Effect of stray fields on Rydberg states in hollow-core PCF probed by higher-order modes.

The spectroscopy of atomic gases confined in hollow-core photonic crystal fiber (HC-PCF) provides optimal atom-light coupling beyond the diffraction limit, which is desirable for various applications such as sensing, referencing, and nonlinear optics. Recently, coherent spectroscopy was carried out on highly excited Rydberg states at room temperature in a gas-filled HC-PCF. The large polarizability of the Rydberg states made it possible to detect weak electric fields inside the fiber. In this Letter, we show that by combining highly excited Rydberg states with higher-order optical modes, we can gain insight into the distribution and underlying effects of these electric fields. Comparisons between experimental findings and simulations indicate that the fields are caused by the dipole moments of atoms adsorbed on the hollow-core wall. Knowing the origin of the electric fields is an important step towards suppressing them in future HC-PCF experiments. Furthermore, a better understanding of the influence of adatoms will be advantageous for optimizing electric-field-sensitive experiments carried out in the vicinity of nearby surfaces.

Efficient anti-Stokes generation via intermodal stimulated Raman scattering in gas-filled hollow-core PCF.

A strong anti-Stokes Raman signal, from the vibrational Q(1) transition of hydrogen, is generated in gas-filled hollow-core photonic crystal fiber. To be efficient, this process requires phase-matching, which is not automatically provided since the group velocity dispersion is typically non-zero and--inside a fiber--cannot be compensated for using a crossed-beam geometry. Phase-matching can however be arranged by exploiting the different dispersion profiles of higher-order modes. We demonstrate the generation of first and second anti-Stokes signals in higher-order modes by pumping with an appropriate mixture of fundamental and a higher-order modes, synthesized using a spatial light modulator. Conversion efficiencies as high as 5.3% are achieved from the pump to the first anti-Stokes band.

Amplification of higher-order modes by stimulated Raman scattering in H2-filled hollow-core photonic crystal fiber.

We report a method for amplifying higher-order guided modes, synthesized with a spatial light modulator, in a hydrogen-filled hollow-core photonic crystal fiber. The gain mechanism is intermodal stimulated Raman scattering, a pump laser source in the fundamental mode providing amplification for weak higher-order seed modes at the Stokes frequency. The gain for higher-order modes up to LP31 is calculated and verified experimentally.

Metrology of laser-guided particles in air-filled hollow-core photonic crystal fiber.

Micrometer-sized particles are trapped in front of an air-filled hollow-core photonic crystal fiber using a novel dual-beam trap. A backward guided mode produces a divergent beam that diffracts out of the core, and simultaneously a focused laser beam launches a forward-propagating mode into the core. By changing the backward/forward power balance, a trapped particle can be selectively launched into the hollow core. Once inside, particles can be optically propelled along several meters of fiber with mobilities as high as 19 cm·s(-1) W(-1) (precisely measured using in-fiber Doppler velocimetry). The results are in excellent agreement with theory. The system allows determination of fiber loss as well as the mass density and refractive index of single particles.

Microfluidic integration of photonic crystal fibers for online photochemical reaction analysis.

Liquid-filled hollow-core photonic crystal fibers (HC-PCFs) are perfect optofluidic channels, uniquely providing low-loss optical guidance in a liquid medium. As a result, the overlap of the dissolved specimen and the intense light field in the micronsized core is increased manyfold compared to conventional bioanalytical techniques, facilitating highly-efficient photoactivation processes. Here we introduce a novel integrated analytical technology for photochemistry by microfluidic coupling of a HC-PCF nanoflow reactor to supplementary detection devices. Applying a continuous flow through the fiber, we deliver photochemical reaction products to a mass spectrometer in an online and hence rapid fashion, which is highly advantageous over conventional cuvette-based approaches.

Reconfigurable optothermal microparticle trap in air-filled hollow-core photonic crystal fiber.

We report a novel optothermal trapping mechanism that occurs in air-filled hollow-core photonic crystal fiber. In the confined environment of the core, the motion of a laser-guided particle is strongly influenced by the thermal-gradient-driven flow of air along the core surface. Known as "thermal creep flow," this can be induced either statically by local heating, or dynamically by the absorption (at a black mark placed on the fiber surface) of light scattered by the moving particle. The optothermal force on the particle, which can be accurately measured in hollow-core fiber by balancing it against the radiation forces, turns out to exceed the conventional thermophoretic force by 2 orders of magnitude. The system makes it possible to measure pN-scale forces accurately and to explore thermally driven flow in micron-scale structures.

Optomechanical nonlinearity in dual-nanoweb structure suspended inside capillary fiber.

A novel kind of nanostructured optical fiber, displaying an extremely high and optically broadband optomechanical nonlinearity, is presented. It comprises two closely spaced ultrathin glass membranes (webs) suspended in air and attached to the inner walls of a glass fiber capillary. Light guided in this dual-web structure can exert attractive or repulsive pressure on the webs, causing them to be pushed together or pulled apart. The elastic deflection of the webs is, in turn, coupled to the electromagnetic field distribution and results in a change in the effective refractive index within the fiber. Employing a pump-probe technique in an interferometric setup, optomechanically induced refractive index changes more than 10^{4} times larger than the Kerr effect are detected. Theoretical estimates of the optomechanical nonlinearity agree well with the experimental results. The dual-web fiber combines the sensitivity of a microoptomechanical device with the versatility of an optical fiber and could trigger new developments in the fields of nonlinear optics, optical metrology, and sensing.

Optofluidic immobility of particles trapped in liquid-filled hollow-core photonic crystal fiber.

We study the conditions under which a particle, laser-guided in a vertically-oriented hollow-core photonic crystal fiber filled with liquid, can be kept stationary against a microfluidic counter-flow. An immobility parameter-the fluid flow rate required to immobilize a particle against the radiation force produced by unit guided optical power-is introduced to quantify the conditions under which this occurs, including radiation, viscous and gravity forces. Measurements show that this parameter depends strongly on the ratio of particle radius a to core radius R, peaking at an intermediate value of a/R. The results follow fairly well the theoretical estimates of the optical (calculated approximately using a ray optics approach) and numerically simulated drag forces. We suggest that the system has potential applications in, e.g., measurement of the diameter, refractive index and density of particles, synthesis and biomedical research.

Single-mode hollow-core photonic crystal fiber made from soft glass.

We demonstrate the first soft-glass hollow core photonic crystal fiber. The fiber is made from a high-index lead-silicate glass (Schott SF6, refractive index 1.82 at 500 nm). Fabricated by the stack-and-draw technique, the fiber incorporates a 7-cell hollow core embedded in a highly uniform 6-layer cladding structure that resembles a kagomé-like lattice. Effective single mode guidance of light is observed from 750 to 1050 nm in a large mode area (core diameter ~30 µm) with a low loss of 0.74 dB/m. The underlying guidance mechanism of the fiber is investigated using finite element modeling. The fiber is promising for applications requiring single mode guidance in a large mode area, such as particle guidance, fluid and gas filled devices.

Structural analysis of photonic crystal fibers by side scattering of laser light.

A side-scattering technique for investigating the inner microstructure of photonic crystal fibers (PCFs) is reported. Multiple scattering is reduced by filling the hollow PCF channels with index-matching fluid. The scattered signal is measured for fixed angles of incidence and detection while the fiber is rotated. A pattern of peaks, unique to each PCF, whether solid or hollow core, correlates closely with the symmetry planes of the PCF structure. As an example of the technique, the twist profile of a structural rocking filter is directly measured.

Doppler velocimetry on microparticles trapped and propelled by laser light in liquid-filled photonic crystal fiber.

Laser Doppler velocimetry is used to measure very accurately the velocity and position of a microparticle propelled and guided by laser light in liquid-filled photonic crystal fiber. Periodic variations in particle velocity are observed that correlate closely with modal beating between the two lowest order guided fiber modes.

Fiber transport of spatially entangled photons.

Entanglement in the spatial degrees of freedom of photons is an interesting resource for quantum information. For practical distribution of such entangled photons, it is desirable to use an optical fiber, which in this case has to support multiple transverse modes. Here we report the use of a hollow-core photonic crystal fiber to transport spatially entangled qubits.

Entangling different degrees of freedom by quadrature squeezing cylindrically polarized modes.

Quantum systems such as, for example, photons, atoms, or Bose-Einstein condensates, prepared in complex states where entanglement between distinct degrees of freedom is present, may display several intriguing features. In this Letter we introduce the concept of such complex quantum states for intense beams of light by exploiting the properties of cylindrically polarized modes. We show that already in a classical picture the spatial and polarization field variables of these modes cannot be factorized. Theoretically it is proven that by quadrature squeezing cylindrically polarized modes one generates entanglement between these two different degrees of freedom. Experimentally we demonstrate amplitude squeezing of an azimuthally polarized mode by exploiting the nonlinear Kerr effect in a specially tailored photonic crystal fiber. These results display that such novel continuous-variable entangled systems can, in principle, be realized.

Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic bandgap fiber.

A great challenge in microfluidics is the precise control of laser radiation forces acting on single particles or cells, while allowing monitoring of their optical and chemical properties. We show that, in the liquid-filled hollow core of a single-mode photonic crystal fiber, a micrometer-sized particle can be held stably against a fluidic counterflow using radiation pressure and can be moved to and fro (over tens of centimeters) by ramping the laser power up and down. Accurate studies of the microfluidic drag forces become possible, because the particle is trapped in the center of the single guided optical mode, resulting in highly reproducible radiation forces. The counterflowing liquid can be loaded with sequences of chemicals in precisely controlled concentrations and doses, making possible studies of single particles, vesicles, or cells.

Dynamic control of higher-order modes in hollow-core photonic crystal fibers.

We present a versatile method for selective mode coupling into higher-order modes of photonic crystal fibers, using holograms electronically generated by a spatial light modulator. The method enables non-mechanical and completely repeatable changes in the coupling conditions. We have excited higher order modes up to LP(31) in hollow-core photonic crystal fibers. The reproducibility of the coupling allows direct comparison of the losses of different guided modes in both hollow-core bandgap and kagome-lattice photonic crystal fibers. Our results are also relevant to applications in which the intensity distribution of the light inside the fiber is important, such as particle- or atom-guidance.

Rydberg atoms in hollow-core photonic crystal fibres.

The exceptionally large polarizability of highly excited Rydberg atoms-six orders of magnitude higher than ground-state atoms--makes them of great interest in fields such as quantum optics, quantum computing, quantum simulation and metrology. However, if they are to be used routinely in applications, a major requirement is their integration into technically feasible, miniaturized devices. Here we show that a Rydberg medium based on room temperature caesium vapour can be confined in broadband-guiding kagome-style hollow-core photonic crystal fibres. Three-photon spectroscopy performed on a caesium-filled fibre detects Rydberg states up to a principal quantum number of n=40. Besides small energy-level shifts we observe narrow lines confirming the coherence of the Rydberg excitation. Using different Rydberg states and core diameters we study the influence of confinement within the fibre core after different exposure times. Understanding these effects is essential for the successful future development of novel applications based on integrated room temperature Rydberg systems.