Microstructure fibers with multiple submicron cores are used to frequency-convert unamplified 0.3-nJ, 80-fs pulses of 800-nm Ti: sapphire laser radiation to the spectral range of 400–500 nm. This frequency-upconverted radiation is then employed to induce reversible changes in the absorption spectrum of spiropyran molecules through photochromic transformations in a solid-phase spiropyran/PMMA sample. Microstructure fibers are thus shown to enhance the capabilities of low-power femtosecond lasers, making unamplified ultrashort pulses suitable for photochemical and micromachining applications.
©2003 Optical Society of America
Microstructure (MS) fibers [1,2] are receiving growing applications for nonlinear-optical spectral transformation of ultrashort laser pulses and generation of broadband radiation. Dispersion of guided modes in such fibers can be tailored by changing their core-cladding geometry . A high refractive index step between the core and the cladding, attainable with such fibers, can strongly confine electromagnetic radiation to the fiber core [4,5]. These unique properties of MS fibers allow the enhancement of a broad class of nonlinear-optical processes , making these fibers an ideal tool for numerous applications . Supercontinuum generation in MS fibers [8,9] is now intensely used in optical frequency metrology [10,11], ultrafast photonics , spectroscopy , and biomedical optics . MS fibers offer much flexibility in phase-matching third-harmonic generation  and four-wave mixing , permitting not only broadband emission, but also isolated spectral components [17, 18] to be generated with a high efficiency.
In this paper, we demonstrate that MS fibers can be employed to frequency-convert unamplified femtosecond laser pulses to the spectral range where light pulses efficiently induce photochromic processes. We start with unamplified 0.3-nJ 80-fs pulses of a Ti: sapphire laser, which are coupled into one of the submicron cores of a multiple-core MS fiber to be frequency-converted to the spectral range of 400–500 nm. This frequency-converted radiation is ideally suited, as shown by the experiments presented below, to induce the photochromism of spiropyran (SP) compounds.
2. Photochromic samples
Photochromism is defined as a light-induced reversible transformation in chemical species between two forms having different absorption spectra. This phenomenon is often routinely understood as light-induced change of color, well-known in every-day life due to photochromic glasses. Applications of photochromism include self-developing photography, optical switching and filtering , 3D optical data storage [20,21], and, since recently, reversible waveguide writing , microfabrication of photonic components , and engineering of photoswitchable biomaterials .
Our experiments were performed with PMMA samples doped with SP molecules (shown in the inset to Fig. 1). The concentration of spiropyran in a 9×10×10 mm3 sample was 1.6·10-2 mol/l. As can be seen from the absorption spectrum of the uncolored form of SP molecules (form A), shown by line 1 in Fig. 1, these molecules efficiently absorb radiation with wavelengths shorter than 410 nm. The photochromic effect, occurring through C—O bond cleavage  (see the inset in Fig. 1) transforms form-A SP molecules into a colored, merocyanine-isomer form (form B), giving rise to a broad absorption band in the visible region (curve 2 in Fig. 1).
3. The laser system and microstructure fibers
The laser system employed in our experiments was based on an argon-laser pumped Ti: sapphire laser (Fig. 2), which generated 80-fs pulses of 800-nm radiation with a pulse energy of 0.3 nJ. These pulses were coupled into an MS fiber where the cladding included two cycles of air holes surrounding the central fiber core with a diameter of about 3 µm (inset 1 in Fig. 3a). MS fibers were fabricated of fused silica using the technology described in detail elsewhere . An array of submicron-size fused silica channels in the form of threads, bounded by the system of air holes in the MS fiber (inset 1 in Fig. 3(a)), serve as additional multiple cores of the fiber, allowing, as shown by earlier experiments , efficient anti-Stokes frequency conversion of femtosecond pulses of 800-nm radiation. The wavelength of the anti-Stokes signal generated under these conditions is determined by the dispersion properties of the waveguide channel. The size of the channel is thus the key parameter, controlling the color of the anti-Stokes signal. Fused silica waveguiding channels with different sizes built into our MS fiber help to achieve wavelength tunability in the frequency conversion of ultrashort pulses.
Figures 3(a) and (b) show the spectra of radiation coming out of two different channels in the MS fiber, displaying intense anti-Stokes components. One of these anti-Stokes signals (the blue line) has a spectrum stretching from 400 to 500 nm with a maximum around 405 nm (Fig. 3(a)). The spectrum of the second anti-Stokes signal (the green line) peaks around 490 nm (Fig. 3(b)). The blue anti-Stokes line falls within the absorption band of the uncolored form of SP compounds (cf. Figs. 1 and 3(a)) and was used in our experiments to initiate photochromism in SP/PMMA samples. The green anti-Stokes line lies within the absorption band characteristic of form-B SP (cf. Figs. 1 and Fig. 3(b)). This component was used to induce the reverse photochromic transformation, recovering the uncolored, form-B spiropyran.
4. Results and discussion
Comparison of the spectra of the blue anti-Stokes line at the input (line 1 in inset 1 to Fig. 3(a)) and at the output (line 2 in the same inset) of the SP/PMMA sample shows that this line is ideally suited to transform SP molecules from the form A to the form B. The short-wavelength part of the anti-Stokes signal is almost completely absorbed by the photochromic material, while the long-wavelength part of the spectrum is absorbed less efficiently, indicating that, with such an emission spectrum, we are still able to avoid undesirable absorption of radiation by form-B spiropyran, stimulating the reverse photochromic reaction.
Absorption of the blue anti-Stokes line generated in the MS fiber initiates the photochromic processes in SP molecules, resulting in the formation of merocyanine isomers (form-B spiropyran). This photochromic transformation is visualized in our experiments by characteristic photoluminescence (PL) within the spectral range of 600–700 nm (the inset to Fig. 4) excited with 532-nm second-harmonic radiation from a diode-pumped 10-mW cw Nd: YAG laser (Fig. 2). Radiation of this laser also stimulates the reverse photochromic transformation, recovering form-A spiropyran in the sample, thus preventing the gradual pulse-to-pulse accumulation of form-B spiropyran in the laser-irradiated area.
As the Ti: sapphire laser is switched on (at t=0 in Fig. 4), the blue anti-Stokes line produced in the MS fiber starts to generate form-B spiropyran in the photochromic sample, visualized by the growth in the PL intensity as a function of time in Fig. 4. With the Ti: sapphire laser switched off (t=16 s in Fig. 4), the PL signal decays, indicating the recovery of form-A SP in the sample under the action of 532-nm second-harmonic radiation of the Nd: YAG laser. The green anti-Stokes line induces the reverse photochromic transformation. The decay of the PL signal in inset 1 to Fig. 3(b) shows how this emission line recovers form-A spiropyran in the area of the photochromic sample previously colored by the blue anti-Stokes line, which is switched off at t=0. The PL excited by the green anti-Stokes line, as shown in inset 2 to Fig. 3(b), can be employed to visualize microstructures and waveguides written  in a photochromic material by femtosecond Ti: sapphire laser pulses through two-photon-absorption-induced photochromism.
Experiments presented in this paper demonstrate that MS fibers allow the frequency of unamplified Ti: sapphire laser pulses to be converted to the frequency range where the initiation of photochromic processes becomes possible. We illustrated this possibility by using MS fibers to convert the frequency of 0.3-nJ 80-fs Ti: sapphire-laser pulses and employing the output anti-Stokes emission to induce photochromism in solid-phase spiropyran/PMMA samples. MS fibers are thus shown to enhance the capabilities of low-energy femtosecond laser systems with the frequency-upconversion option, making photochemical and photobiological applications accessible to unamplified femtosecond pulses.
We are grateful to A.B. Fedotov for valuable help, Yu.N. Kondrat’ev, V.S. Shevandin, A.V. Khokhlov, and K.V. Dukel’skii for fabricating MS fibers, and N.T. Sokolyuk for providing us with spiropyran samples.
This study was supported in part by the President of Russian Federation Grant MD-42.2003.02, the Russian Foundation for Basic Research (projects nos. 03-02-16929 and 02-02-17098), and the Volkswagen Foundation (project I/76 869). This material is also based upon work supported by the European Research Office of the US Army under Contract No. 62558-03-M-0033.
References and links
3. W.H. Reeves, J.C. Knight, P.St.J. Russell, and P.J. Roberts, “Demonstration of ultraflattened dispersion in photonic crystal fibers,” Opt. Express 10, 609–613 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-14-609 [CrossRef] [PubMed]
4. N.G.R. Broderick, T.M. Monro, P.J. Bennett, and D.J. Richardson, “Nonlinearity in holey optical fibers: measurement and future opportunities,” Opt. Lett. 24, 1395–1397 (1999). [CrossRef]
5. A.B. Fedotov, A.M. Zheltikov, A.P. Tarasevitch, and D. von der Linde, “Enhanced spectral broadening of short laser pulses in high-numerical-aperture holey fibers,” Appl. Phys. B 73, 181–184 (2001). [CrossRef]
6. Nonlinear optics of photonic crystals, Feature issue of J. Opt. Soc. Am. B 19, no. 9 (2002).
7. B. J. Eggleton, C. Kerbage, P. S. Westbrook, R. S. Windeler, and A. Hale, “Microstructured optical fiber devices,” Opt. Express 9, 698–713 (2001), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-9-13-698. [CrossRef] [PubMed]
8. J.K. Ranka, R.S. Windeler, and A.J. Stentz, “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,” Opt. Lett. 25, 25–27 (2000). [CrossRef]
9. W.J. Wadsworth, A. Ortigosa-Blanch, J.C. Knight, T.A. Birks, T.P.M. Mann, and P.St.J. Russell, “Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source,” J. Opt. Soc. Am. B 19, 2148–2155 (2002). [CrossRef]
10. D.J. Jones, S.A. Diddams, J.K. Ranka, A. Stentz, R.S. Windeler, J.L. Hall, and S.T. Cundiff, “Carrierenvelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science , 288, 635–639 (2000). [CrossRef] [PubMed]
11. R. Holzwarth, T. Udem, T.W. Hänsch, J.C. Knight, W.J. Wadsworth, and P.St.J. Russell, “Optical frequency synthesizer for precision spectroscopy,” Phys. Rev. Lett. 85, 2264–2267 (2000). [CrossRef] [PubMed]
12. A. Baltuska, T. Fuji, and T. Kobayashi, “Self-referencing of the carrier-envelope slip in a 6-fs visible parametric amplifier,” Opt. Lett. 27, 1241–1243 (2002). [CrossRef]
13. A.B. Fedotov, Ping Zhou, A.P. Tarasevitch, K.V. Dukel’skii, Yu.N. Kondrat’ev, V.S. Shevandin, V.B. Smirnov, D. von der Linde, and A.M. Zheltikov, J. Raman Spectrosc. “Microstructure-Fiber Sources of Mode-Separable Supercontinuum Emission for Wave-Mixing Spectroscopy,” 33, 888–896 (2002). [CrossRef]
14. I. Hartl, X. D. Li, C. Chudoba, R.K. Rhanta, T.H. Ko, J.G. Fujimoto, J.K. Ranka, and R.S. Windeler, “Ultrahigh-resolution optical coherence tomography using continuum generation in an air-silica microstructure optical fiber,” Opt. Lett. 26, 608–610 (2001). [CrossRef]
15. A.N. Naumov, A.B. Fedotov, A.M. Zheltikov, V.V. Yakovlev, L.A. Mel’nikov, V.I. Beloglazov, N.B. Skibina, and A.V. Shcherbakov, “Enhanced χ(3) interactions of unamplified femtosecond Cr: forsterite laser pulses in photonic-crystal fibers,” J. Opt. Soc. Am. B 19, 2183–2191 (2002). [CrossRef]
16. S. Coen, A. Hing Lun Chau, R. Leonhardt, J.D. Harvey, J.C. Knight, W.J. Wadsworth, and P.St.J. Russell, “Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers,” J. Opt. Soc. Am. B 19, 753–764 (2002). [CrossRef]
17. A. Efimov, A.J. Taylor, F.G. Omenetto, J.C. Knight, W.J. Wadsworth, and P.St.J. Russell, “Nonlinear generation of very high-order UV modes in microstructured fibers,” Opt. Express 11, 910–918 (2003), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-8-910. [CrossRef] [PubMed]
18. A.B. Fedotov, I. Bugar, D.A. Sidorov-Biryukov, E.E. Serebryannikov, D. Chorvat Jr., M. Scalora, D. Chorvat, and A.M. Zheltikov, “Pump-depleting four-wave mixing in supercontinuum-generating microstructure fibers,” Appl. Phys. B 77, no. 1/2 (2003). [CrossRef]
19. H. Dürr and H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems (Elsevier, Amsterdam, 1990).
21. D.A. Akimov, N.I. Koroteev, S.A. Magnitskii, A.N. Naumov, D.A. Sidorov-Biryukov, A.B. Fedotov, and A.M. Zheltikov, “Optimizing two-photon three-dimensional data storage in photochromic materials using the principles of nonlinear optics,” Jpn. J. Appl. Phys. 36, 426–428 (1997). [CrossRef]
22. S. Lecomte, U. Gubler, M. Jäger, Ch. Bosshard, G. Montemezzani, P. Günter, L. Gobbi, and F. Diederich, “Reversible optical structuring of polymer waveguides doped with photochromic molecules,” Appl. Phys. Lett. 77, 921–923 (2000). [CrossRef]
23. S.O. Konorov, A.B. Fedotov, and A.M. Zheltikov, “Three-dimensional reversible laser micromachining with subnanojoule femtosecond pulses based on two-photon photochromism,” Appl. Phys. B 76, 707–710 (2003). [CrossRef]
24. I. Willner, “Photoswitchable biomaterials: en route to optobioelectronic systems,” Acc. Chem. Res. 30, 347–356 (1997). [CrossRef]