High- and Low-Fluorescent Photoinitiators for Multiphoton Lithography

Abstract

Multiphoton lithography (MPL), an additive manufacturing method, enables the fabrication of intricate three-dimensional micro- and nanostructures with high spatial resolution, crucial for applications in photonics, micro-optics, and biomedicine. Central to the performance of the MPL is the choice of photoinitiator (PI), which governs polymerization efficiency, resolution, and application-specific functionality. However, conventional PIs often suffer from drawbacks such as high autofluorescence and poor spectral selectivity, limiting their utility in fluorescence-sensitive applications. This work presents a systematic study on the nonlinear optical (NLO) properties of lab-made low-fluorescence PIs (LF, indane-1,3-dione-based push-pull compounds), comparing them to high-fluorescence PIs (HF, triphenylamine-based aldehydes), and examines their effectiveness for MPL. The NLO properties of the PIs were investigated employing the two-beam initiation threshold (2-BIT) method and Z-scan technique both in solution and integrated into the hybrid photoresist SZ2080. The characterization of NLO properties and manufacturing tests were performed within a single optical setup, under similar spectrotemporal laser radiation conditions (pulse width, 150 fs; wavelength, 780 nm). This proposed approach allows for a straightforward and efficient evaluation of the suitability of aPI for MPL. LF-PIs were found to be up to 2 orders of magnitude less fluorescent than HF-PIs, as determined by photoluminescence analysis, and exhibited up to 10-fold higher NLO absorption-related parameters. This indicates that high fluorescence may compete with the NLO performance by interfering with absorption processes essential for effective polymerization. Most importantly, LF-PIs enabled structuring performance comparable to that of SBB (a benchmark material for low-fluorescent MPL-fabricated structures) when embedded in SZ2080, and the resulting printed structures exhibited an improved selective fluorescence response, indicating their strong potential for printing scaffolds in biorelated applications, where a high fluorescent signal usually hinders signal detection and analysis.

Keywords: Multiphoton lithography; fluorescence; nonlinear optical properties; photoinitiators; polymerization.

1

2-BIT/Z-scan/MPL optical system (left) and a detailed schematic of the 2-BIT experimental setup (right): BS, beam splitter; P, polarizer; λ/2, half-wave plate; PBS, polarizer beam splitter; M, mirror; DM, dichroic mirror.

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2-BIT/Z-scan/MPL optical system (left) and a detailed schematic of the Z-scan experimental setup (right). ND: neutral density filter.

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The 2-BIT/Z-scan/MPL optical system (left) and a detailed schematic of the MPL experimental setup (right): AOM, acousto-optic modulator; M, mirror.

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(A) Chemical structures of the investigated PIs. (B) Absorption spectra of HF-PIs and (C) LF-PIs in DCM. The dashed cyan and light-blue lines represent the wavelength for one- and two-photon absorption, correspondingly. (D) Close-up view of the absorption spectrum for PIR and PIG, highlighting the 550–850 nm range.

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2-BIT data for the HF-PIs (PI₁,PI₂,PI₃) and LF-PIs (PIR,PIG). The blue, light-blue, and cyan colored lines are the best fits to eq for three-, two-, and one-photon absorption, respectively, given as a reference. The error bars represent standard deviations derived from multiple measurements.

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(A) OA Z-scans of HF- and LF-PIs: PI₁ (6.6 mM), PI₂ (4.6 mM), PI₃ (6.1 mM), PIR (2.6 mM), and PIG (2.5 mM), under different laser excitation intensities. (B) Values of imaginary part of second-order hyperpolarizability (Im γ) and two-photon absorption cross section (σ) of HF- and LF-PIs, under 150 fs, 780 nm laser excitation. BIS and SBB are mentioned as references for HF-PIs and LF- PIs, respectively.

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Photoluminescence measurements of the cubes (inserted SEM images) fabricated with (A) HF-PIs and (B) LF-PIs incorporated into SZ2080, excited with P = 35 mW at wavelength of 325 nm CW irradiation.

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SEM images of 3D microstructures of LF-PIs. (A) 3D scaffolds shaped in the N (PIR), L (PIG), and L (SBB) letters. (B) 3D scaffolds of the same PIs in cubic shape. Red, green, and black are representing the intrinsic coloration of each PI.