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.

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

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

Photonic CrystalsPrinciples 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 confinelight within a cavity.Photonic band gaps appear in the plane of periodicity and in2D 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 CrystalsThe Principle

“magical oven mitts” forholding and controlling light

3D Photonic Crystal with Defectscan trap light in cavities and waveguides (“wires”)

with photonic band gaps: “optical insulators”

Photonic CrystalsThe 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 requirecurved lens

can exceed classicaldiffraction 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 fromthe 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

ScaleIntensity: -20 dB to -40 dB

X-Y: 37.5 X 30 cm2

Self organized Nano Photonic Crystal

3D IMAGINGIN REAL SPACE

Semiconductor substrate

Negative refraction in Photonic Crystal

Flat lens n = -1

d = u + v

vu

object image

d

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 CrystalsReplacing conventional optical fibres

The Glass Ceiling: Limits of SilicaLoss: 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

devicesRadical modifications to dispersion, polarization effects…tunability is

limited

Long DistancesHigh 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.


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


Microfabrication : By layer by layer lithography.

Colloidal self-assembly.


• 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

LithographyBasic 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 SiO2

1. Photoresist coating

SiO2

Photoresist

Substrate

3. Development

Substrate

Substrate

Mask

Ultra violet lightOpaque

ExposedUnexposed

2. Exposure

1. Photoresist coating

SiO2

Photoresist

Substrate

3. Development

Substrate

Substrate

Mask

Ultra violet lightOpaque

ExposedUnexposed

2. Exposure

• 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

4. Etching

Substrate

Substrate

5. Ion implant

Substrate

6. After doping

diffusion

Lithography (contd.)

The lithographic sequence is repeated for each physical layer

2µm

Lithography at its best

= 780nm

resolution = 150nm

7µm

(3 hours to make)


2µm

(300nm diameter coils, suspended in ethanol, viscosity-damped)

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 (SiO2)

sediment by gravity intoclose-packed fcc lattice!

(evaporate)

Inverse Opalsfcc solid spheres do not have a gap…

…but fcc spherical holes in Si do have a gap

Infiltration

sub-micron colloidal spheres

Template (synthetic opal)3D

Remove Template

“Inverted Opal”

[ figs courtesyD. Norris, UMN ]

In Order To Forma More Perfect Crystal…

meniscussilica250nm

• Capillary forces during drying cause assembly in the meniscus

• Extremely flat, large-area opals of controllable thickness

Heat Source

80C

65C1 micron silica spheresin 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.

THANK YOU !!!

Photonic Materials

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