Wavelength Conversion and All- Optical Switching in …TiAu contact Mirror 8 repeats AlAs/Al0.3Ga0.7As ITQW Ambleside Sept 2007. Non-linear optics for new wavelengths *QCL cavities

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

Wavelength Conversion and All-Optical Switching in QCL’s.

Johannes Gambari, Charalambos Zervos, Mark Frogley and Chris Phillips Experimental Solid State Group,

Physics Dept., Imperial College London.

And

Dymtro Kundys, Luke Wilson John Cockburn and Maurice Skolnick, Sheffield University.


Outline

*Device Design.

*Materials Considerations.

*Frequency mixing experiments.

* THz laser results.

*All-optical switching.

*Concluding Remarks.


Device Design

*Standard (-ish) QCL with window in top contact

* Needs cladding layers transparent to bandgapradiation

* Needs DBR for reflective mode experiments.

0.25m grating Monochromator

PMT

Cry

osta

t

77k CMT photo-diode

Gated Photon Counting Electronics

Linear Polarizer

(b)

ωNIR ωQCL

Window in top contact

ωSB

(a)

Pulsed Current Driver

16.5° Ti

:Sap

phire

lase

r

-5 0 5 10 15 20

-500

01500

2000

2500

ωSideband

ωQCL

ωNIR|hh1>

|e3>

|e2>

|e1>

Ener

gy (m

eV)

Distance (nm)

Contact Layer 0.3 µm GaAs

Graded layer 1 µm Al0.9Ga0.1As

AR 36 periods GaAs/AlAs

Waveguiding layers1.45 µm Al0.2Ga0.8As

SiN dielectric

Incoming Radiation

Bulk GaAs

TiAu contact

Mirror 8 repeats AlAs/Al0.3Ga0.7As





SiN dielectric

Incoming Radiation

Bulk GaAs

TiAu contact



Materials Issues

*Lots of Al, but electrically stable.

* JTh indistinguishable from devices without holes or DBR’s

* DBR looks much like cladding to the QCL mode.

0 2 4 6 8 10012345678

Distance (microns)

Mod

e in

tens

ity0 1 25

1015202530

0

5

10

150 1 2 3 4 5 6 7

Mid

-IR o

utpu

t (a.

u.)

Volta

ge (V

)

Current (A)

Current Density (kA cm-2)

0 2 4 6 8 10012345678

Distance (microns)

Mod

e in

tens

ity





SiN dielectric

Incoming Radiation

Bulk GaAs

TiAu contact






SiN dielectric

Incoming Radiation

Bulk GaAs

TiAu contact



NonNon--linear optics for new linear optics for new wavelengthswavelengths

*QCL cavities have high *QCL cavities have high radiation density and the radiation density and the intrinsic intrinsic χχ(2) of (2) of llllll--VV’’s.s.

*Artificial resonances can be *Artificial resonances can be designeddesigned--in.in.

*Designable dispersion for *Designable dispersion for phase matching.phase matching.

N N OwschinikowOwschinikow et al. PRL, et al. PRL, 90 (4), 04390290 (4), 043902--1 (2003).1 (2003).

S S S S DhillonDhillon et al. APL 87, et al. APL 87, 071101 (2005).071101 (2005).

K L K L VodopyanovVodopyanov et al. APL et al. APL 72(21), 2654 (1998).72(21), 2654 (1998).

Polarisation

E

E Input at ω

P response at ω + 2ω


Earlier FEL Experiments.Earlier FEL Experiments.

Multilayer DBR

Asymetric CQW

THz polariser

CW Ti:sapphire NIR laser

0.75 mmonochro-mator

NIR polarisers

THz radiation from FEL(~1KW, 2µsec, 1 Hz )

12K cryostat

*Easy phase-matching.

*Polarisation insensitive.

*Broad bandwidth.

*telecomms λ’s


Sideband spectra.Sideband spectra.

0.25m grating Monochromator

PMT

Cry

osta

t

77k CMT photo-diode

Gated Photon Counting Electronics

Linear Polarizer

(b)

ωNIR ωQCL

Window in top contact

ωSB

(a)

Pulsed Current Driver

16.5°

Ti:S

apph

ire la

ser

*Gated measurement.

695 700 705 71002468

101214

ω NIR + ω QCL

Spectrometer'sResolution 2nm

Wavelength nm

Cou

nt ra

te [1

05 sec

-1]


THz lasersTHz lasers*∆λ/λ ~ 1%

*Closer to DWFM applications.

*Tunes over wide frequency range.

*Polarisation insensitive.


Sideband generation in THz lasersSideband generation in THz lasers*∆λ/λ ~ 1%

*Sideband has right polarisation dependence


728 730 732 734 736 738 740 7421x104

2x104

3x104

4x104

5x104

6x104

7x104

8x104

Ti saph 743 1Khz chopping QCL 10 sec accumulation time

Num

ber o

f pho

tons

(per

10

sec)

Wavelength

Difference (ON gate)*correction ratio- Off gate


Sideband tracks with incident Sideband tracks with incident wavelength.wavelength.

*∆λ/λ ~ 1%

*Sideband has right energy dependence


740 760 780 800 820730740750760770780790800810Position of Sideband feature with respect to incident Ti

SB W

avel

engt

h

Incident Ti wavelength

Predicted Observed


AllAll--optical switching.optical switching.*Seen only in 25% Al samples.

*QCL Jth lowers when Bandgap radiation is applied.

*100% modulation depth possible @ ~ 50% of Max QCL o/ppower.

1.6 1.8 2.0 2.2 2.4 2.6 2.80

2

4

6

8

0

50

100

NIR off

QCL

Out

put (

arb.

uni

ts)

Current (A)

NIR on

Mod

ulat

ion

Dept

h (%

)


Spectral Dependence.Spectral Dependence.*Follows same spectral sensitivity as Active region PLE

*QCL Jth lowers when Bandgap radiation is applied.

*Implies switching effect originates with photoexcitation in AR.

740 760 780 800 8200

1

2

3

4

0

2

4

6

8

10

λPLPl s

igna

l (ar

b. u

nits

)

C B A

Excitation wavelength (nm)

switc

hed

QCL

out

put (

arb.

uni

ts)


Switching dynamics.Switching dynamics.*Even with PRF down to 200Hz, switching intensity is independent of pulse timings (!)

* Applying 10 nsec / 120 mW asynchronous pulses, need 13 to switch device for PRF’s 10Hz-> 10kHz (!)

*Current pulse “resets” memory of NIR illumination.

*Re-timing function for telecomms.


Switching Mechanism?Switching Mechanism?*Switching fluence 6 x 10 10 photons, i.e ~ 2 x electrons flowing in the 100 nsec J pulse.

* Could be J enhancement, but how come the long memory?

*Of 0, 10, 20, 25% Al clad devices , only 25% ones switched.

*Everything was below ~120K anyway

Two possibilities


DX centres in DX centres in AlGaAsAlGaAs

CONFIGURATION COORDINATE

EL

EΓ

Eb

Ed

Ee

Eo

ED

X

QTQ0

UD

X

UL

UΓ

ELECTRO

NIC PLU

S ELASTIC ENER

GY

(x = 0.32)

* ~10 9 donors under window

* Could hold ~ ½% of switching charge

* Would be field ionised by J pulse, with long recapture time?

* Known to appear only at low T and x > 22%


Charge separation at Charge separation at heterojunctionheterojunction

* Barrier only present for X~ 20% in clad

* Needs “giant ambipolar”diffusion mechanism* to spread charge.

* Would work at high T and in InGaAs.

* Watch this space….

Active region

Wave guide

∆EC

e- motion

NIR

Wave guide

Active region

Wave guide

∆EC

e- motion

NIR

Wave guide

* See K H Gulden et al. PRL 66, 373 (1991).


Concluding remarks

* NL frequency mixing demonstrated

*Electronically modulatable two-terminal device for a variety of telecoms functions. (add-drop, packet switching, high bandwidth, data transparent conversion etc.)

* Principles transferable to telecomms wavelengths.

* That elusive “killer application” for QCL’s?

Wavelength Conversion and All- Optical Switching in …TiAu contact Mirror 8 repeats AlAs/Al0.3Ga0.7As ITQW Ambleside Sept 2007. Non-linear optics for new wavelengths *QCL cavities

Documents

Wavelength Conversion and All- Optical Switching in …TiAu contact Mirror 8 repeats AlAs/Al0.3Ga0.7As ITQW Ambleside Sept 2007. Non-linear optics for new wavelengths *QCL cavities