Optoelectronics:%20the%20opportunity%20-%20optoelectronics%20has%20come%20of%20age!

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Optoelectronics the opportunity-
optoelectronics has come of age!
Professor Wilson Sibbett, University of St Andrews
This perspective is reproduced from a
presentation given at an inauguration
mini-symposium on the Optoelectronics College
held in November 2007 at the Ballathie House
Hotel .
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Introductory remarks

• Electronic devices are all around us but what
  about devices that exploit optoelectronics?
• Everyday optoelectronic technologies range from
  flat-screen displays (TVs, computers, mobile
  phones ) through the checkout bar-scanners to
  internet communications links
• A growing number of healthcare-related
  implementations of optoelectronics are beginning
  to emerge in biology and medicine
• In Scotland, we have notable research strengths
  in optoelectronics and efforts are being made to
  translate these into more widespread and
  practical applications

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Basis of this overview

• Let us start with an historical perspective on
  optoelectronics
• Then, consider semiconductor devices as the
  bridge between electronics and optoelectronics
• Starting with LEDs we proceed to lasers
• We can consider the translation of science to
  technology
• We can look at a few representative applications
  of optoelectronics
• All of this has implications for the teaching of
  optoelectronics

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2007 marks a century of optoelectronics

• HENRY J. ROUND was a key figure in the histroy of
  optoelectronics. He was
• One of Marconis Assistants in England and
  later the Chief of Marconi Research he
  published a 24-line note in Electrical World
  reporting a bright glow from a carborundum
  diode.
• Round, H. J., Electrical World, 49, 308 (1907)
  
• Was Henry Round the discoverer of the LED? Maybe
  not but most definitely he can be credited with
  the discovery of electroluminescence!
• In any case, 1907 can be pinpointed as the year
  of birth for optical electronics or
  optoelectronics!

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Oleg Vladimirovich LOSEV the short life of a
genius

• We must acknowledge the early work of pioneer, Dr
  Oleg Losev (1903-1942)
• He was the son of a Russian Imperial Army Officer
  but the politics of the day denied him any career
  path in Bolshevik Russia!
• Sadly, he died of hunger at the age of 39 during
  the blockade of Leningrad!

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Oleg Losev the discoverer of the LED?

• He was remarkable as self-educated scientist.
  His PhD degree was awarded in 1938 by the Ioffe
  Institute (Leningrad) without a formal thesis
  because he had published 43 journal papers and 16
  patents.
• Working in a besieged Leningrad (1941), he
  discovered that a 3-terminal semiconductor device
  could be constructed to have characteristics
  similar to those of a triode valve but
  circumstances prevented publication! Losev had
  probably invented the TRANSISTOR!
• Mid-1920s Oleg Losev observed light emission
  from electrically-biased zinc oxide and silicon
  carbide crystal rectifier diodes Light Emitting
  Diodes or LEDs!
• Called the inverse photo-electric effect, Losev
  worked out the theory of LED operation and
  studied the emission spectra and even observed
  spectral narrowing at high drive currents
  evidence perhaps of the stimulated emission that
  applies to lasers?!
• Notably, the first significant blue LED
  re-invented in the 1990s used silicon carbide!!

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Semiconductor LEDs and lasers

• LEDs are now commonplace in games consoles,
  remote controls, vehicle lights, traffic lights
  and, increasingly, in domestic lighting
• By the end of this decade, the market value is
  predicted to reach 15B!
• Semiconductor lasers the LED process is at the
  core of this effect and laser action was first
  reported in 1962 by four US research groups (2 at
  GE, IBM, MIT)
• The are many everyday applications of
  semiconductor lasers in barcode readers, CD DVD
  players, optical-carrier sources for
  communications and internet data
• NB The optical frequency for the optimum
  telecommunications wavelength (1500nm) is
  extremely high - equivalent to 200 THz (i.e.
  200,000,000,000,000Hz)!

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Major areas of commercial growth in the
optoelectronics marketplace

• Flat-panel displays recorded sales are up 30
  year on year currently, 8 growth in Europe
  USA and 9 in Japan
• Solid-state vehicle lighting much more than just
  brake lights!
• Solid-state domestic lighting replacement of
  incandescent lighting with LED-based sources
  would reduce CO2 emissions by many millions of
  tonnes worldwide!
• Power generation solar cell technologies are
  progressing steadily for example, in Germany a
  new power station based on solar cells is
  producing 5MW to power up 1800 households

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Recent advances in LEDs for domestic lighting

• By way of background
• Incandescent lights are not efficient and have a
  so-called luminous efficacy of 13-14 lumens/Watt
  (L/W)
• Halogen lighting is a little more efficient at
  17L/W
• Fluorescent lights are significantly better with
  typical luminous efficacies of 60-70L/W
• More recently
• White LEDs have achieved 100L/W and, in the
  laboratory, figures up to 300L/W have been
  reported for tailored warm-white LED lighting!

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Organic semiconductors

• We can now have organic materials that have
  exploitable semiconducting characteristics. These
  feature
• Conjugated molecules
• Novel types of semiconductors
• Easy processing schemes
• LED compatibility
• Physical flexibility

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Organic light emitting diodes (OLEDs)

• These diagrams illustrate the basic OLED
  concepts.

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Examples of some OLED displays
Sony ultra-thin 13 display
Kodak viewfinder
Epson widescreen display
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Photo-dynamic therapy (PDT)
ALA cream is applied to the site of the skin
tumour (5-aminolevulinic acid)
Exposure to light induces the PP9 to produce
singlet molecular oxygen that leads to local cell
kill within the tumour
The sensitised tumour region is then exposed
to intense light from a source such as a laser or
LED
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A typical scar-free outcome from photo-dynamic
therapy or PDT of a skin cancer
Before
After
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Potential of OLEDs for PDT

• OLEDs have the advantages of
• Uniform illumination
• Light weight so can be worn
• Relatively low intensity for longer treatment
• So reduced pain, increased effectiveness
• Low cost - disposable
• Attractive for hygiene
• Widens access to PDT
• A simple wearable power supply
• Ambulatory treatment1
• At work
• At home

1. See for example, Moseley et al,
Brit.Jour.Derm., 154, 747 (2006)
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Typical device application cycle
Device applied
Disposal
Device worn during normal daily activities
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Skin cancer treated with OLED-based PDT
Effective treatment with reduced scarring and pain
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Concept of spontaneous emission

• Consider an excited atom
• This excited atom will relax over some
  characteristic relaxation time
• If photons are produced during the relaxation
  process this is called spontaneous emission
• This emission process is independent of external
  influences

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Concept of stimulated emission
Excited Atom
Incident Photon
Stimulated Transition
Incident Photon
Emitted Photon

• An excited atom can be stimulated to emit a
  photon
• This process is called stimulated emission
• The stimulated photon is an exact copy of the
  photon that induced the transition
• A repeat of this process leads to the optical
  gain which represents the basis of laser action

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A laser or laser oscillator

• Stimulated emission provides optical gain
• Photons reflected by the resonator mirrors cause
  an avalanche of stimulated emission along the
  axis of the resonator
• A high intensity beam of light thus builds up in
  the laser resonator
• A collimated and directional laser beam emerges
  from a partially transmitting exit mirror

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A semiconductor diode laser chip
200mm
3mm
p-type GaAlAs
200nm active GaAs layer
n-type GaAlAs

• Cleaved or cleaved-and-coated facets act as the
  mirrors in a diode laser
• This is the preferred source for optical
  communications

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Absorption of light by major tissue chromophores
23
Illumination of a hand and wrist area with light
in 700nm, 800nm, 900nm spectral regions
illustrates clearly the deeper penetration at the
longer wavelengths into the biological tissue
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Treatment of varicose veins

• The laser used produces green pulses of light for
  strong absorption in blood but having durations
  matched to the tissue thermal relaxation time.

After
Before
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Skin resurfacing using lasers

• Laser skin resurfacing is becoming the method of
  choice
• preferable to chemical peels or dermabrasion
• A pulsed carbon dioxide laser is used

After!
Before
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We can now consider digital optoelectronics

• Lasers can be made to produce either
• - constant
  intensity beams, or
• - sequences of
  discrete optical pulses or optical digits

Pulsed
Intensity
Continuous
Time
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Why might we wish to use optical digits?

• The laser pulses or optical digits can have
  very high peak intensity
• Thus, these light impulses can induce either
  single- photon or rather more interesting
  multiple-photon interactions
• The advantage is strong near-infrared absorption
  (in tissue) with interactions involving two or
  three photons that are equivalent to green or
  blue/uv light
• The average power or heating effect can be at a
  modest level to avoid tissue damage
• Ultrashort pulses picoseconds (10-12s) and
  femtoseconds (10-15s) also imply short exposure
  times and so we have ultrafast (or snapshot)
  photography

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An example of a multiple-photon excitation

• This multi-photon excitation is localised both in
  space and in time
• - interactions occur primarily at the
  beam focus for the ultrashort light pulses
• - penetration of long-wavelength light
  but interaction may involve 2,3 photons!

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Multi-photon excitation for treatment of cancer
tumours (PDT)
For example Melanoma on skin in mice
The laser pulses are in the near-infrared
(1047nm) but 3-photon absorption is exploited for
the photo-dynamic therapy (PDT)
Photogen Inc, Knoxville Tennessee Massachusetts
Eye Ear Infirmary
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Snapshots in the millisecond regimeEadweard
Muybridge Galloping Horse, 1887
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Flash photography with microsecond exposures

• The motion can be effectivelt frozen using
  short pulses of light
• - e.g., using 1 microsecond flashes from a xenon
  flashbulb

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An example of frozen motion! Harold Edgerton,
MIT, 1964
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Concept of prompt imaging

• An ultrashort laser pulse passing through a
  scattering medium carries image information via
  three components as illustrated

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Seeing through raw chicken!
Photograph of two crossed metal needles (0.5mm
diameter)
The needles viewed through a 6mm slab of raw
chicken breast in ordinary illumination
Snapshot image of the needles using femtosecond
illuminating and gating pulses
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Laser beam propagation in optical fibres
many-km-lengths of glass!

• Intensity
• either continuous or pulsed
• Focusability
• efficient coupling propagation of laser beams
  in optical fibres

Optical fibre
Many applications in endoscopy and
tele/data-communications
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Optical fibres
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Optoelectronic communications
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Optoelectronic datacomms at 100Tb/s!

• What data speed does this represent?

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High-speed data transfer - DVDs

Other information media?
gt 600 DVD movies!!
in one second
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An application in biology involves the poration
of cells to provide access to low penetration
drugs
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Corrective eye surgery using laser pulses

• Schematic of a laser-pulse produced flap
• laser pulses focused 160µm below the tissue
  surface to produce micro-cavitations
• subsequent micro-machined cut to provide hinged
  flap

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Femtosecond laser-based eye surgery

• Femtosecond-laser-based Keratomileusis procedure
• Laser pulses are focused and scanned to outline
  with micron precision a lens-shaped block of
  corneal stroma or lenticule
• This lenticule is then removed and the corneal
  flap replaced

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Optoelectronics for peace weapons
decommissioning!

• Femtosecond laser pulses cut pellets of
  high-explosive and metals

Cut in PETN (LX-16) with 500ps laser pulses
Cut in HNS (LX-15) with femtosecond laser pulses
KEY ADVANTAGES - this process offers a high
safety status - there are no solid HE waste
products - this offers decommissioning
opportunities!
F Roeske Jr et al
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Concluding remarks

• Optoelectronic devices have come of age and have
  opened up a wide range of exciting possibilities
  both within science and in the products used in
  everyday life
• These are re-defining many of the boundaries of
  modern life and technology
• Some knowledge of optoelectronics is vital for
  all of us living in the 21st century
• It follows, therefore, that the teaching of some
  practical skills in optoelectronics should now
  form an exciting part of a modern science
  curriculum and education!