High-order harmonic (HOH) beam generated from the intense femtosecond laser pulse always propagates coaxially with the fundamental laser field. Thus, we need to separate the fundamental laser field from the HOH beam whenever we apply the HOH to spectroscopies. Conventional thin foil metal filters can completely eliminate visible or infrared fundamental laser light, while the thin foils are very fragile and easily damaged with the intense fundamental laser pulse. We have already demonstrated that a silicon or silicon carbide plate is useful for splitting the fundamental and HOH beams thanks to the Brewster incident angle to the fundamental laser beam. Nevertheless, the silicon or silicon carbide (amorphous) beam splitter (BS) is not fully satisfactory for the application because the BS is opaque for visible light. Hence, we cannot reuse the transmitted light. Thermally induced effects such as deformation of the surface may not be negligible under the high-power irradiation of the laser field. Thus, we have searched for transparent materials for visible light which should be suitable for the BS of the HOH beam.
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