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Wavelength Conversion and All- Optical Switching in QCL's. - ITQW07

CW Ti:sapphire NIR laserEarlier FEL Experiments.THz radiation from FEL(~1KW, 2µsec, 1 Hz )THz polariserNIR polarisers12K cryostat0.75 mmonochromatorAsymetric CQWMultilayer DBR*Easy phase-matching.*Polarisationinsensitive.*Broad bandwidth.*telecomms λ’sITQW Ambleside Sept 2007.


Sideband spectra.PulsedCurrentDriverWindow in top contact16.5°ω SB*Gated measurement.Cryostat77k CMTphoto-diodeω QCLTi:Sapphire laser0.25m PMTgratingMonochromatorLinearPolarizerω NIR(a)GatedPhotonCountingElectronics(b)Count rate [10 5 sec -1 ]ω NIR+ ω QCL14121086420Spectrometer'sResolution 2nm695 700 705 710Wavelength nmITQW Ambleside Sept 2007.


Sideband generation in THz lasers*∆λ/λ ~ 1%*Sideband has rightpolarisationdependence*Tunes over widefrequency range.Number of photons (per 10 sec)Ti saph 743 1Khz chopping QCL 10 sec accumulation time8x10 47x10 46x10 45x10 44x10 43x10 42x10 4Difference (ON gate)*correction ratio- Off gate1x10 4728 730 732 734 736 738 740 742WavelengthITQW Ambleside Sept 2007.


All-optical switching.*Seen only in 25% Alsamples.*QCL J th lowers whenBandgap radiation isapplied.*100% modulationdepth possible @ ~50% of Max QCL o/ppower.QCL Output (arb. units)864201.6 1.8 2.0 2.2 2.4 2.6 2.8Current (A)NIR onNIR off100500Modulation Depth (%)ITQW Ambleside Sept 2007.


Switching dynamics.*Even with PRF down to 200Hz, switching intensity isindependent of pulse timings (!)* Applying 10 nsec / 120 mW asynchronous pulses, need 13 to switchdevice for PRF’s 10Hz-> 10kHz (!)*Current pulse “resets” memory of NIR illumination.*Re-timing function for telecomms.ITQW Ambleside Sept 2007.


Switching Mechanism?*Switching fluence 6 x 10 10 photons, i.e ~ 2 x electrons flowingin 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% onesswitched.*Everything was below ~120K anywayTwo possibilitiesITQW Ambleside Sept 2007.


DX centres in AlGaAs* ~10 9 donors underwindow* Could hold ~ ½% of switchingcharge* Would be field ionised by Jpulse, with long recapture time?* Known to appear only at low Tand x > 22%ELECTRONIC PLUS ELASTIC ENERGYE LE Γ(x =0.32)U ΓU LE bCONFIGURATIONCOORDINATEE dE eE oE DQ 0Q TU DXXITQW Ambleside Sept 2007.


Charge separation atheterojunction* Barrier only present forX~ 20% in cladWaveguideActiveregionWaveguide* Needs “giant ambipolar”diffusion mechanism* to spreadcharge.NIRe - motion∆E C* Would work at high T and inInGaAs.* Watch this space….* See K H Gulden et al. PRL 66, 373 (1991).ITQW Ambleside Sept 2007.


Concluding remarks* NL frequency mixing demonstrated*Electronically modulatable two-terminal device for a variety oftelecoms functions. (add-drop, packet switching, high bandwidth, datatransparent conversion etc.)* Principles transferable to telecomms wavelengths.* That elusive “killer application” for QCL’s?ITQW Ambleside Sept 2007.

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