We experimentally demonstrated supercontinuum generation through a hollow core photonic bandgap fiber (HC-PBGF) filled with DNA nanocrystals modified by copper ions in a solution. Both double-crossover nano DNA structure and copper-ion-modified structure provided a sufficiently high optical nonlinearity within a short length of hollow optical fiber. Adding a higher concentration of copper ion into the DNA nanocrystals, the bandwidth of supercontinuum output was monotonically increased. Finally, we achieved the bandwidth expansion of about 1000 nm to be sufficient for broadband multi-spectrum applications.
© 2015 Optical Society of America
Supercontinuum generation (SCG), the creation of very broad spectral components through a strong nonlinear interaction between propagating intense pulses and an optical material along a waveguide, has attracted great interests in the applications of optical coherence tomography [1,2], absorption and transmission spectroscopy , confocal microscopy [4,5], telecommunication , and medical imaging . Although the broad supercontinuum generation has been generally realized in in silica-based optical fibers such as photonic crystal fibers and microstructure fibers [8,9], further enhancement of optical nonlinearity of the material comprising an optical waveguide is still being required due to relatively small nonlinear coefficient of silica material. Increasing the nonlinear interaction without structure variation of photonic crystal fibers have been attempted by insertion of highly nonlinear liquid materials such as ethanol , toluene  and water  into hollow cores. It was confirmed that these liquid materials significantly increased nonlinear interaction to modify the spectral compositions of outputs. Furthermore, this scheme can provide a long interaction length with high optical field confinement, which enables generating enhanced nonlinear effects at a relatively lower power level than bulk counterparts .
Recently, previously reported researches have demonstrated that biomaterials such as DNA have shown unusual optical properties, which are not easily replicated from other inorganic materials. For example, it has been reported that the nonlinear refractive index of DNA nanocrystals (2.0x10−15 cm2W−1 at λ = 1300 nm) is about 10 times larger than that of silica . More interestingly, the optical characteristics can be modified by binding other nano-materials into DNA. Furthermore, according to our previously measured z-scan result, nonlinearity of DNA solution was measured to be 1.9x10−15 cm2W−1 while the other value of nonlinearity for DNA with copper was 3.5x10−14 cm2W−1 . The difference among nonlinear optical properties of each fabricated samples seems to be caused by different chirality, which is related to nonlinear optical processes .
In this paper, we successfully generated supercontinuum through a hollow core photonic bandgap fiber (HC-PBGF) filled with copper-ion-modified DNA for the first time to the best knowledge of the authors. Also, we experimentally investigated the impacts of copper ion concentration on the bandwidth of supercontinuum generated by DNA material.
2. Technical approach
Supercontinuum generation (SCG) has been described by the nonlinear-Schrödinger equation (NLSE) as below:17]. Also, TR is defined as
We used lab-built HC-PBGF fabricated by the well-known stack-and-draw technique [18,19]. The HC-PBGF has a core with a diameter of around 13 μm. The photonic crystal structure surrounding the hollow core, composed of 8 rows, has a pitch (hole-hole distance) of 2.5 μm where the air filling fraction of HC-PBGF in the cladding was over 90%. The relative optical transmission by bandgap effect covers the spectral range tens of nm away from the pumping wavelength. The fiber had the zero dispersion at 775 nm. The dispersion of the HC-PBGF without any liquid was estimated to be about −123.08 ps/nm·km at the pumping wavelength of 725 nm. The calculated β2 was about 35.71 ps2/km at λ = 725 nm, we used the following equation to calculate the dispersion length, LD:
In our experiments, we used the short segments of the HC-PBGF with the length from 1 cm to 5 cm, which are significantly shorter than LD. By filling the air holes with DNA solutions, we expect that total dispersion will be slightly changed due to the material dispersion value of DNA solution. Considering the nonlinearity of the DNA solution for a given input power and pulse width of the pulse laser source, we expect that nonlinear length will be smaller than dispersion length in our system. Therefore, the nonlinear process of self-phase modulation (SPM) is expected to be a dominant means to cause the spectral broadening.
The double-crossover (DX) DNA is prepared in the following steps. High-performance liquid chromatography (HPLC) purified synthetic oligonucleotides of DNA strands were purchased from BIONEER (Daejeon, Korea). Complexes were formed by mixing a stoichiometric quantity of eight different DX strands (200 nM concentration) in a physiological 1 × TAE/Mg2+ [40 mM Tris base, 20 mM acetic acid, 1 mM EDTA (pH 8.0), and 12.5 mM magnesium acetate] buffer solution. They were cooled slowly from 95 °C to 25 °C in AXYGEN tubes of a Styrofoam box containing 2 L of boiled water for at least 24 hours to facilitate hybridization . The DNA solution is injected into the HC-PBGF using syringe injection method. When two or three droplets are observed at the other side of the fiber, we cut the fiber by a fiber-optic cleaver. It usually takes 20 ~30 minutes to fully fill the solution in the fiber with a length of 5 cm. The solution is observed to be mostly injected into the core rather than cladding air-structure. We expect that this comes from the difference between the core diameter (13 μm) and the pitch (2.5 μm) of air-structure. The sample injection method and DX DNA sequence are described in Fig. 1. The measured refractive indices of prepared DNA solution and DNA with copper ion (6 mM) solution by a refractometer are 1.3344 and 1.3346 at 590 nm, respectively.
The experimental setup for supercontinuum generation is schematically described in Fig. 2. Ultrafast optical pulse with the pulse duration of 100 fs was launched into the sample fiber by an optical parametric amplifier (OPA) (TOPAS prime, Spectra-Physics). Supercontinuum generated through the fiber was detected through a monochromator (MicroHR, Horiba Jobin Ynon Inc.). Four different sample fibers (Bare fiber, Buffer-filled fiber, DNA-filled fiber, and DNA with modified copper ion-filled fiber), which were cut into the precisely same length, were aligned at the same focal length and height, and moved only in the single axis to maintain the same input power and focal length applied into all the fiber samples.
3. Experimental results and discussion
We measured supercontinuum spectra for four different samples at the same fiber length of 3 cm for the input pulse with the intensity of 109 GW/cm2 at the wavelength of 725 nm. Figure 3 shows the supercontinuum spectra results. The spectrum of the bare HC-PBGF (black line) exhibits the broadened spectrum from 620 nm up to 820 nm. The graph plotted with red line shows the spectrum of the HC-PBGF filled with only buffer of DNA. When we fill the liquid including the DNA solution in the HC-PBGF, the DNA solution is happened to be located at inner side a little bit from the end-facet of the HC-PBGF due to the surface tension. Thus, for better mode-matching between incident laser beam and fundamental guided mode of the HC-PBGF filled with DNA solution, we intentionally move the focal point from the end facet of the HC-PBGF. This makes non-negligible silica-light interaction before meeting the DNA solution, which is responsible for spectral broadening in the absence of the DNA solution. The spectrum of the HC-PBGF filled with DNA (blue line) exhibits the broadened spectrum from 545 nm up to 850 nm. We observed that the spectral bandwidth in the presence of DNA/composite in the HC-PBGF (pink line), which expands from 540 nm to 900 nm, is wider than that of the bare HC-PBGF. The copper ion concentration in this experiment is 8 mM. The supercontinuum spectra indicate that the nonlinear property of DNA is enhanced in the presence of copper ion, similar to our previous report of nonlinear refractive index measured by Z-scan experiment . We expect that copper ion causes the conformational changing of DNA structure because of chromatin binding with copper ion so that purine and pyrimidine ring stacking structure is changed . As a result, the purine and pyrimidine ring stacking structures contribute to the enhancement of nonlinearity because the structural change influence on polarizability with the dipole moment . When considering the refractive index (~1.33) of DNA solution and air-filling fraction of cladding in our fiber, we expect that the waveguiding properties including group delay dispersion can be modified in the presence of DNA solution. However, we found that this does not significantly modify our results of nonlinear spectral broadening because nonlinear propagation is more dominant than dispersive effect in our experimental condition. Therefore, the supercontinuum from the fiber filled with copper ion modified DNA shows the wider bandwidth than that with only DNA.
To investigate the relationship between the nonlinear effect and the copper ion bound into DNA, we measured supercontinuum spectra from the HC-PBGF filled with DNA with different concentrations of copper ion. Figure 4 shows the result for the samples with different copper concentration at the incident pulse intensity of 251 GW/cm2. The input power was properly optimized to clearly visualize the tendency of the spectral broadening in the experiment. Black, red, blue and pink solid lines correspond to spectra broadening caused in the HC-PBGF including the DNA solution bound with the different copper ion concentration of 2 mM, 4 mM, 6 mM, and 8 mM, respectively. Copper ion concentration exceeding 8 mM is not considered since excessive copper ions other than 8 mM are no longer bound into DNA, but simply remains in the buffer. These experimental results show that the width of the spectra progressively increases with the increment of the copper ion concentration. Increasing the copper ion concentration causes larger nonlinearity in DNA sample resulting in the further broadening of supercontinuum spectra.
We further carried out the experiment for the different length of the HC-PBGF while maintaining other experimental conditions to investigate that SPM effect is a dominant factor for nonlinear process. The phase shift induced by SPM can be expressed asFigure 5 shows the experimentally measured spectra from the fibers filled with copper-ion-modified DNA with different interaction lengths (1 cm, 3 cm, and 5 cm) and pumping intensity of 109 GW/cm2. Black, red, and blue lines correspond to the interaction length of 1 cm, 3 cm, and 5 cm, respectively. As shown in Fig. 5, it is clearly observed that the spectral bandwidth becomes broader as the sample length increases from 1 cm to 5 cm. However, the spectral bandwidth in the spectrum does not exactly show linear-dependent behavior although the SPM effect predominantly contributes to the spectral broadening. We expect that this is due to that there is another contribution in nonlinear interaction such as non-negligible silica-light interaction in the HC-PBGF before meeting the DNA solution, which can partially contribute to the spectral broadening in the experiment. Also, it is observed that there is spectral asymmetry in the experiment for Stokes and anti-Stokes sides. We expect that the higher-order dispersion (particularly third-order dispersion) in the waveguide is mostly responsible for this asymmetric behavior in the spectrum. Nevertheless, SPM is assumed to be a dominant factor under current experimental conditions considering all our experimental results.
Finally, broad supercontinuum spectrum expanding nearly octave has been obtained by optimizing input power and copper ion concentration as shown in Fig. 6. The input intensity, fiber length, and the concentration of copper ion of this experimentation are about 376 GW/cm2, 4 cm, and 8 mM, respectably. In the current experiment, copper ion is fully saturated in the DNA solution at the concentration of 8 mM, which limits the enhancement of nonlinearity in our work. The vaporization of DNA sample solution for higher intensity laser pulse can also restrict the performance of our DNA solution-based nonlinear device. Nevertheless, the broadband supercontinuum spectrum of copper modified DNA expanding from 400 nm to 1350 nm is obtained. In comparison with previous report, our spectrum is much wider than the previous report of water-filled HC-PBGF, which shows the bandwidth of 600 nm to 1200 nm at similarity condition  since the nonlinear refractive index of DNA is 10 - 100 times larger than the nonlinear refractive index of water. Further enhancement on spectral width can be possibly obtained if other nano-materials with higher nonlinearity are incorporated with DNA, and HC-PBGF characteristics such as core size and distribution are optimized.
We then conducted numerical simulation based on split-step Fourier method to fully describe the pulse propagation in the presence of copper-ion-modified DNA where the nonlinearity of the sample was regarded as 1.9x10−15 cm2W−1 based on our previous experimental study . In the numerical simulation, the value of fR was set as 0.18. In the numerical simulation result, we found that the nonlinear spectral broadening is less sensitive for the slight variation (- 5 ps2/km ~-25 ps2/km) of the β2. This result indicates that nonlinear pulse propagation is more dominant than the dispersive pulse propagation in our experimental condition. Figure 7(b) compares the numerical results, which are reasonably in good agreement with that of experiment in Fig. 7(a).
We demonstrated broadband supercontinuum generation for the first time to the best knowledge of the authors from a HC-PBGF filled with copper-ion-modified DNA. The fiber with copper-ion-modified DNA shows the broader spectrum compared to that with only DNA and without any material. Furthermore, our experimental results indicate that the concentration of copper ion bound into DNA influence the spectral width of supercontinuum. Adopting DNA-based material, which can be easily modified with other nano-materials, can further enhance supercontinuum spectral width based on optimized conditions that gives a variety of choice for broadband supercontinuum generation. Also, DNA-based material, which has low absorption over wide range from visible to deep IR, can be one of possible candidates for mid-IR source generation.
This work was partially supported by the Center for Advanced Meta-Materials (CAMM) of Global Frontier project (CAMM-NRF-2014M3A6B3063727), Nano·Material Technology Development Program (2012M3A7B4049804), and the Pioneer Research Center Program (2010-0019457) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning, and the KIST Institutional Program (Project No. 2E25382).
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