By Innofocus Photonics Technology Pty Ltd
The answer to this question is at the heart of the 3D femtosecond laser nanofabrication. Compared to single photon absorption based processing, dual photon absorption based processing has the following merits: 1. Single photon absorption based processing can only be used for photoresist materials that have linear absorption for the laser wavelength used. Based on the nature of the photoresist chosen, it is able to achieve single photon polymerization (e.g. negative photoresist. Exposed positions are retained while unexposed positions are washed out by the developer solutions) or single photon bond-breaking (e.g. positive photoresist. Unexposed positions are retained while exposed positions are washed out by the developer solutions). However, dual photon absorption based processing can be applicable to various kinds of materials and is not limited by the linear absorption feature for laser wavelength. Through different reaction mechanisms on different materials, the possible processing mechanisms of dual photon absorption based processing include the following: - It can also be used to achieve dual photon polymerization and bond breaking for photoresist materials. - It enables material modification via dual-photon-driven chemical reactions e.g. photo-reduction and photo-oxidation. - It enables phase change of materials, e.g. from crystalline phase to amorphous phase and it is evident by looking at the change in the material's refractive index. - It enables the stripping and etching processing of materials, e.g. layer-by-layer stripping of 2D materials, etching of metallic materials or semiconductors. - It enables microbursts inside the materials, to realize hollow points or lines around which a high density shell layer will be formed. This allows forming lattice or waveguide structures in the materials, with high refractive index difference. As a result, there are a wealth of mechanisms associated with dual photon absorption based processing that accommodate different materials and application requirements. 2. Since single photon absorption based processing requires that the material to be processed has linear absorption for the selected laser wavelength, it is not possible to focus the laser inside the material for three-dimensional processing. In contrast, two-photon absorption occurs only at the focal point of the laser (subject to the requirement of material threshold), which allows three-dimensional processing with three-dimensional spatial resolution to be performed by focusing the laser inside the material even if it has no linear absorption for that laser wavelength. It enables fabrication of arbitrary three-dimensional structures by scanning the processing path inside the material. Even if the sample has a layer structure, the dual-photon approach can enable modifying or processing of any layer in the material. Such approach offers a significant degree of freedom and convenience. For example, conventional single-photon based processing of layer structures requires pre-processing to expose the desired layer to the surface for single-photon exposure processing, and then cladding the material layer by layer after processing according to the protection requirements. However, the dual-photon approach does not require any pre-processing and post-processing, which effectively reduces steps, shortens time, and lowers costs of processing. 3. For the same laser wavelength, the dual-photon absorption based processing has a higher spatial resolution compared to the single-photon processing. For example, the resolution of the conventional diffraction limit based on the single-photon mechanism can be expressed as: d s=λ/2NA, where λ is the laser wavelength used and NA is the numerical aperture of the focusing microscope lens used. However, based on dual-photon mechanism, this resolution can be improved by a factor of √‾2, to d t= d s/√‾2 =λ/2√‾2 NA. In sum, compared to single-photon absorption based processing using continuous-wave lasers, the dual-photon absorption-based femtosecond laser processing has the following advantages: abundant processing mechanisms, applicable to multiple materials, three-dimensional processing capability and high spatial resolution etc.
