Dilusha Silva

Tunable Fabry-Perot filters operating in the 3 to 5 μm range for infrared micro-spectrometer applications

In this article the design, fabrication and characterization of micro-Fabry-Perot filters operating in the mid-wavelength infrared range is presented. Using surface micromachining techniques, low temperature silicon nitride based structures with distributed Bragg mirrors made of Ge/SiO/Ge layers have been fabricated and tested, both mechanically and optically. The membrane/mirror deflection has been measured using an optical profilometer and is estimated to be of the order of 800nm with voltage bias up to 17V while still preserving good mirror parallelism. The respective optical transmission peak shifted from 4.5μm to 3.6μm. Without antireflection coating at the back of the silicon substrate ~50% maximum transmission has been measured at the resonance peaks. The FWHM was measured to be 210+/-20nm, which is ~20% larger than estimated theoretically. In agreement with theoretical modeling, after crossing 1/3 of the cavity length, the membrane/mirror structure has been found to enter into an unstable region followed by snap-down to the bottom mirror surface. In order to prevent this detrimental effect, membranes with anti-stiction bumps have been fabricated demonstrating repeatable structure recovery from the stage of full collapse.

Ultrathin tunable terahertz absorber based on MEMS-driven metamaterial

The realization of high-performance tunable absorbers for terahertz frequencies is crucial for advancing applications such as single-pixel imaging and spectroscopy. Based on the strong position sensitivity of metamaterials’ electromagnetic response, we combine meta-atoms that support strongly localized modes with suspended flat membranes that can be driven electrostatically. This design maximizes the tunability range for small mechanical displacements of the membranes. We employ a micro-electro-mechanical system technology and successfully fabricate the devices. Our prototype devices are among the best-performing tunable THz absorbers demonstrated to date, with an ultrathin device thickness (~1/50 of the working wavelength), absorption varying between 60% and 80% in the initial state when the membranes remain suspended, and fast switching speed (~27 μs). The absorption is tuned by an applied voltage, with the most marked results achieved when the structure reaches the snap-down state. In this case, the resonance shifts by >200% of the linewidth (14% of the initial resonance frequency), and the absolute absorption modulation measured at the initial resonance can reach 65%. The demonstrated approach can be further optimized and extended to benefit numerous applications in THz technology.

Nanoindentation of Si 1−x Ge x thin films prepared by biased target ion beam deposition

The mechanical properties of Si_1-xGe_x thin films are studied via nanoindentation. The Si_1-xGe_x thin films are prepared with a biased target ion beam deposition (BTIBD) method. We investigate the effect of varying the Si to Ge composition ratio on the elastic modulus and hardness of the resulting alloyed films. In comparison to pure BTIBD Si (E_si = 154GPa, H_si = 9.9GPa), for Si_1-xGe_x a decreasing trend in Youngs modulus and hardness is observed to be associated with the increase in Ge content, and for Si_0.5Ge_0.5 the values of 139 GPa and 8.4GPa are found to represent the Youngs modulus and hardness, respectively.

Recent developments towards low-cost MEMS spectrometers

While optical spectroscopy has shown great promise in a multitude of applications, the cost, size, and fragility of spectrometer instruments have hindered widespread application of the technology. :tvfEMS microspectrometers offer great hope for low-cost, lightweight, and robust spectrometers, paving the way for pervasive use in many fields. In this invited paper, we report on nearly 15 years of development on MEMS spectrometers in our research group, beginning with devices designed for the shortwave infrared (SWIR) and midwave infrared (MWIR), and moving on to our most recent work towards MEMS spectrometers in the visible and near infrared (NIR) as well as the thermal long-wave infrared (LWIR) bands.

Long-wavelength infrared Fabry-Perot etalon for multi-spectral thermal imaging

A miniature Fabry-Perot etalon was designed and fabricated to provide spectral filtering capability at the resonance wavelength of 10 μm. A high transmission peak of 85% and a relatively broad bandwidth of 500 nm are expected based on optical modeling. Optimal deposition conditions for process durable thin film materials were developed and optical constants of these materials were characterized. Fabrication of devices was accomplished using standard surface micromachining technique. Released mirrors exhibited a deflection of 400 nm over a length of 150 μm.

Fast tunable terahertz absorber based on a MEMS-driven metamaterial

We present an experimental study of ultra-thin tunable THz absorbers based on MEMS-driven metamaterials. Using the high mechanical sensitivity of thin subwavelength metamaterial absorbers, we proposed a paradigm to combine meta-atoms and suspended flat membranes to simultaneously maximize the near-field coupling and avoid resonance broadening. We employed a MEMS technology and successfully fabricated THz absorbers based on integration of meta-atoms and MEMS, demonstrating giant tuning of resonant absorption. The devices presented in this paper are among the best-performing tunable THz absorbers achieved to date, particularly in device thickness and tunability characteristics.

Towards MEMS-based long-wavelength infrared tuneable Fabry-Perot filters

This paper describes the realization of LWIR (8-12 μm) Fabry-Perot filters on basis of our previously demonstrated SWIR (1.6-2.5 μm) or MWIR (3-5 μm) filter technique. An innovative filter design employing a single layer of Ge as the top mirror is proposed and optical modelling shows that the filter can potentially achieve spectral characteristics required by the LWIR spectral imaging applications. As a proof of concept, filters with fixed air cavity were fabricated and bowing in the suspended mirror was corrected by depositing a thin SiNx stress compensation layer underneath the mirror. Transmission measurement was carried out on the filter exhibiting the least bowing, despite some spectral degradation compared to the theoretical model of the filer, showing not optimal but adequate transmission and bandwidth for multispectral imaging applications.

Optical actuation of silicon cantilevers: modelling and experimental investigation

This paper reports on the modeling and experimental investigation of optical excitation of silicon cantilevers. In this work, the silicon cantilevers fabricated have dimensions with width of 15 μm, thickness of 0.26 μm, and variable length from 50 to 120 μm. In order to investigate the effect of the laser modulation frequency and position on the temperature at the anchor edge and displacements at the tip of cantilevers, a transient thermal ANSYS simulation and a steady-state static thermal mechanical ANSYS simulation were undertaken using a structure consisting of silicon device layer, SiO2 sacrificial layer and silicon substrate. The dynamic properties of silicon cantilevers were undertaken by a series of experiments. The period optical driving signal with controlled modulation amplitude was provided by a 405 nm diode laser with a 2.9 μW/μm2 laser power and variable frequencies. The laser spot was located through the longitude direction of silicon cantilevers. In factor, simulation results well matched with experimental observation, including: 1) for untreated silicon cantilevers, the maximum of displacement is observed when the laser beam was located half a diameter way from the anchor on the silicon suspended cantilever side; 2) for the both cantilevers, maximum displacement occurs when the optical actuation frequency is equal to the resonant frequency of cantilevers. Understanding the optical excitation on silicon cantilevers, as waveguides, can potentially increase sensing detection sensitivity (ratio of transmission to cantilever deflection).