PhRC NEWSLETTER PHOTONICS'La - Nanyang Technological ...

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Photonic Materials and Devices Research A/P Chin Mee Koy The photonic materials and devices (PM&D) is a broad and diverse program involving about 12 academic staff and 16 research fellows. The material-based research can be classified in three broad categories: (i) compound semiconductors, (ii) glass, solgel and other oxides, and (iii) Polymers and liquid crystals. As usual, no single material is perfect. The best scenario may be to combine the advantages of each material to form hybrid integrated modules and systems, where different materials perform useful functions for which they are eminently suited. The ability to achieve this hinges on the availability of a wide array of materials with novel properties. Materials research provides the foundation for technology development that will lead to important applications. III-V Semiconductor Photonics The largest material-based group is in the compound semiconductors area, especially the III-V group, which has about 7 academic staff and 8 research fellows. Besides a slew of equipment for processing, the group also operates a Metal-Organic Chemical Vapor Deposition (MOCVD) system for the epitaxy of both GaAs and InP-based materials. The MOCVD system (courtesy of Dr Tang Xiaohong) The primary advantage of III-V semiconductors is that they can be used to make active optical devices such as laser diodes with the right wavelengths for optical communication applications. Members of the group are pur- 14 hotonics'a suing high-power lasers, tunable lasers, modulators, and photodetectors. In fact, so many devices can be made from the same material that monolithic integration of all the devices on the same substrate is in principle achievable. Multi-wavelength laser arrays, for example, has been achieved by using a technique called quantum-well intermixing (QWI). Our next goal is to develop photonic integrated circuits (PIC) that combine a broad range of active devices with passive devices in the same chip. Sol-Gel and Silica Photonics Sol-gel is a colloidal suspension of gelled silica particles. It can be chemically purified and consolidated at high temperatures into high purity silica. The silica can then be modified with a variety of dopants to produce new glass properties unattainable by conventional means. One such novel derivative is the photosensitive hybrid sol-gel glass, which has excellent optical properties and usually involves a single-step low temperature fabrication process. This material has provided a low-cost way to fabricate diffractive optical elements, such as microlens arrays that may be used to facilitate coupling between a waveguide array and a fibre array. We have worked on both passive and active optical waveguide devices over a number of years. Silica based materials have been fabricated by the sol-gel technique. This technique enables various oxide dopants such as GeO2, TiO2, B2O3, Al2O3, etc. to be easily incorporated into the SiO2 host to obtain the desired optical properties of the materials (e.g. refractive index). Recently, the SiO2- GeO2 system has been developed for various waveguide based devices. The refractive index can be tailored easily by varying the GeO2 dopant concentration. Low optical absorption has been achieved and the technique to obtain highly photosensitive films has also been developed. High refractive index change upon UV exposure has been obtained.
Polymer Photonics Polymeric materials are broad and specific properties can be engineered for specific applications, including a wide bandwidth of transparency, high electrooptic (EO) coefficients, or high thermo-optic (TO) coefficients, making them suitable for very fast (picosecond) switches as well as low-power, millisecond switches and variable attenuators. Polymeric passive and active devices can be easily integrated on any surface of interest, and passive waveguides can be fabricated easily and at low cost with excellent control of refractive index and geometry. This makes polymer-based channel waveguides ideal for board-level chip-to-chip optical interconnects, and as interlayer dielectrics and protective overcoats. Research on polymer and polymer photonics has been Photonic Integrated Circuits Research A/P Chin Mee Koy Integrated optics, or photonic integrated circuits (PIC), began in Bell Labs over 40 years ago, but has only entered the industry lexicon in the last 10 years. This is because of its emerging importance as the surest way to reduce components cost, which forms a major portion of systems cost. Integration reduces the number of parts and hence the level of packaging and assembly required. It also results in parts with smaller footprints that save valuable space in central offices. Integrated components tend to be more reliable and power efficient and hence cut down operating cost. Because of these advantages, photonic integration is considered a key to opening the floodgate of low-cost optical subsystems that will make longterm sustainable investment in optical networks possible. Just as microelectronic integration has brought about the relentless reduction in the price of IC’s, photonic integration, likewise, can be expected to bring about similar benefits, and ultimately give the whole industry a path to long-term growth. The future networks are also migrating toward alloptical in order to eliminate the optical-electronic-optical (O-E-O) bottleneck. Based on this evolution, the next technology requirements are expected to be dominated by optical switching and routing functionality, with subsystems such as optical cross-connects (OXCs) and optical add/drop multiplexers (OADMs) as the lynchpin products. Many competing switching technologies are being developed, such as micro-electromechanical systems (MEMS), arrayed-waveguide gratings (AWG), liq- carried out jointly in the School of Materials Engineering and the School of Electrical and Electronic Engineering for a number of years. A niche area is highlighted below. Conjugated polymers are organic macromolecules with alternating single and double bonds along the molecular chains. The overlap of the pz orbitals gives rise to two molecular orbitals (HOMO, LUMO) that resemble the conduction and valence bands in semiconductors. So just like semiconductor materials, they can conduct electricity and in some cases, emit or absorb light. This makes them an interesting (and indeed fascinating) class of electronic materials to study. Conjugated polymers are strong candidates for flat panel displays. We have demonstrated organic LEDs (OLED) using both conjugated polymers and molecular complexes. uid crystals, fibre-based Mach-Zehnder, thin-films, and fibre Bragg gratings. However, all of these switches tend to be bulky and space-hungry and require complicated assembly and fibre management. Next-generation product development will need to drive down both cost and space. These requirements provide the motif for integration and favors technologies that are amenable to highdensity integration. Microphotonic integration is more challenging, and hence much less mature, than microelectronic integration. Partly this is due to the large diversity of photonic devices that need to be integrated. Unlike transistors which form the bedrock building block for all of electronics, no photonic equivalent of transistors exists. Figure 1: A popular commercial laser-modulator product is an assembly of 10 or more components aligned in free space with tedious precision. 70% or more of the cost goes into assembly and packaging. Most of this cost could be eliminated if the laser and modulator were integrated in a single chip. Given the strategic significance of photonic integration, we have formed a PIC research group to spearhead research in this area. This will be the first such research PHOTONIC MATERIALS & DEVICES RESEARCH September 2003 15
Page 1 and 2: hotonics'a A Publication of the Pho
Page 3 and 4: My warmest greetings to all the stu
Page 5 and 6: Research Funding: Total cumulative
Page 7 and 8: Biophotonics Research Ast/P Ng Beng
Page 9 and 10: the laser active Q-switching techni
Page 11 and 12: Electrically tunable dispersion com
Page 13: Thermal characterization of GaAsN e
Page 17 and 18: (b) Figure 1: (a) the selective are
Page 19 and 20: Low k material Porous dielectric ma
Page 21 and 22: Liquid Crystal Devices Ast/P Sun Xi
Page 23 and 24: Now let’s take a look at the phys
Page 25 and 26: FEATURE ARTICLE Optical Tweezers Le
Page 27 and 28: References [1] Ashkin, A., Dziedzic
Page 29 and 30: S tudent ACTIVITIES Making presenta
Page 31 and 32: Congratulations to Hery Djie who de
Page 33 and 34: Announcements New Faces The Centre
Page 35 and 36: ZUGO PHOTONICS PTE LTD www.zugo.com

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