Stimulated Emission Peak Research Articles

Femtosecond transient absorption spectroscopy on the thermally activated delayed fluorescence of bis[4-(9-H-carbazole)phenyl] sulfone

Tracking the time-evolution process of singlet-triplet conversion is essential to achieve effective thermally activated delayed fluorescence (TADF). The femtosecond transient absorption (fs-TA) spectroscopy of Bis[4-(9-H-carbazole)phenyl] Sulfone (CZ-DPS) in dichloromethane (DCM) shows that the excited state absorption (ESA) signal (375 nm) and the stimulated emission (SE) signal (430 nm) reach their maximum values within 4.1 ps. Subsequently, along with the decay of the ESA and SE peaks, a noticeable triplet-triplet absorption (TTA) signal appears at 428 nm and the intensity gradually increases. Furthermore, an isosbestic point at 400 nm between ESA and TTA signals indicates the occurrence of the intersystem crossing (ISC) process from singlet to triplet states, and its time constant is 3.3 ns. Moreover, the observed long-lived lifetime of 12.1 μs in DCM solution is assigned to the delayed fluorescence. The large spin-orbit coupling (SOC) constant (1.039 cm−1) facilitates the ISC process of the CZ-DPS from the first excited state (S1) to the second triplet-state (T2). Furthermore, the small energy difference (ΔES1T2 = 0.03 eV) induces the effective reversed ISC process from the T2 to S1 states to achieve delay fluorescence emission. This work presents the time-resolved spectra changes and TADF mechanism of CZ-DPS molecule.

A polarization scheme that resolves cross-peaks with transient absorption and eliminates diagonal peaks in 2D spectroscopy

Two-dimensional (2D) optical spectroscopy contains cross-peaks that are helpful features for determining molecular structure and monitoring energy transfer, but they can be difficult to resolve from the much more intense diagonal peaks. Transient absorption (TA) spectra contain transitions similar to cross-peaks in 2D spectroscopy, but in most cases they are obscured by the bleach and stimulated emission peaks. We report a polarization scheme, <0°,0°,+θ2(t2),-θ2(t2)>, that can be easily implemented in the pump-probe beam geometry, used most frequently in 2D and TA spectroscopy. This scheme removes the diagonal peaks in 2D spectroscopies and the intense bleach/stimulated emission peaks in TA spectroscopies, thereby resolving the cross-peak features. At zero pump-probe delay, θ2 = 60° destructively interferes two Feynman paths, eliminating all signals generated by field interactions with four parallel transition dipoles, and the intense diagonal and bleach/stimulated emission peaks. At later delay times, θ2(t2) is adjusted to compensate for anisotropy caused by rotational diffusion. When implemented with TA spectroscopy or microscopy, the pump-probe spectrum is dominated by the cross-peak features. The local oscillator is also attenuated, which enhances the signal two times. This overlooked polarization scheme reduces spectral congestion by eliminating diagonal peaks in 2D spectra and enables TA spectroscopy to measure similar information given by cross-peaks in 2D spectroscopy.

Phosphors emitting light yellow (laser) and light white through sodium-magnesium-borotellurite glasses activated with Dy3+

Sodium-magnesium-borotellurite glasses doped with 0.05, 0.5, 1.0 and 1.5 mol% of Dy2O3 (BTNMxD, x = 0.05, 0.5, 1.0 and 1.5) were synthetized by the melting quenching method. A spectroscopic investigation of BTNMxD glasses was focused to obtain the main radiative properties of yellow laser application and photometric properties of WLED application. The BTNM1.0D glass excited at 386 nm could be more feasible of obtaining yellow laser emission, since its stimulated emission peak cross section is the highest one among the BTNMxD glasses. The decay time of the yellow emission (572 nm) decreases from 0.639 to 0.429 ms with increase of dysprosium content, as consequence of an increment of energy migration between Dy3+ ions via cross-relaxation processes. Such energy migration could be dominated by electric dipole-dipole interaction. The CIE1931 color coordinates of the overall emission from the BTNMxD glasses excited at 386 nm are into the neutral white region with a correlated color temperature in the 4311–4682 K range.

Ultrafast charge transfer dynamics in the excited state of DCM measured by a 6-fs UV pulse laser

• The transition from LE state to CT state was observed by using UV 6-fs pulse laser. • LE state transfers to CT state in 50 fs. • Stimulated emission of LE state was found to have peak at 2.9 eV. The photoexcited dynamics of a laser dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), in an acetonitrile solution and a dimethyl sulfoxide (DMSO) solution were studied by pump–probe spectroscopy with a 6-fs ultraviolet laser. UV light excites the DCM solved in any of acetonitrile or DMSO into locally excited (LE) state (via S 0 → S 2 excitation), then the LE state relaxes to a charge transfer state. In the previous study, the mechanism of transition from the LE state to the intramolecular charge transfer (ICT) state was not clear just known to proceed faster than 100 fs . Our present work has clarified the primary process determining the time constant ( L E → I C T state) to be 50 fs . Spectral analysis of the transient difference-absorption spectrum has shown that the locally excited state has a stimulated emission peak at 2.9 eV.

Interplay of Purcell Effect, Stimulated Emission, and Leaky Modes in the Photoluminescence Spectra of Microsphere Cavities

A theoretical model for describing the emission spectra of microsphere cavities is presented, and its prediction of detailed lineshapes of emission spectra associated with whispering gallery modes (WGMs) of various orders in ZnO microspheres (MSs) are verified experimentally by photoluminescence (PL) spectroscopy. The interplay of Purcell effect, quality factor, and leaky modes in spontaneous and stimulated emission spectra related to WGMs of all orders is revealed. The key success of the theory is based on the expansion of the full Green function of the MS in terms of all possible resonance modes in complex frequency space, which allows incorporation of contributions from leaky modes, stimulated emission processes, and Purcell effect. We show that the spontaneous emission spectrum calculated according to Mie theory (without Purcell effect) is dominated by the contribution of leaky modes, while the spontaneous and stimulated emission enhanced by Purcell effect are responsible for the main WGM resonance peaks observed experimentally. It is found that the stimulated emission peaks are doubly enhanced by their respective mode quality factor Q: one factor from the Purcell effect and the other factor from the photon number derived from the rate equation. After combining all these effects the theory can provide a quantitative description of fine features of both TE and TM modes (including higher-order modes) observed in the PL spectra of ZnO MSs. Surprisingly, it is found that for ZnO MS with diameter larger than 5 $\mu m$, the PL emission spectrum is dominated by higher-order modes. The quantitative understanding of the interplay of these emission mechanisms should prove useful for optimizing the performance of light-emitting devices based on micro resonators.

Spectroluminescent properties of ytterbium-activated fluorophosphate glasses

In this work, we studied the effect of the concentration of ytterbium ions on the spectroluminescent properties and laser parameters of fluorophosphate glasses of the Ba(PO3)2–MgCaSrBaAl2F14 system. It was shown that the absorption peak corresponds to λp=974 nm, and the stimulated emission peak is located at λ0=1002 nm. It was established that the position of these peaks does not change with an increase in the ytterbium content. The highest values of the stimulated emission cross-section σemi=0.562×10−20 cm2 and the luminescence lifetime τexp=2.1 ms correspond to the YbF3 concentration of 0.5 mol. %. The quantum yield Q for the YbF3 concentration of 0.5 mol. % is 91.6%. The presented glasses have a long luminescence lifetime, and the calculations show the possibility of obtaining large values of gain G.

Direct Signatures of Light-Induced Conical Intersections on the Field-Dressed Spectrum of Na2.

Rovibronic spectra of the field-dressed homonuclear diatomic Na2 molecule are investigated to identify direct signatures of the light-induced conical intersection (LICI) on the spectrum. The theoretical framework formulated allows the computation of the (1) field-dressed rovibronic states induced by a medium-intensity continuous-wave laser light and the (2) transition amplitudes between these field-dressed states with respect to an additional weak probe pulse. The field-dressed spectrum features absorption peaks resembling the field-free spectrum as well as stimulated emission peaks corresponding to transitions not visible in the field-free case. By investigating the dependence of the field-dressed spectra on the dressing-field wavelength, in both full- and reduced-dimensional simulations, direct signatures of the LICI can be identified. These signatures include (1) the appearance of new peaks and the splitting of peaks for both absorption and stimulated emission and (2) the manifestation of an intensity-borrowing effect in the field-dressed spectrum.

Carrier thermalization under stimulated emission in In0.17Ga0.83N epilayer at room temperature

We elucidate a strong room temperature stimulated emission (SE) of In0.17Ga0.83N epilayer grown by molecular beam epitaxy under the subpicosecond pulse excitation. The SE peak at 428 nm emerges on the higher energy side of the spontaneous emission in photoluminescence spectra when the excitation density exceeds the threshold of ∼3.68 mJ/cm2. Nondegenerate transient differential reflectivity measurements show that a multi-stage carrier thermalization from excited states to localized edge states and stimulated emission dominate the decay processes of photogenerated carriers under various excitation densities. Our results indicate that the existence of phonon bottleneck effect could result in a slow thermalization process in the InGaN material even under the condition of stimulated emission.

Anisotropy of the spectroscopic, optical and thermo-mechanical properties of Li_6Eu_1-xGd_x(BO_3)_3 crystals optimized for heat-scintillation cryogenic bolometers

Optical, spectroscopic and thermo-mechanical properties of monoclinic Li6Eu1-xGdx(BO3)3 (x = 0,0.25,0.35) bulk single crystals, grown to be used in the design of heat-scintillation cryogenic bolometers (HSCBs), were investigated. The linear thermal expansion was determined along the a, b, c and c* directions over the temperature range 303–873 K, and its tensor principal coefficients were calculated for both x = 0.25 and x = 0. In addition, the anisotropic thermal conductivity was measured over the temperature range 20-400 K in Li6Eu0.75Gd0.25(BO3)3 (LGEB7, x = 0.25), and the principal components of its tensor at 300 K were established. Spectroscopic properties such as polarized absorption, polarized emission, Raman spectroscopy and optical refractive indices are also reported for the first time. Based on the polarized emission spectra, the line and oscillator strengths, the radiative lifetimes and fluorescent branching ratios were obtained. The intensity parameters Ωλ (λ = 2,4) were obtained and then predicted via the Judd–Ofelt theory. The crystal field parameters and the 7F1 level splitting were discussed using the simple overlap model (SOM) and the method of equivalent nearest neighbours (MENN). The 613 nm emission originates from the 5D0→7F2 transition and the associated stimulated emission peak cross section reaches its maximum value in π-polarization, ≈1.07 × 10−20 cm2 in LGEB7. The thermo-mechanical characterizations and spectroscopic analysis in LGEB7 suggest that this crystal has better optical properties than the Eu3+-doped Li6Y(BO3)3 crystals. However, its potential for solid-state laser applications is still quite speculative, unless a clever thermo-mechanical management of the crystal under laser operation is designed.

Optical Properties and Electronic Energy Relaxation of Metallic Au144(SR)60 Nanoclusters

Electronic energy relaxation of Au144(SR)60(q) ligand-protected nanoclusters, where SR = SC6H13 and q = -1, 0, +1, and +2, was examined using femtosecond time-resolved transient absorption spectroscopy. The observed differential transient spectra contained three distinct components: (1) transient bleaches at 525 and 600 nm, (2) broad visible excited-state absorption (ESA), and (3) stimulated emission (SE) at 670 nm. The bleach recovery kinetics depended upon the excitation pulse energy and were thus attributed to electron-phonon coupling typical of metallic nanostructures. The prominent bleach at 525 nm was assigned to a core-localized plasmon resonance (CLPR). ESA decay kinetics were oxidation-state dependent and could be described using a metal-sphere charging model. The dynamics, emission energy, and intensity of the SE peak exhibited dielectric-dependent responses indicative of Superatom charge transfer states. On the basis of these data, the Au144(SR)60 system is the smallest-known nanocluster to exhibit quantifiable electron dynamics and optical properties characteristic of metals.

Stimulated emission of surface plasmon polaritons on smooth and corrugated silver surfaces

We have observed two regimes of stimulated emission of surface plasmon polaritons (SPPs) propagating at interfaces between dye-doped polymeric films and silver substrates. (i) On top of smooth silver substrates, the lasing threshold is very soft, and the relatively broad stimulated emission peak gradually narrows with increase of the pumping. We infer that this stimulated emission regime is dominated by amplification of SPPs in open paths, with a possible contribution from a diffusion feedback provided by unintentional scattering. (ii) At the same time, the laser action on top of roughened silver substrates is characterized by a sharp threshold and multiple narrow spectral lines. This novel stimulated emission effect has strong coherent feedback supported by strong scattering of SPPs at a roughened silver surface. The observed phenomena provide an addition to the family of surface plasmon lasers, enabling active metamaterials and plasmonics.

Enhanced electron-hole plasma stimulated emission in optically pumped gallium nitride nanopillars

An enhanced stimulated emission was observed in optically pumped GaN nanopillars. The nanopillars were fabricated from an epitaxial wafer by patterned pillar etching followed by crystalline regrowth. Under optical excitation, a strong redshifted stimulated emission peak emerged from a broad spontaneous emission background. The emission is attributed to the electron-hole plasma gain at high carrier density. The emission slope efficiency was greatly enhanced by 20 times compared with a GaN substrate under the same pumping condition. The enhancement is attributed to the better photon and gain interaction from the multiple scattering of photons among nanopillars.

Influence of the miniband on emission mechanism in Zn1−xCdxSe/ZnSe quantum wells

The stimulated emission has been investigated in Zn1−xCdxSe/ZnSe quantum wells with a constant sample width of 60 Å and barrier widths from 47 Å to 500 Å. A redshift of the stimulated emission peak with increasing excitation intensity was observed in samples with barrier widths narrower than 350 Å, and the stimulated emission mechanism in these samples is found to be dominated by exciton-exciton scattering. In contrast, no redshifts were observed for the sample with a barrier width of 500 Å, and the stimulated emission in this sample appears to be dominated by biexcitons. Time-resolved spectroscopy measurements were also performed on samples with barrier width of 47 and 500 Å below the threshold of stimulated emission, and it was found that the exciton lifetime decreases with increasing exciton energy for both of the samples. With a ten times smaller barrier width in the 47 Å sample, the lifetime of high energy exciton is significantly shorter than that of the other sample, while the low energy lifetime becomes longer. This implies that with a narrower barrier width of the quantum wells the excitons move faster to lower energy regions. The narrower the barrier, the easier it is for excitons to move through the miniband. This freedom of movement increases the exciton concentration at low energies in localized areas in the wells. High exciton concentration made exciton-exciton scattering more probable and reduce the stimulated emission threshold value.

Enhancement of stimulated emission in 12-fold symmetric quasi-crystals

In this paper, we investigate the stimulated emission in a 12-fold symmetric quasiperiodic photonic crystal. The stimulated emission peaks in the quasiperiodic photonic crystal are more abundant and stronger than those in a periodic crystal. Also, more stimulated emission peaks appear as the crystal size and the gain increase, and some frequencies of the peaks are independent of the incident direction. These phenomena may be due to wave localization in the quasiperiodic photonic crystal.

Transfer-of-coherence-enhanced stimulated emission and electromagnetically induced absorption in Zeeman split F_g--&gt;F_e=F_g−1 atomic transitions

The probe absorption spectra in single and multiple tripod systems formed when a weak sigma polarized pump and a tunable pi polarized probe interact with a Zeeman split F(g)-->F(e)=F(g)-1 atomic transition are characterized by two interfering stimulated Raman features separated by an electromagnetically induced absorption (EIA) peak at the line center. These Raman features can appear as either sharp stimulated emission peaks or electromagnetically induced transparency windows. In the multitripod systems, the EIA and stimulated emission peaks derive from the combined effects of interference between the stimulated Raman features and transfer of coherence from the excited to ground states.

Controlling Vibrational Excitation with Shaped Mid-IR Pulses

We report selective population of the excited vibrational levels of the T(1u) CO-stretching mode in W(CO)(6) using phase-tailored, femtosecond mid-IR (5.2 microm, 1923 cm(-1)) pulses. An evolutionary algorithm was used to optimize specific vibrational populations. Stimulated emission peaks, indicative of population inversion, could be induced. Systematic truncation of each optimized pulse allowed for increased understanding of the excitation mechanism. The pulses and techniques developed herein will have broad applications in controlling ground state chemistry and enhancing vibrational spectroscopies.

ZnO Nanobelt Arrays Grown Directly from and on Zinc Substrates: Synthesis, Characterization, and Applications

In situ growth of ZnO nanobelt arrays from and on zinc substrates (foils and microparticles) has been accomplished by controlled thermal oxidation in the presence of oxygen. The nanobelts grow approximately perpendicular to the Zn substrate surface along the 110 direction of ZnO, which has a thickness of approximately 3-4 nm, a width tapering from about 50 to 300 nm, and a length of approximately 10-20 mum. On the basis of the structural analysis and kinetic studies, a tip-growth mechanism is proposed, which underlines the transport of Zn from the substrate to the growing tip. The ratio of UV to green photoluminescent emissions of the as-synthesized ZnO nanobelt arrays could be controlled by varying the reaction conditions. Sharp UV stimulated emission peak is also observed at moderate threshold excitation intensity ( approximately 0.7 mJ/cm(2)) showing the high quality of the ZnO nanobelts. The ZnO nanobelts array has also been tested for sensing NH(3) gas, and high sensitivity, reversibility, and rapid response have been demonstrated.

Characteristics of amplified spontaneous emission of high indium content InGaN/GaN quantum wells with various silicon doping conditions

We compared the temperature dependent spectral variations of the amplified spontaneous emission (ASE) between InGaN/GaN quantum well samples of no doping, well doping, and barrier doping of silicon. The comparisons were particularly made between two series of samples with a low and a high indium content. The results show that a multi-peak ASE spectral feature and a low energy stimulated emission peak, existing at the photoluminescence shoulder, could be observed only in the high-indium-content, barrier-doped sample. Such results are supposed to originate from the formation of quantum dots of various sizes, concentrations, and shapes under the condition of barrier doping in the sample. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Indium-incorporation-induced transformation of optical, photoluminescence and lasing properties of InGaN epilayers

A comparative combined study of photoluminescence (PL), PL kinetics, stimulated emission (SE) and photoreflectance (PR) properties in In x Ga 1− x N epilayers is carried out in the composition range 0≤ x≤0.19. In-incorporation up to 4% leads to the sufficient longer radiative recombination decay time due to the decrease in non-radiative recombination channels, which are peculiar to GaN, and band-to-band optical transitions predominate the spontaneous PL spectrum. Further In-incorporation ( x>4%) leads to the localization of carriers and/or excitons at band-tails in the In-rich areas. Correlation between the position of dominant low-energy PR oscillation due to the main band gap and SE peak position shows that band-to-band transitions are responsible for lasing and dominate the PL spectrum in all highly pumped In x Ga 1− x N samples.

Photoluminescent properties of ZnO thin films grown on SiO2/Si(100) by metal-organic chemical vapor deposition

We report on the photoluminescent (PL) properties of ZnO thin films grown on SiO2/Si(100) substrates using low pressure metal-organic chemical vapor deposition. The growth temperature of the films was as low as 400°C. From the PL spectra of the films at 10–300 K, strong PL peaks due to free and bound excitons were observed. The origin of the near bandedge emission peaks was investigated measuring temperature-dependent PL spectra. In addition, the Zn O films demonstrated a stimulated emission peak at room temperature. Upon illumination with an excitation density of 1 MW/cm2, a strong, sharp peak was observed at 3.181 eV.