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Materials Science in Additive Manufacturing Ceramic vat photopolymerization

to resin by projecting an entire slice of the 2D model energy matches the electronic transition energy between
(Figure 1B). For the projection of the slice pattern onto ground and excited states, resulting in a linear photon
the resin, a digital micromirror device (DMD) panel is flux dependence. Conversely, TPA requires simultaneous
used, making the DLP printing process faster than SL and absorption of two lower-energy photons. During TPA,
facilitating quick fabrication of highly accurate parts with the molecule enters a short-lived virtual intermediate
excellent resolution, resembling the products made with state after the first photon absorption. This mechanism
SL. DLP also requires a support platform, and structures enables UV-sensitive photoinitiators (350 – 400 nm) to
need to be removed manually and may require finishing. be activated by near-infrared Ti: Sapphire lasers (700 –
The concept of DLP was first developed in 1996. Further 800 nm) through TPA.
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in 1997, liquid crystal display was replaced by DMDs 17
to create high resolution and contrast in the projected As a third-order nonlinear optical phenomenon,
pattern. DMD is an array of microscopic mirrors that are degenerate TPA exhibits an energy absorption rate
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capable of swift rotation within a range of ±10–12°. expressed as:
2
dW 8
2.3. Two-photon polymerization (TPP) I Im[ ] (III)
2
3 ()
dt cn
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TPP, as illustrated in Figure 3, utilizes the two-photon
absorption mechanism, in which molecules transition where ω represents the angular frequency, c denotes the
from ground to excited states through simultaneous speed of light in a vacuum, and n is the refractive index of
absorption of two laser photons – either with identical the medium. The nonlinear optical response arises from
energy (termed degenerate two-photon absorption the imaginary component of the third-order susceptibility
[TPA]) or distinct energy levels (non-degenerate TPA). (Im[χ ]), with the TPA process exhibiting a quadratic
(3)
This mechanism demands intense photon flux due to dependence on laser intensity. Due to this nonlinear
the molecule’s extremely brief residence (10 s) in the intensity dependence, TPA occurs only at extremely high
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transient virtual state, necessitating ultrashort-pulsed optical intensities, necessitating high-numerical-aperture
infrared lasers to achieve sufficient energy delivery. TPA (NA) microscope objectives for tight laser focusing. The
was theoretically predicted by Göppert-Mayer in 1931 and TPA effect rapidly diminishes away from the focal center,
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the experimental observation was confirmed by Kaiser and resulting in a well-defined 3D voxel that enables intrinsic
Garrett in 1961. Nowadays, the TPA process has become volumetric fabrication in two-photon lithography. The
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extensively important for technological applications, polymerization threshold is determined by the balance
such as multiphoton polymerization, multiphoton optical between radical generation (initiated by TPA) and
limiters, and multiphoton fluorescence spectroscopy. competing deactivation pathways, including quenching,
In conventional single-photon absorption (e.g., in SL internal conversion, and radical termination. Below this
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or DLP printing), excitation occurs when incident photon threshold, insufficient crosslinking occurs, while above

Figure 3. Schematic diagram of two-photon polymerization process (schematic reused under the terms of the Creative Commons CC-BY license)
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Volume 4 Issue 3 (2025) 4 doi: 10.36922/MSAM025200031