Optofluidic notch filter integration by lift-off of thin films

Liquid Core ARROW Waveguides: A Promising Photonic Structure for Integrated Optofluidic Microsensors

In this paper, we introduce a liquid core antiresonant reflecting optical waveguide (ARROW) as a novel optofluidic device that can be used to create innovative and highly functional microsensors. Liquid core ARROWs, with their dual ability to guide the light and the fluids in the same microchannel, have shown great potential as an optofluidic tool for quantitative spectroscopic analysis. ARROWs feature a planar architecture and, hence, are particularly attractive for chip scale integrated system. Step by step, several improvements have been made in recent years towards the implementation of these waveguides in a complete on-chip system for highly-sensitive detection down to the single molecule level. We review applications of liquid ARROWs for fluids sensing and discuss recent results and trends in the developments and applications of liquid ARROW in biomedical and biochemical research. The results outlined show that the strong light matter interaction occurring in the optofluidic channel of an ARROW and the versatility offered by the fabrication methods makes these waveguides a very promising building block for optofluidic sensor development.

Rib waveguides for trapping and transport of particles

Rib waveguides are investigated as an alternative to strip waveguides for planar trapping and transport of microparticles. Microparticles are successfully propelled along the surface of rib waveguides and trapped in the gap between opposing rib waveguides. The trapping capabilities of waveguide end facets formed by a single and opposing waveguide geometries are investigated. The slab beneath a rib waveguide continues to guide light after the end facet of a rib waveguide. Thus particles can be trapped in wider gaps formed by opposing rib waveguides than with strip waveguides. Rib waveguides were found more efficient in trapping a collection of particles in the gap and particles could be moved to different locations in the gap by changing the relative power in the two opposing rib waveguides. Numerical simulations are used to show that the trapping efficiency on the surface of rib and strip waveguides is comparable. The simulations also confirm the advantage of opposing rib waveguides for trapping particles in wide gaps. The low sidewalls of rib waveguides give low propagation losses and make it easy to integrate rib waveguides with other functions in a lab-on-a-chip where particle trapping and transport is required.

Terahertz wave transmission within metal-clad antiresonant reflecting hollow waveguides

We present the transmission characteristics of THz waves in metal-clad antiresonant reflecting hollow waveguides. We have derived the equation for the blueshift of the resonance frequency. The effects of the waveguide structure on the blueshift of the resonance frequency are studied comprehensively. In particular, we find that the blueshift of the resonance frequency is strongly affected by the interval between two dielectric slabs. By changing the interval, we find that the maximum frequency-tuning range is up to 2030 GHz and the maximum sensitivity of the resonance frequency shift is up to 6950 GHz/mm at the resonance order of m=1. When the THz wave is at the near-zero loss frequency, both the loss and the dispersion of the guide modes are very low.

Dual-core optofluidic chip for independent particle detection and tunable spectral filtering

We present the first integration of fluidically tunable filters with a separate particle detection channel on a single planar, optofluidic chip. Two optically connected, but fluidically isolated liquid-core antiresonant reflecting optical waveguide (ARROW) segments serve as analyte and spectral filter sections, respectively. Ultrasensitive detection of fluorescent nanobeads with high signal-to-noise ratio provided by a fluidically tuned excitation notch filter is demonstrated. In addition, reconfigurable filter response is demonstrated using both core index tuning and bulk liquid tuning. Notch filters with 43 dB rejection ratio and a record 90 nm tuning range are implemented by using different mixtures of ethylene glycol and water in the filter section. Moreover, absorber dyes and liquids with pH-dependent transmission in the filter channel provide additional spectral control independent of the waveguide response. Using both core index and pH control, independent filter tuning at multiple wavelengths is demonstrated for the first time. This extensive on-chip control over spectral filtering as one of the fundamental components of optical particle detection techniques offers significant advantages in terms of compactness, cost, and simplicity, and opens new opportunities for waveguide-based optofluidic analysis systems.

Tailoring the spectral response of liquid waveguide diagnostic platforms

Liquid filled waveguides that form the basis for on-chip biophotonics diagnostic platforms have primarily found application in fluorescence and Raman spectroscopy experiments that require sensitive discrimination between weak analyte signals and a variety of background signals. Primary sources of background signal can include light from excitation sources (strong, narrow frequency band) and photoluminescence generated in waveguide cladding layers (weak, wide frequency band). Here we review both solid and liquid core filtering structures which are based on anti-resonant reflection that can be integrated with waveguides for attenuating undesirable optical bands. Important criteria to consider for an optimized biosensor include cladding layer materials that minimize broad-spectrum photoluminescence and optimize layer thicknesses for creating a desired spectral response in both solid and liquid guiding layers, and a microfabrication process capable of producing regions with variable spectral response. New results describing how spurious fluorescence can be minimized by optimized thermal growth conditions and how liquid-core filter discrimination can be tuned with liquid core waveguide length are presented.

Dual-color fluorescence cross-correlation spectroscopy on a planar optofluidic chip

Fluorescence cross-correlation spectroscopy (FCCS) is a highly sensitive fluorescence technique with distinct advantages in many bioanalytical applications involving interaction and binding of multiple components. Due to the use of multiple beams, bulk optical FCCS setups require delicate and complex alignment procedures. We demonstrate the first implementation of dual-color FCCS on a planar, integrated optofluidic chip based on liquid-core waveguides that can guide liquid and light simultaneously. In this configuration, the excitation beams are delivered in predefined locations and automatically aligned within the excitation waveguides. We implement two canonical applications of FCCS in the optofluidic lab-on-chip environment: particle colocalization and binding/dissociation dynamics. Colocalization is demonstrated in the detection and discrimination of single-color and double-color fluorescently labeled nanobeads. FCCS in combination with fluorescence resonance energy transfer (FRET) is used to detect the denaturation process of double-stranded DNA at nanomolar concentration.

Tailorable integrated optofluidic filters for biomolecular detection

Spectral filtering is an essential component of biophotonic methods such as fluorescence and Raman spectroscopy. Predominantly utilized in bulk microscopy, filters require efficient and selective transmission or removal of signals at one or more wavelength bands. However, towards highly sensitive and fully self-contained lab-on-chip systems, the integration of spectral filters is an essential step. In this work, a novel optofluidic solution is presented in which a liquid-core optical waveguide both transports sample analytes and acts as an efficient filter for advanced spectroscopy. To this end, the wavelength dependent nature of interference-based antiresonant reflecting optical waveguide technology is exploited. An extinction of 37 dB, a narrow rejection band of only 2.5 nm and a free spectral range of 76 nm using three specifically designed dielectric layers are demonstrated. These parameters result in an 18.4-fold increase in the signal-to-noise ratio for on-chip fluorescence detection. In addition, liquid-core waveguide filters with three operating wavelengths were designed for Förster resonance energy transfer detection and demonstrated using doubly labeled oligonucleotides. Incorporation of high-performance spectral processing illustrates the power of the optofluidic concept where fluidic channels also perform optical functions to create innovative and highly integrated lab-on-chip devices.

Wavelength-selective filter based on a hollow optical waveguide

In this paper, we describe a theoretical and experimental study of a wavelength-selective filter derived from hollow optical waveguides composed of Bragg reflectors with defect layers on a silicon substrate. The defect states of the transmission filter at wavelengths of 1519 and 1571 nm were realized using one-dimensional photonic crystals (1D PCs) formed from a-Si and SiO(2). The transmission spectra of the filter waveguides and the band structure of the defect 1D PCs were calculated using the two-dimensional finite-difference time-domain and transfer matrix methods, respectively. The device exhibited the narrow bandwidths of 0.5 and 1.1 nm for wavelengths of 1571 and 1519 nm, respectively.

Application of metal-clad antiresonant reflecting hollow waveguides to tunable terahertz notch filter

A novel tunable terahertz notch filter is demonstrated using antiresonant reflecting hollow waveguides with movable metal layers outside dielectric claddings. Based on the Fabry-Pérot resonance of the dielectric cladding, multiple deep notches are observed in a broad THz transmission spectrum. Continuous shift of notch frequencies is for the first time experimentally observed by lateral translation of metal layers from dielectric claddings. The measured maximum frequency-tuning-range approached 60GHz, equaling to 50% of the bandwidth of every passband, and a 20dB rejection notch-depth with a linewidth as narrow as 6GHz at frequency of around 0.2THz was also achieved. Numerical simulations match the measurements and verify the spectral-tuning mechanism.