Modification Of Spontaneous Emission Research Articles

Modulation of upconversion white light emission in PMMA/NaYF(4):Yb(3+), Er(3+), Tm(3+) composite photonic crystals.

In recent years, spontaneous emission modulation of emitters in photonic crystals (PCs) has been widely reported. However, the modulation of PCs on upconversion (UC) white light emission has been rarely studied. In this Letter, triply doped PMMA/NaYF(4):Yb(3+), Er(3+), Tm(3+) composite photonic crystals were successfully fabricated and white light emission was obtained. It is interesting to observe that the chromaticity could be conveniently tuned by changing the photonic stop band (PSB), the incident angle, or the power of the excitation light.

Photoluminescence and Two-Photon Lasing of ZnO:Sn Microdisks

ZnO:Sn microdisks were fabricated by a vapor phase transport method at different pressure conditions. The photoluminescence of the ZnO:Sn microdisks was measured, and the origination of defect emission was attributed to the oxygen vacancy defects. Under the excitation of the 532 nm line of a femtosecond pulsed laser, two-photon absorption induced UV amplified spontaneous emission and whispering-gallery mode lasing were observed from the ZnO:Sn microdisks. The electron–hole plasma induced red-shift and broadening of the amplified spontaneous emission and lasing emission were demonstrated when the excitation power was increased. In addition, the WGM lasing light distribution was simulated by a numerical method.

Spontaneous emission of electric and magnetic dipoles in the vicinity of thin and thick metal

Strong modification of spontaneous emission of Eu(3+) ions placed in close vicinity to thin and thick gold and silver films was clearly demonstrated in a microscope setup separately for electric and magnetic dipole transitions. We have shown that the magnetic transition was very sensitive to the thickness of the gold substrate and behaved distinctly different from the electric transition. The observations were described theoretically based on the dyadic Green's function approach for layered media and explained through modified image models for the near and far-field emissions. We established that there exists a "near-field event horizon", which demarcates the distance from the metal at which the dipole emission is taken up exclusively in the near field.

Modulated photoluminescence of graphene quantum dots in the vicinity of an individual silver nano-octahedron

We propose an effective strategy to enhance and modulate the photoluminescence (PL) of graphene quantum dots (GQDs) in the vicinity of a single silver nano-octahedron (SNO) utilizing three-dimensional finite-difference time-domain calculations. The SNO is designed to act as a multifrequency plasmonic antenna with multiple plasmon resonance modes covering multiple emission peaks of GQDs. The spectral modifications of spontaneous emission are investigated with the variations of the GQD's position and dipole moment orientation relative to the SNO. The PL colour of the GQD can be precisely adjusted between blue and green through the strong interaction with the designed antenna. The multicolour features of GQDs will also facilitate their potential applications as eco-friendly and multifunctional optical probes. The study contributes to a deeper understanding of the PL properties of GQDs near the metallic nanoparticles.

The Purcell factor of nanoresonators

Advances in theoretical nano-optics provide new insights into how nanoscale modification of spontaneous emission can be realized.

A high-harmonic generation source for seeding a free-electron laser at 38 nm

Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the self-amplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics.

RETRACTED: Modification of spontaneous emission from fluorescent spheres by partially disordered photonic crystals

RETRACTED: Modification of spontaneous emission from fluorescent spheres by partially disordered photonic crystals

Dipole Decay Rates Engineering via Silver Nanocones

A full control of the interaction between confined plasmons and point sources of radiation is a central issue in molecular plasmonics. In this paper, a theoretical contribution towards a physical understanding on the localized surface plasmons excited into metallic nanocones by a point dipole is given. A numerical approach based on the discrete dipole approximation is applied to determine the modifications of the dipole decay rates for varying geometrical parameters of the dipole-metal nanoparticle system. Results declare the centrality of the cone aperture to control the plasmon resonances and to handle the effects it induces on the lifetime of a point emitter. A full spectral tuning of the resonances in the decay rates can be achieved by operating on a unique spatial degree of freedom: by tailoring the aperture alone, total decay rates 105 times higher than the free-space value can be obtained at short distances from the metal in a large region of the spectral range. Quite unexpectedly, size dependence of the antenna is found to have a marginal role if only a lifetime manipulation is desired. It becomes, instead, a crucial aspect of the problem when large quantum yields are required. Results presented in this work shed light on spontaneous emission modification due to interaction with plasmonic nanocones of different shapes and are relevant for a number of applications in the fields of nanoplasmonics and fluorescence microscopy.

High reflectance membrane-based distributed Bragg reflectors for GaN photonics

Preparation of highly reflective distributed Bragg reflectors (DBRs) from III-nitrides is an important building block for cavity photonics. In this work, we report the fabrication of a membrane-based GaN/air-gap DBR for blue/green light emitting devices. The formation of membrane DBRs relies on a recently discovered electrochemical procedure in which selective etch is achieved by adjusting the conductivity rather than chemical composition, thus relieving greatly the burden in creating epitaxial DBRs. Micro-reflectance measurement shows over 98% peak reflectance and a wide stopband with only four pairs of GaN/air-gap layers. Micro-photoluminescence spectra of InGaN multiple quantum wells (MQWs) on DBRs show reduced linewidth and improved emission efficiency. After capping the MQWs on DBRs with silver, a significant linewidth narrowing indicates the modification of spontaneous emission due to the presence of a planar microcavity.

Momentum spread in a relativistic electron beam in an undulator

The motion of the relativistic electron beam in the spatially periodic magnetic field of an undulator has been considered taking into account the effect of the incoherent field of the spontaneous undulator radiation on the motion of the electrons. An expression for the rms momentum of the electrons has been obtained. It has been shown that the momentum spread in the ultrarelativistic electron beam increases in the spontaneous incoherent emission mode. Conditions for the self-amplification of the spontaneous undulator radiation in ultrashort-wavelength free-electron lasers have been discussed.

Tuning the spontaneous light emission in phoxonic cavities

The modulation of spontaneous light emission of active centers through elastic waves in Si/SiO2 multilayer phoxonic structures that support dual photonic-phononic localized modes, in the bulk or at the surface, is studied by means of rigorous full electrodynamic and elastodynamic calculations. Our results show that strong dynamic modulation of the spontaneous emission can be achieved through an enhanced acousto-optic interaction when light and elastic energy are simultaneously localized in the same region.

Effect of energy shifts on the spontaneous emission modification via quantum interference

Effect of energy shifts on the spontaneous emission modification via quantum interference

Reversible Modulation of Spontaneous Emission by Strain in Silicon Nanowires

We computationally study the effect of uniaxial strain in modulating the spontaneous emission of photons in silicon nanowires. Our main finding is that a one to two orders of magnitude change in spontaneous emission time occurs due to two distinct mechanisms: (A) Change in wave function symmetry, where within the direct bandgap regime, strain changes the symmetry of wave functions, which in turn leads to a large change of optical dipole matrix element. (B) Direct to indirect bandgap transition which makes the spontaneous photon emission to be of a slow second order process mediated by phonons. This feature uniquely occurs in silicon nanowires while in bulk silicon there is no change of optical properties under any reasonable amount of strain. These results promise new applications of silicon nanowires as optoelectronic devices including a mechanism for lasing. Our results are verifiable using existing experimental techniques of applying strain to nanowires.

Dynamics of spontaneous emission of quantum dots in a one-dimensional cholesteric liquid crystal photonic cavity

We investigate the modulation of recombination lifetimes of CdSe/ZnS quantum dots (QDs) dispersed in a cholesteric liquid crystal (CLC) photonic cavity. Using ultrafast spectroscopic techniques we focus on the time-resolved emission from QD ensembles in CLC matrices with either planar or homeotropic alignment. In the case of planar alignment and a well-defined spectral stop-band (reflection band) we observe the emergence of a second, faster decay time of less than 2 ns. This short recombination pathway is observed only in samples where the QD emission spectrum partially overlaps the CLC stop-band by 50% or more. Samples prepared with homeotropic alignment do not have a stop-band and, consequently, do not lead to spectral or dynamical modulation of the QD emission. Our observations indicate that coupling between the excitonic and the photonic cavity modes results in an enhancement and modulation of spontaneous emission in the liquid crystal medium.

Modification of spontaneous emission from nanodiamond colour centres on a structured surface

Colour centres in diamond are promising candidates as a platform for quantum technologies and biomedical imaging based on spins and/or photons. Controlling the emission properties of colour centres in diamond is a key requirement for the development of efficient single-photon sources having high collection efficiency. A number of groups have achieved an enhancement in the emission rate over narrow wavelength ranges by coupling single emitters in nanodiamond crystals to resonant electromagnetic structures. In this paper, we characterize in detail the spontaneous emission rates of nitrogen-vacancy centres at various locations on a structured substrate. We found a factor of 1.5 average enhancement of the total emission rate when nanodiamonds are on an opal photonic crystal surface, and observed changes in the lifetime distribution. We present a model for explaining these observations and associate the lifetime properties with dipole orientation and polarization effects.

Self-Amplified Spontaneous Emission Free-Electron Laser with an Energy-Chirped Electron Beam and Undulator Tapering

We report the first experimental implementation of a method based on simultaneous use of an energy chirp in the electron beam and a tapered undulator, for the generation of ultrashort pulses in a self-amplified spontaneous emission mode free-electron laser (SASE FEL). The experiment, performed at the SPARC FEL test facility, demonstrates the possibility of compensating the nominally detrimental effect of the chirp by a proper taper of the undulator gaps. An increase of more than 1 order of magnitude in the pulse energy is observed in comparison to the untapered case, accompanied by FEL spectra where the typical SASE spiking is suppressed.

Carrier heating in quantum wells under optical and current injection of electron-hole pairs

Carrier heating in GaAs/AlGaAs quantum wells (QWs) under optical interband pumping in the spontaneous-emission mode has been studied. The electron temperature was determined as a function of the pumping intensity. The effect of the electric field on the photoluminescence spectrum was examined. The change in the carrier concentration with the drive current in the spontaneous- and stimulated-emission modes in InGaAsSb/InAlGaAsSb QWs was determined from electroluminescence spectra. The rise in the temperature of hot carriers, which results in the increase in the carrier concentration with the drive current, was roughly estimated.

Partial-coherence method to model experimental free-electron laser pulse statistics

A general numerical approach is described that allows obtaining model sets of temporal pulse shapes of free-electron lasers (FELs) operating in the self-amplified spontaneous emission mode. Based on a random partial-coherence approach, sets of pulse shapes can be calculated that satisfy statistical criteria of FEL light predicted by established FEL theory. Importantly, the numerically retrieved sets of pulses reproduce the experimentally accessible FEL light characteristics as measured at the Free-electron LASer at Hamburg (FLASH), such as the average spectrum, single-shot spectral shape, and pulse duration. The high-precision agreement with the experimental average spectral shape, without further knowledge of FEL machine parameters, makes this approach a convenient tool for the analysis and theoretical modeling of nonlinear optical or pump-probe experiments with FEL light.

Modification of spontaneous emission from quantum dots by the surface of a three-dimensional photonic crystal

The emission behaviors of CdSe/ZnS core-shell quantum dots (QDs) in proximity to the surface of a three-dimensional photonic crystal (PC) are investigated experimentally and theoretically. Results show that the spontaneous emission (SE) of QDs in such a structure presents a damped periodic oscillation with the distance between QDs and the PC surface, and the most significant modification always appears in the short-wavelength region, resulting from the coefficient of surface modes and interference between the direct SE field and the reflected SE field. These display a wide range of novel optical properties for many applications in optoelectronic devices.

The quantum free electron laser: A new source of coherent, short‐wavelength radiation

AbstractA Free Electron Laser (FEL) operating in the quantum regime can provide a compact and monochromatic X‐ray source. Here we review the basic principles of a high‐gain quantum FEL starting from noise, with special emphasis on the self‐amplified spontaneous emission (SASE) mode of operation. In the first part of the paper, a condition for the neglect of the fermionic character of the electrons is derived and the full quantum theory of the N‐particle and single‐radiation‐mode FEL Hamiltonian is presented. Quantum effects such as cooperative gain, discrete spectrum and line narrowing are described, both in the multi‐particle and in the second quantization formalism. In the second part, propagation effects (i.e. slippage) are described and the main features of the quantum SASE regime are discussed. The broad and spiky radiation spectrum observed in classical SASE reduces in the quantum regime to a series of narrow lines, associated with sequential transitions between adjacent momentum states. A simple interpretation of the discrete nature of the spectrum and of the linewidth of the single spike observed in the quantum regime is presented.