Sum frequency mixing has been demonstrated below 150 nm in KBeBO3F2 by using the fundamental with its fourth harmonic of a 6 kHz Ti: sapphire laser system. The wavelength of 149.8 nm is the shortest ever obtained to our knowledge by phase matching in nonlinear crystals. The output powers were 3.6 μW at 149.8 nm and 110 μW at 154.0 nm, respectively. The phase matching angles measured from 149.8 to 158.1 nm are larger by 3-4 degrees than those expected from the existing Sellmeier equation. The measured transmission spectra of KBeBO3F2 crystals support the generation of coherent radiation below 150 nm.
© 2016 Optical Society of America
Narrow band light sources in the vacuum ultraviolet (VUV) region are attractive for photo lithography  and high resolution photoelectron spectroscopy [2, 3]. A quasi CW 177-nm (7eV) light source have been widely applied to high resolution photoelectron spectroscopy to analyze superconductive materials [2–4]. A 156-nm light was generated by a single mode Ti:sapphire laser . A quasi CW 153-nm (8eV) light source was developed  and now being applied to photoelectron spectroscopy at a μW power level. With use of femtosecond pulses, the second harmonic was generated in the VUV region below 150 nm in the non-phase matching or randomly quasi phase matching conditions [7, 8]. However gas media may be more suitable for practical use if we use femtosecond high peak power laser because they are damage-free. Actually sub mW output power was obtained at 90 nm in Kr under the non-resonant and non-phase matching conditions by using a fs, UV pump source . Phase matching is essential to generate high power VUV lights by using a narrow band, low peak intensity, nanosecond (ns) pump source.
We thought that the absorption edge of KBe2BO3F2 (KBBF) crystals might be possibly shifted to the shorter wavelength when the growth technique of the flux method was changed [10, 11] by comparing the transmission spectra of two papers [5, 12]. Thus we started the experiment to generate VUV lights with a power higher and a wavelength shorter than the previous paper  by an improved laser system with much shorter pulses. In the past, the short wavelength edge by phase-matched frequency conversion was extended to 156.0 nm  in 2004, and 153.4 nm  in 2011. In this paper, we generated VUV lights at 149.8 nm, which is the shortest ever obtained by phase matching in nonlinear crystals. The output powers were improved and reached to 3.6 μW at 149.8 nm and 110 μW at 154.0 nm. The phase matching angles were measured in the wide wavelength range from 158.1 nm to 149.8 nm and were found somewhat larger than those calculated by the Sellmeier equations . The transmittances of three KBBF samples with various thicknesses, two grown in the initial stage and one grown after the new method [10, 11], were measured. In addition, the transmission of two prism-coupled KBBF devices were also measured. The transmittance around the absorption edge does not depend on growth technique of the flux method, but significantly depend on crystal thickness. In thin crystals, the transmission remains well below 150 nm, supporting the present results.
In this paper we describe on (1) a 6 kHz Ti:sappphire laser, (2) a frequency conversion stage and the definition of the phase matching angle in a prism-coupled device, (3) the measurement of phase matching angles in the wide wavelength range, (4) the output power dependence of VUV lights on wavelength, (5) the transmittance measurement of 4 KBBF samples, (6) discussions and finally conclude this paper.
2. 6 kHz Ti:sapphire laser system
A narrow band VUV laser we have developed in this research, consists of a Ti:sapphire laser system and a frequency conversion stage. A schematic diagram of the Ti:sapphire laser system is shown in Fig. 1. This system is composed of an oscillator, a pulse slicer and a 2-stage multipass amplifier. The oscillator with a 1 m cavity length has a flat total reflector and an output coupler with a 1 m radius of curvature and a 90% reflectance. A Brewstar-angled Ti:sapphire crystal (4 mm x 6 mm x 4 mm) was pumped by a part of a frequency-doubled YAG laser (8 W) through the total reflector by a 70 mm focal length lens. We used a birefringence filter and a solid etalon in order to tune the wavelength and to achieve a narrow band width. The birefringence filter consists of three SiO2 crystal plates with the minimum thickness of 330 μm and a thickness ratio of 1:4:16 and was set at the Brewster angle. The solid etalon has a 250 μm thickness and a 70% reflectance on both sides. A prism pair is indispensable for an operation below 750 nm in order to suppress an oscillation at the high gain region around 790 nm. The wavelength was always monitored by a laser spectrum analyzer (High Finesse) and the absolute wavelength along with the band width was measured by a wave meter (High Finesse WS-6UV) with an accuracy of 0.1 pm and a band width resolution of 0.1 GHz. The output power, band width and pulse duration were 250 mW, 1.4 GHz at 749.1 nm and 63 ns (FWHM) respectively at 745.4 nm. The polarization direction of pulses from OC is horizontal. High peak intensity is needed for higher frequent-conversion efficiency in nonlinear crystals. Short pulses are desirable for this purpose. Therefore a Pockels cell (PC) was used to slice output pulses from 63 ns to 1.6 ns. A Faraday isolator was inserted after PC to prevent oscillator instability due to backscattering. The polarization direction of pulses are converted to the vertical direction by using a half wave plate and Faraday isolator after PC and introduced to a 2-stage amplifier. The fundamental power of 2.4 mW was sliced after passing through the PC at 749.1nm.
The input pulses were sent to the 1st stage of the amplifier composed of a quasi-confocal cavity with a 4-pass configuration , which was pumped by a part of a frequency-doubled YAG laser (27 W). The output power dependence on wavelength is shown in Fig. 2
In this experiment, the wavelength of the fundamental was tuned in the range from 790.4 nm to 745.4 nm. Therefore we used concave mirrors (CCM) to focus the fundamental beam without chromatic aberration. The radius of concave of the mirrors are 500 mm.
The 2nd-stage is the same as the 1st stage amplifier. The pump power was 35 W. However the number of passes was increased from 1 to 2 to recover a decreased gain below 750 nm. The output power dependence on wavelength is shown in Fig. 2. The fluctuation of the output power after the 2-stage amplifier was about 120 mW.
The sliced pulse was generated 380 ns after the peak of the 1st pump laser with a width of 80 ns. This timing corresponds to the plateau of the maximum gain of the 1st stage amplifier. The 2nd pump laser was delayed in 100 ns to the 1st one. Pulses go through the 2nd amplifier at a largest gain level with a tolerance of a few hundred nanoseconds.
3. Frequency conversion stage
A schematic diagram of the frequency conversion is shown in Fig. 3. The amplified fundamental (ω) beam was split by a beam splitter (R: 90%, T: 10%). The 90% fundamental was used for the second harmonic generation (SHG) and the rest for the fifth harmonic generation. We employed a LiB3O5 (LBO) crystal for generating the SH (2ω), a prism-coupled KBBF device both for the forth harmonic generation (FHG) and for the sum frequency mixing of FH (4ω) with ω to generate the fifth harmonic (5ω). The LBO crystal has a 20 mm length and an aperture of 5 mm x 5 mm. The KBBF crystals were coupled with CaF2 prisms having an apex angle of 56.8° as shown in Fig. 4. The thicknesses of KBBF are 2.3 mm for the forth harmonic generation and 0.86 mm for the fifth harmonic generation.
Both KBBF crystals were grown by the new method. The apex angle of prisms (56.8°) is designed for SHG from 386 to 193 nm. For 4ω, this angle is adequate to cover from 197 to 186 nm. For 5ω, this angle corresponds to ~155 nm from the Sellmeier equations. The device we used was the only one device with wide incident angles at hand. It turned out that the larger angle was better later in the experiment. However it was not an obstacle to carry out the whole experiment. The reflection loss at an incident angle of 20° is almost the same as the normal incidence. We employed Type-I phase matching. The polarization directions of ω, 2 ω and 4 ω are indicated in Fig. 3. The ω beam was loosely focused on the LBO by a lens with a 1 m focal length. The beam diameters at 1/e2 intensity in LBO were 0.8 mm (H) x 0.6 mm (V) in the horizontal and vertical directions, respectively. The 2ω output power is around 50% of the ω input power corresponding to 90% of the ω output power in Fig. 2
The 2ω was also loosely focused on to the prism-coupled KBBF device. The 2ω beam diameters at the KBBF crystal were 0.6 mm (H) x 0.8 mm (V) in the horizontal and vertical directions, respectively. The KBBF was set in a vacuum chamber, which was equipped with a rotational stage for phase-matching. The chambers for 4ω and 5ω were evacuated.
The ω and 4ω beams were directed coaxially to the surface of the prism for the fifth harmonic generation. The spatial separation at the surfaces of KBBF crystal can be neglected compared with the spot size of the beam. The diameters of the 4 ω and ω beam were 0.7 mm (H) x 0.7 mm (V) and 0.9 mm (H) x 1.2 (V) in KBBF in the horizontal and vertical directions, respectively. The ω and 4ω beams are diverged within 1 degree in KBBF and generate the 5ω beam in the direction where the phase-matching condition is satisfied as Fig. 4. We observed the generation of the 5ω beam by using a fluorescence glass (Sumita Optical Glass, Luminas G9) attached to the inside of the window 210 mm apart from KBBF. Fig. 5(a) shows a fluorescence pattern of 5ω obtained at 149.8nm. The wavelength was determined by one-fifth of the fundamental which is fixed with an accuracy of 0.1 pm. The power was determined separately by exchanging the fluorescence glass with a phototube as described later. The temporal shape of the 5ω pulse was measured by a phototube (Hamamatsu R1328U-54) accommodated in the N2- purged cell attached after a CaF2 window. The 5ω fluorescence spot was introduced to the central region of the window by rotating the two rotational stages loading KBBF and the vacuum chamber. Thereafter the window was exchanged to the CaF2 window accommodating the biplanar photo tube. The full 5ω beam with a diameter of ~1mm can be introduced in a 10 mm active area of the photo tube. A filter (Acton research FN150-N-0.5D) was inserted to reduce the 4ω and ω scattering.
Typical temporal profiles of the sliced pulse after PC (orange), (2) amplified pulse (red), (3) 4ω pulse (blue), and (4) 5ω pulse (violet) are shown in Fig. 5(b) where the wavelength of 5ω is 149.8 nm. The FWHM pulse durations were (1) 1.6 ns, (2) 1.2 ns, (3) 0.4 ns, and (4) 0.4ns respectively. The pulse shortening due to saturation amplification in (2) and due to frequency conversion in (3), (4) were observed. The rise and fall times of photo tubes (R1328 −51,-54) is less than 100 ps and fast enough to follow sub ns pulses. The swing on the tail of the 5 ω pulse may be an artifact induced by the band limit (1GHz) of an oscilloscope (Tektronix DPO 4104B)
4. Measurement of phase matching angle
In the prism-coupled KBBF device, the 4ω and ω beams are not collinear in KBBF except when both beams come to different incidence angles to the surface of the prism. The realization of this condition is unrealistic in the experiment. Then, we used collinear incident beams of ω and 4ω in this experiment. Accordingly we define the phase-matching angle as the refraction angle of the 5ω beam (θ5ω) in Fig. 4. The phase-matching angle is determined from the measured incidence angle of the ω and 4ω beams when the 5ω beam is generated by assuming the refractive index of CaF2  and KBBF  at ω and 4ω. The measured incidence angle (θiω、θi4ω) at phase matching determine the refractive angles (θω) at ω and (θ4ω) at 4ω in KBBF, giving the phase matching angle (θ5ω) by the phase matching condition. The prism-coupled KBBF was set on a rotational stage and contained in a vacuum chamber on another rotational stage. The prism surface was set normal to the 4ω and ω beams initially by observing the reflection from the prism surface and the KBBF device was rotated to generate the fifth harmonic by the rotational stage. We measured angle deviation between the normal reflection and the fifth harmonic generation.
The phase-matching angle versus fifth harmonic wavelength is shown in Fig. 6. The blue solid circles are the present result by the new KBBF crystal. The solid line is the calculation based on the improved Sellmeier equations . The solid triangles indicate the previous data by the old KBBF crystal . The present results are consistent with the previous study within the experimental precision of about 1°. On the other hand, discrepancy between the present data and the calculation is quite large. In the calculation, the 4ω and ω beams coaxially propagate in KBBF and the 5ω is generated along with those beams, while the 4ω and ω beams are not collinear in KBBF as shown in Fig. 5. However the discrepancy of the phase-matching angle between the experiment and calculation is too large (3-4°) to explain by the angle difference between three beams because it is at most 1°. When θiω = θi4ω = 21.0°at 5ω = 149.8 nm, θω = 65.6° and θ4ω = 64.5°, resulting θ5ω = 64.9°.
We measured phase matching angles of SHG down to 170 nm to determine the Sellmeier equations . It should be noted this measurement was done in 2002 by using a KBBF crystal before the new growth method. The improved Sellmeier equations well fit to the data. At the same time, the phase-matching angles at 167.5 and 165 nm in the crystal grown by the new method fit to the Sellmeier equations [16, 17]. The Sellmeier equations well fit to the phase matching angles of SHG down to 165 nm by using a new KBBF crystal [16, 17]. This fact means that the Sellmeier equations are valid down to 165 nm regardless of crystals before and after the new growth method. In Fig. 7, there are no significant discontinuity of data between by the old crystal (156-160nm) and by the new crystal (149.8-158.1 nm). This also support that the growth method does not change the refractive indices. The observed discrepancy in phase matching angles may be because the wavelengths are too short to extend the Sellmeier equations determined above 170 nm but not because of the change of the growth method. The phase matching angle at 153 nm was not measured intensively .
5. Dependence of output power on wavelength below 160 nm
The dependence of 5ω output power on 5ω wavelength is shown in Fig. 7(a) as well as 4ω input power versus 4ω wavelength. The 4ω input power was measured by a power meter (OPHIR 3A) in front of the vacuum chamber where the KBBF device was set for the fifth harmonic generation. We also show the conversion efficiency of 5ω output to 4ω input in Table 1. The 5ω output power was measured by introducing the whole beam into the biplanar photo tube (Hamamatsu photonic R1328-54) in the N3-purged cell after the CaF2 window. The photo tube was calibrated to the power meter (OPHIR 3A) at 193 nm and slightly corrected around 150 nm by considering the spectral response of the photo tube. The decline in 4ω output at 197.5 nm and 190 nm compared with those at other wavelength would just result from the condition of the laser system.
The output power reaches to 110 μW at 154.0 nm when the input was 170 mW. The output was as high as 3.6 μW at 149.8 nm. Such a high power at 154.0 nm and at the short wavelength below 150 nm cannot be expected from the previous report  where the transmittance reaches zero at 154 nm. On the other hand, the transmittance was recently reported to reach zero at 147 nm . We thought that this difference came from the exchange of the growth method. Therefore we measured the transmittances of three samples, (1) two KBBF crystals at the age of previous paper  or before, (2) a KBBF crystal after the new method .
6. Transmittance measurement of KBBF crystals
We measured the transmittance of KBBF. The optical transmission spectra are shown in Fig. 8. The blue solid line corresponds to the crystal (2) with a thickness of 0.23 mm. The green solid line corresponds to the crystal (1)-1 with a thickness of 0.16 mm. The red solid line corresponds to the crystal (1)-2 with a thickness of 0.73 mm. The black curve shows the transmittance of the 1.0-mm thick crystal reported in , Kanai et al. as a reference. Four optical transmission spectra were taken by using the identical spectrophotometer (Bunkoukeiki Co., ltd. VUV-200). The surface reflection and scattering are not corrected. The transmittances were 86-90% at 300 nm besides (1)-1, while the transmittance was 78% for (1)-1. The crystal (1)-1 was cleaved and the surface was unpolished. Then the scattering loss looks large to other samples. However this does not affect the absorption near the edge. The red curve agrees well with the black one , showing the reproducibility of measurement. The most prominent feature of Fig. 8 is that the absorption near the edge seems to depend on thickness rather than on growth technique of the flux method.
We also measured the transmission of two prism-coupled KBBF devices stacked by prisms, (1) one with a thickness of 0.86 mm grown in the age of ref. 5 and (2) the other with a thickness of 0.20 mm grown after the new method. Note that (1) is different from the device used for the 5ω generation although the thickness is accidentally the same. The optical path length through KBBF is estimated as the KBBF thickness / cos (56.8°), although it depends slightly on wavelength. The optical path lengths are 1.6 mm for the device (1) and 0.37 mm for the device (2), respectively. The transmission spectra are shown in Fig. 9. The transmittance near the edge depend on thickness of KBBF and consistent well with Fig. 8. The transmittances at 300 mm 84% for the device (1) and 88% for the device (2) respectively. If the surface losses of CaF2 including Fresnel reflection and scattering are the same as KBBF, the losses in optical contact on both side are estimated to be ~2%. This value shows the satisfactory quality
The absorption coefficients would be more informative in some cases. Thus, we tried to deduce the absorption coefficients from the transmission spectra although there are some ambiguity due to assumptions needed. The transmission spectra T(λ) = (1-R(λ) – β)2exp(-α(λ)l), where R(λ) is the wavelength dependent Fresnel reflection at one surface, β is the surface loss such as scattering at one surface, α(λ) is the wavelength dependent absorption coefficient in cm−1, and l is the crystal length (or optical path length) in cm. We assume the absorption is zero at 300 nm, where β is determined from the measured transmittance T(300nm) and the calculated R(300nm). The calculated R(300nm) agrees well with the measured reflectance and is ~4% at both KBBF and CaF2 surfaces. We assume β is independent on wavelength. The thick crystals were not adequate to deduce the absorption coefficients because the transmittance below 160 nm is very close to zero and brings out a large error in the absorption coefficient. We deduced two absorption curves from the transmission spectra corresponding to the crystal thickness of 0.23 mm (β = 2.1%) and 0.16 mm (β = 7.9%) in Fig. 8. Below 170 nm, both curves agree well with each other, but differ above 170 nm. This is because the 0.16-mm sample is unpolished and β is quite large (7.9%). Thus the assumption of wavelength independent β would be broken.
In Fig. 9, there is shown the absorption coefficient of the 0.20-mm thick KBBF crystal which corresponds to the 0.37-mm optical path length. In the prism-coupled device, the surface is CaF2 and the loss in the interface between KBBF and CaF2 is added, resulting in β = 3%. In Fig. 9, the curve of the 0.23-mm thick sample is also shown for the comparison. Both curves seem to agree well with each other and would show the uncertainty of the method to deduce the absorption coefficient from the transmission curve.
The thickness of KBBF in the prism-coupled device we used in the 5ω experiment was 0.86 mm. However only the thin exit region of KBBF can contribute to the 5ω generation, while the 5ω generated in the entrance part of KBBF will be absorbed completely around the absorption edge. Therefore the effective thickness is quite thin like the crystal (2)-2 whose absorption edge is down to 147 nm. This is the reason why the 149.8 nm light was generated.
The spectrophotometer does not contain a cryostat. Unfortunately we cannot measure the temperature dependence of the absorption edge. Therefore, we cannot decide whether the absorption edge is due to Urbach tail (intrinsic) or impurities. Even if the absorption edge move to the shorter wavelength at a low temperature, we cannot apply to a prism-coupled device because it will be broken by the heat stress at a liquid N2 temperature as was experienced in the previous paper .
Although the absorption edge does not seem to move to the shorter wavelength for these years, thicker KBBF crystals become possible and the quality of optical contact was improved. In the previous report , we used BBO instead of KBBF for the 4ω generation because of low quality in the optical contact between KBBF and CaF2. In the present work, the output power at 4ω (187.3 nm) reaches 160 mW at 6 kHz by using a prism-coupled KBBF device. The improvement of a prism-coupled device also contributed to the 5ω generation and enabled the generation of coherent radiation below 150 nm. Another reason for the present break through to the shorter wavelength is the use of the short pulse driving source. In , Kanai et al., the pulse widths at ω and 5ω were 16 ns and 9.7 ns respectively, while they were 1.2 ns at ω and 0.4 ns at 5ω in the present work respectively. These short pulses also contributed to the generation of lights below 150 nm.
The 149.8 nm light was generated by sum frequency mixing in KBBF by using the fundamental with its forth harmonic of a 6 kHz Ti:sapphire laser. The wavelength is the shortest ever obtained to our knowledge by phase matching in nonlinear crystals. The output powers were 3.6 μW at 149.8 nm and 110 μW at 154.0 nm respectively. The phase matching angles were measured in the wide range (149.8 – 158.1 nm) resulting in the discrepancy by 3-4° to the calculation based on the Sellmeier equation. The optical transmission spectra of some KBBF crystals were measured by the spectrophotometer. The transmittance near the absorption edge support the generation of coherent radiation below 150 nm.
We appreciate Dr. T. Onose for developing the base of the Ti:sapphire laser system.
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