Chapter 4.2: Applications of SFG Flashcards
Whats the point of four wave SFG
Generate:
tunable freq
narrow bandwidth
in VUV region (100-200nm)
Why cant we use crystals in VUV gen and consquence of using other
Crystals absorb strongly in VUV (even if u indeed found one that had a X2 or X3)
need to use metal vapours or noble gasses
-isotropic
centrosymmetric -> lowest NL order is X3
energy diagram fo X3
X3(w3,w1,w1,w2)
2 photons of w1 and 1 photon of w2 annihilated
2w1 is on resonance
Origin on 4 wave SF mixing name
4 waves - 4photons interacting (2w1+w2 = w3)
SFG between w1 w2
2 photon resonance of 2w1
Exp challenge in 4 wave 2 photon resonance
To get X3 high for the desired SFG (2w1+w2) and low for the underired ones (2w2+w1) (2w2-w1), etc
3 main ways of exhancing only desired X3 and suppressing other X3
1) 2 photon mixing
2) incidnet laser polarisation (from coupled WE, prevents 3w1 resonance)
3) phase matching
Why choose a 2 photon rather than a 1 phtoon resonsance
1 photon res (here of w1 or w3) much more strongly attenuates I3 (or I1) than a 2 photon absorption due to resonance
When choose 2w1 2 photon resonance we can stil have other esonacne what are theya dn how can their X3 be reduced
3w1 and 2w1-w2 will also be resonant with the 2w1 resonance,
the 3w1 X3 is reduced by the incidentt circular polarisation of laser beam,
Explain how polarisation fo incident laser beams results in prohibition of 3w1 phton abs wigner eckart
Angular momentum selection rules
mu terms in deno are actually expectation vals of QM WF, iwth each mu(wi) haveing a delta mi acc wigner Ect thm
dm1+dm1+dm2 +dm3 = 0
where dm is the change in teh amgnetic quantum number associated with the electric dipole operator mu(wi)
what is allowed with wigner eckart
A photon cannot carry an angular momentum great than 1 so dm = +-1 only. since we had by wigner eckart from the mu’s
dm1+dm1+dm2+dm3=0
we can set dm1 as +1 (circ polarised input w1 beam)
to statify wigner eckart we must have dm2=dm3=-1 to balance out. so input w1 and w2 beams are polarised in opposite directions
X3(w3,w1,w1,w1) would have
dm1+dm1+dm1+dm3 = 0; but that required dm3=-3 which isnt allowed so this 3w1 third harm gen is prohibited
