Open Waveguide Research Articles

BaF2 Ridge Waveguide Operating at Mid-Infrared Wavelength

We report on the fabrication of optical ridge waveguide in barium fluoride (BaF2) crystal by 15 MeV C5+ ions irradiation with femtosecond laser ablation. The near-field modal profile and propagation loss of the waveguide at mid-infrared wavelength 4 µm are investigated by using end-face coupling system. We implement a series of annealing treatment and it efficiently reduces the propagation loss of the waveguide. The confocal Raman spectra demonstrate that the lattice structure of BaF2 crystal does not change largely after C5+ ion irradiation.

Spontaneous decay of artificial atoms in a multi-qubit system

We consider a one-dimensional chain of N equidistantly spaced noninteracting qubits embedded in an open waveguide. In the frame of single-excitation subspace, we systematically study the evolution of qubits' amplitudes if the only qubit in the chain was initially excited. We show that the temporal dynamics of qubits' amplitudes crucially depend on the value of kd, where k is the wave vector, d is a distance between neighbor qubits. If kd is equal to an integer multiple of π, then the qubits are excited to a stationary level which scales as N−1. We show that in this case, it is the dark states which prevent qubits from decaying to zero even though they do not contribute to the output spectrum of photon emission. For other values of kd the excitations of qubits have the form of damping oscillations, which represent the vacuum Rabi oscillations in a multi-qubit system. In this case, the output spectrum of photon radiation is defined by a subradiant state with the smallest width.

Surface waves in an open circular waveguide filled with inhomogeneous chiral media

Propagation of surface waves in a radially inhomogeneous chiral waveguide is considered. The setting is reduced to a boundary eigenvalue problem for the longitudinal components of the electromagnetic field in Sobolev spaces. To find the solution, variational formulation is used. The variational problem is reduced to the analysis of an operator-valued function. Discreteness of the spectrum is proved and distribution of the characteristic numbers of the operator-valued function on the complex plane is determined. The results of numerical modeling of the spectrum of propagating surface waves in an open chiral waveguide are presented.

Complex modes in optical fibers and silicon waveguides.

In an open optical waveguide, complex modes that are confined around the waveguide core and have a complex propagation constant may exist, even though the waveguide consists of lossless isotropic dielectric materials. However, the existing studies on complex modes are very limited. In this Letter, we consider circular fibers and silicon waveguides, study the formation mechanism of complex modes, and calculate the dispersion relations for several complex modes in each waveguide. For circular fibers, we also determine the minimum refractive-index ratio for the existence of complex modes. Our study fills a gap in optical waveguide theory and provides a basis for realizing potential applications of complex modes.

Investigation of the Leaky-Wave Characteristics of a Cylindrical Dielectric Rod Using the Coefficient Matrix of the System of Characteristic Equations and Davidenko’s Method

An open dielectric waveguide includes both guided propagating modes within the waveguide and radiating and evanescent modes that propagate transversely outside the waveguide. In this study, the leaky-wave characteristics of a cylindrical dielectric rod are investigated using the coefficient matrix of the system of characteristic equations and Davidenko’s method. Using the coefficient matrix of the system of characteristic equations simplified the derivative(s) required to use Davidenko’s method. The results obtained for a 10-mm radius structure with a relative dielectric constant of 4 showed that a second antenna mode region existed for higher-order hybrid HE modes and lower-order transverse magnetic (TM) modes. However, it did not exist for transverse electric (TE) modes. Additionally, the hybrid HE modes had larger leaky-wave spectrum and antenna mode regions than both the TE and TM modes. This demonstrates that for antenna applications, hybrid HE modes supply a wider frequency spectrum.

Integrated superconducting circuit for qubit and resonator protection* *Project supported by the National Key R&D Progarm of China (Grant No. 2016YFA0301802), the National Natural Science Foundation of China (Grant Nos. 61521001, 12074179, and 11890704), and the Key R&D Program of Guangdong Province, China (Grant No. 2018B030326001).

A semi-infinite waveguide acts as a mirror and helps protect the qubit in front of it from decoherence. Here, we investigate the interference effect in an open waveguide consisting of resonators with different decay rates. We find that a lossy resonator works as a mirror and changes the effective decay rate of the other. The spontaneous radiation of qubit is related to its environment, and we can control it by arranging the lossy resonator’s position or frequency. Our approach helps improving the qubit performance, as well as the quantum gate fidelities.

Broadband Folded Transmitarray Antenna With Ultralow-Profile Based on Metasurfaces

A broadband folded transmitarray antenna (FTA) with ultralow-profile, high-gain, and high polarization purity is proposed in this communication. It consists of a focusing metasurface (MS) with the function of reflecting and transmitting two orthogonal linear-polarized waves, a reflection-type linear polarization conversion MS, and an open waveguide as a feed. With elaborate design, a −3 dB gain bandwidth of 43.5% from 9 to 14 GHz, a peak gain of 23.5 dBi, and a maximum aperture efficiency of 32.8% are achieved. Meanwhile, the −20 dB cross-polarization level at the broadside direction covers the entire operating band. Furthermore, it is worth mentioning that the profile of the proposed FTA is reduced to about 1/3 of the focal length and the profile-to-diameter ratio is only 0.22. The significant promotions in performance indicate the remarkable application value in portable communication systems.

Towards lab-on-chip ultrasensitive ethanol detection using photonic crystal waveguide operating in the mid-infrared

Abstract Thanks to the unique molecular fingerprints in the mid-infrared spectral region, absorption spectroscopy in this regime has attracted widespread attention in recent years. Contrary to commercially available infrared spectrometers, which are limited by being bulky and cost-intensive, laboratory-on-chip infrared spectrometers can offer sensor advancements including raw sensing performance in addition to utilization such as enhanced portability. Several platforms have been proposed in the past for on-chip ethanol detection. However, selective sensing with high sensitivity at room temperature has remained a challenge. Here, we experimentally demonstrate an on-chip ethyl alcohol sensor based on a holey photonic crystal waveguide on silicon on insulator-based photonics sensing platform offering an enhanced photoabsorption thus improving sensitivity. This is achieved by designing and engineering an optical slow-light mode with a high group-index of n g = 73 and a strong localization of the modal power in analyte, enabled by the photonic crystal waveguide structure. This approach includes a codesign paradigm that uniquely features an increased effective path length traversed by the guided wave through the to-be-sensed gas analyte. This PIC-based lab-on-chip sensor is exemplary, spectrally designed to operate at the center wavelength of 3.4 μm to match the peak absorbance for ethanol. However, the slow-light enhancement concept is universal offering to cover a wide design-window and spectral ranges towards sensing a plurality of gas species. Using the holey photonic crystal waveguide, we demonstrate the capability of achieving parts per billion levels of gas detection precision. High sensitivity combined with tailorable spectral range along with a compact form-factor enables a new class of portable photonic sensor platforms when integrated with quantum cascade laser and detectors.

Ultimate quantum limit for amplification: a single atom in front of a mirror

We investigate three types of amplification processes for light fields coupling to an atom near the end of a one-dimensional (1D) semi-infinite waveguide. We consider two setups where a drive creates population inversion in the bare or dressed basis of a three-level atom and one setup where the amplification is due to higher-order processes in a driven two-level atom. In all cases, the end of the waveguide acts as a mirror for the light. We find that this enhances the amplification in two ways compared to the same setups in an open waveguide. Firstly, the mirror forces all output from the atom to travel in one direction instead of being split up into two output channels. Secondly, interference due to the mirror enables tuning of the ratio of relaxation rates for different transitions in the atom to increase population inversion. We quantify the enhancement in amplification due to these factors and show that it can be demonstrated for standard parameters in experiments with superconducting quantum circuits.

Reconfigurable bandpass optical filters based on subwavelength grating waveguides with a Ge2Sb2 Te5 cavity

We demonstrate a reconfigurable optical bandpass filter based on subwavelength grating (SWG) waveguide operating in the Bragg reflection regime and integrated with a cavity of the phase-change material G e 2 S b 2 T e 5 (GST). Partial crystallization of GST provides us with an efficient tool to modify the effective optical properties of the GST governed by the effective medium theory. Consequently, the resonance wavelength as well as the transmission peak can be tuned in the designed filters. Numerical simulations indicate that the presented SWG waveguide with a single GST cavity offers up to 8.8 nm redshift while the transmission amplitude can be modulated from 0.544 to 0.007. The presented Fabry–Perot structure can also be used as a nonvolatile optical switch with a high extinction ratio of about 24 dB at the wavelength of 1548.3 nm.

An In-Line Coaxial-to-Waveguide Transition for Q-Band Single-Feed-Per-Beam Antenna Systems

An in-line transition between a coaxial cable and rectangular waveguide operating in Q-band (33–50 GHz) is presented. The aim of the work is to minimize the modifications in the waveguide to the strictly necessary to overcome the manufacturing issues due to the high frequencies involved. In addition, the transition is compact and it does not increase the space occupation on the transverse section, this suggests its application in horn antennas clusters arrangement. The operating principle consists of both a modal conversion and an impedance matching between the devices. The modal conversion is realized in an intermediate region, where the coaxial penetrates in the waveguide: the device geometry is designed so that the electric field in the transition is a trade-off between the TEM mode of the coaxial and the TE10 of the guide. A shaped waveguide backshort and a reactive air gap in the coaxial cable co-participate to achieve the matching. An optimized Chebyshev stepped transformer completes the transition to fulfil the impedance mismatch with the full waveguide. The design issues and technological aspects are considered. The influences of the feeding pin misalignment, the presence of groove is included in the analysis and these practical aspects are discussed and numerically validated via the scattering parameters analysis of the proposed design. The return loss is higher than 25 dB over the whole Q-band.

Waveguide coupled cavity-enhanced light emission from individual carbon nanotubes

We demonstrate an individual single-walled carbon nanotube light emitter integrated onto a microcavity and a waveguide operating in the telecom wavelength regime. Light emission from the carbon nanotube is enhanced at the cavity resonance and is efficiently extracted from the waveguide facet. We have transferred carbon nanotubes to a nanobeam cavity with a dry process, ensuring that an individual carbon nanotube is used. The guided light emission from a chirality-identified single carbon nanotube has a narrow linewidth of less than 1.3 nm and an off-resonance rejection of ∼17 dB. The waveguide-coupled device configuration is compatible with fully integrated on-chip designs and is promising for carbon-nanotube-based photonics.

Smooth approximation of a varying refractive-index profile and its application in the computation of light waves

In this paper, the smooth approximation of light waves is studied for an open optical waveguide with a distinct refractive-index profile, which involves high-precision computation of the eigenmodes and corresponding eigenfunctions. During analysis, the refractive-index function is first approximated by a quadratic spline interpolation function. Since the quadratic spline function is a polynomial of degree two in every sub-interval (sub-layer), it is equivalent to a piecewise polynomial of degree two, based on which, the corresponding Sturm-Liouville eigenvalue problem of the Helmholtz operator in sub-layer can be solved analytically by the Kummer functions. Finally, the approximate dispersion equation is established to the TE case. Obviously, the approximate dispersion equations converge fast to the exact ones, as the maximum value of the sub-interval sizes tends to zero. Furthermore, eigenmodes may be obtained by Müller’s method with suitable initial values. To refine the accuracy, the equidistant partition and the non-equidistant partition are applied to divide the interval. Numerical simulations show that the eigenfunctions of the spline interpolation are much smoother than the ones with piecewise interpolation. In addition, the non-equidistant partition can help improve the accuracy and the order of convergence of general solutions reaches the third.

Waveguide bandgap engineering with an array of superconducting qubits

Waveguide quantum electrodynamics offers a wide range of possibilities to effectively engineer interactions between artificial atoms via a one-dimensional open waveguide. While these interactions have been experimentally studied in the few qubit limit, the collective properties of such systems for larger arrays of qubits in a metamaterial configuration has so far not been addressed. Here, we experimentally study a metamaterial made of eight superconducting transmon qubits with local frequency control coupled to the mode continuum of a waveguide. By consecutively tuning the qubits to a common resonance frequency we observe the formation of super- and subradiant states, as well as the emergence of a polaritonic bandgap. Making use of the qubits quantum nonlinearity, we demonstrate control over the latter by inducing a transparency window in the bandgap region of the ensemble. The circuit of this work extends experiments with one and two qubits toward a full-blown quantum metamaterial, thus paving the way for large-scale applications in superconducting waveguide quantum electrodynamics.

Fast-Reconfigurable Microwave Photonics Phase Shifter Using Silicon Microring Resonators

Thermally controlled silicon photonics microwave phase shifters suffer from slow reconfiguration time that practically limits their application fields. A microwave photonics phase shifter exploiting fast plasma dispersion effect in a silicon microring resonator (MRR) embedding a laterally-doped pn-junction waveguide operating in carrier-depletion mode has been designed and characterized. Proper dimensioning of ring parameters allows moderate insertion loss and limited RF power excursion in correspondence of wide RF phase changes as the space charge region of the junction is modulated by an applied reverse voltage. Experimental results confirm a large modulation bandwidth of more than 5GHz and a phase excursion well in excess of 360° using a cascaded MRRs pair with a residual amplitude modulation down to 2.3 dB.

Features of search for complex solutions of non-self-adjoint boundary value problems of electrodynamics

The features of finding of complex solutions of the dispersion equations of guiding electrodynamic structures waves described by non-self-adjoint operators have been considered. These solutions are found using the combined numerical method for complex roots finding combining the Muller method and the phase variation method. A brief explanation of the combined search method essence has been given, and its advantages have been described. As examples, structures with simple geometry are selected in order to avoid difficult mathematical expressions and focus on the process of complex solutions finding. The guiding electrodynamic structures both without losses and with absorbing metal films have been considered in the approximation of the surface current method and based on its real thickness. A new approach to evaluate the physical correctness and accuracy of the results of solving dispersion problems set in an open form for guiding structures described by non-self-adjoint operators has been shown – by the tendency to zero of the power flow of proper complex waves on average over a period through the cross section of a non-dissipative guide structure. The formulation and the solution features of the adjoint boundary value problem for a round open dielectric waveguide have been considered. The existence possibility of the adjoint waves in this waveguide has been shown.

Исследование задачи о нормальных волнах открытого волновода кругового сечения с неоднородным киральным слоем

Исследование задачи о нормальных волнах открытого волновода кругового сечения с неоднородным киральным слоем

Broadband THz Corrugated Bull’s Eye Antennas

A 3-dB gain and input impedance bandwidth enhancement technique for THz periodically corrugated metallic antennas, also known as Bull's Eye antennas, is presented. A novel design that exploits the depth qualities of the corrugations is introduced, by employing two different depth values for two sets of indented rings. While the gain bandwidth of most single-depth planar corrugated antennas is limited, the 3-dB gain bandwidth of the proposed dual-depth design is expanded along with a high maximum gain. The simulated optimum model of 10 rings at around 300 GHz exhibits a 3-dB gain bandwidth of 8.85% with a maximum gain at broadside of 21.3 dBi. A leaky-wave analysis has been performed via a simple transcendental equation at the level of the gap openings in order to explain the antenna principle of operation. A prototype of the optimum antenna design has been fabricated and measured. The typical subwavelength slot feeding of Bull's eye corrugated antennas is substituted with an easy-to-fabricate open waveguide aperture, yielding a broadband matching performance. Measured results demonstrate a good agreement with simulations and validate the proposed antenna design.

The foam concrete temperature measuring during its microwave heat treatment

The actual problem of accelerating and reducing the energy costs of the foam concrete slabs heat treatment technological process is considered. It is shown that to solve this problem, it is advisable to use the microwave radiation energy as a heat source. The main advantages of the microwave method of foam concrete slabs heat treatment in comparison with traditional methods are considered. The design of a microwave installation for the foam concrete slabs heat treatment has been developed. The installed microwave radiation sources have energy outputs in the form of a rectangular waveguide openings, which are used as radiating antennas. Huygens-Kirchhoff method was used to calculate the temperature distribution on the foam concrete slab surface; the method of loaded long lines was used to calculate the temperature distribution over the foam concrete slab thickness. A method for measuring the foam concrete slab temperature distribution is proposed. The results of theoretical and experimental temperature distribution studies on the surface and the cross-section of foam concrete slab with a width of 1500 mm, a height of 1000 mm and a thickness of 200 mm, a density of 1000 kg/m3 on the electromagnetic fields oscillation frequency of 2450 MHz are shown. The obtained experimental results showed a high efficiency of using microwave radiation for the foam concrete slabs heat treatment technological processes. Microwave technologies can be used for heat treatment of products made of concrete, reinforced concrete and polymer composite materials.

Transparent boundary condition for the calculation of eigenmodes in transversely infinite waveguides

Abstract. Waveguides play one of the key figures in today's electronics and optics for signal transmission. Corresponding simulations of electromagnetic wave transportation along these waveguides are accomplished by discretization methods such as the Finite Integration Technique (FIT) or the Finite Element Method (FEM). For longitudinally homogeneous and transversely unbounded waveguides these simulations can be approximated by closed boundaries. However, this distorts the original physical model and unnecessarily increases the size of the computational domain size. In this article we present a boundary condition for transversely open waveguides based on the Kirchhoff integral which has been implemented within the framework of FIT. The presented solution is compared with selected conventional methods in terms of computational effort and memory consumption.