# Femtosecond Laser-Inscripted Direct Ultrafast Fabrication of a DNA Distributor Using Microfluidics
## 2 Methods
### Micro-Scribing with a Femtosecond Laser  
A femtosecond pulsed laser system was used to fabricate the microfluidic channels. A seed laser (PHAROS PH1-SP; Light Conversion, Vilnius, Lithuania) can deliver a maximum pulse energy of 1.5 mJ, maximum average power of 6.0 W, and full width at a half maximum (FWHM) of 8.2 nm at a wavelength of 1030 nm. The laser system selected the wavelength among the fundamental (1030 nm), second (515 nm), and third (343 nm) harmonic outputs. The resulting beam used for laser micro-scribing was Gaussian with a beam quality of \(M^{2}<1.3\) (TEM\({}_{00}\)) and a wavelength of \(\lambda=343\) nm. The laser wavelength in ultraviolet (UV) region (i.e., 343 nm) has a smaller divergence angle than that of near infrared (NIR) region (i.e., 1030 nm), leading to the increase of the absorption rate of the irradiated laser beam in a transparent material such as PDMS [23]. In addition, when PDMS is scribed with a shorter laser wavelength, ablation and thermal damage can be minimized. Considering these advantages, the wavelength of 343 nm was chosen for laser micro-machining of PDMS.  
When using \(\lambda=343\) nm, maximum average power and maximum pulse energy were 1.8 W and 0.375 mJ, respectively. The laser micro-scribing was performed using a moving stage (ABG 10000, Aerotech, Inc., Pittsburgh, PA, USA) at a speed of 500 mm/s. The laser beam was focused on the sample using a microscopic objective lens (M Plan Apochromat NUV; Mitutoyo, Kawasaki, Japan) with a numerical aperture (NA) of 0.42. The focal length of lens and the beam spot size were 15 mm and 5 \(\upmu\)m, respectively. The central peak of the beam was approximately 1 \(\upmu\)m in diameter. The pulse repetition rate (PRR) and pulse duration were 600 kHz and 190 fs, respectively.  
A 5-mm thick PDMS block was placed on the jig for micro-scribing in an unenclosed environment, as shown in Figure 1a. To find the exact location of the top surface, the sample was scanned using the focal distance of the attenuated beam in the \(z\)-direction. A two-dimensional channel drawing was loaded into the laser system, and laser micro-scribing was performed. To investigate the geometric characteristics of the micro-channels, the PDMS samples were ablated with various laser intensities according to the energy doses of 1%, 5%, 10%, 15% and 20% of the energy peak (Table 1). For each energy dose, the process was performed with 5, 10 and 15 laser beam passes. After the laser micro-scribing, the samples were cross-sectioned using a razor blade. The cross-sectioned surface was sputter-coated with Pt, and then the geometric features were observed using a field-emission scanning electron microscope (FE-SEM; SUPRA 40 VP, Carl Zeiss AG, Oberkochen, Germany). The experiments were repeated three times to confirm reproducibility.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Energy Dose (\%)** & **Pulse Energy (\(\upmu\)J)** & **Laser Fluence (J/cm\({}^{2}\))** \\ \hline
1 & 3.75 & 19.11 \\
5 & 18.75 & 95.54 \\
10 & 37.50 & 191.08 \\
15 & 56.25 & 286.62 \\
20 & 75.00 & 382.16 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the laser micro-scribing parameter at various laser intensities in terms of energy doses.  
Figure 1: \(Cont.\)