N. Lecong, B. Zietz, J. Léonard, O. Crégut and S. Haacke
Ultrafast spectroscopy with a photonic crystal fibre
Collaboration with P. Viale, P. Leproux et V. Couderc XLIM – Photonics group, Limoges, France.
We have built a compact broad-band femtosecond spectrometer based on a probe light supercontinuum generated in a birefringent photonic crystal fiber (see the “experimental techniques” of GONLO). These micro-structured material systems can be tailored for very efficient generation of broadband “white-light” continua using only moderate laser pulse energies derived e.g. from a Titane-Saphire oscillator, rather than from an expensive multi-component amplified laser chain. Previous spectrometers of that kind where however limited to wavelengths > 550 nm or used 800 nm for excitation. The use of multi-channel detection with a CCD operated at 600 Hz read-out rate provides more precise spectral information, and a high signal-to-noise ratio. The ultrafast photo-isomerisation of Malachite Green (MG) is studied in a proof-of-principle experiment.
The system is based on a Ti:Sapphire oscillator with an extended cavity operating at 27 MHz and delivering 40 fs pulses of 15-18 nJ energy. About 20 % of that pulse energy is focused by a microscope objective into a birefringent 8-mm fiber with an elliptical core and 775 nm zero GVD wavelength. The remaining 80 % of laser power is frequency doubled in a 2mm – BBO, so as to yield “pump” pulses in the 1-1.5 nJ range. The birefringent fiber input facet is protected by a thin slice of a multi-mode fiber (≈50 m thickness). This reduces damage and ensures operational stability over weeks. The chirp of the supercontinua is characterized by a two-photon absorption pump-probe experiment in a thin ZnS sample. For short fibers, the output is single-pulsed (no soliton break-up) and the cross correlation has a width of 120-150 fs.
Figure 1 : a) Absorption spectrum of MG in ethanol. b) In false-color scale: time- and spectrally-resolved transient absorption spectra of MG in ethanol promoted in the S2 state. This data set is obtained when using the supercontinuum generated in a photonic crystal fiber as a probe beam, without reference beam. The level of noise is in the range 5*10-5.
The relative absorption changes of MG in ethanol show the different bands associated with the excited S2 the lower-lying S1 and the ground S0 states (Figure 1-b). With a 0.2 absorbance at 430 nm, and a 0.3 mm flow cell, the maximum signal is above 2 %. Averaging 3000 spectra per time step leads to a DA ≈ 5×10-5 noise level, obtained in 1h experiment time. The time evolution of MG’s transient absorption spectrum is rather complex. Various transient absorption bands (positif or negative) can be associated to S2, S1 and S0 successively. The excited state absorption from S2 is observed in the 680 – 1050 nm range. S2-S1 relaxation reduces this signal in 270-320 fs while a stimulated emission signal from S1 rises in the 680 -900 nm range, on the same time scale. Ground state bleach gives rise to the dominant signal at 620 nm (S0-S1 absorption). The decay of the stimulated emission from S1 and the rise of the “hot” ground state absorption set the time scale for the excited state bond flip (600 fs). Detailed data analysis of MG transient signals and further experimental improvements are in progress. The set-up is also being used for the investigation of ultrafast bio-mimetic photo-switches and of bilirubin.