Thesis Title: Pockels effect based on-chip Electro-optic modulators using ferroelectric materials
Name of the Student   : Ms. Anupama T Vasudevan
Degree Registered      : Ph.D. Engineering 
Advisor : Prof. Shankar Kumar Selvaraja, CeNSE 
Date and Time : Wednesday , 24th September 2025 at 3:30 PM 
Venue : Online 

Abstract: 
An electro-optic modulator finds applications in multiple technological fields like optical communication, 
photonic computing, quantum computing, signal processing, neuromorphic computing, programmable photonics,
 and optical sensing. All these fields are moving towards on-chip modulators. An optical modulator changes
 the carrier light's phase or intensity by the electrical signal. Silicon-based nanophotonic devices can
 enable chip-scale photonic systems with integrated devices. Plasma dispersion-based EO modulators in 
silicon are mature and demonstrated an EO bandwidth of over 110 GHz. Despite such a large bandwidth, 
achieving a pure phase modulator is impossible. The modulation comes at the cost of absorption loss. 
Hence, exploring more material systems and modulation mechanisms is essential. Pockels effect is the 
linear EO effect and is an intrinsic property of non-centrosymmetric materials. Ideally the effect 
can be used to modulate light in THz frequency with pure phase modulation. The works discussed in this 
thesis attempt to study material systems that use the principle of Pockels effect for electro-optic modulation.
 The work here spans theoretical presentations, material selection, device optimization, and practical applications.

To choose out of the many materials that show Pockels effect, we have mainly two selection rules: high Pockels 
coefficient and easy integration into current integrated photonics platforms. Accordingly, we explore two materials, 
Barium Titanate (BTO) and Lithium Niobate (LNO), for EO modulation through simulations, material growth, design and
 development of devices. BTO has the highest Pockels coefficient in its bulk form and reports of its growth on Silicon 
are encouraging. On the other hand, LNO is a well-established material for non-linear and EO applications. Lithium 
Niobate on Insulator (LNOI) platform is the new thin-films on-chip photonic platform that is being deeply explored 
for photonic circuits. The first half of the thesis focuses on BTO, while the second on LNO.

Since we are exploring ferroelectric materials for EO modulators, understanding the role of ferroelectric domains
 in EO repose is crucial. We studied this dependence through simulations that showed the optimal geometry of the 
device to extract the best EO response. We present the importance of uniformly oriented domain structures for
 efficient EO modulation. We find that the electro-optic properties are deeply tied to their material properties,
 which must be qualified before making a modulator. Once we understand the system, we can grow it and develop EO
 modulators. Two systems were studied for EO modulator development: BTO thin films and BTO nanoparticles. The thin
 films were deposited using PLD (Pulsed Laser Deposition) technique and nanoparticles were grown by hydrothermal method.
 Material and EO characterisation results of both systems are presented and correlated. Both systems showed a similar
 looking monodirectional EO response. We were able to attribute this discrepancy to the chargers that get trapped in 
the systems. The two systems are also compared in terms of their properties to understand the underlying growth factors 
that affect an EO modulator. The importance of EO characterisation to fully qualify a material for EO modulation application 
is highlighted in this work.

The second half of the thesis focuses on using Lithium Niobate for EO modulation. A Mach-Zehnder-based EO modulator 
using a loaded waveguide structure was designed, fabricated and characterised. The device's EO responses were recorded 
and analysed in the DC and high-speed regimes. We got the EO figure of merit, VπL value of 1.35 V cm, which is the
 lowest reported for the waveguide configuration used here. Our modulator works till 50 GHz, limited by modulating 
RF source frequency. Additionally, the role of ferroelectric domains is also explored, supported by simulation and 
experimental results. These results further emphasize the role of domains in ferroelectric systems for EO applications. 
An on-chip EO modulator finds applications in various fields. Finally, we examine the possibility of using the LNOI platform 
for fiber optic gyroscope application. We integrated the different passive and active components needed for an on-chip 
interferometric fiber optic gyroscope (IFOG) on the LNOI platform.

To summarise, this thesis can be a guide to research works that explore new materials or new material growth techniques 
for EO applications. The effect of fundamental properties of the material on device responses as well as qualifying characterisations 
are discussed. Finally, an EO modulator is presented and its novel applications explored.