Biopolymer-based hybrid systems for lasing

The unique properties derived from its double-helix structure make DNA a very interesting material for photonic applications such as data storage, optical switching, and microlasing. Accordingly, it is a popular subject of study.^1–3 Matrices made of DNA-surfactant complexes containing nonlinear optical or photochromic molecules (i.e., that change color in response to light), such as disperse red 1 (DR1), exhibit very fast and reversible diffraction-grating formation appropriate for dynamic holography. We have proved this behavior for a DNA-cetyltrimethylammonium (CTMA, a cationic surfactant) complex with DR1 through observation of optical correlation and phase-conjugation phenomena.^4–7 We have also tentatively explained the response speed to light excitation by semi-intercalation of a photochromic dye in the DNA-CTMA matrix (see Figure 1) and confirmed our hypothesis by Monte Carlo simulations.⁸

Figure 1. Illustration of the semi-intercalation of a photochromic molecule of disperse red 1 (DR1) in a DNA-cetyltrimethylammonium (CTMA, a cationic surfactant) biopolymer. DR1-trans and -cis: Different configurations of DR1.

It was recently discovered that the same DNA-CTMA matrix loaded with luminescent dye shows strong fluorescence enhancement.^9,10 This finding has application in the construction of thin-film lasers, especially so-called distributed-feedback Bragg (DFB) lasers.¹¹ In these lasers, optical excitation of a dye by another pump laser leads to strong luminescence gain because of suitable periodic perturbations of the medium's refractive index. In other words, the period of the grating (in nanometers) is proportional to the light intensity emitted from the sample. The emitted laser wavelength depends on the period of index or gain modulation and could be tuned by altering of the latter.

We have investigated an amplified-spontaneous-emission (ASE) phenomenon and lasing in a two-layer system, a modified DNA-CTMA polymeric matrix containing rhodamine 6G (Rh6G) dye superimposed on a periodic relief structure formed in a photochromic azo (nitrogen-containing)-functionalized polymer. To prepare the biopolymeric matrix, we used commercially available purified salmon roe DNA. We replaced all DNA salt ions with CTMA, achieving a DNA-CTMA complex. This complex is easily soluble in many organic solvents, including alcohols, and can be processed into good-optical-quality thin films by casting and spin-coating deposition. We then prepared a butanol solution of DNA-CTMA and Rh6G.

A thin film of azo-containing liquid-crystalline polymer was deposited on a glass plate by casting from solution. We inscribed the periodic structure in the film through holographic recording using a temporally stable light-intensity pattern. Atomic-force microscopy revealed a surface-relief grating (SRG) of 20nm amplitude. In a next step, we deposited a luminescent layer of DNA-CTMA:Rh6G (with thickness of ~1μm) from solution by casting, followed by drying. To measure the ASE and DFB lasing phenomena, we assembled an experimental setup wherein linearly polarized light of wavelength λ= 532nm from a pulsed Nd:YAG (neodymium-doped yttrium aluminum garnet) laser was incident onto the top of the sample. ASE or lasing emerging from the edge of the sample was collected by an optical fiber and analyzed using a spectrometer. Figure 2 shows examples of signals emerging from the sample excited by laser pulses of energy density ρ = 3.38mJ/cm². Lasing signals were obtained only from areas in the sample where an SRG was present under the DNA-CTMA:Rh6G layer in the direction along the grating wave vector. The weaker and broader ASE Rh6G signal excited by nanosecond laser pulses was measured from the sample areas containing the DNA-CTMA:Rh6G layer but without the SRG. Note that the same excitation-energy densities exhibit a difference in intensity of emitted light.

Figure 2. Examples of the lasing and amplified-spontaneous-emission (ASE) signals measured for excitation-energy density ρ =3.38mJ/cm², λ= 532nm, pulse width 6ns, and pulse repetition frequency 11Hz. a.u.: Arbitrary units.

We also observed a shift in the maximum position of the wavelength. The laser emission centered near 590nm corresponds to the fourth order of diffraction (m = 4) responsible for the feedback. The full width at half maximum (FWHM) for lasing was Δλ = 4nm. This value results from both the limited resolution of our spectrometer and random mode hopping of the light. The spectral width (FWHM) for ASE amounts to Δλ = 9nm. Figure 3 shows a comparison of the peak intensities of emitted light from the sample under lasing conditions and ASE as a function of the energy density of the excitation light. The threshold lies at a pump energy density of 1.8 and 3mJ/cm² for lasing and ASE, respectively. The emitted signal intensity clearly increases much faster with an increase in energy density for lasing than for ASE. For example, the intensity values for ρ = 6.25mJ/cm² differ by more than one order of magnitude.

Figure 3. Peak intensities as a function of excitation-energy densities in distributed-feedback Bragg lasing and ASE in a DNA-CTMA:rhodamine 6G (Rh6G) system. I: Light intensity.

In summary, DNA-based systems containing highly luminescent molecules deposited over a photochromic material layer with an SRG, recorded holographically, are suitable for ASE and lasing. As a next step, we plan to use the modified biopolymer described here for additional photonic applications such as microlasers and fast optical switches.

The authors wish to thank the Polish Ministry of Science and Higher Education (grants NN 507 475237 and NN 507 474537), the European Commission through the Human Potential Programme (Marie-Curie Research and Training Network BIMORE, grant MRTN-CT-2006-035859), and Wrocław University of Technology for financial support.