Presentation on theme: "Optoelectronics: the opportunity - optoelectronics has come of age!"— Presentation transcript:
1 Optoelectronics: the opportunity - optoelectronics has come of age! Professor Wilson Sibbett, University of St AndrewsThis 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 remarksElectronic 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 linksA growing number of healthcare-related implementations of optoelectronics are beginning to emerge in biology and medicineIn 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 overviewLet us start with an historical perspective on optoelectronicsThen, consider semiconductor devices as the bridge between electronics and optoelectronicsStarting with LEDs we proceed to lasersWe can consider the translation of science to technologyWe can look at a few representative applications of optoelectronicsAll 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 lightingBy 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 dataNB: 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 JapanSolid-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/WFluorescent lights are significantly better with typical luminous efficacies of 60-70L/WMore 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 moleculesNovel types of semiconductorsEasy processing schemesLED compatibilityPhysical flexibility
11 Organic light emitting diodes (OLEDs) These diagrams illustrate the basic OLED concepts.
12 Examples of some OLED displays Sony ultra-thin 13” displayKodak viewfinderEpson widescreen display
13 Photo-dynamic therapy (PDT) The ALA is metabolised to light-sensitive PP9 predominantly within the tumourALA* cream is applied to the site of the skin tumour (*5-aminolevulinic acid)Show OLED demonstrator here.Skin cancer statisitcsExplain 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 tumourThe ‘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 BeforeAfter
15 Potential of OLEDs for PDT OLEDs have the advantages of:Uniform illuminationLight weight – so can be wornRelatively low intensity for longer treatmentSo reduced pain, increased effectivenessLow cost - disposableAttractive for hygieneWidens access to PDTA simple wearable power supplyAmbulatory treatment1At workAt home1. See for example, Moseley et al, Brit.Jour.Derm., 154, 747 (2006)
16 Typical device application cycle Device appliedDisposalDevice 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’ atomThis excited atom will relax over some characteristic relaxation timeIf photons are produced during the relaxation process this is called spontaneous emissionThis emission process is independent of external influencesLevel 2Energy = E2Level 1Energy = E1
19 Concept of stimulated emission Excited AtomStimulated TransitionIncident PhotonIncidentPhotonEmittedPhotonAn excited atom can be stimulated to emit a photonThis process is called stimulated emissionThe stimulated photon is an exact copy of the photon that induced the transitionA 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 gainPhotons reflected by the resonator mirrors cause an avalanche of stimulated emission along the axis of the resonatorA high intensity beam of light thus builds up in the laser resonatorA collimated and directional laser beam emerges from a partially transmitting exit mirror
21 A semiconductor diode laser chip ~200mm3mmp-type GaAlAs200nm active GaAs layern-type GaAlAsCleaved or cleaved-and-coated facets act as the mirrors in a diode laserThis 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.BeforeAfter
25 Skin resurfacing using lasers Laser skin resurfacing is becoming the method of choicepreferable to chemical peels or dermabrasionA pulsed carbon dioxide laser is usedAfter!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”PulsedIntensityContinuousTime
27 Why might we wish to use optical digits? The laser pulses or ‘optical digits’ can have very high peak intensityThus, these light ‘impulses” can induce either single- photon or rather more interesting multiple-photon interactionsThe advantage is strong near-infrared absorption (in tissue) with interactions involving two or three photons that are equivalent to green or blue/uv lightThe average power or heating effect can be at a modest level to avoid tissue damageUltrashort 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 micePrior to treatmentImmediately followingtreatment2 months afterThe 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 illustratedInputdiffusesnake-likeballisticOutput
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 illuminatingand gating pulses
35 Laser beam propagation in optical fibres – many-km-lengths of glass! Intensityeither continuous or pulsedFocusabilityefficient coupling & propagation of laser beams in optical fibresOptical fibreMany applications in endoscopy and tele/data-communications
38 Optoelectronic datacomms at 100Tb/s! What data speed does this represent?~1.7 million xworks of Shakespeare -in one second!~1.5x1012 words100 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’ drugsLaserpulsesWhite lightSampleDichroicmirrorCCDcameraShutter
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-cavitationssubsequent micro-machined cut to provide hinged flap
42 Femtosecond laser-based eye surgery Femtosecond-laser-based Keratomileusis procedureLaser pulses are focused and scanned to outline with micron precision a lens-shaped block of corneal stroma or lenticuleThis lenticule is then removed and the corneal flap replaced
43 Optoelectronics for peace – weapons decommissioning! Femtosecond laser pulses cut pellets of high-explosive and metalsCut in HNS (LX-15) withfemtosecond laser pulsesCut in PETN (LX-16) with500ps laser pulsesKEY 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 remarksOptoelectronic 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 lifeThese are re-defining many of the boundaries of modern life and technologySome knowledge of optoelectronics is vital for all of us living in the 21st centuryIt follows, therefore, that the teaching of some practical skills in optoelectronics should now form an ‘exciting’ part of a modern science curriculum and education!