Two-frequency CARS imaging by switching fiber laser excitation.

To fully exploit the power of coherent Raman imaging, techniques are needed to image more than one vibrational frequency simultaneously. We describe a method for switching between two vibrational frequencies based on a single fiber-laser source. Stokes pulses were generated by soliton self-frequency shifting in a photonic crystal fiber.

Phase noise characterization of a QD-based diode laser frequency comb.

We measure, simultaneously, the phases of a large set of comb lines from a passively mode locked, InAs/InP, quantum dot laser frequency comb (QDLFC) by comparing the lines to a stable comb reference using multi-heterodyne coherent detection. Simultaneity permits the separation of differential and common mode phase noise and a straightforward determination of the wavelength corresponding to the minimum width of the comb line. We find that the common mode and differential phases are uncorrelated, and measure for the first time for a QDLFC that the intrinsic differential-mode phase (IDMP) between adjacent subcarriers is substantially the same for all subcarrier pairs.

Nanophotonic modal dichroism: mode-multiplexed modulators.

As the diffraction limit is approached, device miniaturization to integrate more functionality per area becomes more and more challenging. Here we propose a strategy to increase the functionality-per-area by exploiting the modal properties of a waveguide system. With such an approach the design of a mode-multiplexed nanophotonic modulator relying on the mode-selective absorption of a patterned indium-tin-oxide (ITO) is proposed.

All-optical short pulse translation through cross-phase modulation in a VO₂ thin film.

VO2 is a promising material for reconfigurable photonic devices due to the ultrafast changes in electronic and optical properties associated with its dielectric-to-metal phase transition. Based on a fiber-optic, pump-probe setup at 1550 nm wavelength window, and by varying the pump-pulse duration, we show that the material phase transition is primarily caused by the pump-pulse energy. For the first time, we demonstrate that the instantaneous optical phase modulation of probe during pump leading edge can be utilized to create short optical pulses at probe wavelength, through optical frequency discrimination.

Simulation of the Impact of Si Shell Thickness on the Performance of Si-Coated Vertically Aligned Carbon Nanofiber as Li-Ion Battery Anode.

Micro- and nano-structured electrodes have the potential to improve the performance of Li-ion batteries by increasing the surface area of the electrode and reducing the diffusion distance required by the charged carriers. We report the numerical simulation of Lithium-ion batteries with the anode made of core-shell heterostructures of silicon-coated carbon nanofibers. We show that the energy capacity can be significantly improved by reducing the thickness of the silicon anode to the dimension comparable or less than the Li-ion diffusion length inside silicon.

Bandwidth efficient coherent lidar based on phase-diversity detection.

Bandwidth efficient coherent lidar based on phase-diversity detection is reported for the first time, to the best of our knowledge, which allows the doubling of bandwidth efficiency through the simultaneous utilization of the in-phase (I) and quadrature (Q) components. By maintaining RF phase continuity between linearly frequency-chirped I and Q components through digital signal processing, the range resolution of the lidar system can be improved.

Near-infrared electro-optic modulator based on plasmonic graphene.

We propose a novel scheme for an electro-optic modulator based on plasmonically enhanced graphene. As opposed to previously reported designs where the switchable absorption of graphene itself was employed for modulation, here a graphene monolayer is used to actively tune the plasmonic resonance condition through the modification of interaction between optical field and an indium tin oxide (ITO) plasmonic structure. Strong plasmonic resonance in the near infrared wavelength region can be supported by accurate design of ITO structures, and tuning the graphene chemical potential through electrical gating switches on and off the ITO plasmonic resonance.

Multi-modal label-free imaging based on a femtosecond fiber laser.

We demonstrate multi-mode microscopy based on a single femtosecond fiber laser. Coherent anti-Stokes Raman scattering (CARS), stimulated Raman scattering (SRS) and photothermal images can be obtained simultaneously with this simplified setup. Distributions of lipid and hemoglobin in sliced mouse brain samples and blood cells are imaged.

Fiber-optic acoustic pressure sensor based on large-area nanolayer silver diaghragm.

A fiber-optic acoustic pressure sensor based on a large-area nanolayer silver diaphragm is demonstrated with a high dynamic pressure sensitivity of 160  nm/Pa at 4 kHz frequency. The sensor exhibits a noise limited detectable pressure level of 14.5  μPa/Hz(1/2).

Embedded optical waveguides fabricated in SF10 glass by low-repetition-rate ultrafast laser.

Symmetric embedded waveguides were fabricated in heavy metal oxide SF10 glass using slit-shaped infrared femtosecond laser writing in the low-repetition frequency regime. The impact of the writing parameters on the waveguide formation in the transverse writing scheme was systemically studied. Results indicate that efficient waveguides can be inscribed in a wide parameter space ranging from 500 fs to 1.

Single fiber laser based wavelength tunable excitation for CRS spectroscopy.

We demonstrate coherent Raman spectroscopy (CRS) using a tunable excitation source based on a single femtosecond fiber laser. The frequency difference between the pump and the Stokes pulses was generated by soliton self-frequency shifting (SSFS) in a nonlinear optical fiber. Spectra of C-H stretches of cyclohexane were measured simultaneously by stimulated Raman gain (SRG) and coherent anti-Stokes Raman scattering (CARS) and compared.

Femtosecond multi-beam interference lithography based on dynamic wavefront engineering.

A method for precise multi-spot parallel ultrafast laser material structuring is presented based on multi-beam interference generated by dynamic spatial phase engineering. A Spatial Light Modulator (SLM) and digitally programming of phase masks are used to accomplish the function of a multi-facet pyramid lens, so that the laser beam can be spatially modulated to create beam multiplexing and desired two-dimensional (2D) multi-beam interference patterns. Various periodic microstructures on metallic alloy surfaces are fabricated with this technique.

Complex-optical-field lidar system for range and vector velocity measurement.

A coherent lidar system based on the measurement of complex optical field is demonstrated for the first time. An electro-optic in-phase/quadrature (I/Q) modulator is used in the lidar transmitter to realize carrier-suppressed complex optical field modulation in which the positive and the negative optical sidebands can carry independent modulation waveforms. A fiber-optic 90° hybrid is used in the lidar receiver for coherent heterodyne detection and to recover the complex optical field.

Stressed waveguides with tubular depressed-cladding inscribed in phosphate glasses by femtosecond hollow laser beams.

We report on the single-step fabrication of stressed optical waveguides with tubular depressed-refractive-index cladding in phosphate glasses by the use of focused femtosecond hollow laser beams. Tubelike low index regions appear under direct exposure due to material rarefaction following expansion. Strained compacted zones emerged in domains neighboring the tubular track of lower refractive index, and waveguiding occurs mainly within the tube core fabricated by the engineered femtosecond laser beam.

Polarization behavior of femtosecond laser written optical waveguides in Ti:Sapphire.

Ultrashort pulsed laser photoinscription of Ti:Sapphire crystals may result in the self-organization of nanoscale material redistribution regions in regular patterns within the laser trace and stress-induced birefringence around the laser trace. We report on the formation of anisotropic optical waveguides in Ti:Sapphire by a procedure that involves femtosecond laser inscription of adjacent nonguiding birefringent traces with nanopatterned crosssections and the accumulation of stress birefringence in the region between. Double parallel line structures with a separation of 25μm with vertical and horizontal nanoscale arrangements were written with a choice of orthogonal polarizations.

Surface-passivated plasmonic nano-pyramids for bulk heterojunction solar cell photocurrent enhancement.

We report that self-assembled gold (Au) nanopyramid arrays can greatly enhance the photocurrent of narrow bandgap organic solar cells using their plasmonic near-field effect. The plasmonic enhanced power conversion efficiency exhibited up to 200% increase under the AM 1.5 solar illumination.

Plasmonic graphene transparent conductors.

Plasmonic graphene is fabricated using thermally assisted self-assembly of silver nanoparticles on graphene. The localized surface-plasmonic effect is demonstrated with the resonance frequency shifting from 446 to 495 nm when the lateral dimension of the Ag nanoparticles increases from about 50 to 150 nm. Finite-difference time-domain simulations are employed to confirm the experimentally observed light-scattering enhancement in the solar spectrum in plasmonic graphene and the decrease of both the plasmonic resonance frequency and amplitude with increasing graphene thickness.

Development of nanopatterned fluorine-doped tin oxide electrodes for dye-sensitized solar cells with improved light trapping.

Transparent conductors (TCs) are an important component of optoelectronic devices and nanoscale engineering of TCs is important for optimization of the device performance through improved light trapping. In this work, patterned periodic arrays of nanopillars and nanolines of pitch size of ~700 nm were created on fluorine-doped tin oxide (FTO) using nanoimprint lithography and reactive ion etching using environmentally friendly gases. The patterned FTO exhibits enhanced light trapping as compared to the unpatterned FTO, which agrees well with simulations based on Finite-Difference Time-Domain method for up to a distance of 4 μm.

Fiber laser based two-photon FRET measurement of calmodulin and mCherry-E(0)GFP proteins.

The speed and accuracy of Förster resonance energy transfer (FRET) measurements can be improved by rapidly alternating excitation wavelengths between the donor and acceptor fluorophore. We demonstrate FRET efficiency measurements based on a fiber laser and photonic crystal fiber as the source for two-photon excitation (TPE). This system offers the potential for rapid wavelength switching with the benefits of axial optical sectioning and improved penetration depth provided by TPE.

Tunable excitation source for coherent Raman spectroscopy based on a single fiber laser.

We demonstrate a wavelength tunable optical excitation source for coherent Raman scattering (CRS) spectroscopy based on a single femtosecond fiber laser. Electrically controlled wavelength tuning of Stokes optical pulses was achieved with soliton self frequency shift in an optical fiber, and linear frequency chirping was applied to both the pump and the Stokes waves to significantly improve the spectral resolution. The coherent anti-Stokes Raman scattering (CARS) spectrum of cyclohexane was measured and vibrational resonant Raman peaks separated by 70 cm(-1) were clearly resolved.

Digital subcarrier multiplexing for flexible spectral allocation in optical transport network.

We demonstrate a spectrally efficient digital subcarrier multiplexed (DSCM) coherent optical system for optical transport network. In the proposed system, mutually orthogonal subcarrier channels are digitally generated, which allows a high degree of flexibility in bandwidth allocation and scalability in data rate granularity. The receiver can also dynamically change the number of DSCM channels for detection without changing the system configuration.

[Review on label-free optical bio-sensing technology based on whisper-gallery-mode].

Optical biosensors are becoming an important tool for drug research and life science, and the label-free optical biosensor based on whisper-gallery-mode (WGM) is reviewed in the present paper. The WGM-based sensors are categorized into three types according to the microcavity structure. The biosensor using microsphere got extensive research because of high quality factor, and its response to protein, virus, and bacteria had been studied.

Theoretical and experimental investigation of compatible SSB modulation for single channel long-distance optical OFDM transmission.

We present the first experimental demonstration of compatible single-sideband (compatible-SSB) modulated OFDM optical system at 11.1Gb/s data rate for long distance transmission over 675 km uncompensated standard single-mode fiber. Compatible-SSB modulation employing a simple dual-drive Mach-Zehnder modulator (MZM) in the transmitter and direct-detection at the receiver provides an OFDM system implementation with reduced complexity.

OFDM system implementation using compatible SSB modulation with a dual-electrode MZM.

Single-sideband (SSB) direct-detection optical orthogonal frequency division multiplexing (OFDM) transmission is experimentally demonstrated in a system with 125 km fiber length, which does not need a guard band between the carrier and the OFDM band. The waveforms of compatible SSB modulation are analytically derived and realized through a single dual-electrode Mach-Zehnder modulator. The required optical signal-to-noise ratios for different modulation indices on the OFDM band are discussed for optimal operation.