arXiv subjects

Haruyuki Sakurai

Publications and source records attributed to Haruyuki Sakurai.

7 recordsLinked to original sources

arXiv2026

Femtosecond-to-millisecond holographic imaging of laser ablation dynamics

Femtosecond laser ablation redistributes optically deposited energy across electronic, structural, mechanical, and thermal degrees of freedom over timescales from femtoseconds to milliseconds. However, these coupled processes are usually measured in separate temporal ranges and through different observables, limiting quantitative comparison between early transient dynamics, residual heating, and final morphology. Here we introduce pump-probe holographic imaging that reconstructs amplitude- and phase-resolved optical fields across this full temporal range under matched imaging conditions. Applied to deep-ultraviolet femtosecond ablation of BK7 glass, the method captures the transition from early excitation and removal-stage dynamics to residual substrate heating and permanent modification. Differential phase analysis isolates sub-nanosecond evolution of the transient ablating layer and microsecond residual heating after material removal. Above the ablation threshold, crater depth increases with fluence, whereas the residual thermal signal saturates, indicating that additional absorbed energy is preferentially partitioned into material removal and ablation-related processes rather than retained as substrate heat. These results identify fluence-dependent energy partitioning as a dynamical basis of low-heat-affected femtosecond processing and establish holographic imaging as a route to tracking laser-driven nonequilibrium material transformations.

physics.optics↗

arXiv2026

Tellurium Metasurface Beam Splitter with Pulse Laser-Controlled Anisotropy

Laser-programmable optical anisotropy offers a new route to developing reconfigurable metasurfaces without conventional nanofabrication processes. Here, we demonstrate a lithography-free approach based on spatial control of the crystallographic $c$ axis orientation in tellurium (Te) using pulse laser irradiation. As a proof of concept, we demonstrate a Te metasurface beam splitter by laser-written optical-axis patterning and experimentally confirm that its optical response is in good agreement with theoretical predictions and numerical simulations. By directly programming the local optical anisotropy, this method enables a simple fabrication process while offering the possibility of rewriting and dynamically reconfiguring device functionality. These features make this approach a promising platform for non-resonant active metasurfaces and other reconfigurable flat-optics applications.

cond-mat.mes-hall↗

arXiv2026

3D Printed Alumina as a Millimeter-Wave Optical Element

We present millimeter and sub-millimeter room temperature transmission and loss measurements of 3D printed alumina disc and of a disc with one-sided 3D printed sub-wavelength structures anti-reflection coatings (SWS-ARC). For four bands spanning 158 - 700~GHz we find an index of refraction consistent with $n= 3.107 \pm 0.007$. The loss over the entire frequency band between 158~GHz and 700~GHz spans $ 1 \cdot 10^{-3} \leq \tan δ\leq 2.49 \cdot 10^{-3}$ with 10%-30% uncertainty at the lower range of frequencies shrinking to $\sim\!2\%$ at the higher frequencies. As expected, constructive and destructive interference fringes that are apparent with the flat disc data are absent with the disc that has SWS-ARC. The measured data are consistent with finite element analysis predictions that are based on the measured shape of the SWS. At frequencies between 158~GHz and 200~GHz, below the onset of diffraction effects, reflectance is reduced from a maximum of 64% to about 25%, closely matching predictions. These measurements of the index, loss, and SWS-ARC of 3D printed alumina suggest that the material and fabrication technique could be useful for astrophysical applications.

astro-ph.IM↗

arXiv2026

Silicon-based vacuum window for millimeter and submillimeter-wave astrophysics

We designed, fabricated, and characterized the properties of a silicon-based vacuum window suitable for millimeter-wave astrophysical applications. The window, which has a diameter of 124 mm, optically active diameter of 68 mm, and thickness of about 4 mm, gives an average transmittance and reflectance of 99% and 1%, respectively, a fractional bandwidth of 67%. Absorptive loss is below the detection limit of our measurement. The anti-reflection coating is made with laser ablated sub-wavelength structures (SWS), and the measured transmittance and reflectance values agree with modeling based on the measured SWS shapes. The window has been integrated into DESHIMA v2.0, an astrophysics instrument that took year-long observations with the Atacama Submillimeter Telescope Experiment.

astro-ph.IM↗

arXiv2025

Laser ablated sub-wavelength structure anti-reflection coating on an alumina lens

We used laser ablation to fabricate sub-wavelength structure anti-reflection coating (SWS-ARC) on a 5 cm diameter alumina lens. With an aspect ratio of 2.5, the SWS-ARC are designed to give a broad-band low reflectance response between 110 and 290 GHz. SWS shape measurements conducted on both sides of the lens give 303 $μ$m pitch and total height between 750 and 790 $μ$m, matching or exceeding the aspect ratio design values. Millimeter-wave transmittance measurements in a band between 140 and 260 GHz show the increase in transmittance expected with the ARC when compared to finite element analysis electromagnetic simulations. To our knowledge, this is the first demonstration of SWS-ARC on an alumina lens, opening the path for implementing the technique for larger diameter lenses.

astro-ph.IM↗

arXiv2025

Prototype sub-wavelength structure anti-reflection coating on alumina filters for ground-based CMB telescopes

We present designs and fabrication of sub-wavelength anti-reflection (AR) structures on alumina for infrared absorptive filters with passbands near 30, 125, and 250 GHz. These bands are widely used by ground-based instruments measuring the cosmic microwave background radiation. The designs are tuned to provide reflectance of 2% or less for fractional bandwidths between 51% and 72%, with each of the three primary bands containing two sub-bands. We make the sub-wavelength structures (SWS), which resemble a two-dimensional array of pyramids, using laser ablation. We measure the shapes of the fabricated pyramids and show that for incidence angles up to 20 degrees the predicted in-band average reflectance is 2% or less, in agreement with the design. The band average instrumental polarization is less than $3\times 10^{-3}$.

astro-ph.IM↗

arXiv2022

A Large Diameter Millimeter-Wave Low-Pass Filter Made of Alumina with Laser Ablated Anti-Reflection Coating

We fabricated a 302 mm diameter low-pass filter made of alumina that has an anti-reflection coating (ARC) made with laser-ablated sub-wavelength structures (SWS). The filter has been integrated into and is operating with the MUSTANG2 instrument, which is coupled to the Green Bank Telescope. The average transmittance of the filter in the MUSTANG2 operating band between 75 and 105 GHz is 98%. Reflective loss due to the ARC is 1%. The difference in transmission between the s- and p-polarization states is less than 1%. To within 1% accuracy we observe no variance in these results when transmission is measured in six independent filter spatial locations. The alumina filter replaced a prior MUSTANG2 Teflon filter. Data taken with the filter heat sunk to its nominal 40 K stage show performance consistent with expectations: a reduction of about 50% in filters-induced optical power load on the 300 mK stage, and in in-band optical loading on the detectors. It has taken less than 4 days to laser-ablate the SWS on both sides of the alumina disk. This is the first report of an alumina filter with SWS ARC deployed with an operating instrument, and the first demonstration of a large area fabrication of SWS with laser ablation.

astro-ph.IM↗