Welcome to CIBA

Optical Materials and Devices


Introduction

The development of Photonic Integrated Circuits (PICs) is motivated by the realization that semiconductor technology and electronics is rapidly reaching its limits in terms of speed and the level of integration. At some stage in the not too distant future, Moore’s law will no longer apply and it is clear that a fundamental shift in technology is required in order to keep pace with the ever increasing demand for smaller and more powerful computers and faster communications. One of the possible solutions to many of the issues faced by the semiconductor industry is the use of photonics instead of electronics for integrated optical circuits. The current level of integration for photonic components is nowhere near the level that is possible with electronic components so in recent years, much effort has gone into developing the technology for miniaturizing many of the basic components required for making PICs.


Polymer microlasers

In this work, the fabrication of high quality SU-8 optical microdisk resonators with a Q factor of 10^4 was demonstrated. Simulations carried out using finite difference time domain (FDTD) are in good agreement with experimental results. By introducing rhodamine-B laser dye into a SU-8 resist spiral microdisk cavity lasing was achieved with threshold pump fluence of 127 μJ/mm^2 when pumped with a 532 nm pulsed Nd:YAG laser.



Vanga Sudheer Kumar, Shuvan Prashant Turaga, Ee Jin Teo, Andrew A. Bettiol, Fabrication of optical microresonators using proton beam writing, Microelectronic Engineering 102 (2013) 33-35


Near cut-off photonic guiding for low loss bends

In this work, we investigate the use of metal-assisted photonic guiding in a polymer-metal waveguide as an alternative approach for high density photonic integration at visible wavelengths. We demonstrate high confinement and long propagation length in sub-wavelength dimensions down to 300 nm x 200 nm using leakage radiation microscopy at a wavelength of 632.8 nm. Simulations using the finite element method (FEM) show that the optimum dimension that gives good confinement and propagation length is similar to that of the predicted plasmonic mode supported in the same waveguide. Under such optimum conditions, the metal-assisted photonic mode shows a five times longer propagation length and higher transmission efficiency for all 90° bending radius down to 1 μm compared to the plasmonic mode.



C. Y. Yang, E. J. Teo, T. Goh, S. L. Teo, J. H. Teng, and A.A. Bettiol, Metal-assisted photonic mode for ultrasmall bending with long propagation length at visible wavelengths, Optics Express 20 (2012) 23898-23905

Introduction

Metamaterials are artificial materials that owe their electromagnetic properties to their physical structure rather than their chemical composition. They have attracted tremendous research interest in recent years due to their unique electromagnetic properties. Although metamaterials are themselves made of a non-magnetic material such as copper or gold, they can exhibit a band of negative magnetic permeability (μ) in a region close to their resonance frequency a property not found in naturally occurring materials. Coupled with the negative permittivity (ε) commonly found in metals, it is possible to make a double negative material that has a negative refractive index.The properties of these materials was first predicted by Veselago in 1968 and demonstrated experimentally by Shelby at radio frequencies in 2001 using a split ring resonator design proposed by Pendry a year earlier. The ability to fabricate materials that have a controllable permittivity and permeability opens up numerous applications such as electromagnetic invisibility cloaking,super-lensing and new optics at terahertz (THz) and mid-IR frequencies.

Typically the unit cell dimensions of a metamaterial needs to be on the order of =10 where λ is the wavelength of the incident electromagnetic radiation. This makes it technically challenging to fabricate such materials especially as wavelengths move towards the near infra-red and visible part ofthe spectrum. For this reason, many of the early experiments were performed at microwave and radio frequencies. The vast majority of metamaterials that resonate at near optical frequencies reported in the literature thus far have been fabricated using planar (2D fabrication) technologies such as electron beam lithography or photolithography. In an attempt to increase the interaction length between the impinging electromagnetic radiation and the metamaterial, research has recently moved towards extending fabrication technologies to the third dimension. This is straightforward for microwave frequencies, however it is a difficult challenge for higher frequencies (optical wavelengths).

Our research programme in metamaterials and THz optics has concentrated on applying new fabrication technologies such as proton beam writing and two-photon lithography to making novel three dimensional metamaterials for THz frequencies. Terahertz frequencies (0.1-100 THz) are particularly interesting because there are very few sources, detectors and optics at these frequencies. THz radiation weakly interacts with most materials (except metals) so it has been applied to many areas such as defence and security. The fabrication technologies that I use are well matched to the critical dimensions required for THz applications (1-100 microns).


Study of aspect ratio and substrate effects

Metamaterials owe their unique electromagnetic properties to resonant coupling between electric or magnetic fields from the incident radiation and the metallic structure. As a consequence the measured optical transmission or reflection spectrum can be extremely sensitive to the surrounding dielectric medium. A change in the surrounding medium, especially in regions where fields are concentrated can result in a measurable shift in the resonant peak position. The amount by which a peak can shift depends on various parameters including the design of the structure, the substrate and the height. Our investigations in this area show that high aspect ratio 3D metamaterials have a much larger peak shift and as a consequence, increased sensitivity when compared to thin structures.



S. Y. Chiam, Ranjan Singh, Jianqiang Gu, Jiaguang Han, Weli Zhang and A. A. Bettiol, Increased frequency shifts in high aspect ratio terahertz split ring resonators, Applied Physics Letters 94 (2009) 064102

A new analogue for EIT at THz frequencies

Electromagnetically Induced Transparency (EIT) is a quantum interference phenomenon that occurs in atomic systems. Our recent work in a metamaterial analogue to EIT has shown that we can fabricate structures that have a transparency window centred in a frequency band that is resonant. These types of structures can be used to reduce the loss in metamaterials allowing us to make optically thick structures. We have also measured a group index of about 75 in this transparency region which confirms that we have slow light behaviour.



Sher-Yi Chiam, Ranjan Singh, Carsten Rockstuhl,Weili Zhang, Falk Lederer and A.A. Bettiol, Analogue of electromagnetically induced transparency in a terahertz metamaterial, Physical Review B 80 (2009) 153103

Selective electroless coating of 3D structures

A new selective silver electroless coating method for making 3D metamaterials - We developed method for selective silver coating of SU-8 structures on Si substrates by treating the sample with radio frequency plasma prior to electroless plating. Silver films with high conductivity and low surface roughness of 9 nm have been obtained. When combined with two-photon lithography, this process can be utilized for three-dimensional metamaterials applications. Unlike previous work on selective coating, our process can coat directly on SU-8 photoresist that is widely used for two-photon lithography and does not require any resin modification.



Yuanjun Yan, M. Ibnur Rashad, Ee Jin Teo, Hendrix Tanoto, Jinghua Teng and Andrew A. Bettiol, Selective electroless silver plating of three dimensional SU-8 microstructures on silicon for metamaterials applications, Optical Materials Express 1 (2011) 1548-1554

introduction

In addition to the development of PICs, integrated optics and photonics has a major role to play in many different types of systems including Lab-on-a-chip (LOC) devices. Developing methods and tools for integrating optical components into various types of materials, ranging from polymers, glass, single crystals and semiconductors, will allow for added functionality to be introduced into devices made from these materials. In particular, polymers are an important material when it comes to LOC devices. Polymers are low cost, they can be easily patterned using various types of lithography, they can be doped with non-linear molecules or laser dyes, and they can be used for replication.

Cell sorting based on optical force

In this project, we developed a non-invasive, versatile and effective method for sorting cells or colloidal particles within a microfluidic system. To achieve optical sorting within a microfluidic system we fabricated two devices using soft lithography and masters that were fabricated using proton beam writing. The first device consisted of a three-dimensional channel system that utilized the optical scattering force imposed by a focused laser spot for sorting. The device operated under an inverted biological optical microscope using a laser that was focused onto the channel by an objective lens. The second device had integrated optics that enabled a laser to be introduced into the microfluidic system by an optical fiber and an integrated cylindrical lens. Under optimized conditions, both devices were able to achieve near 100 % sorting or switching efficiency.



Siew-Kit Hoi, Zhi-Bin Hu, Yuanjun Yan, Chorng-Haur Sow, and Andrew A. Bettiol, A microfluidic device with integrated optics for microparticle switching, Applied Physics Letters 97 (2010) 183501.

Integrated filters in Lab-on-a-chip devices using colloidal crystals

We have developed several microfluidic chip designs that combine colloidal crystal filters, microchannels for the flow of samples and most notably optical fibers for introducing an excitation light source and a detection probe. In contrast to conventional top-down probing methods, the optical properties of the colloidal crystalline structure were probed locally and directly using etched optical fibers that were positioned by structures patterned into the PDMS molds. Integrating colloidal crystals with different filtering effects into the microfluidic chip enable selectively transmitting or blocking of a particular range of wavelength locally. In addition, a different combination of double band colloidal crystal filters provide further tunability of the working wavelength for on chip detection applications.



Siew-Kit Hoi, Xiao Chen, Vanga S. Kumar, Sureerat Homhuan, Chorng-Haur Sow and Andrew A. Bettiol, A microfluidic chip with integrated colloidal crystal for online optical analysis, Advanced Functional Materials 21 (2011) 2847-2853