Developments in the field of quantum optics have led to an increased demand for
better sources of entangled photons. Recently discovered ferroelectric nematic liq-
uid crystals (FNLCs) have emerged as promising materials for generating entangled
photon pairs via spontaneous parametric down-conversion (SPDC). Their polar or-
dering leads to second-order nonlinear optical phenomena, such as SPDC. To observe
and detect photon pairs, we assembled a custom experimental setup. With it, we
can adjust the polarization of the pump beam incident on the sample. The resulting
photon pairs are collimated, and only those with a certain polarization are transmit-
ted to the detectors. From the time delay between individual detected events, we
can extract the SPDC signal. Measuring various polarization states enables us to
perform polarization tomography, providing information on the overall polarization
state of the generated photon pairs. Additionally, the setup includes a camera for
microscopy and a part for measuring second harmonic generation (SHG).
The performance and suitability of the setup for measuring the SPDC signal
was verified using a lithium niobate crystal. As the measurement results were con-
sistent with the results from literature, we proceeded with measurements on liquid
crystals. The ratio between the flux of generated photon pairs and the background
was inversely proportional to the pump beam power, which is in accordance with
theoretical predictions. We determined that the coherence length is approximately
three times smaller than the expected theoretical value.
Since the purpose of SPDC is typically to generate entangled photon pairs, I
focused in the second part of my thesis on how the photon entanglement depends
on the properties of the liquid crystal cell and the polarization of the pump beam.
With the simulation developed by Kavčič and Sultanov [1], I examined the theo-
retical dependence of entanglement on the thickness of the liquid crystal cell and
the polarization of the pump beam. The model predicted that entanglement close
to 1 could be achieved only with very thin cells. Consequently, measurements were
conducted at the smallest thicknesses of our wedge-shaped liquid crystal cell, which
was at approximately 1.8 μm. The entanglement of the photon pair was controlled
by the polarization of the pump beam. Among other results, we generated photon
pairs with low entanglement C = 0.12 ± 0.2 and also photon pairs with very high
entanglement C = 0.98 ± 0.07. To the best of our knowledge, we were the first in
Slovenia to have succeeded in creating entangled photons.
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