arXiv subjects

Unė G. Būtaitė

Publications and source records attributed to Unė G. Būtaitė.

4 recordsLinked to original sources

arXiv2026

Miniaturised transmissive multi-plane light converters via laser-written geometric phase holograms cascaded in glass

Multi-plane light converters (MPLCs) are an emerging beam shaping technology capable of deterministically mapping a basis of input spatial light modes to a different basis of output modes. The ability to perform such multi-modal spatial reformatting operations has many future applications in both classical and quantum photonics, spanning from optical communications to photonic computing and advanced imaging. In this work we fabricate miniaturised transmissive MPLCs fully-encapsulated within a fused silica glass chip using single-step 3D direct laser writing. Our approach relies on the formation of femto-second laser induced birefringent nanogratings with a spatially controllable slow-axis orientation. Multiple layers of these nanogratings are laser-written throughout the volume of the glass to create a sequence of axially separated geometric phase holograms which imprint controllable phase patterns onto circularly polarised read-out light propagating through them. We construct and test a range of proof-of-concept laser-written MPLCs operating in the visible (lambda = 633nm). These miniature beam multiplexers are formed from up to 5 separate phase masks of width ~260um, cascaded along a total length of ~2.7mm, thus occupying a compact volume of ~0.15 mm3. We first demonstrate Hermite-Gaussian (HG) mode sorters capable of diverting the energy carried by up to 28 overlapping HG modes into spatially separated output channels. We next create a 7-mode orthogonal speckle sorter, highlighting the universal nature of the spatial transformations it is possible to encode. Finally, we show analogue optical matrix multiplications achieved by passively scattering light through these 3D structured glass elements. Our work begins to merge the concepts of free-space optics with 3D integrated photonics in glass and plots a path towards the rapid prototyping of robust monolithic MPLC technology.

physics.optics↗

arXiv2025

Self-configuring high-speed multi-plane light conversion

Multi-plane light converters (MPLCs) - also known as linear diffractive neural networks - are an emerging optical technology, capable of converting an orthogonal set of optical fields into any other orthogonal set via a unitary transformation. MPLC design is a non-linear problem typically solved by optimising a digital model of the optical system. However, inherently high levels of design complexity mean that even a minor mismatch between this digital model and the physically realised MPLC leads to a severe reduction in real-world performance. Here we address this challenge by creating a self-configuring free-space MPLC. Despite the large number of parameters to be optimised (typically tens of thousands or more), our proof-of-principle device converges in minutes using a method in which light only needs to be transmitted in one direction through the MPLC. Two innovations make this possible. Firstly, we devise an in-situ optimisation algorithm combining wavefront shaping with the principles of wavefront matching that would conventionally be used to inverse-design MPLCs offline in simulation. Secondly, we introduce a new MPLC platform incorporating a microelectromechanical system (MEMS) phase-only light modulator - allowing rapid MPLC switching at up to kiloHertz rates. Our scheme automatically accounts for the physical characteristics of all system components and absorbs any unknown misalignments and aberrations into the final design. We demonstrate self-configured MPLCs capable of mapping random orthogonal speckle input fields to well-defined Laguerre-Gaussian and Hermite-Gaussian output modes, as well as universal mode sorters. Our work paves the way towards large-scale ultra-high-fidelity fast-switching MPLCs and diffractive neural networks, which promises to unlock new applications in areas ranging from optical communications to optical computing and imaging.

physics.optics↗

arXiv2023

Photon-efficient optical tweezers via wavefront shaping

Optical tweezers enable non-contact trapping of micro-scale objects using light. Despite their widespread use, it is currently not known how tightly it is possible to three-dimensionally trap micro-particles with a given photon budget. Reaching this elusive limit would enable maximally-stiff particle trapping for precision measurements on the nanoscale, and photon-efficient tweezing of light-sensitive objects. Here we solve this problem by customising a trapping light field to suit a specific particle, with the aim of simultaneously optimising trap stiffness in all three dimensions. Initially taking a theoretical approach, we develop an efficient multi-parameter optimisation routine to design bespoke optical traps for a wide range of micro-particles. We show that the confinement volume of micro-spheres held in these sculpted traps can be reduced by one-to-two orders-of-magnitude in comparison to a conventional optical tweezer of the same power. We go on to conduct proof-of-principle experiments, and use a wavefront shaping inspired strategy to suppress the Brownian fluctuations of optically trapped micro-spheres in every direction concurrently, thus demonstrating order-of-magnitude reductions in their confinement volumes. Our work paves the way towards the fundamental limits of optical control over the mesoscopic realm.

physics.optics↗

arXiv2022

How to build the optical inverse of a multimode fibre

When light propagates through a multimode optical fibre (MMF), the spatial information it carries is scrambled. Wavefront shaping can undo this scrambling, typically one spatial mode at a time - enabling deployment of MMFs as ultra-thin micro-endoscopes. In this work we go beyond serial wavefront shaping by showing how to simultaneously unscramble all spatial modes emerging from an MMF in parallel. We introduce a passive multiple-scattering element - crafted through the process of inverse design - that is complementary to an MMF and undoes its optical effects. This optical inverter makes possible both single-shot wide-field imaging and super-resolution imaging through MMFs. Our design consists of a cascade of diffractive elements, and can be understood from the perspective of both multi-plane light conversion, and as a physically inspired deep diffractive neural network. This physical architecture can outperform state-of-the-art electronic neural networks tasked with unscrambling light, as it preserves the phase and coherence information of the optical signals flowing through it. Here we demonstrate our MMF inversion concept through numerical simulations, and efficiently sort and unscramble up to ~400 step-index fibre modes, reforming incoherent images of scenes at arbitrary distances from the distal fibre facet. We also describe how our optical inverter can dynamically adapt to see through flexible fibres with a range of experimentally realistic TMs - made possible by moulding optical memory effects into the structure of our design. Although complex, our inversion scheme is based on current fabrication technology so could be realised in the near future. Beyond imaging through scattering media, these concepts open up a range of new avenues for optical multiplexing, communications and computation in the realms of classical and quantum photonics.

physics.optics↗