 2700 in all samples.…”

“…Under these experimental conditions, it is possible to compare the contribution of the TO effect in each OMC by measuring the maximum λ r,1 shift. Since the TO coefficient was determined to be ∂λ r,1 /∂ΔT = 0.09 nm/K, regardless of the crystalline arrangement of the nc-Si films [19], the spectral shifts can be directly related to ΔT (see Table 1). At this point, it is necessary to recall the results of Figure 4C, which show that the dependence of the optical loss rate on n o is similar in all the nc-Si OMCs, i.e., the carrier concentration does not depend on T a .…”

“…The process was developed in the early 80s [17] and is used in Microelectromechanical systems (MEMS) technology due to the relatively easy tuning of mechanical, optical, electrical and thermal properties by doping, tailoring the size of the crystallites and the stress in the films [17,18]. Recently, it was demonstrated that nc-Si is also an excellent and cost-competitive alternative to c-Si for optomechanical devices taking advantage of nonlinear effects while operating in ambient conditions [19]. Nonlinear dynamic functions, such as phonon lasing and chaos, were demonstrated with broader bandwidth than measured in equivalent SOI devices, as well as wafers [19].…”

“…Recently, it was demonstrated that nc-Si is also an excellent and cost-competitive alternative to c-Si for optomechanical devices taking advantage of nonlinear effects while operating in ambient conditions [19]. Nonlinear dynamic functions, such as phonon lasing and chaos, were demonstrated with broader bandwidth than measured in equivalent SOI devices, as well as wafers [19]. The dynamic behavior depends on the optical and mechanical losses and on heat generation and extraction rates of the OMCs [4,20].…”