Functional pixels: a pathway towards true holographic displays using today's display technology.

Today's 3D dynamic holographic display techniques suffer from severe limitations due to an available number of pixels that is several orders of magnitude lower than required by conventional approaches. We introduce a solution to this problem by introducing the concept of functional pixels. This concept is based on pixels that individually spatially modulate the amplitude and phase of incident light with a polynomial function, rather than just a constant phase or amplitude. We show that even in the simple case of a linear modulation of the phase, the pixel count can be drastically reduced up to 3 orders of magnitude while preserving most of the image details. This scheme can be easily implemented with already existing technology, such as micro mirror arrays that provide tip, tilt and piston movement. Even though the individual pixels need to be technologically more advanced, the comparably small number of such pixels required to form a display may pave the way towards true holographic dynamic 3D displays.

Optical anisotropy of self-organized gold quasi-blazed nanostructures based on a broad ion beam.

To circumvent elaborate conventional lithographic methods for realizing metallic nanostructures, it is necessary to develop self-organized nanofabrication methods for suitable template structures and their optical characterization. We demonstrate the potential of ion bombardment with impurity co-deposition to fabricate terraced or quasi-blazed nanostructure templates. Self-organized terraced nanostructures on fused silica were fabricated using Ar+ ion bombardment with iron impurity co-deposition and subsequent Au shadow deposition. The aspect ratios are enhanced threefold, and the range of nanostructure period variation is significantly increased with respect to that of conventional nanostructures realized by pure ion bombardment. We reveal the key features of the method via atomic force microscopy and optical characterization. Variable-profile quasiperiodic nanostructures with periods of 100-450 nm, heights of 25-180 nm, and blaze angles of 10°-25° were fabricated over an area of 20×40mm2, and these exhibited tunable and broadening optical anisotropy across the nanostructured area. Thus, the proposed method is a viable technique for rapid, cost-effective, and deterministic fabrication of variable nanostructure templates for potential optical applications.

Bound states in the continuum for optomechanical light control with dielectric metasurfaces.

We investigate a reconfigurable dielectric metasurface merging optomechanical interaction and quasi-bound states in the continuum promising for all-optical light control light. The surface consists of a dimerized high-contrast grating with a compliant bilayer structure. The optical forces induced by a control light field lead to structural deformations changing the optical response. We discuss requirements for the geometry and optical force distribution to enable an efficient optomechanical coupling, which can be exploited to tune reflectivity, phase and polarization of a beam impinging on the metasurface. Numerical results explore some tunable devices as mirrors, saturable output couplers, phase modulators and retarder plates.

Quasi-bound states in the continuum for deep subwavelength structural information retrieval for DUV nano-optical polarizers.

We demonstrate the retrieval of deep subwavelength structural information in nano-optical polarizers by scatterometry of quasi-bound states in the continuum (quasi-BICs). To this end, we investigate titanium dioxide wire grid polarizers for application wavelengths in the deep ultraviolet (DUV) spectral range fabricated with a self-aligned double-patterning process. In contrast to the time-consuming and elaborate measurement techniques like scanning electron microscopy, asymmetry induced quasi-BICs occurring in the near ultraviolet and visible spectral range provide an easily accessible and efficient probe mechanism. Thereby, dimensional parameters are retrieved with uncertainties in the sub-nanometer range. Our results show that BICs are a promising tool for process control in optics and semiconductor technology.

Soft X-ray varied-line-spacing gratings fabricated by near-field holography using an electron beam lithography-written phase mask.

A fabrication method comprising near-field holography (NFH) with an electron beam lithography (EBL)-written phase mask was developed to fabricate soft X-ray varied-line-spacing gratings (VLSGs). An EBL-written phase mask with an area of 52 mm × 30 mm and a central line density greater than 3000 lines mm-1 was used. The introduction of the EBL-written phase mask substantially simplified the NFH optics for pattern transfer. The characterization of the groove density distribution and diffraction efficiency of the fabricated VLSGs indicates that the EBL-NFH method is feasible and promising for achieving high-accuracy groove density distributions with corresponding image properties. Vertical stray light is suppressed in the soft X-ray spectral range.

Line-edge roughness as a challenge for high-performance wire grid polarizers in the far ultraviolet and beyond.

High-performance nano-optical elements for application wavelengths in the ultraviolet spectral range often require feature sizes of only a few tens of nanometers where line edge roughness (LER) becomes a critical parameter for the optical performance. In this contribution, we explore the influence of LER on the optical performance of wire grid polarizers (WGP) in the far ultraviolet range. Therefore, we present a method, which uses the finite difference time domain method in combination with a comprehensive spatial frequency dependent LER model. The measured LER of 3.6 nm (standard deviation) reduces the WGP's extinction ratio by a factor of 3.6 at a wavelength of 248 nm. We identify a critical range of the correlation length, which maximizes the detrimental effect of LER. The presented method and the results provide the basis for future fabrication technology optimization of WGPs and other optical meta-surfaces in the ultraviolet spectral region or at even shorter wavelengths.

Reducing Rowland ghosts in diffraction gratings by dynamic exposure near-field holography.

Near-field holography (NFH) combined with electron beam lithography (EBL)-written phase masks is a promising method for the rapid realization of diffraction gratings with high resolution and high accuracy in line density distribution. We demonstrate a dynamic exposure method in which the grating substrate is shifted during pattern transfer. This reduces the effects of stitching errors, resulting in the decreased intensity of the optical stray light (i.e., Rowland ghosts). We demonstrate the intensity suppression of ghosts by 60%. This illustrates the potential for dynamic NFH to suppress undesirable periodic patterns from phase masks and alleviate the stitching errors induced by EBL.

Cancellation of lateral displacement noise of three-port gratings for coupling light to cavities.

Reflection gratings enable light coupling to optical cavities without transmission through substrates. Gratings that have three ports and are mounted in a second-order Littrow configuration even allow the coupling to high-finesse cavities using low diffraction efficiencies. In contrast to conventional transmissive cavity couplers, however, the phase of the diffracted light depends on the lateral position of the grating, which introduces an additional noise coupling. Here, we experimentally demonstrate that this kind of noise cancels out once both diffracted output ports of the grating are combined. We achieve the same signal-to-shot-noise ratio as for a conventional coupler. From this perspective, three-port grating couplers in a second-order Littrow configuration remain a valuable approach to reducing optical absorption of cavity coupler substrates in future gravitational-wave detectors.

Silicon wire grid polarizer for ultraviolet applications.

We present a silicon wire grid polarizer operating down to a wavelength of 300 nm. Besides metallic grating materials, semiconductors also offer appropriate material properties to realize wire grid polarizers in the ultraviolet (UV) spectral range. The presented polarizer with a period of 140 nm was realized by means of electron beam lithography and dry etching using amorphous silicon as the grating material. At a wavelength of 365 nm, a transmission of 42% and an extinction ratio of 90 (19.5 dB) are measured. The spectral bandwidth of these polarizers in the UV-spectral range is about 100 nm.

Coupled grating reflectors with highly angular tolerant reflectance.

We report on stacked high-contrast grating reflectors with virtually angular independent reflectance for transverse-magnetic polarized light. The investigated structure consists of two-layer pairs of amorphous silicon and silicondioxide that are designed for a wavelengths of 1550 nm. The large angular tolerance results from coupling of the two involved silicon gratings and is achieved if the modal fields in the reflectors are matched. With this approach, a reflectance of more than 96% in the entire angular spectrum is feasible. Experimentally we demonstrate a reflectance of more than 98% for incidence angles up to 60° and more than 90% up to 80°.

Temperature-dependent photo-elastic coefficient of silicon at 1550 nm.

This paper presents a study on the temperature dependent photo-elastic coefficient in single-crystal silicon with (100) and (110) orientations at a wavelength of 1550 nm. The measurement of the photo-elastic coefficient was performed using a polarimetric scheme across a wide temperature range from 5 to 300 K. The experimental setup employed high-sensitivity techniques and incorporated automatic beam path correction, ensuring precise and accurate determination of the coefficient's values. The results show excellent agreement with previous measurements at room temperature, specifically yielding a value of [Formula: see text] 1/Pa for the (100) orientation. Interestingly, there is a significant difference in photo-elasticity between the different crystal orientations of approximately [Formula: see text]. The photo-elastic coefficient's absolute value increases by approximately 40% with decreasing temperature down to 5 K. These findings provide valuable insights into the photo-elastic properties of silicon and its behavior under varying mechanical stress, particularly relevant for optomechanical precision experiments like cryogenic gravitational wave detectors and microscale optomechanical quantum sensors.

Angular bandpass filters based on dielectric resonant waveguide gratings.

We report on a novel concept for transmissive optical elements based on resonant waveguide gratings (RWGs), which enables the realization of direction selective filters. Hereby, the broadband reflectivity of an RWG for nearly normal incidence angles is combined with high diffractive efficiency in transmission for a specific angle of incidence. Silicon is used as material with high refractive index and good compatibility with semiconductor fabrication. By adjusting the grating parameters different transmission angles and angular widths of the transmission range are feasible. First experimental results of the introduced filters provide a high transmission up to 63% at an incidence angle of 45° with a full width at half maximum of 20°.

Michelson interferometer with diffractively-coupled arm resonators in second-order Littrow configuration.

Michelson-type laser-interferometric gravitational-wave (GW) observatories employ very high light powers as well as transmissively-coupled Fabry-Perot arm resonators in order to realize high measurement sensitivities. Due to the absorption in the transmissive optics, high powers lead to thermal lensing and hence to thermal distortions of the laser beam profile, which sets a limit on the maximal light power employable in GW observatories. Here, we propose and realize a Michelson-type laser interferometer with arm resonators whose coupling components are all-reflective second-order Littrow gratings. In principle such gratings allow high finesse values of the resonators but avoid bulk transmission of the laser light and thus the corresponding thermal beam distortion. The gratings used have three diffraction orders, which leads to the creation of a second signal port. We theoretically analyze the signal response of the proposed topology and show that it is equivalent to a conventional Michelson-type interferometer. In our proof-of-principle experiment we generated phase-modulation signals inside the arm resonators and detected them simultaneously at the two signal ports. The sum signal was shown to be equivalent to a single-output-port Michelson interferometer with transmissively-coupled arm cavities, taking into account optical loss. The proposed and demonstrated topology is a possible approach for future all-reflective GW observatory designs.

Reflective cavity couplers based on resonant waveguide gratings.

We report on a novel concept for reflective diffractive cavity couplers based on resonant waveguide gratings instead of multilayer coatings. The diffracting or rather beam splitting properties are induced to the subwavelength structures by a periodic parameter modulation of the ridges. Since such a perturbation of the highly reflective system also enhances transmission stacks of two and three reflectors are considered to retrieve transmittivities as low as possible. Our calculations show that transmissions of less than 10(-4) are possible for different configurations based on silicon and silica. The results of first technological tests for the realization of stacked T-shape structures are presented. With a total effective layer thickness not exceeding 1.1 µm the discussed approaches are expected to remarkably reduce the urgent problem of coating thermal noise of conventional components for high-precision metrology.

Diffractively coupled Fabry-Perot resonator with power-recycling.

We demonstrate the optical coupling of two cavities without light transmission through a substrate. As the all-reflective coupling component, we use a dielectric low-efficiency 3-port diffraction grating. In contrast to a conventional transmissive coupling component, such an all-reflective coupler avoids all thermal effects that are associated with light absorption in the substrate. An all-reflective scheme for cavity coupling is of interest in the field of gravitational wave detection. In such detectors light that is resonantly enhanced inside the so-called power-recycling cavity is coupled to (kilometre-scale) Fabry-Perot resonators representing the arms of a Michelson interferometer. We realized such an all-reflective coupling in a table-top experiment. Our findings are in qualitative agreement with the theoretical model incorporating the characteristics of the 3-port grating used, and therefore encourage the application of all-reflective cavity couplers in future gravitational wave detectors.

Waveguide grating mirror in a fully suspended 10 meter Fabry-Perot cavity.

We report on the first demonstration of a fully suspended 10 m Fabry-Perot cavity incorporating a waveguide grating as the coupling mirror. The cavity was kept on resonance by reading out the length fluctuations via the Pound-Drever-Hall method and employing feedback to the laser frequency. From the achieved finesse of 790 the grating reflectivity was determined to exceed 99.2% at the laser wavelength of 1064 nm, which is in good agreement with rigorous simulations. Our waveguide grating design was based on tantala and fused silica and included a ≈ 20 nm thin etch stop layer made of Al2O3 that allowed us to define the grating depth accurately and preserve the waveguide thickness during the fabrication process. Demonstrating stable operation of a waveguide grating featuring high reflectivity in a suspended low-noise cavity, our work paves the way for the potential application of waveguide gratings as mirrors in high-precision interferometry, for instance in future gravitational wave observatories.

Widely tunable monolithic narrowband grating filter for near-infrared radiation.

We propose a monolithic narrowband guided-mode grating filter in fused silica that is widely tunable in the near-IR wavelength region. Based on a recently demonstrated approach for a monolithic reflector comprising an encapsulated grating, we theoretically investigate such a device by means of rigorous modeling aimed at a narrow linewidth. It is demonstrated that upon a spatial variation of the filter's grating period its resonance wavelength can be tuned in a remarkably wide range of near-IR radiation with 800 nm<λ(res)< 1600 nm by translating the laser beam relative to the grating area. The filter performance in terms of linewidth and contrast is essentially preserved over the entire tuning interval.

Enhanced angular tolerance of resonant waveguide grating reflectors.

We introduce an approach to enhance the angular tolerance of resonant waveguide gratings by stacking two resonant structures on top of each other. It is shown that reflectivities close to unity can be retrieved over the entire angular spectrum by a double T-shaped grating configuration. Although a combination of silicon as the high-index and diamond as the low-index material is considered, the principles of our new approach can also be used to realize monolithic silicon structures with similar properties. We illustrate that the functionality of the device can be understood by a decomposition into separated elements. Our approach might have compelling applications as new diffractive-reflective optical components with low-coating thermal noise in the field of high-precision metrology.

Pound-Drever-Hall error signals for the length control of three-port grating coupled cavities.

Gratings enable light coupling into an optical cavity without transmission through any substrate. This concept reduces light absorption and substrate heating and was suggested for light coupling into the arm cavities of future gravitational wave detectors. One particularly interesting approach is based on all-reflective gratings with low diffraction efficiencies and three diffraction orders (three ports). However, it was discovered that, generally, three-port grating coupled cavities show an asymmetric resonance profile that results in asymmetric and low quality Pound-Drever-Hall error signals for cavity length control. We experimentally demonstrate that this problem is solved by the detection of light at both reflection ports of the cavity and the postprocessing of the two demodulated electronic signals.

Photonic and Thermal Modelling of Microrings in Silicon, Diamond and GaN for Temperature Sensing.

Staying in control of delicate processes in the evermore emerging field of micro, nano and quantum-technologies requires suitable devices to measure temperature and temperature flows with high thermal and spatial resolution. In this work, we design optical microring resonators (ORRs) made of different materials (silicon, diamond and gallium nitride) and simulate their temperature behavior using several finite-element methods. We predict the resonance frequencies of the designed devices and their temperature-induced shift (16.8 pm K-1 for diamond, 68.2 pm K-1 for silicon and 30.4 pm K-1 for GaN). In addition, the influence of two-photon-absorption (TPA) and the associated self-heating on the accuracy of the temperature measurement is analysed. The results show that owing to the absence of intrinsic TPA-processes self-heating at resonance is less critical in diamond and GaN than in silicon, with the threshold intensity I th = α / ß , α and ß being the linear and quadratic absorption coefficients, respectively.