Selective Mode Excitation Research Articles

Characterization of Mode Coupling in Few-Mode FBG With Selective Mode Excitation

We present the results on fabrication and the characterization of few-mode fiber Bragg gratings (FM-FBGs) inscribed in two mode and four mode step-index fibers. Under the conditions of selective input mode launching, coupling between specific modes of interest can be selectively excited and the self-coupling and cross-coupling properties at the associated resonant wavelengths can be clearly identified and verified by observing the reflected mode intensity profiles. Such FM-FBGs can potentially be used as reflective mode-converters for mode division multiplexed data transmission systems.

Selective excitation of resonances in gammadion metamaterials for terahertz wave manipulation

A gammadion terahertz (THz) metamaterial embedded with a pair of splits is experimentally investigated. By introducing the pair of splits at different arms, the transmitted amplitude at the resonance frequency can be manipulated from 61% to 24%. Broadband static resonance tunability from 1.11 to 1.51 THz is also demonstrated via varying the relative split positions at certain arms. The amplitude change and static resonance tunability are attributed to the introduced split pairs, which enable selective excitation of different resonance modes in the gammadion metamaterials. This work promises a new approach to design THz functional devices.

Excitation of Mesoscopic Plasmonic Tapers by Relativistic Electrons: Phase Matching versus Eigenmode Resonances.

We investigate the optical modes in three-dimensional single-crystalline gold tapers by means of electron energy-loss spectroscopy. At the very proximity to the apex, a broad-band excitation at all photon energies from 0.75 to 2 eV, which is the onset for interband transitions, is detected. At large distances from the apex, though, we observe distinct resonances with energy dispersions roughly proportional to the inverse local radius. The nature of these phenomena is unraveled by finite difference time-domain simulations of the taper and an analytical treatment of the energy loss in fibers. Our calculations and the perfect agreement with our experimental results demonstrate the importance of phase-matching between electron field and radiative taper modes in mesoscopic structures. The local taper radius at the electron impact location determines the selective excitation of radiative modes with discrete angular momenta.

Selective mode excitation in three-chained magnetic vortices

We report on collective dynamics of three-chained magnetic vortices interacting with each other through magnetic dipolar coupling. Three different eigenstates characterized by the phase difference of the rotating cores were clearly observed by means of the spin torque diode effect. We also selectively excited such eigenstates with the same system by tuning the phase difference. These experimental results are well reproduced by analytical calculations based on the linearized Thiele’s equation. Moreover, we demonstrate that the spectra obtained for the selective excitations correspond to a part of the dispersion relation for one-dimensional chained vortices.

Communication: On the competition between adiabatic and nonadiabatic dynamics in vibrationally mediated ammonia photodissociation in its A band.

Non-adiabatic processes play an important role in photochemistry, but the mechanism for conversion of electronic energy to chemical energy is still poorly understood. To explore the possibility of vibrational control of non-adiabatic dynamics in a prototypical photoreaction, namely, the A-band photodissociation of NH3(X̃(1)A1), full-dimensional state-to-state quantum dynamics of symmetric or antisymmetric stretch excited NH3(X̃(1)A1) is investigated on recently developed coupled diabatic potential energy surfaces. The experimentally observed H atom kinetic energy distributions are reproduced. However, contrary to previous inferences, the NH2(Ã(2)A1)/NH2(X̃(2)B1) branching ratio is found to be small regardless of the initial preparation of NH3(X̃(1)A1), while the internal state distribution of the preeminent fragment, NH2(X̃(2)B1), is found to depend strongly on the initial vibrational excitation of NH3(X̃(1)A1). The slow H atoms in photodissociation mediated by the antisymmetric stretch fundamental state are due to energy sequestered in the internally excited NH2(X̃(2)B1) fragment, rather than in NH2(Ã(2)A1) as previously proposed. The high internal excitation of the NH2(X̃(2)B1) fragment is attributed to the torques exerted on the molecule as it passes through the conical intersection seam to the ground electronic state of NH3. Thus in this system, contrary to previous assertions, the control of electronic state branching by selective excitation of ground state vibrational modes is concluded to be ineffective. The juxtaposition of precise quantum mechanical results with complementary results based on quasi-classical surface hopping trajectories provides significant insights into the non-adiabatic process.

Selective mode excitation in finite size plasma crystals by diffusely reflected laser light

The possibility to use diffuse reflections of a laser beam to exert a force on levitating dust particles is studied experimentally. Measurements and theoretical predictions are found to be in good agreement. Further, the method is applied to test the selective excitation of breathing-like modes in finite dust clusters.

Compact multiplexing

Spatial multiplexers (SMUXs) for mode division multiplexing often involve multiple strategies for mode-selective excitation and the minimization of insertion and other losses. Haoshuo Chen, Roy van Uden, Chigo Okonkwo and Ton Koonen, working at the COBRA Institute at the Eindhoven University of Technology in The Netherlands, have reported a SMUX for mode division multiplexing that packages multiple strategies into compact components with a small footprint (Opt. Express 22, 31582–31594; 2014). They demonstrated photonic integrated SMUXs on both silicon-on-insulator (SOI) and indium phosphide (InP) platforms for selectively exciting modes.

Mode-selective control of the crystal lattice.

CONSPECTUS: Driving phase changes by selective optical excitation of specific vibrational modes in molecular and condensed phase systems has long been a grand goal for laser science. However, phase control has to date primarily been achieved by using coherent light fields generated by femtosecond pulsed lasers at near-infrared or visible wavelengths. This field is now being advanced by progress in generating intense femtosecond pulses in the mid-infrared, which can be tuned into resonance with infrared-active crystal lattice modes of a solid. Selective vibrational excitation is particularly interesting in complex oxides with strong electronic correlations, where even subtle modulations of the crystallographic structure can lead to colossal changes of the electronic and magnetic properties. In this Account, we summarize recent efforts to control the collective phase state in solids through mode-selective lattice excitation. The key aspect of the underlying physics is the nonlinear coupling of the resonantly driven phonon to other (Raman-active) modes due to lattice anharmonicities, theoretically discussed as ionic Raman scattering in the 1970s. Such nonlinear phononic excitation leads to rectification of a directly excited infrared-active mode and to a net displacement of the crystal along the coordinate of all anharmonically coupled modes. We present the theoretical basis and the experimental demonstration of this phenomenon, using femtosecond optical spectroscopy and ultrafast X-ray diffraction at a free electron laser. The observed nonlinear lattice dynamics is shown to drive electronic and magnetic phase transitions in many complex oxides, including insulator-metal transitions, charge/orbital order melting and magnetic switching in manganites. Furthermore, we show that the selective vibrational excitation can drive high-TC cuprates into a transient structure with enhanced superconductivity. The combination of nonlinear phononics with ultrafast crystallography at X-ray free electron lasers may provide new design rules for the development of materials that exhibit these exotic behaviors also at equilibrium.

Selective Excitation of Spatial Modes in Two-Dimensional Microcavity Lasers

Selective Excitation of Spatial Modes in Two-Dimensional Microcavity Lasers

Nonlinear optical imaging of single plasmonic nanoparticles with 30 nm resolution.

We show that background free nonlinear optical imaging of Au nanostructures with a resolution down to 30 nm can be achieved. To attain such performance, an ultrafast laser source (110 fs pulse duration) has been integrated into a parabolic mirror assisted scanning near-field optical microscope. Combining nonlinear hyperspectral imaging and the simultaneously obtained topography, the setup allows one to directly correlate/assign locations with nonlinear signals originating from either second harmonic generation or two-photon excitation processes. The contrast mechanisms of the far-field background free nonlinear optical image are discussed based on the different tip-sample coupling schemes and the selective excitation of the plasmonic modes.

Researching of modulation of radiation intensity in multimode polymer fiber under selective excitation of modes

When coherent light is propagated into the multimode fiber, a speckle pattern is formed at the exit face of fiber.When the fiber is vibrated, the speckle pattern is modulated due to mode coupling and phase modulation of the propagating modes.In this paper the effect of fiber bends on theintensity modulation of the off-axis modes in multimode polymer fiber has been investigated. The theoretical analysis of a bend multimode fiber shows that the change of the speckle pattern intensity depends on the length of the fiber perturbed and amplitude of the vibration signal. When a highly coherent light source is used, the speckle pattern modulation is due primarily to phase modulation of the modes, and the component of the induced frequency overcomes the first and second harmonics with a difference of 13 dB and 18 dB, respectively, which allows clear identification of the induced vibration frequency. In contrast, when less coherent sources are used, the difference between fundamental vibration frequency and its harmonic components are small. Experimentally shown that when the off-axis modes are excited in multimode polymer fiber the amplitude of the output signal depends linearly on the amplitude of the vibration signal. The considered modulation mechanism can be effectively used to detection of the vibration and mechanical oscillation.

Compact spatial multiplexers for mode division multiplexing.

Spatial multiplexer (SMUX) for mode division multiplexing (MDM) has evolved from mode-selective excitation, multiple-spot and photonic-lantern based solutions in order to minimize both mode-dependent loss (MDL) and coupler insertion loss (CIL). This paper discusses the implementation of all the three solutions by compact components in a small footprint. Moreover, the compact SMUX can be manufactured in mass production and packaged to assure high reliability. First, push-pull scheme and center launch based SMUXes are demonstrated on two mostly-popular photonic integration platforms: Silicon-on-insulator (SOI) and Indium Phosphide (InP) for selectively exciting LP01 and LP11 modes. 2-dimensional (2D) top-coupling by using vertical emitters is explored to provide a coupling interface between a few-mode fiber (FMF) and the photonic integrated SMUX. SOI-based grating couplers and InP-based 45° vertical mirrors are proposed and researched as vertical emitters in each platform. Second, a 3-spot SMUX is realized on an InP-based circuit through employing 45° vertical mirrors. Third, as a newly-emerging photonic integration platform, laser-inscribed 3D waveguide (3DW) technology is applied for a fully-packaged dual-channel 6-mode SMUX including two 6-core photonic lantern structures as mode multiplexer and demultiplexer, respectively.

Mode selective generation of guided waves by systematic optimization of the interfacial shear stress profile

Piezoelectric transducers are commonly used in structural health monitoring systems to generate and measure ultrasonic guided waves (GWs) by applying interfacial shear and normal stresses to the host structure. In most cases, in order to perform damage detection, advanced signal processing techniques are required, since a minimum of two dispersive modes are propagating in the host structure. In this paper, a systematic approach for mode selection is proposed by optimizing the interfacial shear stress profile applied to the host structure, representing the first step of a global optimization of selective mode actuator design. This approach has the potential of reducing the complexity of signal processing tools as the number of propagating modes could be reduced. Using the superposition principle, an analytical method is first developed for GWs excitation by a finite number of uniform segments, each contributing with a given elementary shear stress profile. Based on this, cost functions are defined in order to minimize the undesired modes and amplify the selected mode and the optimization problem is solved with a parallel genetic algorithm optimization framework. Advantages of this method over more conventional transducers tuning approaches are that (1) the shear stress can be explicitly optimized to both excite one mode and suppress other undesired modes, (2) the size of the excitation area is not constrained and mode-selective excitation is still possible even if excitation width is smaller than all excited wavelengths, and (3) the selectivity is increased and the bandwidth extended. The complexity of the optimal shear stress profile obtained is shown considering two cost functions with various optimal excitation widths and number of segments. Results illustrate that the desired mode (A0 or S0) can be excited dominantly over other modes up to a wave power ratio of 1010 using an optimal shear stress profile.

Selective normal mode excitation in multilayer thin film bulk acoustic wave resonators

A method for selective normal mode excitation in thin film bulk acoustic wave resonators, based on multilayer structures with any number of ferroelectric films in the paraelectric phase, is presented. The possibility to control the excitation of thin film bulk acoustic resonators' normal modes by simultaneous manipulating both the polarities and the magnitudes of the dc bias voltages applied to the ferroelectric layers is demonstrated. The proposed method was verified using the Lakin's model, modified to describe the electro-mechanical behavior of a structure with four active ferroelectric layers.

Suppression of the fundamental mode in a dual-mode photonic crystal fiber

For the first time, the fundamental mode of a dual-mode photonic crystal fiber (PCF) is suppressed by the index-matching between the core and cladding modes. The PCF designed proposes a new solution of selective excitation of second-order modes. The simulation results demonstrate that the confinement loss of the fundamental mode is higher than 1dB/m, while the second-order modes are lower than 10−4dB/m in the C+L waveband. The high feasibility and low wavelength dependence make the fiber of great potential in the mode-division multiplexing (MDM) transmission.

A method for selective excitation of Ince-Gaussian modes in an end-pumped solid-state laser

A method for selective excitation of Ince-Gaussian modes is presented. The method is based on the spatial distributions of Ince-Gaussian modes as well as the transverse mode selection theory. Significant diffraction loss is introduced in a resonator by using opaque lines at zero-intensity positions, and this loss allows to excite a specific mode; we call this method “loss control.” We study the method by means of numerical simulation of a half-symmetric laser resonator. The simulated field is represented by angular spectrum of the plane waves representation, and its changes are calculated by the two-dimensional fast Fourier transform algorithm when it passes through the optical elements and propagates back and forth in the resonator. The output lasing modes of our method have an overlap of over 90 % with the target Ince-Gaussian modes. The method will be beneficial to the further study of properties and potential applications of Ince-Gaussian modes.

Acoustooptic Generation and Characterization of the Higher Order Modes in a Four-Mode Fiber for Mode-Division Multiplexed Transmission

We demonstrate a highly efficient acoustooptic mode converter for selective excitation of the higher-order modes in a four-mode fiber which guides the LP01, LP11, LP21, and LP02 modes at telecommunication wavelengths. The coupling efficiency and the mode extinction ratio are measured by analyzing the output far-field patterns. The acoustic resonance frequencies and the operating bandwidths are compared with the theoretical results, by which the modal characteristics, the core diameter, and the core-cladding index difference are estimated.

Systematic design method of a mobile multiple antenna system using the theory of characteristic modes

In this study, the authors present a systematic design method of a three-port multiple-input-multiple-output antenna system for small terminals, such as smart phones based on the selective excitation of orthogonal chassis modes. The underlying idea is based on the theory of characteristic modes (TCM). Selective excitation of these modes is realised by sets of non-resonant inductive coupling elements integrated at predefined positions into the printed circuit board (PCB) layout. Stripline feed networks also integrated within the multi-layer PCB layout care for a good impedance match to 50 Ω. The design method of this antenna system is a straight forward application of the TCM and is therefore of general interest for the design of small terminal antennas. Furthermore, owing to the electrically small couplers that require only limited area at the outer edge of the PCB the method represents an interesting solution with the clear potential to be extended to more ports. The realised prototype resembles the predicted performance very well and therefore proves the applicability of the proposed method.

Inertial wave dynamics in a rotating liquid metal

AbstractThe dynamics of free and forced inertial waves inside cylinders of different aspect ratios ($\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}A=H_0/2R_0$) were investigated experimentally in this study. The liquid metal GaInSn was chosen as the fluid in order to enable a contactless stimulation of the flow by means of alternating electromagnetic fields. A rotating magnetic field generates the rotating motion of the liquid, whereas periodic modulations of the field strength and short pulses excite specific wave modes. Ultrasound Doppler velocimetry was used to record the flow structure and to identify inertial waves in the set-up. Our experiments demonstrate selective excitation of different inertial wave modes by deliberate variation of the magnetic field parameters. Furthermore, it was found that turbulent perturbations in the boundary layers of the swirling flow are able to induce an inertial wave mode that survives over a long time. Experiments at the fundamental resonance have shown that multiple harmonic wave modes appeared simultaneously. The measured inertial wave frequencies were compared to the predictions of the linear inviscid theory.

Dynamics and Control in Complex Transition Metal Oxides

Advances in the synthesis, growth, and characterization of complex transition metal oxides coupled with new experimental techniques in ultrafast optical spectroscopy have ushered in an exciting era of dynamics and control in these materials. Experiments utilizing femtosecond optical pulses can initiate and probe dynamics of the spin, lattice, orbital, and charge degrees of freedom. Major goals include (a) determining how interaction and competition between the relevant degrees of freedom determine macroscopic functionality in transition metal oxides (TMOs) and (b) searching for hidden phases in TMOs by controlling dynamic trajectories in a complex and pliable energy landscape. Advances in creating intense pulses from the far-IR spectrum through the visible spectrum enable mode-selective excitation to facilitate exploration of these possibilities. This review covers recent developments in this emerging field and presents examples that include the cuprates, manganites, and vanadates.