Three-dimensional (3D) printing technologies have been widely used for over a decade because of their usefulness for rapid modelling and prototyping. Recently, photopolymerization techniques have been the focal points of many research efforts because they enable high resolution fabrication of complex structures. Consequently, they have found applications in photonics , three-dimensional microfabrication , microfluidic , and drug delivery .
Photopolymerization is a form of radical polymerizations that uses light to initiate and propagate a polymerization of a polymer chain. Photopolymers are usually composed of monomer, oligomer, and photoinitiator. Under an irradiation of a photon at specific energy, the photoinitiator will be excited by the irradiation. The excited energy will be passed along to other molecules to initiate the polymerization. The photopolymerization can also be categorized by the types of light sources utilized such as continuous wave  or ultrashort pulsed lasers [1–4, 6, 7]. However, to fabricate sub-micron structures, the two-photon polymerization (TPP) process is required to circumvent the diffraction limit. While the photopolymerization relies on the direct absorption of a photon, the TPP, on the other hand, relies on the simultaneous absorption of two photons at a lower energy level, called two-photon absorption (TPA), where the energy of one photon is insufficient to complete the transition between the ground and excited states. Since the simultaneous absorption of two photons rarely occurs in nature, a specific light source that can provide high photon fluxes such as an ultrashort pulse laser is required.
Although sub-micron 3D printings have been demonstrated in literatures, the processes commonly rely heavily on femtosecond lasers such as Ti: Sapphire lasers which increase both complexity and cost, making it less accessible. In contrast, Neodymium lasers (Nd-lasers) are well-known as an inexpensive alternative that can provide efficient generation of nanosecond and picosecond pulses. However, most of available conventional two-photon photoinitiators have the absorption band in the UV region and not in the visible [8, 9], resulting in them not being suitable for TPA at Nd-laser fundamental wavelength of 1064 nm. Although there were a few investigations into the two-photon polymerization by the Nd-laser at its fundamental wavelength, a specific combination of photoinitiators and resists  or the femtosecond pulses  were required. To our knowledge, no attempt has been made to quantify and compare the TPA of commercialized visible light photoinitiators found in this work under an irradiation of nanosecond pulses provided by Nd-lasers.
To investigate the two-photon absorption of the commercialized visible light photoinitiators under the irradiation of the Q-switched Nd: YAG nanosecond pulsed laser at 1064 nm similar to . Three visible light photoinitiators were selected for this study, i.e., 4,4‘-bis(dimethyl-amino)-benzophenone (Michler’s ketone: MK) , Irgacure-784 (I784) [12, 13], and Indene-1,3-dione (Ind) . Irgacure-784 (I784) is known as a visible light photoinitiator. The photopolymerization under the irradiation at 460 nm was reported in . The photoinitiation mechanism in epoxy resin at 532 nm was presented in , making it an excellent candidate for TPA measurement at 1064 nm. In the case of Ind, a combination of Indene-1,3-dione (Ind) and a Zr-based organic-inorganic hybrid material was recently demonstrated as a photoinitiator for the two-photon polymerization under the excitation of a picosecond pulses . Though the two-photon polymerization was achieved by a common picosecond laser, a part of the combined photoinitiator was not commercially available on the market and difficult to synthesize. Another success was achieved by combining the 4,4‘-bis(dimethyl-amino)-benzophenone with a photoresist (SZ2080) . The polymerization was obtained by an Yb:KGW laser at 1030 nm, still relying on a femtosecond pulsed regime.
In this report, we aim to investigate and quantify the TPA of the commercially available photoinitiators listed above which are suitable for polymerization with Nd-lasers. Linear and nonlinear absorptions were characterized by the UV-vis spectrophotometer and the Z-scan technique, respectively . Although, both MK and I784 have strong linear absorption at 532, only MK showed an excellent nonlinear absorption at 1064 nm, comparable to other common TPP photoinitiators . With the low-cost Nd: YAG Q-switched nanosecond laser together with MK, the sub-micron 3D printing could be more applicable and affordable.