Antilaser Seminar

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B.Tech Seminar Report

ANTI-LASER TECHNOLOGY

Submitted in partial fulfillment for the award of the Degree of

Bachelor of Technology in Electronics And Communication Engineering

Submitted byNITHIN M (Candidate Code:08420021)

Under the guidance of

Mrs. ANJALY SWAPNA

Department of Electronics & Communication EngineeringCOLLEGE OF ENGINEERING AND MANAGEMENT, PUNNAPRA

KERALANOVEMBER 2011


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CERTIFICATE

This is to certify that the thesis entitled ANTI-LASER TECHNOLOGYis a bonafide record of the seminar presented by NITHIN M ( Roll No.08420021)

under my supervision and guidance, in partial fulfillment for the award of Degree

of Bachelor of Technology in Electronics And Communication Engineering from

the University of Kerala for the year 2011.

----------------------------- -------------------------Mrs. ANJALI SWAPNA(Seminar guide) (seminar co-ordinator)Asst.Professor Asst.Professor

Dept. of ECE Dept. of ECE

--------------------------------------

DEEPAK V. K

Asst.Professor & HODDept. of ECE

Place: Alappuzha


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Date:

ACKNOWLEDGEMENT

First and foremost, I wish to place on records my ardent and earnest gratitude to my

seminar guide Mrs. ANJALI SWAPNA, Assistant Professor, Dept. of Electronics And

Communication Engineering. Her tutelage and guidance was the leading factor in

translating my efforts to fruition. Her prudent and perspective vision has shown light on

our trail to triumph.

I am extremely happy to mention a great word of gratitude to Mr. DEEPAK V.K,

Headof the Department ofElectronics And Communication Engineeringfor providing me

with all facilities for the completion of this work.

Finally yet importantly, I would like to express my gratitude to ProfA.J SAJI, the

principal of our institution for providing me with all facilities for the completion of this work.

I would also extend my gratefulness to all the staff members in the Department. I also

thank all my friends and well-wishers who greatly helped me in my endeavour.

NITHIN M


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ABSTRACT

More than 50 years after the invention of the laser, scientists at Yale University

have built the world's first anti-laser, in which incoming beams of light interfere with one

another in such a way as to perfectly cancel each other out. The discovery could pave the

way for a number of novel technologies with applications in everything from optical

computing to radiology. A Coherent perfect absorber (CPA), or anti-laser, is a device

which absorbs coherent light and converts it to some form of internal energy such as heat

or electrical energy. It is the time reversed counterpart of a laser. It is a two-channel CPA

device which absorbs the output of two lasers, but only when the beams have the correct

phases and amplitudes. The initial device was able to absorb 99.4 percent of all incoming

light, but the team behind the invention believes it will be possible to increase this

number to 99.999 percent.

In this time-reversed counterpart to laser emission, incident coherent optical fields

are perfectly absorbed within a resonator that contains a loss medium instead of a gain

medium. The incident fields and frequency must coincide with those of the corresponding

laser with gain. We demonstrated this effect for two counter propagating incident fieldsin a silicon cavity, showing that absorption can be enhanced by two orders of magnitude,

the maximum predicted by theory for our experimental setup. In addition, it shows that

absorption can be reduced substantially by varying the relative phase of the incident

fields. The device, termed a coherent perfect absorber, functions as an absorptive

interferometer, with potential practical applications in integrated optics.
http://en.wikipedia.org/wiki/Coherent_lighthttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Coherent_light


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CONTENTS

Chapter No TITLE Page No

List of Abbreviations i

List of Figures ii

1 INTRODUCTION 1

1.1 CPA (Coherent Perfect Absorber) 1

1.2 Time Reversed Laser or Anti- Laser 2

2 HISTORY OF ANTI-LASER 3

2.1 DASER Concept 3

2.2 Anti-Laser Concept 3

3 LASER 5

3.1 LASER Concept 5

3.2 Components of LASER 5

3.3 Working Principle of LASER 6

4 PRINCIPLE BEHIND ANTI-LASER 8

4.1 Time-Reversed Lasing Action 8

4.2 Time -Reversal Symmetry Property of Optical Systems 8

4.3 Basic Working Design 9


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5 ANTI-LASER THEORY 12

5.1 CPA Theorem 12

5.1.1 S Matrix Concept 12

5.1.2 Interferometric Absorption

5.2 Time-Reversed Lasing and Interferometric 15

Control of Absorption

6 APPLICATIONS OF ANTI-LASER 24

TECHNOLOGY

7 CURRENT RESEARCHES AND 30

ACHIEVEMENTS

8 CHALLENGES TO ANTI-LASER 32

TECHNOLOGY

9 CONCLUSION 33

REFERENCES 34


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ii

List of Abbreviations

GPS Global Positioning System

SCAN Spoken Content-Based Audio Navigation

LCD Liquid Crystal Display

PIC Peripheral Interface Controller

EEPROM Electrically Erasable Programmable Read only Memory


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ii

LIST OF FIGURES

Figure No Title Page No

3.1 Constructional Details of LASER 5

3.2 Basic principle behind Laser action 6

4.1 Working Principle behind the Anti-Laser 9

5.1 Modulation depth of Anti-Laser Output Signal 16

5.2 Phase modulation of beam absorption 18

5.3 Complex refractive indices for the uniform 21

Dielectric slab as a CPA

5.4 Semi-log plot of normalized output intensities 22

6.1 Optical switching by means of laser action 25

6.2 Optical Transistor (Schematic View) 27


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Anti-Laser Technology 1

Dept. of ECE CEM,Punnapra

CHAPTER 1

INTRODUCTION

1.1 CPA (Coherent Perfect Absorber)

An arbitrary body or aggregate can be made perfectly absorbing at discrete

frequencies, if a precise amount of dissipation is added under specific conditions of

coherent monochromatic illumination. This effect arises from the interaction of optical

absorption and wave interference, and corresponds to moving a zero of the S-matrix onto

the real wave vector axis. It is thus the time-reversed process of lasing at threshold. The

effect may be demonstrated in a Si slab illuminated in the 500-900nm range. Coherent

perfect absorbers form a novel class of linear optical elements-absorptive interferometerswhich may be useful for controlled optical energy transfer.

A laser is a physical system which, when subjected to an energy flux (pump), self-

organizes at a threshold value of the pump to produce narrow-band coherent

electromagnetic radiation. In the absence of inhomogeneous broadening and quantum

fluctuations, this radiation has zero line width. Above the first lasing threshold, lasers are

nonlinear systems, but at the first threshold they satisfy a linear wave equation with a

negative (amplifying) imaginary part of the refractive index, generated by the population

inversion due to the pump. In conventional lasers, the gain medium is confined in

resonators with a relatively high quality factor (Q), and the lasing modes are closely

related to passive-cavity modes. However, recent demonstrations of random lasers have

shown that the lasing threshold can be reached and coherent lasing obtained in resonators

with no high-Q passive-cavity modes. It can be rigorously shown within semi classical

laser theory that the first lasing mode in any cavity is an eigenvector of the

electromagnetic scattering matrix (S matrix) with an infinite eigenvalue, i.e., lasing

occurs when a pole of the S matrix is pulled up to the real axis by including gain as a

negative imaginary part of the refractive index. This viewpoint suggests the possibility of

the time-reversed process of lasing at threshold. A specific degree of dissipation (loss

medium) is added to the resonator, corresponding to a positive imaginary refractive

index equal in absolute value to that at the lasing threshold. The system is illuminated


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coherently and monochromatically by the time reverse of the output of a lasing mode,

and the incident radiation is perfectly absorbed. We refer to such an optical system as a

coherent perfect absorber (CPA).

1.2 Time Reversed Laser or Anti-Laser

Now the anti-laser become a reality and on 17 February 2011 Yale physicist A.

Douglas Stone and his team published a study explaining the theory behind an anti-laser,

demonstrating that such a device could be built using silicon, the most common

semiconductor material. But it wasn't until now, after joining forces with the

experimental group of his colleague Hui Cao, that the team actually built a functioning

anti-laser. In the anti-laser, a coherent beam of light is inserted into a loss medium, which

can be the same material as the gain medium or one less likely to emit light, such as

the silicon used in this experiment. Any material will absorb some photons and scatter the

rest, but picking just the right wavelength for the particular material and the length of the

anti-laser cavity ensures that all the photons will be absorbed if they stay in the material

long enough The device could be used to create a signal in a photo detector. But because

the anti-laser works with a specific wavelength in a coherent beam, it wouldnt have any

practical use in solar cells. It also wouldnt help with stealth technology, and its not a

shield against laser beams, Stone points out. The physicists are talking to researchers at

Cornell University, in Ithaca, N.Y., about whether the anti-laser might aid in the

development of a hybrid optical and electronic computer that uses light instead of

electrons for some calculations.
http://spectrum.ieee.org/semiconductors/design/the-silicon-solutionhttp://spectrum.ieee.org/semiconductors/design/the-silicon-solution


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CHAPTER 2

HISTORY OF ANTI-LASER

2.1 DASER Concept

In the 60s, the anti-laser concept was invented alongside the laser concept. It was

a joke then, but recent research shows how interesting physics does lie within this

concept.

In 1960, the laser light amplification by stimulated emission of radiation was

invented. The dasar darkness amplification by stimulated absorption of radiation was also

invented as a parody in Towness lab, of course. Surprisingly, Yidong Chong and

colleagues in the group of Douglas Stone at Yale University (Connecticut, USA) have

now shown that new physics lies within the dasar or anti-laser concept.

In a standard laser, light is forced to bounce back and forth across a material and

is thus amplified in the process. Lasing comes about as a combination of constructive in-

terference the self-interference of the light bouncing back and forth across the amplifying

material and amplification, explains Chong, which, of course, involves the conversion

of electricity, or some other source of energy, into light.

A dasar works in exactly the opposite way as a laser and, at least in the original

playful meaning, can be implemented with several ordinary objects. Take, for example, a

black sheet of paper. If you shine a bright light on it, most of the light will get absorbed

and dissipated through heat, but this is not just it! We were explaining how a laser works

to a visitor some months ago, says Chong, when Prof. Stone pointed out that one way

to understand this is to think about the reverse process, where an incoming light signal is

completely absorbed by a body. We realized that there is some interesting physics lying

within this concept and that work could actually be done.

2.2 Anti-Laser Concept

The proposed device, or Coherent Perfect Absorber (CPA), is described by the

very same equations describing the laser, just with dissipation instead of amplification

and destructive interference instead of constructive interference. Light is therefore


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Principal components are

1) Gain medium

2) Laser pumping energy

3) High reflector

4) Output coupler

5) Laser beam

3.3 Working Principle of LASER

Light of a specific wavelength that passes through the gain medium

is amplified (increases in power); the surrounding mirrors ensure that most of the light

makes many passes through the gain medium, being amplified repeatedly. Part of thelight that is between the mirrors (that is, within the cavity) passes through the partially

transparent mirror and escapes as a beam of light. The process of supplying

the energy required for the amplification is called pumping. The energy is typically

supplied as an electrical current or as light at a different wavelength. Such light may be

provided by a flash lamp or perhaps another laser. Most practical lasers contain additional

elements that affect properties such as the wavelength of the emitted light and the shape

of the beam.

Basic principle behind Laser action can be represented as

Fig 3.2 Basic principle behind Laser action
http://www.thefullwiki.org/Output_couplerhttp://www.thefullwiki.org/Optical_amplifierhttp://www.thefullwiki.org/Light_beamhttp://www.thefullwiki.org/Energyhttp://www.thefullwiki.org/Laser_pumpinghttp://www.thefullwiki.org/Xenon_flash_lamphttp://www.thefullwiki.org/Xenon_flash_lamphttp://www.thefullwiki.org/Laser_pumpinghttp://www.thefullwiki.org/Energyhttp://www.thefullwiki.org/Light_beamhttp://www.thefullwiki.org/Optical_amplifierhttp://www.thefullwiki.org/Output_coupler


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Laser medium is the heart of the laser system and is responsible for producing

gain and subsequent generation of laser. It can be a crystal, solid, liquid, semiconductor

or gas medium and can be pumped to a higher energy state. The material should be of

controlled purity, size and shape and should have the suitable energy levels to support

population inversion. In other words, it must have a metastable state to support stimulated

emission. In a four level laser, the material is pumped to level 4, which is a fast decaying

level, and the atoms decay rapidly to level 3, which is a metastable level. The stimulated

emission takes place from level 3 to level 2 from where the atoms decay back to level 1.

In Four level lasers, the laser transition takes place between the third and second excited

states. Since lower laser level 2 is a fast decaying level which ensures that it rapidly gets

empty and as such always supports the population inversion condition.

We may conclude that, laser action is preceded by three processes, namely,

absorption, spontaneous emission and stimulated emission - absorption of energy to

populate upper levels, spontaneous emission to produce the initial photons for stimulation

and finally, stimulated emission for generation of coherent output or laser.


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CHAPTER 4

PRINCIPLE BEHIND ANTI-LASER

4.1 Time-Reversed Lasing Action

Time-reversal symmetry is a fundamental symmetry of classical electromagnetic

theory and of non relativistic quantum mechanics. It implies that if a particular physical

process is allowed, then there also exists a time reversed process that is related to the

original process by reversing momenta and the direction of certain fields (typically

external magnetic fields and internal spins). These symmetry operations are equivalent to

changing the sign of the time variable in the dynamical equations, and for steady state

situations they correspond to interchanging incoming and outgoing fields. The power of

time-reversal symmetry is that it enables exact predictions of the relationship between

two processes of arbitrary complexity. A familiar example is spin echo in nuclear

magnetic resonance (NMR) : A set of prcising spins in a magnetic field fall out of phase

because of slightly different local field environments, quenching the NMR signal. The

signal can be restored by imposing an inversion pulse at time T, which has the effect of

running the phase of each spin backward in time, so that after 2T they are back in phase,

no matter how complicated their local field environment. Time-reversal symmetry is the

origin of the well known weak localization effect in the resistance of metals, the coherent

backscattering peak in the reflection from multiple scattering media , and the elastic

enhancement factor familiar in nuclear scattering. Effects due to direct generation of

time-reversed waves via special mirrors have been extensively studied for sound waves

and microwave radiation.

4.2 Time -Reversal Symmetry Property of Optical Systems

Recently, several of the authors explored theoretically an exact time-reversal

symmetry property of optical systems: the time-reversed analog of laser emission. In the

lasing process, a cavity with gain produces outgoing optical fields with a definite

frequency and phase relationship, without being illuminated by coherent incoming fields


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at that frequency. The laser is coupled to an energy source (the pump) that inverts the

electron population of the gain medium, causing the onset of coherent radiation at a

threshold value of the pump. Above threshold the laser is a nonlinear device, but at

threshold for the first lasing mode, the laser is described by the linear Maxwell equations

with complex (amplifying) refractive index. Because of the properties of these equations

under time reversal, it follows that the same cavity, with the gain medium replaced by an

equivalent absorbing medium, will perfectly absorb the same frequency of light, if it is

illuminated with incoming waves with the same field pattern. Additional analysis showed

that if the cavity is illuminated with coherent field patterns not corresponding to the time-

reversed emission pattern, it is possible to decrease the absorption well below the value

for incoherent illumination. Such a device, related to a laser by time reversal, was termed

a coherent perfect absorber (CPA). The properties of CPAs point to a new method forcontrolling absorption through coherent illumination. Here we demonstrate both the

strong enhancement and reduction of absorption in a simple realization of the CPA: a

silicon wafer functioning as solid Fabry-Perot etalon.

4.3 Basic Working Design

The working principle behind the Anti-Laser can be schematically represented as

fig.given below

Fig 4.1 Working Principle behind the Anti-Laser


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In the fig 4.1A laser beam from a tunable (800 to 1000 nm) continuous- wave

Ti:sapphire source enters a beam splitter (designated 1). The two split beams are directed

normally onto opposite sides of a silicon wafer of thickness ~110 mm, using a Mach-

Zehnder geometry. A phase delay in one of the beam paths controls the relative phase of

the two beams. An additional attenuator ensures that the input beams have equal

intensities, compensating for imbalances in the beam splitters and other imperfections.

The output beams are rerouted, via beam splitters (designated 2, 3, and 4), into a

spectrometer. The inset is a schematic of the CPA mechanism. The incident beams from

left and right multiply scatter within the wafer with just the right amplitude and phase so

that the total transmitted and reflected beams destructively interfere on both sides,

leading to perfect absorption.

The basic working design is that two identical lasers are fired into a cavity

containing a silicon wafer, a light-absorbing material that acts as a "loss medium." The

wafer aligns the light waves from the lasers so they become trapped, causing most of

the photons to bounce back and forth until they are absorbed and transformed into heat.

Furthermore, many of the remaining light waves are cancelled out by interfering with

each other. In contrast a normal laser uses a gain medium which amplifies light instead of

absorbing it.

In the context of lasers, time reversal isnt a way for scientists to travel back to

childhood and fix their embarrassing mistakes. Its a technique for rewinding and

undoing a process by reversing the mathematics underlying it in this case, by changing a

plus sign to a minus sign to make the energy absorbed by the anti-laser equal to the

energy produced by a laser. Using time reversal of the lasing operation and the source of

the laser, [the Yale researchers] found a very elegant way to make a perfect absorber of

light, says Mathias Fink, a physicist at ESCPI Paris Tech who developed the first time

reversal techniques applied to sound waves.

Lasers work by passing light through a material that amplifies it a piece of crystal,

glass, semiconductor or other gain medium to produce an intense, coherent beam. The
http://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Active_laser_mediumhttp://en.wikipedia.org/wiki/Active_laser_mediumhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Silicon


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Yale contraption runs this process backward. An incoming beam enters a splitter, which

divides it into two beams of equal strength. Mirrors sync up these beams and guide them

into opposite sides of a silicon wafer, a material that absorbs light. The energy not

absorbed by the silicon disappears as the beams collide and interfere with each other; less

than 1 percent of the beams energy escapes the silicon death trap.

Lasers work by using a gain medium, often gallium arsenide or some other

semiconductor, to produce light waves with the same frequency and amplitude. These

waves, which are in step with one another, make up a focused beam of coherent light.

By contrast, the anti-laser utilizes a silicon wafer loss medium. When two laser

beams were shone into a cavity containing that wafer, it aligned the light waves so that

they became perfectly trapped, causing them to ricochet back and forth until they were

absorbed and transformed into heat.


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CHAPTER 5

ANTI-LASER THEORY

5.1 CPA Theorem

A laser is a physical system which, when subjected to an energy pump, self-

organizes at a threshold value of the pump to produce coherent narrow-band

electromagnetic radiation. Above the first threshold, a laser is a non-linear system, but at

the first threshold it satisfies a linear wave equation with a negative (amplifying)

imaginary part of the refractive index, generated by the population inversion. Semi

classical laser theory shows that the first lasing mode in any cavity is an eigenvector of

the scattering matrix (S-matrix) with infinite eigenvalue; i.e. lasing occurs when a pole of

the S-matrix is pulled up" to the real axis by including gain as a negative imaginary part

of the refractive index. This suggests the possibility of the time-reversed process of lasing

at threshold. A specific amount of dissipation is added to the medium, corresponding to a

positive imaginary refractive index equal in absolute value to that at the lasing threshold.

The system is illuminated coherently and monochromatically by the time-reverse of the

output of a lasing mode, and the incident radiation is perfectly absorbed. We refer to such

an optical system as a coherent perfect absorber (CPA) or Anti-laser.

5.1.1 S Matrix Concept

Coherent perfect absorption is a general and robust phenomenon related to the

analytic properties of the S-matrix. For simplicity, we consider scattering in one or two

dimensions, for which the (TM) electric field is a scalar obeying the equation

[

+

( )

( ) = 0

Here k= /c (a scalar), is the frequency, and n = n+ n is the complex refractive

index, with n < 0 for gain and n > 0 for absorption. There is an external region,

extending from some radius rs to infinity, where n = 1. The field in the external region is


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a combination of incoming waves (amplitudesm) and outgoing waves (amplitudes m).

The scattering channel amplitudes are related by

(k) m= m

Continuing k into the complex plane, S(k) possesses a countably infinite set of poles and

zeros, symmetrically placed at

where

Adding gain or dissipation moves the zeros and poles of S(k) in the complex k

plane. There exist several theoretical methods for locating these zeros and poles, such as

the R-matrix" theory of Wigner and Eisenbud. With sufficient dissipation, a zero can

cross the real axis at k = ~km. Radiation incident at each ~km can be completely

absorbed if it corresponds to the specific eigenvector of the S-matrix having eigenvalue

zero. This is the heart of the CPA process, arising from the interplay of interference and

absorption: with specific amounts of dissipation, there exist interference patterns that trap

incident radiation for an infinite time. Even small rates of single-pass absorption can lead

to perfect absorption, albeit within narrow frequency bands.

We now give a more precise statement of the CPA theorem. For simplicity, consider the

scalar wave equation.

[ + ( ) ( ) = 0.1

Where k = w/c, w is the frequency, c is the speed of light, k(r) is the electric field,

and n = n + in is the refractive index (n < 0 for gain and n > 0 for absorption). Outside

of the cavity, n is assumed to be real and constant. Steady-state solutions of these

equations are described by the electromagnetic scattering matrix (S-matrix).Which relates

incoming and outgoing channel states whose weights are represented by complex vectors

, , obeying

S[n(r)] . = 2


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cavity-enhanced photo detectors). In addition, a closely related device, used as an

optical switch or filter, is the critically coupled resonator, which typically has a ring

geometry and is equivalent to two decoupled single-channel CPAs.

5.2 Time-Reversed Lasing and Interferometric Control of Absorption

Our two-channel CPA is qualitatively different from the single-channel case

because it requires two coherent input beams, and perfect absorption is only achieved

when these beams have the correct relative phase and amplitude. Thus, it is not only

sensitive to frequency but to the amplitude and phase of the input light and can function

as an absorptive interferometer, potentially useful as a modulator, detector, or phase

controlled optical switch. Reaching the precise CPA condition of perfect absorptionrequires tuning two parameters (e.g., n and n or nand ); by analogy to the laser, the

CPA must have the correct absorption to reach threshold and also must satisfy an

appropriate interference condition in the cavity. However, simply tuning near the band

gap of a semiconductor can bring the system very close to the CPA condition, increasing

the absorption by many orders of magnitude.

The simplest two-channel CPA has a uniform complex refractive index n

approaching one of the values needed for the CPA condition, connected to a single

propagating mode on the left and another on the right. In our implementation (Fig. 1),

two collimated counter propagating free space laser beams are directed onto opposite

surfaces of a Si wafer, which functions as a low-Q Fabry-Perot etalon based on Fresnel

reflection at the surfaces (Q 840). Although these illumination conditions are not truly

single-channel because of the finite width of the free-space beams, our results indicate

that this is not the main source of deviation from the ideal behavior. The output beams

are collected into a high-resolution spectrometer, and the intensities in each individual

output beam, as well as the total output intensity, are measured.


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Fig 5.2 Phase modulation of beam absorption.

If we work at a wavelength corresponding to a CPA resonance, such as the central

minimum shown in Fig. 5.2A, In which (A) represents the Theoretical plot of normalized

total output intensities as a function of wavelength l for parity-odd (blue) and parity-even

(red) scattering eigenmodes. The dashed black line is the result for incoherent input

beams. (B to D) represents the Theoretical output intensities at three representative values

of l as the relative phase of the input beams is varied, showing intensities emitted to the

right (magenta) and left (green) sides of the slab, and the total intensity (black). Values of

l corresponding to (B) to (D) are marked by vertical lines in (A); (B) is the CPA

resonance. (E to G) Experimental results at values of l approximately corresponding to

(B) to (D). Solid lines are fits to the data, not theory curves; results are normalized to

max(Iout) of the fit then, upon varying the relative phase from 0 to (keeping the two

beam intensities constant and equal), the system goes from enhanced scattering (red

curve) to nearly zero scattering (blue curve). Intermediate values of do not correspond

to a single S-matrix eigenstate, so the scattered intensity interpolates between the

extremal values. The black curve in Fig. 5.2A shows the expected scattered intensity for

incident beams neglecting their interference, 2(R + T)I. At the CPA resonance, it lies


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miss the CPA condition by tuning only one parameter, , leading to a maximum

modulation depth of ~ to .

However, the limiting factors in the experiment are the temporal and spatial

coherence of the laser, reducing M() to ~ . The finite laser line width (0.18 nm)

smears out the CPA resonances, which are optimized for a monochromatic input. This

effect can be partially compensated by filtering the output through a spectrometer

[resolution 0.05 nm (12)]. This finite resolution of the spectrometer can be incorporated

into the analysis, and the resulting theoretical curve, shown in Fig. 5.1A, agrees well with

the experimental data. The dual role of interference in both enhancing and suppressing

absorption can be seen more clearly in Fig. 5.1B, which compares the maximum and

minimum output intensities to 2(R + T), the expected output intensity for two incoherent

input beams. At the CPA resonant wavelength, the minimum output intensity is less than

1% of the input, while the incoherent illumination gives ~35% output. When the phase is

adjusted to maximize the scattering, the output reaches ~70%.

Although we have demonstrated coherent reduction of absorption in our

experiment, this effect should be distinguished from the phenomenon of

electromagnetically induced transparency, in which absorption is suppressed bycoherently driving the absorbing medium itself, instead of by enhancing escape from the

cavity by constructive interference, as in our system. Because this optical effect is easily

realized in silicon, coherent perfect absorbers may enable novel functionalities in silicon

integrated photonic circuits of the type envisioned for next generation optical

communications and computing applications as well as for coherent laser spectroscopy.

The simplest versions of the device immediately would serve as compact on-chip

interferometers, which absorb or scatter the input beams instead of steering them.

Although our current CPA operates near the silicon band edge, it should be possible to

fabricate devices in which an additional parameter tunes the absorption coefficient

independently of (e.g., by free carrier injection or by optical pumping), allowing one to

fix the operating wavelength by design. Directband gap semiconductors also are suitable

materials for CPAs, assuming that fluorescent emission can be tolerated or avoided in a


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specific application. Recent theoretical work has proposed a fascinating extension of the

CPA concept, suitable for directband gap materials: Systems with balanced gain and

loss can function simultaneously as a CPA and as a laser (i.e., as an interferometric

amplifier attenuator). The CPA effect is not immediately applicable to photovoltaic or

stealth technology because it is a narrow-band effect requiring coherent inputs.

The simplest possible CPA is a single port reflector,similar to critically-coupled

fiber-resonator" systems. However, important properties of the CPA are only revealed

with multiple ports and hence non-trivial eigenvectors. We study a two-port case,

consisting of a one-dimensional slab of thickness a and uniform index n, with two input

channels for each k corresponding to incident radiation from left and right. In Fig 5.3, we

plot the{nv(k)} which produces a zero of the S-matrix for a fixed k, where ka = 664.7. In

practice, a material will have a frequency-dependent n(k); when the material is fabricated

and illuminated in a 2-port configuration, one may realize a CPA by scanning k and

looking for coincidences, i.e. n(k)nv(k) for some integers v.

Fig 5.3 Complex refractive indices for the uniform dielectric slab as a CPA


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In the fig 4 Parity-even solutions are shown as hollow blue circles; parity-odd

solutions are omitted for clarity. The green curve shows the refractive index of Si at

different frequencies. Inset: parity-even solutions (hollow blue circles) and parity-odd

solutions (filled red circles) in the region 3.55 < Re(n) < 3.62; the green cross shows the

index of Si at ka = 664.7, a = 100 m (i.e. = 945.3 nm).

Fig. 5.4 shows the S-matrix eigenvalue intensities for a slab of undoped Si, with a

= 100 m where S are S-matrix eigenvalues) vs. the wavelength = 2/k, for a 100 m Si slab.

Solid lines show ( ) for a parity-even (blue) or parity-odd (red) eigenmode. The

dashed line shows 2(r2 + t2), the total output intensity when the two input beams are

incoherent. Inset: upper and lower bounds for s2 over a wide range of . Deep CPA minima

are achieved in the neighborhood of = 945nm, with a maximum intensity contrast 50dB, and more than ten substantial minima are visible in the range 938nm < < 954nm.

The location of these minima is a-dependent and hence tunable within a given material.

Further aspects of the CPA phenomenon can be explored with non-uniform and/or

higher-dimensional systems. For instance, we have shown that it is possible to use a

periodic array of slabs (i.e., a one-dimensional photonic crystal) to optimize the contrast

between the perfectly absorbed and incoherent/reduced-absorption illumination

conditions.

Fig 5.4 Semi-log plot of normalized output intensities


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More generally, the exact time-reversal symmetry property that relates laser

emission to coherent perfect absorption implies that an arbitrarily complicated scattering

system can be made to perfectly absorb at discrete frequencies if its imaginary refractive

index can be tuned continuously over a reasonable range of values, and if appropriate

coherent incident beams can be imposed. Progress in these areas would open up

interesting new avenues for future research and applications.


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CHAPTER 6

APPLICATIONS OF ANTI-LASER TECHNOLOGY

In the anti-laser the absorbed photon energy may flow out of the cavity in a

number of different forms. In direct band gap materials, a large fraction will be reemitted

via fluorescence, which is not generally desirable, whereas in indirect band gap materials

such as Si, it can be extracted in the form of heat and/or photocurrent, either of which

could be useful. Therefore, materials which are not useful as lasing media, such as Si,

make good CPAs, whereas good lasing materials, such as GaAs, make poor CPAs. By

utilising this property and the complete light absorbing capability CPAs can be used in

number of applications and some of them are listed here.

CPAs are potentially useful as transducers, modulators, or optical switches, for example,

in on-chip integrated optical circuits based on Si waveguide or resonator technology. We

have verified that with realistic non optimized parameters a Si single-mode waveguide

with 0.9 m distributed-Bragg-reflector mirrors and a 4 m loss region of pure

intrinsic Si exhibits a CPA absorption resonance at 947 nm with contrast of roughly 90%.

Operation likely can be extended into the communications wavelengths around 1.5 m by

designing devices with index tuning via free carrier injection as has already been

achieved in other Si-based resonant photonic circuits

The main applications of CPA are

1. CPAs are potentially useful as transducers, modulators, or optical switches, for

example, in on-chip integrated optical circuits based on Si waveguide or resonator

technology.

In telecommunication, an optical switch is a switch that enables signals in optical

fibers or integrated optical circuits (IOCs) to be selectively switched from one circuit to

another. Systems that perform this function by physically switching light are often

referred to as "photonic" switches, independent of how the light itself is switched. Away

from the world of telecom systems, an optical switch is the unit that actually switches
http://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Integrated_optical_circuithttp://en.wikipedia.org/wiki/Telecommunication_circuithttp://en.wikipedia.org/wiki/Telecommunication_circuithttp://en.wikipedia.org/wiki/Integrated_optical_circuithttp://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Telecommunication


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2. Coherent perfect absorbers can be used to build absorptive interferometers, which

could be used in detectors, transducers, and optical switches

In general, the goal of absorption spectroscopy is to measure how well a sample

absorbs or transmits light at each different wavelength. Since the CPA are capable of

perfectly absorbing the light of specific wavelength they have an important role in

absorptive interferometers The absortive spectrometer is just a Michelson interferometer

but one of the two fully-reflecting mirrors is movable, allowing a variable delay (in the

travel-time of the light) to be included in one of the beams.

3. The applications of anti-laser could lead to optical switches replacing transistors in

future computers. Optical computers could potentially be much more powerful than

today's computers, given that the size of components could be shrunk beyond the limits

of today's electron-based technologies.

An optical computer (also called a photonic computer) is a device that performs

its computation using photons of visible light or infrared (IR) beams, rather than electrons

in an electric current. The computers we use today use transistors and semiconductors to

control electricity but computers of the future may utilize crystals and metamaterials to

control light.

This is why scientists have been trying for some time to find ways to produceintegrated circuits that operate on the basis of photons instead of electrons. The reason is

that photons do not only generate much less heat than electrons, but they also enable

considerably higher data transfer rates. And the research group has now achieved a

decisive breakthrough by successfully creating an optical transistor with a single

molecule.By using one laser beam to prepare the quantum state of a single molecule in a

controlled fashion, scientists could significantly attenuate or amplify a second laser beam.

This mode of operation is identical to that of a conventional transistor, in which electrical

potential can be used to modulate a second signal.
http://tremblinguterus.blogspot.com/2011/01/future-of-computers-overview.htmlhttp://tremblinguterus.blogspot.com/2011/01/future-of-computers-overview.html


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Fig 6.2 optical transistor schematic view

4. The CPA device could be an integral element in optical computers, a long promised

successor to today's computers that would use light instead of electrons to processinformation.

Researchers have created a new process for making complex miniature

waveguides that can steer optical signals in three dimensions through solid materials. All

optical computers could approach the theoretical speed of a photonic switch which is

estimated to be on the order of petahertz (). They should definitely achieve multi-

terahertz speeds. So 1000 to 1 million times faster than current computers at 4 Gigahertz

(4x).

5. An anti-laser switch could help solve one of the toughest challenges in building anoptical computer, namely the management and manipulation of the light used to encode

information. For instance, a CPA could be used in an optical switch, one that would

absorb light of a particular wavelength while letting light with other wavelengths pass.

An electric current creates heat in computer systems and as the processing speed

increases, so does the amount of electricity required; this extra heat is extremely

damaging to the hardware. Photons, however, create substantially less amounts of heat

than electrons, on a given size scale, thus the development of more powerful processing

systems becomes possible. By applying some of the advantages of visible and/or IR

networks at the device and component scale, a computer might someday be developed

that can perform operations significantly faster than a conventional electronic computer.
http://www.technologyreview.com/Infotech/20036/?a=fhttp://www.technologyreview.com/Infotech/20036/?a=fhttp://nue.clt.binghamton.edu/intro1_5.htmlhttp://nue.clt.binghamton.edu/intro1_5.htmlhttp://nue.clt.binghamton.edu/intro1_5.htmlhttp://nue.clt.binghamton.edu/intro1_5.htmlhttp://nue.clt.binghamton.edu/intro1_5.htmlhttp://nue.clt.binghamton.edu/intro1_5.htmlhttp://nue.clt.binghamton.edu/intro1_5.htmlhttp://www.technologyreview.com/Infotech/20036/?a=fhttp://www.technologyreview.com/Infotech/20036/?a=f


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7. Anti-laser can be usefull in coherent laser spectroscopy.

8. The simplest versions of the anti-laser immediately would serve as compact on-chip

interferometers, which absorb or scatter the input beams instead of steering them.

9. The most potential application is in radiology, where the principle of the CPA might be

used to precisely target electromagnetic radiation inside human tissues for therapeutic or

imaging purposes


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CHAPTER 7

CURRENT RESEARCHES AND ACHIEVEMENTS IN

ANTI-LASER TECHNOLOGY

Now the anti-laser become a reality and on 17 February 2011 Yale physicist A.





anti-laser.

In anti-laser the scientists focused two laser beams with a specific frequency into

a cavity containing a silicon wafer that acted as a "loss medium." The wafer aligned the

light waves in such a way that they became perfectly trapped, bouncing back and forth

indefinitely until they were eventually absorbed and transformed into heat. Using time

reversal of the lasing operation and the source of the laser, [the Yale researchers] found a

very elegant way to make a perfect absorber of light, says Mathias Fink, a physicist at

ESCPI Paris Tech who developed the first time reversal techniques applied to sound

waves.

The anti-laser, officially known as a coherent perfect absorber (CPA), is about

one centimeter across, and capable of absorbing 99.4 percent of incoming light.

According to Yale physicist A. Douglas Stone, however, the current model is merely a

proof-of-concept. He believes that future versions should be able to absorb 99.999

percent of the light, and could be built as small as six microns approximately one-

twentieth the width of a human hair. The current CPA is also limited to absorbing near-

infrared light, but Stone believes that by altering the cavity and the loss medium, future

versions should be able to handle visible and infrared light. The Yale physicist A.



http://www.yale.edu/http://www.yale.edu/


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anti-laser. The team, whose results appear in the Feb. 18 issue of the journal Science,

focused two laser beams with a specific frequency into a cavity containing a silicon wafer

that acted as a "loss medium." The wafer aligned the light waves in such a way that they

became perfectly trapped, bouncing back and forth indefinitely until they were eventually

absorbed and transformed into heat.

Stone believes that CPAs could one day be used as optical switches, detectors and

other components in the next generation of computers, called optical computers, which

will be powered by light in addition to electrons. Another application might be in

radiology, where Stone said the principle of the CPA could be employed to target

electromagnetic radiation to a small region within normally opaque human tissue, either

for therapeutic or imaging purposes. Theoretically, the CPA should be able to absorb

99.999 percent of the incoming light. Due to experimental limitations, the team's current

CPA absorbs 99.4 percent. "But the CPA we built is just a proof of concept," Stone said.

"I'm confident we will start to approach the theoretical limit as we build more

sophisticated CPAs." Similarly, the team's first CPA is about one centimeter across at the

moment, but Stone said that computer simulations have shown how to build one as small

as six microns (about one-twentieth the width of an average human hair). The team that

built the CPA, led by Cao and another Yale physicist, Wenjie Wan, demonstrated the

effect for near-infrared radiation, which is slightly "redder" than the eye can see and

which is the frequency of light that the device naturally absorbs when ordinary silicon is

used. But the team expects that, with some tinkering of the cavity and loss medium in

future versions, the CPA will be able to absorb visible light as well as the specific

infrared frequencies used in fiber optic communications. It was while explaining the

complex physics behind lasers to a visiting professor that Stone first came up with the

idea of an anti-laser. When Stone suggested his colleague think about a laser working inreverse in order to help him understand how a conventional laser works, Stone began

contemplating whether it was possible to actually build a laser that would work

backwards, absorbing light at specific frequencies rather than emitting it.


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CHAPTER 8

CHALLENGES TO ANTI-LASER TECHNOLOGY

However the anti-laser is physically a reality but the application of thistechnology in the existing technologies need more studies and experiments

This technology is only limited to coherent light which reduces the wide

applications of the technology in the field of science.

The CPA effect is not immediately applicable to photovoltaic or stealth

technology because it is a narrow-band effect requiring coherent inputs.

The application of CPA in the computing field is currently very expensive.

The implementation of the technology in the research fields are very complicated

one.


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CHAPTER 9

CONCLUSION

Here we discussed about the Anti-Laser technology (coherent perfect absorber

technology) which exactly the opposite that of the Laser technology in which thedarkness amplification by stimulated absorption of radiation is occurring thats why it is

also called DASER. This technology is based on the optoelectronic principle of Time-

Reversed Lasing and Interferometric Control of Absorption. In the Anti-Laser in which

incoming beams of light interfere with one another in such a way as to perfectly cancel each other

out to form a complete darkness otherwise it causes the complete exactly 99.99% absorption of

the incoming coherent light. The Anti-Laser technology has a tremendous scope in the field of

optical computing which would be the next generation super computers in which light is used to

process the data instead of electrons in the conventional computers .It is also applicable in laser

spectroscopy and many other fields. In future this technology will make a huge impact in

computing and many other areas.


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REFERENCES

1. Time-Reversed Lasing and Interferometric Control of AbsorptionWenjie Wan, Yidong Chong, Li Ge, Heeso Noh, A. Douglas Stone, Hui Cao*

Science 331, 889 (2011); DOI: 10.1126/science.1200735

2. Coherent Perfect Absorbers: Time-reversed Lasers IEEE 2010Y. D. Chong, Li Ge, Hui Cao, and A. D. Stone

Department of Applied Physics, Yale University, New Haven,

OCIS codes: (030.1670) Coherent optical absorbers; (290.5825)

Scattering theory

3. IEEE Spectrum online magazine 2011 feb edition

4. www.wikipedia.com

5. science daily online science magazine 2011 may edition6. www. iee.ucsb.edu

7.Optical computingDamien Woods

Department of Computer Science and Artificial IntelligenceUniversity of Seville, Spain 2008
http://www.wikipedia.com/http://iee.ucsb.edu/http://iee.ucsb.edu/http://www.wikipedia.com/

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