Delayed Fluorescence and Phosphorescence from Polyphenylquinoxalines

A survey of various methods of optical excitation of ionic states, including excimer states, of molecules demonstrates the extensive potential optical pumping and makes it possible to determine the trends that are promising for further searches. Noticeable among them, firstly, is direct optical excitation and its accompanying secondary chemical processes with phototransport of electrons, which offer the researchers the largest choice of specific molecules and excitation mechanisms. We have shown, with molecular chlorine as the example, that the search for working media for optical pumping need not be limited to heavy molecules. The use of a complicated isotopic composition of molecules, Zeeman splitting of levels in a magnetic field, and impact broadening of spectral lines makes it possible to extend, within reasonable limits, the method of wide-band pumping to include also light molecules. As to secondary exchange reactions with charge transfer, in which optically excited molecules participate, it can be seen from the results of estimates of the cross sections that they constitute the largest class of excimerformation reactions and are characterized by additional possibilities of spectrally controlling the course of the reactions for the purpose of obtaining various products. Other promising methods considered here are production of ionic photodissociation states, including polar photodissociation, and also chemical radiative collisions. Note particularly the need for continuing the search for polyatomic excimers. It is obvious that production of such excimers is possible in media having a more complicated molecular composition, where such an advantage of optical pumping as the absence of induced absorption at the lasing wavelength is most strongly manifested.

We present the direct far-infrared optical excitation of NO radicals from the electronic ground state to the spin-orbit excited state. This spin-orbit forbidden transition at a photon energy near 125 cm−1 borrows intensity by a small admixture of character into the wavefunction, and can be saturated using the intracavity free electron laser FELICE. In the state, the NO radical has a negligible magnetic moment, whereas the radicals have a large magnetic moment in the excited state. The direct optical excitation demonstrated here thus effectively 'switches on' the molecule's magnetic moment, and allows for the production of a sample of NO () with almost perfect quantum state purity. These optically prepared well-defined packets of magnetic NO radicals offer interesting prospects for the use of this benchmark molecule in molecular deceleration and trapping experiments.

We describe in detail the characterization of two high-power photonic transmitters based on two different kinds of high-power photodiodes, one a GaAs/AlGaAs based uni-traveling-carrier photodiode (UTC-PD) and the other a separated- transport-recombination photodiode (STR-PD). The diodes operate under optical pulse excitation at the 800 nm wavelength. Both PDs have the same total depletion layer thickness (same theoretical RC-limited bandwidth) and are monolithically integrated with the same broadband micro-machined circular disk monopole antennas to radiate strong sub-THz pulses. However the STR-PD based transmitter exhibits very different dynamic and static performance from that of the UTC-PD based transmitter due to the existence of a low-temperature-grown GaAs (LTG-GaAs) based recombination center inside the active region, and the much thinner thickness of effective depletion layer. Under optical pulse excitation (~ 480 pJ/pulse), the STR-PD based transmitter exhibits a much lower maximum averaged output photocurrent (1.2 mA versus 0.3 mA) than that of the UTC-PD transmitter, although the radiated electrical pulse-width and maximum peak-power, which are measured by the same THz time-domain spectroscopic (TDS) system, of both devices are comparable. These results indicate that although the recombination center in the STR-PD degrades its DC responsivity, it effectively improves the high-speed and output power performance of the device and eliminates the DC component of the photocurrent, which should minimize device-heating problem during high-power operation. The radiated waveforms of both devices under intense optical pulse illumination also exhibit excellent linearity and strong bias dependent magnitude. This suggests their suitability for application as photonic emitters and possibly as a data modulator in sub-THz impulse-radio communication systems.

We report on an experimental study into the effects of surface acoustic waves on the optical emission of dot-in-a-nanowire heterostructures in III-V material systems. Under direct optical excitation, the excitonic energy levels in III-nitride dot-in-a-nanowire heterostructures oscillate at the acoustic frequency, producing a characteristic splitting of the emission lines in the time-integrated photoluminescence spectra. This acoustically induced periodic tuning of the excitonic transition energies is combined with spectral detection filtering and employed as a tool to regulate the temporal output of anti-bunched photons emitted from these nanowire quantum dots. In addition, the acoustic transport of electrons and holes along a III-arsenide nanowire injects the electric charges into an ensemble of quantum dot-like recombination centers that are spatially separated from the optical excitation area. The acoustic population and depopulation mechanism determines the number of carrier recombination events taking place simultaneously in the ensemble, thus allowing control of the anti-bunching degree of the emitted photons. The results presented are relevant for the dynamic control of single photon emission in III-V semiconductor heterostructures.

We examine transport and relaxation dynamics of optically excited electrons in GaAs-based heterostructure layers, involving dilute nitrides in percent-level concentration range. Such heterostructures contain materials with very different mobilities. Drift instead of Hall mobility is determined using a special technique. The value of the mobility of photogenerated electrons in a freestanding, 3% N InGaAsN layer, as part of a two-layer structure of 100 nm GaAs/2 μm nitride, is found to be of the order of 2 cm2/Vs, much lower than other values reported in the literature. The concomitant presence of carriers in the GaAs and nitride layers leads to formation of a barrier at the interface region that hinders electrons to enter in the nitride material. The dwell time of photoexcited electrons in GaAs interfacing the nitride layer is of the order of milliseconds, as seen by photoconductivity transients after pulsed optical excitation, much longer than the resulted time from optical experiments. Comparison of optical with transport properties reveals that the same centers involved in luminescence, that appear to be extended, are responsible also for hopping transport, where they appear as deep states. A theoretical explanation is given.

A new fine structure of red luminescence (RL) was observed in unintentionally doped ZnO at low temperature. The new RL was demonstrated to be different from the one assigned to Fe3+ center by Heitz et al and visible only under the excitation of light capable of generating excitons. The RL can be extremely enhanced at the incident frequency in resonance with the donor exciton and the intensities of some adjacent lines separated by ∼4 meV are thermally populated with the increase in temperature. The new structured RL was studied in the terms of photoluminescence (PL) and PL excitation spectra and the origin was discussed taking into account the internal transitions of transition metal elements, structure defects, and native point defects. There is a great possibility that the new structured RL arises from the excited states converted from bound excitons, for example, the excitons or the electron and hole pairs bounded by the donor and acceptor pairs (DAPs) of O and Zn vacancies, because the binding energy determined by the equation of DAP fluorescence is highly in accordance with the theoretic values reported in the literature.

Using pulsed optical excitation and read-out along with single-phonon-counting techniques, we measure the transient backaction, heating, and damping dynamics of a nanoscale silicon optomechanical crystal cavity mounted in a dilution refrigerator at a base temperature of Tf≈11 mK. In addition to observing a slow (approximately 740-ns) turn-on time for the optical-absorption-induced hot-phonon bath, we measure for the 5.6-GHz “breathing” acoustic mode of the cavity an initial phonon occupancy as low as ⟨n⟩=0.021±0.007 (mode temperature Tmin≈70 mK) and an intrinsic mechanical decay rate of γ0=328±14 Hz (Qm≈1.7×107). These measurements demonstrate the feasibility of using short pulsed measurements for a variety of quantum optomechanical applications despite the presence of steady-state optical heating.Received 6 April 2015DOI:https://doi.org/10.1103/PhysRevX.5.041002This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical Society

In the realm of many-body physics, the study of low-dimensional excitonic complexes has emerged as a compelling area of research, offering tunable modifications and highlighting quasiparticle interactions as key drivers for advancing valleytronics. Building upon extensive studies of simpler few-body systems such as excitons and trions, here we present a comprehensive exploration of the valley dynamics in more complex five-body charged biexciton (XX-) in monolayer WS2 using helicity-resolved ultrafast spectroscopy. We observe a near-unity degree of valley polarization at a moderate temperature of ∼150 K, which persists substantially longer than the population lifetime. Intriguingly, this polarization reveals an unexpected positive correlation with external disturbances such as temperature and pump fluence─behaviors distinct from conventional few-body systems. These phenomena are attributed to the inherent suppression of valley-exchange interactions in XX-, combined with its dual formation mechanisms: direct optical excitation and indirect conversion mediated by trion-trion interactions. Our results demonstrate that multibody excitonic complexes are stable candidates for maintaining valley polarization and could enable valleytronic applications that utilize many-body correlations.

Doping of single-walled carbon nanotubes leads to the formation of new energy levels which are able to participate in optical processes. Here, we investigate (6,5)-single walled carbon nanotubes doped in a solution of hydrochloric acid using optical absorption, photoluminescence, and pump-probe transient absorption techniques. We find that, beyond a certain level of doping, the optical spectra of such nanotubes exhibit the spectral features related to two doping-induced levels, which we assign to a localized exciton X and a trion T, appearing in addition to an ordinary exciton {E}_{1}. We evaluate the formation and relaxation kinetics of respective states and demonstrate that the kinetics difference between E1 and X energy levels perfectly matches the kinetics of the state T. This original finding evidences the formation of trions through nonradiative relaxation via the X level, rather than via a direct optical excitation from the ground energy state of nanotubes.

Direct optical excitation into excited states of Cr 3+ pairs in ruby

Some polarisation effects in photoemission from a cubic crystal

Optical control of electronic state populations via the dynamic Stark effect

Photoluminescence excitation (PLE) spectra of porous silicon have been studied for samples subjected to postanodization photochemical etching in a HF solution. It is shown that the peak energies of the PLE spectra show a large shift of about 1 eV with increasing etching time. Furthermore, the manner in which the PLE spectra shift is irregular; they first move toward lower energies, and then back to higher energies with increasing etching time. This behavior is interpreted by assuming two independent excitation mechanisms competing with each other in luminescent Si nanocrystallites: a direct-gap absorption process similar to the \ensuremath{\sim}4-eV absorption in bulk crystalline Si and the direct excitation of localized states in the crystallites. The latter excitation mechanism is more important in efficiently luminescent samples.

Polymer micro-/nanofibers, due to their low-cost and mechanical flexibility, are attractive building blocks for developing lightweight and flexible optical circuits. They are also versatile photonic materials for making various optical resonators and lasers, such as microrings, networks and random lasers. In particular, for random lasing architectures, the demonstrations to-date have mainly relied on fiber bundles whose properties are hard to tune post-fabrication. Here, we demonstrate the successful implementation of an inverted photonic glass structure with monodisperse pores of 1.28 μm into polymer fibers with diameter ranging from 10 to 60 μm. By doping organic dye molecules into this structure, individual fibers can sustain random lasing under optical pulse excitation. The dependence of lasing characteristics, including lasing spectrum and lasing threshold on fiber diameter are investigated. It is found that the lasing emission red-shifts and the threshold decreases with increasing fiber diameter. Furthermore, owing to the mechanical flexibility, the lasing properties can be dynamically changed upon stretching, leading to a wavelength-tunability of 5.5 nm. Our work provides a novel architecture for random lasers which has the potential for lasing tunability and optical sensing.

29p-SB-7 Coherent and incoherent optical processes in condensed matter under optical pulse excitation