New approaches based on phase separation in soft and hard matter have recently been shown to generate anisotropic nanostructures and monitor pattern evolution in biological systems. Researchers have proposed alternative methods to classical phase separation for manipulating the spatial distribution of liquid crystal molecules in liquid crystal-monomer mixtures, and have applied them in various photonic devices.1 Additionally, multi-photon photopolymerization is now widely used to demonstrate micro- and nanomachining.2
A nonlinear response to ultrafast titanium:sapphire laser pulses can create microgratings in UV-curable monomers due to the two-photon excitation mechanism. Pattern formation caused by the nonlinear optical phenomenon of modulation instability, whereby a waveform transforms itself into a train of pulses, has been reported.3
We have demonstrated polymerization-induced phase separation in a liquid crystal-monomer composite under different scan speeds of an ultrafast pulse with both single- and two-photon excitation.4 The monomers form a polymer chain upon illumination, and the peak power of the pulse is sufficient to arouse the intrinsic nonlinearity of both materials. Thus, photopolymerization and optical nonlinearity effects simultaneously occur in this system. The former dominates when ultrafast laser pulse lithography is performed with single-photon excitation (360nm), while the latter dominates in the two-photon excitation (720nm) scheme. A high optical electric field gives rise to optical nonlinearity, driving molecular motion and liquid crystal reorientation. This self-organization can lead to formation of a high-resolution pattern.
To investigate the optical nonlinear behavior in our system, we perform photolithography with two-photon excitation and observe transverse instability in the process. At a central wavelength of 720nm, the pulse width is around 100fs, the average power is set at 800mW, and the full width at half-maximum is around 12μm with a Gaussian shape. The optical electric field is estimated to be 6×1010eV/pulse), and the laser intensity range is 0.7MW/cm2. The laser polarization is adjusted to be perpendicular to the scanning direction. Two-photon photopolymerization occurs only in the vicinity of the focal point due to high photon density in that region.
An interesting pattern evolution is observed under a pair of parallel polarizers: see Figure 1(a). A straight, uniform line is formed along the lithography path at low scanning speed. As the speed increases, a periodic sinusoidal pattern appears, which then splits into a micrograting structure. This pattern formation can be inferred as a manifestation of modulation instability, which can cause symmetry-breaking patterns in strongly nonlinear systems.
Figure 1. Optical microscopy shows liquid crystal-monomer mixtures scanned with 720nm titanium:sapphire pulses at various scanning speeds observed under (a) parallel polarizers and (b) crossed polarizers.
Under a pair of crossed polarizers, the bright region in Figure 1(b) means that the liquid crystal molecules reoriented themselves to the plane of the glass substrate. The fine structures produced by two-photon excitation reveal clean lines on the micrometer scale. As the writing polarization rotates 90°, becoming parallel to the laser scan direction, the sinusoidal pattern no longer occurs at any lithography speed. We have attained the mathematical model of pattern evolution in this scheme.
We believe this laser lithography method holds promise for creating high-resolution patterns in liquid crystal composites. The technique is suitable for composites grown on plastic substrates, such as for displays, and the patterns are promising for integrated optics with liquid crystal modulation applications. Furthermore, using ultrafast pulses to induce phase separation will help us to investigate liquid crystal-based nonlinear optics.
National Central University
Kuei-Chu Hsu is an assistant professor with the Department of Optics and Photonics. His research interests include liquid crystal optics, fiber sensors and lasers, and integrated photonic crystal devices.
1. H. Ren, S. T. Wu, Y. H. Lin, In situ observation of fringing-field-induced phase separation in a liquid-crystal-monomer mixture, Phys. Rev. Lett. 100, pp. 117801, 2008.
2. C. C. Jeng, Y. Lin, R. C. Hong, R. K. Lee, Optical pattern transitions from modulation to transverse instabilities in photorefractive crystals, Phys. Rev. Lett. 102, pp. 253905, 2009.
3. C. H. Lee, H. Yoshida, Y. Miura, A. Fujii, M. Ozaki, Local liquid crystal alignment on patterned micrograting structures photofabricated by two photon excitation direct laser writing, Appl. Phys. Lett. 93, pp. 173509, 2008.
4. K.-C. Hsu, J.-H. Lin, Ultrashort pulse induced nonlinear photo-polymerization and phase separation in liquid crystal and monomer mixtures, Proc. SPIE
7927, pp. 792712, 2011. doi:10.1117/12.871443