Charles J. Divin

Real-time biomolecular binding detection using a sensitive photonic crystal biosensor.

Real-time measurement of specific biomolecular interactions is critical to many areas of biological research. A number of label-free techniques for directly monitoring biomolecular binding have been developed, but it is still challenging to measure the binding kinetics of very small molecules, to detect low concentrations of analyte molecules, or to detect low affinity interactions. In this study, we report the development of a highly sensitive photonic crystal biosensor for label-free, real-time biomolecular binding analysis. We characterize the performance of this biosensor using a standard streptavidin-biotin binding system. Optimization of the surface functionalization methods for streptavidin immobilization on the silica sensing surface is presented, and the specific binding of biotinylated analyte molecules ranging over 3 orders of magnitude in molecular weight, including very small molecules (<250 Da), DNA oligonucleotides, proteins, and antibodies (>150 000 Da), are detected in real time with a high signal-to-noise ratio. Finally, we document the sensor efficiency for low mass adsorption, as well as multilayered molecular interactions. By all important metrics for sensitivity, we anticipate this photonic crystal biosensor will provide new capabilities for highly sensitive measurements of biomolecular binding.

Power scalable compact THz system based on an ultrafast Yb-doped fiber amplifier

A power-scalable approach for THz generation is demonstrated using optical rectification in GaP pumped by a high power ultrafast Yb-doped fiber amplifier operating at 1.055 mum. A 120-MHz-repetition-rate pulse train of single-cycle THz radiation with 6.5 muW average power is generated using 10 W from a parabolic fiber amplifier. Analysis of the THz power scalability indicates that due to the unique advantages offered by ultrafast optical rectification in GaP and due to the power scalability of fiber lasers, this approach has the potential to generate single-cycle THz pulse trains with average powers up to several mW.

Coherent Control of Ballistic Photocurrents in Multilayer Epitaxial Graphene Using Quantum Interference

We report generation of ballistic electric currents in unbiased epitaxial graphene at 300 K via quantum interference between phase-controlled cross-polarized fundamental and second harmonic 220 fs pulses. The transient currents are detected via the emitted terahertz radiation. Because of graphenes special structure symmetry, the injected current direction can be well controlled by the polarization of the pump beam in epitaxial graphene. This all optical injection of current provides not only a noncontact way of injecting directional current in graphene but also new insight into optical and transport process in epitaxial graphene.

Sensitive molecular binding assay using a photonic crystal structure in total internal reflection

A novel optical sensor for label-free biomolecular binding assay using a one-dimensional photonic crystal in a total-internal-reflection geometry is proposed and demonstrated. The simple configuration provides a narrow optical resonance to enable sensitive measurements of molecular binding, and at the same time employs an open interface to enable real-time measurements of binding dynamics. Ultrathin aminopropyltriethoxysilane/ glutaraldehyde films adsorbed on the interface were detected by measuring the spectral shift of the photonic crystal resonance and the intensity ratio change in a differential reflectance measurement. A detection limit of 6 x 10(-5) nm for molecular layer thickness was obtained, which corresponds to a detection limit for analyte adsorption of 0.06 pg/mm(2) or a refractive index resolution of 3 x 10(-8) RIU; this represents a significant improvement relative to state-of-the-art surface-plasmon-resonance-based systems.

GaP waveguide emitters for high power broadband THz generation pumped by Yb-doped fiber lasers

We demonstrate the generation of broadband THz pulses by optical rectification in GaP waveguides pumped by high power Yb-doped fiber amplifiers. The dispersion of the GaP emitter can be controlled via the geometry of the waveguide; the peak frequency of the emitted THz radiation is tuned by varying the waveguide cross-section. Most importantly, the use of a waveguide for the THz emission increases the coherent buildup length of the THz pulses and offers scalability to higher power; this was investigated by pumping a GaP waveguide emitter with a high power Yb-doped fiber laser system. A 25-MHz-repetition-rate pulse train of THz radiation with 120 muW average power was achieved using 14 W optical power, which represents the highest average power for a broadband THz source pumped by fiber lasers to date.

Generation of radially polarized terahertz pulses via velocity-mismatched optical rectification

We demonstrate the generation of radially polarized terahertz pulses via optical rectification in a Cherenkov geometry exploiting velocity mismatch, contrary to the traditional approach for generating linearly polarized terahertz beams. A compact system is implemented using 001-cut ZnTe pumped by an ultrafast Yb-doped fiber amplifier.

Current relaxation due to hot carrier scattering in graphene

In this paper, we present direct time-domain investigations of the relaxation of electric currents in graphene due to hot carrier scattering. We use coherent control with ultrashort optical pulses to photoinject a current and detect the terahertz (THz) radiation emitted by the resulting current surge. We pre-inject a background of hot carriers using a separate pump pulse, with a variable delay between the pump and current-injection pulses. We find the effect of the hot carrier background is to reduce the current and hence the emitted THz radiation. The current damping is determined simply by the density (or temperature) of the thermal carriers. The experimental behavior is accurately reproduced in a microscopic theory, which correctly incorporates the nonconservation of

Whole spectrum fluorescence detection with ultrafast white light excitation

We use a single supercontinuum laser source to simultaneously excite multiple types of fluorescent dyes and collect the entire emission spectrum ranging from visible to near infrared, without using band pass filters and dichroic mirrors.

Electronic cooling via interlayer Coulomb coupling in multilayer epitaxial graphene

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. As a result, electron–phonon interactions occur primarily within layers and interlayer electrical conductivities are low. In addition, strong covalent in-plane intralayer bonding combined with weak van der Waals interlayer bonding results in weak phonon-mediated thermal coupling between the layers. We demonstrate here, however, that Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport, even though all electronic states are strongly confined within individual 2D layers. This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb coupling containing no free parameters that accounts for the experimentally observed trends in hot-carrier dynamics as temperature and the number of layers is varied.

Time-reversal and model-based imaging in a THz waveguide.

We investigate two approaches to improving the resolution of time-reversal based THz imaging systems. First, we show that a substantial improvement in the reconstruction of time-reversed THz fields is achieved by increasing the systems numerical aperture via a waveguide technique adapted from ultrasound imaging. Second, a model-based reconstruction algorithm is developed as an alternative to time-reversal THz imaging and its performance is demonstrated for cases with and without a waveguide.