Defect modes in two-dimensional periodic photonic structures have found use in a highly diverse set of optical devices. For example, photonic crystal cavities confine optical modes to subwavelength volumes and can be used for Purcell enhancement of nonlinearity, lasing, and cavity quantum electrodynamics. Photonic crystal fiber defect cores allow for supercontinuum generation and endlessly-single-mode fibers with large cores. However, these modes are notoriously fragile: small changes in the structure can lead to significant detuning of resonance frequency and mode volume. Here, we show that a photonic topological crystalline insulator structure can be used to topologically protect the resonance frequency to be in the middle of the band gap, and therefore minimize the mode volume of a two-dimensional photonic defect mode.We experimentally demonstrate this in a femtosecond-laser-written waveguide array, a geometry akin to a photonic crystal fiber. The topological defect modes are determined by a topological invariant that protects zero-dimensional states (defect modes) embedded in a two-dimensional environment; a novel form of topological protection that has not been previously demonstrated.

When a two-dimensional (2D) electron gas is placed in a perpendicular magnetic field, its in-plane transverse conductance becomes quantized; this is known as the quantum Hall effect. It arises from the non-trivial topology of the electronic band structure of the system, where an integer topological invariant (the first Chern number) leads to quantized Hall conductance. It has been shown theoretically that the quantum Hall effect can be generalized to four spatial dimensions, but so far this has not been realized experimentally because experimental systems are limited to three spatial dimensions. Here we use tunable 2D arrays of photonic waveguides to realize a dynamically generated four-dimensional (4D) quantum Hall system experimentally. The inter-waveguide separation in the array is constructed in such a way that the propagation of light through the device samples over momenta in two additional synthetic dimensions, thus realizing a 2D topological pump. As a result, the band structure has 4D topological invariants (known as second Chern numbers) that support a quantized bulk Hall response with 4D symmetry. In a finite-sized system, the 4D topological bulk response is carried by localized edge modes that cross the sample when the synthetic momenta are modulated. We observe this crossing directly through photon pumping of our system from edge to edge and corner to corner. These crossings are equivalent to charge pumping across a 4D system from one three-dimensional hypersurface to the spatially opposite one and from one 2D hyperedge to another. Our results provide a platform for the study of higher-dimensional topological physics.

We experimentally demonstrate topological edge states arising from the valley-Hall effect in twodimensional honeycomb photonic lattices with broken inversion symmetry. We break inversion symmetry by detuning the refractive indices of the two honeycomb sublattices, giving rise to a boron nitride-like band structure. The edge states therefore exist along the domain walls between regions of opposite valley Chern numbers. We probe both the armchair and zig-zag domain walls and show that the former become gapped for any detuning, whereas the latter remain ungapped until a cutoff is reached. The valley-Hall effect provides a new mechanism for the realization of time-reversal invariant photonic topological insulators.Photonic topological insulators (PTIs) are dielectric structures that possess topologically protected edge states that are robust to scattering by disorder [1][2][3][4][5][6][7][8][9][10][11][12]. There are two categories of PTIs: those that break time-reversal symmetry [3,7] and those that preserve it [8,9,11]. In PTIs that break time-reversal symmetry, there exist one-way edge states, which ensure their robustness, due to the lack of counter-propagating partners at same frequency. In those that preserve it, there exist counter-propagating edge states that are protected only against certain classes of disorder. However, the latter can be more straightforward to realize because they do not require strong time-reversal breaking. Photonic topological insulators have been of interest due to the possibility of photonic devices that are less sensitive to fabrication disorder.In the valley-Hall effect, broken inversion symmetry in a two-dimensional honeycomb lattice causes opposite Berry curvatures in the two valleys of the band structure [13,14], and has been realized in solid-state twodimensional materials [15][16][17][18][19]. The valley-Hall effect is time-reversal invariant and has common characteristics with the spin Hall effect [20], where the two valleys in the band structure are used as 'pseudo-spin' degrees of freedom. It was shown theoretically that valley-Hall topological edge states would arise in analogous photonic structures [21][22][23][24][25][26][27]. In addition, valley-Hall topological edge states have also been recently studied in the context of topological valley transport of sound in sonic crystals [28].Here, we present the experimental observation of photonic topological valley-Hall edge states at domain walls between valley-Hall PTIs of opposite valley Chern numbers. The bulk-edge correspondence ensures the presence of edge states: the change in valley Chern number across the domain wall is associated with the existence of counter-propagating edge states [19,29,30]. We realize the photonic valley-Hall topological edge states in evanescently-coupled waveguide arrays, i.e., photonic lattices, fabricated using the femtosecond direct laser writing technique [31]. We probe different types of domain walls, namely the armchair and zig-zag edges. We also enter a fully gapped regime, which is not...

Weyl fermions are hypothetical two-component massless relativistic particles in threedimensional (3D) space, proposed by Hermann Weyl in 1929. Their band-crossing points, called "Weyl points", carry a topological charge and are therefore highly robust. There has been much excitement over recent observations of Weyl points in microwave photonic crystals and the semimetal TaAs. Here, we report on the first experimental observation of Weyl points of light at optical frequencies. These are also the first observations of "type-II" Weyl points for photons at any wavelength, which have strictly positive group velocity along one spatial direction. We use a 3D structure consisting of laser-written waveguides, and show the presence of type-II Weyl points by (1) observing conical diffraction along one axis when the frequency is tuned to the Weyl point; and(2) observing the associated Fermi arc surface states. The realization of Weyl points at optical frequencies allow these novel electromagnetic modes to be further explored in the context of linear, nonlinear, and quantum optics.

Weyl points are isolated degeneracies in reciprocal space that are monopoles of the Berry curvature. This topological charge makes them inherently robust to Hermitian perturbations of the system. However, non-Hermitian effects, usually inaccessible in condensed matter systems, are an important feature of photonics systems, and when added to an otherwise Hermitian Weyl material have been predicted to spread the Berry charge of the Weyl point out onto a ring of exceptional points, creating a Weyl exceptional ring and fundamentally altering its properties. Here, we observe the implications of the Weyl exceptional ring using real-space measurements of an evanescentlycoupled bipartite optical waveguide array by probing its effects on the Fermi arc surface states, the bulk diffraction properties, and the output power ratio of the two constituent sublattices. This is the first realization of an object with topological Berry charge in a non-Hermitian system.

Spontaneous gas-phase Raman scattering using a hollow-core photonic bandgap fiber (HC-PBF) for both the gas cell and the Stokes light collector is reported. It was predicted that the HC-PBF configuration would yield several hundred times signal enhancement in Stokes power over a traditional free-space configuration because of increased interaction lengths and large collection angles. Predictions were verified by using nitrogen Stokes signals. The utility of this system was demonstrated by measuring the Raman signals as functions of concentration for major species in natural gas. This allowed photomultiplier-based measurements of natural gas species in relatively short integration times, measurements that were previously difficult with other systems.

We present fiber Bragg grating pressure sensors in air-hole microstructured fibers for high-temperature operation above 800°C. An ultrafast laser was used to inscribe Type II grating in two-hole optical fibers. The fiber Bragg grating resonance wavelength shift and peak splits were studied as a function of external hydrostatic pressure from 15 psi to 2000 psi. The grating pressure sensor shows stable and reproducible operation above 800°C. We demonstrate a multiplexible pressure sensor technology for a high-temperature environment using a single fiber and a single-fiber feedthrough. © 2010 Optical Society of America OCIS codes: 060.2370, 060.4005, 120.5475, 120.6780. Pressure sensors operated at high temperature above 500°C have many important applications in the energy industry. They ensure safe and efficient energy production during operations of gas turbines, coal boilers, nuclear power plants, and others. The hightemperature environment presents unique challenges to sensing systems. It not only requires robust sensor elements but also demands reliable packaging and wiring techniques rated for high-temperature environments.Fiber-based optical sensors have been considered good candidates for applications within harsh environments. For example, high-temperature pressure sensors based on a Fabry-Perot interferometer (FPI) have been successfully demonstrated [1,2], and fiber pressure sensors with operating temperatures up to 800°C have been achieved using sapphire fibers [1]. On the other hand, conventional fiber Bragg grating (FBG) sensors are generally considered unsuitable for high-temperature operation. Although FBG pressure sensors have been fabricated using UV laser writing, their operational temperature was limited to below 300°C owing to the poor thermal stability of the UV-induced refractive index change [3,4].In contrast to interferometer-based fiber sensors, FBG-based sensors are readily multiplexible, and the fabrication and the packaging of FBG sensors are relatively simple. They can be produced reliably and in large quantities using a phase mask writing technique. These highly desirable advantages have resulted in significant efforts to improve the hightemperature stability of FBG sensors. In particular, the adaptation of novel fibers, such as nitrogen-doped fiber [5], or novel fabrication techniques, such as ultrafast laser writings [6], has potentially improved the operational temperature of fiber grating devices to be on par with FPI-based sensors, approaching or exceeding 800°C.In this Letter, we apply femtosecond-pulsed ultrafast laser writing to produce high-temperature stable FBG in two-hole fibers for pressure sensing. This work demonstrates a significant improvement of the operational temperature of FBG pressure sensors in air-hole microstructured fiber to over 800°C. A large number of high-temperature FBG pressure sensors in microstructured fibers can be fabricated in one fiber using a simple phase mask approach. The multiplexed fiber sensor array in a single fiber can be serviced by...

An ultrafast thulium-doped fiber laser with large net normal dispersion has been developed to produce dissipative soliton and noise-like outputs at 1.9 μm. The mode-locked operation was enabled by using single-wall carbon nanotubes as saturable absorber for all-fiber configuration. Dissipative soliton in normal dispersion produced by the fiber laser oscillator was centered at 1947 nm with 4.1-nm FWHM bandwidth and 0.45 nJ/pulse. The output dissipative soliton pulses were compressed to 2.3 ps outside the laser cavity.