Phys649: Physics ofPhys649: Physics ofOptoelectronic DevicesOptoelectronic Devices

Alexey Belyanin

Room 426 MISTOffice: 845-7785Cell: 324-3071Email: belyanin@tamu.edu

Office hours: any time when I am in the office

Textbook:

E. Rosencher, B. Vinter, Optoelectronics, 2002

• Concise presentation of main sub-disciplines• Emphasis on the key unifying concepts• Utilitarian approach: useful technique as opposed to hard-nosed theory • We won’t cover all chapters (e.g. quantum mechanics) but you can use them as a reference

Policy

• Attendance is required (20% of the grade)– Questions, discussion are encouraged;– I may adjust the contents of the course as we go

• Homework assignments every week (50%)• Short presentations in the end of the

semester on a topic of your choice (30%)

Optoelectronics:• Studies electronic devices that interact with light• Interact = emit, absorb (detect), modulate, switch• Light = any EM radiation• “Electronic” usually means “made of semiconductors”.

Emerging technologies: plasmonics, graphene, carbon nanotubes, molecular devices, …

Interplay between quantum electronics, nonlinear optics, physics of semiconductors, and transport phenomena

It is important not to get lost and to see coherence and connections between different subjects

• Basic principles and device physics

• Examples of state of the art devices

• Challenges and outlook for the future

Plan of study for each kind of devices:

Main devices: - semiconductor lasers and detectors, - nonlinear optical systems - novel devices (carbon-based, plasmonic)

Integrated photonics, nanodevices, quantum optical systems (cryptography, communications, …)

Always with applications in mind, but emphasis on physics perspective

• Stone Age

• Bronze Age

• Iron Age

• Ice Age

What is Information Age?

Physics laid down foundation for the Information Age

Information Age

The cost of the transmission, storage and processing of data has been decreasing extremely fast

Information is available anytime, any place, and for everyone

Information and knowledge became a capital asset

All of this became possible because of several revolutionary ideas

Laser and semiconductor laser

Transistor

Computer

World-Wide Web

Optical fibers

… Are invented by physicists

Integrated circuits

History of the WWW

History of the WWW

• First proposal: Tim Berners-Lee (CERN) in 1989

• 1991: First WWW system released by CERN to physics community; first Web server in the US (SLAC)

• 1993: University of Illinois releases user-friendly Mozaic server • Currently: WWW is one of the most popular Internet applications; >100 million users in the US alone

Invention of Computer

• The first digital electronic computer was invented by Theoretical Physics Prof. John Vincent Atanasoff in 1937. It was built by Atanasoff and his graduate student Clifford Berry at Iowa State College in 1939 ($650 research grant).

Basement of the Physics Dept. buildingwhere the Atanasoff-Berry Computer

(ABC) was built.

ABC•Used base-two numbers (the binary system) - all other experimental systems at the time used base-ten •Used electricity and electronics as it's principal media •Used condensers for memory and used a regenerative process to avoid lapses that could occur from leakage of power •Computed by direct logical action rather than by the enumeration methods used in analog calculators

Implemented principles of modern computersOnly material base has been changed.

ABC Replica

The drum – the only surviving fragment of ABC. It holds 30 numbers of 50 bits each. They are operated on in parallel. It is the first use of the idea we now call "DRAM" -- use of capacitors to store 0s and 1s, refreshing their state periodically.

Card punch and reader

Berry with the ABC

From ABC to ENIAC• 1940s: J. Mauchly and J. Eckert build ENIAC (Electronic Numerical Integrator And Computer). All basic concepts and principles of ENIAC are “borrowed” from Atanasoff’s papers.

• 1972: U.S. Court voids the Honeywell’s patent on the computing principles and ENIAC, saying that it had been “derived” from Atanasoff’s invention.

• 1990: Atanasoff receives the U.S. National Medal of Technology. He dies in 1995 at the age of 91.

From ENIAC to …

ENIAC (1946) weighed 30 tons, occupied 1800 square feet and had 19,000 vacuum tubes.It could make 5000 additions per second

Computers in the future may weigh no more than 1.5 tons. (Popular Mechanics, 1949)

1940's - IBM Chairman Thomas Watson predicts that "there is a world market for maybe five computers".

1950's - There are 10 computers in the U.S. in 1951. The first commercial magnetic hard-disk drive and the first microchip are introduced. Transistors are first used in radios.

1960's-70's - K. Olson, president, chairman and founder of DEC, maintains that "there is no reason why anyone would want a computer in their home." The first microprocessor, 'floppy' disks, and personal computers are all introduced. Integrated circuits are used in watches.

Before diodes and transistors: vacuum tubes

1954-1963: SAGE Air Defense Project

• 23 32-bit computers • Each contains 55,000 vacuum tubes, weighs 250 tons, and consumes 3 Megawatt• Tracks 300 flights• Total cost: $60 billion (double the price of Manhattan Project!)• Performance equivalent to $8 calculator built on transistors!

Impact of semiconductor electronics

Moore’s Law (1965): every 1.5 years the number of transistors on a chip is doubled

Smaller, Denser, Cheaper

Transistor of single-atom size by 2020?

• Limit on the transistor size

• Limit on the manufacturing technology

Pushing Fundamental Limits:Challenges and Bottlenecks

Semiconductors: how small the device can be? Memory and data storage: limits on writing density?

Communications: limits on data rate?

Telecommunications

Shannon-Nyquist Theorem

In a communication channel with bandwidth B, the data rate (number of bits per second) can never exceed 2B

Number of channels = Total bandwidth of the medium/B

Samuel Morse's telegraph key, 1844. Today's information age began with the telegraph. It was the first instrument to transform information into electrical signal and transmit it reliably over long distances.

Alexander Graham Bell’s commercial telephone from 1877.

How it all started …

In 1880 patented a “Photophone” (air-based optical telephone)

Alexander Bell, Helen Keller, and Anne Sullivan, 1894

Speaking into the handset's transmitter or microphone makes its diaphragm vibrate. This varies the electric current, causing the receiver's diaphragm to vibrate. This duplicates the original sound.

• Telephone connection requires a dedicated wire line;• Only one communication is possible at a time

How many channels are possible? How many signals can be transmitted at the same time??

Radio: communication through radio waves

1895

Alexander PopovGuglielmo Marconi

www.nrao.edu

Frequency measured in Hertz1 Hz = 1 cycle/second1 kHz = 1000 cycles/second

Radio stations have to broadcast at different carrier frequencies to avoid cross-talk

Range of frequencies (Bandwidth) needs to be at least 20 kHz for each station

Human ear: 10 Hz-20 kHz

Frequencies of different stations should be at least 20 kHz apart

Even if you transmit only voice, from 0 to 2000 kHz you can squeeze only 2000 kHz/20 kHz = 100 different “talks”.

What if you want to download data?

About 100 bands from 0 to 2000 kHz

Digital transmission: any signal, but transmission speed is still limited by bandwidth!

Binary code is transmitted: “0s” and “1s” – bits of information

Want download speed of 2 Mb/sec? Need bandwidth at least 1 MHz

Mega = million, Giga = billion; 1GHz = 1000 MHz = 109 Hz

Want 100 Mb/sec? need 50 MHz bandwidth just for yourself

Modern cell phones and GPS use gigahertz (GHz) frequenciesBut this is only 1000 MHz/50 MHz = 20 channels at 100 Mb/sec!

Higher carrier frequencies

Wider bandwidth

Higher data rate

Faster, faster, faster

Using optical frequencies?! 100 THz !

What kind of medium can carry optical frequencies?

Air? Only within line of sight; High absorption and scattering

Optical waveguides are necessary!

Copper coaxial cable? High absorption, narrow bandwidth 300 MHz

Glass? Window glass absorbs 90% of light after 1 m.Only 1% transmission after 2 meters.

Extra-purity silica glass?!

Lo

ss p

er k

m, d

B

Wavelength, nm

Maximum tolerable loss

Transmisson 95.5% of power after 1 km P = P(0) (0.995)N after N kmP = 0.01 P(0) after 100 km: need amplifiers and repeaters

Total bandwidth ~ 100 THz!!

Loss in silica glassesWhat is dB? Increase by 3 dB corresponds to doubling of power

How to confine light with transparent material??

Total internal reflection!

n > n’

Dielectric waveguides

n > n’

Optical fiber!1970: Corning Corp. and Bell Labs

Optical fibers

Made by drawing molten glass from a crucible

1965: Kao and Hockham proposed fibers for broadband communication

1970s: commercial methods of producing low-loss fibers by Corning and AT&T.

1990: single-mode fiber, capacity 622 Mbit/s

Now: capacity ~ 1Tbit/s, data rate 10 Gbit/s

Fibers open the flood gate

Bandwidth 100 THz would allow 100 million channels with 2Mbits/sec download speed!

Each person in the U.S. could have his own carrier frequency, e.g., 185,674,991,235,657 Hz.

However, we are using less than 1% of available bandwidth!

And maximum transmission speed is less than 0.00001 of bandwidth

In optical communications, information is transmitted as light signal along optical fibers

However, if we want to modify, add/drop, split, or amplify signal, it needs to be first converted to electric current, and then converted back to photons

This is a slow process (maximum 10 GHz)

Limitations of fiber communications

All-optical switches

Micro-Electro-Mechanical Systems (MEMS)256 micro-mirrors (Lucent 2000)

Futuristic silicon chip with monolithically integrated photonic and electronic circuits

THE DREAM: could we replace electrons with photons, and electric circuits with all-optical circuits?

IBM website

wires waveguides

Possible solutions: photonic crystals, plasmons, etc.

Note T-intersections and tight bends, as in electric wires.You cannot achieve it in usual waveguides!

800x600, 1024x768, 1280x720, and 1920x1080 (HDTV) matrices of microscopic mirrorsOne mirror is only 16 microns!Less than 1/5 of human hair width

Digital Light Processing (DLP)Texas Instruments 1987

http://www.dlp.com/includes/demo_flash.aspx

mini laser projector

Laser mini-projectors

Need microscopic lasers of all colors: red, blue, and green

Fiber communications need mini lasers emitting in the near-infrared

Smallest lasers are semiconductor lasers!

Excellent near infrared and red lasers

Bad blue lasers

Very bad green lasers

Number of transistors grows, but this does not improve the performance as much.

Reason?

We use 21-century semiconductor devices and 19-century copper wires connecting them!

Problem of Interconnects

Electronic circuits: 45 nm wires, 1 million transistors per mm2

Computing speed is limited by inertia of electrons

The interconnect bottleneck• 109 devices per chip• Closely spaced metal wires slow down computation• Huge heat generated due to electric resistance

Can electronic circuits be replaced by photonic ones?!

Using photons as bits of information instead of electrons would speed up the computing

Photons travel much faster and don’t dissipate as much power

Optical computer is still a dream. Even wilder dream is a quantum computer.

However, large size of a photon would make computers 1000 times larger!

Quantum computer

Bit Qubit (quantum bit): state of a single electron or atom

Classical computer: 0 or 1Quantum computer: 0 or 1 or any mixture of 0 and 1

Quantum computer can perform millions of calculations in parallel.

Especially good for searching through huge amount of data or encoding/decoding secret messages

Extremely fragile, needs low temperature

Conclusions

Microelectronics is approaching its fundamental limits. Revolutionary ideas are needed!

- Single-molecule transistors?- new materials, new fabrication technology?

Communication: how to increase speed? - Novel lasers?- All-optical network?- How to make photons smaller?

New principles of computing??