Miniaturised multi-plane light converters via laser-written geometric phase holograms

Multi-plane light converters (MPLCs) are an emerging 3D beam shaping technology capable of deterministically mapping a basis of input spatial light modes to a new basis of output modes. The ability to perform such spatial reformatting operations has many future applications in both classical and quantum photonics, spanning from optical communications to photonic computing and imaging. MPLCs are intricate optical systems consisting of a cascade of inverse-designed diffractive optical elements, typically separated by free-space. In this work we investigate the fabrication of miniaturised fully-encapsulated transmissive MPLCs within a glass chip using single-step direct laser writing. Our approach relies on the formation of femto-second laser induced birefringent nanogratings with a spatially controllable fast axis orientation. The glass chip is internally patterned with layers of these nanogratings to create multiple geometric phase holograms which imprint controllable phase patterns onto circularly polarised light propagating through them. We experimentally demonstrate two proof-of-concept glass-embedded 700x700x2000 micrometer cubed MPLCs: a 3-mode and a 10-mode Hermite-Gaussian mode sorter. We discuss the fabrication challenges and future improvements of these devices. Our work plots a path towards the rapid prototyping of robust monolithic MPLC technology.

💡 Research Summary

Multi‑plane light converters (MPLCs) have emerged as a powerful tool for deterministic spatial mode manipulation, enabling the mapping of an arbitrary set of input transverse modes onto a desired set of output modes. Conventional MPLCs consist of a cascade of diffractive phase masks separated by free‑space, each mask being inverse‑designed to imprint a specific phase pattern on the propagating field. While this architecture offers high flexibility and the ability to process many modes in parallel, it suffers from bulkiness, alignment complexity, and limited scalability.

In this work the authors demonstrate a radically different fabrication strategy: embedding the entire MPLC within a monolithic glass chip using a single‑step femtosecond laser direct‑writing (DLW) process. The key physical mechanism is the formation of birefringent nanogratings (laser‑induced nanostructures) whose fast‑axis orientation follows the linear polarisation direction of the writing beam. By rotating the polarisation during scanning, the authors create spatially varying half‑wave plates that impart a Pancharatnam‑Berry (geometric) phase of 2ϕ to circularly polarised light, while simultaneously flipping its handedness. Each “plane” of the MPLC is therefore a geometric‑phase hologram, and multiple such planes can be stacked at different depths inside the same glass substrate.

The authors first validate the approach by writing a 1 mm², 2 µm‑pixel‑pitch hologram that projects a test image (a tropical island) into the far‑field, confirming that the geometric‑phase concept works with four quantised phase levels. They then design and fabricate two proof‑of‑concept MPLCs: a 3‑mode Hermite‑Gaussian (HG) sorter and a 10‑mode HG sorter. Both devices occupy less than 0.8 mm³ (≈ 700 µm × 700 µm × 2 mm). The phase masks are inverse‑designed using a custom gradient‑descent algorithm that maximises overlap between the simulated output field and a set of target focal spots arranged on a ring. Each mask is realised with two vertically offset nanograting layers (44 µm separation) to approximate a half‑wave plate response, and the continuous phase profiles are quantised to six levels for the 10‑mode device.

Experimental testing employs a high‑fidelity liquid‑crystal SLM to generate the required input HG modes. For the 3‑mode sorter, the measured far‑field intensity patterns show the majority of power concentrated in the intended output channel, but the coupling matrix reveals a diagonal efficiency of only ~20 % (vs. ~98 % in ideal simulations) and noticeable off‑diagonal leakage. The zero‑order (undiffracted) beam accounts for roughly 13 % of the total transmitted power, indicating incomplete conversion to the opposite circular handedness.

The 10‑mode sorter demonstrates the scalability of the approach but also highlights the challenges of higher‑order mode handling. While most modes are still directed to the correct focal spot, the experimental coupling matrix shows an average off‑diagonal intensity of 0.58, with particularly strong crosstalk between adjacent high‑order modes (e.g., HG02↔HG03 and HG11↔HG12). The authors attribute these degradations to (i) imperfect generation of high‑order HG modes on the SLM, (ii) accumulated fabrication errors in nanograting orientation, and (iii) the limited phase‑level quantisation and residual zero‑order leakage.

In the discussion, several routes for improvement are outlined: increasing the number of quantised phase levels to better approximate the continuous phase, optimising nanograting depth and spacing to enhance half‑wave plate efficiency and suppress the zero‑order, refining the linear phase ramps to eliminate the need for additional beam steering, and employing tighter tolerances on the translation stages to reduce inter‑plane misalignment. The authors also note that as the number of modes grows, additional phase planes will be required to maintain low crosstalk, suggesting that multi‑plane stacking beyond two layers is a natural extension of the technique.

Overall, this study provides a compelling proof‑of‑concept that femtosecond laser‑written geometric‑phase holograms can realise compact, robust, and potentially mass‑producible MPLCs. By eliminating the need for free‑space alignment and external optics, the approach opens a pathway toward integrated spatial‑mode processing chips for applications such as space‑division multiplexing in optical communications, high‑dimensional quantum information processing, low‑power optical neural networks, and all‑optical matrix multiplication. The work thus represents a significant step toward practical, chip‑scale implementations of multi‑plane light conversion technology.