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Broadband ultrafast spectroscopy using a photonic crystal fiber: application to the photophysics of malachite green
Optics Express, Vol. 15, Issue 24, pp. 16124-16129 (2007)
http://dx.doi.org/10.1364/OE.15.016124
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– Abstract
1. Introduction
2. Supercontinuum generation and characterization
3. Application to broadband transient absorption spectroscopy
4. Ultrafast spectroscopy of Malachite Green: Results and discussion
5. Conclusion
– References and links
– Abstract
1. Introduction
2. Supercontinuum generation and characterization
3. Application to broadband transient absorption spectroscopy
4. Ultrafast spectroscopy of Malachite Green: Results and discussion
5. Conclusion
– References and links
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Abstract
Femtosecond single-pulsed supercontinua (SC) are produced in a short sub-cm piece of photonic crystal fiber. The SC span from 450 nm to more than 1.1 µm with 1-nJ energy injection. UV light down to 340 nm is observed with increased injection power. Using such a single-pulsed SC we implemented a compact transient absorption spectrometer with broadband detection and 150-fs FWHM time resolution to monitor the ultrafast dynamics of the electronic states of malachite green in ethanol excited to the S2 state. The full spectral evolution is observed from 450 nm to 1050 nm, with high sensitivity and a signal-to-noise ratio as high as 1000.
© 2007 Optical Society of America
1. Introduction
In recent years, ultrabroad spectra of light have been produced by injecting nJ-energy femtosecond pulses obtained from conventional non-amplified mode-locked lasers into micro-structured fibers [1]. These “supercontinua” (SC) have since emerged as attractive light sources for a variety of applications such as optical frequency synthesis, imaging techniques, or ultrafast spectroscopy. For the latter application, not only the ultrabroad spectrum, but also the temporal distribution of the supercontinuum is a critical parameter. The broadest spectra are obtained when high-order soliton fission takes place during propagation in the anomalous dispersion regime [2, 3]. As a result, blue light can be produced in fibers longer than a few tens of cm [4], but the temporal structure of the SC shows multiple pulses extending over picoseconds in time [5]. Under these conditions sub-picosecond spectroscopy is impossible unless the SC is spectrally filtered and single wavelength detection is used, as was demonstrated in earlier work with probe wavelengths >550 nm [6, 7]. However, in order to elucidate the ultrafast dynamics of molecules in solution, where inter-state transitions may spectrally overlap and shift, transient absorption spectroscopy is best performed with broadband detection. This requires ≃400-nm pumping light and single-pulsed probe spectra extending into the blue or better near-UV, for the large majority of molecules of interest.
J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air silica microstructure optical fibers with anomalous dispersion at 800nm,” Opt. Lett. 25, 25–27 (2000). [CrossRef]
A. Husakou and J. Herrmann, “Supercontinuum Generation of Higher-Order Solitons by Fission in Photonic Crystal Fibers,” Phys. Rev. Lett. 87, 203,901 (2001). [CrossRef]
J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78, 1135–1184 (2006). [CrossRef]
L. Tartara, I. Cristiani, and V. Degiorgio, “Blue light and infrared continuum generation by soliton fission in a microstructured fiber,” Appl. Phys. B 77, 307–311 (2003). [CrossRef]
J. M. Dudley, X. Gu, L. Xu, M. Kimmel, E. Zeek, P. O’Shea, R. Trebino, S. Coen, and R. S. Windeler, “Cross-correlation frequency resolved optical gating analysis of broadband continuum generation in photonic crystal fiber: simulations and experiments,” Opt. Express 10, 1215–1221 (2002). [PubMed]
V. Nagarajan, E. Johnson, P. Schellenberg, W. Parson, and R. Windeler, “A compact versatile femtosecond spectrometer,” Rev. Sci. Instrum. 73, 4145–4149 (2002). [CrossRef]
M. Punke, F. Hoos, C. Karnutsch, U. Lemmer, N. Linder, and K. Streubel, “High-repetition-rate white-light pump-probe spectroscopy with a tapered fiber,” Opt. Lett. 31, 1157–1159 (2006). [CrossRef] [PubMed]
In this work, we perform ultra-broadband transient absorption spectroscopy of malachite green (MG), using a compact non-amplified Ti:sapphire oscillator, and SC generated in a photonic crystal fiber (PCF). Remarkably broad spectra are produced in a sub-cm short piece of the PCF with sub-picosecond single pulse output. Pump-induced absorption changes (ΔA) of the probe beam are measured with a sensitivity in the range of 5×10-5. Time-resolved spectra can be recorded over a 300-nm broad spectral range in a single experimental run, with a 150-fs FWHM time resolution, and wavelengths as short as 450 nm are probed. Triphenylmethane (TPM) dyes, of which MG is an example, relax non-radiatively from the S1 state supposedly through the rotation of one or several phenyl group(s), with a rate strongly depending on solvent viscosity [8]. These have been thus described as model systems for radiationless electronic relaxation involving large-amplitude motion with no intramolecular potential barrier. To the best of our knowledge, only time-resolved fluorescence has been used so far to probe the relaxation dynamics of the S2 state [9, 10, 11]. Here we present results of transient absorption spectroscopy of MG in ethanol, excited to the S2 state by a 425-nm pump pulse, and discuss some new findings pertaining to the ultrafast relaxation scenario of MG.
G. Oster and Y. Nishijima, “Fluorescence and Internal Rotation : Their Dependence on Viscosity of the Medium,” J. Amer. Chem. Soc. 78, 1581–1584 (1956). [CrossRef]
M. Yoshizawa, K. Suzuki, A. Kubo, and S. Saikan, “Femtosecond study of S2 fluorescence in malachite green in solutions,” Chem. Phys. Lett. 290, 43–48 (1998). [CrossRef]
Y. Kanematsu, H. Ozawa, I. Tanaka, and S. Kinoshita, “Femtosecond optical Kerr-gate measurement of fluoresence spectra of dye solutions,” J. Lumin. 87–89, 917–919 (2000). [CrossRef]
A. C. Bhasikuttan, A. V. Sapre, and T. Okada, “Ultrafast Relaxation Dynamics from the S2 State of Malachite Green Studied with Femtosecond Upconversion Spectroscopy,” J. Phys. Chem. A 107, 3030–3035 (2003). [CrossRef]
2. Supercontinuum generation and characterization
The laser source is a Ti:Sa oscillator (KML), pumped by a 5-W cw VERDI laser. It delivers 15-nJ, 45-fs pulses with a 27-MHz repetition rate, due to the implementation of a multipass cavity [12]. The central wavelength is adjusted at 850 nm. A few nJ of the pulse energy are used for injection into a sub-cm short piece of a PCF, the cross section of which being displayed on Fig. 1(b). The remaining part of the laser power is frequency doubled in a 2-mm-thick BBO crystal, producing typically 1.5-nJ pulses at 425 nm, which is used as a pump beam for our transient spectroscopy measurements.
S. H. Cho, B. E. Bouma, E. P. Ippen, and J. G. Fujimoto, “Low-repetition-rate high-peak-power Kerr-lens mode-locked Ti:Al2O3 laser with a multiple-pass cavity,” Opt. Lett. 24, 417–419 (1999). [CrossRef]
The PCF was produced at the XLIM laboratory by using the standard stack-and-draw method. The triangular structure of holes with an average diameter d=1.85 µm and spacing Λ=2.6 µm yields an air filling fraction d/λ=0.71. Two lateral bigger holes with diameters of 3.3 µm and 3.6 µm make the core asymmetric and induce strong birefringence. At 850 nm, the effective area of the fundamental mode is ≃4.9 µm2 and four different modes LP01x
, LP01y
, LP11x
and LP11y
can be guided in the core. Their zero-dispersion wavelengths (ZDW’s) are 827, 866, 757 and 764 nm respectively. With a fiber as short as 8 mm, and a 25-kW peak power injected onto the LP01 mode close to the ZDW, the SC extends typically from 450 nm to above 1.1 µm (limited by the CCD-based detection) as displayed by the solid black line of Fig. 1(a). The location of maxima and minima in the spectrum can be tuned by rotating the incident (linear) laser light polarization. The output polarization is wavelength-dependent, a broadband polarizer is used for the spectroscopic application. The spectro-temporal distribution of the SC is investigated by time-gated non-degenerate two-photon absorption in a 0.1-mm thick ZnS plate, where the frequency-doubled pulse is used as the gating pulse. Fig. 1(c) shows, in false-color scale, the spectrally and temporally resolved change of transmission of the SC which is observed for temporal coincidence of gate and SC pulses in ZnS. The SC appears to be a 600-nm-broad single pulse extending over ≃300 fs in time. It is worth noting that the spectrum extends this far towards short wavelengths (450 nm) with such a short propagation distance in the PCF. Numerical computations of the vectorial and modal phase matching conditions for four-wave mixing (FWM) in the fiber indicate that FWM is not expected to transfer energy to wavelengths shorter than 600 nm. Hence wavelengths arising around 500 nm seem to be due to high-order soliton break-up and phase-matched dispersive wave generation [3]. However, most probably due to the short propagation length, the temporal spreading is small, and the single-pulsed SC output is preserved.
J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78, 1135–1184 (2006). [CrossRef]
Under the above injection conditions the piece of fiber deteriorates rather quickly. However, setting a protection window on its front face drastically reduces the aging of the PCF, allowing us to use it as a stable light source for more than 6 months. It is to be noted that, by injecting about 5 times more power at a different angle of incidence, we could observe a SC extending in the UV range down to 340 nm (red dashed curve of Fig. 1-(a)). As suggested by the spatial output pattern, we believe the power was mainly coupled into a leaky mode. The time-frequency distribution showed again a single-pulsed SC, with the UV part (340–400 nm) spreading over about 700 fs in time. However, the remaining laser power available for producing the blue pump pulse was too low to perform pump-probe spectroscopy with this spectrum.
Fig. 1. (Color online) (a) Spectra obtained by coupling incident infrared light into the LP01 transverse mode (solid black curve) and in a supposedly leaky mode (red dashed curve) of the fiber (length 8 mm). (b) Cross-sction of the fiber. (c) Temporal distribution of the spectrum displayed in (a) for the LP01 mode. The solid red curve is a polynomial fit to the spectro-temporal distribution which reveals the chirp of the SC.
3. Application to broadband transient absorption spectroscopy
The high-repetition-rate non-amplified laser system and the SC light source fulfill all requirements for the implementation of a sensitive transient absorption spectrometer. An energy density of 0.25 mJ/cm2 is obtained at 425 nm by focusing the pump beam with a parabolic mirror of 25-mm effective focal length. With MG, this leads to an excitation probability of ≃3%, and a maximum bleach signal ΔA
Bleach ≃-0.025 at 620 nm, as shown in Fig. 2.
The high repetition rate of the laser system may cause multiple excitation of the molecules or accumulation of population in long-lived states. In these cases a non-zero bleach signal is measured at negative pump-probe delays, which is in fact the absorption change induced by the previous pump pulses. Fast circulation of the solution and a small pump beam diameter limit this effect. In our case, the bleach signal measured for MG at large negative delays is in the range of ΔA
<0≃-0.007, which means that on an average less than 1% of the molecules are not in their ground state when the next pump pulse arrives. Hence, after subtraction of the negative-time signal, at least 99% of the remaining transient signal is due to MG molecules initially excited to the S2 state.
In addition, strong focusing of the pump in the solution creates local temperature and index gradients which in turn produce an optical lens. The resulting pump-induced change of the collection efficiency of the probe light simulates a change of absorbance. This effect was taken care of by a fast circulation of the sample and by a moderate focusing of the pump beam.
Further, intensity fluctuations in the probe beam are a critical issue for sensitive pump-probe experiments. We note that, the level of noise in our PCF-based spectrometer is not higher than that observed with supercontinua produced by µJ pulses in Sapphire. Thus, no reference beam has been implemented to correct for the probe fluctuations in our experiment. The SC spectrum is measured with a CCD camera (Roper Scientific, Spec10), at an acquisition rate of 800 spectra per second with exposure times of 0.4 ms. The pump beam is chopped at half the acquisition frequency. It can be easily shown that the noise on the raw signal for the absorption change is in fact the relative shot-to-shot noise in the probe beam, where a “shot” means a background-free record of the probe spectrum measured over the 0.4 ms integration time of the camera. This shot-to-shot noise is typically in the range 0.5 to 1%, depending on the wavelength. Enhanced noise (up to 3%) is observed below 550 nm and around 830–860 nm, due respectively to larger fluctuations in the supercontinuum at specific wavelengths, and to residual scattered light at the fundamental wavelength of the oscillator.
Typical transient absorption data for MG in ethanol excited to the S2 state are displayed in Fig. 2(b). Three experimental runs recorded in 1 hour and covering a 300-nm spectral bandwidth were merged to build Fig. 2(b). Each time point is an average of 2000 spectra. The ultimate sensitivity is typically in the range of 5×10-5 as given by the standard deviation of the residuals to the fits displayed in Fig. 3 (see below). We emphasize that, this noise figure obtained with spectrally resolved detection is a very good result since it reaches that of a single-wavelength lock-in detection. At the intensity minima of the SC, the noise figure degrades, due to the finite dynamical range of the CCD. Indeed, for broadband detection the SC is attenuated with neutral OD filters 2–3, in order not to saturate the detector at more intense wavelengths. The temporal response function of the spectrometer is derived from pump-probe experiment on diphenyl-hexatriene in acetone. It is in the range of 150 fs FWHM.
4. Ultrafast spectroscopy of Malachite Green: Results and discussion
The transient absorption spectra displayed in Fig. 2(b) are obtained with pump and probe beams polarized orthogonally. As the S0–S1 and S0–S2 transition dipole moments are orthogonal, this allows maximizing the S0–S1 bleach signal, which we observe to be the dominant feature between 540 nm and 670 nm. On the low-energy side of the bleach signal and immediately after excitation a positive band appears from 750 nm up to 1050 nm at least. This hitherto unobserved feature is attributed to excited-state absorption (ESA) from S2. Similarly, in the blue of the bleach signal a weak instantaneous gain (hardly visible on Fig. 2(b) because of the color code, best seen at early times in Fig. 3(a)) is detected which we attribute to stimulated emission (SE) from S2. Both bands decay rapidly and are followed by a negative one at 650–950 nm, which has already been identified unambiguously as SE from S1 by several authors. On the same time scale, an ESA signal, spanning from below 450 nm up to 590–600 nm, rises and overlaps with the bleach signal. This ESA band has previously been assigned to a superposition of ESA from the S1 state and an intermediate S
x
state [13]. Finally, a positive signal appears after ≃2 ps between 640 nm and 690 nm. This absorption band has also previously been observed and discussed in different TPM dyes.
M. M. Martin, P. Plaza, and Y. H. Meyer, “Ultrafast conformational relaxation of triphenylmethane dyes: Spectral Characterization,” J. Phys. Chem. 95, 9310–9314 (1992). [CrossRef]
Fig. 2. (Color online) a) Stationary absorption spectrum ofMG in ethanol. The arrow shows the excitation wavelength of 425 nm. b) Temporally- and spectrally-resolved transient absorption spectra (in false colors) of MG excited to the S2 state. These data are reconstructed from 3 different experimental runs with different central detection wavelengths and merged at 540 nm and 850 nm. Here the PCF length is 6.5 mm.
Fig. 3. (Color online) Kinetic traces (in black) and their best fits to a biexponential curve (in red) at a) 490 nm, c) 800 nm, and to a triexponential curve at b) 680 nm. The time scale is linear below 1 ps and logarithmic above. Data are extracted from Fig. 2(b), and averaged on a 10-nm window, that is over 4 adjacent columns of the 2D plot displayed in Fig. 2(b). The standard deviation of the residuals to the fit gives a typical noise level of 5×10-5.
Data analysis at wavelengths shorter than 520 nm and longer than 720 nm is done by fitting a sum of two exponential functions convoluted with a gaussian instrument response function. Excellent agreement is obtained as illustrated by Fig. 3(a) and Fig. 3(c). At wavelengths longer than 900 nm, the decay of the ESA band from S2 is almost pure and fitted with a single exponential decay with a 0.31±0.03 ps time constant. In the 720 to 850 nm range, the same value is obtained when modeling both the decay of S2 and rise of S1 with a unique time constant. In addition, a second time constant of 0.60±0.10 ps is obtained for the decay of the SE signal from S1. Yoshizawa et al. [9] performed time-resolved fluorescence spectroscopy on MG excited to the S2 state in water. They measured 0.27±0.05 ps for the fluorescence life time of S2, and 0.43±0.06 ps and 0.54±0.06 ps for the rise and decay of the fluorescence of S1. Although the solvent is different, the viscosity is virtually the same and our measurements in ethanol agree well with Yoshizawa’s results obtained in water.
M. Yoshizawa, K. Suzuki, A. Kubo, and S. Saikan, “Femtosecond study of S2 fluorescence in malachite green in solutions,” Chem. Phys. Lett. 290, 43–48 (1998). [CrossRef]
At wavelengths shorter than 520 nm, we measure a rise time of 0.46±0.08 ps for the ESA, which also coincides with Yoshizawa’s measurement for the rise of the fluorescence of S1 in water. The measured decay time of this ESA is wavelength-dependent and shifts from 1.1± 0.1 ps around 520 nm up to 1.7±0.3 ps around 460 nm. Based on results obtained for ethyl violet, Martin et al. [13] argued that the ESA band was due to absorption from S 1 and from a dark S
x
intermediate state, the latter dominating at longer times. In line with that, the longer decay time observed at shorter wavelengths supports the existence of two states contributing to this positive transient signal for MG as well.
M. M. Martin, P. Plaza, and Y. H. Meyer, “Ultrafast conformational relaxation of triphenylmethane dyes: Spectral Characterization,” J. Phys. Chem. 95, 9310–9314 (1992). [CrossRef]
Between 660 nm and 720 nm a third time constant is required to account for the ps-lived absorption. It varies from 3.3±0.5 ps at 720 nm to 6.2±0.5 ps at 660 nm. A fitting curve is displayed in Fig. 3(b), along with the experimental kinetic trace at 690 nm. To our knowledge, this wavelength dependence of the absorption signal decay has never been reported. This observation strongly supports the assignment of the absorption band to a distribution of vibrationally-excited ground state molecules, which narrows on a few-picosecond time scale while intermolecular thermalization occurs with the solvent thermal bath [14].
T. Robl and A. Seilmeier, “Ground-state recovery of electronically excited malachite green via transient vibrational heating,” Chem. Phys. Lett. 147, 544–550 (1988). [CrossRef]
In the 530–660 nm range, the transient absorption signal is the superposition of bleach, ESA and SE. In this range, no reliable fit could be obtained with a sum of exponential functions, indicating that a more complex relaxation scenario from S2 may apply [9]. A more detailed analysis is out of the focus of this paper and will be presented elsewhere.
M. Yoshizawa, K. Suzuki, A. Kubo, and S. Saikan, “Femtosecond study of S2 fluorescence in malachite green in solutions,” Chem. Phys. Lett. 290, 43–48 (1998). [CrossRef]
5. Conclusion
Using a non-amplified Ti:Sa oscillator, we produce pulses of white light extending down to 450 nm and spanning over ≃300 fs in time, in a sub-cm short piece of a PCF. The single-pulsed SC can be used directly as a probe beam for ultrafast transient spectroscopy with broadband detection and a 150-fs FWHM time resolution. High shot-to-shot stability and very sensitive pump-induced signal detection are demonstrated. We perform spectrally resolved transient absorption spectroscopy of MG in ethanol, excited to the S2 state. This work illustrates that accurate broadband ultrafast spectroscopy can now be implemented with non-amplified Ti:Sapphire laser sources. This bears many advantages not only for the study of molecules in solution, but also for solid-state spectroscopic surveys. Preliminary data analysis shows excellent agreement with previous observations, and new interesting insights arise from the broadband detection. In particular, the wavelength dependence of the transient absorption signal in the spectral range 660–720 nm strongly supports the presence of a vibrationally “hot” ground state.
Acknowledgments
Many helpful suggestions from B. Zietz and J.-Y. Bigot are gratefully acknowledged. The project is funded by the CNRS and the Université Louis Pasteur, Strasbourg (“BQR 2005”).
References and links
J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air silica microstructure optical fibers with anomalous dispersion at 800nm,” Opt. Lett. 25, 25–27 (2000). [CrossRef] | |
A. Husakou and J. Herrmann, “Supercontinuum Generation of Higher-Order Solitons by Fission in Photonic Crystal Fibers,” Phys. Rev. Lett. 87, 203,901 (2001). [CrossRef] | |
J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78, 1135–1184 (2006). [CrossRef] | |
L. Tartara, I. Cristiani, and V. Degiorgio, “Blue light and infrared continuum generation by soliton fission in a microstructured fiber,” Appl. Phys. B 77, 307–311 (2003). [CrossRef] | |
J. M. Dudley, X. Gu, L. Xu, M. Kimmel, E. Zeek, P. O’Shea, R. Trebino, S. Coen, and R. S. Windeler, “Cross-correlation frequency resolved optical gating analysis of broadband continuum generation in photonic crystal fiber: simulations and experiments,” Opt. Express 10, 1215–1221 (2002). [PubMed] | |
V. Nagarajan, E. Johnson, P. Schellenberg, W. Parson, and R. Windeler, “A compact versatile femtosecond spectrometer,” Rev. Sci. Instrum. 73, 4145–4149 (2002). [CrossRef] | |
M. Punke, F. Hoos, C. Karnutsch, U. Lemmer, N. Linder, and K. Streubel, “High-repetition-rate white-light pump-probe spectroscopy with a tapered fiber,” Opt. Lett. 31, 1157–1159 (2006). [CrossRef] [PubMed] | |
G. Oster and Y. Nishijima, “Fluorescence and Internal Rotation : Their Dependence on Viscosity of the Medium,” J. Amer. Chem. Soc. 78, 1581–1584 (1956). [CrossRef] | |
M. Yoshizawa, K. Suzuki, A. Kubo, and S. Saikan, “Femtosecond study of S2 fluorescence in malachite green in solutions,” Chem. Phys. Lett. 290, 43–48 (1998). [CrossRef] | |
Y. Kanematsu, H. Ozawa, I. Tanaka, and S. Kinoshita, “Femtosecond optical Kerr-gate measurement of fluoresence spectra of dye solutions,” J. Lumin. 87–89, 917–919 (2000). [CrossRef] | |
A. C. Bhasikuttan, A. V. Sapre, and T. Okada, “Ultrafast Relaxation Dynamics from the S2 State of Malachite Green Studied with Femtosecond Upconversion Spectroscopy,” J. Phys. Chem. A 107, 3030–3035 (2003). [CrossRef] | |
S. H. Cho, B. E. Bouma, E. P. Ippen, and J. G. Fujimoto, “Low-repetition-rate high-peak-power Kerr-lens mode-locked Ti:Al2O3 laser with a multiple-pass cavity,” Opt. Lett. 24, 417–419 (1999). [CrossRef] | |
M. M. Martin, P. Plaza, and Y. H. Meyer, “Ultrafast conformational relaxation of triphenylmethane dyes: Spectral Characterization,” J. Phys. Chem. 95, 9310–9314 (1992). [CrossRef] | |
T. Robl and A. Seilmeier, “Ground-state recovery of electronically excited malachite green via transient vibrational heating,” Chem. Phys. Lett. 147, 544–550 (1988). [CrossRef] |
OCIS Codes
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(300.6390) Spectroscopy : Spectroscopy, molecular
(320.7150) Ultrafast optics : Ultrafast spectroscopy
ToC Category:
Spectroscopy
History
Original Manuscript: July 3, 2007
Revised Manuscript: August 24, 2007
Manuscript Accepted: September 2, 2007
Published: November 20, 2007
Citation
Jérémie Léonard, Nhan Lecong, Jean-Pierre Likforman, Olivier Crégut, Stefan Haacke, Pierre Viale, Philippe Leproux, and Vincent Couderc, "Broadband ultrafast spectroscopy using
a photonic crystal fiber: application to
the photophysics of malachite green," Opt. Express 15, 16124-16129 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-24-16124
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References
- J. K. Ranka, R. S. Windeler, and A. J. Stentz, "Visible continuum generation in air silica microstructure optical fibers with anomalous dispersion at 800nm," Opt. Lett. 25, 25-27 (2000). [CrossRef]
- A. Husakou and J. Herrmann, "Supercontinuum Generation of Higher-Order Solitons by Fission in Photonic Crystal Fibers," Phys. Rev. Lett. 87, 203,901 (2001). [CrossRef]
- J. M. Dudley, G. Genty, and S. Coen, "Supercontinuum generation in photonic crystal fiber," Rev. Mod. Phys. 78, 1135-1184 (2006). [CrossRef]
- L. Tartara, I. Cristiani, and V. Degiorgio, "Blue light and infrared continuum generation by soliton fission in a microstructured fiber," Appl. Phys. B 77, 307-311 (2003). [CrossRef]
- J. M. Dudley, X. Gu, L. Xu, M. Kimmel, E. Zeek, P. O’Shea, R. Trebino, S. Coen, and R. S. Windeler, "Crosscorrelation frequency resolved optical gating analysis of broadband continuum generation in photonic crystal fiber: simulations and experiments," Opt. Express 10, 1215-1221 (2002). [PubMed]
- V. Nagarajan, E. Johnson, P. Schellenberg, W. Parson, and R. Windeler, "A compact versatile femtosecond spectrometer," Rev. Sci. Instrum. 73, 4145-4149 (2002). [CrossRef]
- M. Punke, F. Hoos, C. Karnutsch, U. Lemmer, N. Linder, and K. Streubel, "High-repetition-rate white-light pump-probe spectroscopy with a tapered fiber," Opt. Lett. 31, 1157-1159 (2006). [CrossRef] [PubMed]
- G. Oster and Y. Nishijima, "Fluorescence and internal rotation: their dependence on viscosity of the medium," J. Am. Chem. Soc. 78, 1581-1584 (1956). [CrossRef]
- M. Yoshizawa, K. Suzuki, A. Kubo, and S. Saikan, "Femtosecond study of S2 fluorescence in malachite green in solutions," Chem. Phys. Lett. 290, 43-48 (1998). [CrossRef]
- Y. Kanematsu, H. Ozawa, I. Tanaka, and S. Kinoshita, "Femtosecond optical Kerr-gate measurement of fluorescence spectra of dye solutions," J. Lumin. 87-89, 917-919 (2000). [CrossRef]
- A. C. Bhasikuttan, A. V. Sapre, and T. Okada, "Ultrafast relaxation dynamics from the S2 State of Malachite Green studied with Femtosecond upconversion Spectroscopy," J. Phys. Chem. A 107, 3030-3035 (2003). [CrossRef]
- S. H. Cho, B. E. Bouma, E. P. Ippen, and J. G. Fujimoto, "Low-repetition-rate high-peak-power Kerr-lens modelocked Ti:Al2O3 laser with a multiple-pass cavity," Opt. Lett. 24, 417-419 (1999). [CrossRef]
- M. M. Martin, P. Plaza, and Y. H. Meyer, "Ultrafast conformational relaxation of triphenylmethane dyes: Spectral Characterization," J. Phys. Chem. 95, 9310-9314 (1992). [CrossRef]
- T. Robl and A. Seilmeier, "Ground-state recovery of electronically excited malachite green via transient vibrational heating," Chem. Phys. Lett. 147, 544-550 (1988). [CrossRef]
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