An Optoelectronics Perspective, Wilson Sibbett

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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 .
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
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
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!
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!
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!!
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)!
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
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!
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
Organic light emitting diodes (OLEDs)
These diagrams illustrate the basic OLED concepts.
Examples of some OLED displays
Sony ultra-thin 13” display
Kodak viewfinder
Epson widescreen display
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 ALA is metabolised to light-sensitive
PP9 predominantly within the tumour
The ‘sensitised’ tumour
region is then exposed to
intense light from a
source such as a laser or
LED
A typical scar-free outcome from photodynamic therapy or ‘PDT’ of a skin cancer
Before
After
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)
Typical device application cycle
Device applied
Disposal
Device worn during
normal daily activities
Skin cancer treated with OLED-based PDT
Effective treatment with reduced scarring and pain
Concept of spontaneous emission
Level 2
Energy = E2
Level 1
Energy = E1
•
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
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
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
A semiconductor diode laser chip
3mm
~200mm
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
Absorption of light by major tissue
chromophores
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
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
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!
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”
Intensity
Pulsed
Continuous
Time
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
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!
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
treatment
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
Snapshots in the millisecond regime
[Eadweard Muybridge –Galloping Horse, 1887]
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
An example of ‘frozen motion’!
[Harold Edgerton, MIT, 1964]
Concept of prompt imaging
• An ultrashort laser pulse passing through a scattering medium
carries image information via three components as illustrated
Input
diffuse
Output
diffuse
ballistic
snake-like
snake-like
ballistic
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
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
Optical fibres
Optoelectronic communications
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!
High-speed data transfer - DVDs
Other information media?
100 Tbits
in one second
> 600 DVD
movies!!
An application in biology involves the poration of
cells to provide access to ‘low penetration’ drugs
White light
Sample
Dichroic
mirror
Shutter
CCD
camera
Laser
pulses
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
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
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
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!
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