Chalcogenide Photonic Crystal All-Optical Switch
Latest Research News, September 07
We have recently demonstrated double-heterostructure-based microcavities formed within photonic crystal slab (PCS) waveguides after their fabrication. Instead of exploiting a change of the PCS period, the double-heterostructures are created by selectively filling a restricted region of the PCS (see fig 1) using a micropipette approach. This approach offers a flexible way to write microcavities by choosing (i) the index of the infused liquid and (ii) the length and pattern of the infiltrated PCS area. In addition, the reversible nature of these double-heterostructures enabled by fluid mobility offers a “rewrite” potential, paving the way for reconfigurable microphotonic devices and novel sensing architectures.
The fluid-filled double-heterostructures presented here has a fluid-filled section of 9.8 μm length, as shown in figure 2 (a) and is characterized via evanescent probing. Figure 2 (b) shows the experimental transmission through the tapered fiber as it is evanescently coupled to the W1, before and after the liquid infiltration. For both measurements, the tapered optical fiber is in contact with the structure, over a length estimated to 15 μm. Without the liquid, a single dip is obtained at 1570 nm, which corresponds to the coupling into the fundamental mode of the W1. After the liquid infiltration, the spectrum displays additional features at slightly longer wavelengths to the reference fundamental. We attribute these dips to Fabry-Perot (FP) resonances as the fundamental mode of the liquid filled W1 undergoes multiple reflections at the discontinuities between the different PCS sections which act as effective mirrors in the mode-gap window. This work has been accepted for publication in APL
Figure 1. Schematic of a fluid-filled heterostructure microcavity with lattice period a and holes radius R.
Figure 2. (a) Image of W1 PCS structure with a 9.8 μm fluid-filled section, and; (b) trace of i) waveguide before fluid-filling and ii) after fluid-filling with observable FP peaks.
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| Project Manager: Christian Grillet | Science Leader: Barry Luther-Davies |
Contributing staff: Snjezana Tomljenovic-Hanic, Eric C Magi, Benjamin J. Eggleton (Sydney), Steve Madden, Andrei Rode (ANU), Lindsay Botten, Ara Asatryan (UTS)
Students: Darren Freeman (ANU), Michael Lee, Cameron Smith (Sydney), Michael Byrne (UTS)
Project Goals and Motivation
The development of optical devices with similar functionality to that which the transistor provides in electronics is a “holy grail” in photonics. A “photonic transistor” would allow control of high-speed optical signals by light. This would simplify and lower the cost of future optical communications networks. The challenge is to make devices that operate at low optical powers and at speeds of several tens of GHz. In this project we aim to demonstrate all-optical switching in chalcogenide photonic crystals and explore the properties of these materials for fast all-optical processing. We investigate switching due to optical bistability in a high-Q resonator fabricated in two-dimensional photonic crystals of nonlinear chalcogenide glass. If the volume of the resonator is small and the glass nonlinearity is high, the power needed to observe bistability can be very low.
We use chalcogenide because of its ultra-fast nonlinear optical response. The nonlinear optical properties of silicon are based on thermal effects or the generation of free carriers, both relatively slow effects, while those in chalcogenide are based on the Kerr effect, whose response time is far more rapid. Chalcogenide glasses have sufficiently high refractive indices (between 2 and 3) to be useful for photonic crystal fabrication.
CUDOS strategy
We use nonlinear chalcogenide glasses that have a high index of refraction (allowing light to be trapped in small waveguides or resonators), large ultra-fast third order nonlinearity and low linear and nonlinear optical losses. We use these materials to fabricate 2-D photonic crystals in which light is tightly confined in a high-Q optical resonator to achieve a nonlinear optical response. This will lead to switching at speeds limited only by the Q of the resonator, at exceptionally low power and without interference from thermal or free-carrier induced effects.
The research skills and facilities in CUDOS provide a world-leading platform to carry out this project. UTS and Sydney in collaboration with Dr Mike Steel (RSoft) have a strong device design and modeling capability. At ANU we now produce unique chalcogenide-based planar photonic devices using proprietary deposition, lithography and ion beam etching capabilities. At Sydney we have a novel evanescent coupling process for getting light in and out of these microphotonic devices and a set of characterization capabilities (micro-alignment rigs, high power lasers, and sophisticated optical measurement and data acquisition systems) to measure their optical performance.
Recent Achievements and Highlights
Sufficient light must be coupled into the cavity to induce bistable behavior. We achieved sufficient evanescent coupling from a novel low-loss fiber taber (Figure 1) into L3 photonic crystal nanocavity resonators (so-called because three holes are removed) with different end-hole shift and diameters. Q values predicted from 3D FDTD simulations are greater than 10,000 for the optimal geometry.
Figure 1. Coupling scheme used: schematic showing the coupling from a tapered fibre to PhC nanocavity.
Structures have been manufactured using either FIB milling [1-3] or e-beam lithography plus chemically assisted ion beam etching [4]. Figure 2 shows results of experimental measurements performed on a cavity with both a side-hole shift and diameter reduction. A Q value as high as 10,000 was measured for a separation of the fibre from the resonator of 800 nm. As this separation decreases and the loading of the cavity increases, the measured Q factor also decreases (as expected) down to 2000 and the depth of the transmission dip increases up to 1.5 dB.
Figure 2. Transmission spectra through the tapered fibre for coupling to a modified L3-type nanocavity as a function of fibre to PhC separation.
These data indicate that simple chalcogenide PC resonators can exhibit sufficiently high Q to make all-optical switching feasible, even though the transmission depth is in this case restricted to a few dB, which limits the contrast ratio of the expected switching device.
We investigated ways in which a high-Q nanocavity in a photonic crystal slab (PCS) could be formed. There are two usual approaches: either as a point cavity or by forming a “heterostructure”. We considered both; our results indicate that nanocavities with the highest Q values that may be realised in a chalcogenide-based PCS are photonic crystal double-heterostructures, where regions of slightly different lattice constant are combined in a single slab to create a cavity. We designed cavities of this type with Q=7×105, comparable to the best results reported to date in silicon-based PCS [6].
One of the highlights of 2006 was the development and demonstration of a novel concept for creating high-Q cavities in PCS of photosensitive material. Spatially selective post exposure to light in a photosensitive uniform photonic crystal slab alters the refractive index permanently and was shown to yield high-Q nanocavities [7]. These high-Q cavities (up to Q=1×106) can be achieved with photo-induced index changes that are consistent with those seen in chalcogenide glasses.
We successfully demonstrated this photosensitive post tuning of a planar photonic crystal device [5]. We used the photosensitivity of AMTIR-1 chalcogenide glass to modify the optical properties of a photonic crystal waveguide (Figure 3a). A W1 PC waveguide was exposed to 633 nm light at an intensity of 1.3 W/cm2. The resulting change in the dispersion of the modes of the waveguide was detected using an evanescent probing technique, which yielded a shift of 5 nm in the wavelength for resonant coupling (Figure 3b).
Figure 3. a) Schematic showing the principle of operation of the photosensitive post tuning of a Chalcogenide photonic crystal waveguide. b) Shift in coupling wavelength versus exposure fluence at 633nm of the PhC waveguide.
Modelling: We continued to study Fano resonances in PCS [8]. We improved the robustness of the algorithms in a sophisticated new tool based on Bloch mode and multipole techniques which we use for modelling and characterising Fano resonances in photonic crystal slabs. As a result, the capabilities of the method were substantially extended. The method now accommodates both square and hexagonal lattices as well as multiple layer structures (e.g., composite chalcogenide-nitride layers) in both normal and off-normal incidence configurations. This latter extension required the development and implementation of generalised Fresnel matrices that characterise the reflection and transmission of Bloch modes on either side of the interface of two inhomogeneous PC media.
Fabrication: 2D Photonic Crystals were produced by focused ion beam machining (Figure 4) and, in collaboration with KAIST, electron beam lithography (Figure 5).
Figure 4. SEM micrograph of a) a photonic crystal L3 cavity b) a photonic crystal waveguide fabricated by FIB milling.
Figure 5. SEM micrograph of a photonic crystal L3 cavity with shifted end holes fabricated using ebeam lithography at KAIST, Korea.
References
[1] Grillet C, Smith C, Freeman D, Madden S, Luther-Davies B, Magi E, Moss D, Eggleton B Efficient coupling to chalcogenide glass photonic crystal waveguides via silica optical fiber nanowires OPTICS EXPRESS 14, 1070-1078 2006
[2] Freeman D, Madden S, Luther-Davies B Fabrication of planar photonic crystals in a chalcogenide glass using a focused ion beam OPTICS EXPRESS 13, 3079-3086 2005
[3] Grillet C, Freeman D, Luther-Davies B, Madden S, McPhedran RC, Moss DJ, Steel MJ, Eggleton BJ Characterization and modeling of Fano resonances in chalcogenide photonic crystal membranes OPTICS EXPRESS 14, 369-376 2006
[4] Ruan Y, Kim MK, Lee YH, Luther-Davies B, Rode A Fabrication of high-Q chalcogenide photonic crystal resonators by e-beam lithography APPLIED PHYSICS LETTERS 90, 071102 2007
[5] Lee MW, Grillet C, Smith CLC, Moss DJ, Eggleton BJ, Freeman D, Luther-Davies B, Madden S, Rode A, Ruan Y, Lee Y-h Photosensitive post tuning of chalcogenide photonic crystal waveguides OPTICS EXPRESS 15, 1277-1285 2007
[6] Kuramochi, E. , et al. Ultrahigh-Q photonic crystal nanocavities realized by the local width modulation of a line defect APPLIED PHYSICS LETTERS 88, 041112 (2006)
[7] Tomljenovic-Hanic S, Steel MJ, de Sterke CM, Moss DJ High-Q cavities in photosensitive photonic crystals OPTICS LETTERS 32, 542-544 2007
