Raman detection of cell proliferation probes with antiresonance-guiding hollow fibers.

Antiresonance-guiding hollow-core fibers are shown to enable highly sensitive detection of cell proliferation probes using Raman scattering within the region where the cellular Raman activity is minimal. We demonstrate that such fibers can substantially reduce the level of the background compared to standard index-guiding optical fibers, thus radically improving the sensitivity of Raman detection of DNA synthesis in cells and offering a powerful tool for fiber-based live-cell imaging.

Multiwatt octave-spanning supercontinuum generation in multicore photonic-crystal fiber.

High-power supercontinuum spanning over more than an octave was generated using a high power femtosecond fiber laser amplifier and a multicore nonlinear photonic crystal fiber (PCF). Long multicore PCFs (as long as 20 m in our experiments) are shown to enable supercontinuum generation in an isolated fundamental supermode, with the manifold of other PCF modes suppressed due to the strong evanescent fields coupling between the cores, providing a robust 5.4 W coherent supercontinuum output with a high spatial and spectral quality within the range of wavelengths from 500 to 1700 nm.

Plasmonic waveguiding in a hexagonally ordered metal wire array.

We propose the inclusion of a structured pattern of nanoscale metal wires in a silica fiber to form a symmetric plasmonic waveguide. The surface plasmon polariton modes within the waveguide are studied by varying the wire diameter and spacing. Simulation results show that hybridization of the single-wire mode and the gap plasmon mode can yield a hybrid mode with optimum propagation lengths comparable to those reported for other structures but with better light confinement. The fiber can be easily doped with a gain material to offset the loss so that the resultant waveguide will be useful for integration with electronic circuits at nanometer dimensions.

Generation of 150 MW, 110 fs pulses by phase-locked amplification in multicore photonic crystal fiber.

We use seven-core Yb-doped large-mode-area (LMA) photonic crystal fiber (PCF) to demonstrate phase-locked amplification of 0.7 W, 1 MHz, 1.9 ps laser pulses delivered by an LMA-PCF-laser-LMA-PCF-preamplifier system. Compression of the 24 W output of the multicore LMA-PCF amplifier with a grating compressor yields 110 fs pulses with a peak power up to 150 MW.

Hybrid multicore photonic-crystal fiber for in-phase supermode selection.

We examine a hybrid multicore photonic-crystal fiber, where the cores are separated by high-index solid rods and the microstructure cladding is built on a hexagonal lattice of air holes in silica. Antiresonant reflection from high-index solid rods is shown to assist the field confinement in the cores of such a fiber. When the cores are doped with a laser-active material, the maximum gain is achieved for the in-phase supermode, which translates into a high-quality Gaussian-like beam profile in the far field.

Stabilized soliton self-frequency shift and 0.1- PHz sideband generation in a photonic-crystal fiber with an air-hole-modified core.

Fiber dispersion and nonlinearity management strategy based on a modification of a photonic-crystal fiber (PCF) core with an air hole is shown to facilitate optimization of PCF components for a stable soliton frequency shift and subpetahertz sideband generation through four-wave mixing. Spectral recoil of an optical soliton by a red-shifted dispersive wave, generated through a soliton instability induced by high-order fiber dispersion, is shown to stabilize the soliton self-frequency shift in a highly nonlinear PCF with an air-hole-modified core relative to pump power variations. A fiber with a 2.3-microm-diameter core modified with a 0.9-microm-diameter air hole is used to demonstrate a robust soliton self-frequency shift of unamplified 50-fs Ti: sapphire laser pulses to a central wavelength of about 960 nm, which remains insensitive to variations in the pump pulse energy within the range from 60 to at least 100 pJ. In this regime of frequency shifting, intense high- and low-frequency branches of dispersive wave radiation are simultaneously observed in the spectrum of PCF output. An air-hole-modified-core PCF with appropriate dispersion and nonlinearity parameters is shown to provide efficient four-wave mixing, giving rise to Stokes and anti-Stokes sidebands whose frequency shift relative to the pump wavelength falls within the subpetahertz range, thus offering an attractive source for nonlinear Raman microspectroscopy.

Two-dimensional coherent superposition of blue-shifted signals from an array of highly nonlinear waveguiding wires in a photonic-crystal fiber.

Frequency-shifted dispersive optical waves generated as a result of soliton dynamics of 30-fs Ti: sapphire-laser pulses in an array of waveguiding wires, implemented on a platform of a photonic-crystal fiber (PCF), are shown to produce regular stable interference patterns with high visibility, indicating a high coherence of frequency-shifted fields. For a hexagonal array of waveguides built into a silica PCF, the field intensity at the main peak of a six-beam interference pattern was found to be a factor of 22 higher than the intensity of a frequency-shifted signal from an individual waveguide in the array and 3.7 times higher than the field intensity attainable through an incoherent superposition of the same fields.

Analysis and design of terahertz photonic crystal fibers by an effective-index method.

An effective-index method (EIM) is used to analyze and design photonic crystal fibers (PCFs) for the terahertz radiation. By building an analogy between a conventional optical fiber and a PCF, the EIM solves the effective index of the fiber cladding and the effective modal index of a PCF analytically. The EIM is first validated by comparison with available data in the reference, showing that the role of material dispersion is negligible at higher frequencies. Terahertz PCFs with flattened dispersion are designed based on this method and the scaling property of the Maxwell equations.

Mode-selective mapping and control of vectorial nonlinear-optical processes in multimode photonic-crystal fibers.

We demonstrate an experimental technique that allows a mapping of vectorial nonlinear-optical processes in multimode photonic-crystal fibers (PCFs). Spatial and polarization modes of PCFs are selectively excited in this technique by varying the tilt angle of the input beam and rotating the polarization of the input field. Intensity spectra of the PCF output plotted as a function of the input field power and polarization then yield mode-resolved maps of nonlinear-optical interactions in multimode PCFs, facilitating the analysis and control of nonlinear-optical transformations of ultrashort laser pulses in such fibers.

Tunable supercontinuum generation in a high-index-step photonic-crystal fiber with a comma-shaped core.

A fused silica high-index-step photonic-crystal fiber with a comma-shaped core is shown to upport two different types of guided modes with bell-shaped intensity profiles, efficiently transforming unamplified 30-fs Ti: sapphire laser pulses into supercontinuum emission through two different physical mechanisms. The modes of the first type provide broadly spanning supercontinuum emission with a smooth spectrum stretching from 450 to 1400 nm. The initial stage of supercontinuum generation in these modes involves four-wave mixing around the wavelength of zero group-velocity dispersion, leading to the depletion of the pump field. The modes of the second type generate supercontinuum with an enhanced short-wavelength wing, dominated by intense spectral lines centered at 400--450 nm. The two regimes of supercontinuum generation and the two types of output spectra are switched by displacing the input end of the fiber with respect to the laser beam in the transverse direction.

A hollow beam from a holey fiber.

A high-quality spectrally isolated hollow beam is produced through a nonlinear-optical transformation of Ti: sapphire laser pulses in a higher order mode of a photonic-crystal fiber (PCF). Instead of a doughnut shape, typical of hollow beams produced by other methods, the far-field image of the hollow-beam PCF output features perfect sixth-order rotation symmetry, dictated by the symmetry of the PCF structure. The frequency of the PCF-generated hollow beam can be tuned by varying the input beam parameters, making a few-mode PCF a convenient and flexible tool for the guiding and trapping of atoms and creation of all-fiber optical tweezers.

Perturbative and phase-transition-type modification of mode field profiles and dispersion of photonic-crystal fibers by arrays of nanosize air-hole defects.

Based on the results of a fully vectorial finite-difference analysis, we identify three important regimes of field-profile and dispersion management of photonic-crystal fibers with a solid core modified by arrays of nanosize air-hole defects. In the first regime, very small air holes act as weak perturbations, slightly modifying the field profiles of fiber modes and red-shifting the wavelength of zero group-velocity dispersion (GVD). In the second regime, larger holes reduce the effective mode area, tightening the confinement of the light field in the fiber core and blue-shifting the zero- GVD wavelength. Finally, in the third regime, the nanosize air-hole defects with diameters above a critical value induce a phase-transition-type behavior of mode field profiles, dramatically reducing the localization of the field in the fiber core and increasing the radiation power in the fiber cladding. This phase transition in mode field profiles qualitatively modifies the wavelength dependence of the effective mode area and dispersion parameters of fiber modes, especially in the long-wavelength range, suggesting an attractive strategy for fiber dispersion and mode area engineering.

Effective surface plasmon polaritons on the metal wire with arrays of subwavelength grooves.

In this paper we explore the existence of electromagnetic surface bound modes on a perfect metal wire milled with arrays of subwavelength grooves. The surface modes are axially symmetric transverse magnetic (TM) waves and have the same polarization state with the dominant propagating surface plasmon polaritons on the real metal wires. The dispersion of the fundamental surface mode has close resemblance with the dispersion of the surface plasmon polaritons. Moreover, we note that for TM polarization this metallic structure can be equivalent to a dielectric coated metal wire with defined geometrical parameters and effective refractive index of the dielectric coating. This metallic structure is expected to have some potential applications in the optical research in microwave or THz region.

Wavelet-transform analysis of spectral shearing interferometry for phase reconstruction of femtosecond optical pulses.

We introduce a novel method for retrieving the phase from a spectral shearing interferogram, based on wavelet-transform technique. We demonstrate with both theoretical and experimental data that this technique provides an alternative and reliable technique for phase retrieval, particularly for highly structured pulse spectra.

Polarization-demultiplexed two-color frequency conversion of femtosecond pulses in birefringent photonic-crystal fibers.

Birefringent photonic-crystal fibers provide efficient polarization-sensitive anti-Stokes frequency conversion of unamplified 35-fs Ti: sapphire laser pulses, giving rise to a doublet of intense blue-shifted emission spectral lines centered at 490 and 510 nm. We show that this anti-Stokes doublet can be wavelength-demultiplexed by a polarizationseparating prism. Generation of the 510-nm signal is decoupled from frequency conversion to 490 nm by accurately polarizing the pump field along one of the principal axes of the elliptically deformed fiber core.

Honeycomb photonic bandgap fibers with and without interstitial air holes.

The photonic bandgaps (PBGs) of honeycomb photonic bandgap fibers (HPBFs) with and without interstitial air holes (IAHs) are numerically investigated. It is shown that the IAHs can increase the width of PBGs in HPBFs, and also that at the same moderate total air filling fraction, HPBFs with IAHs produce more uniform PBGs than those without IAHs. The bandgap behavior is qualitatively explained using the node-and-vein concept.

Frequency-tunable anti-Stokes line emission by eigenmodes of a birefringent microstructure fiber.

Birefringent microstructure fibers are shown to allow efficient generation of frequency-tunable anti-Stokes line emission as a result of nonlinear-optical spectral transformation of unamplified femtosecond Ti: sapphire laser pulses. Femtosecond pulses of 820-nm pump radiation polarized along the fast and slow axes of the elliptical core of the microstructure fiber generate intense blue-shifted lines centered at 490 and 510 nm, respectively, observed as bright blue and green emission at the output of a 10-cm microstructure fiber.

Multiplex frequency conversion of unamplified 30-fs Ti: sapphire laser pulses by an array of waveguiding wires in a random-hole microstructure fiber.

An array of fused silica waveguiding channels with randomly distributed transverse sizes in a disordered microstructure fiber is shown to allow a highly efficient broadly tunable frequency conversion of low-energy ultrashort laser pulses. Dispersion can be switched in such waveguide arrays by coupling the pump field into waveguiding wires with different diameters. Microstructure-fiber-integrated random arrays of waveguides with diameters ranging from 0.6 up to 1.5 mum can frequency-convert unamplified subnanojoule Ti: sapphire laser pulses with an initial duration of 30 fs to any wavelength within a broad spectral range from 400 up to 700 nm, suggesting interesting fiber-optic strategies for multiplex frequency conversion and sensing.