In nature, arthropods have a remarkably sophisticated class of imaging systems, with a hemispherical geometry, a wideangle field of view, low aberrations, high acuity to motion and an infinite depth of field. There are great interests in building systems with similar geometries and properties due to numerous potential applications. However, the established semiconductor sensor technologies and optics are essentially planar, which experience great challenges in building such systems with hemispherical, compound apposition layouts. With the recent advancement of stretchable optoelectronics, we have successfully developed strategies to build a fully functional artificial apposition compound eye camera by combining optics, materials and mechanics principles. The strategies start with fabricating stretchable arrays of thin silicon photodetectors and elastomeric optical elements in planar geometries, which are then precisely aligned and integrated, and elastically transformed to hemispherical shapes. This imaging device demonstrates nearly full hemispherical shape (about 160 degrees), with densely packed artificial ommatidia. The number of ommatidia (180) is comparable to those of the eyes of fire ants and bark beetles. We have illustrated key features of operation of compound eyes through experimental imaging results and quantitative ray-tracing-based simulations. The general strategies shown in this development could be applicable to other compound eye devices, such as those inspired by moths and lacewings (refracting superposition eyes), lobster and shrimp (reflecting superposition eyes), and houseflies (neural superposition eyes).
Compound eyes in arthropods demonstrate distinct imaging characteristics from human eyes, with wide angle field of view, low aberrations, high acuity to motion and infinite depth of field. Artificial imaging systems with similar geometries and properties are of great interest for many applications. However, the challenges in building such systems with hemispherical, compound apposition layouts cannot be met through established planar sensor technologies and conventional optics. We present our recent progress in combining optics, materials, mechanics and integration schemes to build fully functional artificial compound eye cameras. Nearly full hemispherical shapes (about 160 degrees) with densely packed artificial ommatidia were realized. The number of ommatidia (180) is comparable to those of the eyes of fire ants and bark beetles. The devices combine elastomeric compound optical elements with deformable arrays of thin silicon photodetectors, which were fabricated in the planar geometries and then integrated and elastically transformed to hemispherical shapes. Imaging results and quantitative ray-tracing-based simulations illustrate key features of operation. These general strategies seem to be applicable to other compound eye devices, such as those inspired by moths and lacewings (refracting superposition eyes), lobster and shrimp (reflecting superposition eyes), and houseflies (neural superposition eyes).
We show that the optical modes of thin polymer slabs doped with J-aggregating dye molecules are strongly coupled
exciton - waveguide photon modes. The hybridization appears as splitting in the dispersion relation of the fundamental
transverse electric (TE) and transverse magnetic (TM) modes of the system. It is shown that these modes can be
controlled by changing the thickness of the optical waveguide and by changing the concentration of the dye molecules.
Leaky conical emission from the strongly coupled modes with radial and azimuthl polarizations is captured.
The rapid development of microfluidic devices in recent years has led to a huge number
of applications in chemistry, biology and interdisciplinary areas. This is because they
act as miniaturized platforms in which sorting, mixing, reaction and measurement can
be achieved in a precise and rapid manner. Being able to both understand and measure
the pressure of fluids inside these devices is very important, especially in the cases
where multiphase flows are involved. For example, certain advanced micromixing
technologies demand accurate evaluations of bubble-induced extra pressure, since the
pressure contribution from one bubble is likely to impact the velocity and residence time
of others, affecting the mixing efficiency and quality in a complicated manner. Similarly,
in some microfluidics-based biochemical analysis, extra pressure brought about by
droplets is a critical factor in the design of on-chip pumping, as high throughput
experiments involving continuous supply of large numbers of droplets often require a
considerable enhancement in the pumping pressure necessary to maintain the droplet
flow3. Last, state-of-the-art microfluidic logic devices rely heavily on the pressure
distribution inside the channels, which automatically controls the paths of each droplet
in the microfluidic network and as a result determines the "on" and "off" of each switch.
A few techniques to measure pressure change or pressure drop in microfluidic channels
have been developed. Examples include connecting the device to commercially
available pressure sensors and comparing pressures of different areas by analyzing the position of fluid-fluid interface. However, all of those methods have intrinsic drawbacks in one or more aspects that considerably limit their applications. A significant one is that they are primarily aiming at measuring or comparing pressures over relatively long channels (~10 mm), and are hence only designed to work in the highpressure range, i.e. to detect a pressure change on the order of tens or hundreds of Pascals. Moreover, the long channels make it rather challenging to look into the detailed dynamics of pressure variations caused by inhomogeneous emulsions, since such a long section invariably contains multiple elements, for instance droplets, of the emulsion flow, and the measurements average out the behavior of one single element. Consequently, to further reveal the characteristics of flows in microfluidics, it is highly desirable for a pressure measurement device to work in the low-pressure range, and to resolve pressure changes "locally", i.e. within small spatial regions.
We demonstrate an on-chip microparticle passive sorting device employing a 3-dB optical splitter that consists of a slot
waveguide and a conventional channel waveguide. Simulations indicate that the optical force in the vertical direction
exerted on small particles (d < 1 μm) by the slot waveguide is larger than that by the channel waveguide. On the other
hand, the channel waveguide provides a stronger vertical force on large particles (d > 1 μm) than the slot waveguide. The
in-plane optical force provides a double-well trapping potential for small particles only. We perform experiments in
which small (320 nm diameter) and large (2 μm diameter) particles are brought to the splitter by the channel waveguide.
Due to a structural perturbation provided by a stuck bead, the small particles are transferred to the slot waveguide. The
large particles remain on the channel waveguide. The automated nature of this method, along with the low guided power
employed (20 mW in these experiments), makes this a promising approach for sorting sub-micron particles.
We describe a means for improving the performance of CMOS image sensors using vertical waveguides, known
as light pipes. We describe experimental results on the etching of silicon nitride pillars, and the fabrication of
This work describes the development and fabrication of a novel nanofluidic flow-through sensing chip that utilizes a
plasmonic resonator to excite fluorescent tags with sub-wavelength resolution. We cover the design of the microfluidic
chip and simulation of the plasmonic resonator using Finite Difference Time Domain (FDTD) software. The fabrication
methods are presented, with testing procedures and preliminary results.
This research is aimed at improving the resolution limits of the Direct Linear Analysis (DLA) technique developed by
US Genomics . In DLA, intercalating dyes which tag a specific 8 base-pair sequence are inserted in a DNA sample.
This sample is pumped though a nano-fluidic channel, where it is stretched into a linear geometry and interrogated with
light which excites the fluorescent tags. The resulting sequence of optical pulses produces a characteristic "fingerprint"
of the sample which uniquely identifies any sample of DNA. Plasmonic confinement of light to a 100 nm wide metallic
nano-stripe enables resolution of a higher tag density compared to free space optics. Prototype devices have been
fabricated and are being tested with fluorophore solutions and tagged DNA. Preliminary results show evanescent
coupling to the plasmonic resonator is occurring with 0.1 micron resolution, however light scattering limits the S/N of
the detector. Two methods to reduce scattered light are presented: index matching and curved waveguides.
We report the development of new fabrication techniques for creating high aspect ratio optical lightpipes in SiO2 layers
of 10μm thickness and above. A dielectric photo mask was used for deep reactive ion etching. Our experiments show
that CF4-based reaction gases were best for deep etching with high selectivity and etch rate. Trenches with diameters or
width of 1.5μm were demonstrated, with an aspect ratio of 7.2:1 and a sidewall angle of 87.4 degrees. We also present
the lift-off process of the etch masks and the via-filling procedures for the lightpipes. These structures are useful for
image sensors, vertical interconnect and waveguiding applications.
Nanoscale metal stripes support plasmonic modes with strongly confined fields. When combined with a waveguide-coupled
microfluidic system, these stripes can provide highly sub-wavelength excitation regions for single biomolecule
sensing, e.g. dark-field fluorescence detection of tagged DNA. Using a prism coupled geometry, we experimentally
characterize the dispersion of plasmon modes in 80-500 nm wide metal stripes. Our results agree well with numerical
modeling. We investigate how the stripe morphology affects the mode distribution and dispersion, and consider the
implications for integrated near field fluorescence excitation.
Using a Fresnel zone plate, we demonstrate optical trapping with a larger numerical aperture than is commonly available with commercial objective lenses. The zone plate is fabricated onto the inner wall of the fluidic cell and, consequently, focusing is free from on axis aberrations due to an absence of dielectric interfaces. Using zone plates with extremely large focusing angles, we observe an enhanced ellipticity in the trapping volume. For a zone plate with a numerical aperture of 0.986nwater (1.308), we observe a trapping stiffness that is more than four times stiffer perpendicular to the polarization than parallel to the polarization. By rotating the incident linear polarization state, the trapping stiffness along a given direction can be modulated by a factor of four. The ellipticity in the focal volume is due to the presence of an axial field component whose magnitude is proportional to the sine of the focusing angle of the lens.
We report on the fabrication, experimental characterization and modeling of atomic force microscope (AFM) probes
with pyramidal optical antennas fabricated at the ends of the tips. These are being developed for tip-enhanced near-field
scanning optical microscopy. We use focused ion beam milling to etch a gold-coated Si3N4 AFM tip, resulting in a
pyramidal gold nanoparticle (188 - 240 nm long) at the end of the tip. Using finite-difference time-domain (FDTD)
simulations, we estimate the electric field distribution around the nanoparticle as a function of incident wavelength for
nanoparticles of various lengths. We experimentally measure the scattering spectra of fabricated probes and show
enhanced scattering associated with the localized surface plasmon resonance of the tip. Both simulations and
experiments show that an increase of the tip length results in a redshift of the tip resonance wavelength. These pyramidal
metal nanoparticle tips could be used for either mapping the field distribution of nanophotonic devices or high spatial
We present a technique in which atomic force microscopy (AFM) at ultrasonic frequencies is used to measure the contact stiffness between an AFM tip and thin films on silicon substrates. In this method, the resonance frequencies of the cantilever flexural modes are used to determine the tip-sample contact stiffness. We present experimental results, showing that the contact stiffness is highly sensitive to the thickness of thin metal and polymer films. These results are compared with those from out theoretical model, which we call the Contact Stiffness Algorithm (CSA), that may be used to calculate the contact stiffness between an AFM tip and an arbitrarily layered sample. Unlike transmission electron microscopy (TEM) or scanning electron microscopy (SEM) on a cross-section of the sample, this film thickness measurement technique is non- destructive. It is also capable of high lateral spatial resolution, provided that a sharp AFM tip is used. We present images of a photoresist film on silicon with contrast resulting from the elastic properties of the sample.
Circumventing the optical diffraction limit is important for data storage, photolithography, and imaging. The Solid Immersion Lens (SIL), in which light is focussed through a high refractive-index lens held close to a sample, offers a method for improving resolution. Microfabrication of SILs enables the use of lenses as small as a few wavelengths in diameter. When these are integrated onto cantilevers, the SIL can be scanned over topography with control of lens- sample separation. We have extended Mie theory to investigate aberration in small lenses and the effect of lens size on a converging beam. We also report progress on the fabrication and first results from micromachined silicon SILs.