Demonstration of a Monolithic Dielectric Microstructured Surface with a Reflectivity of 99.8%

Frank Brückner, Tina Clausnitzer, Thomas Käsebier, Ernst-Bernhard Kley, Andreas Tünnermann Institut für Angewandte Physik, Friedrich-Schiller-Universität Jena,

Max-Wien-Platz 1, 07743 Jena, Germany frank.brueckner@uni-jena.de

Abstract—We present first experimental data on a highly reflective monolithic dielectric microstructured surface that has been proposed recently to open a route to monolithic (micro-) opto-mechanical systems such as high precision interferometric experiments.

I. INTRODUCTION For the design and fabrication of micro-opto-mechanical

systems one has often to abandon the freedom of using different materials due to thermal mismatch, increased thermal noise, or the involvement of adverse material properties such as optical absorption. Hence, a desired optical function such as efficient reflection must be provided by a monolithic material configuration. For opto-mechanical systems in high precision measurements, thermally driven motion of optical components give rise to thermal noise [1], and is becoming a limiting factor for the sensitivity of interferometric experiments such as gravitational wave detection [2], laser cooling of mechanical oscillators [3], the generation of entangled test masses [4], and frequency stabilization of lasers with rigid cavities [5]. Outstanding optical quality of the mirrors surface as well as high mechanical quality factors of the test mass vibrational modes are required to reduce the influence of thermal noise [6]. In current approaches the surface of the mechanical device is composed of a multilayer dielectric coating. Reflectivities of up to 99.9998% have been demonstrated [7]. To achieve high mechanical quality factors, monocrystalline materials such as quartz or silicon are used. However, recent theoretical and experimental research revealed that multilayer dielectric coatings result in a significant reduction of quality factors, mainly caused by the mechanical loss of the alternately coated layers [6]. Thus, the simultaneous realization of high optical and mechanical quality is a nontrivial problem.

Here, we present a new approach as well as very promising experimental data for a highly reflective dielectric surface without the need of adding any other lossy material to the substrate. Our alternative mirror architecture is solely based on periodically microstructuring its monolithic surface [8]. By properly utilizing resonant light coupling, reflectivities of up to 100% can be theoretically achieved for the proposed subwavelength grating structure. For a processed silicon substrate, we experimentally observed an outstanding value of resonant reflection of 99.8% for a wavelength of 1550 nm under normal incidence.

II. RESONANT WAVEGUIDE GRATINGS The new approach is based on resonant waveguide gratings

which have been suggested before to reduce the optical and mechanical loss [9]. These structures comprise a periodically microstructured high-index layer attached to a low-index substrate. Though they reduce the thick dielectric layer stack of conventional mirrors to a thin waveguide layer, at least one residual coating step is involved for the fabrication, thus causing thermal mismatch and/or a reduction of the mechanical quality. The fundamental principle of waveguide gratings is illustrated in Fig. 1. In case of normal incidence, the three following parameter inequalities have to be fulfilled to allow for resonant reflection:

d < λ (to permit only zeroth order in air), (1)

λ/nH < d (first orders in high-index layer), (2)

d < λ/nL (only zeroth order in substrate), (3)

where d is the grating period, λ is the light’s vacuum wavelength, and nH and nL are the higher and lower refractive indices, respectively. In this setup, higher diffraction orders experience total internal reflection (at the boundary layer to the low-index substrate) and excite resonant waveguide modes. If d, the groove depth g, the grating fill factor f (ratio between ridge r and period d), and the high-index layer thickness s with respect to the refractive index values are designed properly, all transmitted light can be prompted to interfere destructively. It has been shown theoretically as well as experimentally that this functionality does not break down in case of a zero waveguide layer thickness and the reduction of the low-index substrate to a thin layer [10]. Such a grating configuration is illustrated on the left hand side of Fig. 2.

Figure 1. Fundamental principle of resonant waveguide gratings in a simplified ray picture.

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III. MONOLITHIC IMPLEMENTATION Our monolithic implementation is realized by introducing

an effective medium low-index layer instead of a homogeneous one that is built on an additional low fill factor grating beneath the diffracting one; see right hand side of Fig. 2. For further explanation we refer to [8]. A particular design consideration in [8] that aimed perfect reflectivity of a 1.55 µm wavelength normally incident onto a silicon substrate resulted in a T-shaped grating profile with a 700 nm period. Due to the high refractive index contrast between silicon and air (Δn = 2.5), the spectral as well as the angular reflectance is very broad. This, in turn, determines convenient parameter tolerances for the fabrication process. Being a standard material for telecommunication integrated micro-opto-mechanical systems, silicon was considered here as well as it is supposed to replace fused silica as the substrate material in the next generation high precision experiments due to lower absorption and higher mechanical quality at cryogenic temperatures. However, our approach can also be expanded to other materials and wavelength regions, respectively. Note, that with decreased refractive index contrast (between the used material and air) the spectral bandwidth of high reflectance as well as the parameter tolerance is decreasing. In fact, this interrelation makes the monolithic reflector architecture also suitable for narrowband filter applications.

Figure 2. Monolithic implementation of a resonant grating structure by introducing an effective media low-index layer (low fill factor grating).

IV. FABRICATION PROCESS For the fabrication, firstly a 700 nm period grating was

defined on a silicon substrate by the use of electron beam lithography for an area of (7.5 x 13) mm2. With regard to the desired T-shaped grating profile, a two-step ICP (Inductively-Coupled-Plasma) etching process was utilized. Here, a sidewall passivation after the first etching process enabled the undercut of the upper grating to generate the low fill factor grating beneath. Fig. 3 depicts a SEM (scanning electron microscope) cross-sectional view on the grating that has been characterized within this work.

Figure 3. SEM cross-sectional view on a 700 nm period T-shaped grating that has been characterized within this work.

V. CHARACTERIZATION The spectral reflectance under normal incidence (0 ± 1°)

was measured using a fiber-based tunable diode laser with a spectral range from 1.26 µm < λ < 1.63 µm. The measured data is shown in Fig. 4 and reveals a reflectivity of higher than 95% for a broad spectral range from 1.32 µm < λ < 1.62 µm. This is in close agreement to the simulated plots in [8], verifying the unique capability of the T-shaped grating structure of broad and high reflectance by using only a monolithic dielectric surface. The peak reflectance is located closely to the wavelength of 1.55 µm with a record value of resonant reflection of 99.8%, where an error of ± 0.3% needs to be taken into account due to the measurement setup. With our results we are sure to open a new prospect to monolithic micro-opto-mechanical devices and to dramatically reduce the influence of coating thermal noise for high precision measurements.

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Figure 4. Spectral reflectance of the T-shaped grating from Fig. 3 under normal incidence (0° ± 1°).

ACKNOWLEDGMENT This work is supported by the Deutsche Forschungs-

gemeinschaft within the Sonderforschungsbereich TR7.

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