Photoluminescence Excitation Experiments Research Articles

Probing the optical near-field interaction of Mie nanoresonators with atomically thin semiconductors

Optical Mie resonators based on silicon nanostructures allow tuning of light-matter-interaction with advanced design concepts based on complementary metal–oxide–semiconductor (CMOS) compatible nanofabrication. Optically active materials such as transition-metal dichalcogenide (TMD) monolayers can be placed in the near-field region of such Mie resonators. Here, we experimentally demonstrate and verify by numerical simulations coupling between a MoSe2 monolayer and the near-field of dielectric nanoresonators. Through a comparison of dark-field (DF) scattering spectroscopy and photoluminescence excitation experiments (PLE), we show that the MoSe2 absorption can be enhanced via the near-field of a nanoresonator. We demonstrate spectral tuning of the absorption via the geometry of individual Mie resonators. We show that we indeed access the optical near-field of the nanoresonators, by measuring a spectral shift between the typical near-field resonances in PLE compared to the far-field resonances in DF scattering. Our results prove that using MoSe2 as an active probe allows accessing the optical near-field above photonic nanostructures, providing complementary information to sophisticated near-field microscopy equipment.

Hot exciton effect in photoluminescence of monolayer transition metal dichalcogenide

AbstractHot excitons are usually neglected in optical spectroscopy in two‐dimensional semiconductors for the sake of momentum conservation, as the majority of hot excitons are out of light cones. In this letter, we elaborate on the contribution of hot excitons to optical properties of monolayer Molybdenum diselenide (MoSe2) with photoluminescence (PL) and PL excitation (PLE) spectroscopy. With the excitation‐intensity‐dependent PL, temperature‐dependent PL and PLE experiments combined with the simulations, we experimentally distinguish the influences of the exciton temperature and the lattice temperature in the PL spectrum. It is concluded that the acoustic phonon‐assisted PL accounts for the non‐Lorentzian high energy tail in the PL spectrum, and the hot exciton effect is significant to linear optical properties of transition metal dichalcogenides. Besides, the effective exciton temperature is found to be several tens of Kelvin higher than the lattice temperature at non‐resonant optical excitation. It indicates that the exciton temperature needs to be carefully taken into account when considering the exciton‐related quantum phase phenomena such as exciton condensation. It is experimentally demonstrated that the effective exciton temperature can be tuned by excitation energy.Key points The acoustic phonon‐assisted photoluminescence (PL) accounts for the non‐Lorentzian high‐energy tail in the PL spectrum. “Hot” excitons play a significant role in optical properties of two‐dimensional transition metal dichalcogenides. The effective exciton temperature could be tuned by excitation energy.

Investigations of Electron-Electron and Interlayer Electron-Phonon Coupling in van der Waals hBN/WSe2/hBN Heterostructures by Photoluminescence Excitation Experiments.

Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe2/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′1 (WSe2) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe2 monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe2 monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX0 at double resonance condition is observed for the first time.

(Invited) Rare Earth Doped Layers Fabricated By Atomic Layer Deposition

Many approaches, developments, structures and materials have been proposed these last years to offer innovative solutions allowing the decrease of the CO2 production and the energy consumption while increasing the efficiency of the properties which are aiming for. Over the last decade, among the approaches developed, a growth process has experienced an important boom related to the possibility it offers on the development of high quality layers. It concerns the Atomic Layer Deposition technique (ALD) that allows growing conformal layer on a substrate having high surface/volume ratio. Although such a technique is well developed and used for microelectronic and photovoltaic applications, the works on photonic devices are still at an early stage. Indeed, for the former, Al2O3, SiNx, HfO2 materials are currently deposited using ALD for microelectronic devices. Besides, the high quality alumina passivation layer is deposited on the Si solar cell bottom that allows the increase of the performance of the solar cell. However, for photonic devices, the crucial issue is to succeed in incorporating and managing the dopant concentration during the process.ALD process is a, at least, two steps process requiring a precursor containing the element of the layer to grow and oxidation step. The most used sequence for Al2O3 growth is a trymethylaluminium (TMA) precursor pulse followed by an oxidation step using water pulse. For doping such layer, a suitable precursor allowing to grow a dopant atomic layer in a temperature range and under oxidizing conditions that are appropriate with the host matrix growth has to be found.The objective of this paper is to present our recent investigations on the growth of RE-doped Al2O3 layer by ALD using different RE precursors. For growing such doped layer, four steps process is required corresponding to the TMA precursor/oxidation sequence followed by the RE-precursor/oxidation one. Fabrication conditions using either water, O2 plasma or ozone as oxidant agent have been investigated to find the ALD window leading to a suitable Growth Per Cycle (GPC) rate in a low deposition temperature range. After deposition, different annealing treatments have been applied to optimize the microstructural and the optical properties of the films considering their potential application, notably in photovoltaic domain. The most attention was paid to Tb- and Er-doped Al2O3 thin layers growth and to the optimization of the processing conditions to achieve required optical properties for the photon management in solar cell.As an example, Er-doped Al2O3 layers have been produced varying the number of Er precursor cycles with respect to the TMA one in order to manage the Er concentration in the layer. Our thin films containing 4.0 to 13 at.% of Er in the host matrix were found to be homogeneous not only prior to the annealing treatment but also after an annealing at 650°C during 15 minutes in nitrogen, conditions that are suitable for solar cell processing. For the highest concentration, phase separation has been observed upon annealing temperature at high temperature. Photoluminescence and photoluminescence excitation experiments have been carried out to study the optical response of such a layer. Indeed, considering that 100nm-thick Al2O3 ALD layer is actually used at the bottom of the Si Cell, succeeding to incorporate optically active Er ions in such a layer and achieving an upconversion process to convert the IR non-absorbed photons by the cell would be a way to increase the Si cell efficiency without modifying a well-established industrial process.

Comment on “Observation of Fourier transform limited lines in hexagonal boron nitride”

Recently, emission linewidths of single defect centers in layered hexagonal boron nitride down to 50 MHz were reported, close to the lifetime limit. The linewidths were extracted from time traces of laser scanning photoluminescence excitation experiments. At the same time, temporal blinking was affecting the measured time traces even in the absence of a frequency scanning the excitation laser. I argue here that the reported temporal blinking timescale is consistent with the observed transients, implying that the interpretation as spectral shapes and thus the extracted linewidths are invalid.

Photoluminescence excitation spectroscopy of InAs/InGaAs/GaAs quantum dot arrays in a wide temperature range

Detailed photoluminescence and photoluminescence excitation (PLE) studying in temperature range 10-300 K with variation of detection energy of self-organized InAs/InGaAs/GaAs quantum dots is presented for the first time. Abnormal quantum well-like behavior of quantum dot arrays in PLE experiment at raised temperatures is observed. Changing in properties of quantum dots with temperature is attributed to modification of the type of the mechanism of dot population by charge carriers from non-equilibrium one to equilibrium one. Activation of thermally assisted carrier transport between quantum dots is found to destroy dot selection mechanism by detection energy in PLE experiment.

Synthesis, Luminescence and Crystal Field Calculations of Mn4+- Doped Li2TiO3 Deep Red Phosphor

Red emission of the Mn4+ ions incorporated in different materials is used in various applications, such as lighting, holographic recording, thermoluminescence dosimetry, thermometry, etc. Mn4+ emission can improve color-rendering-index of phosphor-converted white-light-emitting diodes, and it is a promising candidate to replace the well-established rare-earth Eu activator in red phosphors. Here, synthesis, structure, detailed spectroscopic and crystal field analysis of Mn4+ doped Li2TiO3 are presented. We have successfully synthesized Li2Ti0.995Mn0.005O3 powder by a solid state synthesis. All precursors were mixed with stoichiometric ratio Li : Ti = 2 : 1 and calcined at 800⁰C for 5 h to produce pure-phase. Structural analysis was done by X-ray diffraction. It confirmed monoclinic Li2TiO3 structure with space group C2/2, in which the octahedral sites are occupied by Ti4+ and Mn4+. Optical characterization included measurements of photoluminescence excitation and emission spectra, emission decay analyses and diffuse reflection measurements. Optical spectra showed emission and absorption bands originating from d–d electronic transitions (d3 configuration). Kubelka-Munk function of the measured diffuse reflectance spectrum showed characteristic absorption of Mn4+. Deep red emission at 680 nm under λexc= 490 nm is a result of the spin-forbidden 2Eg→4A2 and 4T2→ 4A2 electron transition of the tetravalent manganese ions. The energy levels of the Mn4+ ions in Li2TiO3 were calculated using the crystal field exchange-charge model. Three parameters are needed to describe completely each energy level: the Racah parameters B and C, which determine the positions of the free ion energy levels, and the crystal field strength denoted by Dq, which describes the splitting of the free ion terms. These parameters were calculated and showed good agreement with the experimental photoluminescence excitation and emission spectra. Figure 1

Observation of exciton-phonon coupling in MoSe2 monolayers

We study experimentally and theoretically the exciton-phonon interaction in MoSe2 monolayers encapsulated in hexagonal BN, which has an important impact on both optical absorption and emission processes. The exciton transition linewidth down to 1 meV at low temperatures makes it possible to observe high energy tails in absorption and emission extending over several meV, not masked by inhomogeneous broadening. We develop an analytical theory of the exciton-phonon interaction accounting for the deformation potential induced by the longitudinal acoustic phonons, which plays an important role in exciton formation. The theory allows fitting absorption and emission spectra and permits estimating the deformation potential in MoSe2 monolayers. We underline the reasons why exciton-phonon coupling is much stronger in two-dimensional transition metal dichalcodenides as compared to conventional quantum well structures. The importance of exciton-phonon interactions is further highlighted by the observation of a multitude of Raman features in the photoluminescence excitation experiments.

Resonant excitation of infrared emission in GaN:(Mn,Mg)

By combining experimental photoluminescence excitation spectroscopy and calculations based on density functional theory and many-body Green's functions, the most efficient excitation channels of infra-red (IR) emission from Mn-Mg$_{k}$ paramagnetic complexes stabilized in GaN:(Mn,Mg) are here identified. Moreover, a Tanabe-Sugano energy diagram for 3$d^{2}$Mn$ ^{5+}$ is reconstructed and Mn-Mg$_{3}$ are singled out as the predominant configurations responsible for the IR emission. The correlation of intensity of the individual emission lines as a function of temperature and excitation energy, allows assigning them to well defined and specific optical transitions.

Photon conversion in Tb,Yb:CaxSr1-xAl2O4 nanocrystals

In this work the fabrication and the photoluminescence properties of CaxSr1-xAl2O4 nanocrystals doped with Ca, Tb and Yb are investigated. The photoluminescence quantum yield measured at 980 nm of the Yb doped CaxSr1-xAl2O4 nanocrystals is found to be 8% for an excitation in the ultraviolet range. However the quantum efficiency decreases with the insertion of Tb. Photoluminescence excitation experiments and time-resolved photoluminescence both conclusively demonstrate that no energy transfer between Tb and Yb occurs in this host matrix.

(Invited) Photoluminescence from an Individual Double-Walled Carbon Nanotube

In this communication, we report direct and unambiguous evidence of the existence of inner semiconducting tube photoluminescence (PL) from measurements performed on four individual freestanding index-identified double-walled carbon nanotubes (DWNTs). Based on thorough Rayleigh scattering, Raman scattering, and PL experiments, we are able to demonstrate that the inner semiconducting tube PL is observed with a quantum yield estimated to be a few 10-6 independent of the semiconducting or metallic nature of the outer tube. This result is mainly attributed to ultrafast exciton transfer from the inner to outer tube. Furthermore, by carrying out PL excitation experiments on the (14,1)@(15,12) DWNT, we show that the inner semiconducting tube PL can be detected through the optical excitation of the outer tube, indicating that the exciton transfer can also occur in the opposite way.

Photoluminescence from an individual double-walled carbon nanotube

We report direct and unambiguous evidence of the existence of inner semiconducting tube (ISCT) photoluminescence (PL) from measurements performed on four individual freestanding index-identified double-walled carbon nanotubes (DWNTs). Based on thorough Rayleigh scattering, Raman scattering, and PL experiments, we are able to demonstrate that the ISCT PL is observed with a quantum yield estimated to be a few ${10}^{\ensuremath{-}6}$ independent of the semiconducting or metallic nature of the outer tube. This result is mainly attributed to ultrafast exciton transfer from the inner to outer tube. Furthermore, by carrying out PL excitation experiments on the (14,1)@(15,12) DWNT, we show that the ISCT PL can be detected through the optical excitation of the outer tube, indicating that the exciton transfer can also occur in the opposite way.

Simultaneous Determination of Electron and Hole Mobilities in InP/InGaAsP/InAs/InP Laser Heterostructure by Admittance Spectroscopy

Temperature- and field-dependent electron and hole mobilities were simultaneously derived from admittance measurements on an InP/InGaAsP/InAs/InP long wavelength laser structure. Each carrier’s mobility was separated in the frequency domain due to the corresponding different relaxation times from ${C}$ versus log ${f}$ curves. Derived electron and hole mobilities followed the Poole–Frenkel-type field dependence, indicating the transport of carriers through hopping. Extracted thermal energy gap ( ${\Delta }$ ) and disorder temperature ( ${T_{0}}$ ) values were in mutual agreement with previous current density–voltage measurements and from the energy band diagram of the present structure, constructed from other studies in the literature by photoluminescence and photoluminescence excitation experiments.

Synthesis, Structure and Luminescence of Mn4+- Doped ATiO3 (A=Ca, Mg, Zn) Deep Red Phosphors

Red emission of the Mn4+ ions incorporated in different materials is used in various applications, such as lighting and holographic recording. Mn4+ emission can improve color-rendering-index of phosphor-converted white-light-emitting diodes, and it is a promising candidate to replace the well-established rare-earth Eu activator in red phosphors. Here, synthesis, structure, morphology, and detailed spectroscopic and crystal field analysis of Mn4+ doped metal titanates ATiO3 (A = Ca, Mg, Zn) are presented. Materials were prepared by combining sol-gel and molten salt methods. In the first step a typical sol-gel procedure is applied in which acetic acid, absolute ethanol, tetrabutyl titanate and metal compound were mixed at the molar ratio of 3:16:1:1 with appropriate amount of manganeze nitrate hexahydrate. In the second step, obtained dry gels were mixed with eutectic salt NaCl/KCl = 1:1 at molar ratio of 1:10, and then calcined at 750oC. Structural analysis was done by X-ray diffraction for all samples. Optical characterization included measurements of photoluminescence excitation and emission spectra, emission decay analyses and diffuse reflection measurements. Optical spectra showed emission and absorption bands originating from d–d electronic transitions (a d3 configuration). The energy levels of the Mn4+ ions in the crystal field of ATiO3 are calculated using the exchange charge model and showed nice matching with the experimental photoluminescence excitation and emission spectra.

Investigation on the structural and photoluminescent properties of chromium-doped ceramics cordierite

This work presents the investigation about the structural and optical properties of a doped-chromium system containing cordierite (Mg2Al4Si5O18) as main phase. The sample composition and the structural data were obtained from X-ray diffraction (XRD) measurements and the results were analyzed by Rietveld method. Photoluminescence (PL) spectra under several excitation wavelengths and photoluminescence excitation (PLE) measurements were performed. The obtained results from PL and PLE experiments are evidences that Cr3+ occupies octahedral sites in this ceramic system. By correlating the optical results, the crystal field parameter (Dq) and Racah interelectronic repulsion parameters (B and C) were calculated and discussed according to the Tanabe-Sugano (TS) theory for d3 transition metals in octahedral sites.

Laser-induced REE3 + photoluminescence of selected accessory minerals — An “advantageous artefact” in Raman spectroscopy

Laser-induced photoluminescence of trivalent rare-earth elements (REEs), which is obtained as analytical artefacts in Raman spectra of selected accessory minerals, was studied. Spectra of natural titanite, monazite—(Ce), xenotime—(Y), and zircon samples from various geological environments were compared with emission spectra of synthetic, flux-grown analogues doped with REEs. The latter is of great importance to identify potentially mistakable bands as either Raman or PL signal, and to assign them to certain REE centres. In the samples investigated, various REE centres are excited selectively using 473, 514, 532, 633, and 785nm laser excitation. Their assignment was verified by photoluminescence-excitation experiments. Luminescence spectral patterns of zircon and titanite vary in dependence of trace-REE concentrations, hence reflecting geochemical growth conditions. “REE artefacts” in Raman spectra of accessory minerals may be used as fingerprint tool for mineral phase-identification. The distribution of REE emission-intensities, revealed by hyperspectral mapping, opens up the opportunity to visualise mineral textures, complementary to cathodoluminescence imaging-techniques.

Polarization and time-resolved photoluminescence spectroscopy of excitons in MoSe2 monolayers

We investigate valley exciton dynamics in MoSe2 monolayers in polarization- and time-resolved photoluminescence (PL) spectroscopy at 4 K. Following circularly polarized laser excitation, we record a low circular polarization degree of the PL of typically ≤5%. This is about 10 times lower than the polarization induced under comparable conditions in MoS2 and WSe2 monolayers. The evolution of the exciton polarization as a function of excitation laser energy and power is monitored in PL excitation experiments. Fast PL emission times are recorded for both the neutral exciton of ≤3 ps and for the charged exciton (trion) of 12 ps.

Giant enhancement of the optical second-harmonic emission of WSe(2) monolayers by laser excitation at exciton resonances.

We show that the light-matter interaction in monolayer WSe_{2} is strongly enhanced when the incoming electromagnetic wave is in resonance with the energy of the exciton states of strongly Coulomb bound electron-hole pairs below the electronic band gap. We perform second harmonic generation (SHG) spectroscopy as a function of laser energy and polarization at T=4 K. At the exciton resonance energies we record an enhancement by up to 3 orders of magnitude of the SHG efficiency, due to the unusual combination of electric dipole and magnetic dipole transitions. The energy and parity of the exciton states showing the strong resonance effects are identified in 1- and 2-photon photoluminescence excitation experiments, corroborated by first principles calculations. Targeting the identified exciton states in resonant 2-photon excitation allows us to maximize k-valley coherence and polarization.

Deep-Red Emitting Mn4+ Doped Mg2TiO4 Nanoparticles

Synthesis, structure, morphology, and detailed spectroscopic and crystal-field analysis of Mn4+ doped Mg2TiO4 nanoparticles (NPs) are presented. These Mg2TiO4:Mn4+ NPs are obtained through a Pechini-type polymerized complex route and calcination at 600 °C, and are approximately 10 nm in diameter and loosely agglomerated into 1-μm particles, as evidenced from transmission electron microscopy. These NPs exhibit strong, sharp red emission at 658 nm (with a 1.2 ms emission decay) as a result of the spin-forbidden 2Eg → 4A2g electron transition of the tetravalent manganese ions. No signatures of the presence of manganese ions in divalent or trivalent valence states are observed in the NPs with either photoluminescence or diffuse reflection spectroscopy. The energy levels of the Mn4+ ions in a trigonal crystal field of Mg2TiO4 are calculated using the exchange-charge model and are well matched with the experimental photoluminescence excitation and emission spectra. The absolute values of the calculated crystal-...

Exciton dynamics in WSe2 bilayers

We investigate exciton dynamics in WSe2 bilayers in time-resolved photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for both the direct exciton with τD ≲ 3 ps and the indirect optical transition with τI ≈ 25 ps. For temperatures between 4 to 150 K τI remains constant. Following polarized laser excitation, we observe for the direct exciton transition at the K point of the Brillouin zone efficient optical orientation and alignment during the short emission time τD. The evolution of the direct exciton polarization and intensity as a function of excitation laser energy is monitored in PL excitation experiments.