Opal and inverse opal structures for optical device applications
An All-Optical Silicon Transistor Utilizing Opal/Inverse Opal Based Heterostructure as a Photonic Crystal Mohammad Faisal Halim (Faissal) Department of Electrical Engineering, The City College of New York, City University of New York, 140th ST. and Convent Ave., New York, NY 10031 Abstract:For some time photonic crystals have been used as photonic nanocavities for confininglight, both for guiding it, and for using it as an optical switch. Thus far, most of this kindof work has been carried out using materials and structures with dimensions that lieabove the regime of nanomaterials (200-420nm). More recently, however, inverse opalshave been reported to have been used as photonic crystals, and also, inverse opals havebeen reported to have been fabricated in dimensions in the nano regime. This, combinedwith the interest in silicon based inverse opals opens up the possibility of rapidlyimplementing all optical, ultrafast (nanosecond and picosecond time intervals), switchingtechnology into future computer chips, since the technology for fabricating silicondevices is so mature, and silicon based photonic structures can be readily integrated intosilicon chips without expensive bonding technologies. This paper will investigatephotonic bandgap materials and photonic nanocavities in the light of silicon and siliconbased materials in inverse opals and opals research, and the photonic band structuresdesirable for an all-optical switching device (in this case, all optical transistor) and willsuggest approaches that could be taken for the realization of such a device. This devicewill use low dimensional (nano structure regime, so as to take advantage of quantumeffects) opals or inverse opals for photonic bandgap materials in which switching will beperformed by varying the applied electric field thus changing the refractive index of thematerial. The advantage of using nanostructures is that the resulting quantumconfinement of light and electrons is known, from other studies, to increase the powerefficiencies of devices, having low thresholds and for having very low powerrequirements.Keywords: PC, PhC, PBG, nanocavity, inverse opals, opals, silicon photonics
BackgroundSilicon has dominated the computing landscape for a long time, and silicon devices,interconnected by copper interconnects, are the staple of most of the microprocessorindustry. In order to extend Moore’s Law (Figure 1), however, this technology (and itsparadigm) will need to be abandoned, as silicon devices are facing more and moreproblems with power and reliability as the devices are being made smaller. Also, thecopper interconnects in the chips are fast approaching their information carrying limits. Figure 1: Moore’s LawThis situation has motivated scientists and engineers to look at other carriers ofinformation, and other methods of performing digital logic operations. One promisingsolution is the use of light (since its speed in a dielectric is much faster than the speed ofelectrons in a copper wire) as the replacement for electricity (currents and voltages),which would mandate the use of optical switching devices for logic operations.
IntroductionPhotonic crystals (PCs, or PhCs) have been investigated for quite some time, for use inall optical circuits. Photonic crystals can act, both, as more efficient waveguides (usingdefect modes), as well as digital logic devices. Photonic crystals, also called photonicbandgap materials (PBGs), are macroscopic (their minimum size depends on thewavelength ranges that the device made from the crystal will be used for) structures thateither possess transmission bands (i.e., they only allow certain ranges of EM wavelengthsthrough), or stop bands (i.e., they block certain ranges of wavelengths – corresponding toonly certain ranges of energies of incoming photons – from getting through the device).Most devices encountered in academia and research laboratories that are run bycorporations focus on band pass (also called band stop) materials/crystals, and so thispaper will focus on band stop materials (see Figure 2). The bandgap has resulted in PCsbeing described as optical semiconductors, since semiconductors have bandgaps. Figure 2: Source: MODELING OF PHOTONIC BAND GAP STRUCTURES AND PROPOSED SYNTHESIS SCHEMES, By Srivatsan Balasubramanian, RPI, 2002
Clearly, from Figure 2, photonic crystals need to be periodic arrays of dielectricstructures. They need to be periodic, with the periodicity chosen to block the chosenrange of wavelengths, and they need to be dielectric structures because dielectrics allowthe transmission of light. While photonic crystals with microscopic (and evenmacroscopic) elements (i.e., the dimensions of the periodic structures are large, ratherthan in the nanoscale) can be created, their application will be for wavelength of thatscale. In other words, PCs block wavelengths comparable to the dimensions of theirperiodicities. So, such devices will be large and bulky, and impractical as a replacementfor silicon components that are already in the nanoscale. If PC devices are made for usingoptical wavelengths, however, then the dimensions of the entire crystal that is being usedas a device can be shrunk to very small dimensions, and the features of the crystals,themselves, will be in the nanoscale. Although a lot of work has been done towards therealization of PCs, there are some major challenges in the area: 1. The easiest kind of photonic crystal that can be made is a one dimensional crystal, and so it only has a photonic bandgap in one direction. It is physically (structurally) identical to the simple conceptual picture of PCs shown in Figure 2. It will be easy to fabricate these structures in the manner in which the electronics industry manufactures diode lasers, however, that would still entail layer by layer deposition, which is not as versatile as self assembly. 2. With two dimensional PCs the growth process may not be as cumbersome as for the 1D PC. Also, the PC may be fabricated separately from the rest of the circuit, and then just placed into position, for use. Such PCs have been receiving a lot of attention in theoretical and experimental research. For example, the work “Two-
Dimensional Optical Interconnection Based on Two-Layered Optical Printed Circuit Board,” IEEE Photon. Technol. Lett., vol. 19, no. 6, pp 411-413, Mar. 15, 2007, done by Hwang, Cho, Kang, Lee, Park, and Rho utilizes this kind of structure. 3. 3D PCs allow the greatest flexibility in use. They have the further advantage that nanoscale 3D PC structures can be fabricated by self assembly, thus easing, and speeding, mass production. Further, calculations and experiments with opals and inverted opals, with infiltrated electro-optic materials, have yielded promising results that could be applied towards devices. 4. A further challenge is the migration to all optical, or at least party optical, technologies. This challenge is two pronged: a. Bringing industrial process technologies up to speed with PC manufacturing. It will be more cost effective for the industry to make PC from silicon based materials, since silicon process technologies are very mature, and that will reduce the time that it takes PCs to make itto the market. b. Integration with the silicon on the chips. This problem can be solved, at least in part, by doing more research work in creating PCs using silicon, and related materials. If the PCs are made of the same of the same material as the underlying substrate then there is no need to invest in expensive, and cumbersome, wafer bonding technologies.In conclusion, the industry needs silicon based 3D PCs, in order to realize the goal of alloptical computing quickly, and with the least number of technological challenges.
Opals and Inverted OpalsFigure 3 shows an SEM image of an inverted opal PC and is photonic bandgap (theshaded region).Figure 3: Source: Source: MODELING OF PHOTONIC BAND GAP STRUCTURES AND PROPOSED SYNTHESIS SCHEMES, By Srivatsan Balasubramanian, RPI, 2002So, if light is shone on the PC with a frequency within the shaded region then that lightwill get blocked by the PC. In other words, light within the range of shaded wave-vectorsshown will be the reflected light, as shown in Figure 4. Figure 4: Source: Inverse Opal Photonic Crystals – A Laboratory Guide, Schroden and Balakrishnan
Theory of Making Opals and Inverse OpalsOpalsOpals are made by depositing beads of the chosen opal material into a cavity, where theyform a regular structure, often an FCC (face centered cubic) [PHYSICAL REVIEW B72, 205109 2005]. The spaces between the spheres/beads can then be infiltrated by apolymer in solution, that can subsequently get the structure to set. Also, the infiltratedmaterial can have electro-optic properties, in which case the system can be used inelectro-optic applications, like digital logic operations.Inverse OpalsOnce the structure in an opal system is set/fixed, then the beads themselves can bedissolved away in solution, leaving air spaces. The resulting system can be a PC withentirely different properties. Also, if these new air spaces are infiltrated with electro-opticmaterials, like liquid crystals, then the optical properties of the PC can actually be tuned.Examples of this effect can be seen in published papers like: IEEE Transactions onDielectrics and Electrical Insulation Vol. 13, No. 3; June 2006.Figure 5a shows (in green) the region that would be a occupied by beads in an opal. Therest of Figure 5 shows the space left (in the interstices) when the bead is dissolved away,leaving behind a structure that is an inverse opal.
Figure 5: Source: D. GAILLOT, T. YAMASHITA, AND C. J. SUMMERS PHYSICAL REVIEW B 72, 205109 2005) Figure 6: The effects of Voltage on infiltrated PCs made of Opals and Inverse OpalsFigure 6 shows the electro-optic effects of infiltrated inverse opals, as published in: IEEETransactions on Dielectrics and Electrical Insulation Vol. 13, No. 3, June 2006.
Current Work at the NanoscaleMost of the current work on opals and inverse opals have been done on materials otherthan silicon related materials. This could present a significant integration challenge forsilicon technologies. Current work at the nanoscale includes work on Tungsten Nitride,among other materials, as shown in Figure 7, from the paper Nano Lett., Vol. 3, No. 9,2003. The system is at the nanoscale because the interstitial walls are less than 100nm inthickness. Figure 7.Most current work on silicon based materials are still at a scale higher than nanoscale, butprogress is being made towards the nanoscale so that visible light may be utilized forinformation processing, rather than infra red (IR). Such work can be seen in papers likeOPTICS EXPRESS 2678 / Vol. 13, No. 7 / 4 April 2005.
ConclusionAlthough most silicon related work at the nanoscale is being done for IR wavelengths,progress is being made towards going into the nanoscale, for utilizing visible light insilicon based PCs, as seen in papers like: IEEE Transactions on Dielectrics and Electrical Insulation Vol. 13, No. 3; June 2006 APPLIED PHYSICS LETTERS 87, 151112 (2005) Figure 8: Source: IEEE Transactions on Dielectrics and Electrical Insulation Vol. 13, No. 3; June 2006
AppendixPHYSICAL REVIEW B 72, 205109 2005Inverse Opal Photonic Crystals – ALaboratory Guide, Schroden andBalakrishnan MODELING OF PHOTONIC BAND GAP STRUCTURES AND PROPOSED SYNTHESIS SCHEMES, By Srivatsan Balasubramanian, RPI, 2002“Two-Dimensional Optical InterconnectionBased on Two-Layered Optical PrintedCircuit Board,” IEEE Photon. Technol.Lett., vol. 19, no. 6, pp 411-413, Mar. 15,2007, done by Hwang, Cho, Kang, Lee,Park, and RhoIntel CorporationIEEE Transactions on Dielectrics andElectrical Insulation Vol. 13, No. 3; June2006D. GAILLOT, T. YAMASHITA, AND C. J.SUMMERS PHYSICAL REVIEW B 72,205109 2005)Nano Lett., Vol. 3, No. 9, 2003OPTICS EXPRESS 2678 / Vol. 13, No. 7 / 4April 2005