Optoelectronics: the opportunity - optoelectronics has come of age!

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. Electronic devices are all around usbut what aboutdevices that exploit ‘optoelectronics’? Everyday optoelectronic technologies range fromflat-screen displays (TVs, computers, mobile phones …)through the checkout bar-scanners to internetcommunications 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 Introductoryremarks

3. 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 Basis of this overview

4. HENRY J. ROUNDwas 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 electronicsoroptoelectronics! 2007 marks a century of optoelectronics

5. 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!

6. 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 theTRANSISTOR! Mid-1920s:OlegLosev 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!! Oleg Losev – the discoverer of the LED?

7. 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)! Semiconductor LEDs and lasers

8. 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 Major areas of commercial growth in the optoelectronics marketplace

9. 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! Recent advances in LEDs for domestic lighting

10. 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 Organic semiconductors

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

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

13. The ALA is metabolised to light-sensitive PP9 predominantly within the tumour 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

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

15. 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 Potential of OLEDs for PDT 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. Level 2 Energy = E2 Level 1 Energy = E1 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

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

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 laserchip ~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. The laser used produces green pulses of light for strong absorption in blood but having durations matched to the tissue thermal relaxation time. Treatment of varicose veins After Before

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. Lasers can be made to produce either: - constant intensity beams, or - sequences of discrete optical pulses or “optical digits” We can now consider “digital optoelectronics” Pulsed Intensity Continuous Time

27. 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 Why might we wish to use optical digits?

28. 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! An example of a multiple-photon excitation

29. Prior to treatment Immediately following treatment 2 months after treatment 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

30. Snapshots in themillisecondregime[Eadweard Muybridge –Galloping Horse, 1887]

31. The motion can be effectivelt ‘frozen’ using short pulses of light - e.g., using 1 microsecond flashes from a xenon flashbulb Flash photographywithmicrosecond exposures

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

33. An ultrashort laser pulse passing through a scattering medium carries image information via three components as illustrated Input Output diffuse diffuse ballistic snake-like ballistic snake-like Concept ofprompt imaging

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. Intensity either continuous or pulsed Focusability efficient coupling & propagation of laser beams in optical fibres Laser beam propagation in optical fibres – many-km-lengths of glass! Optical fibre Many applications in endoscopy and tele/data-communications

36. Optical fibres

37. Optoelectronic communications

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

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

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

41. 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 Corrective eye surgery using laser pulses

42. 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 Femtosecond laser-based eye surgery

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

44. 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! Concluding remarks