Even-order Modes Research Articles

Defect-induced activation of symmetry forbidden infrared resonances in individual metallic nanorods

We report on the observation of second-order infrared (IR) plasmon resonances in lithographically prepared gold nanorods investigated by means of far-field microscopic IR spectroscopy. In addition to the fundamental antennalike mode, even and odd higher order resonances are observed under normal incidence of light. The activation of even-order modes under normal incidence is surprising since even orders are dipole-forbidden because of their centrosymmetric charge density oscillation. Performing atomic force microscopy and calculations with the boundary element method, we determine that excitation of even modes is enabled by symmetry breaking by structural deviations of the rods from an ideal, straight shape.

Role of spatial mode function in the presence of two-atom events in a micromaser

In this paper, we discuss the effects of spatial mode function in an one-photon micromaser in the presence of two-atom events. It is shown that two-atom events allow us a possibility to study the effects of different cavity eigenmodes in a micromaser. We find that squeezing properties of the radiation field depend upon the parity (odd or even) and order (lower or higher) of cavity eigenmodes. For example, squeezing can be obtained for odd-order cavity eigenmodes which completely vanishes for even-order modes. Our results also show that effects similar to self-induced transparency are never obtained in the presence of two-atom events. Finally, we consider the effect of pump fluctuations and cavity losses in our system.

Design and modeling of inversion layer ultrasonic transducers using LiNbO 3 single crystal

Using inversion domain engineering controlled by heating temperature, the LiNbO 3 (LNO) piezoelectric plate with both odd and even-order thickness-extensional modes can be excited simultaneously. Therefore, the inversion layer ultrasound transducer is expected to be capable of operating over a wider frequency range. In this paper, the electrical impedance and the acoustic characteristics of LiNbO 3 (LNO) inversion layer transducer have been studied by finite element modeling (FEM). The transducer designed for this study uses a 36° rotated Y-cut LiNbO 3 thin plate with an active element thickness of approximately 100 μm. First the electrical and elastic properties of the 36° rotated Y-cut LNO were obtained by transforming a basic piezoelectric matrix for Z-cut LNO. In order to validate the FEM using the transformed properties several pieces of pure and 50% inversion layer LNO were tested on the electrical impedance analyzer. The modeled impedance characteristics were consistent with the measured data. Next the model was used to design 50–60 MHz transducers using pure and 30% inversion LNO. Two λ/4 matching layers and a Tungsten loaded epoxy backing were used in these designs. The modeled results show that an over 90% bandwidth transducer can be made with proper matching and 30% inversion layer.

LiNbO3 Ultrasonic Transducers with an Inverted-Domain Layer for Radiation to Solid Media

Heat treatment of lithium niobate (LiNbO3) plates induces domain inversion, thereby yielding a ferroelectric inversion layer. In such a piezoelectric plate with an inversion layer, even-order thickness-extensional modes, as well as odd-order modes, can be excited piezoelectrically. Therefore, ultrasonic transducers using such a piezoelectric plate are expected to operate over a wide frequency range. In this paper, it is shown both theoretically and experimentally that broadband ultrasonic transducers for radiation to solid media can be obtained when the inversion layer is on the side of the acoustic load medium and the inversion layer-to-plate thickness ratio is around 0.3. A 50–110 MHz ultrasonic transducer was fabricated using 36°-rotated Y-cut LiNbO3 plates with an inversion layer. The conversion loss characteristic was evaluated when the transducer was bonded to an acoustic medium of fused silica by a low-temperature Au diffusion bonding technique. The 3 dB specific bandwidth was as large as 74% and the minimum conversion loss was as low as 2 dB.

Broadband ultrasonic transducers using a LiNbO3 plate with a ferroelectric inversion layer.

Some heat treatment of a lithium niobate (LiNbO3) plate induces domain inversion, thereby yielding a ferroelectric inversion layer. In such a piezoelectric plate with an inversion layer, even-order thickness-extensional modes, as well as odd-order modes, can be excited piezoelectrically. Therefore, the ultrasonic transducer using such a piezoelectric plate is expected to operate over a wide frequency range. In this paper, it is shown that broadband ultrasonic transducers can be obtained at a certain thickness ratio of inversion layer to plate, and that the transducer characteristics differ depending on whether the inversion layer is on the front side or on the backside. The broadband characteristics are experimentally demonstrated by fabricating transducers with 9 MHz or 75 MHz center frequency using 36 degrees rotated Y-cut LiNbO3 plates with a ferroelectric inversion layer.

Semiconductor space switches based on multimode interference couplers

It is shown theoretically that selective perturbation by electrorefractive effect of the even-order modes in III-V semiconductor multimode interference couplers gives rise to high-performance optical switching. The calculated performance of the proposed switch is 3.4 GHz/Vmm, compared to 0.85 GHz/Vmm for an equivalent Mach-Zehnder interferometer (MZI) switch. The effects of electroabsorption and dimension variations are analyzed to demonstrate the feasibility of the proposed device.

Broadband Transducers Using Effectively Graded Piezoelectric Plates for Generation of Short-Pulse Ultrasound

Broadband ultrasound transducers that utilize piezoelectric materials having an effectively graded piezoelectric parameter and/or an excitation electric field applied across the piezoelectric material are investigated. These transducers are constructed by forming a number of fine V-grooves on one surface of a piezoelectric plate. The broadband frequency characteristic of the proposed transducer is demonstrated by experiments of wave generation in water. Analysis by the finite-element method proves that even-order resonance modes are also excited in the plate with a grooved configuration.

Propagation of finite amplitude sound in a waveguide with a parabolic sound velocity profile.

The propagation of finite amplitude sound in a waveguide with a parabolic sound velocity profile is investigated theoretically. It is assumed that the primary wave propagates in a single mode at a frequency that is large compared with the cutoff frequency. The acoustic energy is therefore concentrated near the axis of the sound channel. Both the primary wave and the nonlinearly generated second harmonic component have mode shapes that are described by Gauss–Hermite eigenfunctions. For a primary wave in mode m, the second harmonic component is generated in all even-order modes up to 2m, with most of the energy contained in mode 2m. A nonlinear Schrödinger equation is derived for the envelope of a narrow-band pulse that propagates in a single mode. Analytical expressions are derived for the coefficients in the Schrödinger equation, and it is found that only dark envelope solitons can propagate without distortion in the waveguide. [DER was supported by the ARL Naval Academy Scholarship, EAZ and MFH by the David and Lucile Packard Foundation and by the Office of Naval Research.]

Zero field steps in Josephson junctions

A study is made of the d.c. current steps of resonant Josephson tunnel junctions in zero applied magnetic field. The results are in good agreement with the experimental observations. The existence of even-order modes, the cut-off voltage and the temperature-dependent current steps are clearly demonstrated.

The experimental determination of higher-order mode conversion in a multimode waveguide

An overmoded rectangular waveguide system which operates in the vicinity of 30 GHz is used to investigate multimode propagation problems. By careful design of the input waveguide either a relatively pure TE10 or a combination of TE10 and higher modes may be launched into a 3 m long overmoded test section of the system. The overmoded section is terminated in a matched load. The waveguide will support TEn0 modes where n is any odd integer between 1 and 13. Even-order modes are normally suppressed, if they arise at all, by a longitudinal slot along the centre of one of the broad faces of the waveguide. Electric field amplitude and phase profiles within the overmoded waveguide are measured continuously, either transversely or longitudinally. Experimental measurements of the field within the waveguide enable mode content to be determined for comparison with theoretical data. In this way the amplitudes and phases of the higher-order propagating modes are determined.