Spontaneous Emission Regime Research Articles

Stimulated emission from hexagonal silicon-germanium nanowires

Hexagonal crystal phase silicon-germanium (hex-SiGe) features efficient direct bandgap emission between 1.5 and 3.4 µm. For expanding its application potential, the key challenge is to demonstrate material gain for enabling a hex-SiGe semiconductor laser. Here we report the transition from the spontaneous emission regime to the stimulated emission-dominated amplified spontaneous emission regime in the optically excited part of a hexagonal Si0.2Ge0.8 nanowire. We observe narrow resonance peaks arising above a spontaneous emission background, which show lasing signatures such as a threshold and a superlinear increase of the emission. A Hakki-Paoli analysis of the height of the cavity resonances provides the gain spectrum of hex-SiGe, showing evidence for a positive material gain. Measurements of the cavity line widths provide an independent assessment of the total cavity loss. While lasing has not been reached, the observation of optical amplification and amplified spontaneous emission provides a clear roadmap toward lasing in hexagonal SiGe. This opens a new pathway for the monolithic integration of a Si-compatible laser within electronic chips.

On the relationship between electrical and electro-optical characteristics in vertical cavity surface emitting lasers for chip scale Rb atomic clocks

We report on a comprehensive study of the electrical and electro-optical properties of 795 nm vertical cavity surface emitting lasers (VCSELs) designed for chip scale Rb atomic clocks. We highlight several key findings including the observation that the current flow at moderate bias levels comprises several parallel paths which are identified by an analysis of the I−V characteristic also confirmed by a numerical simulation. Resistance is a key parameter in any VCSEL. We analyze it in detail at all bias levels and find that above transparency, when the VCSEL enters the high injection regime, the current flow mechanism is modified significantly from an exponential to a power law characteristic. Consequently, resistance attains a nonlinear contribution which is quadratic in the spontaneous emission regime and quasi-linear above the threshold. This nonlinear contribution is not considered in common models. The optoelectronic properties are strongly correlated with the electrical characteristics what allow to explain several peculiarities of the VCSEL performance. We designed and fabricated the VCSELs according to the requirements of miniature Rb atomic clocks, including optimal operation at high temperatures. Their minimum threshold occurs at 363 K where they emit at 794.7 nm. The modal and polarization discrimination in the bias range where these VCSELs operate in practical miniature atomic clocks is well above 30 dB.

Self‐Assembly of Hydrogen‐Bonded Organic Crystals on Arbitrary Surfaces for Efficient Amplified Spontaneous Emission

Organic lasers attract much attention due to their high efficiency, low energy consumption, and structural flexibility. However, long‐term stability and the creation of the lasers on arbitrary surfaces remain a challenge. Here, a synthesis of amide‐based organic molecules that provides packing into hydrogen‐bonded organic crystals (OCs) is reported. The resulting OCs demonstrate an amplified spontaneous emission (ASE) regime with 0.55 μJ cm−2 threshold under the normal conditions due to 5%–13% quantum yield and high emission rate (1.02 ns). The simple process of self‐assembly of the hydrogen‐bonded OCs and highly stable ASE (over 30 min of continuous operation) allow fabricating fibers, flexible polymers, and hard planar periodic optical systems based on them, which paves the way to creating organic laser diodes of an arbitrary design.

Generation of ultrahigh-brightness pre-bunched beams from a plasma cathode for X-ray free-electron lasers

The longitudinal coherence of X-ray free-electron lasers (XFELs) in the self-amplified spontaneous emission regime could be substantially improved if the high brightness electron beam could be pre-bunched on the radiated wavelength-scale. Here, we show that it is indeed possible to realize such current modulated electron beam at angstrom scale by exciting a nonlinear wake across a periodically modulated plasma-density downramp/plasma cathode. The density modulation turns on and off the injection of electrons in the wake while downramp provides a unique longitudinal mapping between the electrons’ initial injection positions and their final trapped positions inside the wake. The combined use of a downramp and periodic modulation of micrometers is shown to be able to produces a train of high peak current (17 kA) electron bunches with a modulation wavelength of 10’s of angstroms - orders of magnitude shorter than the plasma density modulation. The peak brightness of the nano-bunched beam can be O(1021A/m2/rad2) orders of magnitude higher than current XFEL beams. Such prebunched, high brightness electron beams hold the promise for compact and lower cost XEFLs that can produce nanometer radiation with hundreds of GW power in a 10s of centimeter long undulator.

Compact LWFA-Based Extreme Ultraviolet Free Electron Laser: Design Constraints

The combination of advanced high-power laser technology, new acceleration methods and achievements in undulator development offers the opportunity to build compact, high-brilliance free electron lasers driven by a laser wakefield accelerator. Here, we present a simulation study outlining the main requirements for the laser–plasma-based extreme ultraviolet free electron laser setup with the aim to reach saturation of the photon pulse energy in a single unit of a commercially available undulator with the deflection parameter K0 in the range of 1–1.5. A dedicated electron beam transport strategy that allows control of the electron beam slice parameters, including collective effects, required by the self-amplified spontaneous emission regime is proposed. Finally, a set of coherent photon radiation parameters achievable in the undulator section utilizing the best experimentally demonstrated electron beam parameters are analyzed. As a result, we demonstrate that the ultra-short, few-fs-level pulse of the photon radiation with the wavelength in the extreme ultraviolet range can be obtained with the peak brilliance of ∼7×1028 photons/pulse/mm2/mrad2/0.1%bw.

Polarization dependent beaming properties of a plasmonic lattice laser

We study beaming properties of laser light produced by a plasmonic lattice overlaid with organic fluorescent molecules. The crossover from spontaneous emission regime to stimulated emission regime is observed in response to increasing pump fluence. This transition is accompanied by a strong reduction of beam divergence and emission linewidth due to increased degree of spatial and temporal coherence, respectively. The feedback for the lasing signal is shown to be mainly one-dimensional due to the dipolar nature of the surface lattice resonance. Consequently, the beaming properties along x and y directions are drastically different. From the measurements, we obtain the M 2 value along both principal directions of the square lattice as a function of the pump fluence. Our work provides the first detailed analysis of the beam quality in plasmonic lattice lasers and reveals the underlying physical origin of the observed strong polarization dependent asymmetry of the lasing signal.

A Review of High-Gain Free-Electron Laser Theory

High-gain free-electron lasers, conceived in the 1980s, are nowadays the only bright sources of coherent X-ray radiation available. In this article, we review the theory developed by R. Bonifacio and coworkers, who have been some of the first scientists envisaging its operation as a single-pass amplifier starting from incoherent undulator radiation, in the so called self-amplified spontaneous emission (SASE) regime. We review the FEL theory, discussing how the FEL parameters emerge from it, which are fundamental for describing, designing and understanding all FEL experiments in the high-gain, single-pass operation.

Short-time emission of higher-angular-momentum photons by atomic transitions

The short-time regime of spontaneous light emission by few-electron ions is examined in detail, with a specific emphasis on the angular momentum of the emitted light. It is found that, in general, photons carrying a higher angular momentum are emitted with important probabilities, at short times, in transitions that are not of the electric dipole type. The probability of emission of such photons is found to be parametrically non-negligible in this time regime, and even numerically dominant for some cases. It is also found that, in all time regimes, the emission of electric 2 n+1-pole fields is typically numerically dominant over the emission of magnetic 2 n -pole fields by many orders of magnitude. These results refine and deepen our understanding of the emission of angular momentum-carrying light by simple atomic systems.

Superthermal-light emission and nontrivial photon statistics in small lasers

Photon statistical measurements on a semiconductor microlaser, obtained using single-photon counting techniques, show that a newly discovered spontaneous pulsed emission regime possesses superthermal statistical properties. The observed spike dynamics, typical of small-scale devices, is at the origin of an unexpected discordance between the probability density function and its representation in terms of the first moments, a discordance so far unnoticed in all devices. The impact of this new dynamics is potentially large, since coincidence techniques are presently the sole capable of characterizing light emitted by nanolasers.

The regime of multi-stage trapping in free-electron lasers operating in the super-radiant and SASE regimes

The use of electron–wave interaction systems consisting of several tapered sections is considered as a method of efficiency enhancement of free-electron lasers (FELs) operating in different regimes of emission of a short wave pulse from a short electron bunch. These regimes are the principally multi-frequency self-amplified spontaneous emission regime traditionally used in short-wavelength FELs and the regime of electron–wave group synchronism, in which super-radiation of a quasi-monochromatic wave packet propagating together with the electron bunch occurs. In both the cases, the use of multi-stage trapping of electrons in the bunch by the radiated wave provides a significant (at least by an order of magnitude) increase in efficiency as compared to the saturated-stage efficiency in regular systems.

All-optical control of exciton flow in a colloidal quantum well complex

Excitonics, an alternative to romising for processing information since semiconductor electronics is rapidly approaching the end of Moore’s law. Currently, the development of excitonic devices, where exciton flow is controlled, is mainly focused on electric-field modulation or exciton polaritons in high-Q cavities. Here, we show an all-optical strategy to manipulate the exciton flow in a binary colloidal quantum well complex through mediation of the Förster resonance energy transfer (FRET) by stimulated emission. In the spontaneous emission regime, FRET naturally occurs between a donor and an acceptor. In contrast, upon stronger excitation, the ultrafast consumption of excitons by stimulated emission effectively engineers the excitonic flow from the donors to the acceptors. Specifically, the acceptors’ stimulated emission significantly accelerates the exciton flow, while the donors’ stimulated emission almost stops this process. On this basis, a FRET-coupled rate equation model is derived to understand the controllable exciton flow using the density of the excited donors and the unexcited acceptors. The results will provide an effective all-optical route for realizing excitonic devices under room temperature operation.

Demonstration of critical coupling in an active III-nitride microdisk photonic circuit on silicon

On-chip microlaser sources in the blue constitute an important building block for complex integrated photonic circuits on silicon. We have developed photonic circuits operating in the blue spectral range based on microdisks and bus waveguides in III-nitride on silicon. We report on the interplay between microdisk-waveguide coupling and its optical properties. We observe critical coupling and phase matching, i.e. the most efficient energy transfer scheme, for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the spontaneous emission regime. Whispering gallery mode lasing is demonstrated for a wide range of parameters with a strong dependence of the threshold on the loaded quality factor. We show the dependence and high sensitivity of the output signal on the coupling. Lastly, we observe the impact of processing on the tuning of mode resonances due to the very short coupling distances. Such small footprint on-chip integrated microlasers providing maximum energy transfer into a photonic circuit have important potential applications for visible-light communication and lab-on-chip bio-sensors.

Spontaneous super-radiative cascade undulator emission from short dense electron bunches

We propose to use super-radiative self-compression of a short dense electron bunch to provide the cascade two-undulator regime of spontaneous emission from the bunch. At the first stage of this cascade, the spontaneous super-radiative emission of a relatively long-wavelength wave results in compression of the bunch by the radiated field. This results in high-efficiency spontaneous radiation of a short-wavelength wave at the second stage. According to the simulations performed for electron bunches with the parameters typical for modern photoinjectors, the cascade regime ensures radiation in the subterahertz frequency range with efficiencies from 10% (in regular systems) up to 30%–50% (in profiled systems).

Coherence and pulse duration characterization of the PAL-XFEL in the hard X-ray regime

We characterize the spatial and temporal coherence properties of hard X-ray pulses from the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL, Pohang, Korea). The measurement of the single-shot speckle contrast, together with the introduction of corrections considering experimental conditions, allows obtaining an intrinsic degree of transverse coherence of 0.85 ± 0.06. In the Self-Amplified Spontaneous Emission regime, the analysis of the intensity distribution of X-ray pulses also provides an estimate for the number of longitudinal modes. For monochromatic and pink (i.e. natural bandwidth provided by the first harmonic of the undulator) beams, we observe that the number of temporal modes is 6.0 ± 0.4 and 90.0 ± 7.2, respectively. Assuming a coherence time of 2.06 fs and 0.14 fs for the monochromatic and pink beam respectively, we estimate an average X-ray pulse duration of 12.6 ± 1.0 fs.

Tunable mid-infrared fiber laser source based on acetylene-filled hollow-core fiber

We report here a 3.1 µm band tunable fiber gas laser source based on acetylene-filled nodeless hollow-core fibers with low losses at both the pump and laser wavelengths. The gas is pumped in the first-order rotational–vibrational overtones near the 1.5 µm region by a modulated, amplified narrowband and accurately tunable diode laser; then 3 µm lasing transmissions are generated in the single-pass amplified spontaneous emission regime. The laser wavelength is step-tunable in the range of 3.09–3.21 µm when the pump laser is precisely tuned to different absorption lines of the P-branch of acetylene molecules. Furthermore, this system can be extended to other selected gases to generate tunable output in the spectral band up to 5 µm.

Spectral Analysis of the Amplified Spontaneous Emission in the Second Positive System of the N2 Molecule.

The spectrum of the 0-0 band of the [Formula: see text] electronic transition of the N2 molecule presents a considerable difference in its distribution of intensities, as a function of the wave number, when the emission spectrum by glow discharge is compared to an amplified spontaneous emission (ASE) regime spectrum, commonly known as the N2 "laser". In the present paper, this particularity, due to gain of the transition, is analyzed from an experimental and theoretical point of view, and for the first time has its experimental and theoretical intensities fully compared. An experimental rotational spectrum is obtained for this transition and a model for the ASE intensities has been carried out in order to retrieve the experimental conditions. The theoretical calculations of the gain have been carried out through a model proposed by other authors, as explained in the article. For the comparison among the ASE experimental and theoretical intensities, the fast and slow relaxation approximations proposed have been used, being the first one that best reproduces the experimental spectrum. For the first time, the experimental and theoretical spectra are compared directly, allowing the precise determination of the vibrational coefficient of inversion and temperature, showing the possible problems arising from the approximation. A good agreement between experimental and theoretical results is observed showing the reasonable validity of the model for the gain.

Mid-infrared 1 W hollow-core fiber gas laser source.

We report the characteristics of a 1W hollow-core fiber gas laser emitting CW in the mid-IR. Our system is based on an acetylene-filled hollow-core optical fiber guiding with low losses at both the pump and laser wavelengths and operating in the single-pass amplified spontaneous emission regime. Through systematic characterization of the pump absorption and output power dependence on gas pressure, fiber length, and pump intensity, we determine that the reduction of pump absorption at high pump flux and the degradation of gain performance at high gas pressure necessitate the use of increased gain fiber length for efficient lasing at higher powers. Low fiber attenuation is therefore key to efficient high-power laser operation. We demonstrate 1.1W output power at a 3.1μm wavelength by using a high-power erbium-doped fiber amplifier pump in a single-pass configuration, approximately 400 times higher CW output power than in the ring cavity previously reported.

1/f Noise in Optical Output and Non-Gaussianity in Voltage Noise of GaAs Nanoscale Light-Emitting Structures

The 1/[Formula: see text] noise investigation in nanoscale light-emitting diodes (LEDs) and lasers, based on GaAs and its solid solutions, is presented here. Spectra of 1/[Formula: see text] noise [Formula: see text] in the voltage across terminals of device, and [Formula: see text] in emitted light intensity, as well as the cross-spectrum [Formula: see text], were measured. It was found that the noise in the leakage current, and in the additional nonlinear current, of devices is transformed into the noise in optical output intensity in spontaneous emission regime. We measured the histogram as the estimate of the probability density function of the voltage noise. The obtained data in lasers characterize the non-Gaussianity and/or nonstationarity of the noise in the regime of induced radiation. In contrast, in LEDs the deviation of noise from Gaussianity practically was not observed.

A few-emitter solid-state multi-exciton laser

We report on non-conventional lasing in a photonic-crystal nanocavity that operates with only four solid-state quantum-dot emitters. In a comparison between microscopic theory and experiment, we demonstrate that irrespective of emitter detuning, lasing with {g}^{mathrm{(2)}}=1 is facilitated by means of emission from dense-lying multi-exciton states. In the spontaneous-emission regime we find signatures for radiative coupling between the quantum dots. The realization of different multi-exciton states at different excitation powers and the presence of electronic inter-emitter correlations are reflected in a pump-rate dependence of the β-factor.

Ultrafast Room-Temperature Single Photon Emission from Quantum Dots Coupled to Plasmonic Nanocavities.

Efficient and bright single photon sources at room temperature are critical components for quantum information systems such as quantum key distribution, quantum state teleportation, and quantum computation. However, the intrinsic radiative lifetime of quantum emitters is typically ∼10 ns, which severely limits the maximum single photon emission rate and thus entanglement rates. Here, we demonstrate the regime of ultrafast spontaneous emission (∼10 ps) from a single quantum emitter coupled to a plasmonic nanocavity at room temperature. The nanocavity integrated with a single colloidal semiconductor quantum dot produces a 540-fold decrease in the emission lifetime and a simultaneous 1900-fold increase in the total emission intensity. At the same time, the nanocavity acts as a highly efficient optical antenna directing the emission into a single lobe normal to the surface. This plasmonic platform is a versatile geometry into which a variety of other quantum emitters, such as crystal color centers, can be integrated for directional, room-temperature single photon emission rates exceeding 80 GHz.