Ultrafast Laser Microfabrication of Transparent Materials
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2021-03-28
From handheld and wearable displays to communication and computing systems, photonic devices are now ubiquitous worldwide. The essential materials for manufacturing photonic components are transparent substances—such as glass, polymers, and crystals—typically requiring properties like broad-band transparency, stability, and versatility in composition. Focused ultrashort laser pulses can induce nonlinear absorption effects within these transparent materials, enabling precise micromachining both on their surfaces and deep inside. This technique, known as ultrafast laser micromachining, has already found a wide range of applications, including direct writing of features like cuts, micro-holes, waveguide couplers, and splitters; creating optical dynamic memory systems; and even advanced bonding or joining technologies that seamlessly fuse interfaces between glass and glass, glass and metal, or glass and ceramic.
Ultrafast laser pulses hold significant potential for applications in material processing and micromachining. In ultrafast laser pulses, the intense peak power is achieved because the optical energy is confined to an extremely short duration. Compared to continuous-wave and long-pulse lasers used in micromachining, ultrafast lasers offer several key advantages: they enable the creation of intricate microstructures with minimal collateral damage to the surrounding environment, deliver a clean, pristine processing outcome, generate a tiny heat-affected zone (HAZ), preserve the intrinsic properties of the material, and even allow for the modification of transparent materials—whether on their surfaces or within their internal structures. Ultrafast laser micromachining is a rapidly evolving field within the broader application of ultrafast lasers. Moreover, since the process relies on nonlinear absorption rather than linear absorption of the laser wavelength, virtually any dielectric, metal, or mechanically robust material can be processed using the same laser beam—for tasks ranging from surface ablation to precise internal modifications.
Ultrafast lasers are versatile tools for micromachining a wide range of materials, including metals, polymers, semiconductors, and transparent materials. However, the properties of the material—particularly its optical and thermal characteristics—must be carefully considered to select the appropriate laser parameters for optimal results. To achieve precise ultrafast laser micromachining on a material, it’s essential to choose the right laser operating parameters, such as laser wavelength, repetition rate, laser power, scanning fluence, pulse duration, polarization, beam spot size, and beam quality. The images below illustrate how these laser parameters influence the micromachining process of transparent materials.
Figure 1. The laser fluence (with the energy per pulse given in parentheses) varied as follows: (a) 12.1 J/cm² (170 μJ), (b) 12.9 J/cm² (180 μJ), (c) 15.0 J/cm² (210 μJ).
Figure 2. SEM micrographs showing debris accumulated inside and outside the craters (a and b) as well as within and around the fragments (c and d) after laser drilling and cutting.
Figure 3. SEM micrograph of a 50-micron-thick borosilicate glass cut produced by 10 consecutive 10-ps laser pulses at a wavelength of 515 nm.
Figure 4 shows the SEM image of grooves created by laser, demonstrating the influence of strain on laser processing quality.
A notable feature of ultrafast laser pulse micromachining is the ability to perform internal microfabrication within transparent materials. When near-infrared ultrafast laser pulses are focused inside glass, the intensity within the focal volume becomes high enough to induce nonlinear absorption, resulting in localized structural changes to the glass at the focal point (as shown in the figure below).
Microfabrication techniques using ultrashort laser pulses are being employed to create photonic devices within transparent materials. By precisely scanning the focus of an ultrashort laser pulse through various types of glass, this technology has enabled the integration of photonic components—such as waveguides, couplers, and gratings—directly in three-dimensional space. Additionally, as a method for creating voids within transparent materials, 3D optical data storage has been demonstrated, where the presence of voids or nanogratings encodes binary values, while their absence represents another binary state.
An array of voids formed in bulk glass using ultrafast laser pulses.
One of the most compelling applications of ultrafast laser glass micromachining is the direct fabrication of biochips, such as microfluidic, optofluidic, and lab-on-a-chip devices, designed to perform biochemical sample reactions, detection, analysis, separation, and even synthesis. To create three-dimensional microfluidic structures inside glass, two methods have been widely adopted: liquid-assisted ultrafast laser drilling and ultrafast laser–assisted wet chemical etching.
In liquid-assisted ultrafast laser drilling, as shown in the figure below, the ultrafast laser 3D ablation begins from the back surface of the glass that is in contact with distilled water or another liquid. The wetting liquid infiltrates the channels created during the laser drilling process, significantly enhancing the removal of ablation debris trapped within the narrow microfluidic channels formed. This, in turn, dramatically reduces the issue of debris clogging that typically occurs during deep drilling.
Schematic Diagram of Liquid-Assisted Ultrafast Laser Drilling
Schematic illustration of ultrafast laser-assisted wet chemical etching. (a) Latent image induced by ultrafast laser irradiation, (b) modification zone resulting from thermal treatment, (c) microchannels formed via chemical etching.
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