High Harmonic Generation in ZnO with a High-Power Mid-IR OPA

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1 High Harmonic Generation in ZnO with a High-Power Mid-IR OPA Shima Gholam-Mirzaei 1, John Beetar 1 1, a), and Michael Chini 1 Department of Physics, University of Central Florida, Orlando FL 32816, USA We generate high-order harmonics in a-cut (11-20) ZnO at a high repetition rate of 50 khz, using tunable mid-ir pulses (3-4 µm wavelength) from a high-power optical parametric amplifier (OPA). For driving laser pulses with 3.8 µm central wavelength, we observe nonperturbative harmonic spectra that well exceed the material band gap. The harmonic spectra depend strongly on the orientation of the crystal with respect to the laser polarization, with odd harmonics exhibiting periodicities of π/2 for a polarization within the (11-20) crystal plane. Energy conversion efficiencies of ~10-6 per harmonic are measured for the 9 th -13 th harmonics, yielding an average power of more than 0.2 µw for the 13 th harmonic. a) Author to whom correspondence should be addressed. Electronic mail: Michael.Chini@ucf.edu 1

2 High-order harmonic generation (HHG), resulting from the interaction of an intense laser field with an atomic or molecular gas, has led to significant advances in ultrafast science. In combination with the development of few-cycle laser technology, HHG has enabled the generation of isolated attosecond pulses 1-4 and their application to pump-probe measurements of electron dynamics 5-9. Furthermore, the recollision process 10 central to HHG encodes information about the target, allowing tomographic imaging of a molecular orbital 11 as well as measurements of rotational 12 and vibrational dynamics 13. Scaling gasphase HHG to high average powers has proven challenging, however, and has required the use of state-ofthe-art laser sources or intercavity geometry. Both of these methods have yielded harmonics of nearinfrared lasers with microwatt average power 14-17, and recently power approaching the milliwatt level has been obtained using visible driving lasers 18, 19. HHG from a crystalline solid was observed for the first time in 2011, by focusing intense mid-ir (λ = 3.25 µm) laser pulses into a bulk ZnO crystal 20. This new development, like gas-phase HHG before it, promises to open new frontiers in ultrafast science. In solids, nonperturbative harmonics and isolated attosecond pulses 21 can be generated with driving laser intensities as low as a few TW/cm 2, suggesting the potential for compact attosecond sources with high average power. Furthermore, high harmonic spectroscopy techniques have recently been applied to HHG in crystalline solids 22, demonstrating the capability to reconstruct the electronic band structure 23 and valence electron orbitals 24 using all-optical measurements. However, much is still unknown about HHG in solids. Measurements using visible 25, mid- IR 20,22, and multi-thz 26,27 driving pulses each point to different generation mechanisms, and the tools for high harmonic spectroscopy are still at the early stages of development. Moreover, application of solidstate HHG as a high-power attosecond source will require reasonable conversion efficiency without damaging the crystal 28, neither of which have been demonstrated at high average power. In this Letter, we produce HHG from single-crystal ZnO (11-20) using a tunable mid-ir optical parametric amplifier (OPA) operating at a repetition rate of 50 khz. We observe the generation of ultraviolet harmonics extending well above the band gap (Δ gap = 3.3 ev), and confirm the nonperturbative scaling for near- and above-threshold harmonics. In agreement with past measurements 20, we find that the harmonic spectrum depends strongly on the orientation of the crystal with respect to the laser polarization, as the yield of odd harmonics is modulated with a π/2 periodicity when the crystal orientation is rotated with respect to the laser polarization within the (11-20) crystal plane, with even harmonics appearing for laser polarization along the c-axis. With a measured conversion efficiency of ~10-6 per harmonic above the band gap, the high repetition rate of the OPA allows scaling of the average flux of nonperturbative harmonics directly to the microwatt level. In the experiments, mid-ir pulses are produced by a commercial OPA (Light Conversion ORPHEUS-ONE) pumped by a 20 W Yb:KGW (λ pump = 1.03 µm) regenerative amplifier (Light Conversion PHAROS) at a repetition rate of 50 khz. The OPA idler is tunable in wavelength between µm, with representative spectra shown in Fig. 1(a). We optimized the HHG signal for an idler wavelength of λ idler = 3.8 µm, for which a maximum pulse energy of 14 µj was obtained with a FWHM spectral bandwidth exceeding 0.5 µm. The pulses from the OPA were compressed using anti-reflection (AR) coated (3-5 µm) Si plates, and the pulse duration was measured to be below 100 fs at the target location for a central wavelength of 3.8 µm. The mid-ir pulses were focused using an AR-coated Si lens (f = 100 mm) to a FWHM spot size of 60 µm, measured using a knife-edge scan, and the generated harmonics were imaged by an aluminumcoated mirror onto the entrance slit of a UV-enhanced high-resolution spectrometer (Ocean Optics HR2000+ES). The position of the ZnO crystal (300 µm thickness) was varied through the focal spot in order to optimize the yield of the above-gap harmonics. The highest harmonic signal was obtained for a 2

3 laser pulse energy of ~5.5 µj, corresponding to an intensity of ~1.4 TW/cm 2 ; above this energy we observed an increase in the band fluorescence yield and eventually crystal damage. The harmonic spectra were measured using two different integration times as indicated in the figure captions, in order to capture the full dynamic range of both below- and above-gap harmonics. The HHG spectrum was measured for different idler wavelengths, as shown in Fig. 1(b). Odd harmonics up to the 17 th order were obtained for driving laser central wavelengths ranging from µm. Here, the silicon thickness and laser pulse energy were optimized for HHG from the 3.8 µm driving laser and were not varied when the OPA wavelength was tuned. As expected, we observe the harmonic peaks to shift with the driving laser wavelength, while the band fluorescence peak (3.3 ev; 800 THz) remains at a constant frequency 26. The non-perturbative scaling 29 of solid-state HHG was verified by measurements of the relative harmonic yields as a function of the driving laser intensity, as shown in Fig. 2. The intensity was varied using an iris, and the intensity was estimated from the measured pulse energy and focal spot size, assuming a pulse duration of 100 fs. As the focal spot size increases by <15%, while the intensity drops by a factor of more than 2, we neglect the increase in the number of emitters due to the larger spot size at low intensities. We find that while the below-gap 5 th and 7 th harmonics follow perturbative scaling laws, as shown in Fig. 2 (a), both the near-gap (Fig. 2(b)) and above gap (Fig. 2(c)) harmonics deviate from this behavior at high intensities as observed in previous measurements under similar irradiation intensities 20. We additionally measured the dependence of the high-order harmonic spectrum on the crystal orientation, by rotating the crystal about the laser propagation axis. For polarization lying in the (11-20) plane, the angle-dependent harmonic spectrum is shown in Fig. 3(a). As the crystal s optic axis lies in the surface plane, inversion symmetry is broken and the even harmonics are generated when the laser polarization is along the optic axis (rotation angle θ = 0 or π). The yield of odd harmonics, on the other hand, exhibits a periodicity of π/2 as the crystal is rotated, maximizing at rotation angles θ = 0, π/2, and π as observed in previous measurements 20. Finally, the high average power of the OPA source allowed direct measurement of the harmonic flux using a calibrated Si photodiode sensor (Newport 818UV/DB), which was found to be insensitive to the mid-ir driving laser. The integrated harmonic power was measured in tandem with the harmonic spectrum, allowing us to determine the average power and energy conversion efficiency for each harmonic. The results are shown in Fig. 4. We find that the conversion efficiency for the 11 th -17 th harmonics varies from 10-6 to 10-7, similar to HHG driven by 0.8 µm lasers in noble gases 30. Coupled with the large reduction in the required driving laser intensity, this result suggests solid-state HHG as a path towards harmonic sources with unprecedented average power, for example by using high-power Tm:fiber driving lasers 31, 32. In conclusion, we generate high-order harmonics in ZnO using tunable mid-infrared laser pulses from a high-power OPA operating at a repetition rate of 50 khz. At the intensity for which HHG is optimized, we observe no indication of damage, suggesting solid-state HHG to be a viable technique for developing attosecond sources with high average power. We find the conversion efficiency for above-gap harmonics in ZnO to be as high as 10-6 per harmonic, and obtain microwatt-level average power for the 13 th harmonic. Coupled with the relatively low driving laser intensities needed to generate HHG in crystal, this result suggests the potential of solid-state HHG for the development of attosecond sources with unprecedented average power. We would like to thank Dr. Shambhu Ghimire for helpful discussions, Dr. Yanchun Yin for assistance with the IR spectrometer, and Dr. Zenghu Chang for making specialized equipment available to 3

4 us. This material is based on work supported by the Air Force Office of Scientific Research under award number FA

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FIG. 1. (a) Representative normalized spectra of the OPA idler when tuned from 3.0 to 4.2 µm. At a wavelength of 3.8 µm, the spectral bandwidth exceeds 0.5 µm with a pulse energy of 14 µj.

8 FIG. 1. (a) Representative normalized spectra of the OPA idler when tuned from 3.0 to 4.2 µm. At a wavelength of 3.8 µm, the spectral bandwidth exceeds 0.5 µm with a pulse energy of 14 µj. The absorption feature near 4.25 µm is due to CO 2 in air. (b) HHG spectra measured for different driving laser wavelengths. The dispersion and intensity were optimized for HHG from the 3.8 µm output. Low ( THz) and high ( THz) frequency spectra were obtained with spectrometer integration times of 1 ms and 400 ms, respectively. The band fluorescence, which is present for all driving wavelengths at a frequency of ~800 THz, is indicated by the red dashed line. Harmonic orders of above-gap harmonics are indicated in the figure. 8

9 FIG. 2. Intensity-dependence of the high-order harmonic yield for (a) below-gap, (b) near-gap, and (c) above-gap harmonics. While below-gap harmonics scale perturbatively with I q (dashed lines), the nearand above-gap harmonics deviate from this behavior for intensities above ~1 TW/cm 2. 9

10 FIG 3. (a) Schematic of the wurtzite ZnO crystal, showing the relevant (11-20) plane (blue). The laser polarization is indicated by the black arrow, and the rotation angle θ is indicated. (b) Angle-dependent HHG spectrum for laser polarization in the (11-20) plane. Due to the inversion symmetry breaking for laser polarization along the crystal s optic axis, even harmonics are generated at rotation angles θ = 0 and π. The yield of odd harmonics is maximized with a periodicity of π/2, as indicated by the dashed lines. Low ( THz) and high ( THz) frequency spectra were obtained with spectrometer integration times of 1 ms and 500 ms, respectively. 10

11 FIG. 4. Conversion efficiency and average power of visible and UV harmonics, measured using a calibrated Si photodetector. Conversion efficiencies as high as 10-6 are obtained for near- and above-gap harmonics, yielding average powers approaching microwatt per harmonic. 11

Effects of driving laser jitter on the attosecond streaking measurement

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