Air-core Waveguides Research Articles

20 dB improvement utilizing custom-designed 3D-printed terahertz horn coupler.

Terahertz band is envisaged to provide substantially higher capacity and much lower latency for wireless communications in contrast to microwave frequencies. Moving to higher frequencies comes with its own unique challenges to be addressed, such as poor coupling efficiency from free space into and out of planar air-core waveguides. Here, we propose a framework for rapid design and low-cost fabrication of terahertz horn couplers. The horn couplers are first designed by maximizing the field overlap integral on apex and aperture interfaces, then fabricated exploiting 3D printing technique, and finally sputtered with a thin layer of gold. A 28~μm standard deviation of the surface roughness height of the 3D printed horn couplers is calculated. Experimental demonstrations show that the proposed horn coupler improves the transmittance of a hybrid photonic crystal waveguide by 20 dB in comparison with the previous pinhole-based coupling configuration. This work provides a fast, convenient and economical approach for design and fabrication of customized couplers for any waveguide size, with a cost of only 5% of commercially available counterparts, and could be integrated in 3D-printed terahertz devices during fabrication.

Property Variation in Wavelength-thick Epsilon-Near-Zero ITO Metafilm for Near IR Photonic Devices

Thin indium tin oxide (ITO) films have been used as a medium to investigate epsilon-near-zero (ENZ) behavior for unconventional tailoring and manipulation of the light-matter interaction. However, the ENZ wavelength regime has not been studied carefully for ITO films with thicknesses larger than the wavelength. Thick ENZ ITO film would enable the development of a new family of ENZ-based opto-electronic devices that take full advantage of the ENZ behavior. Here, we demonstrated wavelength-thick ITO films reaching the ENZ regime around a wavelength of 1550 nm, which permit the design of such devices operating in the common optical telecommunications wavelength band. We discovered that the permittivity of the film was non-uniform with respect to the growth direction. In particular, after annealing at a sufficiently high temperature, the real part of the permittivity showed a step change from negative to positive value, crossing zero permittivity near the middle of the film. Subsequently, we conducted comprehensive microanalysis with X-ray diffraction, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) to investigate the correlation of the permittivity variation with variations in the ITO crystallite morphology and relative concentrations of different atom species. The result of this study will allow us to design a new family of opto-electronic devices where ITO can be used as the cladding that guides light within an air-core waveguide to provide a new platform to explore ENZ properties such as environment insensitivity, super-coupling, and surface avoidance. We have also provided a comprehensive method to determine the permittivity in a non-uniform ENZ material by using an advanced physical model to the fit experimental data.

Low-loss ARROW waveguide with rectangular hollow core and rectangular low-density polyethylene/air reflectors for terahertz waves

Designing low-loss waveguides for terahertz waves is challenging as most materials are very lossy in this frequency band. Most scientists simply consider transmitting the waves through low-loss air, which however also has its own difficulties as index-guiding is not possible. In this paper, we report on the design of low-loss waveguides for terahertz waves and associated results by using a finite element leaky mode solver. These results show that waveguides designed using ARROW (anti-resonant reflecting optical waveguide) approach yield a low combined absorption and leakage loss down to only 0.05[Formula: see text]dB/cm for the q-TE[Formula: see text] fundamental mode using realistic values of refractive index at 1 THz operating frequency. The structure employs rectangular hollow-core and low-density polyethylene/air anti-resonant reflecting bilayers, which can be easily fabricated. These results are compared with those of other structures, i.e., a photonic crystal fiber-like structures using the same materials with rectangular holes, which is shown to give a higher loss of 3[Formula: see text]dB/cm and a suspended air-core waveguide with TOPAS vein offering a loss of 1[Formula: see text]dB/cm.

PD-0036 OPTIMIZED OUTPUT FACTOR MEASUREMENT METHOD FOR CYBERKNIFE COLLIMATORS

than 10 mm. Our approach is to develop a phantom that contains an embedded array of plastic scintillators coupled to square air core light guides. The prototype array allows for accurate verification of fields as small as 4 mm, with high spatial and temporal resolution and without the need for correction factors. Materials and Methods: The 16 element linear array was constructed from custom cut PMMA sheets. Plastic scintillation detectors (BC400) of volume 0.8 mm3 were placed in 1 x 1 mm silvered square air core waveguides to avoid the production of a Cerenkov background signal in the primary field. The 16 detector elements have a center-tocenter spacing of 2 mm and an edge-to-edge distance of 1 mm and provide high resolution profiles of small treatment fields and of high dose gradients.Scintillation light from the air core array was measured with a photomultiplier tube (PMT) array detector, enabling measurements with high temporal resolution (10 Hz). This scintillation light readout system and user interface software was optimized for clinical use.The array was placed in a water equivalent medium in order to measure the beam output factors, percentage depth dose and beam profiles of radiation fields produced by a Varian Novalis TX linac. The array was then displaced laterally three times for measuring the beam profile, increasing the array's spatial resolution to 0.5 mm. Results: The PMMA light guides of 1 mm square cross section and 180 mm length show 5 times more light transmission than the previously used circular light guides of 1 mm diameter of the same length. The dose measurements were compared to those measured using a single air core dosimeter and other commercially available dosimeters. This experimentally confirms the suitability of the air core array for small field dosimetry with no corrections needed for angular dependence or dose perturbation as currently required by commercial arrays.

Hollow Bragg waveguides fabricated by controlled buckling of Si/SiO_2 multilayers

We describe integrated air-core waveguides with Bragg reflector claddings, fabricated by controlled delamination and buckling of sputtered Si/SiO2 multilayers. Thin film deposition parameters were tailored to produce a desired amount of compressive stress, and a patterned, embedded fluorocarbon layer was used to define regions of reduced adhesion. Self-assembled air channels formed either spontaneously or upon heating-induced decomposition of the patterned film. Preliminary optical experiments confirmed that light is confined to the air channels by a photonic band-gap guidance mechanism, with loss ~5 dB/cm in the 1550 nm wavelength region. The waveguides employ standard silicon processes and have potential applications in MEMS and lab-on-chip systems.

Hollow ARROW Waveguides on Self-Aligned Pedestals for Improved Geometry and Transmission.

Micrometer-sized hollow antiresonant reflecting optical waveguides on silicon substrates have been previously demonstrated with liquid and gas-filled cores. Previous designs have nonideal geometries, with nonuniform lateral layers around the hollow core, resulting in higher loss than could potentially be achieved. A new design and fabrication process has been developed involving hollow waveguide fabrication on a self-aligned pedestal (SAP) using anisotropic plasma etching. With the SAP structure, the hollow core is surrounded by uniform layers and a terminal layer of air on three sides, resulting in air-core waveguide loss of 1.54 cm(-1) at 785 nm and high fabrication yield.

Mode Propagation in X-Ray Waveguides

Single-mode propagation conditions of X-ray waveguides are investigated by numerical calculations in order to understand the importance of waveguide design parameters, such as core thickness and the optical constants of waveguide materials, on the transmission and coherence properties of the waveguide. The simulation code for mode analyzing is developed based on a numerical solution of the parabolic wave equation. The initial boundary value problem is solved numerically using a finite-difference scheme based on the Crank-Nicolson scheme. The E-field intensities in a core layer are calculated at an X-ray energy of 8.0 keV for air and beryllium(Be) core waveguides with different cladding layers such as Pt, Au, W, Ni and Si to determine the dependence on waveguide materials. The highest E-field intensity radiated at the exit of the waveguide is obtained from the Pt cladded beryllium core with a thickness of 20 nm. However, the intensity from the air core waveguide with Pt cladding reaches 64% of the Be-Pt waveguide. The dependence on the core thickness, which is the major parameter used to generate a single mode in the waveguide, is investigated for the air-Pt, and Be-Pt waveguides at an X-ray energy of 8.0 keV. The mode profiles at the exit are shown for the single mode at a thickness of up to 20 nm for the air-Pt and the Be-Pt waveguides.

Out-of-plane coupling at mode cutoff in tapered hollow waveguides with omnidirectional reflector claddings

We describe the theoretical and experimental analysis of light propagation in tapered, air-core waveguides with omnidirectional reflector claddings. For light within the omnidirectional band, nearly vertical out-ofplane radiation at wavelength-dependent positions along the length of the taper was observed. The coupling positions correspond to the core sizes at which individual modes approach cutoff. The leaky nature and low scattering loss of the waveguides enabled the direct imaging of modal interference and standing waves. The out-coupling experiments were corroborated by in-coupling experiments and by a theoretical analysis. The mechanism described might find application to three-dimensional optical integration, on-chip spectroscopy, and wavelength division multiplexing.

Conditions for designing single-mode air-core waveguides in three-dimensional photonic crystals

The authors present a general procedure that allows the design of single-mode air-core waveguides in three-dimensional photonic crystals. The procedure involves analyzing the modal profile of the band edge mode in the perfect crystal, identifying the regions of maximal electric-field intensity, and placing the air defects to enclose these regions. As an illustration, they present a detailed design of air-core waveguides in a recently proposed silicon body-center-cubic crystal structure that possesses a 25% complete band gap.

Self-assembly of three-dimensional photonic-crystals with air-core line defects

We have introduced line defects into a three-dimensional (3D) photonic crystal by combining the techniques of optical photolithography, self-assembly, and template replication. The template is a colloidal crystal film that is grown on a silicon or glass substrate patterned with photoresist lines, and the 3D photonic crystal with air-core line defects is obtained by infiltrating the colloidal crystal film with silica and removing the template by calcination. SEM and optical transmission spectra show that the resulting 3D PC structure has high structural integrity and crystalline quality. This approach can introduce defects at the desired places of 3D photonic crystals, which may be useful in making 3D photonic crystal-based air-core waveguides.