Sanja Hadzialic

Two-Dimensional Photonic Crystals Fabricated in Monolithic Single-Crystal Silicon

We describe the fabrication of photonic crystals (PCs) in monolithic single-crystal Si on standard wafers without the need for silicon-on-insulator materials, and we demonstrate two types of PC devices based on this technology. The first is a PC mirror that exhibits broadband high reflectivity in the telecommunication wavelength band (> 90% from 1420 to 1520 nm). The second device shows large changes in reflectivity over narrow wavelength regions and has a sharp (3.5 nm wide) reflection minimum around 1550 nm, making the PC well suited for sensor applications. In both types of PC devices, guided resonances in the PC slab are exploited to obtain the desired reflection response. The PCs are fabricated in a process consisting of a series of oxidation and etch steps, with only one initial lithographic mask. The resulting PC slabs are monolithic structures, with the advantages of very thin and flat membranes with low internal stress, no temperature expansion coefficient mismatch, and compatibility with microelectromechanical systems and complementary metal-oxide-semiconductor processing.

Single-film Broadband Photonic Crystal Micro-mirror with Large Angular Range and Low Polarization Dependence

We experimentally demonstrate a photonic crystal (PC) slab dielectric mirror with reflectivity higher than 90% at optical communication wavelengths. The mirror shows low sensitivity to polarization and incident angle of the input beam.

Monolithic Photonic Crystals

A method for making monolithic 2-D silicon photonic crystals is introduced. We demonstrate a structure with broad-band reflectivity (> 90% from 1420 to 1520 nm), and a structure with a sharp (3.5 nm) reflection minimum.

Displacement Sensing With a Mechanically Tunable Photonic Crystal

We demonstrate a mechanically tunable photonic crystal (PC) designed for highly position-sensitive reflection. Our device consists of a multilayer PC membrane attached to a silicon microelectromechanical system structure for lateral actuation. Inside each of the PC holes there is a pillar, which is attached to the substrate. By moving the PC membrane with respect to the pillars we change the shape of the PC holes and thereby modulate the PC reflectivity. Our measurements show a reflectivity change of more than 80% for a 115-nm lateral displacement.

Double-layered monolithic silicon photonic crystals

Double-layered, self-aligned, silicon photonic crystals are fabricated using directional and isotropic etches - a potential step towards 3D PCs. One structure shows sharper resonances compared to corresponding single layer structure. Another shows high, broadband reflectivity.

Reflectivity and polarization dependence of polysilicon single-film broadband photonic crystal micro-mirrors

We report on the fabrication of 2-D photonic crystal (PC) micro-mirrors, and Finite Difference Time Domain (FDTD) simulations and measurements of their reflectance spectra and polarization dependence at normal incidence. The PC mirrors were fabricated in free-standing thin polysilicon membranes supported by silicon nitride films for stress compensation. Greater than 90% reflectivity is measured over a wavelength range of 35 nm from 1565 nm to 1600 nm with small polarization dependence. Our FDTD simulations show that fabrication errors on the order of tens of nanometers can strongly affect the reflection spectra. When the fabrication errors are kept below this level, FDTD simulations on perfectly periodic structures accurately predict the reflection spectra of the fabricated PC mirrors, despite their sensitivity to the fabrication errors.

Interface quality control of monolithic photonic crystals by hydrogen annealing

We demonstrate that the optical characteristics of silicon photonic crystals can be modified by hydrogen annealing. Hydrogen annealed PCs show reduced surface roughness and improved structural uniformity, leading to increased reflectivity and sharper resonance peaks.

Displacement Sensing with a Mechanically Tunable Photonic Crystal

We demonstrate a mechanically tunable photonic crystal designed for highly position sensitive reflection. Our measurements show that the reflectivity changes from 5.8 to 69% for a 90 nm displacement.

Miniature, low-cost, 200 mW, infrared thermal emitter sealed by wafer-level bonding

Infrared (IR) thermal emitters are widely used in monitoring applications. For autonomous systems, miniaturized devices with low power consumption are needed. We have designed, fabricated and tested a novel device design, packaged on the wafer level by Al-Al thermo-compression bonding. 80 μm wide Aluminium frames on device and cap wafers were bonded in vacuum at 550°C, applying a force of 25 kN for 1 hour. The bond force translated to a bond pressure of 39 MPa. Subsequent device operation showed that the seals were hermetic, and that the emitters were encapsulated in an inert atmosphere. The emitters were optimized for radiation at λ=3.5 μm. Emission spectra by Fourier Transform Infrared Spectroscopy showed high emissivity in the wavelength range 3 – 10 μm at 35 mA driving current and 5.7 V bias, i.e. 200 mW power consumption. The emitter temperature was around 700 °C. The rise and fall times of the emitters were below 8 and 3 ms, respectively. The low thermal mass indicates that pulsed operation at frequencies around 100 Hz could be realized with about 90 % modulation depth. The measured characteristics were in good agreement with COMSOL simulations. Thus, the presented devices have lower power consumption, an order of magnitude higher modulation frequency, and a production cost reduced by 40 – 60%1-4 compared to available, individually packaged devices. The patented device sealing provides through-silicon conductors and enables direct surface mounting of the components.

Photonic Crystal Mirrors for Free-Space Communication and Fiber-Optic Sensors

The use of Photonic Crystals (PCs) to control electromagnetic fields enables unprecedented scaling of low-loss, free-space optical devices. In this paper, we describe the operation, design, and fabrication of PC mirrors, and demonstrate their applications in microoptical systems.