NTU's Fully-funded PhD Studentship Scheme 2022
Project ID: S&T33
Plasmonics has promised revolutionary advancements in many fields. However, the plethora of expectations has been hit by practical roadblocks causing many applications to be still lagging. The fundamental limitations are associated with optical losses and spectral restrictions of metals, the cornerstone materials in plasmonics. To break free from these limitations, the most disruptive technology has been viewed to be the “all-dielectric plasmonics”. Polar dielectrics can be used to couple an electromagnetic field to collective lattice oscillations, namely optical phonons. Similarly to their metallic counterparts, these oscillations can only be supported when ε1<0. This happens at the so-called Reststrahlen band. Naturally, all the optical nanoscale phenomena occur within this region while its extent defines the spectral range for which Surface Phonon Polaritons (SPhP) and Localised Surface Phonon Resonances (LSPhR) occur. Optical phonons exhibit larger mean free times than the free carriers and thus optical losses of phononic materials are significantly lower than their plasmonic counterparts, enabling higher quality factors.
We aim to generate the necessary knowledge to establish metal-oxide perovskites as viable nanophotonic components for mid- and far-infrared (IR) applications. Starting with SrTiO3 (STO) and BaTiO3 (BTO) we will follow a combined theoretical and experimental methodology to determine the performance (in terms of the spectral range of operation and electric field (E-field) enhancement capability) of nano-architectures based on those two materials. These architectures will serve as core components in proof-of-principle IR photodetection devices. Both STO1 & BTO2 are well-established technological materials.1 However, they have been overlooked as core components for IR nanophotonics. Recently, our attention was caught by the line shape of STO’s and BTO’s dielectric permittivity because it uniquely supports negative values of real permittivity (ε1) across a wide range of wavelengths (a prerequisite for nanophotonic elements), unlike most polar dielectrics that present fundamentally narrow range where ε1<0. “MOPIN” will throw STO & BTO directly into centre stage for future IR nanophotonic applications.
In our recent theoretical work,3 STO has been suggested as a viable material for nanophotonics over mid- and far-IR wavelengths. The ability to induce enhanced light-matter interactions via coupling of light to optical phonons is known for a while, however, the fact that we can achieve that in a broad spectral range is new knowledge that has not been applied yet. This range offers a plethora of applications, for example, the mid-IR range is considered the molecular fingerprint region of biochemical building blocks such as proteins, lipids and DNA.4 However, the optical signal of those bio-elements is extremely weak and the proposed materials are excellent candidates to enhance their detection. Far-IR wavelengths become important for photodetection relevant to molecular rotational transitions, space applications, far-IR sources (which are currently limited, exceptionally bulky and require cryogenic cooling) and security applications (including imaging through opaque objects), to name a few.
“MOPIN” builds upon this preliminary but highly innovative work with an emphasis on the design optimisation of nanoarrays, scalable fabrication of highly crystalline thin films and delivery of proof-of-principle IR photodetectors. Commercially available, room-temperature (RT) far-IR photodetectors (pyroelectric detectors and Golay cells) are reasonably sensitive with a noise equivalent power (NEP) of ~1 nW/√Hz.5 However, their responsivity is very low: 100 and 30 ms, respectively.5 Bolometer far-IR detectors can be highly sensitive (NEP~0.5pW/√Hz) with a fast responsivity (~50 ps) but require cryogenic temperatures and suffer from narrow dynamic range.5 Schottky diodes, although combining high responsivity and high sensitivity (NEP~10-100pW/√Hz), have a small dynamic range. Hence, current technology fails to meet simultaneously the following criteria: sensitivity, speed, operating temperature, and spectral range. A route for responding to this challenge is to utilise a more appropriate set of materials; here, we propose the utilisation of the MOP material platform via STO & BTO.
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How to apply
For guidance and to make an application, please visit our studentship application page. The application deadline is Friday 14 January 2022.
Fees and funding
This is part of NTU's 2022 fully-funded PhD Studentship Scheme.
Guidance and support
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