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Here we describe our current projects (as of October 2024) and our experimental capabilities.
Nanophotonics with van der Waals materials
Nanophotonics with van der Waals materials
Over the last two decades, layered crystals, often referred to as van der Waals materials, have attracted tremendous interest due to their unique properties in mono- and few-layer forms. Semiconducting transition metal dichalcogenides (TMDs) exhibit robust excitons with high oscillator strengths as well as direct bandgaps in monolayers making them attractive for integration in various photonic structures. Here we are interested in the quasi-bulk counterparts of 2D materials that have been much less explored but have recently started attracting considerable attention for their favourable optical properties with a potential for photonic applications.
Similar to monolayers, van der Waals layers of any thickness from few atomic layers up to 100s of nanometres and lateral sizes up to few 100 micrometres can be readily fabricated via mechanical exfoliation. Thanks to van der Waals forces, the exfoliated flakes can easily adhere to a wide range of of substrates without the need for chemical bonding or lattice matching. By now, there are many demonstrations of standard electron-beam lithography followed by wet or dry etching used to pattern van der Waals materials.
Compared to Si or III-V semiconductors, semiconducting TMDs exhibit higher refractive indices, enabling confinement of light to smaller volumes; far larger birefringence values, attractive for light polarization control and nonlinear optics; transparency in the visible and near-infrared; out-of-plane van der Waals adhesive forces which offer additional post-fabrication tuning and novel approaches to structure fabrication such as vertical layer and structure stacking and twisting similar to few-atomic-layer thick van der Waals heterostructures, which may enable the realization of previously inaccessible photonic structures. Finally, strong room temperature excitonic transitions in most semiconducting van der Waals materials open their potential for nonlinear nanophotonic elements.
Our recent papers explored a wide range of van der Waals materials and structures, focussing on tungsten disulphide (WS2) where we reported on 0D photonic nanoantennas ACS NANO 16, 6493 (2022); LASER & PHOTONICS REVIEW 17, 2200957 (2023); ACS NANO 18, 16208 (2024); their use for creating single photon emitters in monolayer TMDs https://arxiv.org/abs/2408.01070; and more recently on 2D topological photonic insulators https://arxiv.org/abs/2407.05908 (to appear in ACS NANO in 2024).
Our current focus is on 1D subwavelength grating waveguides, where we explore the ways to realise various (exotic) photonic states (including those with topological protection) and control light-matter interaction with other materials that we integrate with such gratings. The ultimate goal is to create nonlinear polariton devices for on-chip integration.
Strong light-matter interaction in van der Waals 2D materials
Strong light-matter interaction in van der Waals 2D materials
Here we focus on studies of the strong light-matter interaction in 2D materials embedded in optical microcavities and coupled to various photonic structures.
New states of the matter, exciton-polaritons emerge in these structures, which provide a particularly rich phenomenology in atomically thin transition metal dichalcogenides (TMDs), as we have shown in our recent papers in:
Further unexplored strong-coupling phenomena, for example in high density electron gas in the extreme 2D limit will be studied in this project opening unprecedented possibilities to explore new non-linear optical phenomena and device applications.
Nano-magnetism in atomically thin 2D quantum materials and their heterostructures
Nano-magnetism in atomically thin 2D quantum materials and their heterostructures
In a large family of layered crystals the properties range from superconductors and metals, to semiconductors and insulators. Properties of such quantum materials in a few-atomic-layer form are strongly influenced by the quantum confinement of the electronic excitations due to the extreme crystal thinness.
Surprisingly, a family of layered ferromagnetic materials exists, which preserve their ferromagnetic properties even in an atomic monolayer form. Such 2D materials will revolutionise electronics, memory devices, and will have broad applications in quantum technologies, particularly in combination with other layered semiconductors.
In this PhD project we will discover novel magnetic few-atomic-layer materials, work on advancing their fabrication, and develop methods for combining such materials with other 2D monolayer crystals such as transition metal dichalcogenides (TMDs).
The goal is to fabricate and explore novel types of opto-electronic devices taking advantage of various magnetic proximity effects generated by 2D magnetic materials on the nanoscale, as shown in our work in Nature Communications.
We also investigate new magnetic materials: NPJ 2D MATERIALS AND APPLICATIONS 8, 6 (2024).
More recently, our focus has been on CrSBr, a fascinating magnetic semiconductor with strong exciton signatures, suitable for realisation of strongly tunable magneto-exciton-polaritons.
Topological photonic crystal made from a 70 nm thick flake of WS2. Flakes up to 300 micron in size are deposited on a carrier substrate (SiO2 in this case, but can be gold or any other dielectric of metal). Following electron beam lithography, the tiny triangular holes (side about 120 nm) are etched using reactive ion etching.
Arrays of nanoantennas with diameters down to 200 nm etched in a 40 nm thick WS2 flake deposited on a gold substrate. Parts of unetched flake are visible on the right of the image.
Photoluminescence (PL) from a monolayer WSe2 deposited on WS2 nanoantennas placed on gold. Bright yellow spots correspond to the positions of nanoantennas
Strong light-matter interaction observed in a tunable microcavity with a bilayer MoS2 (schematic in panel a). Collaboration with ultrafast spectroscopy group in Milan and theorists in Exeter. MoS2 bilayers are quite unique as in addition to the intralayer excitons they also have interlayer excitons formed from electrons and holes residing in different monolayers, and thus called dipolar excitons (after the permanent electric dipole of the exciton). The interlayer excitons in MoS2 bilayers have a large oscillator strength, so they can strongly interact with light and form dipolar polaritons (panel c above), which as we find, show stronger nonlinear properties.
Results of the collaborative project between our group, 2D materials specialists from Manchester and theorists from INL (Portugal). a Microscope image of the sample: multi-layered CrBr3, monolayer MoSe2, and hBN encapsulation layers. b Photoluminescence spectrum (T=4.2K) from MoSe2 monolayer attached to ferromagnetic CrBr3. c, d Schematics of a MoSe2/CrBr3 heterobilayer structure used in the density functional theory calculations, viewed from the side (c) and top (d), where the supercell is highlighted. e The DFT calculated electronic band structure of the MoSe2/CrBr3 heterobilayer, projected on the host material.
Our labs
Our labs
Our expertise is in photonics and magneto-optics of nano-structured semiconductors. The group occupies three dedicated high-spec optical laboratories (including a state-of-the-art vector-magnet magneto-optics and Raman set-up) and shares access to several other state-of-the-art optics laboratories in the LDSD group. We have established a 2D materials fabrication facility including a glovebox that ensures high quality heterostructure assembly as well as allows working with air-sensitive 2D materials. Finally, we have launched a new Near-field Optical Imaging and Spectroscopy Centre (NOSC), where we have access to a range of tip-enhanced optical techniques and nano-spectroscopy.
Several of our set-ups are quite unique, even among the world leading optics groups. For example, we have a tunable Fabri-Perot microcavity set-up installed in a vector magnetic field cryostat where 4.5T can be rotated in 2D plane. This cryostat also has 9T maximum vertical magnetic field. The NOSC lab has a unique combination of excitation lasers and detectors in a wide range of wavelength including visible, near-infrared and mid-infrared.
The Attocube cryostat with a vector magnet (9T vertical and 4.5T rotated in the vertical plane) houses our unique cryogenic scanning microcavity set-up. The lab is equipped with a spectrometer and liquid nitrogen cooled CCD.
Scott Dufferwiel is working on a set-up combining a vector magnet and a scanning microcavity. June 2016. The lab is now further equipped with a supercontinuum laser tunable from 300 to 2000 nm.
Microscopes in one of the labs used for sample fabrication and for characterisation of nano-photonic devices in ambient conditions.
Set-up for cryogenic ultra-low frequency Raman spectroscopy uniquely designed for studies of magnetic and superconducting layered and atomically thin materials. Oscar Hutchings is aligning the set-up. Autumn 2020.
A typical micro-photoluminescence and micro-reflectance set-up for spectroscopy from cryogenic to room temperature, a work-horse of our experiments replicated in several labs.
Flake transfer set-up in a nitrogen-purged glovebox. The set-up will be equipped with optical characterisation allowing to measure air-sensitive materials and structures as soon as they've been made. July 2022.
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