Spatially coherent, phase matched, high-order harmonic EUV beams at 50 khz

Spatially coherent, phase matched, high-order harmonic EUV beams at 50 khz M.-C. Chen, 1* M. R. Gerrity, 1 S. Backus, 2 T. Popmintchev, 1 X. Zhou, 1 P. Arpin, 1 X. Zhang, 2 H.C. Kapteyn 1 and M. M. Murnane 1 1 Department of Physics and JILA, University of Colorado and National Institute of Standards and Technology, Boulder, Colorado, 80309-0440 2 Kapteyn-Murnane Labs Inc., Boulder, CO 80301, USA *mchen@colorado.edu Abstract: By focusing a high repetition rate (50kHz), compact, femtosecond laser system with low pulse energy (25µJ) using a tightfocusing geometry, we demonstrate fully phase matched high-order harmonic generation for the first time at very high repetition rates, resulting in EUV light with full spatial coherence. The result is a practical, singlebox, coherent source useful for applications in metrology, ultrafast spectroscopy, imaging and microscopy. The soft x-ray flux can be improved further by increasing the laser pulse energy and/or repetition rate. 2009 Optical Society of America OCIS codes: (320.7090) Ultrafast lasers; (320.7160) Ultrafast technology; (140.7240) UV, EUV, and X-ray lasers References and links 1. W. Li, X. B. Zhou, R. Lock, S. Patchkovskii, A. Stolow, H. C. Kapteyn, and M. M. Murnane, Time-resolved dynamics in N2O4 probed using high harmonic generation, Science 322(5905), 1207 1211 (2008). 2. A. S. Sandhu, E. Gagnon, R. Santra, V. Sharma, W. Li, P. Ho, P. Ranitovic, C. L. Cocke, M. M. Murnane, and H. C. Kapteyn, Observing the creation of electronic feshbach resonances in soft x-ray-induced O2 dissociation, Science 322(5904), 1081 1085 (2008). 3. P. Salières, A. L Huillier, and M. Lewenstein, Coherence control of high-order harmonics, Phys. Rev. Lett. 74(19), 3776 3779 (1995). 4. T. Ditmire, E. T. Gumbrell, R. A. Smith, J. W. G. Tisch, D. D. Meyerhofer, and M. H. R. Hutchinson, Spatial Coherence Measurement of Soft X-Ray Radiation Produced by High Order Harmonic Generation, Phys. Rev. Lett. 77(23), 4756 4759 (1996). 5. A. Rundquist, C. G. Durfee 3rd, Z. Chang, C. Herne, S. Backus, M. M. Murnane, and H. C. Kapteyn, Phasematched generation of coherent soft X-rays, Science 280(5368), 1412 1415 (1998). 6. Y. Tamaki, J. Itatani, Y. Nagata, M. Obara, and K. Midorikawa, Highly Efficient, Phase-Matched High- Harmonic Generation by a Self-Guided Laser Beam, Phys. Rev. Lett. 82(7), 1422 1425 (1999). 7. R. A. Bartels, A. Paul, H. Green, H. C. Kapteyn, M. M. Murnane, S. Backus, I. P. Christov, Y. W. Liu, D. Attwood, and C. Jacobsen, Generation of spatially coherent light at extreme ultraviolet wavelengths, Science 297(5580), 376 378 (2002). 8. D. G. Lee, J. J. Park, J. H. Sung, and C. H. Nam, Wave-front phase measurements of high-order harmonic beams by use of point-diffraction interferometry, Opt. Lett. 28(6), 480 482 (2003). 9. R. L. Sandberg, D. A. Raymondson, C. La-O-Vorakiat, A. Paul, K. S. Raines, J. Miao, M. M. Murnane, H. C. Kapteyn, and W. F. Schlotter, Tabletop soft-x-ray Fourier transform holography with 50 nm resolution, Opt. Lett. 34(11), 1618 1620 (2009). 10. P. Balcou, P. Salieres, A. L Huillier, and M. Lewenstein, Generalized phase-matching conditions for high harmonics: The role of field-gradient forces, Phys. Rev. A 55(4), 3204 3210 (1997). 11. F. Lindner, W. Stremme, M. G. Schatzel, F. Grasbon, G. G. Paulus, H. Walther, R. Hartmann, and L. Struder, High-order harmonic generation at a repetition rate of 100 khz, Phys. Rev. A 68(1), 013814 (2003). 12. A. R. Libertun, X. Zhang, A. Paul, E. Gagnon, T. Popmintchev, S. Backus, M. M. Murnane, H. C. Kapteyn, and I. P. Christov, Design of fully spatially coherent extreme-ultraviolet light sources, Appl. Phys. Lett. 84(19), 3903 3905 (2004). 13. C. G. Durfee, A. R. Rundquist, S. Backus, C. Herne, M. M. Murnane, and H. C. Kapteyn, Phase matching of high-order harmonics in hollow waveguides, Phys. Rev. Lett. 83(11), 2187 2190 (1999). 14. M.-C. Chen, M. Gerrity, T. Popmintchev, S. Backus, X. Zhang, M. M. Murnane, and H. C. Kapteyn, Practical Compact Spatially-Coherent, Phase-Matched Extreme UV Source at 50 khz, in Advanced Solid-State Photonics(Optical Society of America, 2009), p. MF7. 15. D. M. Gaudiosi, E. Gagnon, A. L. Lytle, J. L. Fiore, E. A. Gibson, S. Kane, J. Squier, M. M. Murnane, H. C. Kapteyn, R. Jimenez, and S. Backus, Multi-kilohertz repetition rate Ti:sapphire amplifier based on downchirped pulse amplification, Opt. Express 14(20), 9277 9283 (2006). (C) 2009 OSA 28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 17376

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In this paper, we show that by increasing the pulse energy by a moderate factor (3 ) and the gas pressure (~40 ) compared with previous work [11], we can implement fully phasematched high harmonic conversion at very high (50 khz) repetition rates for the first time [14]. As a result, we generate excellent mode quality, fully spatially coherent, HHG beams at very high repetition rates for the first time. We get a photon flux of 2 10 9 photons per second at a photon energy of 45 ev, using a compact driving laser that integrates the pump lasers and ultrafast oscillator and amplifier system into a single 2 6 box. This photon flux is at the theoretical limit for the density-length gas medium we can use. The observed HHG flux scales quadratically with gas pressure, as expected for phase matched high harmonic conversion. Finally, we also show that the HHG flux scales approximately linearly with repetition rate between 10 and 50 khz, demonstrating that gas recombination times and heating effects are not a limiting factor. 2. Experimental setup A schematic of our experimental setup for generating harmonics at very high repetition rates is shown in Fig. 1. An ultrafast regenerative laser amplifier employing downchirped pulse amplification [15] (Wyvern, KMLabs Inc.) provides 45-fs-duration pulses, with an pulse energy of 25 µj, at 50 khz repetition rate, at a wavelength of ~800nm and with good beam quality (M 2 <1.3). The laser beam is focused into a gas cell using a ~f/15 focusing lens. The gas cell consists of a hollow glass capillary with inner diameter 400µm and outer diameter 1.2mm, through which two 100 µm holes were drilled in the walls using a CO 2 laser (see Fig. 1). One end of the capillary is pressurized with Ar, while the other is sealed. The design of the gas cell functions as a good spatial filter for the incoming laser beam with a very high damage threshold. Based on a flow simulation, 90% of gas can be confined in the laser-gas interaction region. The gas cell also significantly decreases the loading for the vacuum system. The lens is translated to precisely control the focus position in the gas cell to obtain optimal phase matching conditions. The focus diameter of 22 µm results in a peak intensity of 2.6 10 14 W/cm 2, over a confocal parameter distance of 950 µm. At these laser intensities, Ar is approximately 18% ionized at the temporal peak of the pulse. The scattered laser light and harmonics orders below the 11th are blocked by two aluminum filters (each of them is 200 nm). The spatial profile of the harmonic beam was recorded with an x-ray CCD camera (Andor, DO420-BN) at a distance of 50 cm past the focus. Fig. 1. Setup for HHG at high repetition rates. To measure the spatial coherence of the HHG beam, a pinhole pair (50-µm pinholes placed 150 µm apart) was placed 20 cm after the focus, where the HHG beam was sampled at 15% of its beam diameter. The interference pattern was then recorded at a distance of 90 cm after the pinhole pair. The depth of modulation of the fringes depends on the correlation between the local phase of the wavefront of the HHG beam at the two points where it is sampled by the pinhole pair. The HHG spectrum was measured by inserting a concave ruled reflectance grating (1200 grooves/mm, Newport 52025BK) to separate and partially refocus the individual harmonics. In this setup, the x-ray CCD camera was placed at the focal plane, approximately 16 cm from the grating. (C) 2009 OSA 28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 17378

3. Results and discussion Figure 2 (a) shows an image of the HHG beam 50 cm after the gas cell, for an Ar pressure of 200 torr, and with the focal position approximately 10 µm after the gas cell. The diameter of the HHG beam is 1.25 mm at the 1/e 2 points, with a slight ellipticity due to imperfections in the sidewalls of the gas cell. The HHG beam size is consistent with a small HHG source diameter of ~8 µm (laser focus diameter = 22 µm), and with a diffraction limited HHG beam divergence of ~5 mrad. The harmonic photon flux is estimated at ~2 10 9 photons/s, which was obtained by summing the accumulated number of counts on the CCD camera, while considering the quantum efficiency of the CCD camera and the transmission of the Al filters. Fig. 2. (a) Spatial profile of the harmonic beam 50 cm from the gas cell. (The spot on the lower right is due to debris on the surface of CCD camera) (b) Interferogram of the EUV beam diffracted by 50 µm pinholes placed 150 µm apart, using a 300s acquisition time, together with the lineout of the image. The excellent fringe contrast at the center indicates full spatial coherence. The imperfect visibility of the central fringe is due to the ~5 mrad divergence of the wavefront and the spectrum consisting of 4 harmonic orders (c) Simulations assuming the central wavelength of the measured spectrum, and 100% spatial coherence with a beam divergence of 5 mrad indicate the expected fringe visibility is 87% in agreement with our experiemental value of 85%. The interferogram of the EUV beam is shown in Fig. 2 (b), taken with an acquisition time of 300 s (1.5 10 7 laser shots). The interference pattern contains two circular Airy distribution patterns with fringes in between. The two Airy distributions are separated by ~750 µm on the CCD camera in the far field. The visibility of the central fringe is 85%. The slight decrease in fringe visibility is primarily due to the measurement, and not to less-than full coherence of the beam. Simulations shown in Fig. 2(c) assuming the central wavelength of the measured spectrum, 100% spatial coherence, and a beam divergence of 5 mrad indicate the expected fringe visibility in this geometry is 87%. Since the EUV spectrum contains 4 harmonic orders, the visibility of off-center fringes is expected to diminish quickly. The measured contrast thus illustrates both the long-term wavefront stability as well as the nearly 100% spatial coherence of the HHG beam. To optimize HHG conversion efficiency, a coherent build-up of the HHG signal must occur over some macroscopic pressure-length product (phase-matching) [16]. In a free-focus geometry, the phase mismatch is the sum of a pressure-dependent positive contribution from neutral atoms, a negative contribution from the free-electron dispersion, and a pressureindependent negative contribution from Gouy phase, in addition to any intrinsic atomic phase contribution [3,5,10,13]: 2π 2 k = q P [ 1 η] δ η [ Natm re λl] θ I. λl b Here q is the harmonic order, P is the pressure, η is the ionization level, r e is the classical electron radius, N atm is the number density of atoms at 1 atm, δ is the difference between the indices of refraction of the gas at the fundamental and harmonic wavelengths, b is the confocal parameter, θ is the slope constant of the intrinsic atomic phase, and I is the intensity (C) 2009 OSA 28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 17379

distribution of a free focus. In a tight focusing geometry, the dominant contribution to the phase mismatch is due to the Gouy phase, which results in a situation where phase matching strongly depends on the position of the medium (gas cell) relative to the position of the laser focus [3]. Therefore, to optimize phase matching, the distance between the laser focus and the gas cell was varied by moving the lens position along the propagation direction. Figure 3 (a) shows the conversion efficiency for the 29th harmonic order (45 ev) as a function of laser focus position, at a backing pressure of 200 torr of Ar. Two different phase matching regions can be identified, where the different contributions to the total k equation compensate each other ( k = 0). On axis, collinear phase matching is seen when the laser is focused 50 µm before the gas cell, while non-collinear off-axis phase matching is seen when the laser is focused 10 µm after the gas cell. Fig. 3. (a)experimental conversion efficiency for the 29th (45 ev) harmonic as a function of the focus position, 200 torr Ar. (b)hhg spectrum through two 200 nm Al filters as a function of pressure (b-top) The transmission curve for 400 nm of thickness in Al, and 400 µm of thickness of Ar at 50 torr. (c) Lineouts showing that the pressure dependence of the HHG signal at the 27th (29th, 35th) order saturates at 190 (190, 150) torr likely due to free electron induced defocusing. The 27th and 29th harmonic grow quadratically at low pressure, as expected for the phase matched case, while the 35th is 100 times weaker and grows linearly since it is not phase matched. The y-axis (intensity) on this graph is separately normalized for each harmonic order. Another important method to increase the HHG flux would be to increase the backing pressure, since in this tightly focused geometry, phase matching is only weakly dependent on pressure. Thus, in theory the HHG flux can be increased by increasing the pressure (the length of the medium cannot be increased due to the short confocal parameter). Figure 3 (b) shows the HHG spectrum through two 200 nm Al filters as a function of pressure. The HHG flux at the 27th and 29th orders increases quadratically with gas pressure (density), which is consistent with phase matched HHG signal buildup as a significantly higher number of atoms (C) 2009 OSA 28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 17380

contribute coherently to the emitted HHG signal. In contrast, the 35th order is 100 times weaker and increases linearly with pressure, indicating incoherent build-up. As discussed below, phase matching is possible for harmonic orders less than order 33rd, for which ionization in the medium balances the neutral atom dispersion. Therefore, these observations support our claim of fully phase matched high harmonic conversion at photon energies below 50 ev. The lineouts (c) of Fig. 3 also show that the HHG flux at the 27th, 29th and 35th orders saturate at pressures above 190, 190, and 150 torr respectively, due to a combination of defocusing of the laser by the generated plasma and reabsorption of the harmonics by the gas. Comparison of the observed saturation density-length product (200(torr) 0.04(cm) = 8 (torr cm) at 45 ev) with the theoretical absorption-limited harmonic generation (~5 P L = ~5/σ 45eV = ~130 (torr cm)) [16 18] reveal that the saturation pressure here (200 torr) is ~15 times smaller than the theoretical absorption-limited pressure. We estimate that at a pressure of 250 torr, the plasma-induced defocusing length may be as short as 550 µm, or ~1.2 the Rayleigh range [19,20]. In a tight focusing geometry, the harmonic yield at 45 ev is likely limited by the ionization-induced defocusing of the fundamental laser beam, which reduces the peak laser intensity, restricting the maximum achievable HHG signal. The other possible limitation is that if the beam diverges from the focus while still in the high-pressure region, the HHG light generated only near the focus will be partially reabsorbed before reaching the vacuum. Fig. 4. HHG spectrum using an Ar backing pressure of 50 torr (thick black line), and 250 torr (thin blue line). Green dashed line shows the theoretical phase matching cutoff, where the pressure-tuned phase matching cannot achieved above 50 ev. The selective enhancement of harmonic order at 27th and 29th with increasing pressure indicates good phase matching. By tightly focusing the laser beam ~10 µm after the center of the gas cell, at an optimum pressure of 200 torr, the maximum HHG conversion efficiency of ~2 10 9 photons per second at 45 ev is achieved. This conversion efficiency is at the theoretical limit of efficiency for this case of a short confocal parameter (several hundred microns) and density-length product (see Table 1.). For comparison, using driving laser energies of 0.5 mj in a few cm long waveguide (150 µm diameter), photon flux of ~10 11 photons per second at 45 ev is achieved at repetition rates of 1 khz. In this case, the increased density-length product possible for higher pulse energies compensates for the lower repetition rate. Similarly high density-length products will be possible at 50 khz repetition rates in the future by using slightly higher laser pulse energies around 250 µj [21]. Figure 4 shows the full HHG spectrum at argon pressures of 50 and 250 torr. The selective enhancement (quadratic growth) of the 27th and 29th orders with increasing gas pressure indicates phase matching. The dashed green line of Fig. 4 shows the phase-matching cutoff, (C) 2009 OSA 28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 17381

which is the highest HHG photon energy that can be generated before the ionization of medium exceeds the critical ionization level: η cr = (1 + N atm λ L2 r e / (2 π δ)) 1 (3.8% in Ar) [5,13,22]. For ionization levels above critical, the free-electron dispersion is too high to be compensated by the neutral atom dispersion. Since the geometric phase (Gouy phase shift) has the same sign as the free-electron contribution to the dispersion, phase matching of the HHG process is thus not possible above photon energies of 50 ev. Fig. 5. The total flux of HHG signal as a function of the repetition rate. Figure 5 shows the total HHG flux as a function of laser repetition rate, demonstrating that it scales approximately linearly from 10 to 50 khz. The slight deviation from a linear dependence is the result of a slightly changing thermal lensing in the Ti:sapphire regenerative amplifier, that shifts the focal position [15,23]. This is supported by a slight change in the divergence angle of the HHG beam, from 6.5 to 5 mrad, as the repetition rate is increased. Geometry /Ref. Gas jet / [11] Gas cell / This work Waveguid e / [5,13] 4. Future Table 1. Flux comparison for high rep. rate, high harmonic generation at 45 ev. The photon flux obtained is between 10 2 and 10 4 greater than previous work at very high repetition rate although the rep. rate is 2 lower and the laser energy only 3 higher. The increased HHG flux is possible due to the fully phase matched geometry in a higher density length product medium. Pulse Energ y Rep. Rate Pressure Length (torr) (cm) Conv. Efficienc y Beam Divergen ce HHG Photons/ s 7 µj 100 ~7.5 0.07 <<10 9 10 5-10 7 khz 0.01 5 (~10 pw) 25 µj 50 200 8 10 8 5 mrad 10 9 khz 0.04 (~10 nw) 500 µj 1 khz 30 5 150 10 5 < 1 mrad 10 11 (~1 µw) Improvements in the overall HHG flux at very high repetition rates can be obtained by a moderate increase in pulse energy, combined with further increases in the repetition rate of the driving laser. In the current geometry, although the HHG process is fully phase matched at the theoretical limit for the density-length product used, the very short confocal parameter that must be used in order to obtain the required laser intensity at low pulse energy, combined with (C) 2009 OSA 28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 17382

pressure limitations due to ionization-induced defocusing in a tight focusing geometry, limit the obtainable efficiency. Based on the very significant (100-10,000 ) increase in flux that we obtained by using 25 µj energy compared with previous work using 7µJ pulse energies (see Table 1.), as well as our previous work generating harmonics in waveguides using laser pulse energies in the 100-500 µj regime [21], we expect that a phase-matched waveguide geometry should be able to reach optimum, absorption-limited, efficiency using ~100 300 µj driving laser pulse energies. Up to 10 13 (~100 µw) photons per second at 45 ev can be expected by focusing ~250 µj laser pulses at repetition rates of 100 khz into hollow waveguides with ~100 µm inner diameter. Such a system is a feasible next step for demonstrating a compact, single-box, ultrafast Ti:sapphire driven, fully coherent HHG source. Other laser technologies such as fiber lasers can also obtain very high average power at high repetition rate, and may be suitable for high-flux HHG sources in the future. However, the available pulse durations and pulse temporal contrast are issues that have a major impact on the efficiency of HHG, and phase matching of harmonics generated by fiber lasers has not been demonstrated to date [24]. The range of photon energies generated under full phase-matching conditions can also be extended further by using mid-infrared driving lasers, to well beyond the experimental phase matching cutoffs of 50 ev in Ar, 100 ev in Ne, and 130 ev in He using 800 nm driving pulses [22,25,26]. Recent experimental work using long wavelength driving lasers has demonstrated full phase matching in the water window around 330 ev, while in theory, full phase matching of the HHG process in the hard x-ray region of the spectrum around 1 kev will be possible in the future. 5. Conclusion We have demonstrated fully phase matched, fully spatially coherent, high harmonic beams at very high repetition rates (50 khz) for the first time. The generated harmonic photon flux is at the theoretical limit for the density-length products used. We have also demonstrated that the phase matched HHG flux scales linearly with the laser repetition rate from 10 to 50 khz. By focusing modest laser pulse energies (25 µj) in to a short gas cell at high pressures, full phase matching of the HHG process is possible, limited only by ionization-induced defocusing of the laser beam in a tight focusing geometry. This compact, single-box, laser system delivers HHG flux at 45 ev with 2 10 9 photons per second, which will be useful for applications in photoelectron spectroscopy, coincidence molecular imaging, metrology, imaging, and microscopy. Acknowledgments The authors gratefully acknowledge funding from the NSF Engineering Research Center for EUV Science and Technology, the Department of Energy, and Kapteyn-Murnane Laboratories Inc. for contributing use of the laser for this experiment. (C) 2009 OSA 28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 17383