Rob Ilic

Silicon-on-sapphire integrated waveguides for the mid-infrared

Silicon waveguides are now widely used to guide radiation in the near-infrared, mainly in the wavelength range of 1.1 - 2.2 microm. While low-loss waveguides at longer wavelengths in silicon have been proposed, experimental realization has been elusive. Here we show that single-mode integrated silicon-on-sapphire waveguides can be used at mid-infrared wavelengths. We demonstrate waveguiding at 4.5 microm, or 2222.2 cm(-1), with losses of 4.3 +/- 0.6 dB/cm. This result represents the first practical integrated waveguide system for the mid-infrared in silicon, and enables a range of new applications.

Silicon waveguides and ring resonators at 5.5 μm

We fabricate and characterize silicon waveguides and ring resonators around 5.5 µm on the Silicon-on-Sapphire (SOS) platform. The waveguide loss is 4.1 ± 0.7 dB/cm, and a Q value of 1.4k is achieved.

All optical reconfiguration of optomechanical filters

Reconfigurable optical filters are of great importance for applications in optical communication and information processing. Of particular interest are tuning techniques that take advantage of mechanical deformation of the devices, as they offer wider tuning range. Here we demonstrate reconfiguration of coupled photonic crystal nanobeam cavities by using optical gradient force induced mechanical actuation. Propagating waveguide modes that exist over a wide wavelength range are used to actuate the structures and control the resonance of localized cavity modes. Using this all-optical approach, more than 18 linewidths of tuning range is demonstrated. Using an on-chip temperature self-referencing method, we determine that 20% of the total tuning was due to optomechanical reconfiguration and the rest due to thermo-optic effects. By operating the device at frequencies higher than the thermal cutoff, we show high-speed operation dominated by just optomechanical effects. Independent control of mechanical and optical resonances of our structures is also demonstrated.

Size and frequency dependent gas damping of nanomechanical resonators

We examine size and frequency dependent gas damping of nanobeam resonators. We find an optimal beam width that maximizes the quality factor at atmospheric pressure, balancing the dissipation that scales with surface-to-volume ratio and dominates at small widths, against the interaction with the underlying substrate via the air that dominates the behavior of the wider devices. This latter interaction is found to affect the Knudsen number corresponding to a transition out of the molecular damping regime. We examine higher order modes and tune tension mechanically to vary the frequency of individual resonators, to resolve size and frequency effects.

Single particle detection in CMOS compatible photonic crystal nanobeam cavities

We report the label-free detection of single particles using photonic crystal nanobeam cavities fabricated in silicon-on-insulator platform, and embedded inside microfluidic channels fabricated in poly-dimethylsiloxane (PDMS). Our system operates in the telecommunication wavelength band, thus leveraging the widely available, robust and tunable telecom laser sources. Using this approach, we demonstrated the detection of polystyrene nanoparticles with dimensions down to 12.5nm in radius. Furthermore, binding events of a single streptavidin molecule have been observed.

Enhanced photoluminescence from embedded PbSe colloidal quantum dots in silicon-based random photonic crystal microcavities

The experimental observation of enhanced photoluminescence from high-Q silicon-based random photonic crystal microcavities embedded with PbSe colloidal quantum dots is being reported. The emission is optically excited at room temperature by a continuous-wave Ti-sapphire laser and exhibits randomly distributed localized modes with a minimum spectral linewidth of 4nm at 1.5μm wavelength.

MultiFocus Polarization Microscope (MF-PolScope) for 3D polarization imaging of up to 25 focal planes simultaneously

We have developed an imaging system for 3D time-lapse polarization microscopy of living biological samples. Polarization imaging reveals the position, alignment and orientation of submicroscopic features in label-free as well as fluorescently labeled specimens. Optical anisotropies are calculated from a series of images where the sample is illuminated by light of different polarization states. Due to the number of images necessary to collect both multiple polarization states and multiple focal planes, 3D polarization imaging is most often prohibitively slow. Our MF-PolScope system employs multifocus optics to form an instantaneous 3D image of up to 25 simultaneous focal-planes. We describe this optical system and show examples of 3D multi-focus polarization imaging of biological samples, including a protein assembly study in budding yeast cells.

Out-of-plane scattering from vertically asymmetric photonic crystal slab waveguides with in-plane disorder

We characterize optical wave propagation along line defects in two-dimensional arrays of air-holes in free-standing silicon slabs. The fabricated waveguides contain random variations in orientation of the photonic lattice elements which perturb the in-plane translational symmetry. The vertical slab symmetry is also broken by a tilt of the etched sidewalls. We discuss how these lattice imperfections affect out-of-plane scattering losses and introduce a mechanism for high-Q cavity excitation related to polarization mixing.

Modeling, design, and characterization of multisegment cantilevers for resonant mass detection

The work presents the analytic modeling, design, and the experimental and numerical characterization of multisegment cantilevers’ bending and torsion resonant responses with the aim of evaluating the amount and position of matter which deposits on these elastic structures. The cantilevers may comprise any number of geometrically different segments that are serially connected, and actual results are presented for the two-segment, circularly notched configuration. The generic analytical model, which formulates the bending and torsion resonant frequencies of both the original cantilever and the one with the attached mass, is derived by means of Rayleigh’s quotient approach. Relationships are formulated between nondimensional parameters characterizing the geometry, resonant frequencies, and deposited mass amount and position for both bending and torsion. The sensitivity of the frequency shift to the landing parameters and mass amount are also studied. Analytical model, finite element, and experimental testing...

Single molecule correlation spectroscopy in continuous flow mixers with zero-mode waveguides

Zero-Mode Waveguides were first introduced for Fluorescence Correlation Spectroscopy at micromolar dye concentrations. We show that combining zero-mode waveguides with fluorescence correlation spectroscopy in a continuous flow mixer avoids the compression of the FCS signal due to fluid transport at channel velocities up to approximately 17 mm/s. We derive an analytic scaling relationship [equation: see text] converting this flow velocity insensitivity to improved kinetic rate certainty in time-resolved mixing experiments. Thus zero-mode waveguides make FCS suitable for direct kinetics measurements in rapid continuous flow.