My group works in femtosecond laser micromachining of transparent materials. Our research activity exploits our capability to produce 3D optical and microfluidic components in glass, crystals and polymers for different applications. One activity is the development of integrated photonic circuits for quantum computing and quantum information processing. The possibility to combine optical waveguides and microfluidic channels on the same chip is exploited to develop innovative microfluidic and optofluidic components for lab-on-a-chip sensing and manipulation of single cells. Two-photon-polymerization is also employed to produce micro/nano-structures of arbitrary geometry, as 3D scaffolds that we use to study the evolution of cultured stem cells.
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In this work, we present the realization of an 8-mode UPP using direct femtosecond laser writing (FLW) as the fabrication platform. FLW allows rapid and cost-effective prototyping of waveguides in glass-based substrates, achieving low insertion losses (down to 0.13 dB cm−1 for propagation and 0.2 dB per facet for coupling), a critical requirement for quantum applications.
By incorporating compact curved deep isolation trenches and stable, efficient thermal phase shifters, we have reduced the size of the MZI unit cell compared to the current state-of-the-art in FLW fabrication. This reduction improves integration density and circuit complexity with respect to the current state-of-the-art devices for this fabrication platform. The phase shifters exhibit minimal power dissipation (∼ 38mW) and thermal crosstalk (∼ 20 %). The device operates at a wavelength of 925 nm, making it compatible with state-of-the-art quantum dot single-photon sources. It features 28 MZIs and 56 thermal phase shifters, with total insertion losses below 3 dB. Additionally, we describe a calibration process combining conventional methods with a machine learning optimization procedure, enabling the realization of unitary transformations with an average amplitude fidelity surpassing 0.99, showcasing the high precision required for quantum photonic applications.
Low-power reconfigurable photonic integrated circuits fabricated by femtosecond laser micromachining
We present the progress in the development of a six-inputs, J-band interferometric beam combiner based on the discrete beam combiner (DBC) concept. DBCs are periodic arrays of evanescent coupled waveguides which can be used to retrieve simultaneously the complex visibility of every baseline from a multi-aperture interferometer. Existing, planned or future interferometric facilities combine or will combine six or more telescopes at the time, thus increasing the snapshot uv coverage from the interferometric measurements. A better uv coverage will consequently enhance the accuracy of the image reconstruction. DBCs are part of the wider project Integrated astrophotonics that aims to validates photonic technologies for utilisation in astronomy.
Before manufacturing the component we performed extensive numerical simulations with a coupled modes model of the DBC to identify the best input configuration and array length. The 41 waveguides were arranged on a zig-zag array that allows a simple optical setup for dispersing the light at the output of the waveguides.
The component we are currently developing is manufactured in borosilicate glass using the technique of multi-pass ultrafast laser inscription (ULI), using a mode-locked Yb:KYW laser at the wavelength of 1030 nm, pulse duration of 300 fs and repetition rate of 1 MHz. After annealing, the written components showed a propagation loss less than 0.3 dB/cm and a negligible birefringence at a wavelength of 1310 nm, which makes the components suitable for un-polarized light operation. A single mode fiber-to-component insertion loss of 0.9 dB was measured. Work is currently in progress to characterize the components in spectro-interferometric mode with white light covering the J-band spectrum.
In this work, we propose integration of FLM with micro-injection moulding (μIM) as a novel route towards the cost-effective and flexible manufacturing of polymeric Lab-on-a-Chip (LOC) devices. In particular, we have fabricated and assembled a polymethylmethacrylate (PMMA) microfluidic optical stretcher by exploiting firstly FLM to manufacture a metallic mould prototype with reconfigurable inserts. Afterwards, such mould was employed for the production, through μIM, of the two PMMA thin plates composing the device. The microchannel with reservoirs and lodgings for the optical fibers delivering the laser radiation for cell trapping were reproduced on one plate, while the other included access holes to the channel. The device was assembled by direct fs-laser welding, ensuring sealing of the channel and avoiding thermal deformation and/or contamination.
The modifications produced by the focused laser beam into the bulk material have been firstly investigated depending on the laser process parameters aiming to produce continuous melting. Results have been evaluated based on heat accumulation models. Finally, fs-laser welding of PMMA samples have been successfully demonstrated and tested by leakage tests for application in direct laser assembly of microfluidic devices.
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