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Reflection-reduced encapsulated transmission grating Tina Clausnitzer,* Thomas Kämpfe, Frank Brückner, Roland Heinze, Ernst-Bernhard Kley, and Andreas Tünnermann Institut für Angewandte Physik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany *Corresponding author:
[email protected] Received June 3, 2008; revised July 25, 2008; accepted July 29, 2008; posted August 5, 2008 (Doc. ID 96986); published August 25, 2008 We present the design and realization of a highly dispersive dielectric transmission grating with 97.0% diffraction efficiency. This grating is embedded in fused silica, which allows for an efficient suppression of any reflection losses. It is very easy to handle and clean, and its monolithic layout gives rise to high resistance against laser-induced damage and long-term stability comparable to conventional fused silica components. © 2008 Optical Society of America OCIS codes: 050.0050, 230.1950.
In the rapidly growing field of high-power femtosecond lasers, commercially available metal-coated reflection gratings are being replaced by more effective dielectric gratings. They usually exhibit a significantly higher damage threshold and can be realized with very high diffraction efficiencies [1]. In a similar manner to the metallic gratings, these gratings can be designed to work reflectively by using a highly reflective layerstack [2,3]. Diffraction efficiencies up to 99.6% have been presented at a 1.064 m wavelength by illuminating the grating under its Littrow angle [4]. In this configuration the −1st reflected order propagates antiparallel to the incident wave. For many applications, one has to deviate from this angle in order to separate the incident and the diffracted beams. This can cause a significant decrease of the achievable diffraction efficiency. Furthermore, the wavelength and angular spectrum of reflective gratings often suffer from breakdowns of diffraction efficiency owing to resonant interactions with the layerstack. Within recent years there also have been many efforts made to realize highly efficient transmission gratings that overcome these limitations. Common techniques are holographic structuring of polymer surfaces [5], holographic exposure of dichroitic gelatin (volume phase holographic gratings [6]), or etching of a rectangular profile into the surface of fused silica substrates [7]. Owing to their monolithic structure, the latter possess by far the best resistance against laser-induced damage. Diffraction efficiencies up to 95.5% have been experimentally demonstrated [8]. Unfortunately, this high efficiency decreases significantly when high dispersion is required. The reason for this is the rapidly rising reflection loss when the grating period approaches half of the wavelength. Though such small periods provide very high dispersion, the incidence angle has to be increased, resulting in a larger reflection. As an example, at a wavelength of 1.064 m (which will be assumed throughout the paper), the profile of a rectangular fused-silica transmission grating (refractive index 1.45) with a period of 1 m (groove density 1000 grooves/ mm) can be theoretically optimized 0146-9592/08/171972-3/$15.00
to diffract ⬎98% to the −1st transmitted order. On the contrary, a grating with a 0.6 m period 共⯝1867 grooves/ mm兲 cannot exhibit more than 93% diffraction efficiency. In a pulse compression setup, the light passes the gratings a total of four times. Hence, the efficiency of the entire compressor decreases significantly with every percent less in the efficiency of each grating. It would be less than 75% for the highly dispersive 0.6 m period gratings. The missing energy is diffracted to reflected diffraction orders, and thus is lost for the compressor system. This drawback of rectangular transmission gratings could be overcome by embedding them in their dielectric substrate material. Recently we demonstrated that with the help of this approach transmission gratings without any reflection losses can be designed, allowing for a diffraction efficiency of 100% [9]. In this Letter we present the experimental realization of such gratings. Figures 1(a) and 1(b) show the numerical simulation of the diffraction efficiency of an encapsulated transmission grating and its conventional surfacerelief counterpart as a function of the groove depth and the fill factor (the ratio between the ridge width and the period). The grating period is 0.6 m in both cases. The gratings are illuminated under the Littrow angle (62.5° in air) by TE polarized light where
Fig. 1. (Color online) (a) Diffraction efficiency of a conventional rectangular surface relief transmission grating and (b) the corresponding encapsulated counterpart as a function of the fill factor and the groove depth (0.6 m period, 1.064 m wavelength, illumination in Littrow configuration, TE polarization). © 2008 Optical Society of America
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the electric field vector oscillates parallel to the grating lines. The calculations were made using a Fourier modal method code [10]. In contrast to the conventional surface relief grating, the diffraction efficiency of the encapsulated device exhibits several broad maxima higher than 95%. It even reaches 100% for some particular parameter pairs, e.g., a fill factor of 0.57 and a groove depth of 1.45 m. For those parameters the symmetrical assembly of the grating device results in a complete suppression of any reflection similar to a symmetrical Fabry–Perot resonator (for a more detailed explanation see [9]). One possible route for the realization of an encapsulated grating is to generate a profile with the parameters found in Fig. 1(b) in the surface of a fusedsilica substrate and to cover it afterwards by a second plane fused-silica substrate by means of wafer bonding [11]. To this end we etched an electron-beamgenerated grating pattern with a size of 10 mm ⫻ 10 mm into a 20 mm⫻ 20 mm, 2-mm-thick fusedsilica substrate. Before bonding, a diffraction efficiency of 82.0% was measured in the −1st order, while the zeroth order was 0.7% in case of TE polarized illumination (measurement accuracy of about ±0.5% of each value). As the cover we chose a circular standard laser mirror substrate (25 mm diameter, 3 mm thickness) with a surface flatness of / 10 at = 0.633 m. Both the cover and the grating were cleaned thoroughly by high-pressure cleaning and surface activated by an oxygen plasma. Afterwards, both parts were contacted and pressed against each other for 72 h by applying a 5 kg weight at 100° C. Both surfaces of the sandwiched sample (Fig. 2) were finally coated with an antireflection layerstack in order to avoid losses due to Fresnel reflection at the glass–air interfaces. The results of the spectral and angular measurements of the diffraction efficiency are illustrated in Figs. 3 and 4. The measured efficiencies represent an average over a measurement spot of about 2 mm ⫻ 4 mm on the sample. For comparison, the gray curves illustrate theoretical simulations of an encapsulated grating (solid curves, fill factor 0.57, groove depth 1.54 m) and a conventional surface relief structure (dashed curves, fill factor 0.45, groove depth 1.45 m), respectively. The spectral and angu-
Fig. 2. (Color online) Encapsulated transmission grating device realized by wafer-direct bonding.
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Fig. 3. Diffraction efficiency as a function of the wavelength for a 62.5° incidence angle.
lar behavior of the antireflection coatings were included in the simulations. The measurements in both figures show a peak diffraction efficiency of 97.0%. It is 4% higher than what is theoretically possible for a surface grating. In Fig. 3 the spectrum is shifted by about 5 nm to larger wavelengths, which can be explained by a slight groove depth deviation of only 20 nm. The diffraction efficiency is ⬎90% within an 80 nm bandwidth. The grating is also very tolerant with respect to the incidence angle. The efficiency is higher than 92% within the measured angular variation of ±5° from the Littrow angle. Note that a deviation from the Littrow angle is usually not necessary in a setup with transmission gratings, which is in contrast to reflection gratings. A pulse compressor that is composed of such gratings would exhibit a peak efficiency of higher than 88% and a very high group velocity dispersion of 2 = −60 ps2 / m. The gratings do not suffer from dust or contamination and can be cleaned like any other coated optical devices. They are well protected against mechanical damage like scratches, which is a very important issue for conventional surface relief gratings (reflective as well as transmissive). Further investigations will be concerned with the enlargement of the grating area. The difficulty of void-free bonding increases significantly with increasing sample sizes. The bonding quality strongly depends on the flatness (or the flexibility) and the
Fig. 4. Diffraction efficiency as a function of the incidence angle at a 1.064 m wavelength.
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cleanliness of available substrates. Other possibilities for the realization of large-area encapsulated gratings are currently under investigation. This work has been supported by the German Federal Ministry of Education and Research (BMBF) within the projects 03ZIK455 “onCOOPtics” and 13N9051 “OKULAS.” Furthermore, the authors thank Gerd Jäger (Jenoptik Laser, Optik, Systeme GmbH) for the spectral characterization of the sample. References 1. M. G. Moharam and T. K. Gaylord, J. Opt. Soc. Am. 72, 1385 (1982). 2. L. Li and J. Hirsh, Opt. Lett. 20, 1349 (1995). 3. J. A. Britten, I. Jovanovic, W. A. Molander, M. D. Aasen, C. G. Brown, T. C. Carlson, C. R. Hoaglan, L. M. Jones II, H. T. Nguyen, J. D. Nissen, B. C. Stuart, L. J. Summers, and C. P. J. Barty, in Conference on Lasers and Electro-Optics Quantum Electronics and Laser Science and Photonics Applications Systems
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7. 8. 9. 10. 11.
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