ITQW ITQW Ambleside Ambleside 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.
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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
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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 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
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
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 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
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
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
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 mmonochro-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
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]
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 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
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 820730740750760770780790800810Position of Sideband feature with respect to incident Ti
SB W
avel
engt
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/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 (%
)
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 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)
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
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%
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 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).
ITQW ITQW AmblesideAmbleside Sept 2007.Sept 2007.
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.