Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy, 2A Max-Born-Strasse, Berlin D-12489, Germany. *Corresponding author: trabs@mbiâberlin.
486
OPTICS LETTERS / Vol. 38, No. 4 / February 15, 2013
Generation of tunable sub-45 femtosecond pulses by noncollinear four-wave mixing Masood Ghotbi, Peter Trabs,* Marcus Beutler, and Frank Noack Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy, 2A Max-Born-Strasse, Berlin D-12489, Germany *Corresponding author: trabs@mbi‑berlin.de Received October 11, 2012; revised December 12, 2012; accepted December 21, 2012; posted January 2, 2013 (Doc. ID 175481); published February 8, 2013 Generation of sub-45 fs vacuum UV (VUV) pulses tunable across the spectral range of 146–151 nm at 1 kHz repetition rate is reported. The pulses are produced using noncollinear difference-frequency four-wave mixing between the third-harmonic of an amplified Ti:sapphire laser and the signal wavelength of an infrared optical parametric amplifier (ωVUV 2ωTH − ωIR ) in krypton and argon. The generated VUV pulses have energies as high as 90 nJ. Pulse duration measurements are realized by cross correlation between the VUV pulses and the laser fundamental wavelength using pump-probe ionization in xenon. © 2013 Optical Society of America OCIS codes: 320.7110, 190.4380, 260.7210.
Ultrashort vacuum ultraviolet (VUV) pulses are of great interest for time-resolved spectroscopy of many atoms, molecules, and molecule clusters because of the subpicosecond relaxation times of their electronic transitions in the UV-VUV spectral range [1,2]. Different physical and technical limitations have restricted the success of the efforts for the generation of energetic (>100 nJ) VUV pulses with sub-100 fs duration for many years (see the [3,4]). Recently, we have demonstrated sub-50 fs pulses at 160 nm with up to 240 nJ energy by collinear four-wave difference frequency mixing (FWDFM) in argon at 1 kHz repetition rate [3]. Later, by the application of noncollinear configuration in the FWDFM process, we improved the efficiency by an order of magnitude and increased the pulse energy to 2.5 μJ [4]. Also, by applying shorter idler pulses, generated by spectral broadening inside the filament, in the FWDFM process, we were able to produce sub-20 fs VUV pulses with more than 400 nJ energy at 160 nm [5]. Until now, our results were limited to the special wavelength of 160 nm as the fifth harmonic of a Ti:sapphire laser. Here we generated tunable VUV pulses in a new spectral range by the application of a tunable idler in the noncollinear FWDFM process. Intense, tunable, ultrashort pulses are very favorable for time-resolved spectroscopy of small molecules and clusters with their first absorption band in this spectral region. Different experiments have been performed to produce tunable pulses in the VUV. The early efforts in this direction were based on frequency mixing in gas media using the pulses of dye lasers in the nanosecond regime. As the first method, frequency tripling of intense dye laser radiation provided an effective way of generating tunable VUV radiation of narrow spectral width [6]. Later, resonant and nonresonant sum-difference frequency mixing were applied for this purpose [7–9]. With the presence of high-energy femtosecond laser sources, they were applied in different experiments for the generation of subpicosecond VUV pulses [10–13]. Free-electron lasers also have been a source for the generation of femtosecond VUV pulses. Recently, by four-wave mixing between the signal pulses of a visible optical parametric amplifier (OPA) and the third harmonic (TH) of a Ti:sapphire laser in an argon-filled capillary, femtosecond pulses tunable in the 0146-9592/13/040486-03$15.00/0
spectral range of 168–182 nm were generated [14]. The VUV pulses were nearly 100 fs long, but the lack of a sensitive temporal characterization method prevented the accurate measurement of the pulse duration. In this Letter we report the generation of sub-45 fs VUV pulses tunable across the spectral range of 146–151 nm by noncollinear phase matching (PM) in a FWDFM process between the TH of a 1 kHz Ti:sapphire laser amplifier and the signal pulses of an infrared OPA system [15] in krypton and argon. The generated VUV pulses have durations as short as 34 fs and energies up to ∼90 nJ. Prechirping of the IR pulses was applied for the compression of the generated VUV pulses. The schematic of the experimental setup is shown in Fig. 1. The pump laser provides up to 3 mJ pulses at 800 nm with 42 fs duration. Half of the pulse energy is used to generate the TH pulses in a tripling stage. To have a compromise between the duration and the energy of the generated TH pulses, we applied a 0.3 mm thick type I β‐BaB2O4 (BBO) crystal (θ 29°) for the second harmonic generation (SHG) and a 0.1 mm thick type I BBO (θ 44°) as the third
Fig. 1. (Color online) Experimental setup of the generation of tunable VUV pulses by FWDFM between the TH and signal output of an IR-OPA. BS, beam-splitter; HWP, half-wave plate; CP, compensation plate; TOF, time-of-flight mass spectrometer. SHG, second harmonic generation. © 2013 Optical Society of America
February 15, 2013 / Vol. 38, No. 4 / OPTICS LETTERS
harmonic generation (THG) crystal. Both crystals have AR coating for the interacting wavelengths. We introduced a half-wave plate for rotating the polarization directions and a calcite plate for group velocity compensation between the BBOs as well. With this configuration the THG stage provides up to ∼90 μJ pulses at 266 nm with a duration of ∼80 fs. The generated TH is combined with the signal pulses of an IR OPA [15] that is pumped by the other 50% of the fundamental pulse energy of the laser. The OPA generates signal pulses by the amplification of white-light-continuum (WLC) in two stages with BiB3O6 (BIBO) crystals. The crystal applied in the first stage is a 3 mm long BIBO cut at θ 42° for type II (o → e o) PM inside the optical xz plane and the second stage crystal is a 3 mm long BIBO cut at θ 11.4° for type I (e → o o) PM inside the same optical plane. Both crystals are uncoated. The signal output of the OPA covers the spectral range of 1150–1550 nm with spectral bandwidths supporting sub-40 fs output pulses [Fig. 2(a)]. For the compression and further control of the chirp content of the generated signal pulses, we applied a prism pair compressor in a double-pass configuration consisting of two Brewster-angled prisms of SF11 with a separation of 30–50 cm depending on the wavelength. The IR-OPA signal is focused with an f 75 cm BK7 lens while the focusing of the TH is realized by a highly reflective (R > 98%) curved dielectric mirror with a
Fig. 2. (Color online) (a) Typical spectra and pulse energy of the signal output of the two-stage WLC seeded BIBO IR-OPA across the tuning range and (b) the spectra and pulse energy of the generated VUV pulses across the tuning range.
487
focal length of 60 cm. To separate the generated VUV pulses from an IR signal and TH we applied special dielectric high-reflecting VUV mirrors (Layertec GmbH) with R > 90% across the spectral range of 145–155 nm. These mirrors are also used for collimation and focusing of the beam into the experimental chamber. Using an MgF2 window, the FWDFM chamber is isolated from the second experimental chamber that is designed for pulse characterization. The thickness of the window is only 100 μm to minimize the dispersion and broadening of the generated VUV pulses. In this experiment we applied noncollinear configuration because of its remarkable effect on the efficiency for VUV generation that previously was demonstrated [4]. This is a result of increasing the PM pressure and consequently, the nonlinear coefficient of the gas as the interaction medium. At each signal wavelength, the experiment was initially performed in collinear configuration. The interaction angle between the IR signal and TH beams was increased gradually, and at each angle the conversion efficiency for VUV generation was optimized by changing the pressure and correcting the delay between the two pulses. The optimized conditions for the experiment across the spectral tuning range were at pressures of 150–250 mbar and the interaction angles of ∼20–25 mrad. The calculated PM angle for noncollinear FWDFM using the Sellmeier equation at the pressure of 200 mbar in the IR spectral range of 1150–1550 nm is in the range of 21–27 mrad, increasing with the wavelength that shows good agreement with the experimental results [4]. The measured energy of the generated VUV pulses as a function of the wavelength is shown in Fig. 2(b). Because of the low energy of the generated VUV at the longest wavelength of 151 nm (∼10 nJ) in krypton, we applied argon (P 150 mbar) at this wavelength, which improved the pulse energy to ∼90 nJ. At the other wavelengths across the tuning range, the efficiency of krypton was higher than argon. The energy of the corresponding IR pulses at each VUV wavelength is shown in the Fig. 2(a). As can be seen, the energy of the VUV pulses increases from 40 to 90 nJ by increasing the IR pulse energy across the tuning range. The spectral measurements were performed by a McPherson 0.2 m Monochromator Model 234/302 in combination with a VUV-optimized CCD Camera Andor D0420-BN-995. Typical spectra of the VUV pulses for some wavelengths across the tuning range are shown in Fig. 2(b). The spectra cover the range of 146–151 nm corresponding to the IR spectral range of 1550–1150 nm for the FWDFM process. The bandwidths of the VUV pulses from 147 to 148 nm follow the bandwidths in the IR and are about 1.5 nm. The slightly broadened spectra at 149 and 151 nm are based on self-phase modulation and cross-phase modulation during the FWDFM process due to the high pulse energies in the IR and are as large as 3 nm. Because of some hot spots in the signal beam of the OPA at 1150 nm, we used argon in the FWDFM process at this wavelength. The higher ionization potential minimizes the losses by plasma generation and lead to higher conversion efficiency. According to the complications of the compression of the VUV pulses for compressing the generated pulses, we took advantage of the negative prechirping of these
488
OPTICS LETTERS / Vol. 38, No. 4 / February 15, 2013
Fig. 3. (Color online) Cross-correlation trace between the fundamental and VUV pulses at 148 nm. The measurement was performed in a xenon-filled TOF mass spectrometer.
pulses for precompensation of the positive chirp that is produced after passing through the gas and MgF2 window between the FWDFM and measurement chambers. As it was explained in [5], if we start the FWDFM process (ωVUV 2ωTH − ωIR ) with a chirped idler (here IR) pulse we can produce a chirp with opposite sign in the VUV pulse. Using this technique, the generated signal pulse of the OPA at each wavelength was compressed to the shortest possible duration by the prism pair compressor. Then by gradual increase of the positive chirp of IR pulse (by inserting more glass path with the second prism) we achieved the optimum duration for the VUV pulse. The temporal measurement of the generated VUV pulses was performed inside the second chamber with a continuous Xe jet at a background pressure of about 10−5 to 10−4 mbar. Pulse duration measurement was realized by crosscorrelation using a time-of-flight (TOF) mass spectrometer. A small part of the fundamental frequency (FF), as probe beam, was focused collinearly with the VUV pulse into the interaction region of the TOF. The pulse duration of the FF probe pulse at the position of the measurement is ∼45 fs. The cross-correlation signal is provided by the measured ion rate. The produced signal has linear dependence on the VUV intensity and cubic dependence on the FF intensity. The results of the temporal measurement of the compressed VUV pulses show
sub-45 fs durations across the spectral tuning range. A typical cross-correlation trace at the wavelength of 148 nm is shown in Fig. 3. The FWHM of 43 fs for the Gaussian fit corresponds to a pulse duration of 34 fs and a time-bandwidth product of 0.44. In conclusion, we have demonstrated the generation of sub-45 fs VUV pulses tunable across the spectral range of 146–151 nm at 1 kHz repetition rate using a simple experimental set up by noncollinear PM in a FWDFM interaction in krypton and argon. Continuous tunability of the generated VUV pulses, together with their moderate pulse energy and shorter duration, compared with the previous experimental results, make this source attractive for application in time-resolved spectroscopy of small molecules. The good stability and short duration of the generated VUV pulses has already allowed the first application of our new source for investigation on the dynamic behavior of NO molecules. References 1. I. V. Hertel and W. Radloff, Rep. Prog. Phys. 69, 1897 (2006). 2. H. T. Liu, J. P. Müller, M. Beutler, M. Ghotbi, F. Noack, W. Radloff, N. Zhavoronkov, C. P. Schulz, and I. V. Hertel, J. Chem. Phys. 134, 094305 (2011). 3. M. Beutler, M. Ghotbi, F. Noack, and I. V. Hertel, Opt. Lett. 35, 1491 (2010). 4. M. Ghotbi, M. Beutler, and F. Noack, Opt. Lett. 35, 3492 (2010). 5. M. Beutler, M. Ghotbi, and F. Noack, Opt. Lett. 36, 3726 (2011). 6. R. Hilbig and R. Wallenstein, Appl. Opt. 21, 913 (1982). 7. G. Hilber, A. Lago, and R. Wallenstein, J. Opt. Soc. Am. B 4, 1753 (1987). 8. H. Wallmeier and H. Zacharias, Appl. Phys. B 45, 263 (1988). 9. J. P. Marangos, N. Shen, H. Ma, M. H. R. Hutchinson, and J. P. Connerade, J. Opt. Soc. Am. B 7, 1254 (1990). 10. A. Tünnermann, C. Momma, K. Mossavi, C. Windolph, and B. Wellegehausen, IEEE J. Quantum Electron. 29, 1233 (1993). 11. J. Ringling, O. Kittelmann, F. Noack, G. Korn, and J. Squier, Opt. Lett. 18, 2035 (1993). 12. S. P. Le Blanc, Z. Qi, and R. Sauerbrey, Opt. Lett. 20, 312 (1995). 13. A. Nazarkin, G. Korn, O. Kittelmann, J. Ringling, and I. V. Hertel, Phys. Rev. A 56, 671 (1997). 14. M. Mero and J. Zheng, Appl. Phys. B 106, 37 (2012). 15. M. Ghotbi, V. Petrov, and F. Noack, Opt. Lett. 35, 2139 (2010).
