Parylene anti-reflection coating of a quasi-optical hot-electron-bolometric mixer at terahertz frequencies
Introduction
Pure single crystalline silicon shows very low absorption at frequencies between 1 and 5 THz. This makes it an excellent material for optical components in this frequency range. An important application is the lens of the hybrid antenna of a quasi-optical mixer such as the hot-electron bolometer (HEB). At frequencies above 1 THz, it is usually made from pure silicon. The HEB and a planar antenna are fabricated onto a substrate, which in turn is glued to the backside of an elliptical or hemispheric silicon lens. The planar antenna and the lens form the hybrid antenna for coupling the local oscillator and the signal radiation to the HEB [1], [2], [3], [4]. However, the high refractive index of silicon (nSi=3.42) causes a high reflection loss of about 30% which in turn results in a 30% increase of the noise temperature of the receiver. The integration time for achieving a certain signal-to-noise ratio with a heterodyne receiver is proportional to the square of its noise temperature. Therefore, minimizing the reflection loss of the silicon lens will result in an almost twice efficient use of observing time. This is of prime importance for future airborne and spaceborne missions. Current astronomical projects which involve THz heterodyne receivers are the Stratospheric Observatory For Infrared Astronomy (SOFIA) [5], [6] and the Far-Infrared and Submillimetre Telescope (FIRST) [7]. Many important emission lines, which will be observed with these observatories, are between 1 and 5 THz. In atmospheric research airborne THz heterodyne receivers are used for the detection of trace gases such as OH, which has a rotational transition at 2.5 THz [8]. In addition, future space missions are likely to employ THz heterodyne receivers.
The reflection loss of a lens can be minimized by applying an appropriate anti-reflection coating to its surface. For a HEB mixer this coating should fulfill several requirements: it should be stable at a temperature of 4 K and should not degrade over many cooling cycles. The required bandwidth should be about 10%. This is sufficient for most applications in astronomy and atmospheric research. It should be simple and cheap in fabrication and the absorption coefficient should be small. The bandwidth requirement can be fulfilled by using a single layer anti-reflection coating with an optical thickness of a quarter of a wavelength. The ideal index of refraction of the coating material for a transition from vacuum to silicon is the square root of the index of refraction of silicon, which is about 1.85. A widely used material in the far-infrared is low-density polyethylene (LDPE). With a refractive index of about 1.52 and an absorption of 0.67 cm−1 it is suitable although not optimal. A major problem with LDPE is to achieve the appropriate thickness and to get it adhesive to the silicon. LDPE is available in films of standard thickness which makes it difficult to realize a non-standard thickness. A silicon window with a LDPE anti-reflection coating was realized by Englert et al. [9]. They achieved a transmission of 90% for a plane parallel 1.5 mm thick sample with a 18.5 μm thick AR coating on both sides. The coating was achieved by stretching a 20 μm thick LDPE film around the silicon sample. However, the coating was not tested at 4 K. Aluminia loaded epoxy that is diamond machined to the correct thickness has been used up to 1 THz [10], but no data available above that. In addition, machining of the layer becomes more critical with increasing frequency.
An alternative material for AR coating is Parylene. Parylene has a refractive index of about 1.62 and should thus be better fitted to reduce the reflection. It can be deposited from the gas phase and therefore a film of almost any thickness with a very high homogeneity can be produced. The absorption coefficient of Parylene in the far-infrared is not well known but in the order of several 10 cm−1. This is higher than for LDPE and therefore part of the advantage of the higher refractive index is lost by absorption. In this article, we report on an investigation of Parylene as an AR coating at THz frequencies for silicon.
Section snippets
Anti-reflection coating with Parylene
Parylene is the generic name for a family of thermoplastic polymers made from para-xylylene. It can be deposited from the gas phase at room temperature at virtually any thickness up to several hundreds of micrometers. The vacuum deposition technique allows to control the thickness of the film with an accuracy of about 1 μm. In addition, vacuum deposited Parylene films are conformal, uniform, free of pinholes or defects, and chemically inert. Since vacuum deposition requires no catalyst or
The hot-electron-bolometric mixer
In order to investigate the effect of a Parylene AR coating on the noise temperature of a HEB mixer with a hybrid antenna from silicon two HEB mixers were investigated. Both were fabricated from a 3.5 nm thin superconducting NbN film. The film was deposited by d.c. reactive magnetron sputtering of Nb in a N2 atmosphere onto a 350 μm thick high resistivity silicon substrate. The bolometer itself is a 1.7 μm wide and 0.2 μm long bridge. The typical transition temperatures were around 9 K and the
Noise temperature and anti-reflection coating
For two HEB mixers the noise temperature was measured using the coated lens as well as the uncoated lens. The current–voltage characteristic of one of the HEB mixers is shown in Fig. 2. During the experiments no degradation was observed. A far-infrared gas laser was used as the local oscillator. It was operated at four different frequencies: 0.7, 1.4, 1.6 and 2.5 THz. In this frequency range, the properties of the log-spiral antenna are independent of the frequency. The signal radiation from a
Conclusion
We have investigated Parylene C as AR coating for silicon at THz frequencies. The transmittance of silicon can be improved by about 30% by a single layer coating with a quarter wavelength optical thickness. The 10% bandwidth extends from 1.5 to 3 THz for a center frequency of 2.3–2.5 THz, where the transmittance is constant. This bandwidth is sufficient for most practical applications of a heterodyne receiver. By coating the silicon lens of the hybrid antenna of a HEB mixer with a quarter
Acknowledgements
The authors would like to thank H. Riemann from the Institut für Kristallzüchtung in Berlin for providing the plane-parallel silicon samples. This work was financially supported by the German Federal Ministry of Education and Research (WTZ RUS 149-97) and the NATO Science Programme (PST.CLG.975239).
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Present address: Max-Planck Institut für Extraterrestrische Physik, 85740 Garching, Germany.