Elucidating complex triplet-state dynamics in the model system isopropylthioxanthone

Two- and three-photon processes during photopolymerization in 3D laser printing

The performance of a photoinitiator is key to control efficiency and resolution in 3D laser nanoprinting. Upon light absorption, a cascade of competing photophysical processes leads to photochemical reactions toward radical formation that initiates free radical polymerization (FRP). Here, we investigate 7-diethylamino-3-thenoylcoumarin (DETC), belonging to an efficient and frequently used class of photoinitiators in 3D laser printing, and explain the molecular bases of FRP initiation upon DETC photoactivation. Depending on the presence of a co-initiator, DETC causes radical generation either upon two-photon- or three-photon excitation, but the mechanism for these processes is not well understood so far. Here, we show that the unique three-photon based radical formation by DETC, in the absence of a co-initiator, results from its excitation to highly excited triplet states. They allow a hydrogen-atom transfer reaction from the pentaerythritol triacrylate (PETA) monomer to DETC, enabling the formation of the reactive PETA alkyl radical, which initiates FRP. The formation of active DETC radicals is demonstrated to be less spontaneous. In contrast, photoinitiation in the presence of an onium salt co-initiator proceeds via intermolecular electron transfer after the photosensitization of the photoinitiator to the lowest triplet excited state. Our quantum mechanical calculations demonstrate photophysical processes upon the multiphoton activation of DETC and explain different reactions for the radical formation upon DETC photoactivation. This investigation for the first time describes possible pathways of FRP initiation in 3D laser nanoprinting and permits further rational design of efficient photoinitiators to increase the speed and sensitivity of 3D laser nanoprinting.

Stimulated Emission Depletion Inspired Sub-100 nm Structuring of Epoxides Using 2-Chlorothioxanthone as Photosensitizer

Until very recently, the enhancement of multiphoton-based optical lithography by stimulated emission depletion (STED) inspired techniques was limited mostly to (meth)acrylates. Epoxides, which play an important role in semiconductor clean-room technology, were basically excluded from capitalizing on STED-inspired lithography, and if they were successfully used in STED-inspired lithography, the achievable structure sizes remained at 125 nm and above. We now found that using 2-chlorothioxanthone (CTX) as a sensitizer for a sulfonium salt acting as the photoinitiator allows for shrinking the structure size down to 83 nm. Compared to the previously used sensitizer 2-isopropylthioxanthone, the triplet lifetime of CTX within the epoxide monomers is supposed to be prolonged by 40%, which renders an optical depletion via excited triplet state absorption more efficient, leading to a sub-100 nm structuring capability.

A 3D nanoscale optical disk memory with petabit capacity

High-capacity storage technologies are needed to meet our ever-growing data demands1,2. However, data centres based on major storage technologies such as semiconductor flash devices and hard disk drives have high energy burdens, high operation costs and short lifespans2,3. Optical data storage (ODS) presents a promising solution for cost-effective long-term archival data storage. Nonetheless, ODS has been limited by its low capacity and the challenge of increasing its areal density4,5. Here, to address these issues, we increase the capacity of ODS to the petabit level by extending the planar recording architecture to three dimensions with hundreds of layers, meanwhile breaking the optical diffraction limit barrier of the recorded spots. We develop an optical recording medium based on a photoresist film doped with aggregation-induced emission dye, which can be optically stimulated by femtosecond laser beams. This film is highly transparent and uniform, and the aggregation-induced emission phenomenon provides the storage mechanism. It can also be inhibited by another deactivating beam, resulting in a recording spot with a super-resolution scale. This technology makes it possible to achieve exabit-level storage by stacking nanoscale disks into arrays, which is essential in big data centres with limited space.

Sub-diffraction optical beam lithography based on a center-non-zero depletion laser

Photoinhibition (PI) mechanisms have been introduced in nanofabrication which allows breaking the diffraction limit by large factors. Donut-shaped laser is usually selected as a depletion beam to reduce linewidth, but the parasitic process has made the results of the experiment less than expected. As a result, the linewidth is difficult to achieve below 50 nm with 780 nm femtosecond and 532 nm continuous-wave lasers. Here, we propose a new, to the best of our knowledge, method based on a center-non-zero (CNZ) depletion laser to further reduce linewidth. By constructing a smaller zone of action under the condition of keeping the maximum depletion intensity constant, a minimum linewidth of 30 nm (λ / 26) was achieved. Two ways to construct CNZ spots were discussed and experimented, and the results show the advantages of our method to reduce the parasitic process to further improve the writing resolution.

STED-Inspired Cationic Photoinhibition Lithography

Direct laser writing by two-photon lithography has been enhanced substantially during the past two decades by techniques borrowed from stimulated emission depletion (STED) microscopy. However, STED-inspired lithography was so far limited to radical polymerizations, mostly to acrylates and methacrylates. Cationic polymers did not derive benefits from this technique. Specifically, epoxide polymerization, which plays a paramount role in semiconductor clean-room technology, has not yet been reported with a second, depleting laser focus in the outer rim of the point spread function. We now found that using a thioxanthone as a sensitizer and sulfonium or iodonium salts as photoinitiators enables at least partial optical on/off switching of two-photon polymerization and, in the case of the sulfonium salt, allows for writing epoxy lines with widths shrunk by approx. two-thirds compared to lines written with two-photon polymerization alone.

Low-Fluorescence Starter for Optical 3D Lithography of Sub-40 nm Structures

Stimulated emission depletion (STED) has been used to break the diffraction limit in fluorescence microscopy. Inspired by this success, similar methods were used to reduce the structure size in three-dimensional, subdiffractional optical lithography. So far, only a very limited number of radical polymerization starters proved to be suitable for STED-inspired lithography. In this contribution, we introduce the starter Michler's ethyl ketone (MEK), which has not been used so far for STED-inspired lithography. In contrast to the commonly used 7-diethylamino-3-thenoylcoumarin (DETC), nanostructures written with MEK show low autofluorescence in the visible range. Therefore, MEK is promising for being used as a starter for protein or cell scaffolds in physiological research because the autofluorescence of DETC so far excluded the use of the green emission channel in multicolor fluorescence or confocal microscopy. In turn, because of the weak transitions of MEK in the visible spectrum, STED, in its original sense, cannot be applied to deplete MEK in the outer rim of the point spread function. However, a 660 nm laser can be used for depletion because this wavelength is well within the absorption spectrum of transient states, possibly of triplet states. We show that polymerization can be fully stopped by applying transient state absorption at 660 nm and that structure sizes down to approx. 40 nm in the lateral and axial directions can be achieved, which means 1/20 of the optical wavelength used for writing.

Model for polymerization and self-deactivation in two-photon nanolithography

A mathematical model is developed to describe the photochemical processes in two-photon nanolithography, including two-step absorption leading to initiation and self-deactivation of the photoinitiator by laser irradiance, polymer chain propagation, termination, inhibition, and inhibitor and photoinitiator diffusion. This model is solved numerically to obtain the concentrations of the reaction species as a function of time and space as a laser beam is scanned through a volume of photoresist, from which a voxel size or linewidth is determined. The most impactful process parameters are determined by fitting the model to experimentally measured linewidths for a range of laser powers and scanning speeds, while also obtaining effective nonlinearities that are similar to previously measured values. The effects and sensitivities of the different process parameters are examined. It is shown that the photopolymerization process is dominated by diffusion of photoinitiators and oxygen inhibitors, and that self-deactivation can lead to higher effective nonlinearities in two-photon nanolithography.