Fundamental Cavity Mode Research Articles

OPTIMAL DESIGN OF SINGLE-PHOTON SOURCE EMISSION FROM A QUANTUM-DOT IN MICRO-PILLAR MICROCAVITY

We have modelled wavelength scale micro-pillar microcavities of group III-V semiconductor materials using the 3-D finite difference time domain (FDTD) method. A broad band dipole source within the microcavity probes the microcavity mode structure and spectrum. We then investigated the modifications to spontaneous emission of photons form narrowband emitters (e.g. quantum dots) at the centre of the resonance. We find strongly enhanced emission due to small modal volumes and high quality factor (Q-factor). A large fraction of the quantum-dot spontaneous emission is coupled into the fundamental cavity mode. Increasing the number of mirror pairs in the bottom distributed Bragg reflector (DBR) obviously reduces the bottom light leakage, leading to light collection efficiency up to 90%. Moreover, we are now looking at more sophisticated structures with both lateral and perpendicular confinements based on annular and photonic crystal defect cavities in order to suppress the remaining sidewall scattering.

Spectral tuning of the coupling between isolated InAs quantum dots and the fundamental micropost cavity mode

The emission from two InAs quantum dots is coupled to the fundamental cavity mode of a micropost cavity. The emission from each quantum dot is spectrally separated by approximately 2 nm. By adjusting the sample temperature the single exciton emission from each quantum dot is spectrally tuned in and out of resonance with the cavity mode. The ratio of the spontaneous emission decay rate at on and off resonance conditions is nearly 4 for both quantum dots.

CRRES electric field study of the radial mode structure of Pi2 pulsations

The radial mode structure of Pi2 pulsations in the inner magnetosphere (L < 7) and its relation to the plasmapause are studied using data acquired by the Combined Release and Radiation Effects Satellite (CRRES) between August 1990 and September 1991. Low‐latitude Pi2 pulsations detected on the ground at Kakioka (L = 1.25) are used as the reference signal to determine the relative amplitude and phase of the electric field oscillations detected at CRRES. The plasmapause is identified using electron density inferred from the plasma wave spectra observed on CRRES. Pi2 events at CRRES are defined to be 10‐min intervals of high coherence between oscillations in the Kakioka horizontal northward magnetic field (H) and CRRES dusk‐to‐dawn electric field (Eφ) components within the Pi2 band (6–25 mHz). The Eφ component represents the poloidal oscillation of the geomagnetic field lines for satellite local times near midnight. Fifty‐five high‐coherence Eφ‐H Pi2 events occurred when both CRRES and Kakioka were within 3 hours of magnetic midnight. For these events CRRES was on L shells ranging from 2 to 6.5 and was either in the plasmasphere or in the close vicinity of the plasmapause, providing evidence for the plasmaspheric origin of low‐latitude Pi2 pulsations. The amplitude of Eφ varied significantly but there is an indication of a maximum near L = 4. The phase of Eφ (relative to Kakioka H) remained near −90° at all distances. These properties are consistent with the radial structure of the fundamental cavity mode oscillations confined in the plasmasphere. For some events observed at L > 3.5 it was also possible to determine the amplitude and phase of the compressional component Bz at CRRES. In contrast to Eφ, the phase of Bz (relative to H) was clustered both at ∼180° and ∼0 for events occurring near the plasmapause. This observation still is consistent with the cavity mode according to a numerical simulation using a dipole magnetic field and a realistic plasmapause plasma density structure, which indicates that the node of Bz is located near the plasmapause. Depending on the satellite position relative to the node, the phase can be either –180° or 0. A negative correlation is found between the Pi2 frequency and the distance of the plasmapause, which is additional support for the cavity mode origin of low‐latitude Pi2 pulsations.

Far-field radiation from quantum boxes located in pillar microcavities

Far-field radiation for quantum boxes located in pillar microcavities was investigated spatially and spectrally at room temperature. We have found that small-diameter pillars show directional emission for the fundamental cavity mode together with a spectral behavior dominated by the pillar's discrete modal structure. These results may be important in the context of single-photon emitters for quantum communications.

Observations of multipass transverse modes in an axially pumped solid-state laser with different fractionally degenerate resonator configurations

We experimentally show, for the first time to our knowledge, that a diode-pumped Nd:YVO4 laser can operate with multipass transverse (MPT) modes that self-reproduce after several round trips in a plano–concave cavity that has fractionally degenerate resonator configurations when the pump beam waist is sufficiently smaller than that of the fundamental cavity mode. The MPT mode is found to exhibit multiple beam waists located at different positions and to experience a lower pumping threshold than the single-pass transverse mode. With off-axis pumping, the N-pass transverse mode forms a symmetric pattern for even N and an asymmetric pattern for odd N. This result can be explained as being due to the introduction of MPT modes but not to superposition of the standard cavity modes.

An explicitly solvable case of the construction of the fundamental cavity mode in the form of a Gaussian beam with a complex astigmatism

An analysis is carried out of the ring cavities whose fundamental modes represent Gaussian beams with complex astigmatism. The analysis is made for the case where the block B or C in the cavity ray matrix ABCD of dimension 4×4 is a symmetric nondegenerate matrix. Explicit formulas are obtained, which permits the expression of beam characteristics directly in terms of the cavity ray matrix without the laborious process of finding its eigenvectors. The results obtained in the study can be used for controlling astigmatism in lasers and laser systems, the calculation of polarization of three-dimensional active media in linear and ring optical cavities of complex configuration, etc.

Possible evidence of damped cavity mode oscillations stimulated by the January, 1997 magnetic cloud event

A sudden drastic change in the solar wind dynamic pressure is one proposed mechanism for the excitation of cavity mode resonances in the plasmasphere. To study the possibility of cavity mode occurrence following a 180 cm−3 solar wind density pulse on January 11, 1997, a numerical MHD simulation was run on a dipole grid. The simulated spectra recorded by a virtual satellite are compared with those produced from the Polar magnetic time signals during the interval 0228–0304 UT. In both sets of spectra, power is concentrated between 2 and 5 mHz; observed FLR‐like spectral peaks in the range 6–20 mHz are in rough agreement with simulated plasmaspheric FLRs between 7–21 mHz. In the simulation, the fundamental plasmaspheric cavity mode is at 3.5 mHz; phase analysis of observed 3.5 mHz electric and magnetic field signals from the interval 0230–0237 UT supports the possibility of a heavily damped standing wave at this frequency.

Tunable fluid-loaded free-electron laser in the low-electron-energy and long-wavelength extreme

A tunable fluid-loaded free-electron laser (FEL) oscillator is demonstrated. This FEL-type experiment employs low-energy electrons, down to 0.4 keV. Therefore, it interacts with extremely long radio waves, in the VHF range $(\ensuremath{\lambda}g1\mathrm{m}).$ The device consists of a folded-foil planar wiggler $({\ensuremath{\lambda}}_{W}=4\mathrm{cm}),$ and a double-stripline cavity. The variable dielectric loading is implemented by distilled water in glass pipes situated on both sides of the stripline structure. Coherent oscillations are observed at the fundamental cavity mode, around 270 MHz. By varying the fluid dielectric loading, the operating frequency is tuned in a range of 10 MHz. This paper presents (a) the effect of a variable dielectric loading on the FEL tunability in general, and (b) an extremely low-electron-energy and long radiation wavelength, in the lowest end of the known FEL operating spectrum.

Harmonically mode-locked fiber laser with an acousto-optic modulator in a Sagnac loop and Faraday rotating mirror cavity

We have demonstrated a new scheme of harmonically mode-locked fiber laser utilizing an all-fiber acousto-optic modulator, which is composed of a two-mode fiber, as an active mode-locker. The harmonically mode-locked lasing occurs when a harmonic frequency of the modulator driven frequency matches with one of the harmonics of the cavity modes at the least common multiple frequency value. The mode-locked laser pulses were obtained at the 7th harmonics of the cavity's fundamental mode frequency.

Supermode selection of a subterahertz-harmonic colliding-pulse mode-locked semiconductor laser using continuous-wave light injection

The supermode of a 192-GHz sixth-harmonic colliding-pulse mode-locked semiconductor laser can be selected by continuous-wave (CW) light injection adjusted to the unlocked fundamental cavity modes. This selection scheme offers a technique for wide-range simultaneous multiwavelength allocation at each fundamental mode spacing and will be useful for providing standard light sources for wavelength-division multiplexed (WDM) systems.

Electromagnetic wave scattering by large-scale inhomogeneities in a bragg cavity

We study the single scattering of a plane electromagnetic wave by random inhomogeneities in a plane periodically nonuniform layer with a reflecting rear boundary. It is shown that when the incident wave satisfies the condition of excitation of the fundamental Bragg cavity mode and its field is large in the layer depth, the average angular spectrum of the backscattered wave field at the cavity output has a narrow maximum proportional to the exponent of the quadruple optical thickness of the periodic structure. This maximum corresponds to the condition of excitation of the fundamental Bragg cavity mode by the scattered field. The other spectral maxima have small amplitudes and intensities. Such scattering takes place if the characteristic size of random inhomogeneities in the direction perpendicular to the layer boundary exceeds the scale of multiple scattering on a periodic structure.

Free-electron-laser-type interaction at 1 meter wavelength range

A free-electron-laser (FEL)-type interaction is observed between a low-energy electron beam (∼ 0.7 keV) and a radio wave in the VHF range ( λ = 1.1 m). The experimental device consists of a transmission-line (TEM-mode) cavity, and a folded-foil planar wiggler ( λ w = 4 cm). Coherent oscillations are observed in the fundamental-cavity mode (0.28 GHz) and in higher-order modes. The FEL-type mechanism is demonstrated here in a new regime, λ ⪢ λ w.

Spectrum of photons generated in a one-dimensional cavity with oscillating boundary

The spectrum of photons generated in a one-dimensional resonator whose wall is oscillating with twice the frequency of the fundamental mode in both long- and short-time limits is found. It is shown that for short-times photons are generated mainly in the fundamental cavity mode. In the long-time limit photons are generated in a wide frequency band whose width grows exponentially fast in time.

Two-fold mode-locking by amplitude modulation at neighbouring harmonics of the laser-cavity fundamental frequency

A model is presented for the pulse-shaping dynamics in an actively mode-locked laser in which the losses are AM modulated at two harmonic frequencies differing by the cavity fundamental mode spacing and is applied to the case of third and fourth harmonics. Coupling to eight side modes is demonstrated. Computer simulations are implemented for a linear resonator which show the effects of modulation and gain for different initial conditions of field-amplitude and phases.

Quantum trajectories and classical attractors in second-harmonic generation.

We numerically investigate the classical limit of quantum trajectories in optical second-harmonic generation. This is a dissipative system of two nonlinearly coupled harmonic oscillators, corresponding to the fundamental and second-harmonic optical cavity modes. Classically it is described by a system of nonlinear differential equations which have a range of attractors: fixed points, limit cycles, and chaotic attractors. We consider the field-amplitude mean values corresponding to the quantum trajectory wave functions. We find that in the classical limit of large photon number the trajectories of these mean values approach the corresponding classical attractors. This is because the mean values of operator products factorize into products of mean values in the classical limit.

Light detection sensitivity of a vertical cavity structure used in an optical switch device

The light detection characteristics in a vertical cavity surface-emitting laser-type vertical-to-surface transmission electrophotonic device (VC-VSTEP) have been evaluated. To realize a low energy optical interconnection, not only the switching threshold energy but also the light detection sensitivity must be optimized. Maximum sensitivity is achieved when the input light wavelength and the field distribution coincide with fundamental cavity mode characteristics. Under the best matching condition, the input light energy necessary to switch the VC-VSTEP state is easily lower than 1 pJ.

Development of a second cyclotron harmonic gyrotron operating at submillimeter wavelengths

The development of a high-frequency, step-tunable gyrotron operating at submillimeter wavelengths is described. The gyrotron design was optimized for operation at the second harmonic of the electron cyclotron frequency in the TE261 cavity mode, whose resonant frequency is 384 GHz. Experimental results show that second harmonic operation can occur without mode competition as long as the beam current is low (Ib ≲0.8 A), but as the current is increased, the fundamental TE231 cavity mode increases and eventually (Ib ≳1 A) suppresses the second harmonic. The competition between the two modes is studied in detail. The starting current for second harmonic operation is also studied experimentally and compared with calculated results. Other resonances have also been examined. With the present superconducting magnet, the maximum frequency achieved is 402 GHz (second harmonic operation in the TE551 cavity mode) at several kilowatts.

Stabilization of laser beam alignment to an optical resonator by heterodyne detection of off-axis modes

We demonstrate a method for real time alignment of a Gaussian beam to an optical resonator. While thefrequency of a source laser is stabilized to a fundamental cavity mode resonance, phase modulation sidebands are applied at the off-axis mode frequencies. Asymmetrical transmission of the sideband at the frequency of each off-axis mode produces amplitude modulated optical signals and indicates the extent of the misalignments. Phase sensitive detection of these optical signals provides the error signals which are minimized by a control system that steers the input beam. In this way, optimum coupling of an injected source beam can be maintained to the fundamental mode of the resonator. This active alignment technique has demonstrated a sensitivity to tilts of 0.1 nrad/ radicalHz and to lateral beam displacements of 0.08 nm/ radicalHz in the ~1-Hz-1-kHz frequency range. These values correspond to 2 parts in 10(7) radicalHz for both the far-field divergence angle and the beam waist size. Such performance is within a factor of 2 of the shot noise limitation of the error signal measurement for a detected power of 160 microW.

Design of a high voltage multi-cavity 35 GHz phase-locked gyrotron oscillator

This paper describes the design for an experimental high power, phase-locked gyrotron oscillator. The electron beam is generated by a 1 MV pulseline accelerator, and the reference signal is provided by a 35 GHz, 20 kW magnetron. The expected output power is in the range 1 to 10 MW. The design is based on a solid 1 MeV, 100 Amp, 4 mm electron beam with a momentum pitch ratio ∞ of 0 · 75. The locking signal from the magnetron is introduced via a prebunching cavity. A second (passive) bunching cavity is used to increase the locking frequency bandwidth obtainable with a given locking power. The bunching cavities are designed to operate in the fundamental TE111 cylindrical cavity mode. Some competition from the TE112 higher order axial mode could not be avoided owing to the constraint on the minimum drift tube diameter set by the requirement to propagate the electron beam. The bunching cavities include two axial slots to control the cavity Q factor and suppress competing modes. Additional slots and apertures a...

Magnetospheric cavity modes and field-line resonances: The effect of radial mass density variation

The relationship between magnetospheric cavity modes and field-line resonances (FLRs) is examined as a function of the radial plasma mass density distribution in the dayside magnetosphere. Using a numerical model of wave coupling we find that, for monotonie mass densities of the form L − q , the radial position and amplitude of cavity-mode driven FLRs increases as q decreases. This makes the afternoon sector of Magnetic Local Time the most favourable for the detection of such resonances, and suggests that FLRs observed at high magnetic latitudes are not driven by global cavity modes. We also find that the maximum damping of the fundamental cavity mode by coupling to a FLR appears to occur for q ≅ 4 and m ≅ 3, where m is the azimuthai wave number. Using models for morning and afternoon mass densities which include a small plasmapause at L ≈ 4, the structure of wave fields is examined in detail. We find that the base of the plasmapause should be a favoured position for the occurrence of long-period hydromagnetic wave activity. In addition to double FLRs which have been described previously, quasi-FLRs should be present at the plasmapause base. These occur when a cavity mode period (or the period of any other monochromatic driving wave) is less than the minimum field-line eigenperiod at the plasmapause base. Field-lines are driven off-resonance, but a relatively large amplitude peak still results when the driving period is close to the minimum eigenperiod. The distinguishing feature of a quasi-FLR is that there is no polarization reversal across the amplitude peak, and the polarization is not linear at the peak. It is suggested that amplitude peaks with no polarization reversal across the peak may be indicators of the plasmapause position in ground-based observations.