We present a suspended-element silicon photonic crystal membrane sensor with a diaphragm area of 12 μm by 15 μm suitable for free-space ultrasonic acoustic measurements. The sensor is based on the position-dependent directional coupling between neighbouring photonic crystal edges and exhibits compatibility with broadband optical sources, low insertion loss, and more freedom in the mechanical design. By tuning the wavelength such that both time-averaged outputs of the directional coupler were balanced, a differential detection scheme was implemented where common noise sources were suppressed while enhancing the sensor signal without additional photonic components. Using a coiled speaker, the dynamic acoustic response of the sensor from 10 kHz to 80 kHz was measured and we report a peak system response in the difference signal at 40 kHz with a sensitivity of 40 μV/Pa. The compact size and improved mechanical frequency response of these integrated photonic sensors make them suitable for array applications, particularly for the study of fluid dynamics involving high Reynolds number phenomena.
Optomechanical coupling produced by high finesse optical micro-cavities has led to a plethora of nonlinear observations in silicon photonics due to the coherent nature of the interaction. We have investigated optomechanical coupling in the dielectric-like cavity modes of photonic crystal edge defects. These structures exhibit extreme sensitivity to changes of the edge defect width, allowing for scaling of the edge length and a reduction of the optical finesse required to produce different phenomena independent of the edge width. The edge defect structures presented have relatively simpler fabrication relative to other optomechanical designs of comparable coupling strength. We report frequency combing in long linear edge defects and optical bi-stability in shorter, hexagonally coupled edge defect devices. These results indicate that photonic crystal edge defects offer an exciting platform for the development of new optomechanical devices.
2-D slab photonic crystal multiple line defect waveguides have been designed for optical power splitting
application which has numerous applications in photonic integrated circuit. The operation of the device is
based on multimode interference effects and self-imaging phenomenon. The proposed structure consists of
multiple photonic crystal line defect waveguides which are formed in the Γ-K direction by removing several
entire rows of air-holes. The adjacent photonic crystal waveguides are separated by a row of air-holes. In this
scheme a 1×3 power splitter is designed which involves three photonic crystal line defect waveguides
multimode region, five photonic crystal line defect waveguides multimode region and one separation region.
The entire structure is verified by 3-D finite difference time domain method. The transmitted power achieved
at each output channel i.e. CH1, CH2 and CH3 are about 26.3%, 26.8% and 26.3% respectively. The total
transmitted output power of 1×3 power splitter is 79.4% at target wavelength of 1.55μm.
Photonic crystals (PhCs) have recently been the focus for the developing micro- and nano-optical sensors, due to its
capability to control and manipulate light on planar devices. This paper presents a novel design of micro-optical pressure
sensor based on 2-dimensional PhC slab suspended on Si substrate. A line defect was introduced to the PhC slab to guide
and reflect light with frequency in the photonic bandgap in the plane of the slab. The structure, with certain surface
treatment, can be used in miro-scale pressure catheters in heart ablation surgeries and other biomedical applications. The
working principle of the device is to modify light reflection in the PhC line defect waveguide by moving a substrate
vertically in the evanescent field of the PhC waveguide. Evanescent field coupling is the critical step that affects light
transmission and reflection. High resolution electron-beam lithography and isotropic wet etching have been used to
realize the device on the top layer of a Si-On-Insulator (SOI) wafer. The PhC slab is released by isotropic wet etch of the
berried oxide layer. The output reflection spectrum of the device under different pressure conditions is simulated using
3-dimensional finite difference time domain (FDTD) method. The result showed that when the PhC slab is close enough
to the substrate (less than 400 nm), the reflected light intensity decreases sharply when the substrate moves towards the
PhC slab. Mechanical response of the sensor is also studied.
A detail study was done on the sensitivities of 1-D photonic crystal (PC) and 2-D PC coupled cavity sensors with
changing sensing layer parameters of thickness and refractive index (RI). Though both refractive index and thickness are
interrelated they have significant individual affects on device response. In 1-D PC shifts in normal transmission peak due
to surface change in thickness and RI and in 2-D PC coupled cavity shifts in transmission dip due to surface changes are
observed. Here sensitivity analysis in change in thickness and RI on these devices was done for four cases; case 1:
change in thickness from 2nm-10nm on PC sensors, case 2: change in thickness from 75nm-175nm on PC sensors, case
3: change in RI in thin film (6nm) on surface and case 4: change in RI in thick film (100nm) on sensors surface.
Sensitivities due to change in thickness (St) of 1-D PC and 2-D PC coupled cavity were calculated from the slope of the
sensitivity curves and found to be (for RI of 1.4) 1.423nm/nm and 2.285nm/nm for case 1 and 0.455nm/nm and
0.801nm/nm for case 2. Sensitivities due to change in RI (Sr) of 1-D PC and 2-D PC coupled cavity were obtained from
the transmission peak and dip shifts due to change in RI from 1(air) to 2. Sr for 1-D PC and 2-D PC coupled cavity were
found to be 70nm/RIU and 103nm/RIU for case 3 and 143nm/RIU and 213nm/RIU for case 4. The results are based on
Transmission and coupling mechanisms in photonic crystal waveguides have been extensively studied in the last few years to optimize photonic crystal designs. Previous numerical results, using 2D FDTD methods have shown that the technique of tapering the photonic crystal lattice can improve coupling efficiencies up to 80%, as compared to 30-40% efficiencies in butt-coupled waveguides. However, for the 3D structures such as photonic crystal slabs, 2D calculations do not take into consideration the losses in the vertical direction and hence 3D simulations are necessary to obtain more accurate results which can be better compared with experimental data. In this paper 2D and 3D FDTD calculation results obtained for a finite height hexagonal silicon photonic crystal slab waveguide (air as top and bottom cladding) with air holes embedded in a silicon dielectric matrix are presented. Various coupling design configurations were investigated using 3D FDTD and coupling efficiencies of 78% in the photonic crystal waveguide and 72% through the output conventional waveguide were obtained for a conventional waveguide of width 3μm coupled to a step tapered PC waveguide on the input and output ports. Furthermore some designs which show excellent efficiencies with 2D calculations are clearly shown to have significant losses in the vertical direction in 3D simulations.
A relatively simple technique for fabrication of GaAs-based quasi-3D photonic crystals has been investigated. Selective impurity-induced layer disordering and wet oxidation techniques are utilized. The feasibility of this technique is successfully demonstrated and a photonic bandgap material with its bandgap around 1.18 micrometers has been fabricated. The electro-optic coefficients have been measured for the first time in such a medium. The process is reproducible an lends itself to integration with other optoelectronic and electronic devices on the same substrate, which might be required for pumping, electrical injection or other functions.