This post will introduce a roll call of classic papers about direct laser writing (DLW) in a chornological order, and talk briefly about their significance. Over the years there have been many papers that defined our understanding of this field of technology and spanning physics, mechanical engineering, and material science. It is with advances in all fronts that has led to the technology to its current standing.
1. 1990: Two-photon laser scanning fluorescence microscopy (doi: 10.1126/science.2321027)
W Denk, JH Strickler, WW Webb, Science 248, 4951, 73-76 . This paper based on a patent by the same authors combining two-photon absorption and laser scanning. This laid foundation of two-photon fluorescence microscopy and eventually two-photon lithography or direct laser writing.
Abstract: Molecular excitation by the simultaneous absorption of two photons provides intrinsic three-dimensional resolution in laser scanning fluorescence microscopy. The excitation of fluorophores having single-photon absorption in the ultraviolet with a stream of strongly focused subpicosecond pulses of red laser light has made possible fluorescence images of living cells and other microscopic objects. The fluorescence emission increased quadratically with the excitation intensity so that fluorescence and photo-bleaching were confined to the vicinity of the focal plane as expected for cooperative two-photon excitation. This technique also provides unprecedented capabilities for three-dimensional, spatially resolved photochemistry, particularly photolytic release of caged effector molecules.
2. 1991: Two-photon excitation in laser scanning fluorescence microscopy (doi:10.1117/12.47787)
JH Strickler, WW Webb, Proc of SPIE Vol. 1398 CAN-AM Eastern ’90 / 107. The same authors in 1991 explored the possibility of using two-photon laser scanning to fabricated microstructures in these proceedings.
Abstract: Simultaneous absorption of two red photons from a strongly focused subpicosecond, colliding pulse, mode-locked dye laser stimulates visible fluorescence emission from fluorophores having their normal absorption in the ultraviolet1. The quadratic increase of the two-photon excitation rate with excitation intensity restricts fluorescence emission to the focal volume, thus providing the same depth resolution as does confocal microscopy. Image degradation due to out of focus backround is thus avoided. Photobleaching and most cellular photodamage are similarly confined to the focus, thereby minimizing sample degredation during acquisition of the multiple sections required for 3-d image reconstruction. Fluorescence images of living cells and other thick photolabile fluorescence labled assemblies illustrate the depth discrimination of both two-photon fluorescence excitation and photobleaching.
3. 1997: Three-dimensional microfabrication with two-photon-absorbed photopolymerization (doi:10.1364/OL.22.000132)
S. Maruo, O. Nakamura, and S. Kawata, Optics Letters, 22, 2, 132 . This paper from Osaka university researchers was the point of inflexion for DLW research as they described two-photon direct laser writing of three-dimensional spiral structures from urethane acrylate based photoresists. In this paper they also discussed the possible use of this techniques to make light driven mircomover, which in todays terms would be an optically driven micro-robot.
Abstract: We propose a method for three-dimensional microfabrication with photopolymerization stimulated by twophoton absorption with a pulsed infrared laser. An experimental system for the microfabrication has been developed with a Ti:sapphire laser whose oscillating wavelength and pulse width are 790 nm and 200 fs, respectively. The usefulness of the proposed method has been verified by fabrication of several kinds of microstructure by use of a resin consisting of photoinitiators, urethane acrylate monomers, and urethane acrylate oligomers.
4. 1998. Design of Organic Molecules with Large Two-Photon Absorption Cross Sections (doi:10.1126/science.281.5383.1653)
M. Albota, D. Beljonne, J.-L. Bre´das, J. E. Ehrlich, J.-Y. Fu, A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder, D. McCord-Maughon, J. W. Perry,* H. Ro¨ckel, M. Rumi, G. Subramaniam, W. W. Webb,* X.-Li Wu, C. Xu, Science 281,1653. This landmark paper explored the role of dipolar as well as quadrupolar organic molecules as strong-two photon absorption. The fundamental understanding of high cross section two-photon absorbers has then on led to the development of highly efficient two-photon absorbers and hence increased the energy efficiency, and controllability of direct laser writing.
Abstract: A strategy for the design of molecules with large two-photon absorption cross sections, 𝛿, was developed, on the basis of the concept that symmetric charge transfer, from the ends of a conjugated system to the middle, or vice versa, upon excitation is correlated to enhanced values of 𝛿. Synthesized bis(styryl)benzene derivatives with donor-π-donor, donor-acceptor-donor, and acceptor-donor- acceptor structural motifs exhibit exceptionally large values of 𝛿, up to about 400 times that of trans-stilbene. Quantum chemical calculations performed on these molecules indicate that substantial symmetric charge redistribution oc- curs upon excitation and provide 𝛿 values in good agreement with experimental values. The combination of large 𝛿 and high fluorescence quantum yield or triplet yield exhibited by molecules developed here offers potential for un- precedented brightness in two-photon fluorescent imaging or enhanced pho- tosensitivity in two-photon sensitization, respectively.
5. 1999. Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication (doi: 10.1038/17989 )
B. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I.-Y. Sandy Lee, D. M.-Maughon, J. Qin, H. Roeckel, M. Rumi, X. -L. Wu, S. R. Marder, J. W. Perry, Nature 398, 51. This paper demonstrates the application of some of the materials developed in the previously listed paper.
Abstract: Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order 3 (where is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of ;-conjugated compounds that exhibit large (as high as 1, 250 10-50 cm4s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.
6. 2000. Structure-Property Relationships for Two-Photon Absorbing Chromophores: Bis-Donor Diphenylpolyene and Bis(styryl)benzene Derivatives (doi:10.1021/ja994497s )
M. Rumi, J. E. Ehrlich, A. A. Heikal, J. W. Perry, S. Barlow, Z. Hu, D. McCord-Maughon, T. C. Parker, H. Roekel, S. Thayumanavan, S. R. Marder, D. Beljonne, and
J.-L. Bre´das, J. Am. Chem. Soc. 122, 9500. As the name suggests this work explores the structure-property relationships in two-photon absorbers. This classic paper established many ground rules in the design of high-two photon absorbers.
Abstract: The two-photon absorption properties of a series of bis dialkylamino- or diarylamino-substituted diphenylpolyenes and bis(styryl)benzenes have been investigated. Two-photon absorption cross sections, δ,as large as 1420 × 10^-50 cm^4 s/photon-molecule have been observed for molecules with this general bis-donor structure. The effect of the type and length of the conjugated chain and of dialkylamino or diarylamino substitution on the position and magnitude of the peak two-photon absorptivity is reported. The transition dipole moments for the transitions between the ground state and the first excited singlet state (Mge) and between the first and second excited singlet states (M^ee′) have been estimated using experimental data from the one- and two-photon spectra. It was found that increases in chain length result mainly in an increase in Mge, whereas the addition of donor end groups or going from diphenylpolyene- to phenylene-vinylene-type bridges leads primarily to an increase inM^ee′. The trends in the energy of the lowest excited singlet states and in the transition moments for the diphenylpolyene series as a function of chain length are in agreement with those calculated by quantum mechanical methods. These results furnish a link between structural features in these classes of molecules and the electronic dipole couplings and state energies that control the strength of the two-photon absorption. In bis(aminophenyl)polyenes containing up to four double bonds (m) the lowest excited singlet state is a Bu state, as opposed to the case of simple polyenes and diphenylpolyenes, for which it is an Ag state for m > 2. The relationship of the state ordering in these systems with the observed values of the radiative and nonradiative decay rates is also discussed.
7. 2001. Finer features for functional microdevices (doi:10.1038/35089130)
S. Kawata, H.-B. Sun, T. Tanaka, K. Takada, Nature, 412, 697. Through this brief communication Osaka university researchers brought DLW to the wider scientific audience through this paper and kick stated the current growth of the field.
Abstract: Compared with light or electron-beam lithography, the virtue of two-photon photopolymerization1 as a tool for making microdevices lies in its threedimensional capability, which has found application in photonic devices and micromachines with feature sizes close to the diffraction limit. Here we show that the diffraction limit can be exceeded by nonlinear effects to give a subdiffraction-limit spatial resolution of 120 nanometres. This allows functional micromachines to be created and shifts the working wavelength of photonic and opto-electronic devices into the visible and near-infrared region.
8. 2003. Scaling laws of voxels in two-photon photopolymerization nanofabrication (doi:10.1063/1.1599968)
H.-B. Sun, K. Takada, M.-S. Kim, K.-S. Lee and S. Kawata, Appl. Phys. Lett. 83,1104. This paper established the ground rules for the formation and evolution of the voxel size. It discusses the interplay between power and exposure time in the formation and growth of voxels.
Abstract: Understanding the performance of individual voxels is critical to establish two-photon photopolymerization as a nanoprocessing tool. In this letter, we report that at near-threshold exposure condition voxel shape scaling follows different laws in the courses of varying laser power and changing exposure duration. The voxel aspect ratio, when exposure enhanced, monotonically increases and reaches a saturation status for the two processes that are conventionally regarded as equivalent for voxel volume tuning. A growth model that deals with fine photopolymerization process occurring at the submicron-focal volume is proposed and applied to interpret the unexpected phenomenon. © 2003 American Institute of Physics
9. 2004. SU-8 for real three-dimensional subdiffraction-limit two-photon microfabrication
Abstract: We report the inherent utility of two-photon-absorption (TPA) in the fabrication of real three-dimensional (3D) structures with subdiffraction-limit resolution, based on SU-8 as the threshold polymer media. We exploit the nonlinear velocity dependence of TPA photopolymerization as the shutter mechanism for disruptive 3D lithography. We show that low numerical aperture optics can be used for the rapid microfabrication of ultrahigh-aspect ratio photoplastic pillars, planes, and cage structures.
10. 2005. Effect of low numerical-aperture femtosecond two-photon absorption on (SU-8) resist for ultrahigh-aspect-ratio microstereolithography
Abstract: We report the quantitative characterization and analysis on the solidification of SU-8, a chemically amplified near-ultraviolet ultrathick resist, based on two-photon-absorbed (TPA) near-infrared photopolymerization. The resolution of TPA photopolymerized SU-8 voxels and lines is studied as a function of laser-pulse energy, single-shot exposure time, and scanning speed. Two-photon microstereolithography using SU-8 as the matrix material was verified by the fabrication of SU-8 photoplastic structures with subdiffraction-limit resolution. We show that the nonlinear velocity dependence of TPA photopolymerization can be used as the shutter mechanism for disruptive three-dimensional (3D) lithography. This mechanism, when combined with low numerical-aperture optics is exploited for the rapid 3D microfabrication of ultrahigh-aspect-ratio (up to 50:1) photoplastic pillars, planes, and cage structures.
The above two papers driven by researchers at IBM established the applicability of the popular cationic photoresist SU8 for DLW. They also feature a systematic study of parameters involved in microfabrication which are very useful for optimization of fabrication parameters.
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