Low-loss Photonic Crystal Waveguides Research Articles

All-dielectric rod antenna array for terahertz communications

The terahertz band holds a potential for point-to-point short-range wireless communications at sub-terabit speed. To realize this potential, supporting antennas must have a wide bandwidth to sustain high data rate and must have high gain and low dissipation to compensate for the free space path loss that scales quadratically with frequency. Here we propose an all-dielectric rod antenna array with high radiation efficiency, high gain, and wide bandwidth. The proposed array is integral to a low-loss photonic crystal waveguide platform, and intrinsic silicon is the only constituent material for both the antenna and the feed to maintain the simplicity, compactness, and efficiency. Effective medium theory plays a key role in the antenna performance and integrability. An experimental validation with continuous-wave terahertz electronic systems confirms the minimum gain of 20 dBi across 315–390 GHz. A demonstration shows that a pair of such identical rod array antennas can handle bit-error-free transmission at the speed up to 10 Gbit/s. Further development of this antenna will build critical components for future terahertz communication systems.

Toward low-loss photonic crystal waveguides in InP/InGaAsP heterostructures

Line-defect photonic crystal waveguides exhibit severe propagation losses if they are implemented in semiconductor heterostructures with a weak refractive index contrast. We present, for what we believe is the first time, experimental structures for which we have evidence that fabrication imperfections are not the limiting factor in terms of propagation losses. We demonstrate a loss figure of 335±5 dB/cm, which is an improvement by a factor of about 2 with respect to state-of-the-art values. Simulations show that even lower losses can be obtained with different waveguide geometries. In other words, the dominant loss mechanism is related to the waveguide design, and losses are not expected to decrease upon further optimization of the fabrication process.

Fabrication of low loss dispersion engineered chalcogenide photonic crystals

We demonstrate low loss photonic crystal waveguides in chalcogenide (Ge(33)As(12)Se(55)) glasses. The measured losses are as low as 21 dB/cm. We experimentally determine the refractive index of the thin film chalcogenide glass to be n = 2.6 and demonstrate that dispersion engineering can be performed up to a group index of ng = 40 in this relatively low refractive index contrast system.

Optomechanical coupling in photonic crystal supported nanomechanical waveguides

We report improved optomechanical properties of a nano-mechanical beam resonator embedded in an optical race-track resonator. Precise control of the mechanical resonator is achieved by clamping the beam between two low-loss photonic crystal waveguide couplers. The low insertion loss and the rigid mechanical support provided by the couplers yield both good mechanical and optical Q-factors for improved signal quality.

Detailed analysis by Fabry-Perot method of slab photonic crystal line-defect waveguides and cavities in aluminium-free material system

A single-line-defect low-loss photonic crystal waveguide based on a perforated GaAs membrane in an aluminium-free material system is demonstrated. The GaInP lattice is matched to GaAs as the cladding/sacrificial layer. Fabry-Perot resonances are analyzed to obtain the group velocity dispersion for a 1-mm long guide. The losses are deduced to be close to 5 dB/cm, taking into account the wavelength dependent reflectivity of the guide extremities. In this framework, side-coupled nanocavities are also investigated. Feasibility of low-loss photonic-crystal-based devices combined with a reliable industrial material systems is thus demonstrated.

Time-delay measurement in singlemode, low-loss photonic crystal waveguides

The group delay in low-loss, singlemode, 1 mm-long photonic crystal slab waveguides is measured using the phase shift technique for the first time. Low group velocities (<c/10) resulting from the flatter bands of these waveguides demonstrated. Time-domain dispersion up to 20 ps is reported in a 20 nm bandwidth. This opens new perspectives towards compact and massively integrated photonic devices for optical signal processing not suffering limitations of Fabry-Perot resonances.

Near-field characterization of low-loss photonic crystal waveguides

A scanning near-field optical microscope is used to directly map the propagation of light in the wavelength range of 1500--1630 nm along straight photonic crystal waveguides (PCWs) fabricated on silicon-on-insulator wafers. The PCWs were formed by removing a single row of holes in the triangular 428-nm-period lattices with different filling factors (0.76 and 0.82) and connected to access ridge waveguides. Using the near-field optical images we investigate the light propagation along PCWs for TM and TE polarizations (the electric field is perpendicular/parallel to the sample surface). Efficient guiding (for both samples) of the TM-polarized radiation is observed in the whole range of laser tunability. For TE polarization, the efficient guiding is limited to the wavelengths shorter than 1552 or 1570 nm for the PCW with the filling factor of 0.76 or 0.82, respectively. For longer wavelengths, we observe drastic and rapid deterioration of the waveguiding and PCW mode confinement, ending up with the complete disappearance of the PCW mode once the wavelength exceeds the cutoff value by merely 2 nm. Using averaged cross sections of the intensity distributions along the PCW axis, the propagation loss is evaluated and found to be in good agreement with the corresponding transmission spectra. Considering spatial frequency spectra of the intensity variations along the PCW axis, we determine (for both polarizations) the dispersion of the PCW mode propagation constant and, thereby, the mode group and phase velocities.

Investigation of Photonic Crystals with a Low In-Plane Index Contrast

The investigation of photonic crystal waveguides and bends realized in glass-like amorphous materials has been performed theoretically and experimentally. Efficient photonic crystal waveguide bends with transmissions up to 75% per bend and low loss photonic crystal waveguides with propagation losses as low as 1.7 dB/mm were realized.

Low loss single line photonic crystal waveguide on InP membrane

We report on spectroscopic measurements of efficient low loss photonic crystal waveguide. Diffracted photoluminescence experiments realized on close waveguides and numerical calculations are associated to analyze propagation properties of this waveguide.

Out-of-plane losses of line-defect photonic crystal waveguides

A numerical model is presented for computing the out-of-plane losses of a general class of two-dimensional photonic crystals superimposed. onto a slab confinement structure. The usefulness of this model is demonstrated by calculating the propagation loss of a single-row-defect waveguide composed of hexagonal air holes etched into two different slab structures. The results are interpreted in terms of a simple coupled-mode-theory picture in which loss is due to resonant coupling between the fundamental and certain radiative slab modes. These calculations show that low-loss photonic crystal waveguides should be possible by carefully designing the slab waveguide.