Degenerate Cavity Modes Research Articles

Circularly‐polarized cavity‐backed slot antenna array with simplified feeding structure

A class of circularly-polarized cavity-backed slot antenna arrays with simplified feeding structure are proposed. Power dividing network is an essential feeding structure for the design of conventional antenna arrays, which results in complicated antenna structure. A novel feeding technique based on the electric field of cavity modes is introduced to feed the array elements. The slots on the top wall are directly fed by the electric field without additional feeding structure. Crossed slots with different lengths are introduced to achieve the self-phased CP radiation as they produce different perturbation on the two degenerate cavity modes. Therefore, high efficiency and simple antenna structure of the CP slot antenna array are achieved. Then, two CP slot arrays with 3 × 3 and 8 × 8 elements are presented to show the designed feasibility. Finally, the 3 × 3 CP antenna array is fabricated and measured to validate the design concept, which can achieve 12.6 dBic gain, 94% total efficiency, 80% aperture efficiency, and −25 dB cross polarization.

Enhanced emission from a single quantum dot in a microdisk at a deterministic diabolical point.

We report on controllable cavity modes by controlling the backscattering by two identical scatterers. Periodic changes of the backscattering coupling between two degenerate cavity modes are observed with the changing angle between two scatterers and elucidated by a theoretical model using two-mode approximation and numerical simulations. The periodically appearing single-peak cavity modes indicate mode degeneracy at diabolical points. Interactions between single quantum dots and cavity modes are then investigated. Enhanced emission of a quantum dot with a six-fold intensity increase is obtained in a microdisk at a diabolical point. This method to control cavity modes allows large-scale integration, high reproducibility and flexible design of the size, the location, the quantity and the shape for scatterers, which can be applied for integrated photonic structures with scatterer-modified light-matter interaction.

Simultaneous All-Optical or and xor Logic Gates Based on the Bimodal Photonic Cavity Containing a Quantum Dot

We theoretically present a design for simultaneous all-optical or and xor logic gates based on single quantum dot (QD)-bimodal cavity system working in low-photon-number regime. The coupling between the QD and two degenerate cavity modes establishes new indirect excitation paths. Driven by single pulsed laser, both mode transmissions are observed. The delay between the peaks of the transmitted pulses is regarded as the extra transition interval (ETI) in the indirect path. When driven by two orthogonally polarized pulsed lasers with delay same as ETI, the coupling system suppresses the latter mode transmission due to the destructive interference effect. The or and xor logic gates based on presence of light are built and the function can be switched between two detectors by simply adjusting the driving pulse sequence. The results show that the proposed logic gates work well within a large range for coupling strength with comparable cavity decay rate. When taking two nonidealities (the detuning between two cavity modes and nonidentical coupling strengths) into consideration, the proposed system shows acceptable tolerances. Moreover, the robustness of the performance on parameters of the driving lasers, including pulse delay, width of pulse, absolute driving strength, and relative driving strength ratio, are demonstrated.

Dual-wavelength filters based on two-dimensional photonic crystal degenerate modes with a ring dielectric rod inside the defect cavity.

A tunable dual-wavelength filter based on degenerate modes with a ring dielectric rod inside the single point defect cavity is proposed. The band structures and mode profiles are computed by the plane-wave expansion method. The normalized transmission spectra for this structure are investigated by using the two-dimensional finite-difference time-domain method. The two orthogonal output modes of the filter can be regarded as the combination of the original degenerate cavity modes and both excited due to the introduced perturbation rod. The influences of the perturbation rod on the localized modes, band separations, and the tuning ranges are all investigated and analyzed based on the perturbation theory. With the ring dielectric rod being introduced into the defect, a narrower wavelength bandwidth and a wider wavelength tuning range can be obtained, which are superior to that of the filter based on a cavity with a larger radius rod. Besides, the perturbation rod can also be used for splitting degenerate modes for multiwavelength filters. The proposed filter has a simple structure and may be potentially applied in various integrated circuits, such as multiwavelength filters.

Role of cavity induced decay-interference effect on vacuum-Rabi splitted spectrum

We have shown that an ultranarrow dark region can be associated with the feature of significant amount of vaccuum-Rabi splitting in the absorption spectrum of a weak coherent field probing a two-level atom strongly coupled to two degenerate cavity-modes with preselected polarizations. The cavity-induced transparency is produced by the decay-interference of the two independent cavity modes interacting with the same reservoir. It has been shown that the resonant probe transparency is independent of the individual decay rates of the cavity modes. The present model exhibits the possibility of obtaining the narrow dark transparency associated with the significant amount of Rabi splitting in the absorption spectrum with the appreciable value of the cavity-losses.

Ultra-compact highly-selective dual-mode pseudoelliptic filters

An innovative ultra-compact dual-mode waveguide filter is presented. The basic idea consists of using a pair of TMmn0 degenerate cavity modes, while non-resonant modes provide a direct input-to-output coupling. In this manner, the single cavity provides two reflection and two transmission zeros, and is at the same time very compact in the longitudinal direction. Theoretical and experimental results of a two-cavity filter with four transmission zeros demonstrate the feasibility of the approach.

Purcell-Factor-Enhanced Scattering from Si Nanocrystals in an Optical Microcavity

Scattering processes in an optical microcavity are investigated for the case of silicon nanocrystals embedded in an ultra-high-Q toroid microcavity. Using a novel measurement technique based on the observable mode splitting, we demonstrate that light scattering is highly preferential: more than 99.8% of the photon flux is scattered into the original doubly degenerate cavity modes. The large capture efficiency is shown to result from the Purcell enhancement of the optical density of states over the free space value, an effect that is more typically associated with spontaneous emission. The experimentally determined Purcell factor has a lower bound of 171.

Alternate oscillations in quasistadium laser diodes

We found alternate oscillations in AlGaAs/GaAs single-quantum-well quasistadium laser diodes (QSLDs). Unlike the alternate oscillations in conventional ring lasers, which are explained by interaction between two counterpropagating modes, the alternate oscillations in QSLDs arise from two nearly degenerate cavity modes due to its geometric structure. The lasing signals in QSLDs are directly obtained from the emitted beams which spread over the space without the use of waveguides. We also showed that the alternate oscillation frequencies decrease linearly as the amplitudes of the two different cavity modes increase.

Entangled two-photon source using biexciton emission of an asymmetric quantum dot in a cavity

A semiconductor based scheme has been proposed for generating entangled photon pairs from the radiative decay of an electrically-pumped biexciton in a quantum dot. Symmetric dots produce polarisation entanglement, but experimentally-realised asymmetric dots produce photons entangled in both polarisation and frequency. In this work, we investigate the possibility of erasing the `which-path' information contained in the frequencies of the photons produced by asymmetric quantum dots to recover polarisation-entangled photons. We consider a biexciton with non-degenerate intermediate excitonic states in a leaky optical cavity with pairs of degenerate cavity modes close to the non-degenerate exciton transition frequencies. An open quantum system approach is used to compute the polarisation entanglement of the two-photon state after it escapes from the cavity, measured by the visibility of two-photon interference fringes. We explicitly relate the two-photon visibility to the degree of Bell-inequality violation, deriving a threshold at which Bell-inequality violations will be observed. Our results show that an ideal cavity will produce maximally polarisation-entangled photon pairs, and even a non-ideal cavity will produce partially entangled photon pairs capable of violating a Bell-inequality.

Enhanced atom capturing in a high-Q cavity by help of several transverse modes

We predict a strong enhancement of the capture rate and the friction force for atoms crossing a driven high-Q cavity field if several near degenerate cavity modes are simultaneously coupled to the atom. In contrast to the case of a single TEM00 mode, circular orbits are not stable and damping of the angular and radial motion occurs. Depending on the chosen atom-field detuning the atoms phase lock the cavity modes to create a localized field minimum or maximum at their current positions. This corresponds to a local potential minimum, which the atom drags along with its motion. The stimulated photon redistribution between the modes then creates a large friction force. The effect is further enhanced if the atom is directly driven by a coherent field from the side. Several atoms in the field interact via the cavity modes, which leads to a strongly correlated motion.

Photon statistics anomalies in adiabatic transfer between two cavity modes

Adiabatic transfer between two degenerate cavity modes is investigated theoretically using a counter-intuitive pulse sequence. Several measures of successful state transfer are shown to correlate strongly with anomalous values of the photon statistics of the initial mode. The detailed behaviour of theses anomalies originate from the way the state approaches vacuum and their features are found to give a good indication of the effciency and robustness of the state transfer.

Dynamic generation of maximally entangled photon multiplets by adiabatic passage

The adiabatic passage scheme for quantum state synthesis, in which atomic Zeeman coherences are mapped to photon states in an optical cavity, is extended to the general case of two degenerate cavity modes with orthogonal polarization. Analytical calculations of the dressed-state structure and Monte Carlo wave-function simulations of the system dynamics show that, for a suitably chosen cavity detuning, it is possible to generate states of photon multiplets that are maximally entangled in polarization. These states display nonclassical correlations of the type described by Greenberger, Horne, and Zeilinger (GHZ). An experimental scheme to realize a GHZ measurement using coincidence detection of the photons escaping from the cavity is proposed. The correlations are found to originate in the dynamics of the adiabatic passage and persist even if cavity decay and GHZ state synthesis compete on the same time scale. Beyond entangled field states, it is also possible to generate entanglement between photons and the atom by using a different atomic transition and initial Zeeman state.

Microwave Faraday Rotation: Design and Analysis of a Bimodal Cavity

The design and analysis of a bimodal cavity for the observation of microwave Faraday rotation is presented. An equivalent circuit of lumped elements is developed and the coupling between degenerate cavity modes is expressed in terms of the elements of the susceptibility tensor of the material producing the rotation. The theory is checked against experimental results with a paramagnetic salt and substantial agreement is obtained. A cavity of this type when used in conjunction with superheterodyne detection appears to provide a high sensitivity spectrometer for the observation of magnetic resonance.

Circularly Polarized Microwave Cavity Filters

A new group of circularly polarized microwave cavity filters is described. With a single circularly polarized cavity, a reflectionless filter is achieved that couples nearly 100 per cent of the energy from the main waveguide at the cavity resonant frequency. Two degenerate cavity modes may be excited, to produce a circularly polarized field, by coupling to the transverse and longitudinal waveguide magnetic fields or to the transverse electric and magnetic waveguide fields. A theoretical analysis is presented as well as experimental results. The loss between the band-pass terminals of the reflectionless circularly polarized filter is identical with the loss in a conventional reflectiontype band-pass filter with the same bandwidth and cavity-wall losses. The null at resonance between the band-elimination terminals of the reflectionless circularly polarized filter is limited only by the asymmetries of the cavity and not by the cavity-wall losses. Design equations and curves are given for eight of the lower order, circularly cylindrical, degenerate cavity modes that are coupled to a rectangular waveguide at the point of circularly polarized waveguide magnetic fields.