Bragg Structures Research Articles

Enhancing X-Ray Generation by Electron-Beam–Laser Interaction in an Optical Bragg Structure

We demonstrate that x-ray radiation emitted by relativistic electrons scattered by a counter-propagating laser pulse guided by an adequate Bragg structure surpasses by about 2 orders of magnitude the energy generated by a conventional free-space Gaussian-beam configuration, given the same e beam and injected laser power in both configurations.

Cherenkov masers with two-dimensional distributed feedback

The possibility of using a two-dimensional (2D) distributed feedback to generate spatially coherent radiation by straight sheet and annular electron beams is studied. The schemes of sectional Cherenkov masers with the transverse synchronization of radiation performed in a 2D Bragg structure, which is arranged at the cathode end of the interaction space and conjugates the longitudinal and transverse (azimuthal) wave flows, is analyzed. By introducing dissipation into this structure, it is possible to increase the transverse dimensions of masers to a level on the order of 103 wavelengths with the corresponding increase in the total output radiation power without changing spectral characteristics and decreasing the energy conversion efficiency.

Continuous wave Terahertz spectrometer with coherent detection

In this paper a continuous wave (cw) spectrometer in the far infrared range covering frequencies between several tens Gigahertz and one Terahertz is presented. The spectrometer employed a coherent detection scheme and therefore measures both the amplitude and phase of THz radiation. First spectroscopic results of Si-wafer and a test Bragg structure are presented.

Two-dimensional distributed feedback lasers with excitation of TE waves in the active medium

Two-dimensional Bragg resonators with the coupling of TE- and TM-polarised waves are proposed. The selective properties of such resonators are analysed. Within the semiclassical approach, the nonlinear dynamics of laser radiation with a two-dimensional (in the xy plane) distributed feedback is studied, at which TE-polarised waves, propagating in the ±z directions, are amplified in the active medium (in particular, based on quantum wells). The latter, on a two-dimensional Bragg structure, are scattered into TM-polarised waves propagating in the ±x directions. These partial wave flows do not interact with the active medium but provide the spatial radiation synchronisation. The conditions of the solution self-similarity are obtained with increasing the dimensions of the active medium and the corresponding increase in the integral output power. It is shown that when an additional end mirror is mounted, almost unidirectional radiation coupling can be realised.

Trapping of surface-plasmon polaritons in a graded Bragg structure: Frequency-dependent spatially separated localization of the visible spectrum modes

We theoretically demonstrate that a metallic film covered by a dielectric grating of graded thickness can strongly slow light as the propagation velocities of surface plasmon polaritons (SPPs) are reduced over a large frequency bandwidth at visible frequencies. Since the dispersive relation of SPPs is dependent on the dielectric grating thickness, the guided SPPs at different frequencies can be localized at different spatial positions of the plasmonic grating. We numerically demonstrate that a true rainbow from violet to red colors can be separately localized, resulting in the spatial separation of the visible spectrum on a chip.

Photonic Polymer Replicas Containing CdSe Nanoparticles Based onPorous Silicon

A photonic crystal containing a Bragg structure was prepared by applying a computer-generated periodic square current waveform and result in a mirror with high reflectivity in a specific narrow spectral region. This paper describes a method for the fabrication of photonic polymer replicas containing CdSe nanoparticles based on distributed Bragg reflector (DBR) porous silicon (PSi). Well-defined one-dimensional photonic polymethly methacrylate (PMMA) replicas containing CdSe nanoparticles showing a reflectivity at 571 nm and a photoluminescence at 560 nm were successfully obtained by removing the DBR PSi template from the PMMA composite film. XRD measurements indicated that the oxidized DBR PSi was completely removed from the composite films. The photonic polymer replicas exhibited a sharp resonance with a full width at half maximum (FWHM) of 20 nm in the reflectivity spectrum. The optical characteristics of the photonic polymer replicas indicated that the surface of the polymer film had the negative structure of DBR PSi. These replicas were stable in aqueous solutions for several days without any degradation.

On the modal characteristics of surface plasmon polaritons at a metal-Bragg interface at optical frequencies

A detailed mathematical analysis along with a theoretical model for the modes supported at the interface of a metal and periodically stratified medium (Bragg structure) is presented. The modes that are supported at the interface of a plasmon active metal (such as gold) and a Bragg structure are commonly known as surface plasmon-Bragg modes. We found that these modes have effective indices lower than any of the material indices of the layers comprising the Bragg structure, and they are highly dispersive when compared to the conventional surface plasmon modes that are supported at the metal and dielectric interface. The plausible physical explanation behind the strong dispersive behavior of the surface plasmon-Bragg mode is provided. Finally, the comparison of dissipation loss for the surface plasmon-Bragg modes is investigated and it has been shown that there is more than fivefold enhancement in the magnitude of propagation lengths as compared to the conventional surface plasmon mode.

Frequency stabilization in free-electron masers with 2D and 1D distributed feedback

We analyze the dynamics of free-electron masers (FEMs) with two-mirror hybrid Bragg resonators using the coupling between running and quasi-critical modes in the input mirror. Such a mirror may have the form of a 2D Bragg structure with coaxial geometry or a segment of a cylindrical waveguide with axisymmetric corrugation having a period close to the wavelength. The output mirror has the traditional Bragg structure coupling two counterpropagating running waves (the corrugation period is close to half the wavelength). It is shown that a stable unimodal lasing with a radiation frequency close to the cutoff frequency of the quasicritical mode excited in the input mirror can be attained using this scheme under optimal conditions. Such a regime is insensitive to variations of the electron beam parameters. Simulation of experimentally implemented FEMs and those being developed is carried out.

Acoustoelectronic microwave structures: Principles of operation, similarity with optics

Many technical solutions in microwave acoustics are similar to these in optics. In this paper Bulk Acoustic Wave (BAW) microwave resonators and filters are discussed together with the devices that use the peculiarities of BAW propagation through semitransparent acoustic Bragg structures. An acoustic analogue of the resonator Fabri-Perott and acoustic resonant multilayered structures are considered. The use of a piezoelectric semiconductor resonant layer with electron drift provides the possibility of the amplification and generation of acoustic waves.

Light confinement in coherently driven defect states in solids

AbstractInhomogeneously broadened optical transitions in nitrogen‐vacancy color centers in diamond and Pr3+ doped yttrium orthosilicate may be employed to attain all‐optically controlled Bragg structures with photonic band‐gaps of high reflectivities and large bandwidths. Such a control mechanism is valuable for improving performance and scalability of solid quantum memory devices. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2722–2726, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24723

Short-wave sectioned free-electron masers with Bragg resonators based on the traveling and quasi-critical wave coupling

We consider the possibility of implementation of a free-electron maser with a two-mirror resonator composed of modified and conventional Bragg mirrors, operated in the short-wave part of the millimeter-wave range. The use of a modified Bragg mirror based on the traveling and quasicritical wave coupling at the input of the interaction space permits the transverse-index selection of modes. Amplification of the synchronous co-propagating wave by an electron beam is reached mainly in the regular part of the resonator. Even slight reflections from the conventional output Bragg cavity, which directly couples the co- and counter-propagating traveling waves, turn out to be sufficient for generation of self-excited oscillations. It is shown that the new scheme of a free electron maser ensures the oscillation frequency stabilization with respect to the electron-energy variation. With the optimal choice of the parameters, the oscillation frequency is close to the cutoff frequency of a quasi-critical wave excited in the modified Bragg structure.

Free electron laser with terahertz band Bragg reflectors

Periodical Bragg structures may be considered as an effective way of controlling the electromagnetic energy fluxes and provision of spatially coherent radiation in the free electron lasers with oversized interaction space. A new scheme of terahertz band FEL with hybrid Bragg resonator is proposed consisting of advanced input Bragg mirror and traditional output Bragg mirror. An advanced Bragg mirror exploiting the coupling between the two counterpropagating modes and the quasicutoff one provides mode selection over the transverse index. The main amplification of the wave by the electron beam takes place in the regular section of the resonator. Small reflections from the output traditional Bragg mirror are sufficient for oscillator self-excitation.

Possibility of surface plasmon-polaritons amplification by direct current in two-interface systems with 2D Bragg structure on the surface

The method for calculation of the local field factor that describes electrodynamical properties of three-layered systems with 2D photonic crystal is proposed. Analytical calculations are performed in the frame of Green function method on the base of the effective susceptibility concept. The possibility of surface waves amplification by direct electrical current in the systems under consideration is studied. Numerical analysis clearly proves the presence of convective instability. It means that three-layered systems can be considered as a surface waves’ amplifier. Because of periodical structure the phase synchronism domain lies in the range of entirely accessible in practice values of wave vector and frequency. The experimental setup and real systems parameters for observation of surface waves amplification effect are discussed.

Liquid crystal based electrically switchable Bragg structure for THz waves

We demonstrate the first electronically switchable Bragg structure for THz frequencies. The structure works as stop-band filter and as mirror. It exhibits the 60 GHz broad stop-band around 300 GHz which can be removed by reorienting liquid crystal molecules in an external electric field. Our first proof-of-principle experiments agree very well with transfer matrix calculations.

Effect of ripple shape and taper on frequency response of reflectivity and transmission in a coaxial Bragg structure

By making use of the CST software, a comparative study is carried out for the effect of three ripple shapes (sinusoidal, rectangular and triangular ripples) on the frequency response in a coaxial Bragg structure. Results show that the rectangular ripple leads to the widest bandwidth while the triangular ripple narrows the bandwidth. Moreover, based on the mode-coupling model, numerical analysis is presented to demonstrate the influence of the ripple tapers. It is shown that the interval between the band gaps of the competing mode and the desired working mode is narrowed by the use of a positive taper, but is expanded if a negative taper is employed.

Using two-dimensional Bragg structures for the synchronization of radiation in planar backward wave oscillators

We propose to use two-dimensional Bragg resonance structures for the spatial synchronization of emission in planar backward wave oscillators (BWOs) pumped by wide sheet electron beams. It is demonstrated that the synchronization regime is stable with respect to changes in the detuning between the free-running BWO generation frequency and the Bragg frequency.

Conservative and dissipative fiber Bragg solitons (a review)

We present a comparative review of two classes of optical solitons—conservative and dissipative solitons—propagating in single-mode optical fibers in which refractive-index gratings are induced such that their period is comparable with the radiation wavelength. Fibers that have the Kerr nonlinearity and negligibly small losses and that do not gain radiation (conservative system) are described by traditional equations of the approximation of slowly varying amplitudes, and effects caused by the nonlinearity of the medium, such as nonlinear switching, optical bistability, and formation of conservative Bragg solitons are considered. It is shown that the passage beyond the scope of the approximation of slowly varying amplitudes makes it possible to describe new important effects, including localization of soliton centers near maxima of the refractive-index grating. Bright and dark conservative solitons are demonstrated, which are formed when the Kerr nonlinearity is replaced by the nonlinearity of two-level atomic systems. The properties of conservative solitons in resonance semiconductor Bragg structures with quantum wells are considered. Results of experimental studies of nonlinear effects in fibers with Bragg gratings are presented. For an active single-mode fiber with a Bragg refractive-index grating and nonlinear gain and absorption, dissipative solitons are described using the approximation of slowly varying amplitudes and inertialess nonlinearity. It is shown that the dissipative factors qualitatively change the properties of solitons compared to the conservative case. Using the Maxwell-Bloch equations, it is demonstrated that the ratio between the gain and absorption relaxation times significantly affects the stability of localized structures. The interaction of dissipative optical Bragg solitons is described. It is shown that, beyond the scope of the approximation of slowly varying amplitudes, the average velocity of propagating dissipative Bragg solitons acquires only discrete values, and formation of pairs of solitons with two values of the phase difference becomes possible. For a birefringent fiber, dissipative vector optical Bragg solitons are demonstrated.

Fabrication of photonic crystals with functional defects by one-step holographic lithography

A one-step introduction of functional defects into a photonic crystal is demonstrated. By using a multi-beam phase-controlled holographic lithography, line-defects in a Bragg structure and embedded waveguides in a two-dimensional photonic crystal are fabricated. Intrinsic defect introduction into a 3-dimensional photonic crystal is also proposed. This technique gives rise to a substantial reduction of the fabrication complexity and a significant improvement on the accuracy of the functional defects in photonic crystals.

Generation of spatially coherent radiation in free-electron masers with two-dimensional distributed feedback

The results of theoretical and experimental studies on the generation of spatially coherent electromagnetic radiation in a planar free-electron maser with two-dimensional distributed feedback are reported. A two-dimensional Bragg structure is used at the initial part of the interaction space to ensure the transverse synchronization of the radiation. The possibility of the narrowband generation in the 75-GHz frequency band is demonstrated experimentally for a sheet kiloampere electron beam whose width is 20 times larger than the wavelength.

Generation of a self-pulsed picosecond solitary wave train from a periodically amplifying Bragg structure

Self-pulsed lasing in the form of a coherent solitary wave train from a periodically amplifying Bragg structure is proposed. It is shown both analytically and numerically that the generated pulse train propagates in the periodically amplifying Bragg structure chip with superluminal group velocity. Its stability and coherence are in contrast with the inherent instability and chaoticity of self-pulsing from a uniform gain medium embedded in a Bragg reflector. Bragg-periodic semiconductor quantum-well heterostructures are candidates for realizing the required periodically amplifying Bragg structure.