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Intracavity high harmonic generation was utilized to generate high average-power coherent radiation at vacuum ultraviolet (vuv) wavelengths. A ytterbium-doped fiber-laser based master-oscillator power-amplifier (MOPA) system with a 10 MHz repetition frequency was developed and used as a driving laser for an external cavity. A series of odd-order harmonic radiations was generated extending down to ∼ 30 nm (41 eV in photon energy). The 7th harmonic radiation generated was centered at 149 nm and had an average output power of up to 0.5 mW. In this way, we developed a sub-mW coherent vuv-laser with a 10 MHz repetition frequency, which, if used as an excitation laser source for photo-electron spectroscopy, could improve the signal count-rate without deterioration of the spectral-resolution caused by space-charge effects.
© 2015 Optical Society of America
1. Introduction
Cavity enhanced high harmonic generation (HHG) has been one of the most promising methods to generate coherent pulse-trains at vacuum ultraviolet (vuv) wavelengths with high repetition frequencies exceeding the MHz range [1–8]. In the cavity enhanced HHG experiment, a high repetition rate pulse train from a seed-laser system is sent to an external cavity. The intracavity pulse energy and the average power are passively enhanced until they become sufficiently high for HHG to take place inside the external cavity. Since the intracavity pulse energy can be enhanced by several orders of magnitude within the external resonator, the conversion efficiency of HHG can be significantly improved, similarly to the more conventional case of cavity enhanced second harmonic generation with continuous-wave lasers. Optical frequency combs are coherent pulse-trains generated from mode-locked oscillators that show a comb-like spectrum in the frequency domain and can be precisely phase-stabilized with active feed-back control. When a near-infrared frequency comb is used as a driving laser for HHG with an external cavity, the resulting vuv pulse-train retains the phase properties of the seed and displays a high degree of pulse-to-pulse phase coherence. Intracavity HHG can therefore be applied to generate frequency combs in the vuv wavelength region [9]. Thus-generated vuv frequency combs have been applied to high precision spectroscopy of atomic targets [10,11] and are expected to play a major role in vuv frequency-metrology.
In this work, we propose the frequency up-conversion of the emission from a high power ytterbium (Yb)-doped fiber-laser system by intracavity high-order harmonic generation which results in a vuv radiation suitable for multiple applications carried out in this wavelength region. The system was designed to have a 10 MHz repetition frequency. Typically, intracavity HHG has been performed with pulse repetition rates ranging from 50 MHz to 200 MHz, in order to realize sufficiently large longitudinal mode separation for comb metrology applications. On the other hand, for a given average power and pulse duration, the peak intensity inside the external cavity can be increased by an order of magnitude by reducing the repetition frequency from 100 MHz down to 10 MHz. For cavity-enhanced HHG experiments carried-out at approximately 100 MHz of repetition rate, it has been reported that the non-linear phase response of the plasma at the cavity focus limits the available output power of the vuv comb [6, 12, 13]. We consider this plasma effect is mitigated for a 10 MHz repetition rate due to the several reasons. Firstly, by reducing the repetition rate down to 10 MHz, significant amounts of plasma can be removed from the interaction site before the next pulse arrives, since the pulse-to-pulse temporal separation is larger. Consequently, the steady state plasma density can be decreased and smaller disturbances on the cavity resonance can be expected. Secondly, since the available pulse energy of the fundamental beam is an order of magnitude larger than that of a 100 MHz system for given average power, weaker focusing can be employed to achieve a non-linearity limited peak intensity at the focus. Accordingly, the interaction volume is increased and the phase matching of HHG is improved, thus enabling increased conversion efficiency.
In addition to frequency-comb related applications, the output of such a system can be used as a source for photoelectron spectroscopy (PES). Synchrotron radiation obtained from accelerated electrons in a storage-ring, has been one of the standard and widely used excitation sources for PES, since the emission ranges from the infrared to the hard x-ray regions, is strongly directional, and provides high brilliance and photon density over a tight focusing. High-repetition rate high harmonic radiation from an external cavity is able to offer similar advantages to synchrotron radiation for PES, with the additional benefit of being a table-top system. In addition to being diffraction-limited, the HHG beam from an external-cavity can be short-pulsed and possesses pure linear polarization. The radiation of the higher order harmonics generated can be extended down to the soft-X-ray region. Furthermore, the ability to provide repetition rates higher than the MHz-level would eliminate the deformation of the electron energy spectrum caused by electron-electron interactions (space charge effect) [14,15]. For these reasons, high-repetition rate high harmonic radiation can be considered as an ideal for PES. The table-top vuv-laser based on intracavity HHG developed in this work can be a convenient alternative light source for applications that have so far been restricted to the use of synchrotron radiation and have necessitated to be in close proximity to electron accelerator facilities.
2. Experimental setup
2.1. Seed laser system
The schematic diagram of the experimental setup that comprises of a Yb-fiber laser system seeding an external cavity is shown in Fig. 1.
Fig. 1 Schematics of the experimental setup for intracavity high harmonic generation at 10 MHz repetition frequency. The seed laser for the external cavity was a Yb-doped fiber-laser based master-oscillator power-amplifier (MOPA) system. The external cavity consisted of four mirrors and had an optical path length of 30 m in vacuum to achieve 10 MHz of repetition frequency. See text for details.
The seed laser system is described in detail in [16]. To achieve mode-locking at a repetition frequency of 10 MHz, the cavity of the Yb-fiber oscillator was extended by introducing a ∼20-m long single-mode fiber. Stable mode-locking was initiated and sustained by the nonlinear polarization-rotation effect generated by the intracavity fiber. The typical average output power from the oscillator was 2 mW. The intracavity group delay dispersion was compensated for by a grating pair and optimized for the broadest spectral bandwidth of the output pulses, which was approximately 10 nm centered at a wavelength of 1040 nm. The transform-limited pulse duration of the oscillator output was 160 fs. Before pulse amplification by the Yb-doped fiber based pre-amplifiers and power amplifier, a pulse stretcher consisting of a transmission grating, prolonged the pulse duration to approximately 500 ps, so as to avoid nonlinear effects that may arise at the amplification stages. The stretcher had a throughput of 65 %. Two fiber-based pre-amplifiers were used to obtain 600 mW of average power without introducing amplified spontaneous emission (ASE). The pre-amplifiers consisted of single-mode Yb-doped gain fibers with core diameters of 6 μm (single cladding) and 10 μm (double cladding). The power amplifier, was an Yb-doped polarization-maintaining photonic-crystal fiber with a mode diameter of 30 μm (NKT photonics). The gain fiber was pumped by a high power laser diode at 976 nm. A maximum output power of 27 W was measured after the optical isolator for a pump power of 54 W. The amplified pulses were compressed down to ∼200 fs by a pair of transmission gratings and the average power was 23 W. The temporal pulse shape measurement by FROG is shown in Fig. 2. For both the pulse stretcher and compressor, highly efficient large-scale transmission gratings (180× 40 mm) manufactured by Canon Inc. were used.
2.2. 10-MHz Enhancement cavity
To passively enhance the pulse train from the amplifier system at a 10 MHz repetition frequency, the external cavity needed to have a 30 m optical path length in vacuum. The four-mirror ring cavity consisted of a high reflective plane mirror, two focusing mirrors (ROC 400 mm) and an input coupler. All four mirrors were mounted on PZT-controlled mounts and fixed onto the optical table. These mounts offered the freedom to rotate, tip and precisely translate the mirrors to control the length of the cavity. The distance between the plane high-reflective mirror and the input coupler (long arm of the bow-tie cavity) was ∼15 m. In order to hold such a large scale arrangement, the optical table, that was 15.4 m across and 1.5 m in length, was specially designed and supported by air-damped legs (Nippon Boushin Industry). The optical table had a thickness of 640 mm to achieve high stiffness. Both top and bottom surfaces of the table were fitted with ∼ 2 cm thick layers filled with temperature controlled water to actively stabilize the temperature of the optical table. The top surface of the table was temperature stabilized to within 0.02 K, while the room temperature could only be controlled to within 0.1 K. The ability to finely regulate the temperature of the optical table led to reproducible day-to-day operation of the large scale external cavity. The chamber that kept the external cavity under vacuum consisted of three compartments connected to one another via vacuum-sealed tubes, as shown in Fig. 1. Urethane foam was placed between the vacuum chamber and the optical table to keep possible vibrations of the cavity to a minimum from the turbo-molecular pumps attached to the vacuum chamber. Air evacuation by the two turbo-pumps at a pumping speed of 450 l/s, 10−4 Pa of pressure was obtained inside the chamber. The large-scale bow-tie configuration led to a significantly large beam cross-sectional area incident on all of the cavity mirrors and reduced possible damages incurred by high intracavity field intensities [17, 18]. The beam radius at the focus of the resonator was estimated to be ∼21 μm. In our previous report, the 10 MHz enhancement cavity was realized with a smaller arrangement of 3.5 m in length by utilizing a folded configuration and ten mirrors in total [3]. However, in the present configuration, with four mirrors, the alignment of the external cavity was carried out more reliably and reproducibly, which also significantly improved the long-term stability of the setup. One of the intracavity mirrors in the fiber oscillator was actuated by a piezo-electric (PZT) actuator, so as to lock the laser cavity to the external cavity by utilizing the Pound-Drever-Hall technique [19]. In order to perform HHG, a capillary nozzle with an opening diameter of ∼150 μm was installed to supply gas at the focus of the resonator. The capillary nozzle was made of fused silica that has a melting point of > 1600 °C, which is sufficient to avoid damaging the capillary tip due to the high temperatures generated at the focus. In order to efficiently remove the gas from the vacuum chamber after HHG, a gas-catch, which consisted of a 6-mm diameter cylinder, was placed vertically in front of the capillary gas nozzle. One end of the cylinder was sealed and the other was connected to a roughing pump (scroll pump) that had a pumping speed of 1000 l/min. The gas-catch had a small opening (3.5 mm in diameter) on the side of the cylinder to absorb gas from the capillary nozzle. The distance between the gas catch and the tip of capillary nozzle was set to less than 0.5 mm. The vacuum chamber pressure was reduced by an order of magnitude when the gas-catch was in operation during HHG. In this way, the turbo-pumps operated under less stringent conditions, which reduced the amplitude of the vibrations introduced to the system. Typical backing pressures of 1.5 atm, 2 atm and 3 atm were employed when using xenon, krypton and argon gases, respectively, in order to maximize the average power of the generated harmonic radiation. To couple the high harmonic radiation out of the external cavity, various methods have been investigated so far [3, 6, 20–26]. In this work, we employed an outcoupling plate placed after the resonator focus at Brewster’s angle for infrared wavelengths [1]. The profile of the outcoupled beam was free of spatial chromatic dispersion. The incident surface of the Brewster plate was dielectric coated and functioned as a dichroic mirror with a high reflectivity for the 7th order harmonic radiation, and low loss for the cavity beam. After trying various materials, a single magnesium oxide crystal was found to withstand the highest intracavity power under the experimental conditions. The outcoupling efficiency (reflectance) was 50 to 75 % at 149nm, depending on the design of the Brewster plate.
3. Results
3.1. Cavity performance in the presence of gas for HHG
When the 1 % transmission input coupler was employed and the enhancement cavity was driven with 10 W of seed power, a typical intracavity power of 1.0 kW was obtained in the absence of gas for HHG. The peak intensity at the focus was estimated to >3.6×1013 W/cm2. An intracavity pulse energy as high as 100 μJ was achieved. The gas was introduced into the cavity via the capillary nozzle described in the previous section. The photographs of Fig. 3 show the external cavity at the focus when HHG took place with xenon, krypton and argon gases.
Fig. 3 Photographs of the external cavity at the focus when HHG was performed with xenon (a), krypton (b) and argon (c) gases. The bright spots and flashes correspond to plasma fluorescence.
When the krypton (xenon) gas was introduced inside the cavity, the intracavity power decreased to ∼70 % (∼50 %) of the power level in the absence of gas. This was due to strong plasma generation at the focus, which introduced nonlinear phase shifting and spatial deformation of the intracavity beam. The latter was confirmed via the astigmatic profile of the beam leakage from the external cavity through one of the high reflective mirrors. In order to reduce the deterioration of the intracavity power due to plasma nonlinearity, the input coupler of 1 % transmission was changed to one of 3 % transmission. With such a change in the transmission, the enhancement factor was expected to decrease to ∼50 % of the original value under the cavity configuration. On the other hand, since the linewidth of the cavity longitudinal-mode was larger, in accordance with the lowered cavity finesse, the cavity was less sensitive to the disturbances due to ionization at the focus. As expected, with 14 W of seed power an intracavity power of 940 W was obtained in the absence of gas. The intracavity power decreased to 780 W when xenon gas was introduced for HHG. It should be noted that the power decrease due to plasma-nonlinearlity was less significant in the case of the 3 % transmission input coupler, than in the case of the 1 % transmission one. In addition, the stability of the cavity-lock was significantly improved with the 3 % transmission input coupler. Hereinafter, all the results reported were obtained with 3 % transmission input coupler, unless stated otherwise.
3.2. Spectral measurement of the output coupled vuv beam
The outcoupled high harmonic radiation was spectrally dispersed by a vuv troidal-grating (HORIBA Jobin Yvon) and re-focused onto a fluorescent plate. Gold mirrors were used to direct the beam from the external cavity and to precisely adjust the angle of incidence onto the troidal grating in order to achieve astigmatism-free focusing beam spots at the fluorescent plate. The visible spots that appear on the fluorescent plate were recorded with a digital-camera. Figure 4 shows the typical recorded image of the fluorescent plate when HHG was performed with xenon, krypton and argon gases. The discrete harmonic orders produced individual spots on the fluorescent screen. No clear harmonic signal was observed when neon gas was utilized for HHG due to the lack of sufficiently high peak intensities to drive the neon gas that has a larger ionization potential and smaller dipole moment. The obtained two-dimensional images were integrated over the vertical direction to yield one-dimensional spectra of the outcoupled harmonic radiation, as shown in Fig. 5.
Fig. 4 The image recorded by digital camera of the fluorescent plate when HHG was performed with xenon (a), krypton (b) and argon(c) gases.
Fig. 5 The spectra of outcoupled high harmonic radiation obtained from the images of the fluorescent plate shown in Fig. 4. Xenon (a), krypton (b) and argon (c) were the gases used for HHG. It can be seen that the cutoff energy grows with decreasing gas atomic weight. The 7th harmonic radiation appears particularly strong due to enhanced outcoupling efficiency for that wavelength. Harmonic orders as high as the 35th were obtained, which corresponds to ∼30 nm in wavelength. It should be noted that the spectra, as well as the fluorescent plate images, feature higher order diffraction contributions. The first row (from the top) of black dots and their drop-down lines indicate the expected wavelengths of the first order diffraction. The second and third rows correspond to the wavelength positions of the second and third orders, respectively.
From the spatial geometry of the fluorescent plate and the grating, the corresponding harmonic orders of each spot were identified and used to calibrate the wavelength axis of the obtained spectra. The spatial positions of the spots on the fluorescent plate were in close agreement with the predicted ones, and each of the harmonic order was identified without ambiguity. To fully explain the spectra, the diffraction grating orders, up to the third one, had to be taken into account.
The 7th harmonic radiation was comparatively stronger than that of the other harmonic orders, due to an enhanced outcoupling efficiency at 149 nm from the dielectric coating of the Brewster plate. For the spectrum acquired with xenon gas, the absence of the 9th harmonic radiation was due to strong reabsorption by xenon atoms. When comparing the spectra obtained with the three different gases of xenon, krypton and argon, the cutoff energy for HHG increased with decreasing atomic weight. This was partly due to the high ionization potential of lighter gases that have a larger ponderomotive energy which enables to generate high-energy harmonic radiation. The larger ionization potentials of lighter gases also prevents the deterioration of the intracavity power due to plasma formation at the focus, which contributes to the extension of the cutoff-wavelength. In the case of HHG with argon gas, harmonic orders as high as the 35th which reached a wavelength below 30 nm, are clearly resolved on the spectrum. The photon energy of the 35th harmonic corresponds to ∼42 eV. The highest observable order of the harmonics was also limited by the low efficiency of the diffraction grating for wavelengths below ∼30 nm. It should be noted that the wavelength dependence of the diffraction grating efficiency, the luminescent yield of the fluorescent material, the reflectance of the gold mirror and reabsorptions by the gases were not taken into account in the plotting of the spectra of Fig. 5.
3.3. Absolute power measurement of the output coupled vuv beam
In order to estimate the average output power of our vuv laser system, the outcoupled high harmonic radiation was guided to a phototube (Hamamatsu R1187). The phototube operated with a bias voltage of 45 V. The outcoupled radiation was attenuated with a partially reflective mirror to avoid the saturation of the detector. The attenuation by the partially reflective mirror was calibrated separately. To investigate the amount of background signal, a mechanical shutter was placed in front of the detector to block the vuv beam. Ionized atoms around the lead of the detector contributed to the detected signal. Such a background ion-current signal was suppressed to a negligible amount after careful isolation of the lead wires. The phototube showed little sensitivity to the radiations of the 3rd and the 5th harmonic orders. Harmonics higher than the 11th order were blocked from the detection, since the window of the detector was made out of MgF2 which does not transmit those wavelengths. The contribution from the 9th harmonic radiation was expected to be an order of magnitude smaller than that of the 7th because of the enhanced outcoupling efficiency of the Brewster plate for the 7th order. Therefore the photocurrent, as measured by the phototube, was considered to be exclusively contributed by the 7th harmonic radiation. The outcoupled power of the 7th harmonic radiation was measured to be 0.5 mW using the 3 % transmission input coupler and xenon gas for HHG. It should be noted that the outcoupled power decreased to 120 μW and 210 μW for krypton and xenon gases respectively, when the 1 %transmission input coupler was employed. This measurement also confirmed that a cavity with a lower finesse was advantageous for mitigating disturbances caused by plasma nonlinearities. From the measured absolute outcoupled power of the 7th harmonic radiation, the approximate average power of each of the other higher order radiations was determined from the spectra of Fig. 5. When using xenon gas for HHG, we consider 10 μW is a safe estimate of the average power of the single harmonic order that lies in the wavelength region between 55 and 70 nm (19th to 15th orders).
3.4. Long-term stability
Figure 6 shows the outcoupled power of the 7th harmonic radiation generated with xenon gas. Approximately 200 μW of harmonic radiation was continuously coupled out over a period of up to 20 min. The continuous operation was cut short by an accidental disturbance of the cavity-locking caused by electronic noise and/or acoustic vibration. Once lost, the cavity-locking could be immediately recovered. The value of the outcoupled power shown in Fig. 6 was smaller than those mentioned in the previous section, as a result of the Brewster plate coating, which was designed to have a rather low reflectance of 48 %, and the lower intracavity power employed in this measurement. The fast fluctuation in the output power was mainly due to the drift of the carrier-envelope-offset frequency of the laser system. The dielectric-coated outcoupler was showing signs of degradation, which reduced the intracavity power and the outcoupled vuv power accordingly. The time-scale of the degradation varied between several minutes to a few days, depending on the type of the coating on the Brewster plate, the coating material and the generated (v)uv power. The degradation of the output coupler seemed to proceed faster when attempting to generate higher harmonics of stronger power. Therefore we considered the wearing problem was due to strong (v)uv beams incident on the dielectric-coated outcoupling plate. No degradation was observed when non-coated MgO was used as the outcoupling medium. We currently do not have an explanation for the degradation process and need to regularly replace the outcoupling plates after several hours of operation. An electrical actuator that would slowly displace the outcoupling plate with respect to the cavity beam may practically solve this degradation issue.
Fig. 6 Time-trace of the average output power of the 7th harmonic radiation. ∼0.2 mW of output power was sustained for over >15 min. The fast fluctuation of the output power was mainly due to the drift of carrier-envelope-offset frequency of the seed laser system.
4. Discussion
In this work, a pulsed vuv-laser with an emission at 149 nm and with 0.5 mW of average output power was developed. The system was operated at 10 MHz repetition frequency to simultaneously achieve a quasi-continuous vuv-radiation together with a high pulse energy for efficient intracavity HHG. Such a high power source with a high photon energy can be used as a driving laser for angle resolved photoelectron spectroscopy (ARPES), as it covers the entire first Brillouin zone of typical iron-based superconductors. Since the pulse duration of the vuv radiation was expected to be shorter than that of the fundamental pulse duration of ∼200 fs, the developed vuv-laser could also be applied to time-resolved photoelectron spectroscopy with femtosecond temporal resolution.
Originally, intracavity high harmonic generation was mainly developed to generate vuv frequency combs that required repetition frequencies in excess of 10 MHz. It is believed that conventional low-repetition rate amplifier systems that run at several kHz repetition frequency are more appropriate and advantageous for the realization of higher average power high harmonic radiation, owed to the improvement of the conversion efficiency of the HHG process caused by increased driving pulse energy. In the experimental setup for intracavity HHG at 10 MHz repetition frequency, the ratio between the average outcoupled power of the harmonic radiation (0.5 mW) and the infrared seeding power (14 W) is 3.5×10−5. This conversion efficiency is comparable to, or even higher still, than that of the typical single pass HHG experiment with a low-repetition rate amplifier system. This work demonstrates that the intracavity HHG could be utilized not only to realize high repetition rate vuv radiation, but could also be used to improve the frequency conversion efficiency and the average output power correspondingly. However, such high repetition rate vuv pulses have lower pulse energies. Here, we would like to point out that average power, rather than pulse energy affects more significantly the count-rate or S/N in many of applications, such as photoelectron spectroscopy, linear absorption spectroscopy and x-ray diffraction.
In comparison to synchrotron radiation at 149 nm, the developed vuv laser system produces comparable performance characteristics. The 7th harmonic radiation generated with our 10 MHz enhancement cavity has a brilliance of 1.9×1016(photons/s/mm2/mrad2/0.1%BW), which is similar to that of the synchrotron radiation at the same wavelength generated by a wiggler at SPring-8 in Harima, one of the state-of-the-art synchrotron radiation facilities in Japan. Assuming a pulse duration of ∼ 100 fs, a peak brilliance of ∼1022(photons/s/mm2/mrad2/0.1%BW) is expected.
When considering the application of photoelectron spectroscopy, the photon-flux at the sample is a more appropriate quantity to compare rather than the brilliance, since the signal count-rate is proportional to the average power and photon-flux. In order to estimate the on-sample photon-flux in a photoelectron spectroscopy setup driven by synchrotron radiation, specific experimental parameters of spectral filtering with gratings have to be taken into account. As a typical example, we compare our system to that used for high resolution photoemission spectroscopy at the Hiroshima Synchrotron Radiation Center (HiSOR) at Hiroshima University. High-energy-resolution ARPES measurements can be performed with a photon-energy tunable system in the beamline BL-9A of HiSOR [27]. In this facility, the reported photon flux of 2.3×1012 (photons/s) at 9 eV was obtained after spectral filtering with a bandwidth of 1 meV. This can be compared to the photon flux of 3.8×1014 (photons/s) of the 7th harmonic radiation generated with our system. The bandwidth of the 7th harmonic was estimated to ∼30 meV, which if applied to photoelectron spectroscopy, would limit the energy resolution. For an improved energy resolution, additional spectral filtering or operation of the enhancement cavity with a longer pulse duration could be employed. In the case of the vuv-radiation obtained from HHG, a tightly focused diffraction-limited beam spot can be used on the sample, while a 0.1 mm × 2 mm beam cross-section is specified for the HiSOR beamline [27]. For a typical photoemission spectroscopy experiment carried out with synchrotron radiation at the wavelength of discussion, the developed vuv laser system shows similar performance abilities. We believe a vuv-laser system based on intracavity HHG is a reliable light source for the next generation of ultraviolet photoelectron spectroscopy methods that combine high photon-flux with table-top systems.
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