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1. Optoelectronics: the opportunity
- optoelectronics has come of age!
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 .
Professor Wilson Sibbett, University of St Andrews
2. 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
3. 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
4. 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!
5. 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!!
6. 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)!
7. 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
8. 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!
9. 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
10. Organic light emitting diodes (OLEDs)
These diagrams illustrate the basic OLED concepts.
11. Examples of some OLED displays
Sony ultra-thin 13” display
Kodak viewfinder
Epson widescreen display
12. Photo-dynamic therapy (PDT)
The ‘sensitised’ tumour
region is then exposed to
intense light from a
source such as a laser or
LED
Exposure to light induces
the PP9 to produce singlet
molecular oxygen that leads
to local cell kill within the
tumour
ALA* cream is applied to the
site of the skin tumour
(*5-aminolevulinic acid)
The ALA is metabolised to light-sensitive
PP9 predominantly within the tumour
13. Before After
A typical scar-free outcome from photo-
dynamic therapy or ‘PDT’ of a skin cancer
14. 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)
16. Skin cancer treated with OLED-based PDT
Effective treatment with reduced scarring and pain
17. Concept of spontaneous emission
Level 1
Energy = E1
Level 2
Energy = E2
• 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
18. Concept of stimulated emission
• 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
Excited Atom
Incident
Photon
Stimulated Transition Incident Photon
Emitted
Photon
19. • 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
A laser or ‘laser oscillator’
20. A semiconductor diode laser chip
200nm active
GaAs layer
3mm p-type GaAlAs
n-type GaAlAs
~200mm
• Cleaved or cleaved-and-coated facets act as the mirrors in a
diode laser
• This is the preferred source for optical communications
22. 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
23. 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.
Before After
24. 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
Before After!
25. 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
Continuous
Time
Intensity
26. 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
27. 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!
28. Multi-photon excitation for treatment of
cancer tumours (PDT)
Photogen Inc, Knoxville Tennessee & Massachusetts Eye & Ear Infirmary
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)
Prior to treatment Immediately following
treatment
2 months after
treatment
29. Snapshots in the millisecond regime
[Eadweard Muybridge –Galloping Horse, 1887]
30. 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
31. An example of ‘frozen motion’!
[Harold Edgerton, MIT, 1964]
32. Concept of prompt imaging
• An ultrashort laser pulse passing through a scattering medium
carries image information via three components as illustrated
Input
diffuse
snake-like ballistic
snake-like
ballistic
diffuse Output
33. 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
34. 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
Many applications in endoscopy and tele/data-communications
Optical fibre
37. Optoelectronic datacomms at 100Tb/s!
What data speed does this represent?
100 Tbits ~1.5x1012 words ~1.7 million x
works of Shakespeare -
in one second!
38. High-speed data transfer - DVDs
Other information media?
100 Tbits > 600 DVD
movies!!
in one second
40. 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
41. 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
42. Optoelectronics for peace – weapons
decommissioning!
• Femtosecond laser pulses cut pellets of high-explosive and metals
F Roeske Jr et al
Cut in HNS (LX-15) with
femtosecond laser pulses
Cut in PETN (LX-16) with
500ps laser pulses
KEY ADVANTAGES
- this process offers a high safety status
- there are no solid HE waste products
- this offers decommissioning opportunities!
43. 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!
