Preferential Emission Research Articles

Nd:YVO4 Random Laser with Preferential Emission at 1340 nm over 1064 nm

Nd:YVO4 Random Laser with Preferential Emission at 1340 nm over 1064 nm

Preferential Emission of Microplastics from Biosolid-Applied Agricultural Soils: Field Evidence and Theoretical Framework

Preferential Emission of Microplastics from Biosolid-Applied Agricultural Soils: Field Evidence and Theoretical Framework

Ultrasound harmonic generation and atomic layer deposition of multilayer, deep-UV mirrors and filters with microcavity plasma arrays

In honor of Professor Kurt Becker’s pioneering contributions to microplasma physics and applications, we report the capabilities of arrays of microcavity plasmas in two emerging and disparate applications. The first of these is the generation of ultrasound radiation in the 20–240 kHz spectral range with microplasmas in either a static or jet configuration. When a 10times 10 array of microplasma jets is driven by a 20-kHz sinusoidal voltage, for example, harmonics as high as m = 12 are detected and fractional harmonics are produced by controlling the spatial symmetry of the emitter array. The preferential emission of ultrasound in an inverted cone having an angle of pm ,45^circ with respect to the surface normal of the jet array’s exit face is attributed to interference between spatially periodic, outward-propagating waves generated by the arrays. The spatial distribution of ultrasound generated by the arrays is analogous to the radiation patterns produced by Yagi-Uda phased array antennas at RF frequencies for which radiation is emitted broadside to arrays of parallel electric dipoles. Also, the nonperturbative envelope of the ultrasound harmonic spectrum resembles that for high-order harmonic generation at optical frequencies in rare gas plasmas and attests to the strong nonlinearity provided by the pulsed microplasmas in the sub-250-kHz region. Specifically, the relative intensities of the second and third harmonics exceed that for the fundamental, and a “plateau” region is observed extending from the 5th through the 8th harmonics. A strong plasma nonlinearity appears to be responsible for both the appearance of fractional harmonics and the nonperturbative nature of the acoustic harmonic spectrum. Multilayer metal-oxide optical filters designed to have peak transmission near 222 nm in the deep-UV region of the spectrum have been fabricated by microplasma-assisted atomic layer deposition. Alternating layers of ZrO_2 and Al_2O_3, each having a thickness in the 20–50 nm range, were grown on quartz and silicon substrates by successively exposing the substrate to the Zr or Al precursor (tetrakis(dimethylamino) zirconium or trimethylaluminum, respectively) and the products of an oxygen microplasma while maintaining the substrate temperature at 300 K. Bandpass filters comprising 9 cycles of 30-nm-thick ZrO_2/50-nm-thick Al_2O_3 film pairs transmit 80% at 235 nm but < 35% in the 250–280 nm interval. Such multilayer reflectors appear to be of significant value in several applications, including bandpass filters suppressing long wavelength (240–270 nm) radiation emitted by KrCl (222) lamps.Graphical abstract

Enhanced Spontaneous Emission of CsPbI3 Perovskite Nanocrystals Using a Hyperbolic Metamaterial Modified by Dielectric Nanoantenna.

In this work, we demonstrate, theoretically and experimentally, a hybrid dielectric-plasmonic multifunctional structure able to provide full control of the emission properties of CsPbI3 perovskite nanocrystals (PNCs). The device consists of a hyperbolic metamaterial (HMM) composed of alternating thin metal (Ag) and dielectric (LiF) layers, covered by TiO2 spherical MIE nanoresonators (i.e., the nanoantenna). An optimum HMM leads to a certain Purcell effect, i.e., an increase in the exciton radiative rate, but the emission intensity is reduced due to the presence of metal in the HMM. The incorporation of TiO2 nanoresonators deposited on the top of the HMM is able to counteract such an undesirable intensity reduction by the coupling between the exciton and the MIE modes of the dielectric nanoantenna. More importantly, MIE nanoresonators result in a preferential light emission towards the normal direction to the HMM plane, increasing the collected signal by more than one order of magnitude together with a further increase in the Purcell factor. These results will be useful in quantum information applications involving single emitters based on PNCs together with a high exciton emission rate and intensity.

Interplay of Zn(OAc)2 concentration, morphology, and emission in hydrothermal-grown ZnO nanostructures

We report the interplay of zinc acetate [Zn(CH3COO)2, Zn(OAc)2] concentration, morphology, and emission in hydrothermal-grown ZnO nanostructures. Highly oriented nanorods with intense, 54 to 96 ps near-band-edge ultraviolet (UV) emissions are successfully fabricated using 25 to 75 mM concentrations. On the other hand, larger and randomly oriented nanocrystals are fabricated using 100 to 150 mM concentrations. Although all nanostructures exhibit hexagonal wurtzite crystal structures, increasing the Zn(OAc)2 concentration affects their preferential orientation, morphology, and UV emission. The peculiar nanostructure emissions observed are also attributed to a reabsorption phenomenon among adjacent nanostructures. For potential scintillator applications, particularly for x-ray diagnostics, our results suggest that less than 100 mM Zn(OAc)2 concentrations should be used to reduce the UV emission reabsorption in hydrothermal-grown ZnO nanostructures.

Iodization-enhanced fluorescence and circularly polarized luminescence for dual-readout probe design

Circularly polarized luminescence (CPL) is a preferential emission from chiral systems. Introducing CPL to traditional small fluorescence probes will greatly enhance selectivity and sensitivity due to avoiding the background fluorescence. Small-molecule probes are practically useful for bioimaging and sensing but always suffer from weak CPL properties. To optimize chiral perturbation, the study of structure-CPL correlation is an urgent necessity. Here, we investigated the influence of 3, 3′-substituent of BINOL on optical properties and found that iodization could significantly enhance the fluorescence quantum yield (ΦF) and dissymmetry factor (|glum|). Based on this finding, we presented a strategy to design dual-readout probes emiting both robust total fluorescence and CPL upon activation. As an application example, we first designed a pair of thiols probes, R/S-P1, with the fluorescence readout showing good linear response to cysteine, and CPL signal displaying significant enhancement with |glum| from undetectable value to 0.7 × 10−3.

Parity-time symmetric gratings in 1550 nm distributed-feedback laser diodes: insight on device design rules

We build on former results from our work on parity-time symmetric gratings implemented in 1550 nm distributed-feedback laser diodes to address the design issues raised by the first trends we observed. These laser diodes are of a complex-coupled nature, with modulations of both real and imaginary parts of the effective index, with a relative phase related to the parity-time symmetry. The unidirectionality of the photonic behavior in the reflection mode is dependent on the level of extra losses incurred by the metallic grating used to implement fixed loss modulation onto a nearly uniform gain, either as a first-order or a third-order grating. The observed behavior suggests that facets play a large role in setting the desired unidirectional lasing operation points, with preferential emission on one side. We explore this issue, which is of generic interest for the coupling of parity-time symmetric structures to open space.

Effects of photoperiod and light spectra on growth and pigment composition of the green macroalga Codium tomentosum

Codium tomentosum is a marine green macroalga with multiple value-added applications that is being successfully used as an extractive species in sustainable integrated multi-trophic aquaculture systems. Nonetheless, growth conditions of this species at an early development phase still require optimization. The present study addresses, under controlled laboratory conditions, the effects of photoperiod (long vs. short-day) and light spectra (white, blue, and red light) on growth and pigment composition of C. tomentosum. Relative growth rate was approximately 2× higher under long-day photoperiod (average of 39.2 and 20.1% week−1 for long and short-day, respectively). Concentrations per dry weight of major pigments such as chlorophyll a (Chla) and siphonoxanthin (Siph) were significantly higher under long-day photoperiod. Relative growth rates were higher under red light, intermediate under white light, and lower under blue light. These last results were rather surprising, as Siph-Chla/Chlb light harvesting complexes of Codium have increased absorption in the blue-green region of the light spectra. Changes in carbon allocation patterns caused by the spectral composition of light and overgrowth of green microalgae in blue light cultures could explain the differences recorded for relative growth rate. Long-day photoperiod and light sources with preferential emission at the red region of the light spectra were identified as optimal for growth of C. tomentosum at early development stages. These lighting conditions can reduce the time required to reach the necessary biomass before transfer to grow-out systems. Overall, these findings can shorten production time, increase macroalgal productivity, and enhance aquaculture revenues.

Ultrafast Excited-State Dynamics of Tricarbocyanine Dyes Probed by Two-Dimensional Electronic Spectroscopy: Polar Solvation vs Photoisomerization.

Photophysical properties of tricarbocyanine dyes in various solvents have been widely investigated using a variety of spectroscopic tools. However, the presence of several ground-state isomers and interconversion between these isomers on an ultrafast timescale upon photoexcitation render unambiguous assignment of spectral features quite difficult. In this work, ultrafast excited-state dynamics of two tricarbocyanine dyes in two solvents, DNTTCI and IR140, in ethanol and ethylene glycol, are studied by two-dimensional electronic spectroscopy (2DES). We present a detailed discussion on design and calibration of the 2DES apparatus and on the method for data processing by phase-cycling. For DNTTCI we report a method to obtain solvation correlation function, the nature of which is found to be strongly dependent on the excitation frequencies; a blue-shifted spectrum at early time is observed and explained based on preferential emission from a subset among various isomers having overlapping spectral features. For IR140 in ethanol, four isomers with distinct spectral features are identified, and most importantly, three of these isomers were found to interconvert upon photoexcitation which completes within 100 fs and is explained based on a kinetic model of consecutive chemical reaction. Density functional theory calculations show the presence of several ground-state isomers for both these dyes. Through this work we demonstrate how 2DES can help us to decipher distinct excited-state photophysics in two carbocyanine dyes, polar solvation and photoisomerization, by resolving spectral congestion without sacrificing time resolution.

Synthesis of Enantiopure Lanthanide Complexes Supported by Hexadentate N,N'-Bis(methylbipyridyl)bipyrrolidine and Their Circularly Polarized Luminescence.

We report the synthesis of lanthanide complexes supported by enantiopure N,N'-bis(methylbipyridyl)bipyrrolidine and subsequent characterization through luminescence studies. Complexes of this ligand with the visibly emissive lanthanides Sm, Eu, Tb, and Dy are luminescent (ϕf of ≤0.32) and demonstrate strong preferential emission of circularly polarized light in all four cases (|glum| of ≤0.26). Notably, all four possess at least one transition with a |glum| of >0.2, and the strongest preferential emission is measured from the complexes of Sm and Dy.

Induction of circularly polarized electroluminescence from achiral poly(fluorene-alt-benzothiadiazole) by circularly polarized light

Without any chiral additives, achiral F6BT could be induced to directly give a preferential emission of specific handed CP EL.

All-optical spatio-temporal control of electron emission from SiO2 nanospheres with femtosecond two-color laser fields

Field localization by nanostructures illuminated with laser pulses of well-defined waveform enables spatio-temporal tailoring of the near-fields for sub-cycle control of electron dynamics at the nanoscale. Here, we apply intense linearly-polarized two-color laser pulses for all-optical control of the highest energy electron emission from SiO2 nanoparticles. For the size regime where light propagation effects become important, we demonstrate the possibility to control the preferential emission angle of a considerable fraction of the fastest electrons by varying the relative phase of the two-color field. Trajectory based semi-classical simulations show that for the investigated nanoparticle size range the directional steering can be attributed to the two-color effect on the electron trajectories, while the accompanied modification of the spatial distribution of the ionization rate on the nanoparticle surface has only a minor effect.

Effect of metal–dielectric substrates on chemiluminescence kinetics

We studied chemiluminescence of rhodamine 6G (R6G) dye in a series of R6G:DNPO:PMMA films deposited onto a variety of metallic, metal–dielectric, and dielectric substrates. We found that chemiluminescence kinetics on top of Ag and Au films covered by an insulating MgF2 layer had longer decay times than those on top of purely dielectric substrates, MgF2 films on glass. We infer that Ag- and Au-based substrates with an insulating MgF2 layer on top affected the reaction rates by influencing polarization energies of reactants and surrounding molecular dipoles. At the same time, the chemiluminescence kinetics on top of Ag and Au surfaces (without an insulating MgF2 spacer) were substantially shorter than those on top of dielectric substrates, probably because of conventional chemical catalysis. The peak emission intensities on top of metallic substrates (with and without the MgF2 layer on top) were lower than those on top of purely dielectric substrates, likely due to inhibition of the energy transfer to acceptors (R6G dye molecules) known in the vicinity of metals. The further reduction of chemiluminescence intensity on top of lamellar metal–dielectric stacks could be due to preferential emission of light inside the volume of the metamaterial rather than outside.

Demonstration of extrinsic chirality of photoluminescence with semiconductor-metal hybrid nanowires

Chiral optical response is an inherent property of molecules and nanostructures, which cannot be superimposed on their mirror images. In specific cases, optical chirality can be observed also for symmetric structures. This so-called extrinsic chirality requires that the mirror symmetry is broken by the geometry of the structure together with the incident or emission angle of light. From the fabrication point of view, the benefit of extrinsic chirality is that there is no need to induce structural chirality at nanoscale. This paper reports demonstration extrinsic chirality of photoluminescence emission from asymmetrically Au-coated GaAs-AlGaAs-GaAs core-shell nanowires fabricated on silicon by a completely lithography-free self-assembled method. In particular, the extrinsic chirality of PL emission is shown to originate from a strong symmetry breaking of fundamental HE11 waveguide modes due to the presence of the asymmetric Au coating, causing preferential emission of left and right-handed emissions in different directions in the far field.

Separation of valley excitons in a MoS2 monolayer using a subwavelength asymmetric groove array

Excitons in monolayer transition metal dichalcogenides are formed at K and K′ points at the boundary of the Brillouin zone. They acquire a valley degree of freedom, which has been explored as an alternative information carrier, analogous to charge or spin. Two opposite valleys in transition metal dichalcogenides can be optically addressed using light with different helicity. Here, we demonstrate that valley-polarized excitons can be sorted and spatially separated at room temperature by coupling a MoS2 monolayer to a subwavelength asymmetric groove array. In addition to separation of valley excitons in real space, emission from valley excitons is also separated in photon momentum-space; that is, the helicity of photons determines a preferential emission direction. Our work demonstrates that metasurfaces can facilitate valley transport and establish an interface between valleytronic and photonic devices, thus addressing outstanding challenges in the field of valleytronics. Helicity of photons is exploited for preferential emission direction and sorting of valley-polarized excitons is achieved at room temperature.

Energy-modulated x-ray fluorescence and luminescence emissions from therapeutic nanoparticles

In this study, we have investigated the possibility of modulating x-ray fluorescence (XF) and x-ray luminescence (XL) emissions from therapeutic nanoparticles (NPs) by fine-tuning the energy of incident x-rays from benchtop x-ray sources. We have carried out detailed experimental studies to determine the strength of XF and XL emissions from Y2O3:Eu3+ and LaF3:Tb3+ NPs being irradiated with x-rays from benchtop x-ray sources operated with different tube-voltages and coupled to various filter configurations. These studies demonstrated that low-energy x-rays with average energy at around 10–15 keV are the most efficient to stimulate XL emission from the Y2O3:Eu3+ and LaF3:Tb3+ NPs. The efficiency falls quickly when x-ray energies go above or below the optimum energy range. As one would expect, x-rays with average energy just above the corresponding absorption edge of the target metal would be the most efficient in inducing XF emission. In this study, we have also demonstrated that one could fine-tune the incident x-ray energy to modulate the XL and XF emissions, such as (a) selectively inducing either XL or XF emission from the same type of NPs, (b) inducing preferential XL activation of Y2O3:Eu3+ over LaF3:Eu3+ or controlling the ratio of XL activation of these two types of NPs, and (c) introducing preferential XF emission from one type of NPs over the other. As a potential application, one could optimize the energy-characteristics of the incident x-rays to facilitate multiplexed combinatorial delivery of photodynamic therapy (X-PDT), where different agents could be administrated and then selectively activated in user-defined spatial and temporal patterns to fulfill combinatorial therapeutic effects. The understanding gained through this study could prove critical for enhancing the therapeutic delivery in X-PDT, and for attaining high-quality x-ray fluorescence computed tomography (XFCT) and x-ray luminescence computed tomography (XLCT) images while minimizing the x-ray dose to the sample.

Valley selective optical control of excitons in 2D semiconductors using a chiral metasurface [Invited

Recent advances in condensed matter physics have shown that the valley degree of freedom of electrons in 2D materials with hexagonal symmetry, such as graphene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDs), can be efficiently exploited, leading to the emergent field of valleytronics, which offers unique opportunities for efficient data transfer, computing and storage. The ability to couple the valley degree of freedom of electrons with light can further expand the ways one manipulates this degree of freedom, thus envisioning a new class of solid-state-photonic interfaces and devices. Besides this expansion of control of valley by light-waves, coupling of photons with valley-polarized electrons can dramatically expand the landscape of available optical responses, which may bring new means of controlling light in photonic devices. In this work we design such hybrid solid-state photonic metasurface integrating 2D TMD and photonic all-dielectric metasurface. While TMD is naturally endowed with the property of valley to optical-polarization coupling, the photonic metasurface is designed to produce chiral field which selectively couples to the valley degree of freedom of solid-state TMD component. We experimentally demonstrate that such coupling leads to controlled valley polarization due to the coupling of 2D materials with the chiral photonic metasurface. The measured emission from valley excitons in this hybrid system yields the preferential emission of specific helicity.

The Lick AGN Monitoring Project 2011: Dynamical Modeling of the Broad-line Region

Abstract We present models of the Hβ-emitting broad-line region (BLR) in seven Seyfert 1 galaxies from the Lick AGN Monitoring Project 2011 sample, drawing inferences on the BLR structure and dynamics as well as the mass of the central supermassive black hole. We find that the BLR is generally a thick disk, viewed close to face-on, with preferential emission back toward the ionizing source. The dynamics in our sample range from near-circular elliptical orbits to inflowing or outflowing trajectories. We measure black hole masses of for PG 1310−108, for Mrk 50, for Mrk 141, for Mrk 279, for Mrk 1511, for NGC 4593, and for Zw 229−015. We use these black hole mass measurements along with cross-correlation time lags and line widths to recover the scale factor f used in traditional reverberation mapping measurements. Combining our results with other studies that use this modeling technique, which brings our sample size to 16, we calculate a scale factor that can be used for measuring black hole masses in other reverberation mapping campaigns. When using the root-mean-square (rms) spectrum and using the line dispersion to measure the line width, we find pred = 0.57 ± 0.19. Finally, we search for correlations between f and other AGN and BLR parameters and find marginal evidence that f is correlated with M BH and the BLR inclination angle, but no significant evidence of a correlation with the AGN luminosity or Eddington ratio.

Zeeman Splitting and Inverted Polarization of Biexciton Emission in Monolayer WS_{2}.

Atomically thin semiconductors provide an ideal testbed to investigate the physics of Coulomb-bound many-body states. We shed light on the intricate structure of such complexes by studying the magnetic-field-induced splitting of biexcitons in monolayer WS_{2} using polarization-resolved photoluminescence spectroscopy in out-of-plane magnetic fields up to 30T. The observed g factor of the biexciton amounts to about -3.9, closely matching the g factor of the neutral exciton. The biexciton emission shows an inverted circular field-induced polarization upon linearly polarized excitation; i.e., it exhibits preferential emission from the high-energy peak in a magnetic field. This phenomenon is explained by taking into account the hybrid configuration of the biexciton constituents in momentum space and their respective energetic behavior in magnetic fields. Our findings reveal the critical role of dark excitons in the composition of this many-body state.

Influence of disorder on electromagnetically induced transparency in chiral waveguide quantum electrodynamics

We study single photon transport in a one-dimensional disordered lattice of three-level atoms coupled to an optical waveguide. In particular, we study atoms of \Lambda-type that are capable of exhibiting electromagnetically induced transparency (EIT) and separately consider disorder in the atomic positions and transition frequencies. We mainly address the question of how preferential emission into waveguide modes (chirality) can influence the formation of spatially localized states. Our work has relevance to experimental studies of cold atoms coupled to nanoscale waveguides and has possible applications to quantum communications.