1. 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 .

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. 2007 marks a century of optoelectronics
HENRY J. ROUND was a key figure in the histroy of optoelectronics. He was:
‘One of Marconi’s 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!

5. Oleg Vladimirovich LOSEV – the short life of a genius
We must acknowledge the early work of pioneer, Dr Oleg Losev ( )
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!

6. 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!!

7. 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)!

8. 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

9. Recent advances in LEDs for domestic lighting
By way of background:
Incandescent lights are not efficient and have a so-called luminous efficacy of 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!

10. 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

11. Organic light emitting diodes (OLEDs)
These diagrams illustrate the basic OLED concepts.

12. Examples of some OLED displays
Sony ultra-thin 13” display
Kodak viewfinder
Epson widescreen display

13. Photo-dynamic therapy (PDT)
The ALA is metabolised to light-sensitive PP9 predominantly within the tumour
ALA* cream is applied to the site of the skin tumour (*5-aminolevulinic acid)
Show OLED demonstrator here.
Skin cancer statisitcs
Explain bad points of bulky light sources.
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

14. A typical scar-free outcome from photo-dynamic therapy or ‘PDT’ of a skin cancer
Before
After

15. 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. Typical device application cycle
Device applied
Disposal
Device worn during normal daily activities

17. Skin cancer treated with OLED-based PDT
Effective treatment with reduced scarring and pain

18. 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
Level 2
Energy = E2
Level 1
Energy = E1

19. Concept of stimulated emission
Excited Atom
Stimulated Transition
Incident Photon
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

20. 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

21. 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

22. 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

24. 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

25. 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

26. 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

27. 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

28. 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!

29. Multi-photon excitation for treatment of cancer tumours (PDT)
For example: Melanoma on skin in mice
Prior to treatment
Immediately following
treatment
2 months after
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

30. Snapshots in the millisecond regime [Eadweard Muybridge –Galloping Horse, 1887]

31. 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

32. An example of ‘frozen motion’! [Harold Edgerton, MIT, 1964]

33. 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
Output

34. 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

35. 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

36. Optical fibres

37. Optoelectronic communications

38. Optoelectronic datacomms at 100Tb/s!
What data speed does this represent?
~1.7 million x
works of Shakespeare -
in one second!
~1.5x1012 words
100 Tbits

39. High-speed data transfer - DVDs
Other information media?
100 Tbits
> 600 DVD movies!!
in one second

40. An application in biology involves the poration of cells to provide access to ‘low penetration’ drugs
Laser
pulses
White light
Sample
Dichroic
mirror
CCD
camera
Shutter

41. 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

42. 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

43. Optoelectronics for peace – weapons decommissioning!
Femtosecond laser pulses cut pellets of high-explosive and metals
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!
F Roeske Jr et al

44. 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!