Glass has always possessed a dual nature, oscillating between a long history of creativity and craftsmanship and compelling technological applications. It is an ancient material, but still widely used in modern industry due to its desirable mechanical properties, chemical inertness, thermal stability associated with a low expansion coefficient, electrical insulation and, most of all, excellent optical transparency. It finds wide applications in chemical and pharmaceutical processes, electronic and electrical technologies, high-temperature operations, light and laser technology, and optical system components. A simple example is its use in everyday products such as in smartphone screens and household equipment, another is its less obvious yet revolutionary use in communications as optical fibres.

The qualities of glass are strongly tied to its manufacturing techniques and each advance in technology can open new opportunities for its uses. Silica glass parts can be manufactured, for example by blowing, moulding and casting, from the melt at substantially higher temperatures than that forconventional soda-lime glass, but their short working window significantly limits the structures that can be obtained. Corning developed its multi-step Vycor process in the 1930s as a way to enable the production of (almost) pure silica components with a wider range of shapes using traditional fabrication technologies1, but challenges in obtaining high precision, small scale and complex architectures still remain. Additive manufacturing, which has developed rapidly and is becoming mature for plastics, enables the fabrication of parts with an almost unlimited variety of geometries, especially those not obtainable with traditional forming technologies. Much higher processing temperatures are required to form glass than to form plastics, therefore glass made a later entrance in the field. Different additive manufacturing techniques have been developed and demonstrated for the manufacturing of glass components, including melt extrusion2, filament extrusion3, paste extrusion4, stereolithography5, digital light processing6,7 and two-photon polymerization (2PP)8. However, the feature resolution for the complex components printed is still limited to the micrometre scale. Now, writing in Nature Materials, ** the suspension with photoluminescent rare-earth elements.

Two-photon polymerization has been demonstrated to be a suitable manufacturing approach for the production of specialized components, including metamaterials, photonic crystals, opto- and microfluidics, drug delivery devices, bioscaffolds for cell cultivation, freeform lenses and micro-optics. Indeed, the functionality of these structures is determined both by the architecture of the objects as well as by the material properties. The possibility of fabricating nanoscale three-dimensional parts with arbitrary shapes based on transparent silica glass thus greatly extends the range of applications enabled by 2PP technology. Although Wen and co-workers only presented a proof of concept, the exceptional sensitivity and the portability of whispering gallery resonators is of great interest for the production of biosensors and point-of-care diagnostic tests. A whispering gallery resonator made of erbium-doped silica has been applied for the detection of virus particles in air10, a promising result with potential application for the prevention and control of epidemics such as the COVID-19 pandemic.

Despite the relevance of the work, challenges still remain in the accurate control of the shape resulting from the sintering process, as viscous flow leads to sagging and undesired deformations, as well as in obtaining full density and defect-free parts. Moreover, the use of a substrate, which is required by the 2PP technology, generates additional issues in terms of thermal expansion mismatch with the sample, as well as in limiting the type of components that can be fabricated. The use of a hybrid additive manufacturing approach, in which 2PP is coupled with digital light processing to obtain free-standing parts11, could provide a solution.