# High-Throughput DNA Analysis Platform Based on an Optofluidic Ring Resonator Laser
## 3 Results  
The laser emission spectra from the target DNA assay and base mismatched DNA samples are shown in Figure 2. While microdroplets of each DNA sample were generated in separate fluidic tubing systems, both droplet samples were connected to the same OFRR and optically pumped with an energy density of 2.1 mJ/mm\({}^{2}\). The curves were shifted vertically for clarity. The target DNA samples exhibited a clear laser emission with typical multi-mode lasing from the OFRR cavity. However, the base mismatched DNA samples presented virtually zero optical signals. Although a free-space coupled fluorescence emission from the base mismatched samples can be observed with the naked eye, there is no observable optical signal from the detection portion. This is because our system, with a fiber taper coupler, only detects the laser emission, excluding the undesired \begin{table}
\begin{tabular}{c c c} \hline
**ssDNA** & **Target** & **Base Mismatched** \\ \hline Sequence 1 & 5\({}^{\prime}\)-ACA ACA AAG AAC ATA CAT AGG-3\({}^{\prime}\) & 5\({}^{\prime}\)-ACA ACA AAG AAC ATA CAT AGG-3\({}^{\prime}\) \\ Sequence 2 & 5\({}^{\prime}\)-CCT ATG TAT GTT CTT TGT-3\({}^{\prime}\) & 5\({}^{\prime}\)-CCT ATG TAT AT CTT TGT TGT-3\({}^{\prime}\) \\ \hline \end{tabular}
\end{table} 
Table 1: DNA sequences of target and base mismatched samples. fluorescence background. The pump energy density of 2.1 mJ/mm\({}^{2}\) is above the lasing threshold of the target DNA laser and below that of the base mismatched DNA laser. Thus, the target DNA was distinguishable from non-target DNA using only a single pulse of laser emission. To further investigate the lasing threshold difference between the target and base mismatches, the laser emission intensities were measured by varying the pump energy density. Figure 3 illustrates the spectrally integrated laser emission intensity as a function of pump energy density. The lasing threshold derived from the linear fit of the laser intensity was approximately 0.62 mJ/mm\({}^{2}\). This is slightly higher than that of the DNA laser with a continuous phase but comparable to previous reports. When microdroplets of the DNA sample solution are inserted into the OFRR portion, the droplets are elongated and the DNA solution comes into contact with the capillary OFRR wall. This is because the hydrophilic phase of the aqueous solution expels the hydrophobic continuous phase (silicon oil). Thus, this is likely not the main reason for the slight increase in the laser threshold. In fact, laser characteristics can be affected by several factors, such as the intrinsic Q factor of the cavity. The most important caveat of this experiment is that lasing from the base mismatched DNA (non-target DNA) is not measurable, even at a high pump energy density. However, the laser threshold of the base mismatched DNA is significantly higher than that of the target at room temperature. Therefore, distinguishing the 21-base-pair target DNA from non-target samples is a straightforward process with our laser-based DNA sequence analysis system. The most distinctive advantage of our DNA analysis system Is that it can rapidly detect the target DNA in different samples. To prove this concept, each microdroplet should be analyzed separately; thus, they can be regarded as individual reaction chambers with small volumes. We generated alternating microdroplets of the target DNA and base-mismatched DNA with two T-junctions in the fluidic tubing portion, as previously described. Figure 4 shows the optical signals observed using the spectrometer at the distal end of the fiber taper as a function of time. When the target DNA sample microdroplets passed the analysis point of the OFRR, clear laser emission was observed, and there was no optical signal when the base-mismatched microdroplets were analyzed. This directly proves that our OFRR DNA  
Figure 2: Laser emission spectra from the target DNA and base mismatched DNA when pumped with the same energy density. Note that the curves were vertically shifted for clarity.