Metal-Oxide Perovskites for Infrared Nanophotonics

Project ID: SST6

Motivation: Plasmonics has promised revolutionary advancements in many fields but there are fundamental limitations associated with optical losses and spectral restrictions of metals, the cornerstone materials. 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.

Vision: The mid-infrared (MIR) spectral region is of significant interest for several applications such as security, healthcare, drug identification, and environmental monitoring. This is due to how chemical compounds can be identified through their unique vibrational modes, which absorb in the molecular “fingerprint” region (6.7–20 μm). We aim to pave the way for a new free-space MIR detection device that will reduce the size and possibly cost compared to commercially available systems. The active components will be SrTiO₃ (STO) and BaTiO₃ (BTO). 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.

We will follow a combined theoretical and experimental methodology to determine the performance of nano-architectures based on those two materials. These architectures will serve as core components in proof-of-principle IR photodetection devices.

Background: 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.

Aims: “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 STO and BTO material platform.

Supervisory Team:

Dr Nikolaos Kalfagiannis

Dr Demosthenes Koutsogeorgis