WHAT WE DO

The ultimate goal of the GRAIL project is, in broad strokes, to demonstrate the feasibility of the 3D laser writing (3DLW) technique to develop, with high level of repeatability, fully monolithic photonic devises micro-structured from a solid piece of material, and pave the way towards its fabrication in a mass production scale.

HOW WE DO IT

The 3DLW method essentially consists on tightly focusing femtosecond laser pulses inside transparent media to create 3D optical structures directly inside the material due to localized multiphoton absorption to fabricate 3D photonic, microfluidic or micromechanic devices. The sample is displaced under the beam in such a way that the structure is «inscribed» by precisely localized controlled damage in the crystalline structure of the material. Many different parameters play an important but yet not fully understood role in the resulting damage such as deposited energy, pulse repetition rate (PRR), pulsing mode (pulse train/burst mode), sample displacement speed, overlapping among others.

Although the damaged areas already present a refractive index change, it is too small (in the order of 10^-3 to 10^-4) to effectively produce nanophotonic devices. For this reason, a wet-etching step has to be done. In this stage the micro-structured sample is chemically treated to “reveal” the structure, similarly to what is applied in the electronic industry to fabricate printed circuits. This is possible because the damage produced in the material during the 3DLW process makes it more prone to be attacked by acidic solutions. This big wet etching selectivity allows for the creation of hollow nanopores and basically any type structure. It is important to note that, the thinner the pores are (i.e., smaller pore diameter), the closer the pores can be and the higher the fabrication freedom is. This is directly related to the 3DLW parameters, the etching rate and, to some extent, to the temperature during the process.

A critical aspect of making sub-micron sized structures repeatable is the ability to move the sample with high precision throughout the writing process. For this we use a computerized 3D stage with a precision of nanometers in the X-Y and Z axes, which allow us to control the position of the sample and the depth at which the laser focus will be satisfactorily. GRAIL aims to, thanks to this 3D nanolithography technique, enable a new field on 3D sub-wavelength engineering of solid-state laser media, with prototype demonstrations on sub-λ metaoptics and Photonic Crystal Waveguides (PCWs) all inside a crystal.

THE MEDIUM, THE DEVICE

Our target photonic device we intended to fabricate is a Single-Frequency and Single-Mode laser photonic crystal YAG laser (SFSMPCL) with emission in the wavelength range between 1550 nm and 1650 nm. This is, a monolithic and crystalline laser with a single longitudinal mode (~MHz-scale linewidth) and single transversal mode (pure TEM₀₀ mode). The reason to choose the aforementioned wavelength range is the wide variety of applications (sensing, metrology, telecom, etc), while YAG is an accessible and inexpensive material suitable for harsh environment photonic applications with exceptional and well known physical and optical properties (high transparency from UV to mid-IR range, stable in oxidizing and reducing environments, high hardness, high fracture strength, high thermal conductivity, and a high melting temperature) that has been shown to be micro-structurable with the 3DLW + wet etching technique. Considering the wavelength and material selection, Er:YAG (erbium doped YAG) is the chosen material for the SFSMPCL prototype intended in the GRAIL project.

A SFSMPCL as a demonstrator prototype may seem a quite complex device (and it certainly is!), but there is a good reason to aim for it in GRAIL: Its manufacture can be done sequentially, with a gradual increase in complexity that allows evaluating not only the manufacturing process itself, but also the characteristics of the structure manufactured at different stages of the process. The prototype implementation contemplates 3 main stages:

1. Creation of simple shapes (pores, lines, planes and angles). To optimize setup and manufacturing parameters. These parameters are essentially energy, pulse duration and pulse repetition frequency, sample motion speed during the writing process, and pulse mode (pulse train or burst mode). In addition, the chemical etching rate will be studied as a function of the energy deposited during the writing process and temperature during the chemical treatment.
2. Implementation of the Photonic Crystal Waveguide (PCW, i.e., the structures required for single-mode guidance properties), inspired by the technology used in Photonic Crystal Fibers (PCF) and exceptional capability to be endlessly single-mode. These waveguides are fundamental in a large number of photonic devices. Therefore, its manufacture, reproducibility, repeatability of the process and quality of the result is a fundamental pillar to justify the suitability of the manufacturing method. After the PCWs are fabricated, their guidance properties will be studied in terms of propagation losses and beam quality at lab environment and at high temperature/pressure. The characterization results will be used to improve the PCW a design iteration.
3. Integration in the already existing PCW of a distributed feedback (DFB) cavity. By doing this it is to turn the active PCW into a laser. The DFB architecture requires the implementation of gratings with particularly small periods for the case of single frequency radiation. In this way the limits of the manufacturing process will be explored (at least in the laboratory demonstrator). This is a way to test the fabrication process when it comes to demanding structures, and will allow to evaluate the precision of the manufacturing process by characterizing the lasing properties. The precision with which DFB gratings need to be engineered will depend on the minimum pore width achievable and on the precision with which the grating period can be defined. This will allow to tune the Bragg wavelengths.

THE TECHNOLOGY TRANSFER

The 3D laser nanolithography technique being applied in GRAIL makes use of three-axis linear nanopositioning stages to move the samples in 3D. This approach is very practical for fundamental development at research stage, but may not provide the necessary fabrication speed for future mass production. The industrial partner LightFab GmbH developed an award-winning system that combines sample translation with synchronous high-dynamic laser focus micro-scanning. This technology allows developing more complex 3D components in a shorter time, something crucial for transferring the technology to a pilot stage, but also will allow to increase the device complexity in future incremental development steps: as an example, metalenses could be embedded within the crystal to minimize losses and increase robustness. The final goal of GRAIL from the technological transfer point of view is to tackle the potential for mass-production of the new technology, a feature that is of paramount importance in order to enable commercial translation of technology and achieve true societal impact.