Spontaneous emission in microcavities

The transverse distributions of intensity inside and outside an optical cavity produced by spontaneous emission from atoms inside the cavity are reviewed. The mode properties inside a closed passive one-dimensional cavity are summarised and the spatial distributions of emitted light for a passive cavity with partially transmitting mirrors are discussed. A qualitative theory describes the effects of an amplifying medium inside the cavity, covering the regimes both below and above the lasing threshold. Expressions are derived for the angular spread of the output light in the presence of gain. A theory based on rigorous general relations between gain and noise provides more complete results for the spontaneous emission noise from an amplifying slab within an optical cavity.     

Enhanced electron field emission from carbon nanotubes irradiated by energetic C ions

The field emission performance and structure of the vertically aligned multi-walled carbon nanotube arrays irradiated by energetic C ion with average energy of 40 keV have been investigated. During energetic C ion irradiation, the curves of emission current density versus the applied field of samples shift firstly to low applied fields when the irradiation doses are less than 9.6 x 10(16) cm(-2), and further increase of dose makes the curves reversing to a high applied field, which shows that high dose irradiation in carbon nanotube arrays makes their field emission performance worse. After energetic ion irradiation with a dose of 9.6 x 1016 cm(-2), the turn-on electric field and the threshold electric field of samples decreased from 0.80 and 1.13 V/microm to 0.67 and 0.98 V/microm respectively. Structural analysis of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy indicates that the amorphous carbon nanowire/carbon nanotube hetero nano-structures have been fabricated in the C ion irradiated carbon nanotubes. The enhancement of electron field emission is due to the formation of amorphous carbon nanowires at the tip of carbon nanotube arrays, which is an electron emitting material with low work function.     

Spontaneous emission from a three-level atom in two-band photonic crystals

Abstract

We investigate the spontaneous emission from a V three-level atom embedded in two-band isotropic photonic crystals. The dipoles of the two transitions from the two upper levels to the lower level are parallel. Due to the quantum interference between the two transitions and the existence of two bands, the populations in the upper levels display some novel properties, such as anti-trapping and continuous oscillation, which differ from that of a two-level atom (with two bands) and also differ from that if only one band (for three-level atom) is considered. The spontaneous emission field is composed of two parts: localized field and travelling field. The localized field is composed of one or two localized modes, and the travelling field is composed of no, one, two or three propagating modes depending on different conditions. The conditions for different combinations of localized modes and propagating modes are discussed.     

Microwave emission from explosive plasma jets

Microwave emission from explosive plasma jets excited by RF pulses has been observed. Plasma jets excited by frequency-modulated signals have been studied, and the distributions of the number and duration of emission pulses over the microwave frequency range have been determined.     

Negative ion emission accompanying the field electron emission from amorphous hydrogenated carbon

It was found that the field electron emission from the surface of amorphous hydrogenated carbon is accompanied by the emission of negative ions of hydrogen and hydrocarbons. This ion emission is explained by the formation and degradation of the local surface emission centers.     

Secondary electron emission from nanocomposite carbon films

The secondary electron emission (SEE) properties of sulfur-incorporated nanocomposite carbon (n-C) films were studied. Maximum SEE yield (δ _max) values obtained ranged from 3.27 to 6.98, which are between those for graphite (δ _max ~ 1) and high purity diamond films in their as-grown condition (δ _max ~ 9), and are consistent with the composite nature of the films. It was found that δ _max values of n-C films are mostly determined by the atomic oxygen concentration on the surface of the films, which appears to control the probability of escape of the secondary electrons from the surface of the films, as inferred by employing Ascarelli’s model for SEE (J Appl Phys 89:689, 2001). Also, mean escape depth values for the secondary electrons were obtained using this model, and their significance as bulk parameters for the films is discussed.     

Phonon-assisted electron emission from individual carbon nanotubes

A question of how electrons can escape from one-atom-thick surfaces has seldom been studied and is still not properly answered. Herein, lateral electron emission from a one-atom-thick surface is thoroughly studied for the first time. We study electron emission from side surface of individual electrically biased carbon nanotubes (CNTs) both experimentally and theoretically and discover a new electron emission mechanism named phonon-assisted electron emission. A kinetic model based on coupled Boltzmann equations of electrons and optical phonons is proposed and well describes experimentally measured lateral electron emission from CNTs. It is shown that the electrons moving along a biased CNT can overflow from the one-atom-thick surface due to the absorption of hot forward-scattering optical phonons. A low working voltage, high emission density, and side emission character make phonon-assisted electron emission primarily promising in electron source applications.     

Spontaneous emission from a planar microcavity below threshold and the wolf effect

Abstract

Equivalence between a planar microcavity below threshold and a partially coherent primary source is pointed out for the first time. Spatial coherence properties of spontaneous emission from the planar microcavity are controllable by varying two cavity parameters of the cavity length and the mirror reflectivity. The spontaneous emission gives rise to the Wolf effect which is spectral changes induced by the spatial coherence. It is clearly shown from our theoretical analyses that the spectrum in the far field shifts toward higher frequencies than that on the emitting plane of the planar microcavity. In particular, the blue shift of the far-field spectrum takes a maximum value at the centre of the far-field plane and decreases gradually far away from the centre. Moreover, the blue shift increases with decreasing values of the two cavity parameters.     

Spontaneous emission from a three-level atom placed in three-dimensional photonic crystals in the long-time limit

The paper describes the spontaneous emission from a three-level atom placed in a periodic dielectric microstructure which exhibits a complete three-dimensional photonic band gap. By using the Euler approach, the upper level population of the atom is calculated for a wide range of relationships between the Rabi frequency and the detuning of the atomic transition frequency from the upper band edge. The results indicate that there are three cases of the relationship between Rabi frequency and detuning, which determine distinctive states of the atomic population in the long-time limit. When the detuning is greater than the Rabi frequency, the upper level has a zero steady-state atomic population, which leads to enhancement of spontaneous emission. When the magnitude of the detuning is less than the Rabi frequency, the upper level has a nonzero steady-state atomic population, which leads to suppression of spontaneous emission. When the negative detuning is greater than the Rabi frequency, the upper level has a nondecaying oscillatory-state atomic population due to long-time atomic splitting. These three properties of the spontaneous emission are relevant to several optical devices on an atomic scale, such as optical memories, switches and clocks.     

Time-dependent resonating plasma treatment of carbon nanotubes for enhancing the electron field emission properties

Journal of Materials Science: Materials in Electronics - Vertically aligned carbon nanotubes having the advanced atomic configuration, structural properties, and electronic conductivity, which...     

Field emission from a selected multiwall carbon nanotube

The electron field emission characteristics of individual multiwalled carbon nanotubes were investigated by a piezoelectric nanomanipulation system operating inside a scanning electron microscopy chamber. The experimental set-up ensures a precise evaluation of the geometric parameters (multiwalled carbon nanotube length and diameter and anode-cathode separation) of the field emission system. For several multiwalled carbon nanotubes, reproducible and quite stable emission current behaviour was obtained, with a dependence on the applied voltage well described by a series resistance modified Fowler-Nordheim model. A turn-on field of ～30?V?μm(-1) and a field enhancement factor of around 100 at a cathode-anode distance of the order of 1?μm were evaluated. Finally, the effect of selective electron beam irradiation on the nanotube field emission capabilities was extensively investigated.     

Field emission from diamond and diamond-like carbon films

Field emission from diamond and diamond-like carbon thin films deposited on silicon substrates has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. The diamond-like carbon films were deposited using the radio frequency chemical vapor deposition method. Field emission studies were carried out using a sphere-to-plane electrode configuration. The results of field emission were analyzed using the Fowler-Nordheim model. It was found that the diamond nucleation density affected the field emission properties. The films were characterized using standard scanning electron microscopy, Raman spectroscopy, and electron spin resonance techniques. Raman spectra of both diamond and diamond-like films exhibit spectral features characteristic of these structures. Raman spectrum for diamond films exhibit a well-defined peak at 1333cm^?1. Asymmetric broad peak formed in diamond-like carbon films consists of D-band and G-band around 1550 cm^?1 showing the existence of both diamond (sp³ phase) and graphite (sp² phase) in diamond-like carbon films.     

High-Current-Density field emission from self-heating printed carbon nanotubes

High-current-density field emitters are considered as the potential and necessary components for compact high definition x-ray sources and high-power cold cathode microwave amplifiers. In this report, high-current-density field emission from self-heating printed carbon nanotubes is introduced. Large emission current causes large heat that increases the temperature of the emitters. The temperature is estimated to be more than ~1600 K. Localized surface field and high temperature both drive more electrons escaping from the emitters, and the maximum current density is larger than ~2.7 A/cm² that will satisfy the need of most vacuum electron devices including x-ray sources and microwave electron devices.     

Anomalous microwave emission from a stationary plasma thruster

The results of ground radio engineering tests on the thruster modules of a combined power unit for Yamal-100 spacecraft are presented. A pulsed electromagnetic radiation component was detected, which is explained by nonstationary processes of the electron emission from plasma of a hollow compensator cathode.     

Study of the ability to field emission from diamond-like carbon layers and carbon nanotubes

The aim of this work was to study the relationship between parameters of the electron field emission and the film deposition method. In this study two methods were applied: classical radio frequency plasma-assisted chemical vapor deposition (RF PACVD) to produce diamond-like carbon (DLC) layers and chemical vapor deposition (CVD) to produce carbon nanotubes (CNT). DLC layers were grown on n-type silicon substrates and CNT were grown on n-type and p-type silicon substrates.Atomic force microscopy (AFM) and Raman spectroscopy were used to investigate the physical and chemical parameters of DLC films after deposition process. The electrical parameters of capacitors with the DLC layer as an insulator were extracted from the capacitance-voltage (C-V) and current-voltage (I-V) characteristics. Measurements of the field emission were performed after characterization of the layer properties.     

Magnetic field effect on electron emission from plasma

The influence of external magnetic field on the significant parameters of electrons from laser induced plasma (LIP) is investigated. A Q-switched Nd:YAG laser (1064 nm, 9-14 ns, 10 mJ, 1.1 MW) is focused on annealed, 4N pure (99.99%) Silver target (2 × 2 × 0.2 cm³) for production of plasma under vacuum ∼10⁻³ torr. Temperature, density and energy measurements for electrons were made by self fabricated Langmuir probe both in the absence and presence of external magnetic field (∼1.2 T) at different positions. The signals are recorded on 200 MHz UTT 2202 digital storage oscilloscope. The results thus obtained reveal decrease in electron temperature, energy and density in presence of external magnetic field. Confinement of plasma is also observed.     

Oxygen plasma assisted end-opening and field emission enhancement in vertically aligned multiwall carbon nanotubes

This paper highlights the changes in micro-structural and field emission properties of vertically aligned carbon nanotubes (VACNTs) via oxygen plasma treatment. We find that exposure of very low power oxygen plasma (6 W) at 13.56 MHz for 15–20 min, opens the tip of vertically aligned CNTs. Scanning electron microscopy and transmission electron microscopy images were used to identify the quality and micro-structural changes of the nanotube morphology and surfaces. Raman spectra showed that the numbers of defects were increased throughout the oxygen plasma treatment process. In addition, the hydrophobic nature of the VACNTs is altered significantly and the contact angle decreases drastically from 110° to 40°. It was observed that the electron field emission (EFE) characteristics are significantly enhanced. The turn-on electric field (ETOE) of CNTs decreased from ∼0.80 V μm⁻¹ (untreated) to ∼0.60 V μm⁻¹ (oxygen treated). We believe that the open ended VACNTs would be immensely valuable for applications such as micro/nanofluidic based filtering elements and display devices.     

Laser-induced carbon plasma emission spectroscopic measurements on solid targets and in gas-phase optical breakdown

We report measurements of time- and spatially averaged spontaneous-emission spectra following laser-induced breakdown on a solid graphite/ambient gas interface and on solid graphite in vacuum, and also emission spectra from gas-phase optical breakdown in allene C3H4 and helium, and in CO2 and helium mixtures. These emission spectra were dominated by CII (singly ionized carbon), CIII (doubly ionized carbon), hydrogen Balmer beta (Hbeta), and Swan C2 band features. Using the local thermodynamic equilibrium and thin plasma assumptions, we derived electron number density and electron temperature estimates. The former was in the 10(16) cm(-3) range, while the latter was found to be near 20000 K. In addition, the vibration-rotation temperature of the Swan bands of the C2 radical was determined to be between 4500 and 7000 K, using an exact theoretical model for simulating diatomic emission spectra. This temperature range is probably caused by the spatial inhomogeneity of the laser-induced plasma plume. Differences are pointed out in the role of ambient CO2 in a solid graphite target and in gas-phase breakdown plasma.     

Field emission from carbon nanotubes in DC and pulsed mode

Multi-wall Carbon Nanotube (CNT) emitters were tested in a combined diode-RF electron gun. Field emission of the nanotubes was observed at 5-30 MV/m, using a 250 ns FWHM long pulse with a peak voltage of 80-470 kV. The field emission threshold is compatible with that found from previous DC testing. We have extracted from a continuous field emitter up to a nanoCoulomb of charge and measured an emittance of 4 mm mrad with a 2 pC electron beam. The total charge emission during RF operation, using the 1.5 GHz, 2 cell RF structure, was found dependent on its period. RF operation showed that back bombarding electrons with up to 5 MeV did not impair the emission stability of the CNTs.     

Spontaneous emission from a double V-type four-level atom in a double-band photonic crystal

Abstract

Quantum interferences in spontaneous emission from a double V-type four-level atom in a double-band photonic crystal have been investigated. The double V-type transitions from the two upper levels to the two lower levels can interact not only with modes near the edges of a double-band photonic band gap, but also with free vacuum modes. The resulting two types of quantum interferences give rise to not only dark lines, but also a narrow spontaneous line in the spectrum from the transition coupled to the free vacuum modes. The dark lines and narrow spontaneous line are shown to be dependent on the width of the forbidden gap, relative position of the upper levels from the band edges, coupling constants, and initial coherent superposition state of the system.