Disordered noble-metal nanoparticle films exhibit highly localized and stable non-linear light emission from sub-diffraction regions upon illumination by near-infrared femtosecond pulses.Such hot spot emission spans a continuum in the visible and near-infrared spectral range. Strong plasmonic enhancement of light-matter interaction and the resulting complexity of experimental observations have prevented the development of a universal understanding of the origin of light emission. Here, we study the dependence of emission spectra on excitation irradiance and provide the most direct evidence yet that the continuum emission observed from both silver and gold nanoparticle aggregate surfaces is caused by recombination of hot electrons within the conduction band. The electron gas in the emiting particles, which is effectively decoupled from the lattice temperature for the duration of emission, reaches effective temperatures of several thousand Kelvin and acts as a sub-diffraction incandescent light source on sub-picosecond time

2 A hallmark of wave-matter duality is the emergence of quantum-interference phenomena when an electronic transition follows different trajectories. Such interference results in asymmetric absorption lines such as Fano resonances 1 , and gives rise to secondary effects like electromagnetically induced transparency (EIT) when multiple optical transitions are pumped 2-5 . Few solid-state systems show quantum interference and EIT 5-11 , with quantum-well intersubband transitions in the IR 12,13 offering the most promising avenue to date to devices exploiting optical gain without inversion 14,15 . Quantum interference is usually hampered by inhomogeneous broadening of electronic transitions, making it challenging to achieve in solids at visible wavelengths and elevated temperatures. However, disorder effects can be mitigated by raising the oscillator strength of atom-like electronic transitionsexcitonswhich arise in monolayers of transition-metal dichalcogenides (TMDCs) 16,17 . Quantum interference, probed by second-harmonic generation (SHG) 18,19 , emerges in WSe2, without a cavity, splitting the SHG spectrum. The splitting exhibits spectral anticrossing behaviour, and is related to the number of Rabi flops the strongly driven system undergoes. The SHG powerlaw exponent deviates strongly from the canonical value of 2, showing a Fano-like wavelength dependence which is retained at room temperature. The work opens opportunities in solid-state quantum-nonlinear optics for optical mixing, gain without inversion and quantum-information processing.We consider the wavelength dependence of SHG from monolayer WSe2 crystals as sketched in Fig. 1a. Under excitation at 724 nm with pulses of 80 fs length at 80 MHz repetition rate, Fig. 1b reveals two peaks in the SHG spectrum, at 360 nm and 364 nm,

With organic light-emitting diodes (OLEDs) emerging in ever more applications, such as smart phones, televisions, and lighting, it is easy to forget that the present technology is based on a brilliantly simple patch to an inherent problem of fluorescent hydrocarbons: three quarters of the electrically generated energy is dissipated as heat by triplet excitons. Radiative decay from the triplet state via phosphorescence is generally very weak, and has only been resolved in transient spectroscopy at low temperatures in select organic semiconductors. [1] The solution to this problem has been to incorporate metal-organic emitters in OLEDs, [2] which mix spin by enhancing intersystem crossing through spin-orbit coupling: the heavy-atom effect. As this approach relies on the longevity of triplet excitons and the associated diffusion lengths, it is highly effective: in a suitably homogeneous environment even ppm concentrations of covalently bound metal atoms are sufficient to activate electrophosphorescence. [3] The second conceivable approach to harvesting energy from triplets is based on endothermic conversion [4] to a fluorescent singlet by reverse intersystem crossing. [5] This method necessitates control not only over spin-orbit coupling, requiring a heavy atom or a carefully engineered charge-transfer state, but also over the singlet-triplet exchange gap, which can be tuned by excitonic confinement. [6] Although progress has been made recently, conceptually it parallels the former approach: all excitations are converted to either triplets or singlets, thereby losing information on the underlying spin correlations of charge carriers. Evidence is emerging, however, that spin correlations in excitonic electron-hole precursor pairs can be used for exquisitely sensitive measurements of magnetic fields [7] and possibly even for quantum coherence phenomenology, [7b] with analogies to avian radical-pair photomagnetosensory processes. [8] To quantify such spin correlations, it is desirable to develop materials without heavy-atom spin mixing that show both intrinsic fluorescence and phosphorescence.The third approach to triplet harvesting has not been explored previously: tuning spin-orbit coupling without heavy atoms such that non-radiative internal conversion from the triplet excited state to the singlet ground state is suppressed and phosphorescence is the only remaining relaxation mechanism. Even in low-atomic-order-number compounds such as hydrocarbons, the orbital component of the wavefunction can give rise to substantial magnetic moments, leading to non-negligible spin-orbit energy terms. The effect is well-studied in carbon nanotubes and graphene, where zero-field splitting correlates with nanoscale curva

Gold nanoparticles emit broad-band upconverted luminescence upon irradiation with pulsed infrared laser radiation. Although the phenomenon is widely observed, considerable disagreement still exists concerning the underlying physics, most notably over the applicability of concepts such as multiphoton absorption, inelastic scattering, and interband vs intraband electronic transitions. Here, we study single particles and small clusters of particles by employing a spectrally resolved power-law analysis of the irradiation-dependent emission as a sensitive probe of these physical models. Two regimes of emission are identified. At low irradiance levels of kW/cm, the emission follows a well-defined integer-exponent power law suggestive of a multiphoton process. However, at higher irradiance levels of several kW/cm, the nonlinearity exponent itself depends on the photon energy detected, a tell-tale signature of a radiating heated electron gas. We show that in this regime, the experiments are incompatible with both interband transitions and inelastic light scattering as the cause of the luminescence, whereas they are compatible with the notion of luminescence linked to intraband transitions.

The method of current extraction under linear increasing voltages (CELIV) allows for the simultaneous determination of charge mobilities and charge densities directly in thin films as used in organic photovoltaic cells (OPV). In the past, it has been specifically applied to investigate the interrelation of microstructure and charge transport properties in such systems. Numerical and analytical calculations presented in this work show that the evaluation of CELIV transients with the commonly used analysis scheme is error prone once charge recombination and, possibly, field dependent charge mobilities are taken into account. The most important effects are an apparent time-dependence of charge mobilities and errors in the determined field dependencies. Our results implicate that reports on time-dependent mobility relaxation in OPV materials obtained by the CELIV technique should be carefully revisited and confirmed by other measurement methods.