Photonic Materials

PHOTONIC MATERIALS Rakesh Kumar Sinha

Photonics

"Photonics" comes from "photon" which is the smallest unit of light just as an electron is the smallest unit of electricity. "Photonics is the generation, process and manipulation of photon to achieve a certain function.

Why Do We Need Photonics instead of Electronics?

An “All - Pervasive” Technology

1) Uninhibited light travels thousands of times faster than electrons in computer chips. Optical computers will compute thousand of times faster than any electronic computer can ever achieve due to the physical limitation differences between light and electricity.

Can pack more wavelengths (that is information channels) into a optical fibre so that the transmission bandwidth is increased than conventional copper wires.

Light encounters no electromagnetic interference than that of electron in copper wires.

Photonic Crystals Principles and Applications

SO WHAT ARE PHOTONIC CRYSTALS ?

Photonic crystals are a new type of materials displaying unusual and attractive properties in the interaction with light

Due to periodic modulation of the refractive index of a material, it is possible to create tailored dispersion relations and stop bands of light propagating through the material.

Photonic crystal with a complete band gap when index contrast is large enough

Features of a photonic crystal

• Made of low-loss periodic dielectric medium • Optical analog to the electrical semiconductors • Able to localize light in specified areas by preventing light from propagating in certain directions – optical bandgap.

In 2D photonic crystal structures it is possible to confine light within a cavity . Photonic band gaps appear in the plane of periodicity and in 2D we can achieve linear localization . By introducing a defect, i.e. removing one column, we may obtain a peak in the density of states localized in the photonic band gap – similar to semiconductors. The defect mode cannot penetrate the crystal in the xy-plane because of the band gap but extends in the z-direction

Photonic Crystals The Principle with photonic band gaps: “ optical insulators ” “ magical oven mitts” for holding and controlling light can trap light in cavities and waveguides (“wires”)

Photonic Crystals The Principle

Periodic arrangement of ions on a lattice gives rise to energy band structure in semiconductors ,which control motion of charge carriers through crystal.

Similarly ,in photonic crystals ,the periodic arrangement of refractive index variation ,controls how photons are able to move through crystal.

Analogy with semiconductors

Semiconductors : potential periodicity

Photonic crystals : dielectric periodicity

1D

Photonic band gap crystals -- a history

Idea of " photonic band gap structure " was first advanced in 1987 by Eli Yablonovitch , now a professor at the University of California at Los Angeles.

In 1990, he built the first photonic crystal , baseball-sized to channel microwaves useful in antennae applications

Photonic Band Gaps

Photons with a certain range of wavelengths do not have an energy state to occupy in a structure

These photons are forbidden in the structure and cannot propagate

Analogous to forbidden energy gaps in semiconductors

Carefully engineered line defects could act as waveguides connecting photonic devices in all-optical microchips, and infiltration of the photonic material with suitable liquid crystals might produce photonic bandgap structures (and hence light-flow patterns) fully tunable by an externally applied voltage

Properties of Photonic Crystals

Negative Refraction opposite of ordinary lens: only images close objects does not require curved lens can exceed classical diffraction limit

Properties of Photonic Materials: Why no Scattering?? forbidden by gap (except for finite- crystal tunneling ) forbidden by Bloch ( k conserved)

Properties of Photonic Crystals: Wide Angle Splitters

Applications Of Photonic Crystals

Perfect mirror :There are materials that reflect the frequency range of interest, with essentialy no loss at all. Such materials are widely available all the way from the ultraviolet regime to the microwave.

Nonlinear effects : Using non-linear properties of materials for construction of photonic crystal lattices open new possibilities for molding the flow of light. In this case the dielectric constant is additionally depend on intensity of incident electromagnetic radiation and any non-linear optics phenomena can appeared.

Photonic integrated circuits based on photonic crystals. The key driving force of this research is the miniaturisation and increase of the functionality of photonic circuits. The application of photonic crystal technology allows us to design waveguides, couplers, and routers on a much smaller scale than previously possible, thus allowing the design of high-density integrated circuits Applications of this work are in telecommunications, e.g. for devices that manage high-speed, high volume data (e.g. internet) traffic.

A part of Photonic integrated circuit

Novel Semiconductor lasers and light emitters : Efficient light sources (e.g. for lighting ,displays) and novel types of optical sensors ,e.g. for biomedical applications.

Imaging by a Flat Slab of Photonic crystal

Source 2. 25 cm from PC

Image 2. 75 cm

Subwavelength image

2D flat lens

F= 9.3 GHz

Wavelength=3.22 cm

Scale Intensity: -20 dB to -40 dB X-Y: 37.5 X 30 cm 2

Self organized Nano Photonic Crystal Negative refraction in Photonic Crystal 3D IMAGING IN REAL SPACE Semiconductor substrate

Flat lens n = -1 d d = u + v v u object image Normal lens: Resolution cannot be greater than  Flat lens : no limitation on the resolution Image resolution Normal lens

E H H E PIM PIM Turning light on its Head Positive Refraction PIM NIM E H E H Negative Refraction

Conventional Optical lens Photonic Crystal lens Advantages of Photonic Crystal lens Optical axis Limited aperture cannot NO Optical axis No limitation on aperture size Subwavelength imaging (evanescent wave amplification) PC : Scalability to sub-micron dimensions -> applications at optical frequencies

Photonic transistor

A transistor is a switch that is turned on and off by signals from other switches. They perform logic, store information and are the work horses of digital computing. Photonic transistors use light to perform the switching functions that are performed by electronic transistors in conventional computers.

Applications Of Photonic Crystals Replacing conventional optical fibres The Glass Ceiling : Limits of Silica Loss : amplifiers every 50–100km … cannot use “exotic” wavelengths like 10.6µm Nonlinearities : after ~100km, cause dispersion, crosstalk, power limits (limited by mode area ~ single-mode, bending loss) also cannot be made (very) large for compact nonlinear devices Radical modifications to dispersion, polarization effects …tunability is limited Long Distances High Bit-Rates Dense Wavelength Multiplexing (DWDM) Compact Devices

Future Applications

Highly efficient photonic crystal lasers

High resolution spectral filters

Photonic crystal diodes and transistors

High efficiency light bulbs

Optical computers

Telecommunication & computer networks

Photonic clothes and candy bars??

Fabrication Of Photonic Crystals An example of a two-dimensional photonic crystal. The distance between the 200 nm wide pillars is about 500 nm and the pillars are 1500 nm long.

Fabrication Of Photonic Crystals Waveguide bend in a two-dimensional array of rods. The waveguide bend is defined by removing a row rods.

Fabrication Of Photonic Crystals Microfabrication : By layer by layer lithography. Colloidal self-assembly.

Fabrication Of Photonic Crystals

The Biological Option

Layer-by-Layer Lithography • Fabrication of 2d patterns in Si or GaAs is very advanced (think: Pentium IV, 50 million transistors) So, make 3d structure one layer at a time

The Woodpile

Lithography

Basic sequence

The surface to be patterned is:

spin-coated with photoresist

the photoresist is dehydrated in an oven ( photo resist : light-sensitive organic polymer)

The photoresist is exposed to ultra violet light:

For a positive photoresist exposed areas become soluble and non exposed areas remain hard

The soluble photoresist is chemically removed (development).

The patterned photoresist will now serve as an etching mask for the SiO 2

The SiO2 is etched away leaving the substrate exposed:

the patterned resist is used as the etching mask

Ion Implantation :

the substrate is subjected to highly energized donor or acceptor atoms

The atoms impinge on the surface and travel below it

The patterned silicon SiO2 serves as an implantation mask

The doping is further driven into the bulk by a thermal cycle

Lithography (contd.) The lithographic sequence is repeated for each physical layer

2µm Lithography at its best  = 780nm resolution = 150nm 7µm (3 hours to make)

Lithography at its best 2µm ( 300nm diameter coils, suspended in ethanol, viscosity-damped )

Lithography at its best

Mass Production by Holographic Lithography absorptive material Four beams make 3d-periodic interference pattern (1.4µm) k -vector differences give reciprocal lattice vectors ( i.e. periodicity) beam polarizations + amplitudes (8 parameters) give unit cell

Holographic Lithography :The Results huge volumes , long-range periodic, fcc lattice…

Mass Production :Colloids microspheres (diameter < 1µm) silica (SiO 2 ) sediment by gravity into close-packed fcc lattice ! (evaporate)

Inverse Opals fcc solid spheres do not have a gap… … but fcc spherical holes in Si do have a gap [ figs courtesy D. Norris, UMN ] Infiltration sub-micron colloidal spheres Template (synthetic opal) 3D Remove Template “ Inverted Opal”

In Order To Form a More Perfect Crystal… meniscus silica 250nm

Capillary forces during drying cause assembly in the meniscus

Extremely flat, large-area opals of controllable thickness

Heat Source 80C 65C 1 micron silica spheres in ethanol evaporate solvent

A Better Opal

Inverse-Opal Photonic Crystal

Manufacturing Photonic Materials: The Biological Option

With the help of DNA very complex structures can be manufactured .

But even then present technology cannot produce crystals producing a more spectacular effect than in the butterfly’s wings.

The Mitoura Grynea butterfly Electron micrograph of a broken scale taken from mitoura grynea revealing a periodic array of holes responsible for the colour

Recent Developments (taking it a step ahead)

MIT researchers reported in the October 9,1998 issue of Science that they have proven through experiments their long-standing theory on a new way to manipulate light waves. Their photonic crystals do what no other waveguide has managed to do: guide light around a 90-degree turn without losing even an iota of efficiency.

The picture depicts waveguide bend exhibiting 100% transmission

MIT researchers reported in the November 27,1998 issue of Science that they have made an important new advance in an age-old device -- the mirror.

The new kind of mirror developed at MIT can reflect light from all angles and polarizations , just like metallic mirrors, but also can be as low-loss as dielectric mirrors.

The "perfect mirror" as quoted by the researchers, is trapping light for longer than ever before possible. This would open up a myriad of technological and research possibilities.

Jan. 10,2003 --MIT researchers have created a low-loss optical fiber that may lead to advances in medicine, manufacturing, sensor technology and telecommunications.

Photonic chips go 3D The dream of building computer chips that use light signals rather than electricity has entered the realm of serious research in recent years with the advent of photonic crystal , a material that blocks and channels light within extremely small spaces.

Research teams from the Massachusetts Institute of Technology and from Kyoto University have made devices that meet all three challenges.

This image shows the microscopic structure of a three-dimensional photonic crystal that is capable of channeling and emitting light in the visible and telecommunications ranges .

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