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 have

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.

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

Outline

*Device Design.

*Materials Considerations.

*Frequency mixing experiments.

* THz laser results.

*All-optical switching.

*Concluding Remarks.

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

Device Design

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

* Needs cladding layers transparent to bandgap radiation

* Needs DBR for reflective mode experiments.

0.25m grating Monochromator

PMT

C ry

os ta

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 ap

ph ire

la se

r

-5 0 5 10 15 20

-500

0 1500

2000

2500

ωSideband

ωQCL ωNIR

|hh1>

|e3>

|e2>

|e1>

En er

gy (m

eV )

Distance (nm)

Contact Layer 0.3 µm GaAs

Graded layer 1 µm Al0.9Ga0.1As

AR 36 periods GaAs/AlAs

Waveguiding layers 1.45 µm Al0.2Ga0.8As

SiN dielectric

Incoming Radiation

Bulk GaAs

TiAu contact

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

Contact Layer 0.3 µm GaAs

Graded layer 1 µm Al0.9Ga0.1As

AR 36 periods GaAs/AlAs

Waveguiding layers 1.45 µm Al0.2Ga0.8As

SiN dielectric

Incoming Radiation

Bulk GaAs

TiAu contact

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

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

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 10 0 1 2 3 4 5 6 7 8

Distance (microns)

M od

e in

te ns

ity0 1 2 5

10 15 20 25 30

0

5

10

15 0 1 2 3 4 5 6 7

M id

-IR o

ut pu

t ( a.

u. )

Vo lta

ge (V

)

Current (A)

Current Density (kA cm-2)

0 2 4 6 8 10 0 1 2 3 4 5 6 7 8

Distance (microns)

M od

e in

te ns

ity

Contact Layer 0.3 µm GaAs

Graded layer 1 µm Al0.9Ga0.1As

AR 36 periods GaAs/AlAs

Waveguiding layers 1.45 µm Al0.2Ga0.8As

SiN dielectric

Incoming Radiation

Bulk GaAs

TiAu contact

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

Contact Layer 0.3 µm GaAs

Graded layer 1 µm Al0.9Ga0.1As

AR 36 periods GaAs/AlAs

Waveguiding layers 1.45 µm Al0.2Ga0.8As

SiN dielectric

Incoming Radiation

Bulk GaAs

TiAu contact

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

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

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

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

Earlier FEL Experiments.Earlier FEL Experiments.

Multilayer DBR

Asymetric CQW

THz polariser

CW Ti:sapphire NIR laser

0.75 m monochro- mator

NIR polarisers

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

12K cryostat

*Easy phase-matching.

*Polarisation insensitive.

*Broad bandwidth.

*telecomms λ’s

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

Sideband spectra.Sideband spectra.

0.25m grating Monochromator

PMT

C ry

os ta

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

ap ph

ire la

se r

*Gated measurement.

695 700 705 710 0 2 4 6 8

10 12 14

ω NIR + ω QCL

Spectrometer's Resolution 2nm

Wavelength nm

C ou

nt ra

te [1

05 s

ec -1 ]

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

THz lasersTHz lasers *∆λ/λ ~ 1%

*Closer to DWFM applications.

*Tunes over wide frequency range.

*Polarisation insensitive.

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

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

*Sideband has right polarisation dependence

*Tunes over wide frequency range.

728 730 732 734 736 738 740 742 1x104 2x104 3x104 4x104 5x104 6x104 7x104 8x104

Ti saph 743 1Khz chopping QCL 10 sec accumulation time

N um

be r o

f p ho

to ns

(p er

1 0

se c)

Wavelength

Difference (ON gate)*correction ratio- Off gate

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

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

*∆λ/λ ~ 1%

*Sideband has right energy dependence

*Tunes over wide frequency range.

740 760 780 800 820 730 740 750 760 770 780 790 800 810 Position of Sideband feature with respect to incident Ti

SB W

av el

en gt

h

Incident Ti wavelength

Predicted Observed

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

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/p power.

1.6 1.8 2.0 2.2 2.4 2.6 2.8 0

2

4

6

8

0

50

100

NIR off

Q CL

O ut

pu t (

ar b.

u ni

ts )

Current (A)

NIR on

M od

ul at

io n

De pt

h (%

)

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

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.

740 760 780 800 820 0

1

2

3

4

0

2

4

6

8

10

λPLP l s

ig na

l ( ar

b. u

ni ts

)

C B A

Excitation wavelength (nm)

sw itc

he d

Q CL

o ut

pu t (

ar b.

u ni

ts )

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

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.

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

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

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

DX centres in DX centres in AlGaAsAlGaAs

CONFIGURATION COORDINATE

EL EΓ

Eb

Ed

Ee

Eo

ED X

QTQ0

UD X

UL

UΓ

ELECTR O

N IC PLU

S ELASTIC EN ER

G Y

(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%

ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.

Charge separation at Charge separation at heterojunctionheterojunction

* Barrier only present for X~ 20% in clad

* Needs “giant ambipolar” diffusion mech