Chapter 4.1 Applications of SHG

Question 1

Auto correlator NLO involved

Answer 1

SHG in BBo crystal - blocked
SFG in BBO - detected (due to momemntum cons i.e. phase matching, goes stright, hence Backgroiund Free AC )

Question 2

DEF
autocorrfunction

Answer 2

A(T) = int (-infty,infty) [I(t) I(t-T)] dt

Require time dep intensity deistruution

Question 3

What is the goal of autocorrelator

Answer 3

Measure length of fs pulse with slow PMT elctronics
-split pulse into 2
- delay one
-focus into BBO crys
- block SHG allow SFG only at overlap

Question 4

SFG relation to how much is generated vs what comes in

Answer 4

dA_2 proportional to E(t) * E(t-Tau) dz

incident E filed in BBO crystal is
Etot(t) = E(t) +E(t-Tau)
A2 prop E(t)E(t-Tau) L
I_(SFG) prop to I(T)I(t-T)

Question 5

What does detector easure in autocorr

Answer 5

Slow detector meausre total Energy of pulse proportioanl to A(tau)

Question 6

data process for autocorrelator

Answer 6

Use th measured autocorreclation function from slow detector
- fit it to autocorr of Gaussian pulse
-use FWHM to extract delta t (pulse width in time)

Question 7

NB things to note in auto corr

Answer 7

ensure that stage movement introduces small enough delay time T such that you can map out the 35fs pulse
(1um ok prod 3fs diff)

Question 8

link symmetry fo crystal fo X gen

Answer 8

oppositeways
Centrosymmetric - > X1 (no x2)
Noncentrosymm -> gives X2
(think of SiO breaking symm giving non zero X2)

Question 9

setup for Surface SHG to analyse crystal surfaces

Answer 9

Fs pulse in
polariser 1 to select P of laser
sample at 45*deg to beam, rotated through phi
(crystal sample typically has X2 i.e. NOT centrosym)
output w and 2w
polariser 2 to select 2w polarisation
w1 filtered out
q2 detected plot as function of phi

Question 10

results from surface SHG

Answer 10

Plot I (shg) as a func of phi ofr diff polarisations
#lobes show #fold symm in crystal lattice
p-p s-p, p-s s-s relative sizes tell how X dep on diff orientation of crystal