Organic Semiconductor Lasers and Optical Amplifiers

G.A. Turnbull, Y. Yang, D. Amarasinghe, G. Tsiminis,A. Ruseckas and I.D.W. Samuel

Organic Semiconductor CentreSchool of Physics and Astronomy, University of St Andrews

www.st-and.ac.uk/~polyoptoidws@st-andrews.ac.uk

Organic Semiconductor Centre

►Light-emitting dendrimers for OLEDs

►Photophysics – fs to µs luminescence

►Exciton diffusion/solar cells

►Lasers and optical amplifiers

►A new light source for medicine

Outline: Part I

►Introduction – towards polymer photonics

►Review of laser physics

►Gain measurements

►Feedback structures

►Distributed feedback lasers

►Exercise (for you!)

Organic Semiconductors

►Conjugated molecules

►Novel semiconductors

►Easy to process

►Can tune properties

►Can emit light

►Flexible

OLED Displays

Sony ultra-thin 13” display

Kodak Viewfinder

Epson Widescreen Display

MED Ltd

Plastic Photonics

ActivePassive

Laser Physics

Light

Amplification byStimulatedEmission of Radiation

Generic LASER:optical amplification

with feedback

Excitation

Output

mirrors

lνpν

Laser Rate Equations

( )cav

ppN

L

cl

dt

dp

τσ −+= 1

Amplification Decay cLcav βτ /2=

L

lβ

υσ

τ h

INN

dt

dN −−Λ=

ExcitationSpontaneous

emissionStimulated emission

ΛΛΛΛN2

N1

ττττ

12 NNN −=

Steady-state Lasing Characteristics

ΛΛΛΛ

ΛΛΛΛ

N

p

► Increased spectral and spatial coherence ►Narrowing of emission spectrum►Directional output beam

ΛΛΛΛTH

►Population inversion (spontaneous emission) clamped at threshold level by stimulated emission

►Power rises linearly with pumping

Threshold and Slope Efficiency

►Gain exceeds loss

►Photon mode concept►106-1010 spatial and spectral photon modes►Above threshold all energy stimulated into one

mode (or at least very few modes)

Threshold:

( )THINRp

loout PPP −= η

νν

ββ

Efficiency:

Fraction of useful losses

Photon energies

Radiative efficiency

Pump rate above threshold

βσ >= Nl2gain

Pulsed Operation

► Dynamic solution of rate equations

► Gain medium has fluorescence lifetime τ to build population inversion

► If tΛ < τ -pump energy density important► If tΛ > τ -pump intensity important

ΛΛΛΛpN

ΝΝΝΝTH

► Pulse build-up time

► Time required for pulse to be macroscopically amplified

► 1 photon (= 3x10-19 J) to 1 nJ pulse - 100 dB gain required

► Build-up time must be less than excited state lifetime/pump duration

► Short cavities required

time

Pulsed Operation: A Femtosecond Polymer Laser

2D DFB laser with MEH-PPV as active medium

Pumped by 100 fs pulses

Pulse width decreases at high excitation density

Minimum pulse width of 700 fs

0 1 2 3 4 50.00

0.25

0.50

0.75

1.00

Nor

mal

ised

Inte

nsity

(ar

b)

Time (ps)

Response 2.8 µJcm-2

4.8 µJcm-2

6.2 µJcm-2

13 µJcm-2

Potential for Polymer Lasers +Amplifiers

► Absorption and emission separated - 4 level system

► Strong absorption ~105 cm-1

- Enormous gain

► Little concentration quenching

► Broad spectra - broad bandwidth

► Compatible with polymer fibretransmission windows (500–560 and ~660 nm),

► Scope for low cost manufacture

► Possibility of electrical pumping

3.2 3 2.8 2.6 2.4 2.2 2 1.8

C8H17H17C8

n

O

O

n

n

Pho

tolu

min

esce

nce

(arb

.)

Energy (eV)

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

O

O

n

Abs

orpt

ion

(105 /c

m)

Wavelength (nm)

Photolum

inescence (arb.)

Chemical Reviews 107 1272 (2007)

Organic Semiconductor Laser Materials

► Conjugated polymers

► solution processed

► wide range of materials

n

C8H17C8H17

O

O

n C6H13

n

C6H13

C10H21

C10H21

O

N

CNNC

AlO

N

O

N

O

N

► Dye doped Alq3

► co-evaporated

► conventional laser dyes

in energy transfer host

O

O O

O

► Dendrimers

► Spiro molecules

► Truxenes►Small solution processed

molecules

(4)

(1) (3)

(2)

Organic Laser Energy states

3.0 2.5 2.0 1.50.0

0.2

0.4

0.6

0.8

1.0

400 500 600 700 800

Abs

orba

nce

/P

hoto

lum

ines

cenc

e (a

rb.)

Energy (eV)

Wavelength (nm)

O

O

n

0S

1S

(4)

(3)

(2)

NS

NT

1T

Measuring Gain 1:Transient Absorption

►Excite sample with short laser pulse►Measure change in transmission with time-delayed probe►Gain (if present) will increase in transmission

SampleOptical delay

ChopperExcitation

Probe Detector

∆T/T

Probing Optical Gain (Stimulated Emission) in Materials

►Excite with short pulse to S1 state.

►Send a probe pulse resonant with PL.

► Probe is amplified by stimulated emission

S0

S1

∆T/T~ N1(σSE- σESA ) - Npσp

SE

ESA

Transient Absorption of PPV (1992)

With F. Raksi and R.H. Friend

Transient Absorption ~1997

► Snapshot of absorption ∆Ainduced by pump pulse

► Photo-induced absorption at low energies

► Gain (-ve absorption) mirrors PL spectrum

► Gain present for a few hundred picoseconds

-0.004

-0.002

0.000

0.002

1200 1000 800 600

t=200 fs

Laser

PL

∆A

Wavelength, nm

Photon energy, eV

Abs

1.0 1.5 2.0 2.5

n( )S

Data from Arvydas Ruseckas

Measuring Gain 2: Waveguide Amplifiers

Polymer film (100-400 nm) forms

optical waveguide

Pump λ absorbed in ~100 nm

Total internal reflection confines light

in polymer film

Signal amplified by 1000 times in 1

mm propagation through film

Amarasinghe et al., App Phys Lett 89, 201119; Adv Mat 21, 107 (2009)

Pump Input Probe

polymer

Silica

Amplified output

0.4 – 1 mm

0 50 100 1500

10

20

30

1022 µm

822 µm

422 µm

Gai

n (d

B)

Pump energy density (µJ/cm2)

ADS233YE F80.9BT0.1

Measuring Gain 3: ASE

► Excite stripe on film and observe emission from edge

Helitois et al., Appl Phys Lett 81, 415 (2002)

n

C8H17C8H17

► Above characteristic energy:

► Super-linear increase in output intensity

►Dramatic narrowing of spectrum

► Signatures of gain narrowing via amplified spontaneous emission

Increasing pump power

420 440 460 480 500 5200.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ised

inte

nsity

Wavelength (nm)

Measuring Gain 3: ASE

►Net gain and waveguide losses can be inferred from the SLN measurements

►Gain: output related to stripe length

►Loss: measured by varying length of unpumped region

( ) ( )( )[ ]1−∝ lgp e

g

II λ

λλ

l

( ) ( )zeI λαλ −∝

z

Ip

Measuring Gain 3: ASE

►Net gains of 10 cm -1, 21 cm-1 and 30 cm-1 (up to 130 dBcm-1) for different pump densities

►Loss coefficient: 3 cm-1

in unpumpedwaveguide

105

2 105

3 10 5

0 0.02 0.04 0.06 0.08 0.1

Experimental DataExponential Fit

Ou

tpu

t S

ign

al (

a.u

.)

Stripe Position (cm)

1000

10 4

10 5

0 0.5 1 1.5 2 2.5

E=3.3µJE=2.1µJE=1.3µJ

Ou

tpu

t S

ign

al a

t λA

SE (

a.u

.)

Excitation Length (mm)

Helitois et al., Appl Phys Lett 81, 415 (2002)

n

C8H17C8H17

Lasing in MEH-PPV Solution (1991-92)

Polymer Laser Resonator Geometries

► Solution processing- high quality waveguides

► Sub-micron structures- laser microresonators

► Strong absorption – transverse pumping possible

microspheremicroringmicrocavity

1-D DFB2-D DFB /

photonic bandgaprandom scatterers

Typical Experimental Configuration

Pumplaser

Variable grey filter

Lens

Polymer laser in vacuum chamber

CCD spectrograph

Energy meter/ beam profiler

Polymer Microcavity Lasers

► Resonant wavelengths:► Few spectral modes

► At high pump energies emission is stimulated preferentially into one spectral mode

Tessler et al., Nature 382, 695 (1996)

dm m 2=λ

Surface-emitting Distributed Feedback Lasers

Air

Polymer (100nm)

Silicaλ

2nd order scattering provides distributed feedback

1st order scattering provides surface output coupling

Air

Polymer (100nm)

Silicaλ

Pulsed pump laser

excitation

2-D distributed feedback

1µm1µm

e.g. Phys Rev B 67,165107 (2003); J Appl Phys 98, 023105 (2005)

4 µµµµm

Angle-Dependence of Bragg scattering

Λ= effnλ

Λ< effnλ Λ> effnλ

Λ

Photoluminescence dominated by Bragg-scattered emission from waveguide modes

Angle Dependent Photoluminescence

Photonic stopband evident at normal incidence

-10 -5 0 5 10

580

600

620

640

660

-10 -5 0 5 10Angle (deg)

660

640

620

600

580

Wav

elen

gth

(nm

)

540 560 580 600 620 640 660 680 7000.0

0.2

0.4

0.6

0.8

1.0

10o

5o

0o

UntexturedMEH:PPV (x 5)

Inte

nsity

(ar

b.)

Wavelength (nm)

O

O

n

AirMEH-PPVSilica

φ

θ

Distributed Feedback Lasers

Air

Polymer

Silica

Λ

► Wavelength scale corrugations cause Bragg scattering

► Counter-propagating waveguide modes couple together at λB

Λ= eff2nm Bλ

Questions – For You to AnswerYou wish to make an MEH-PPV DFB laser operating at 640 nm. The refractive index of the waveguide mode is 1.6.

(a) Write down the Bragg condition for the lasing modes(b) For the first two diffracted orders

(i) Calculate the grating period required(ii) Determine whether the laser is surface or edge-emittingWhat are the advantages and disadvantages of each configuration?

(c) What would the emitted beam look like for(i) A one-dimensional grating?(ii) A two-dimensional grating?

1-D and 2-D DFB Laser Structures

1-D 2-D

Emission Pattern of 1-D and 2-D DFB Lasers

1-D 2-D

1-D versus 2-D feedback

n

C8H17C8H17

Blue lasing in PFO2-D mode confinement leads to lower threshold lasing and improved slope efficiency

G Heliotis et al., Adv Fun Mat 14, 91 (2004)

Outline: Part II

►Tuning polymer lasers, simple fabrication►Reducing size of pump – towards practical lasers►Indirect electrical pumping►Sensing explosives►Polymer optical amplifiers►Optical switching of lasers and amplifiers

Tuning a Polymer Laser by Microstructure

Array of 15 grating periods from 350 to 420 nm

95 nm grating depth, unity mark to space ratio

420 415 410 350355360405 365

3 mm

5µm

5µm

0.2µm

Atomic force microscope image of grating

Tuning of Laser Emission

• Laser wavelength tuned by translating the substrate

• Stopband wavelength changes linearly with grating period

• Laser emission on band edge ~4nm longer in wavelength

• 20 nm tuning range using a single substrate (1 mm translation)

390 395 400 405 410 415 420605

610

615

620

625

630

635

Grating Period (nm)W

avel

engt

h (n

m)

Laser EmissionBragg DipTheory

Elastomeric mould

Simple Fabrication of Polymer Nanostructures► Hot embossing► Solvent assisted micro-moulding► UV-nanoimprint lithography

Appl Phys Lett 81, 1955 (2002); Appl Phys Lett 82, 4023 (2003);

Polymer film

Solvent

Elastomeric mould

120

seco

nds

AFM image of 400 nm period, 90 nm deep eggbox corrugation

Shrinking Polymer Laser systems

1995 Regenerative amplifier(Tessler)

~2000 Q-switched Nd:YAG

2003 Microchip laser2006 Diode

pumped2008 LED pumped

Yang et al., App Phys Lett 92, 163306 (2008)

► Pulsed microchip pump laser

► 1 ns pulses at 532 or 355 nm

► Up to 100 nJ to 1 µJ pulse energy

► 5 kHz rep rate

Compact Polymer Lasers

Materials Today 7, 28 (2004)Chemical Reviews 107 1272 (2007)

Diode laser pumped polymer lasers► GaN diode laser

► λ = 407 nm► Epulse = 0.7 nJ

► tpulse ~ 1.2 nsecEMISSIONEMISSION

20 µm cavity

C-102 / MEH-PPV (120nm thick)

Silica

GaN diode

EMISSIONEMISSION

20 µm cavity

C-102 / MEH-PPV (120nm thick)

Silica

GaN diode

620 630 6400

2

4

30 µµµµm

outp

ut in

tens

ity (

a.u.

)

wavelength (nm)

1.25 x threshold 1 x threshold 0.8 xthreshold

DBR microcavity with a few resonant modes

Above threshold (8µJ/cm2), lasing peak grows rapidly

Single lasing peak dominates spectrum at higher pump powers

See also: Riedl et al. Appl. Phys. Lett. (2006)Karnutsch et al. Phot Tech Lett (2007)

Vasdekis et. al., Opt. Express 14, 9211 (2006)

Electrically Pumped Organic Lasers?

Main Challenges

► High projected threshold current density (> 100s A/cm2)

► Low carrier mobilities

► Losses from metal contacts

(10X to 100X intrinsic waveguide loss)

► Polaron and triplet absorption losseshttp://en.wikipedia.org/wiki/Laser

Chemical Reviews 107 1272 (2007); Nature Photonics 3, 546 (2009)

Why?

Novel, tuneable, simple to fabricate, (flexible) lasers

Without need for expensive (and bulky) separate pump laser

Alternative: Indirect Electrical PumpingTry a Nitride LED as the Pump

www.lumileds.com

Advantages

► Exploits high mobility of InGaN

► Separates charge injection from gain medium

► Much lower cost than laser pumped system

► Very compact, electrically controlled

All the benefits from direct electrical pumping

www.modding-faq.de/

Challenges

► Incoherent light source hard to focus

► CW intensities < 30 W/cm2

0 20 40 60 80 100 120 140 1600

50

100

150

200

250

Pea

k in

tens

ity (W

/cm

2 )

Peak drive current (A)

Lumiled K2 emitter cw spec: (475mW @ 1A at 455 nm)

Under ns pulsed operation:

► 47 ns pulses

► Up to 255 W/cm2 (275 nJ) for 160 A drive current (8 kAcm-2)

High Power Pulsed Nitride LEDs

www.lumileds.com

-50 0 50 100 150

0.0

0.5

1.0

1.5

2.0

Drive current (normalised and offset )

LED emission (normalised)

Time (ns)

Yang et al., App Phys Lett 92, 163306 (2008);

Polymer Gain Medium

500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

Ligh

t Em

issi

on (n

orm

.)

Wavelength (nm)

PL ASE

0.0 0.5 1.0 1.50.1

1

Nor

mal

ised

PL

Time (ns)

PLQY: 80%

lifetime: 3.4 ns

Requirement for gain medium

► Absorption matches LED emission

► Long exciton lifetime

Yang et al., App Phys Lett 92, 163306 (2008);

300 350 400 450 5000.0

0.5

1.0

1.5

2.0

2.5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Abs

orba

nce

Wavelength (nm)

LE

D e

mis

sion

(nor

m.)

ADS 233YEF80.9BT0.1

►1D Distributed feedback resonator►355 nm period grating► In-plane feedback and surface

output coupling

Polymer DFB Lasers

Air

ADS233YE (350 nm)

Silica

560 570 580 5900

1

2

3

4

wavelength (nm)

0 5 10 15 20 25 30 35 400

1

2

3

0 50 100 150 200 250 300

outp

ut a

t 568

nm

(arb

.)

pump energy (nJ)

Pump intensity (W/cm 2)

►Pumped with 450 nm from 4 ns OPO

►DFB lasing in TE0 and TM0

transverse modes

Low Threshold Polymer DFB Lasers

►Dependence on pump beam area

►Threshold intensity falls with increasing excitation area

►Saturates for areas above ~1 mm2

0.1 1 100

2

4

6

8

10

Thre

shol

d in

tens

ity (

kW/c

m2 )

Pump beam area (mm 2)

0 10 20 300

1

2

3

4

5

6

70 50 100 150 200

Out

put (

arb.

)

Pump energy (nJ)

Pump intensity (W/cm 2)

►Minimum threshold ~200 W/cm2

►Threshold reduced to 160 W/cm2

by double-passing pump light550 560 570 580

0

1

2

3

568 nmPumped by OPO at 450 nm

Wavelength (nm)

LED Pumped Polymer Lasers

Light emitting area: 1.5 mm by 1.5 mm

InGaN LED

Laser Output

Pulsed current source

CYTOPADS233YE

Silica

Pump filter

InGaN LED

Laser Output

Pulsed current source

CYTOPADS233YE

Silica

Pump filter

Yang et al., App Phys Lett 92, 163306 (2008); Lupton, Nature (N&V) 453, 459 (2008)

LED Pumped Polymer Laser

0 25 50 75 100 125 1500

100

200

300

400

Ligh

t em

issi

on (

arb.

)

Peak drive current (A)

568 nm (TE0 mode)

555 nm (TM0 mode)

Onset of lasing around 150A

► Sharp laser peak at 568 nm

► Change in output slope

550 560 570 580

0

100

200

300

400

500

600

700

800

900

L

ight

em

issi

on (

arb.

)

160 A

156 A

152 A

148 A144 A120 A

80 A

40 A

Wavelength (nm)

Yang et al., App Phys Lett 92, 163306 (2008); Lupton, Nature (N&V) 453, 459 (2008)

HYPIX

Hybrid organic semiconductor-GaN-CMOS smart pixel arrays

(St Andrews, Strathclyde, Edinburgh, Imperial College)

GA Turnbull, IDW Samuel

µLEDarray

CMOS device

2 mm

B.R. Rae et al J. Phys. D 41 (2008) H.X. Zhang et al Opt. Exp.16 (2008) APL 92, 163306 (2008);

Optical Concentrators for Organic Lasers

Aim: increase power density from LED

► Use luminescent optical concentrators as used in solar panels

► Miniaturised version designed for

concentrating LED power density

Yang et al., Adv Mat 21, 3205 (2009)

Pump beam

Dye doped polymer

Silica

CY

TO

PM

EH

-PP

V

Sili

caLa

ser

outp

ut

► 5-fold reduction in lasing threshold for optimised concentrators

Fluorescent explosives detection

►Many common explosives include TNT, DNT etc

►Nitroaromatic compounds are strong electron acceptors

► Introduction of nitroaromatic molecule leads to dissociation of exciton and quenches emission

S0

S1

S0

S1

n

C8H17C8H17

Swager et al., Chem Rev 107, 1339 (2007) Richardson et al., APL 95 063305 (2009)

Sensitivity enhancement with polymers

►Reversable binding of nitroaromatic molecule with fluorophore

►Only fluorescence from the bound molecules is quenched

Toal et al., J Mater Chem, 16, 2871

►Conjugated polymer- series of connected fluorescing sites

►One nitroaromatic molecule can quench emission from whole chain

Toal et al., J Mater Chem, 16, 2871

Fluorescence sensing with polyfluorene

►Polyfluorene film exposed to ~10ppb dinitrobenzene vapour in air

►15% drop in fluorescence

►Fluorescence recovers to original value when purged in nitrogen

400 450 500 550 6000.0

0.2

0.4

0.6

0.8

1.0

Flu

ores

cenc

e

wavelength (nm)

before exposure after 2min exposure

n

C8H17C8H17

PFO

DNB

Polyfluorene Laser Explosives Sensor

► Polyfluorene DFB laser exposed to ~10ppb DNB vapour in air

► 85% drop in emission

► Factor 3 change in slope efficiency

► Lasing recovers to original performance when purged in N2

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e ou

tput

pow

er (

arb.

)

Pump energy (uJ)

Laser output before exposure after 20 mins exposure after recovery

0 5 10 15 20 25 300.4

0.6

0.8

1.0

Fra

ctio

nal d

rop

in

em

issi

on

Pump energy (uJ)

0 10 20 30 400.0

0.2

0.4

0.6

0.8

1.0

Purge

ExposureExposure

Lase

r int

ensi

ty (n

orm

alis

ed)

time (min)

Yang et al., Adv. Func. Mater. (2010)

Some Future Applications

► Integrated tunable lasers sources for spectroscopy

► Intrinsic chemical sensing

► Optical amplifiers/modulators

A. Rose et al., Nature 434, 876 (2005)

0 50 100 150

0

10

20

30

40

Inte

nsity

(ar

b)Time (ps)

control off control on 2

400 nm

580 nm

800 nm800 nm

Amarasinghe et al., APL 92, 083305 (2008)

Advanced Polymer Photonics:Optical Amplifiers

Pump Signal

IN Signal OUT

Grating coupled

Need for low cost amplifiers e.g compensate splitting losses

Compatibility with polymer optical fibre, planar lightwavecircuits

Scope for very broad amplification bandwidth

Solution Polymer Optical Amplifier

Nitrogen Laser337 nm (0.5 ns)

TuneableOscillator 580–630 nm

PolymerOptical Amplifier

Amplify a weak light pulse to a strong one

First demonstrated in solution

Solution Polymer Amplifier: High Gain, Broadband

APL 80, 3036 (2002)and 85, 6122 (2004)

540 560 580 600 620 6400

10

20

30

40

50

PPV derivativeSt Andrews

Gai

n (d

B)

Wavelength (nm)

F8BTImperialSt Andrews

Gain over 30 dB in 1 cmBandwidth >50 nmor 50 THz

Solid State Polymer Amplifiers

560 580 600 620 640 660 6800

5

10

15

20

Red-FImperialSt AndrewsMEH-PPV

St Andrews

F8BT St Andrews

Gai

n(dB

)

Wavelength (nm)

Pump Signal

INSignal OUT

Grating coupled

Gain up to 21 db: signal amplified 100 times

Wide wavelength range

A Polymer Optical Amplifier Using F8BT : Amplification of a Pulse Sequence with 70 ps Spacing

Gain characteristics of multiple pulsesσ=6x10-17 cm2

0 20 40 60 80 100 120 140 1600

100

200

300

400

P o

w e

r (

a r

b .

u n

i t s

)Time(ps)

Three pulses in a 140 ps window. Amplification > 20 dB. Repetition rate 5 kHz. Pulse shape not affected.

Pump Signal

IN Signal OUT

Grating coupled

Input

Push pulse

1

2

PL

5

3

4

500 nm

800 nm

Optical Switching of Gain

Outputfrom amplifier

Optical Switching of Gain in a Co-polymer

-3 -2 -1 0 1 2 3 40.0

0.5

1.0

τ2=1 ps (a

2=0.14)

τ1=0.05 ps (a

1=0.86)

Push

∆T/T

(ar

b un

its)

Time (ps)

Full recovery in 2 ps.

Amarasinghe et al. Adv. Mater (2009)

Optical Switching of Copolymer Amplifier

0 50 100

0

10

20

30

0 50 100

0

10

20

30

Inte

nsity

(a.

u.)

Time(ps)

Amplified Push blocked- amplified again

Input

Switched off by push pulse

APL 92, 083305 (2008).

Organic Semiconductor Lasers and Optical Amplifiers: Conclusion

►Organic Semiconductor Lasers:►Compact, tuneable visible lasers►Simple fabrication►Direct pumping by InGaN LED demonstrated

►Polymer Optical Amplifiers►Strong gain >20 dB/mm over broad bandwidth►Potential compatibility with POF

►Optical Switching►All-optical switching of gain and lasing demonstrated►Promising building blocks for polymer photonics

See Chemical Reviews April 2007; Nature Photonics Oct 2009

Further Reading

►Samuel and Turnbull Chem. Rev. 2007

►Yang,Turnbull, Samuel APL 2008►Lupton Nature 2008

►Samuel, Namdas, Turnbull, Nature Photonics 2009

►Yang et al Adv. Func. Mater. 2010