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Fabrication of optical waveguides for integrated optic and optofluidic device applications

ByPrathul Nath P P15PH62R06Under the supervision ofProf. Shivakiran Bhaktha B.N

Department of Physics,Indian Institute of Technology Kharagpur

OutlineAim.

Introduction .

Fabrication Techniques.

Experimental Techniques.

Co-doped Waveguide system.

Optical Trapping.

Conclusion and Discussion.

Reference.

AIM

The aim of this project is to fabricate a device based on Co-doped waveguide system which can emit light of different wavelengths and to use it as a source to achieve optical trapping or to study the interaction of light with particles which is flowing through a micro fluidic channel.

IntroductionSystem of light controlling components.

Aim is to create miniature optical circuits.

Data can be processed at much higher speeds.

Fig 1: Lab on a chip biosensing deviceCourtesy : http://onlinelibrary.wiley.com

Fabrication TechniquesSol Gel Synthesis: Involves formation of colloidal suspension (sol gel).

Dip Coating: A simple and effective coating technique.

Micro fluidic Channel Fabrication: PDMS based micro fluidic channel fabrication.

1.Sol Gel SynthesisWet-chemical synthesis technique for preparation of oxide gels, glasses, and ceramics at low temperature.

Involves mixing of different precursors in a specific order and stirring under a specific temperature for a certain amount of time.

Then the prepared sol is filtered out using 0.2 m Whatman puradisc syringe filter and then taken for dip coating.

2. Dip CoatingThe dip-coating method consists of soaking the substrate (Silicon wafer) in the solution and withdrawing it at a constant speed.

Compared to spin coating, the film is more uniform and smooth.Around 20-25 layers are coated for a standard sample making it time consuming.After each coating sample is kept inside a heater at 900 degree for annealing.

Fig 2: Dip coating stagesCourtesy: http://www.solgel.com

3. Micro Fluidic Channel FabricationPDMS has become virtually the default material for forming micro- fluidic devices. It comes with a master and cross linker with 10:1.

We made use of simple technique to make micro channel design which is by using an optical fiber.

Optical fiber is glued on both sides inside a petri dish and PDMS is poured over it and then solidified to get required channel design.

Fig 3: Microscopic image of micro fluidic channel.

Experimental Techniques Prism CouplingPrism coupling couples a substantial fraction of the power contained in a beam of light like laser into a thin film based on evanescent wave formation.

Evanescent waves are formed between the face of high refractive index prism and thin film waveguide which is put closed to each other and it is coupled onto the waveguide.

Used to find the refractive index, thickness and propagation loss of waveguides.Fig 4: Prism coupling on Si-Hf waveguide (refractive index =1.565, thickness= 1m and propagation loss = 0.5 dB/cm)

Butt Coupling and PL SpectraButt coupling : Coupling light from laser passing through an objective lens directly to the waveguide.

Plays important role in the optical trapping experiment.

PL Spectra : light emission from any form of matter after the absorption ofphotons initiated by photon excitation.

Important technique for analyzing the optical properties of Co-doped waveguides.Fig 5: Light coupled to the micro channel waveguide by butt coupling

Co-doped WaveguidesPrepared hybrid Nanocrystal embedded glass ceramic waveguides (SiO2(70%) - HfO2(23%) - ZnO(7%)) and co-doped it with Europium and Terbium (1 mol%).

This was done by sol gel synthesis and dip coating after that.

ZnO is considered as a promising host for optoelectronic device applications because of large exciton binding energy and optical transparency in the visible and near IR.

We have done PL Spectra to determine the peak energy emissions and thereby we can actually observe the effects of RE dopants over emissions.

PL SpectraFig 9: PL Spectra for Eu-Tb doped on Si-Hf-ZnO ternary waveguides at 325nm excitation.

PL SpectraFig 10: PL Spectra for Eu-Tb doped on Si-Hf-ZnO ternary waveguides at 355nm excitation.

Future WorkEmissions of different wavelengths can be channeled out separately using Monochromator and can be used to study interaction of micro sized particles flowing through a micro fluidic channel with varying wavelength.

Possibility of making white light source from the co-doped system as wavelengths from red, green and blue regions are available if proper mixing of RGB can be done. This is extremely challenging to achieve.

Optical TrappingTrapping of particles by making use of light and to study the interaction.

The forces acting on a particle in an evanescent field are the gradient force and the scattering force.

The evanescent field is decaying exponentially from the waveguide surface, and the transverse decaying part of the field is the source of the gradient force.Fig 6: Showing force acting on a particle by lightCourtesy: Oystien Ivar Helle, Arctic University of Norway

Experimental SetupWe used polystyrene beads (size around 1 m) and Si-Hf micro channel waveguides for guiding light.

The polystyrene particles in DI water is put on the focused spot of microscope in channel waveguide using a syringe.

A small piece of cover slip is placed on the drop which allows the particles to flow for certain time on the surface of waveguide and we observe the same on the computer.

Fig 7: Experimental setup for optical Trapping

Observations and Future workFig 8: Microscopic images (5X zoom) of channels (Light coupled) with illumination and without illumination under particle flow.

It was observed that concentration of particles are large enough to determine if there are any influence on the particle due to light and also uniform flow of particles over micro channels not achieved.

One effective way is to bond a micro fluidic channel perpendicular to the direction of propagation of light through micro channels (newly fabricated long Si-Hf waveguide micro channel )and to pass particle solution through the fluidic channel to observe the change in behavior of particle due to light.

Conclusion and DiscussionWork in this project itself defines the importance and wide applications of the field Integrated optics.

Co-doped waveguides proves itself extremely useful in the field of Integrated optics as it can be the key to fabricate on chip light sources and lasers. It is also under study for making white light sources though challenging.

To achieve Optical Trapping is extremely challenging but its applications are plenty and can bring lots of effective advancements in the field of science and medical technology.

ACKNOWLEDGEMENT My sincere thanks to my Supervisor Dr. Shivakiran Bhaktha for his constant and timely guidance. I also thank Prof A.Dhar and all faculties for the motivation. I thank all my lab mates for their valuable suggestions and guidance.

References An Introduction to Integrated Optics,Herwig Kogelnik. Scriven, L.E. (1988). "Physics and applications of dip coating and spin coating". Better ceramics through chemistry III. pp. 717729. Fabrication of microfluidic devices using PDMS, James Frienda and Leslie Yeo.Theory of Prism-Film Coupler and Thin-Film Light Guides,P. K. TIEN AND R. ULRICH. Trapping of Nanoparticles with Optical Waveguides,Firehun Tsige Dullo.Optical trapping on waveguides, Olav Gaute Helles, University of Troms Norway.Optofluidic trapping and transport on solid core waveguides within a microfluidic device, Bradley S. Schmidt1, Allen H. J. Yang2, David Erickson3, and Michal Lipson1.Eu-doped ZnO-HfO2 hybrid nanocrystals embedded glassceramic waveguides as blue-light emitting source,Subhabrata Ghosh and Shivakiran Bhaktha B.N.Eu-doped ZnOHfO2 hybrid nanocrystalembedded low-loss glass-ceramicwaveguides, Subhabrata Ghosh and Shivakiran Bhaktha B N. Righini G C, Brenci M, Forastiere M A, Pelli S, Ricci G,Conti G N, Peyghambarian N, Ferrari M and Montagna M. Peled A, Chiasera A, Nathan M, Ferrari M and Ruschin S 2008, Appl. Phys. Lett. 92 221104.

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