Optical measurements of the silicon vacuum window with anti-reflective sub-wavelength structure for ASTE Band 10

Makoto Nagai; Shohei Ezaki; Ryo Sakai; Keiko Kaneko; Hiroaki Imada; Takafumi Kojima; Wenlei Shan; Yoshinori Uzawa; Shinichiro Asayama; Shinichiro Asayama

doi:10.1364/AO.496027

Applied Optics
Vol. 62,
Issue 23,
pp. 6287-6296
(2023)
•https://doi.org/10.1364/AO.496027

Optical measurements of the silicon vacuum window with anti-reflective sub-wavelength structure for ASTE Band 10

Makoto Nagai, Shohei Ezaki, Ryo Sakai, Keiko Kaneko, Hiroaki Imada, Takafumi Kojima, Wenlei Shan, Yoshinori Uzawa, and Shinichiro Asayama

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Makoto Nagai, Shohei Ezaki, Ryo Sakai, Keiko Kaneko, Hiroaki Imada, Takafumi Kojima, Wenlei Shan, Yoshinori Uzawa, and Shinichiro Asayama, "Optical measurements of the silicon vacuum window with anti-reflective sub-wavelength structure for ASTE Band 10," Appl. Opt. 62, 6287-6296 (2023)

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- Instrumentation and Measurements
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About this Article
History
- Original Manuscript: June 1, 2023
- Revised Manuscript: July 20, 2023
- Manuscript Accepted: July 20, 2023
- Published: August 8, 2023

Abstract

For millimeter and submillimeter-wave astronomy, it is highly desirable to have vacuum windows within the receiver cryostat that exhibit low reflection, low loss, and a wide bandpass. The use of antireflective (AR) sub-wavelength structures (SWSs) on substrates has expanded the possibilities for creating new vacuum windows. Recently, a novel method of fabricating AR SWS on a silicon-on-insulator wafer has been proposed, and a vacuum window with a two-layer AR SWS has been developed for use with the Atacama Submillimeter Telescope Experiment Band 10 receiver. To thoroughly assess the characteristics of the silicon window sample, we conducted transmittance measurements using terahertz time-domain spectroscopy, and noise and beam measurements using an Atacama Large Millimeter/submillimeter Array (ALMA) Band 10 receiver. We found that the silicon window sample exhibits characteristics comparable to the quartz window of the ALMA Band 10 receiver. The result strongly encourages applications of AR silicon windows in receivers with wider bandwidths.

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Layer	Refractive Index	Thickness [µm]	Loss Tangent
AR 1	1.26	64.5	$1.8 \times 10^{- 3}$
AR 2	2.42	34.8	$1.8 \times 10^{- 3}$
Handle	3.437	100.7	$1.8 \times 10^{- 3}$

	Window Body
Model	Refractive Index	Thickness (µm)	Loss Tangent	THz-TDS Bin Width (GHz)
Design	3.242	2600	$9.6 \times 10^{- 5}$	–
Modified	3.242	2644	$2.0 \times 10^{- 4}$	3.81

Frequency	808 GHz		879 GHz		943 GHz
Polarization	P0	P1	P0	P1	P0	P1
Silicon window	$- 33$	$- 31$	$- 31$	$- 29$	$- 34$	$- 31$
Quartz window	$- 34$	$- 31$	$- 34$	$- 30$	$- 29$	$- 30$

Frequency	808 GHz		879 GHz		943 GHz
Polarization	P0	P1	P0	P1	P0	P1
Co-pol. similarity [%]	99.13	98.79	99.54	99.31	99.12	99.01
Cross-pol. similarity [%]	58.38	63.29	64.86	78.69	58.45	68.72

Frequency		808 GHz		879 GHz		943 GHz
Polarization		P0	P1	P0	P1	P0	P1
Gaussicity	Silicon	97.92	97.83	98.19	98.01	97.95	97.92
[%]	Quartz	97.99	98.10	97.99	97.98	97.58	97.64
Waist $x$	Silicon	2.244	2.250	2.079	2.053	1.930	1.945
[mm]	Quartz	2.240	2.223	2.052	2.028	1.897	1.910
Waist $y$	Silicon	2.206	2.201	2.042	2.049	1.917	1.869
[mm]	Quartz	2.221	2.251	2.015	2.000	1.884	1.906
$Z$ offset	Silicon	69.62	69.27	68.71	69.75	67.70	68.23
[mm]	Quartz	70.37	72.04	69.06	70.16	68.65	69.47
$X$ offset	Silicon	$- 0.178$	$- 0.134$	$- 0.188$	$- 0.283$	$- 0.052$	$- 0.185$
[mm]	Quartz	$- 0.191$	$- 0.225$	$- 0.182$	$- 0.293$	0.002	$- 0.221$
$Y$ offset	Silicon	0.148	0.117	0.384	0.207	0.235	0.028
[mm]	Quartz	0.374	0.147	0.414	0.228	0.322	$- 0.075$
$X$ tilt	Silicon	$- 0.875$	$- 0.932$	$- 0.809$	$- 0.824$	$- 0.975$	$- 0.872$
[degree]	Quartz	$- 0.883$	$- 0.879$	$- 0.815$	$- 0.812$	$- 1.029$	$- 0.852$
$Y$ tilt	Silicon	0.154	0.098	0.015	$- 0.081$	0.060	0.113
[degree]	Quartz	0.028	0.147	$- 0.036$	$- 0.072$	$- 0.009$	0.237

Abstract

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Figures (10)

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Equations (1)

Applied Optics