Femtosecond laser micromachining

The use of ultrashort laser pulses to modify the surface or bulk of materials and induce volumetric changes through chemical reactions has garnered significant attention in recent years. This approach offers the potential to tailor material properties for applications in optics and photonics.

Waveguides in nonlinear glasses

Femtosecond laser microfabrication has been established as a versatile and efficient technique for fabricating 3D microstructures in transparent materials. In this approach, the nonlinear optical interaction limits the light's energy transfer to the material focal volume. A valuable result of such a process is the permanent modification of the refractive index in specific regions, providing a method of fabricating waveguides in the volume of materials. Such waveguides are fundamental components of devices for photonics, finding applications in telecom systems, sensors, integrated photonics, optical isolators, and quantum information.

Figure: (left column) displays the experimental mode profiles obtained for three distinct waveguides fabricated with n = 4, in which light guiding in different modes were observed. As mentioned before, light confinement in Type II waveguides occurs due to an increase in the refractive index in the region between the micromachined tracks. Finite element calculations, using Comsol Multiphysics, were carried out to perform modal analysis of such waveguide.

In our lab, we've successfully inscribed homogeneous waveguides in Tb³⁺ and Yb³⁺-doped CaLiBO glass using femtosecond laser writing. These waveguides support single-mode guiding at 632.8 nm and efficiently guide Tb³⁺ emission, producing green visible light. By adjusting the Tb³⁺ concentration up to 2 mol%, we've enhanced fluorescence and light guiding without impacting waveguide loss or fabrication conditions.

Figure: Optical microscopy images (cross-section view of the input face and top view) of waveguides produced in the glass system, along with the near-field output profile (mode profile) of light guided at 632.8 nm. For samples CaLiBO (a), CaLiBOx01 (b) and CaLiBOx1 (c). Scale bar in each image corresponds to 5μm.

Polymeric 3D structures

An extensively used technique in ultrashort laser-based fabrication is direct laser writing through two-photon polymerization (2PP). Unlike traditional micro/nanofabrication methods, 2PP is a maskless patterning tool that operates without stringent processing conditions or the need for cleanrooms. By leveraging the nonlinear nature of two-photon absorption, 2PP excels in fabricating 3D polymeric microstructures with arbitrary geometries and sub-diffraction features. In one of our projects, we used 2PP to fabricate high-Q WGM microresonators incorporating an azoaromatic chromophore. These microresonators were tuned by external resonant light and coupling in the telecommunication range was achieved by tapered fibers. The tuning, driven by thermo-optical effects was analyzed as a function of light irradiance and interpreted through finite element simulations.

Figure: (a) Transmission spectra of the microresonator for increasing external irradiances (Ar-ion laser at 514 nm). (b) SEM micrography of a microresonator fabricated direct by femtosecond laser writing via 2P. (c) Simulation of the radial displacement map of the resonator.

Nitrogen-vacancy (NV) and other color centers in diamond have attracted much attention as non-photobleaching quantum emitters and quantum sensors. Since microfabrication in bulk diamonds is technically difficult, embedding nanodiamonds with color centers into designed structures is a way to integrate these quantum emitters into photonic devices. We have demonstrated a method to incorporate fluorescent nanodiamonds (ND) into engineered microstructures using 2PP. We studied the optimal concentration of nanodiamonds in the photoresist to achieve structures with at least one fluorescent NV center and good structural and optical quality. Our results show the feasibility of fabricating microstructures embedded within fluorescent nanodiamonds via 2PP for photonics and quantum technology applications.

Figure: (a) Confocal scanning image of structures doped with 0.02 wt% of ND, with several fluorescent spots. (b) Two different planes of a structure doped with 0.002 wt% ND; three fluorescent spots in different focal planes are observed. (c) A 3D image composition of the structure depicted in (b).

Diamond

Diamond, known for its unique set of properties, is a highly sought-after material for many electronic, optical and even quantum novel technologies. Among the many proposed processing techniques to produce such devices, femtosecond laser micromachining distinguish itself given its high precision in generating microstructures with minimal collateral damage. Here, the minimum fluence necessary to produce such structures was studied at different experimental conditions in order to further optimize such procedure, where an example can be seen in Figure.

Figure: SEM image of micromachined lines with distinct fluences (a) and squared half-width of micromachined lines as a function of the peak fluence of CVD diamond (b) for 1300 pulses at 1030 nm. Source: Nolasco et al., 2023.

Additionally, the generation of spatially localized NV centers was demonstrated, where microfabrication was responsible for creating vacancies in the lattice that can bind with nitrogen impurities during the sample’s annealing. However, the generation process proved to be stochastic, since no NV centers were observed at certain regions, independently of the irradiation conditions.

Figure: (a) Room-temperature photoluminescence images from an irradiated sample (scale bar 7 µm) at different numbers of pulses and (b) corresponding photoluminescence spectra. Laser excitation at 532 nm. Source: Oncebay et al., 2022.

Silicon Carbide

One significant advantage of fs-direct-laser-writing is the study of the conditions for material modification, enabling the determination of parameters that make it reproducible. This control has emerged as a friendly alternative for the localized induction of color centers with minimal lattice stress. This capability aligns synergistically with one of the paths toward scalability in quantum technologies: the combination of deterministic tools for producing solid-state qubits and using materials with easy industrial translation. Our group has been working with 4H silicon carbide (4H-SiC), a wide-bandgap semiconductor already in the electronic industry and capable of hosting various optically addressable solid-state spin defects, notably near telecommunication spectral regions. The recent generation of spin-controlled indistinguishable photons and photon-spin entanglement from silicon vacancies in electron-irradiated 4H-SiC positions the material as one of the most promising candidates for new quantum applications.

Figure: Femtosecond Laser-induced modifications in 4H-SiC. (A) Damage threshold fluence dependence on the number of pulses, expressing a memory process known as the incubation effect. (B) Production of photoluminescent defects in regions of single-photon emitters. (C) Photoluminescence (PL) of defects in regions irradiated with an increasing number of pulses (R1-R8).

Our team focuses on the study of microfabrication protocols, the investigation of nonlinear optical and structural properties, and the production of color centers in pure and doped 4H-SiC bulk, as well as in 4H-SiC with photonic nanocavities that induce Purcell-enhanced spin-photon coupling.

Perovskites

Perovskite materials processing is highly relevant due to their unique optical and electrical properties, making them ideal for photonic, optoelectronic, and energy-harvesting applications. Crystallinity is key to optimizing devices like light emitters and photodetectors, as it impacts optical efficiency and charge mobility. Femtosecond laser processing offers precise control over energy distribution, enabling localized crystallization with minimal thermal effects. This addresses the limitations of conventional methods, which often result in less stable amorphous films. In Er³⁺/Yb³⁺-doped BaTiO3 (BTEY) thin films, fs-laser processing enhances crystallization, confirmed by Raman and fluorescence confocal microscopy. The process improves luminescence intensity and surface potential (up to 65 mV), reduces defects, and increases grain size. This technique enables the development of advanced devices like photonic modulators, waveguides, and sensors for future technologies.

Figure: (a) Illustration of the process of micromachined/crystallized region. (b) Process of the optical response on excitation wavelength at 445 nm. (c) Confocal images of microstructures at different pulse energies using 50 µm/s for 1 kHz. (d) Luminescence spectra at 488 nm for 1 kHz.