The Narrow-band Optical Parametric Amplifier

The aquisition of Raman spectra with high frequency resolution and decent signal/noise requires a source for spectrally narrow Raman pulses of reasonable brightness. Around 776 nm, these requirements can be easily met by filtering part of the output of the Clark CPA2001 amplifier system in a monochromator. With this approach Raman pulses with energies of 3.7µJ and spectral widths of 5.5 cm⁻¹ are achieved. However, the simulation of Raman signals in chapter 2.4.2 and the experimental Raman spectra of flavin in chapter 6 demonstrate that the measured signal strongly depends on the reso-nance conditions of the Raman pulse. To study and optimize resoreso-nance conditions, it is necessary to generate Raman pulses also at other wavelengths. This can be achieved by applying nonlinear optical processes. Narrowband pulses at 400 nm were previously ob-tained by sum frequency generation of counter-chirped fundamental pulses.^[16] Whereas this approach is limited to a narrow frequency range around 400 nm, optical parametric amplification is a convenient tool to provide tunable pulses in the visible and near-infrared region.^[69,70] Nowadays, sub-10 fs pulses can be routinely obtained, spanning a frequency range of several thousand wavenumbers at a time.^[71–74] In comparison, the development of amplification schemes for the creation of spectrally narrow pulses still lags behind. The conversion process is usually not efficient enough that filtering of the output pulses of optical parametric amplifiers would be a conceivable approach to obtain spectrally narrow pulses. In a recent publication by Shimet al. the direct amplifation of narrowband pulses was demonstrated.^[75] The obtained output pulses featured energies of up to 2.5µJ and a spectral width of 36 cm⁻¹. However, the total conversion efficiency was small, considering that input pulse energies of 800µJ were necessary.

Raman spectra of chromophores with low symmetry are higly congested. Hence, for a reliable tracking of relaxation in biological photoreceptors, one should strive for an improvement of the spectral resolution to ~10 cm⁻¹ . On the other hand, in the current experiment only 450µJ of the the fundamental was available for the generation of Raman pulses. A key requirement was therefore the development of an efficient narrow-band optical parametric amplifier (nb-OPA).

The setup of the nb-OPA is presented in 3.4; it is pumped by a 450 µJ portion of the Clark CPA 2001 output. The first part of the setup resembles a common two-stage non-collinear optical parametric amplification in a 2 mm BBO crystal (type I,θ= 29^◦).^[70]A small fraction (ca. 2%) of the incoming light is separated with a beam splitter and focused with a lens (f = 50 mm) into a 2 mm thick sapphire plate to generate a continuum; the white light is recollimated with a second lens (f = 50 mm) and focused into the BBO crystal for amplification. In order to decrease the spectral width of the amplified beam, the continuum seed is chirped by 5 cm of SF10 glass. The residual

~98 % of the fundamental is frequency doubled in a 2 mm thick BBO crystal (type I, θ= 29^◦) resulting in 190 µJ of blue light. Optimal conversion is achieved by decreasing the near-infrared beam size with a 2:1 telescope to a diameter of 3 mm. For pumping the first stage it is sufficient to separate the residual from the previous second-harmonic

44

3.5 The Femtosecond Stimulated Raman Spectrometer

Figure 3.5: First two stages of amplification in thenb-OPA. The 400 nm pump beams are focused 1 cm in front of the BBO crystal (2 mm, 29^◦) with lenses L₁and L₂ (f = 200 mm). The continuum seed ist focused into the BBO and overlapped with pump 1 for amplification. Thereafter, it is passed a second time through the crystal, thereby collimated and refocused with lens L₃ (f = 200 mm).

generation with a dielectric mirror and frequency-double it again in another BBO crystal.

For the subsequent amplification stage 20% of the main blue light is separated with a beam splitter. Both amplification stages were realized in the nonlinear crystal with a folded geometry (see figure 3.5). The seed is collimated after the first pass through the crystal with lens L₃ (f = 200 mm) and back-reflected by mirror M₁ to pass the crystal a second time, thereby again being focused by lens L₃. The pump beams are focused with lenses L₁ and L₂ (f = 200 mm) to points 1 cm before the BBO crystal, and overlapped with the seed inside the crystal. Typically, 11 µJ of visible pulses with a bandwidth of 100 cm⁻¹ are obtained. Due to the time-bandwidth relationship the minimal spectral width is limited by the pump pulse duration. As already demonstrated by Mathies et al., temporal stretching of the blue light pulse enables the amplification of a narrowband seed.^[75]Here, temporal broadening is achieved by passing the residual blue light through 1 m of quartz glass. Experimentally, this is realized by sending the blue light five times through a quartz block with 20 cm length (Heraeus HOMISIL), whoose end surface are cut at Brewster angle. To avoid continuum generation inside the glass, the beam diameter is enlarged with a telescope to 30 mm. The outcoming pulses are then used to drive the third amplification stage. As seed, the output from the previous amplification stage is filtered in a grating monochromator. The monochromator has a conventional 4f arrangement, consisting of a holographic grating with 2400 lines/mm (Newport), a lens

3 Experimental Section

Figure 3.6: Tunability of the Raman pulse demonstrated for stimulated state Raman spectra of a toluene/acetonitrile (50/50) mixture. Insets show the width of the 1004 cm⁻¹ band.

with f = 300 mm, and a back-reflecting mirror. Amplification of the narrowband seed in a 2 mm BBO crystal (type I,θ= 29^◦) with nearly collinear pump/seed geometry results

46

3.5 The Femtosecond Stimulated Raman Spectrometer

Figure 3.7: a)Layout of the chopper wheel in thenb-OPA. Designed for aNew Focus chopper model 3501, model 3501. b) Raman pulse sequence: In a repeated cycle of 6 submeasurements, transient absorption, ground and transient Raman spectra can be obtained. The sequence was achieved with two choppers: one in the monochromator of the narrowband OPA and one in the pump beam. Each square represents a single pulse.

in 10-20µJ of visible pulses with 30 cm⁻¹ spectral width (FWHM). In a second grating monochromator the pulses are further filtered to match the resolution of the Raman spectrograph (7.5–15 cm⁻¹ in the visible), leaving 1–2 µJ pulse energy. The tunability of the Raman pulses is demonstrated in figure 3.6 for stimulated Raman spectra of a toluene/acetonitrile (50/50) mixture. A close-up of the 1004 cm⁻¹ band in the inset shows the spectral resolution.

To facilitate the evaluation, the monochromators are equipped with a double slit (3.4, inset), enabling the amplification of the seed at two wavelengths which differ by 30–60 cm⁻¹. A chopper (New Focus3501) in thenb-OPA selects between the two (Raman1 and Raman2); for the design of the chopper wheel see figure 3.7a.