Tunable thin-film dye lasers based on DNA complexes

Due to its double-helical structure, DNA can bind to dye molecules, resulting in unusual optical phenomena such as fluorescence enhancement. These DNA-dye complexes have the potential to improve the performance of dye-based devices, such as lasers. In fact, recent studies have demonstrated that DNA-surfactant aggregates can be used to enhance dye laser performance, even when the original fluorescence yield of the dye is relatively poor.¹

Here, we observe amplified spontaneous emission (ASE) and demonstrate tunable lasing from thin films composed of a hemicyanine dye and DNA-cetyltrimethylammonium (CTMA) complex.²

Figure 1 depicts the molecular structures of the dyes used in this work: DiQC₂(1) and its longer conjugation analogue DiQC₂(3). We prepare DNA-CTMA complexes by replacing the sodium ions in DNA salt (from purified salmon milt) with cationic CTMA. The complex and the dye are then prepared so that the molar ratio of DNA base pairs to the dye is 20, and subsequently dissolved in a 4:1 mixture of chloroform and ethanol. Films 4∼5μm in thickness are fabricated using a spin coating method.

Figure 1 also shows a schematic of the optical setup, in which a distributed feedback (DFB) dynamic grating is formed when two coherent beams are irradiated from different incident angles. With a dynamic grating, the optical gain or refractive index is periodically modulated, which leads to wavelength selective feedback.³ This setup is advantageous in that the dye excitation and grating formation occur simultaneously. Furthermore, the lasing wavelength can be continuously tuned by simply varying the incident angles of the excitation beams. This use of a dynamic grating can eliminate complicated device fabrication processes, and is adequate for disposable dye laser components.

Figure 1. Molecular structure of DiQC₂(n) and a schematic illustrating the formation of a dynamic grating from two-beam interference.

We produce interference fringes by overlapping two beams with equal energy generated from a frequency-doubled Nd³⁺:YAG laser. Because the coherence length of the laser is about 10mm, adjustments are made to keep the optical path lengths almost the same. The size of the interference region on the films is 1×1mm² for DiQC₂(1) and 1×5mm² for DiQC₂(3). The angle 2θ between the two beams is varied in order to tune the emission wavelength by moving the positions and directions of mirrors. Based on the fluorescence range of the dyes and the refractive index of the DNA complex (1.5∼1.51), the required incident angles are estimated to be 41–43^° and 33–35^° for DiQC₂(1) and DiQC₂(3), respectively.

Figure 2. Lasing wavelength is shown as a function of the angle between two pumping beams for the two dyes. The dots are the experimental data, and the solid curve shows the relationship calculated by using an effective refractive index value of 1.505 for the DNA complex. The inset shows examples of the lasing spectra.

Figure 2 shows the relationship between the incident angle and lasing wavelength for the two dyes. For DiQC₂(1), lasing activity occurs between wavelengths of 570 and 610nm, while for for DiQC₂(3) it occurs between wavelengths of 670 and 710nm. The experimental values coincide well with the predicted curve, which is also shown in Figure 2. Some examples of the lasing spectra are plotted in the inset of Figure 2 as well. The widths of the emission peaks are about 1.0 nm, and primarily set by the resolution of the spectrometer. Well-defined thresholds are observed at 3.2 and 5.0mJ/cm² for DiQC₂(1) and DiQC₂(3), respectively. In contrast, we did not observe any laser emission or ASE from uncomplexed dyes in a conventional polymer like poly(methyl methacrylate) (PMMA).

Figure 3. a) Changes in the absorption peak intensity over time as samples are subjected to 30-second laser pulses over the course of 10 minutes for the dyes in the DNA-complexed film and PMMA. b) Absorption spectra of DiQC₂(3) in both matrices measured before and after the irradiation experiments.

Besides enhancing fluorescence, the complexation of DNA with organic dyes is expected to improve the durability of the dye-based lasers. We compare the degradation process of dye-doped films containing DNA complexes to that of dyes in PMMA through absorbance measurements. Figure 3(a) shows the change in absorption peak intensities over time for DiQC₂(1) and DiQC₂(3) in both matrices as they are subjected to 30-second laser pulses (532nm, 15mJ/cm², 10Hz) over the span of 10 minutes. These results show that the degradation of DiQC₂(1) is clearly suppressed by the DNA complex. A similar improvement is observed in DiQC₂(3), although its lifetime is shorter.

Figure 3(b) shows the absorbance spectra of DiQC₂(3) doped in both matrices before and after degradation. In PMMA, the dye shows strong absorbance peaks in the shorter wavelength region, indicating contributions f from dimers or dye aggregates. After the degradation experiments however, the dye absorbance in the PMMA is significantly reduced. The dye-doped film with DNA complexes, however, show weaker absorbance at shorter wavelengths suggesting a lesser extent of dye aggregation. After the degradation study, the dye in the DNA-complexed material still retains key features of its absorbance spectra. The dye robustness may be partially due to the prevention of aggregation by the DNA.

We successfully generated tunable laser emission from weakly fluorescent cyanine dyes doped in DNA complexes, demonstrating the applicability of DNA as an optical material. The robustness of the laser dye in the DNA matrix is also improved when compared to the dye performance in a conventional polymer such as PMMA. Incorporating wavelength tunability into compact lasers will pave the way for new applications such as environmental sensing and biomedical imaging. Future work will focus on incorporating dynamic gratings with higher modulation depth in order to realize tunable surface emission.