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Abstract
We demonstrate a 10.7-eV (λ = 115.6 nm) laser with mW levels of average power and a 1-MHz repetition rate, which was driven by the third harmonic radiation (THG), at 347 nm, of an Yb:fiber chirped pulse amplifier (CPA) laser. The 347 nm ultraviolet radiation was obtained by frequency conversion of the high power output of a 1-MHz Yb:fiber CPA, using beta barium borate (BBO) nonlinear crystals. The frequency converted output was focused down into a gas cell filled with a mixture of Ar and Xe, and was subjected to a second THG frequency conversion. The generated 10.7-eV laser was separated from the fundamental beam using a LiF prism and no further separation from other harmonic waves was required. The highest measured output power was ~80 μW, which corresponded to an average power of ~1.25 mW inside the gas cell when the transmission coefficients of the LiF optics were taken into account. The corresponding conversion efficiency from 347 nm down to 115.6 nm was ~2.5 × 10−4.
© 2017 Optical Society of America
1. Introduction
Laser based angle-resolved photoemission spectroscopy (ARPES) is a powerful and unique technique in condensed matter research and is used to explore the electronic structures of solid state materials [1,2]. To effectively probe materials, vacuum ultraviolet (VUV) and extreme-ultraviolet (EUV) lasers, which have photon energies that are greater than the work functions, have to be employed. Usually, the well-established Ti:Sapphire laser is used as the fundamental driver and is combined with nonlinear crystals or rare gases for frequency conversion, to provide such kind of light sources [3–6]. Due to the severity of thermal effects of Ti:Sapphire crystals at high average powers (exceeding 10 W), the available repetition rate is limited to a few tens of kilohertz (~kHz), however, to efficiently carry out space-charge-free measurements by limiting the number of emitted electrons per pulse, the repetition rate needs to be higher [7,8].
In recent years, enabled by newly-developed ytterbium (Yb) doped high average power, high repetition rate ultrafast lasers, which typically can be operated with repetition rates ranging from 100 kHz to 100 MHz [9–11], high repetition rate single pass high harmonics generation (HHG) based VUV or EVU lasers have gained popularity. In this respect, remarkable progress has been made [12–17] as illustrated by the following examples: 0.5 mW of output power at 8.32 eV was achieved with a repetition rate of 10 MHz [12], 0.143 mW at 30 eV was obtained with a repetition rate of 0.6 MHz [13], 50 μW at 27.7 eV was realized with a repetition rate of 10 MHz [14], 0.8 mW at 21.6 eV was attained with a repetition rate of 0.12 MHz [15], 1 μW at 50 eV was reached with a repetition rate of 0.1 MHz [16], and 1 μW at 68.6 eV was secured with a repetition rate of 0.1 MHz [17]. However, the photon energy of 20~100 eV corresponds to the minimum inelastic mean free path of electrons (typically ~0.5-1 nm), which makes ARPES highly surface sensitive [1,18,19]. While this is beneficial in surface science, it presents itself as a drawback when studying bulk-physics properties of a medium, such as superconductivity. To improve the bulk sensitivity of ARPES the photon energy could be increased to a few thousands of eV, but this would come at the expense of high energy and momentum resolution. Another solution might be to use low photon energies to increase the bulk sensitivity. This would allow a mean free path of ~1-5 nm and has been demonstrated in [1,2] using a ~7 eV laser, however, this photon energy was only sufficient to access a limited momentum space. Therefore, in order to simultaneously both access the entire first Brillouin Zone (BZ) and allow good bulk sensitivity, increasing the photon energy to >10 eV is necessary [20]. A concomitant benefit is the ability to use Lithium Fluoride (LiF) optics, which have a cut-off edge at ~12 eV. Thus the energy window of 10~12 eV is a region that could allow high bulk sensitivity and that does not require super high vacuum operation.
Over 40 years ago, the effective generation of a 10.49 eV (λ = 118.2 nm) laser was demonstrated by A. H. Kung et al. [21,22], and the system was based on a 25 ps laser that had undergone a non-resonant third harmonic generation (THG) in a Xe/Ar gas mixture. The conversion efficiency was as high as 1.4 × 10−3, but the repetition rate was only 1 Hz [21,22]. Subsequently, similar systems were widely adopted and used for mass spectroscopy [23,24], however they all had low repetition rates limited by the used nanosecond high energy Nd:YAG lasers or dye lasers, typically on the order of magnitude of tens of Hz. Only recently, high repetition rate cases have been reported [19,20,25]. M. H. Berntsen et al. reported on a 10.5 eV laser for ARPES experiments, which was driven by a 10-ps 355-nm Nd:YVO4 laser with a repetition rate of 0.2 MHz [19]. The power at 10.5 eV was ~15 μW and the conversion efficiency was ~10−6. Limited by the Nd:YVO4 gain medium itself, the pulse duration could not be further decreased. Using the resonant method, Y. He et al. reported on a 10.897 eV laser with sub-nanosecond pulse durations and a high repetition rate of 10 MHz [20], which delivered an average power of ~3.5 μW, corresponding to a conversion efficiency of ~10−6. This source led to a high energy resolution thanks to the relatively longer pulse duration, but it was not applicable to time resolved ARPES (tr-ARPES). Consequently, the same group proposed to develop an 11.271 eV source using a 1-MHz Ti:sapphire femtosecond laser as the driver and based on the resonant method as usual [26], however such a system has yet to be reported. F. Cilento et al. reported on a 9.3 eV source pumped by a 0.25-MHz frequency doubled Ti:sapphire laser. An average power of 1.2 nW was obtained, corresponding to a conversion efficiency of ~10−9 [25]. This low efficiency was because of the fact, that the used rare gas Xe has normal dispersion at the generated wavelength of 9.3 eV, where phase matched THG cannot occur. Instead, a higher nonlinear process was involved, leading to a significantly lower conversion efficiency. Indeed, the dispersion properties of the involved rare gases play an important role in fulfilling the phase matching condition of the THG process [27–29]. For the frequently used fundamental driver wavelength of 355 nm, Xe has negative dispersion at its THG wavelength of 118 nm, which is also the reason why the efficiency reported in [20] and [21] was much higher than that stated in [25]. R. A. Ganeev et al. investigated the wavelength regions where the dispersion in Xe is negative [30–32]. When the wavelength of the driver is within 340.5~351 nm and 352.8~357.6 nm, effective THG can take place. Outside these regions, where Xe has positive dispersion, THG is possible, but the mechanism involved is different [25,33,34], which is six wave mixing [25]. As a result, the conversion efficiency is significantly lower, typically on the order of magnitude of ~10−10-10−9. Other rare gases and metal vapors also have their own negative dispersion regions, which favor the THG process over various different and specific wavelength regions [35]. Therefore, to the best of our knowledge, a high average power ~10-12 eV VUV femtosecond laser source using a gas with a negative dispersion for non-resonant THG has yet to be reported.
In this manuscript, we report on an mW-level single order harmonic generation at 10.7 eV (λ = 115.6 nm). A 1-MHz femtosecond Yb:fiber chirped pulse amplifier (CPA) was used as the pump for THG with BBO crystals, which led to a 347-nm UV laser driver for non-resonant THG in a Xe/Ar gas mixture that featured a negative dispersion at the target wavelength. The generated 10.7-eV laser beam was separated from the fundamental driver light using a LiF prism and no additional separation from the radiation of others harmonics was necessary. Using a VUV photodiode, the power was measured to be ~80 μW, which suggested an average power of ~1.25 mW in the gas cell when taking the transmission coefficients of the LiF optics into account. The corresponding conversion efficiency was 2.5 × 10−4. More specifically, the femtosecond pulse duration and 1 MHz repetition rate could ensure high statistics tr-ARPES in an almost space-charge-free capacity.
2. Experimental setup
Figure 1 shows the schematic diagram of the experimental setup used for the generation of the 10.7 eV laser source. It consisted of three sub-systems: a high power 1-MHz Yb:fiber CPA laser system, a first THG frequency stage using BBO crystals, and a second THG frequency stage using a Xe/Ar gas mixture.
Fig. 1 The schematic diagram of the experimental setup used for the generation of the 10.7 eV laser source, which consisted of three sub-systems: a 1-MHz high power Yb:fiber CPA laser system, a first THG frequency conversion stage based on BBO crystals, and a second THG frequency conversion stage based on a Xe/Ar gas mixture. CPA, chirped pulse amplifier; M, mirror; HWP, half wave plate; PBS, polarization beam splitter; SHG, second harmonic generation; THG, third harmonic generation; L, lens; AXUV100G, VUV photodiode. M1&M2, broadband HR mirrors for 1040 nm; M3&M4, HR coated at 347 nm and AR coated at 1040 nm & 520 nm; M5&M6, broadband HR mirrors at 347 nm.
The fundamental laser was a 100-W Yb-fiber CPA laser system built in-house, which had a repetition rate of 1 MHz and a pulse duration of 270 fs. A brief description is given here and more details can be found in [36]. The CPA system started out as a 64-MHz mode locked fiber oscillator, the output of which was directed to a grating stretcher to stretch the pulse duration to ~1 ns. After that, the pulse repetition rate was lowered to 1 MHz using a fiber coupled acousto-optic modulator (AOM). Multi-stage fiber amplifiers were then used to increase the average power to 120 W. Finally, the pulses were compressed using large-scale dielectric transmission gratings, which led to an average power of 100 W and a pulse duration of 270 fs.
To generate the third harmonic of the fundamental laser light, its second harmonic was firstly generated. As shown in the Fig. 1, after the beam passed through a power controller, which was composed of a polarization beam splitter (PBS) and a half wave plate (HWP), the laser radiation was softly focused down by a lens with a focal length of 1000 mm. Another HWP, situated before the lens, was used to adjust the polarization direction of the incident beam. A 1-mm thick BBO crystal cut at an angle of θ = 23.4˚ to allow type-I phase matching was used for frequency doubling. Both surfaces of the BBO crystal were coated with protective coating due to its low dispersion and high laser damage threshold. For precise adjustment, the BBO crystal was installed on a stage which could be rotated, tilted, and linearly moved along the axis of propagation of the incident beam. Finally, the generated green light was separated from the fundamental one using two dichroic mirrors (HR coated at 520 nm, AR coated at 1040 nm), which are not shown in Fig. 1. After the process of second harmonic generation (SHG) was optimized, a second BBO crystal was introduced in the setup to generate the third harmonic wave at 347 nm. The 1-mm thick BBO crystal was cut at an angle of θ = 40.1˚ to allow type-II phase matching and it also had a protective coating. In the same way as the BBO crystal used for SHG, the one used for THG was installed on a stage which could be rotated, tilted, and translated along the direction of propagation of the beam. The generated UV laser beam was separated from the fundamental and green ones using two dichroic mirrors M3 and M4 (HR coated at 347 nm, AR coated at 1040 nm and 520 nm).
Once the third harmonic wave at 347 nm was generated, it was directed to the gas cell using two mirrors, M5 and M6, for steering. A lens with focal length of 150 mm was used to tightly focus the UV laser down into the center of the gas cell and a HWP was used to adjust the polarization direction. The input end of the gas cell was fitted with an AR coated UVFS window (4 mm in thickness) and the output end was sealed with a LiF window (1 mm in thickness). Apart from those two windows, the gas cell featured another four openings which were used for observation, the introduction of the rare gas, and for monitoring the gas pressure, none of which are shown in the diagram of Fig. 1. To make the system compact, the gas cell was attached directly onto the vacuum chamber. Two gas cylinders (Ar and Xe) were connected with the gas cell, which could be controlled separately for filling it. The generated harmonics and fundamental wave propagated collinearly through the LiF window, a LiF lens with a focal length of 200 mm, and a LiF prism with an apex angle of 63.9˚. After the prism, the generated third harmonic was separated from the fundamental UV beam, while other higher order harmonic waves were not transmitted by the LiF optics. The separation angle was calculated to be 19.0˚. The remaining fundamental UV laser radiation was directed to a beam dump. The generated harmonic wave was first made to be incident to a fluorescence plate for observation. After ensuring the generation of the harmonic wave, the fluorescence plate was removed from the beam path and the average power could be measured with a XUV photodiode (AXUV100G, OptoDiode Corp).
3. Experimental results
This section summarizes the experimental results, including those of the SHG, the THG and the generation of the 10.7 eV laser.
Figure 2(a) shows the power performance of the 520 nm laser. When the incident power at 1040 nm was approximately 90 W, over 44 W of green laser power was obtained and the SHG conversion efficiency was of 49%. The inset of Fig. 2(a) is the spectrum of the output at 520 nm (taken with a HR4000 spectrometer, 0.26 nm resolution, Ocean Optics), where the bandwidth was of ~3 nm. This corresponded to a transform limited pulse duration of 132 fs when Δt·Δυ = 0.44 is considered. A pulse duration measurement of the 520 nm output was carried out using an auto-correlator and a 1-mm thick BBO crystal (type I, θ = 49.3°) for rough estimation. The pulse duration was measured to be approximately 200 fs. However, it should be noted that thinner crystal should be used for precise characterization of pulse duration. We also tested a lithium triborate (LBO) crystal for SHG, but the results were no better than those obtained with the BBO crystal used. Figure 2(b) shows the power performance of the 347 nm laser. An average power of 10 W was obtained when the fundamental power was 70 W, corresponding to a conversion efficiency of 14% from the fundamental beam up to 347 nm one. To further improve the conversion efficiency, time delay compensation plate could be considered in the future. In the experiment, the HWP placed before the BBO crystal for SHG was adjusted for an optimal power ratio between the 1040 nm and the 520 nm laser beams. The inset of Fig. 2(b) is the spectrum of the 347 nm output which had a bandwidth of ~1.85 nm (taken with a HR4000 spectrometer, 0.26 nm resolution, Ocean Optics). This corresponded to a transform limited pulse duration of 98 fs when Δt·Δυ = 0.44 was considered. Although the real pulse duration of the UV laser was not characterized experimentally, we do not expect significant pulse duration broadening.
Fig. 2 Experimental results for (a) the SHG, and (b) the THG processes. Inset of both (a) and (b) shows the corresponding spectrum at 520 nm and 347nm, respectively.
When the UV laser was focused down into the gas cell without any gas loaded in, nothing was observed in the gas cell and no beam cross-section was visible on the fluorescence plate. Then, Xe gas was introduced into the gas cell with gradually increasing pressures. When the incident power was ~5 W, bright plasma was observed, as shown in Fig. 3(a) which was acquired over an integration time of 1 s. Figure 3 (b) shows a picture of the plasma when both Xe and Ar gases were present in the cell which was acquired over an integration time of 5 s and at an incident power of ~5 W.
Fig. 3 Photographs of the plasma generated in the gas cell. In case (a), the gas was Xe alone. In case (b), the gas was a mixture of Xe and Ar. The laser direction was from left to right.
With respect to the power of the 10.7 eV laser, it was measured with a XUV photodiode. The results are shown in Fig. 4. In this case, the incident UV power was approximately 5 W (as measured after the M4 mirror) and the beam diameter ~5 mm. The focused beam diameter was calculated to be ~16 μm with a lens of 150-mm focal length. The UV laser pulse duration was estimated to <500 fs at most and the peak power intensity was determined to be ~5 × 1012 W/cm2. From Fig. 4, we can observe two regions. The left “Xe gas” region corresponded to the instance when only Xe gas was present in the cell, and the right “Ar gas” region corresponded to the case where both Xe and Ar gases were loaded into the cell. When only the Xe gas was introduced into the cell, the power of the 10.7 eV laser firstly increased and then decreased back down to almost zero. This was due to the phase matching of the frequency conversion process being linked to the gas density and the optimal pressure was inversely proportional to the confocal parameter, b, of the focused fundamental beam [37]. Thus, with tighter focusing, the optimal pressure should shift towards larger values. Three lenses of varying focal lengths 100 mm, 150 mm and 200 mm, were tried and the above-mentioned shift in optimal pressure was observed. The inset Fig. 4(a) shows the typical beam profile on the fluorescence plate acquired in the presence of Xe alone in the gas cell. To relax the limitations of the usable maximum Xe gas density, other positively dispersive buffer gases with negligible nonlinearities could be used to mix with Xe in order to simultaneously allow higher pressures of Xe in the gas cell and more optimal phase matching conditions [21,35]. Therefore, after the power of the 10.7 eV laser decreased to zero, the gas cell containing Xe gas was loaded with just Ar gas. With the increased gas pressure inside the cell, the power of the 10.7 eV laser increased once more, reaching a second peak power of ~80 μW, which was achieved with a mixed gas pressure of 2.2 mbar. After this peak, further increasing the Ar gas pressure led the average power of the 10.7 eV laser to decrease back down to zero. The inset Fig. 4(b) shows the typical beam profile on the fluorescence plate acquired when both Xe and Ar gases are present in the cell. It is visibly much brighter than that of inset Fig. 4(a).
Fig. 4 The power performance of the 10.7 eV laser, measured with an AXUV100G photodiode. The inset beam profile (a) corresponds to the condition where there is only Xe introduced into the gas cell. The inset beam profile (b) corresponds to the condition where there is both Xe and Ar introduced into the gas cell.
To calculate the real power of the 10.7 eV laser inside the gas cell, the transmission coefficients of the LiF optics have to be taken into account. The transmittance as specified by the manufacture, was expected to be 55% for wavelengths >120 nm, but no data for wavelengths below 120 nm was provided. In addition, we noticed that in [19], it was mentioned that LiF optics had a transmittance of 20%~40% at 118.2 nm [19]. Thus, the estimation of the power of the 10.7 eV laser beam inside the gas cell, using a transmission coefficient of 40%, is within a reasonable region of acceptance. Since three pieces of LiF optical components were used (an LiF window with a thickness of 1 mm, an LiF lens with a center thickness of 2.57 mm and an LiF prism), the average power of the 10.7 eV laser inside the gas cell was conservatively estimated to be 80/(0.4 × 0.4 × 0.4) = 1250 μW. This corresponded to a conversion efficiency of 2.5 × 10−4 from a 347 nm beam down to a 115.6 nm one.
4. Discussions
In the following section, a summary of selected best results of VUV and EUV lasers with repetition rate higher than 0.1 MHz is made to illustrate in which category our laser system falls into, as well as the general tendency in the development of high repetition rate (>0.1 MHz) VUV and EUV laser sources, especially those based on SHG or SFG using Potassium Fluoroboratoberyllate (KBBF) crystals and HHG in rare gases. Furthermore, we will discuss how to improve our system in terms of the average power and the repetition rate scaling.
Figure 5 shows the summary diagram. There are three unmarked threads running through the figure, which are labeled in blue triangles, red squares and black circles, to indicate KBBF crystal based VUV sources [38–40], sources based on HHG using a femtosecond enhancement cavity (fsEC) [12,41–44], and sources employing single pass HHG [13–17,19, 20,25,45], respectively.
Fig. 5 A summary diagram of high repetition rate (>0.1 MHz) VUV and EUV sources from selected publications to show the general trends. There are three unmarked threads running throughout the figure, which are labeled in blue triangles, red squares and black circles, respectively. fsEC, femtosecond enhancement cavity; SP-HHG, single pass high harmonic generation; NC, nonlinear compression; NC-2, two stages of nonlinear compression. Note: not all related papers are included here.
For KBBF crystals based VUV lasers, the typical pump sources were picosecond lasers with high repetition rate (>1 MHz) [38–40]. However, the achievable photon energy region was limited to a little above 8 eV, owing to the transmission properties of KBBF crystals. Like that shown in Fig. 5, as the photon energy approaches the KBBF cut-off edge, the yield decreases rapidly.
To obtain higher photon energies and inherit the full repetition rate of the oscillator, external enhancement cavity was adopted, which allows the photon energy to range between 8.32 eV to 108 eV with repetition rates that span over 10 MHz to 250 MHz [12,41–44]. However, according to Fig. 5, it seems with this method, the achieved average power for photon energies beyond 50 eV is rather limited (<1 μW), and further extension to the soft-X ray region has not been demonstrated.
Although fsECs have demonstrated to be the superior option for HHG at very high repetition rates, they do not easily operate continuously over long periods of time. Meanwhile, for high average powers (100~1000 W) and high repetition rates (0.1-100 MHz), Yb-doped ultrafast lasers seem to dominate and they enable high repetition rate single pass HHG (SP-HHG) experiments to be carried out. In Fig. 5, the point indicated with a significantly bigger black circle is used to show where our developed laser falls in the diagram. It can be seen that the high repetition rate SP-HHG is able to cover the entire photon energy range from ~9 eV to 350 eV [13–17,19,20,25,45]. Compared with the results of fsEC lasers, the extension into the soft-X ray region has been achieved, however the repetition rate is typically 10~100 times smaller. Within the region between the cut-off edges of both KBBF crystals and LiF optics, all four results (including this work) were not typical HHG. Although they made use of rare gases as the nonlinear interaction medium, the mechanism was based on THG. From ~20 eV to ~350 eV, the tendency seems to be that the average power decreases from ~mW down to ~pW, which is expected for HHG. Another point worth noting is that the repetition rates were almost below 1 MHz, aside for that reported in [14]. And almost all of these results were based on high average power post-nonlinear compressed lasers using one or two stages of compression [13–15,17,45].
The average power of our developed 10.7 eV laser is the highest between the cut-off edges of KBBF crystals and LiF optics. This is thanks to the THG in the rare Xe gas which has negative dispersion at the target wavelength. This type of mechanism relaxes the requirement of very short pulse durations, which is in turn needed to reduce the ionization level of the gas involved in HHG. Hence, moderately longer pulse durations of a few hundreds of fs could be considered to carry out the experiment, which would avoid using post-nonlinear compression and optical parametric chirped-pulse amplification (OPCPA) techniques. To further improve the power performance of the laser, one can consider shifting the center wavelength of the fundamental laser driver from 1040 nm to 1067 nm. Based on the findings in [32], it would seem an increment of two orders of magnitude could be expected. The reason for this is that the oscillator strength of the 5p6 1S-7s [11/2]0 (λ = 117.0 nm) is weaker than that of the 5p6 1S-5d [11/2]0 (λ = 119.2 nm) [32]. In addition, considering the fact the efficiency is high, one could increase the repetition rate from 1 MHz to 10 MHz, while maintaining a pulse energy on the order of a few ~μJ, which would further facilitate the experiments that require high repetition rates and improve the measurements.
5. Conclusion
In conclusion, we have demonstrated a high repetition rate mW-level single order harmonic generation at 10.7 eV (λ = 115.6 nm) driven by a 1-MHz 347-nm UV femtosecond Yb:fiber CPA pump laser which was based on non-resonant THG in a gas mixture of Xe/Ar. This VUV laser source could greatly facilitate ARPES experiments in terms of a wider coverage of the first BZ due to the suitable photon energy, and improve the measurement statistics resulting from the high repetition rate of 1 MHz. The ways in which to improve the power performance was also discussed. We expect higher average powers could be obtained by slightly shifting the driver wavelength.
Funding
Photon Frontier Network Program and Photon and Quantum Basic Research Coordinated Development Program from Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
Acknowledgments
We would like to thank Dr. Alissa Silva, from National Physical Laboratory (United Kingdom), for her careful proof reading. Z. Zhao would like to thank Mr. Hironori Igarashi for his kind assistance on the vacuum equipment used in the experiments.
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