This paper presents a comprehensive theory of the cyclotron resonance maser (CRM) interaction in a circular waveguide. The kinetic theory is used to derive the dispersion relationships for both TE and TM modes. The TE mode case has been investigated by several authors, but there has been comparatively little work on the TM mode case. However, the TM mode interaction competes effectively with the TE mode interaction at relativistic electron energies. The conditions for maximum temporal and spatial growth rates are shown. The TM mode growth rates are found to vanish when the RF wave group velocity equals the beam axial velocity (‘grazing incidence’). The single particle theory is used to derive a compact set of self-consistent non-linear equations for the TE and TM mode interactions. These equations are particularly appropriate for the cyclotron auto-resonance maser (CARM) regime but applicability extends to other regimes as well. The conditions for optimum efficiency are investigated for oscillator and amp...

Linear and nonlinear theory of the Doppler-shifted cyclotron resonance maser based on TE and TM waveguide modes

We propose a new approach leading to giant gain enhancement. It is based on unconventional slow-wave resonance associated with a degenerate band edge (DBE) in the dispersion diagram for a special class of photonic crystals supporting four modes at each frequency. We show that the gain enhancement in a Fabry-P\'erot cavity (FPC) when operating at the DBE is several orders of magnitude stronger when compared to a cavity of the same length made of a standard photonic crystal with a regular band edge (RBE). The giant gain condition is explained by a significant increase in the photon lifetime and in the local density of states. We have demonstrated the existence of DBE operated special cavities that provide for superior gain conditions for solid-state lasers, quantum cascade lasers, traveling wave tubes, and distributed solid-state amplifiers. We also report the possibility to achieve low-threshold lasing in FPC with DBE compared to RBE-based lasers.

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Giant gain enhancement in photonic crystals with a degenerate band edge

We report on experimental realization of the high fidelity of electric pulses passing through normal or anomalous dispersive cascaded electronic circuit systems (CECSs). It is found that the positive or negative group delay can be regarded as a good concept to describe the evolution of electric pulses as long as the fidelity of the output electric pulses is high. Here we systematically demonstrate that the fidelity changes in both normal and anomalous dispersive CECSs strongly depend on the different truncations or pulse shapes encoded in electric signals. Our experiment confirms that before a critical number of cascaded circuits, the pulse fidelity is high initially and changes slowly, however it suffers an exponential decrease beyond that critical number. For a certain electric input pulse, this critical number in normal dispersive CECSs is usually much larger than that in anomalous dispersive CECSs. For a fixed CECS, the weaker the input pulse is truncated, the larger the critical number is. Our results may be useful to the potential applications of positive or negative group delay in designing electronic circuit systems to compensate group delay.

High fidelity of electric pulses in normal and anomalous cascaded electronic circuit systems

This article proposes a scannable leaky-wave antenna (LWA). The antenna consists of a ferrite-blade waveguide with a longitudinal slot on the upper wall, which has a tunable composite right-/left-handed (CRLH) property. The left-handed (LH) behavior of this structure is based on the negative effective permeability of the ferrite material and the negative effective permittivity of the waveguide excited below its cutoff frequency. First, based on the modal analysis and calculated dispersion relation, the unbalanced CRLH response of the ferrite-blade waveguide is investigated. Second, an LWA is presented by using a longitudinal slot on the upper wall of the proposed ferrite-blade waveguide, where the tunable balance CRLH response is achieved and analyzed by the transverse resonance method (TRM). The proposed LWA can provide backward, broadside, and forward radiations by a change in the operation frequency or the magnetic bias, independently. The given results show a +59° to −57° sweep in the radiation beam when the magnetic bias of the ferrite changed from 1400 to 1600 Oe at the sampling frequency of 6.4 GHz. Moreover, the results show that the frequency tuning range is more than 600 MHz (6.2–6.8 GHz). The peak gain and efficiency of the proposed antenna are 7.1 dBi and 78.5% at 6.6 GHz, respectively.

Scannable Leaky-Wave Antenna Based on Ferrite-Blade Waveguide Operated Below the Cutoff Frequency

Fifty years ago Hartman studied the barrier transmission time of wave packets (J Appl Phys 33:3427–3433, 1962). He was inspired by the tunneling experiments across thin insulating layers at that time. For opaque barriers he calculated faster than light propagation and a transmission time independent of barrier length, which is called the Hartman effect. A faster than light (FTL or superluminal) wave packet velocity was deduced in analog tunneling experiments with microwaves and with infrared light thirty years later. Recently, the conjectured zero time of electron tunneling was claimed to have been observed in ionizing helium inside the barrier. The calculated and measured short tunneling time arises at the barrier front. This tunneling time was found to be universal for elastic fields as well as for electromagnetic fields. Remarkable is that the delay time is the same for the reflected and the transmitted waves in the case of symmetric barriers. Several theoretical physicists predicted this strange nature of the tunneling process. However, even with this background many members of the physics community do not accept a FTL signal velocity interpretation of the experimental tunneling results. Instead a luminal front velocity was calculated to explain the FTL experimental results frequently. However, Brillouin stated in his book on wave propagation and group velocity that the front velocity is given by the group velocity of wave packets in the case of physical signals, which have only finite frequency bandwidths. Some studies assumed barriers to be cavities and the observed tunneling time does represent the cavity lifetime. We are going to discus these continuing misleading interpretations, which are found in journals and in textbooks till today.

On the Traversal Time of Barriers

A strong and broadband negative group delay (NGD) is demonstrated in a circular waveguide with an asymmetric cross-shaped slotted structure in the millimeter-wave region. For an input of 45° linear polarization (relative to the orientation of the cross-shaped slots), the NGD fractional bandwidth is as high as 3.57%, in which the optimal group advancement obtained is nearly 24 times greater than the delay traveling with the speed of light in vacuum. This further indicates a very high fractional advancement of 12.10%. In addition, the NGD can be controlled by adjusting the polarization azimuthal angle of the incident pulse. The observed tunable NGD effect is attributed to the destructive cross interference between the two polarization eigenstates, which effectively reduces the released field energy at the observed polarization channel. A general theory is provided with excellent agreement in the experimental and simulation results.

Tunable Negative Group Delay in a Birefringent Fabry–Pérot-Like Cavity With High Fractional Advancement Induced by Cross-Interference Effect

http://absimage.aps.org/image/MAR13/MWS_MAR13-2012-020612.pdf

Frequency-dependent cavity lifetime and apparent superluminality in Fabry-Perot-like interferometer

This study examines the transverse magnetic (TM) waveguide modes, which have long been considered as the unsuitable ones for the operation of the electron cyclotron maser. The beam-wave coupling strength of the TM modes, as expected, is found to be relatively weak as compared with that of the transverse electric (TE) waveguide modes. Unlike TE modes, surprisingly, the linear behavior of the TM modes depends on the sign of the wave number kz. The negative kz has a much stronger linear efficiency than that of the positive kz. The bunching mechanism analysis further exhibits that the azimuthal bunching and axial bunching do not compete but cooperate with each other for the backward-wave operation (negative kz). The current findings are encouraging and imply that TM modes might be advantageous to the gyrotron backward-wave oscillators.

Asymmetric linear efficiency and bunching mechanisms of TM modes for electron cyclotron maser

We synthesized BaTiO3–epoxy nanocomposites (particle size < 100 nm) with volume fractions up to 25 vol. %, whose high-frequency complex permittivity was characterized from 8.2 to 12.5 GHz. The maximum dielectric constant approaches 9.499 with an acceptable loss tangent of 0.113. The dielectric loss gradually saturates when the particle concentration is higher than 15 vol. %. This special feature is an important key to realizing high-k and low-loss nanocomposites. By comparing the theoretical predictions and the experimental data, four applicable effective-medium models are suggested. The retrieved dielectric constant (loss tangent) of 100-nm BaTiO3 nanopowder is in the range of 50–90 (0.1–0.15) at 8.2–12.5 GHz, exhibiting weak frequency dispersion. Two multilayer microwave devices—total reflection and antireflection coatings—are designed based on the fabricated nanocomposites. Both devices show good performance and allow broadband operation.

Dielectric Properties of BaTiO3–Epoxy Nanocomposites in the Microwave Regime

This study investigates the interaction between the relativistic electrons and the waves in cavities with fixed field profiles. Both the transverse electric (TE) and the transverse magnetic (TM) cavity modes are examined, including three first-axial modes, TE011, TM011, and TM111, and two zero-axial modes, TM010 and TM110. The first-axial modes have the same resonant frequency, so a direct comparison can be made. By sweeping the electron pitch factor (α) and the electron transit angle (Θ), the optimal converting efficiency of TM modes occurs at α = 1.5 and Θ = 1.5π, unlike the TE mode of α = 2.0 and Θ = 1.0π. The converting efficiencies of both the first-axial TM modes are much lower than that of TE011 mode. The starting currents of TM011 and TM111 modes are four times higher than that of TE011 mode, indicating that these two TM modes are very difficult to oscillate. This evidences that under the traditional operating conditions, the TM-mode gyrotrons are insignificant. However, the two unique, zero-axial...

Hsin-Yu Yao

Papers

Tunable Negative Group Delay in a Birefringent Fabry–Pérot-Like Cavity With High Fractional Advancement Induced by Cross-Interference Effect

Frequency-dependent cavity lifetime and apparent superluminality in Fabry-Perot-like interferometer

Asymmetric linear efficiency and bunching mechanisms of TM modes for electron cyclotron maser

Dielectric Properties of BaTiO3–Epoxy Nanocomposites in the Microwave Regime

Nonlinear oscillations of TM-mode gyrotrons