The Science of Information Technology

1

Roland [email protected]

Department of Physics, Applied Physics, and Astronomy

The Science of Information Technology

Computing with Light• the processing of signals• properties of light• building a photonic computer• future trends ?

2



Signals in IT

time

volta

ge

(0)

(1)

time

volta

ge

(0)

(10)

(5) (7)(9)

not applicablebinary system: 01100101

3



Making a Byte out of Bits

11000010 = 194

channel 1

channel 2

channel 3

channel 4

channel 5

channel 6

channel 7

channel 8

understanding:computing problems can be separated into processing of single bits.

tools are:• transport• comparison• storage

4



Signal Processing in IT

transport of bits:

switching:

0 1 0 1 1 0

0 1 0 1 1 0

logic operation

switch

input 1

output0 1 0 1 1 0

input 2

0 1 0 1 1 00 1 0 1 1 0

distance, connectorinput output

5



What is a Bit ?

0 50 100 1500.00

0.01

0.02

0.03

0.04

0.05 one bit in frequency-domain

Am

plitu

de (a

rb. u

nits

)

Frequency (arb. units)

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0one bitin time-domain

Sig

nal (

arb.

uni

ts)

Time (arb. units)

Fourier transform

6



The cut-off frequency

0.2 0.4 0.6 0.8-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Sig

nal (

arb.

uni

ts)

Time (arb. units)0 20 40 60 80 100 120 140

0.00

0.01

0.02

0.03

0.04

0.05 cut-offfrequency

cut-offfrequency

Am

plitu

de (a

rb. u

nits

)

Frequency (arb. units)

7



Electronics

transport of bits:

switching:

cut-off= R / L

metal wire

Source

Gate

Drain

p-type S ilicon Wafer

n-type

Oxide

n-type

cut-off = R*C

8



Cut-off frequency vs. clock frequency

0 20 40 60 80 100 120 1400.00

0.01

0.02

0.03

0.04

0.05 cut-offfrequency

cut-offfrequency

Am

plitu

de (a

rb. u

nits

)

Frequency (arb. units)0.2 0.4 0.6 0.8

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

clock

clock

clock

Sig

nal (

arb.

uni

ts)

Time (arb. units)

9



Clock Frequency of Computers

1970 1980 1990 2000 2010 2020 2030

106

107

108

109

1010

1011

1012

1013

physical limit

PCsafter Malone (1995)

technological limit

C

lock

Fre

quen

cy (H

z)

Year

10



The heat problem

11



Clock Frequency of Computers

1970 1980 1990 2000 2010 2020 2030

106

107

108

109

1010

1011

1012

1013

physical limit

PCsafter Malone (1995)

technological limit

C

lock

Fre

quen

cy (H

z)

Year

12



Photonics

Idea: substitute electrical currents with light

cut-off = ?

( 30*1012 Hz )

glass fiber

cut-off= R / L

( 30*108 Hz )

metal wire

electrons

13



Let’s build a photonic computer

semiconductor laser modulator

clock

bit stream

modulatorbit stream

information

modulatorbit stream

information

photonicswitch

(AND)

output to detector

14



Semiconductor laser

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Output of a laserrapidly oscillating electromagnetic field

0 2 4 6 8

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

= 800 nm

Fiel

d (a

rb. u

nits

)

Time (fs)

1 fs = 10 –15 s = 0.000000000000001 s

16



Desired: short pulses and pulse trains

0 20 40 60 80 100 120-1

0

1

2

3

= 800 nm = 30 fs

Fiel

d (a

rb. u

nits

)

Time (fs)

0 50 100 150 200 250

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Sig

nal (

arb.

uni

ts)

Time (fs)

17





clock

bit stream

modulatorbit stream

information

modulatorbit stream

information

photonicswitch

(AND)

output to detector

18



Opto-electronic modulationSearch: Interface between optical & electrical pulses

Electro-optic modulators• example liquid crystals:

• get dark when electrical bias is applied • very slow

• Pockels-effect:• index of refraction depends on applied voltage• very fast

19



Using a Mach-Zehnder interferometer

t

lithium tantalate

20



Constructive & destructive interference

0 2 4 6 8

constructive interference

branch 2

branch 1

Fiel

d (a

rb. u

nits

)

Time (fs)

0 2 4 6 8

destructive interference

Fiel

d (a

rb. u

nits

)

Time (fs)

21



Integration of intensity modulators

material: lithiumniobate

22





clock

bit stream

modulatorbit stream

information

modulatorbit stream

information

photonicswitch

(AND)

output to detector

23



All-optical switching

the problem:light doesn’t interact with light

24



Absorption saturation

idea: use matter (electrons) to mediate the light-light interaction

atom: • electrons in orbits/states• Pauli-rule: up to 2 electrons

per state are allowed• transitions by light absorption

25



Optical transition of electronsen

ergy

fille

d st

ates

empt

yst

ates

atom in ground state

atom in excited state

absorption ofa photon

atom fully in excited state

saturatedabsorption

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All-optical switching by saturated absorption

pulse #1

pulse #2

transmissionsignal

A

B

C

A B C

00 0

0 01 0

101 1 1

AND-gate:

27



Excitation of bulk semiconductors

ener

gy

thickness

valenceband

conductionband

ener

gy

absorption

electron

28



Better: semiconductor heterostructuresen

ergy

layer thickness

valenceband

conductionband

hole state

electronstate

ener

gyabsorption

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AlGaAs-Switch

30



We are done: a photonic computer (???)


clock

bit stream

modulatorbit stream

information

modulatorbit stream

information

photonicswitch

(AND)

output to detector

31



Keep the information for some time

Solution: bistable devices

Electronics: Flip-Flop

Input

Out

put

10

1

0

Time

Inpu

t

a

1

0

b c d

ab

cd

Time

Out

put

a

1

0

b c d

32



The SEED (self-electro-optic effect device)

Ene

rgy

Layer Thickness

Ene

rgy

Layer Thickness

Ene

rgy

Layer Thickness

apply voltage with photo carriers

33



Photoinduced absorption

Lase

r

Energy

Abs

orpt

ion

Ene

rgy

Layer Thickness

apply voltage

Ene

rgy

Layer Thickness

with photo carriers

34



Demonstration of concepts

The first steps towards photonic computing: efficient transfer of data by fibers

rates up to 30 THz switching times as fast as 100 fs low switching energies

close to switching energies in electronic high repetition rates

> 100 GHz factor 100 higher as in PCs

35



Technological problems interface electronics-optics

usually slow (10 GHz) expensive ( ~ 100 US$)

micro integration devices of dimension 0.03 – 10 mm for parallel processing arrays of several cm

hybrid technologies expensive not acceptable

36



The market

assume for 10 years: 500 Mio Computers 100 US$ for photonic components

50 billion US$

more important: relation between market

potential and risk:50 billion US$

risk = ?

37



Research at Rensselaer

optical on chip interconnects fiber optical connects (Persans) terahertz optoelectronics (Zhang, Shur, Kersting)

38



The electromagnetic spectrum

1 kHz 1 MHz 1 GHz 1 THz 1 PHz

1 ms 1 s 1 ns 1 ps 1 fs

time frequency

HiFi

radio waves

ITvisiblelight

39



THz pulses

Properties: THz pulses are information carrier

measure the field very short light pulses possible propagate free space & on metal

wires fibers are no longer necessary

switching medium : semiconductors can be tailored for THz pulses no hybrid technologies

-1 0 1 2 3

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Fiel

d (a

rb. u

nits

)

Time (ps)

40



Logic operations with THz pulses

THz phase modulator

output C

input A

input B

A B C

0011

0101

0001

phase shift

41



THz semiconductor devices

Science fiction ?

our work:THz modulator• operating @ 3THz

42



Terahertz differentiatoranalog computer:• calculates the first time-derivative• operates at THz frequencies

-0.5 0.0 0.5 1.0 1.5

-1.0

0.0

1.0

2.0

3.0

calculation

transmitted pulse

incident pulsex0.1

Ele

ctric

Fie

ld (a

rb. u

nits

)

Time (ps)

silicon substrate

metallic grating

inputTHz pulse

d ~ 10 m

outputTHz pulse

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The Science of Information Technology