Jean-Michel Ménard

Broadband robustly single-mode hollow-core PCF by resonant filtering of higher-order modes.

We report a hollow-core photonic crystal fiber that is engineered so as to strongly suppress higher-order modes, i.e., to provide robust LP01 single-mode guidance in all the wavelength ranges where the fiber guides with low loss. Encircling the core is a single ring of nontouching glass elements whose modes are tailored to ensure resonant phase-matched coupling to higher-order core modes. We show that the resulting modal filtering effect depends on only one dimensionless shape parameter, akin to the well-known d/Λ parameter for endlessly single-mode solid-core PCF. Fabricated fibers show higher-order mode losses some ∼100 higher than for the LP01 mode, with LP01 losses <0.2  dB/m in the near-infrared and a spectral flatness ∼1  dB over a >110  THz bandwidth.

Non-thermal separation of electronic and structural orders in a persisting charge density wave

The simultaneous ordering of different degrees of freedom in complex materials undergoing spontaneous symmetry-breaking transitions often involves intricate couplings that have remained elusive in phenomena as wide ranging as stripe formation, unconventional superconductivity or colossal magnetoresistance. Ultrafast optical, X-ray and electron pulses can elucidate the microscopic interplay between these orders by probing the electronic and lattice dynamics separately, but a simultaneous direct observation of multiple orders on the femtosecond scale has been challenging. Here we show that ultrabroadband terahertz pulses can simultaneously trace the ultrafast evolution of coexisting lattice and electronic orders. For the example of a charge density wave (CDW) in 1T-TiSe2, we demonstrate that two components of the CDW order parameter--excitonic correlations and a periodic lattice distortion (PLD)--respond very differently to 12-fs optical excitation. Even when the excitonic order of the CDW is quenched, the PLD can persist in a coherently excited state. This observation proves that excitonic correlations are not the sole driving force of the CDW transition in 1T-TiSe2, and exemplifies the sort of profound insight that disentangling strongly coupled components of order parameters in the time domain may provide for the understanding of a broad class of phase transitions.

Imaging the spin Hall effect of light inside semiconductors via absorption

The opposite transverse shifts for the right and left circular polarization components of a 100 fs 820 nm linearly polarized pulse focused onto GaAs are observed in situ via absorption. A time-delayed normally incident probe pulse scanned across the excitation spot detects the differential circular dichroism associated with the pump-induced transfer of spin angular momentum from light to electrons. More generally, we show that for a nonnormally incident probe, one can observe the spin Hall effect for probe light through a variety of pump-induced changes to a materials optical properties.

Coherently controlled ballistic currents in single-walled carbon nanotubes and graphite

Ballistic electrical currents are optically injected into aligned single-walled carbon nanotubes and bulk graphite at 300 K via quantum interference between single and two photon absorption of phase-related 700 and 1400 nm, 150 fs pulses. The transient currents are detected via the emitted terahertz radiation. Optical phase and power dependence are consistent with the quantum interference optical process. Under similar excitation conditions, the peak current for a forest of nanotubes, with a diameter distribution of approximately 2.5 +/- 1.5 nm, is 9 +/- 1 times larger than that in graphite. At peak focused intensities of 10 GW cm(-2) (1400 nm) and 0.15 GW cm(-2) (700 nm), the peak current is approximately 1 nA per nanotube. The peak current for pump light polarized along the tubes is approximately 3.5 times higher than that for light polarized perpendicular to the tubes.

Refractive index measurements of planar chalcogenide thin film

Abstract We report on the measurements of the refractive index of As–S–Se chalcogenide glasses near 1.55 μm. The measurements were made on annealed and non-annealed samples of thermally evaporated thin films. The data for two different series of glasses are presented: the compositions As40S60−xSex and the compositions AsxS(100−x)/2Se(100−x)/2 where the ratio of sulfur to selenium is kept constant (1:1). It has been found that replacing sulfur by selenium in the first series increases the refractive index from 2.4 to 2.8 and increasing the arsenic content in the second series increases the refractive index. In all cases, it has been found that annealing the samples increase the refractive index. The accuracy in the refractive index measurement is ±0.2%.

Single-beam differential z-scan technique

We report a single-beam, differential z-scan technique with improved sensitivity for the determination of nonlinear absorption and refraction of materials. A sample is scanned in the direction of beam propagation as usual, but, in addition, its longitudinal position is dithered, producing a detector signal proportional to the spatial derivative of only the nonlinear transmission and therefore giving a background-free signal; the nonlinear transmission for any spatial position of the sample can be recovered by simple integration. For both open and closed aperture scans in GaP, we find an improvement in the signal-to-noise ratio of >5 x compared with a balanced z-scan setup, but this can be improved with apparatus optimization. Nonlinear phase distortions

Revealing the dark side of a bright exciton-polariton condensate

Condensation of bosons causes spectacular phenomena such as superfluidity or superconductivity. Understanding the nature of the condensed particles is crucial for active control of such quantum phases. Fascinating possibilities emerge from condensates of light–matter-coupled excitations, such as exciton–polaritons, photons hybridized with hydrogen-like bound electron–hole pairs. So far, only the photon component has been resolved, while even the mere existence of excitons in the condensed regime has been challenged. Here we trace the matter component of polariton condensates by monitoring intra-excitonic terahertz transitions. We study how a reservoir of optically dark excitons forms and feeds the degenerate state. Unlike atomic gases, the atom-like transition in excitons is dramatically renormalized on macroscopic ground state population. Our results establish fundamental differences between polariton condensation and photon lasing and open possibilities for coherent control of condensates.

Shot noise reduced terahertz detection via spectrally postfiltered electro-optic sampling

In ultrabroadband terahertz electro-optic sampling (EOS), spectral filtering of the gate pulse can strongly reduce the quantum noise while the signal level is only weakly affected. The concept is tested for phase-matched electro-optic detection of field transients centered at 45 THz with 12 fs near-infrared gate pulses in AgGaS2. Our new approach increases the experimental signal-to-noise ratio by a factor of 3 compared to standard EOS. Under certain conditions an improvement factor larger than 5 is predicted by our theoretical analysis.

Higher-order mode suppression in twisted single-ring hollow-core photonic crystal fibers

A hollow-core single-ring photonic crystal fiber (SR-PCF) consists of a ring of capillaries arranged around a central hollow core. Spinning the preform during drawing introduces a continuous helical twist, offering a novel means of controlling the modal properties of hollow-core SR-PCF. For example, twisting geometrically increases the effective axial propagation constant of the LP01-like modes of the capillaries, providing a means of optimizing the suppression of HOMs, which occurs when the LP11-like core mode phase-matches to the LP01-like modes of the surrounding capillaries. (In a straight fiber, optimum suppression occurs for a capillary-to-core diameter ratio d/D=0.682.) Twisting also introduces circular birefringence (to be studied in a future Letter) and has a remarkable effect on the transverse intensity profiles of the higher-order core modes, forcing the two-lobed LP11-like mode in the untwisted fiber to become three-fold symmetric in the twisted case. These phenomena are explored by means of extensive numerical modeling, an analytical model, and a series of experiments. Prism-assisted side-coupling is used to measure the losses, refractive indices, and near-field patterns of individual fiber modes in both the straight and twisted cases.

Phase-matched electric-field-induced second-harmonic generation in Xe-filled hollow-core photonic crystal fiber.

Second-order nonlinearity is induced inside a Xe-filled hollow-core photonic crystal fiber (PCF) by applying an external dc field. The system uniquely allows the linear optical properties to be adjusted by changing the gas pressure, allowing for precise phase matching between the LP01 mode at 1064 nm and the LP02 mode at 532 nm. The dependence of the second-harmonic conversion efficiency on the gas pressure, launched pulse energy, and applied field agrees well with theory. The ultra-broadband guidance offered by anti-resonant reflecting hollow-core PCFs, for example, a kagomé PCF, offers many possibilities for generating light in traditionally difficult-to-access regions of the electromagnetic spectrum, such as the ultraviolet or the terahertz windows. The system can also be used for noninvasive measurements of the transmission loss in a hollow-core PCF over a broad spectrum, including the deep and vacuum UV regions.