High Contrast and High Damage threshold
EUV/Visible Dichroic Mirror
NTT Advanced Technology Corporation, Atsugi, Kanagawa, Japan
E-mail: nano-sales@ml.ntt-at.co.jp
Web: https://keytech.ntt-at.co.jp/
Abstracts: A high-contrast EUV/visible dichroic mirror has been developed for beam separating andcombining of high-order harmonics light and its pump-beam. This grazing incident angle AR coatingbased dichroic mirror has p-polarization reflectivity more than 40% at EUV region and less than 1%
at 800 nm. This contrast is comparable with conventional method such as metal thin-film filters andBrewster’s angle-based beam separators, and the dichroic mirror, in principle, has higher damage
threshold than the others. This easy-customized optical components will be useful for all high-orderharmonic light sources including VUV and soft x-ray region.

High-order harmonics (HH) is a powerful light source for many kinds of research applications,such as spectroscopy, microscopy, and ultrafast dynamics measurements in VUV, EUV and soft
x-ray region. The system is mainly constructed with pump laser, high-order generation, spectralfilter, beam steering mirror, focusing mirror and spectrometer. In addition, some systems have aninteraction chamber. In this chamber, focused HH beam is interacted with many kinds of targets,for example gasses, liquids, solid surface, thin films, particles, and clusters.
To obtain small signal from interaction between HH beam and target materials, to increase S/Nratio is one of the most important techniques in this field and considering low-conversion
efficiency from pump-beam to HH beam, high-performance EUV filters have been required. Fora long time, metal thin-film filters were only-one solution in this field [1], but in 2004,
Brewster’s angle beam separator is designed and demonstrated [2-4]. This beam separator arelarge size, high-contrast and higher damage threshold comparing with those of metal thin-film
filters and have been used in many HH beam lines. However, because of recent developments ofhigh-power compact HH source systems, much higher irradiation tolerance filters are required.
The beam separator is typically based on Si or SiC plate, and their light absorption is sometime
cause of shape deformation or damage.
In this paper we propose and demonstrate a dichroic mirror for EUV and visible laser beam.
This mirror is based on AR coating on fused silica substrate and no absorption for visible light,
and the coating is corresponding to high-intense femtosecond laser pulses. In addition, XUV lightis total reflected from surface layer of coating. This technology has been already proposed inacademic field [5], but limited only academic demonstrations. We NTT-AT have developed themas commercial products both standard and customized model. Note that this technology applied
not only EUV region but also for VUV or soft x-ray region.
Calculated reflectivity profiles of our standard dichroic mirror P/N DM-20/800-2002 at NIR
region and EUV region are shown in figure 1 (a) and (b), respectively. The calculations are
assumed grazing incident angle of 12 degrees and p-polarization. The results shows thatreflectivity of less than 1% at NIR region and more than 40% at EUV region are obtained. Notethat inter-layer roughness and surface roughness are not included in these calculations, but themeasured reflectivity profiles are almost as same as calculated values.

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To understanding detail properties of our dichroic mirror, we show NIR reflectivity dependedon polarization and incident angle in Fig. 2 (a) and (b), respectively. The results show that thedichroic mirror is high dependence on polarization and broadband capability for incident angle.

Fig. 1 Calculated reflectivity of DM-20/800-2002, (a) visible region and (b) EUV region. Assuming
grazing incident angle of 12 deg., and p-polarization.

Fig. 2 (a) incident angle dependence and (b) polarization dependence of DM-20/800-2002.

Fig. 3 Calculated reflectivity of DM-30/800-2002, (a) visible region and (b) EUV region. Assuminggrazing incident angle of 12 deg., and p-polarization.

In addition, reflectivity profiles of another standard P/N DM-30/800-2002 are shown in Fig. 3.
Specifications of standard dichroic mirrors include outer dimensions are shown in table 1.
In table 2, we show capability of customized dichroic mirror. The dichroic mirrors are basedon dielectric multilayer AR coating technology, thus, easily designed depend on requirement,such as target EUV wavelength, visible/NIR wavelength, and incident angle. For example, higherCopyright© 2023 NTT Advanced Technology Corporation

EUV reflectivity will be obtained at smaller grazing incident angle, but visible/NIR reflectivity
also will become higher at smaller grazing incident angle. It is trade off and depend on
customers’ requirements. In Fig. 4, we show customized dichroic mirror calculations as examples.
As shown in these figures, they will work not only for EUV region but also VUV and soft x-ray
region.
Table 1. Specification of standard dichroic mirrors
P/N

DM-20/800-2002

DM-30/800-2002

AOI

78 degpolarizationpNIR reflectivity~2% @800 nm+/- 50 nm

XUV reflectivity~60% @20 nm

Coating
Substrate material
Dimensions~2% @800 nm+/- 50 nm~45% @30 nm
Dielectric AR

Fused silica
2” dia. x 0.2" thick

Table 2. Specifications of customized dichroic mirror
AOI
polarization
target AR wavelength
HR wavelength
Coating
Substrate material
Dimensions

75 deg. - 87 deg.
p, s400 nm, 515 nm, 800 nm, 1030 nm, etc.
3-5 nm, 13.5 nm, 30 nm, 60 nm, 110 nm, etc.
Dielectric AR
Fused silica
Round (1" dia. - 4" dia.) rectangler (~100 mm length)

Although there are no experimental data of damage thresholds, in principle, damage thresholdof this dichroic mirror is higher than that of conventional filter/beam separators. It is because all
coating materials have no absorption for visible/NIR wavelength and substrate quality is highenough for high-intensity beam irradiation.

Fig. 4 Customized dichroic mirror example. Assuming gazing incident angle is 3 deg., andcalculated reflectivity at 1030 nm is <20%.

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In conclusion, this dichroic mirrors have many advantages comparing with conventional filtersand beam separators, high optical performance, easy customized, and high damage threshold.
This new optics will be used for all high-order harmonics light source for wide-range wavelength
region. Especially, it will be powerful tool for high intensity high-order harmonics sources.
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