< > [Additive](../index.html) > Multiphoton Polymerization
## Multiphoton Polymerization (MPP)
###Photopolymerization
A <b>photopolymer</b> typically consists of:
<ul>
<li>a reactive diluent
<li>a cross-linker
and
<li>a photoinitiator.
</ul>
However, most formulations contain additional components all influencing its reactivity, viscosity, reaction mechanism and the properties of the resulting polymer. The key component is the PI, which during photpolymerisation, dissolves into radicals that break the double bonds of the monomers (cross-linkers & diluents) and initiates the solidification process. Designing photpolymerizable formulations requires understanding of the PI's decay into radicals and the process of radical chain growth polymerization.
### A comparison with classical stereolithogrpahy
Similar to stereolithography, 2PP is a laser-scanning approach. However, polymerization is not limited to the surface of the photoresin, the focal point can be moved anywhere in the volume causing polymer curing along its trace. Thus, MPP can be conidered a true 3D writing process eliminating all the drawbacks (high viscosity of the photoresins leading to high surface tension, the necessecity of recoating, the need for supporting structures) related to the surface polymerization in layer by layer fabrication processes.
The basic building unit, where polymerisation takes place (volumetric pixel or voxel) defines the resolution to be as high as 65 nm. This is an effect of a non-linear activation principle, substantially different from one-photon activation in other lithography-based AM porcesses.
### History of Origin
####From Two Photon Absorption (2PA)/Multiphoton Absorption (MPA) to Two Photon-Polymerization (2PP)/Multiphoton Absorption
<ul>
<li> 1931: Theroretical description of simultaneous <i> two photon absorption (2PA) </i> by Maria Goppert-Mayer - <a href = 'https://onlinelibrary.wiley.com/doi/epdf/10.1002/andp.19314010303'> Original Paper in German </a> - <a href='https://onlinelibrary.wiley.com/doi/epdf/10.1002/andp.200910358'>Translated Paper in English </a>
<li> 1960s: 2PA experimental validation at Bell Labs - <a href='https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.7.229'> Original Paper
<li> 1980s: Solid-state femtosecond pulsed lasers become available
<li> 1992: <a href ='https://www.spiedigitallibrary.org/conference-proceedings-of-spie/1674/1/Two-photon-lithography-for-microelectronic-application/10.1117/12.130367.full?SSO=1'> 2PP process as a lithographic technique </a>
<li> 1997: <a href='https://www.osapublishing.org/view_article.cfm?gotourl=https%3A%2F%2Fwww%2Eosapublishing%2Eorg%2FDirectPDFAccess%2FC09B5705-A21D-200E-37959D416D8D06AC_35919%2Fol-22-2-132%2Epdf%3Fda%3D1%26id%3D35919%26seq%3D0%26mobile%3Dno&org=Massachusetts%20Institute%20of%20Technology%20Libraries'> 2PP stereolithography</a> further developed by Maruo and co-workers by using a pulsed Ti:sapphire laser with computer-driven galvano mirrors to direct the beam within the a volume of a photoresin and build high-resolution free-standing structures orders of magnitude smaller compared to other lithographic techniques.
</ul>
from <a href='http://www.ub.tuwien.ac.at/diss/AC07815668.pdf'> <b> here: </b></a>
"The combination of several photons can excite a molecule to an energy electroninc state higher than caused by one photon only. The frequencies of the photons can be different but must sum up to the resonance frequency of the molecule. The simplest version of Goppert-Meyer's prediction is 2PA. In 2PA, two photons interact with a molecule producing an energy state similar to an excitation with one photon of higher energy.
2PA differs from 1PA in the resonance time of the molecule. In a 1PA, the electric field of the photon is in resonance with the molecule for a longer period; it oscillates in phase with the polarisation resulting in a finite transition probability. In 2PA, however, the molecule is only in resonance for a short time rendering no probability for a 1PA.This depends on the photons interacting with a molecule nealry simulataneously (within a time frame of 10^-15s). Following Lambert-Beer's law for 1PA, the absorption of a light beam in 2PA can be expressed and shows that the proabbility of 2PA depends quadratically on the laser intensity. <b>Under tight focusing of a pulsed laser, the absorption is limited to the focal point as the number of molecules excited decreases rapidly with the distance from the intensity maximum. This results in spatial resolutions below the difraction limit of light within a confined 3D volume inside amedium.</b>
A good comparison between 1PA and 2PA is given in the right image of the following figure. A UV laser is focused into a Rhodamine B solution. The sample is excited over the entire length of the beam rendering a double cone like fluorescence. The smallest diameter of this fluorescing volume is in the focal point of the microscope objective. If we focus a femtosecond pulsed near-infrared laser emitting at 100 kHZ repetitions rate and 800 nm wavelength into the same sample we only get a fluorescence in the focal volume, where the square of the intensity is sufficient to cause 2PA. In any other plane of the sample, the intensity is insufficient, and no fluorescence is obtained."
2PP is an AM process with different referall names:
<ul>
<li> Two-Photon Absorbed Photopolymerization
<li> Two-Photon Induced Polymerization
<li> Two-Photon Lithography
<li> Two-Photon Laser Scanning Lithography
<li> Multiphoton-excited Microfabrication
<li> 3D Multiphoton Lithography
<li> 3D Laser Lithography
<li> Direct Laser Writing
</ul>
The term <b>"Multiphoton"</b> refers to the fact that the simultaneous absorption of three or more photons could be realized (minimal probability) and cause crosslinking of the photoresin.
### Operating Principle & System Components of Multiphoton Polymerization (MPP)
To trigger the nonlinear two-photon absorption process, light sources with very high photon density are required. Most currently MPP setups are based on pulsed femtosecond-lasers with pulse durations between 50 and 150 fs. Amplified laser systems allow benefits such as tunable wavelength, pulse duration and intensity, but are limited with regard to maximum repetition rate, which is typically on the order of several kHz. The low repetition rate limits the maximum writing speed of MPP, because at least one laser pulse is required per voxel to trigger polymerization. For this reason, nonamplified lasers are more common for MPP, based either on Ti:sapphire or on fiber lasers. Laser powers vary between 10 and 700 mW, with pulse durations typically around 100 fs and repetition rates of 10−100 MHz.
1) Laser, 2) AOM, 3) Pinhole, 4) Beam expander, 5) Galvanoscanner, 6) Microscope objective, 7) Specimen lamp, 8) XYZ tranlsational axes, 9) CCD camera, 10) Laser power meter
A typical setup for MPP is depicted in the Figure above. The laser beam passes first through a collimator and then through an acousto-optical modulator (AOM), which disperses the beam
into zero- and first-order diffractions. The first-order output can be turned on and off by switching the AOM. The first-order output is fed through a λ/2 wave-plate, which can be rotated to
adjust the laser power. The beam is finally directed through a microscope objective to focus it into the sample holder containing the photopolymerizable formulation. A camera can be positioned behind a semitransparent mirror to allow online observation of the polymerization process. By illuminating the sample with the appropriate lighting (e.g., red light emitting diodes), imaging of the sample is further improved.
1) Laser, 2) acousto-optic modulator and 3) laser power meter
Positioning of the laser focus for MPP can be achieved by two different methods:
<ul>
<li>The positioning of the laser beam in the xy-plane is controlled with piezo-actuated or linear airbearing stages.
<li>The laser beam passes through a galvanoscanner, which is positioned before the microscope objective.
</ul>
1) Galvanoscanner, 2) microscope objective, 3) XYZ axes, 4) CCD Camera and 5) sample illumination
The drawbacks are mostly related to the limited build size: For high-resolution structures,
immersion-oil objectives with high magnification (typically 100×) have to be used. In combination with a galvanoscanner, this setup is limited to build sizes of approximately 30 × 30 μm.
Piezo-actuated stages allow slightly larger scan areas (around 200 × 200 μm), while high-precision air-bearing stages cover significantly larger build areas (up to 100 × 100 mm).
Scan speeds up to 1000 mm s−1 are possible when using highly reactive resins with suitable photoinitiators and appropriate optics. For parts that require very high resolution and precision,
100 μm s−1 up to 1 mm s−1 are commonly used writing speeds.
The spatial resolution of microstructures built by MPP is unrivaled, with feature sizes well below 100 nm being common.
The Figure below illustrates the relation of laser intensity and voxel size on feature resolution defined by the volume in which polymerization occurs. Two important power boundaries are defined:
<ul>
<li>the polymerization/fabrication threshold (Pth)
<li>the damage threshold (Pdam), which dictate the useful range of power for the laser.
</ul>
For clarification, the resin will polymerize as soon as the density of initiating radicals exceeds a certain minimum concentration but will be destroyed when the laser power is too high. Pth is lower in resins with efficient photoinitiators, and Pdam will depend on heat transport and stability of the resin. Feature resolution may be further reduced (to roughly 65 nm) with the addition of radical quenchers, which limit propagation and thus spatially confine polymerization.
Effect of the laser intesity on the voxel diameter
Basic principle of tracing the focal point with a galvanoscanner mirror in 2PP; the movement from position a) to b) creates a polymer line inside the photopolymerisable formulation.
###Videos
<b> High Speed Fabrication of a race car </b>
https://www.youtube.com/watch?v=5y0j191H0kY
<b> Castle on a pencile tip </b>
https://www.youtube.com/watch?v=mdup3w7DCZE&feature=youtu.be
<b> 2PP with sub-50 nm resolution by sub-10 fs laser pulses. </b>
https://www.youtube.com/watch?v=IqdwrZtmL0M
<b> Direct laser writing of 3D helices</b>
https://www.youtube.com/watch?v=NaTI7OYRqb0
<b> Microscale Spaceship</b>
https://www.youtube.com/watch?v=wThtfAtB5U8
<a href='https://www.nanoscribe.de/files/9215/1912/9367/ApplicationFlyer.pdf'> Application Exmaples using the Nanoscribe system </a>
###References
If interested more in the topic:
<ul>
<li> A BOOK: <a href='https://onlinelibrary.wiley.com/doi/book/10.1002/9783527682676'> LINK
<li> A THESIS: <a href='http://www.ub.tuwien.ac.at/diss/AC07815668.pdf'> LINK
</ul>
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