Sub-20-fs pulses tunable across the visible from a blue-pumped single-pass noncollinear parametric converter

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Sub-20-fs pulses tunable across the visible from a blue-pumped single-pass noncollinear parametric converter T. Wilhelm, J. Piel, and E. Riedle Institut fur ¨ Medizinische Optik, Ludwig-Maximilians-Universitat ¨ Munchen, ¨ Barbarastrasse 16, D-80797 Munchen, ¨ Germany Received May 13, 1997 Femtosecond pulses with center wavelengths between 470 and 750 nm are generated in a single-stage type I BBO optical parametric amplifier pumped by a frequency-doubled 1-kHz Ti:sapphire amplifier. A high-quality white-light continuum is used as the seed. Pulse durations as short as 16 fs and pulse energies of as much as 11 mJ are observed. The quantum efficiency is ,25% for both 7- and 40-mJ pump pulses. This unique combination of ultrashort pulse duration and high conversion is made possible by noncollinear phase matching that permits a sufficiently large amplification bandwidth. Simultaneously the group velocities of the signal and the idler are effectively matched. As a result widely tunable sub-20-fs pulses can be generated in a nonlinear crystal as thick as 2 mm.  1997 Optical Society of America

Intense ultrashort pulses at predetermined visible and UV wavelengths are desirable for many experiments in physics, chemistry, and biology. Previous investigations have shown that tunable pulses of 50-fs duration can readily be generated in the near IR by optical parametric generation and optical parametric amplification. The fundamental output of a kilohertz Ti:sapphire regenerative amplif ier (RGA) was used as pump source.1 To generate visible pulses the optical parametric generator or optical parametric amplifier (OPA) can be pumped by a frequency-doubled RGA. However, pulses with a minimal length of ,100 fs are observed, presumably because of the increased dispersion at shorter wavelengths and the resulting large group-velocity mismatch (GVM).2,3 To circumvent this problem a number of authors have upconverted short near-IR pulses at the cost of having to use a multistage setup.4 – 6 Somewhat shorter visible pulses were generated in a 200-kHz system consisting of a blue-pumped OPA seeded with high-quality continuum pulses.7 A further improvement was achieved in a three-stage OPA when slightly different spectral regions were amplif ied in each pass.8 The shortest visible pulses ( less than 20 fs) were reported for a synchronously pumped optical parametric oscillator using a frequency-doubled Ti:sapphire laser as pump source and a noncollinear phase-matching geometry.9 The underlying concept of noncollinear phase matching was investigated in great detail,10,11 and it was demonstrated that GVM could be decreased in this scheme. In this Letter we show that sub-20-fs visible pulses can be produced with high efficiency in a single-stage noncollinear OPA pumped by the frequency-doubled output of a kilohertz RGA. For such short pulses a suff iciently broad spectrum has to be generated and amplified. We therefore seed the OPA with continuum pulses and arrange the OPA for the large amplification bandwidth. If wave-vector mismatch Dk is expanded in powers of seed wavelength detuning Dl1 , Dk ­ Dk0 1

≠Dk 1 ≠2 Dk Dl1 1 Dl1 2 1 . . . , ≠l1 2 ≠l1 2 0146-9592/97/191494-03$10.00/0

a vanishing value of ≠Dky≠l1 is required besides the usual phase-matching condition Dk0 ­ 0. This is equivalent to12 vg,1 ­ cossVdvg,2 , where vg,1 and vg,2 are the group velocities of the seeded signal and the generated idler pulses and V is the angle between them. For a BBO type I OPA the required internal angle c between seed and 405-nm pump wave vectors k1 and kp varies from 2.2± for l1 ­ 480 nm, to 3.7± at 620 nm, to 3.3± at 700 nm. For nonvanishing GVM between the idler and the signal, the idler pulse that is generated simultaneously with the amplification of the seeded signal walks away from the signal. The subsequent amplification of the idler then produces a signal that is temporally offset from the original pulse and hence lengthens the resulting signal. In a noncollinear geometry only the projection of the idler group velocity onto the seed is relevant. Since this projection is exactly equal to the signal group velocity in the situation described above, no lengthening occurs. In contrast to the common situation in collinear frequency doubling or optical parametric amplification, a long nonlinear crystal can be used for the conversion even for extremely short pulses. Pump pulses are generated in our setup by a homebuilt 1-kHz Ti:sapphire RGA (300 mJ of energy; pulse length, 65 fs). A 0.5-mm BBO crystal converts the red pulses to 75-mJ pulses at 405 nm with a spectral width of 5 nm and a smooth transversal mode sM 2 ø 2d. A fraction amounting to either 7 or 40 mJ is used to pump the OPA. The duration of the blue pulses is assumed to be close to the length of the fundamental ones. A small part of the RGA pulses generates a white-light continuum in a 3-mm sapphire crystal whose optical axis is oriented perpendicular to the surface.7 A combination of half-wave plate and polarizer is used to attenuate the pump pulses to just above the continuum threshold. The chirp of the seed continuum at the OPA amounts to a relative delay of individual spectral components of 10.6 fsyTHz at 500 nm and 6.4 fsyTHz at 700 nm.  1997 Optical Society of America

October 1, 1997 / Vol. 22, No. 19 / OPTICS LETTERS

A schematic of the OPA stage is shown in Fig. 1. The blue pump light is steered onto the focusing mirror by a f lat dielectric mirror placed beside the amplifier crystal. The f ­ 250 mm mirror is located directly below the seed beam and focuses the vertically polarized pump beam to a spot , 25 mm in front of the BBO crystal. The resulting diameter of the pump beam in the crystal is of the order of 500 mm. Displacement of the crystal allows fine adjustment of the pump intensity, and a slight movement of the steering mirror is used to set the delay between the chirped seed and the pump pulses. The horizontally polarized seed beam is focused with a f ­ 300 mm lens and overlapped with the pump beam in the BBO. We used 1- and 2-mm BBO crystals, both cut at 26± and protected by a p coating. The phase-matching angle u (between 27± and 31± internal angle) was adjusted by rotation of the crystal around a horizontal axis. Care was taken to orient the crystal axis in such a way that the walk-off (,4±) of the pump beam helped to compensate for the noncollinear arrangement [compare Figs. 1(a) and 1(b); the dotted and dashed lines denote the optical axes]. The resulting good spatial overlap between the seed and the pump beam ensures a high parametric gain.9,12 The chirp of the amplified pulses was compensated in a fused-silica prism compressor. For steering of the continuum and the broadly tunable amplif ied light, highly ref lecting overcoated silver mirrors were used. Because of the noncollinear geometry no dichroic optics were required. The temporal shape of the pulses was measured by background-free autocorrelation in a 100-mm-thick BBO crystal. With the 2-mm BBO OPA crystal we were able to generate broad-bandwidth pulses with center wavelengths between 470 and 750 nm and between 870 and 1500 nm (see Fig. 2). The spectral widths were slightly larger than 22 THz, the minimum needed to support a 20-fs Gaussian pulse. For the 1-mm crystal narrower spectra were obtained, as explained below. The quantum efficiency of the parametric conversion was ,25%, independent of the pump energy. However, tighter focusing is needed for the weaker pump pulses. The maximum visible output achieved was 11 mJ at 490 nm. The prism compressor was optimized for 485- and 640-nm pulses to prove that the pulses can indeed be compressed to less than 20 fs. The resulting autocorrelation traces are shown in Fig. 3 together with the pulse spectra. The solid curves in Figs. 3(a) and 3(b) and in the insets are fits assuming sech2 pulse shapes. It can be seen that clean pulses of 16- and 17-fs duration were produced. The time – bandwidth products Dt Dn of 0.41 for 485-nm pulses and 0.45 for 640-nm pulses indicate that the pulses are nearly Fourier-transform limited. The spatial profile of the pulses was measured with a CCD camera and found to be nearly Gaussian. We measured M 2 values of between 1.5 and 2.0; i.e., the output beam is close to diffraction limited. Typical peak-to-peak energy f luctuations were less than 10%. For the results shown in Figs. 2 and 3 the noncollinear angle c was optimized together with the

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phase-matching angle u and the continuum delay for every center wavelength. We were, however, also able to f ind angle parameters that allowed variation of the wavelength throughout the visible solely by variation of the delay. By the combination of crystal length, angle adjustment, and chirp of the continuum the generated pulses length could be varied by as much as 100 fs. This is demonstrated for 640-nm pulses in Fig. 4. The 17-fs pulses [Figs. 4(a) and 4(d)] were generated in 2-mm BBO, the 33-fs ones [Figs. 4(b) and 4(e)] in 1-mm BBO, and the 78-fs ones [Figs. 4(c) and 4(f)] in 2-mm BBO with largely increased chirp of the continuum seed pulses. The respective values of the time–bandwidth product Dt Dn for the 17-fs, 33-fs, and 78-fs pulses are 0.45, 0.43, and 0.37. The experimental results prove that noncollinear parametric amplif ication pumped by a frequencydoubled Ti:sapphire amplif ier yields clean sub-20-fs pulses across the visible spectrum. This is possible despite the considerable GVM in BBO. The mismatch between the signal and the idler group velocity is effectively corrected by the noncollinear phase matching

Fig. 1. Schematic of the noncollinear parametric converter (c), (d ). The angle c between wave vectors k1 (seed pulse) and kp (405-nm pump) is close to the walk-off angle (a). The projection of the group velocity vg,2 onto the seed direction equals vg,1 for group-velocity matching (b). o.a., optical axis of the BBO crystal.

Fig. 2. Spectra of femtosecond pulses generated by noncollinear parametric amplif ication in a 2-mm BBO crystal.

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Fig. 3. (a), (b) Autocorrelation traces, and (c), (d ) spectra of visible sub-20-fs pulses. The solid curves are f its assuming sech2 pulses shapes.

be overcome by the microjoule pump energies in the kilohertz system. Previous investigations have pointed out that tilted pulse fronts are to be expected in noncollinear phasematched optical parametric generation and optical parametric amplification.3 This does not pertain to the signal pulse in our arrangement since the untilted seed pulse is amplified and retains its shape. We do, however, believe that the idler pulse is tilted and also possesses a transversal chirp. In summary, we have demonstrated that sub-20-fs pulses that are tunable across the visible can be generated in a single-pass type I BBO OPA pumped by the frequency-doubled output of a kilohertz Ti:sapphire RGA. This is made possible by noncollinear phase matching. The setup is highly stable and easy to align. It should therefore be an ideal tool for ultrafast spectroscopy, particularly since the pulse lengths can also be varied continuously. Scaling of pump and output energy is possible by simple translation of the amplifier crystal and thereby optimization of the pump intensity. Finally, we point out that the OPA produces pulses that are shorter by a factor of 4 than the output pulses of the RGA. This drastically reduces the need for RGA’s with pulse lengths in the 20-fs regime if spectroscopic experiments are to be performed with such an extremely high temporal resolution. The authors thank Wolfgang Zinth for continuous support of this study and the Deutsche Forschungsgemeinschaft for financial support. References

Fig. 4. (a)–(c) Autocorrelation traces and (d )– (f ) spectra of 640-nm pulses with varying duration.

and the resulting angle between the signal and the idler beam. The pump pulse merely serves to amplify the seeded signal pulse and does not generate visible light by itself. Blocking the seed reduces the observed visible light by many orders of magnitude. The pump propagates significantly more slowly through the crystal (delay of 37 fsymm compared with propagation of a 500-nm signal and 147 fsymm compared with propagation of a 700-nm signal) and sweeps over a wide range of spectral components in the chirped seed pulse. Consequently, narrower spectra are observed for the shorter crystal and the longer nonlinear crystal produces shorter pulses, contrary to most other observations in femtosecond technology. This is possible only because of the high-quality seed pulses and would not be observed for a parametric generator. The chirp of the seed can be compensated for after amplif ication. The nearly complete compensation of the transversal walk-off of the pump beam and the effective lack of walkaway (GVM) between the signal and the idler give rise to amplification of ,105 . The limiting inf luence of the transversal walk-off of the idler that is found in quasi-cw optical parametric oscillators13 can easily

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