Optical properties of organic NLO chromophores for terahertz applications : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physics at Massey University, Palmerston North, New Zealand
This thesis was motivated by the potential of organic nonlinear optical (NLO) zwitterionic
chromophores to be used as active components in THz emitters and detectors.
This study presents the results from measurements of the electro-optic (EO)
coefficient, photostability and the THz properties of organic NLO chromophores.
An EO coefficient of over 700 pm/V was measured for some EO polymers using
the Teng-Man modulation ellipsometry technique at low frequencies. A roll off
in the value of the EO coefficient was observed for frequencies over 10 kHz and
converging to the theoretically estimated value of ∼ 2 pm/V. The large over estimate
of the EO coefficient at low frequencies using the Teng-Man method is attributed
to contributions from the cavity resonance and interference effects.
The EO coefficient was also measured using a new method based on the electric
field induced modulation of diffraction gratings. The measured EO coefficient using
this technique is an order of magnitude lower than that measured using the Teng-
Man method but it is an order of magnitude higher that the theoretically expected
value. By investigating DC and AC measurements it was possible to deduce that
the large apparent EO coefficient is due to the inverse piezoelectric effect and thin
film interference. The large EO coefficient at low frequencies may also have a contribution
from dipole clusters. The small EO coefficient at high frequency shows that
the chromophores may not be particularly useful for THz applications. However,
they have potential in low frequency applications such as modulators, and dense
wavelength division multiplexing.
The current photodegradation models are shown to be inadequate and the photodegradation
can be modeled by considering a distribution in the photodegradation
quantum efficiencies. The addition of a singlet oxygen quencher has been found to
enhance the photostability of some chromophores by up to 6 times. Structural modifications
also affect the photostability. A correlation between the photostability
and bond length alternation (BLA) was found where there is a trade-off between
the chromophores molecular NLO effect and photostability.
Calculations based on the THz dielectric properties showed that the EO polymers
have a reasonable coherence length for THz generation using optical rectification.
Low wavenumber Raman spectroscopy measurements made on recrystallised compounds
exposed some theoretically predicted modes that were not been reported
found experimentally. Low temperature and low energy Raman spectroscopy measurements
made on some selected organic compounds showed a strong temperature
dependence of the low energy vibrational modes. The temperature dependent redshift
and line broadening have been primarily attributed to thermal expansion and
a distribution in the distance between monomers.