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    Discussion about THz sources for H̄ deexcitation

    → Europe/Zurich
    H* weekly meeting
    Zoom Meeting ID
    61669489432
    Description
    H* meeting room
    Host
    Chloe Malbrunot
    Passcode
    18506744
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    Join via phone
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    Participants: Jean-Francois, Daniel, Tim, Fredrik, Maxime, Chloé

    1) slides by Jean Francois
    to get the power needed with a lamp in the 200GHz-1THz range we would need  330 000K equiv temperature.
    As expected lamps are not a solution for 200  GHz to 1 THz band
    But higher frequency reachable with the 5kK Hg lamp and 1kK Globar using filters to allow more discrete spectrum (avoid direct ionization of higher n states)

    Filters:
    Bandpass of few % to 10% is not too difficult to get
    Easier to combine high pass and low pass than broadband bandpass 
    few 100 GHz : resonant filter made from etched metal films
     

    A solution for higher the ~200 GHz - 1THz region: 
    Photoswitch

    Typical values for femtosecond lasers
    time structure: 100MHz rate -> THz pulse separated by 10ns
    frequency: 100 MHz comb structure --> continuous structure seen by the atoms

    femtosecond rates can be lower or higher
    - 1GHz possible, probably similar energy per tooth (to be verified) but less teeth

    standard dipoles are not OK for us (integrated power ~1.5uW)

    Large aperture THz emitters

    Ti:Saph laser on GaAs -> 180uW (probably close to limit of destruction)
    380mW input

    Erbium laser: InGaAs:Rhodium
    Kohlhaas et al. APL 2020 (group in Berlin) -> JF has a collaboration with them on other doping 

    28mW input at 550  -> 640uW (limited by input laser)
    spectral density unknown

    Proposal by JF to initiate a collaboration with them and measure the absolute spectral power density 
    Measurement of interest also in view of comparing photo switch to other THz sources

    Possibility to use a 200 mW input at 1.5um at 80MHz (and also higher rep. rate: 1GHz)

    measurements of absolute power changes with repetition rate can also be interesting: thermal powers are different
    ------------

    photomixer: 
    1) continuum source: Amplified continuous emission from Erbium
    -->less powerful compared to a pulsed source

    ------------

    Photodiode could be an option to generate a continuum but less efficient at high frequencies than photoswitch
    ---------

    2) Fredrik
    filters: low pass to remove visible, UV, IR & low cost: black polyethylene
    --> very transparent in ~300 GHz range. At ~2THz only 50% transmission. Above 5% it is absorbing
    very  efficient in mid-infrared 
    40um films 

    -> JF will send us some black polyethylene sheets 

    Grid bandpass filters used to be bought at Thorlabs (expensive and very fragile) -> they stopped

    Metalized etched Silicon: made by the startup at IEMN (Vmicro)
    other potential suppliers: TIDEX,  Virgina diodes - VDI (passive components), QMC  Limited (UK) - mostly doing bolometers (maybe mostly low pass filters)

    THz power sensor
    Maybe calibration done at in mid IR -> to check 

    thermopile (drift of the zero), calorimeter, pyroelectric (more sensitive) -> better for our application (using not too low chopper frequency)
    SLT pyro (chop a 20 Hz)

    Be careful about sensor diameter
    ask if the absorber is efficient in the THz range
    not flat ->only SLT  is flat

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