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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 have

Mar 15, 2020

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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 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

    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

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