Excitonic Optical Transitions Research Articles

Exciton-Phonon Interactions in Strained Domes of Monolayer MoS2 Studied by Resonance Raman Spectroscopy.

This work describes a resonance Raman study performed in the domes of monolayer MoS2 using 23 different laser excitation energies covering the visible and near-infrared (NIR) ranges. The multiple excitation results allowed us to investigate the exciton-phonon interactions of different phonons (A'1, E', and LA) with different excitonic optical transitions in biaxially strained monolayer MoS2. The analysis of the intensities of the two first-order peaks, A'1 and E', and the double-resonance 2LA Raman band as a function of the laser excitation furnished the values of the energies of the indirect exciton and the direct excitonic transitions in the strained MoS2 domes. It was noticed that the out-of-plane A'1 phonon mode is significantly enhanced only by the indirect exciton I and the C exciton, whereas the in-plane E' mode is only enhanced by the C exciton of the MoS2 dome, thus revealing the weak interaction of these phonons with the A and B excitons in the strained MoS2 domes. On the other hand, the 2LA Raman band is significantly enhanced at the indirect exciton I and by the A (or B) exciton but not enhanced by the C exciton, thus showing that the LA edge phonons that participate in the double-resonance process in MoS2 have a weak interaction with the C exciton.

Band Structure and Exciton Dynamics in Quasi‐2D Dodecylammonium Halide Perovskites

AbstractFemtosecond transient absorption spectroscopy (FTAS) is an important tool to investigate the physics of halide perovskites having different dimensionality, morphology, and architectures, giving insights into the electronic and excitonic optical transitions. Here, FTAS on monolayer (n = 1) and multilayer (n = 2, 3) quasi‐2D perovskites in the Ruddlesden–Popper phase: DA2MAn‐1PbnI3n+1 (DAMAPI) is presented, with the dodecylammonium (DA = CH3‐(CH2)11‐NH3+) as the spacer and methylammonium (MA = CH3NH3+) as the organic cation for samples with n > 1. The measurements, performed at 77 K and room temperature using several pump energies and excitation densities, allow the observation of different absorption bleaching energies. Those energies are compared with the results of first‐principles theoretical simulations based on density functional theory, the GW method, and the Bethe–Salpeter equation and assigned to transitions involving excitons with principal quantum numbers 1s and 2s. The temporal analysis of the absorption bleaching indicates the exciton–exciton annihilation as the main relaxation mechanism in the first picoseconds after excitation, while exciton radiative recombination is observed at longer time delays (>100 ps). Therefore, FTAS allows the study of the carrier dynamics and, given its high sensitivity to carrier density changes, the observation of spectral features not observable with steady‐state measurements.

(Invited) Quantum Well Architecture to Realize High Optical Gain Lasing Media

Photonic applications of semiconductor quantum dots (QDs) had significant drawbacks due to their ultrafast Auger decay of multiple excitons. These nonradiative processes have hindered the performance of electroluminescent and lasing devices. The emergence of the colloidal quantum well (QW) architecture where semiconductor quantum shell with an inverted QD geometry has led to suppressed Auger recombination. Ultra long biexciton lifetime (>10 ns) and large biexciton quantum yields has been achieved through spatial separation promoted by the QW geometry. Moreover, architecture induced exciton-exciton repulsions have led to the splitting of exciton and biexciton optical transitions which resulted in the single exciton gain mode along with the biexciton gain mode. High biexciton lifetime and the longest reported single exciton gain lifetime (>6 ns) has rendered this geometry a worthy candidate for the development of optically and electrically pumped gain media

Tuning absorption and emission in monolayer semiconductors: a brief survey

We review the physical mechanisms that allow tuning of absorption and emission characteristics in monolayer semiconductors. We use the model system of transition metal dichalcogenide mono-layers such as MoSe 2 or WSe 2 due to their very efficient light-matter interaction and availability of high quality samples. For monolayers encapsulated in hexagonal boron nitride both homogeneous and inhomogenous contributions to the exciton optical transition linewidth can be tuned opening up pathways for tailoring the reflectivity and absorption in van der Waals heterostructures.

Quantum Shells Boost the Optical Gain of Lasing Media.

Auger decay of multiple excitons represents a significant obstacle to photonic applications of semiconductor quantum dots (QDs). This nonradiative process is particularly detrimental to the performance of QD-based electroluminescent and lasing devices. Here, we demonstrate that semiconductor quantum shells with an "inverted" QD geometry inhibit Auger recombination, allowing substantial improvements to their multiexciton characteristics. By promoting a spatial separation between multiple excitons, the quantum shell geometry leads to ultralong biexciton lifetimes (>10 ns) and a large biexciton quantum yield. Furthermore, the architecture of quantum shells induces an exciton-exciton repulsion, which splits exciton and biexciton optical transitions, giving rise to an Auger-inactive single-exciton gain mode. In this regime, quantum shells exhibit the longest optical gain lifetime reported for colloidal QDs to date (>6 ns), which makes this geometry an attractive candidate for the development of optically and electrically pumped gain media.

Transverse magnetic routing of light emission in hybrid plasmonic-semiconductor nanostructures: Towards operation at room temperature

We study experimentally and theoretically the temperature dependence of transverse magnetic routing of light emission from hybrid plasmonic-semiconductor quantum well structures where the exciton emission from the quantum well is routed into surface plasmon polaritons propagating along a nearby semiconductor-metal interface. In II-VI and III-V direct band semiconductors the magnitude of routing is governed by the circular polarization of exciton optical transitions, that is induced by a magnetic field. For structures comprising a (Cd,Mn)Te/(Cd,Mg)Te diluted magnetic semiconductor quantum well we observe a strong directionality of the emission up to 15% at low temperature of 20 K and magnetic field of 485 mT due to giant Zeeman splitting of holes mediated via the strong exchange interaction with Mn$^{2+}$ ions. For increasing temperatures towards room-temperature the magnetic susceptibility decreases and the directionality strongly decreases to 4% at T = 45 K. We also propose an alternative design based on a non-magnetic (In,Ga)As/(In,Al)As quantum well structure, suitable for higher temperatures. According to our calculations, such structure can demonstrate emission directionality up to 5% for temperatures below 200 K and moderate magnetic fields of 1 T.

Optical properties of Sub-30 nm-thick ZnS films studied by spectroscopic ellipsometry

The sub-30 nm-thick ZnS thin films with different thicknesses were deposited on Si substrates by electron beam evaporation (EBE). The thickness and optical constants of ZnS films were obtained by fitting spectroscopic ellipsometry (SE) data using Tauc-Lorentz Model. The variation of refractive index, extinction coefficient, optical band gap, the complex dielectric function of the samples with film thickness was analyzed. Due to the increase of film density, the refractive index increases with the thickness, and the extinction coefficient has the same trend. Due to the quantum size effect, the optical band gap decreases from 3.56 eV to 3.23 eV with increasing film thickness. Two peaks (E0 and E1) are obtained from the complex dielectric function spectrum, which most likely corresponds to the central energy of the exciton optical transition. Three critical points (CP) energies (EA, EB, and EC) and the types of interband optical transitions were extracted from the standard linear analysis of the second-order derivatives of the dielectric function. The three CPs energies have a significant thickness dependence, which is mainly attributed to quantum size effects. This result is beneficial for the design and application of photonic and optoelectronic devices with sub-30 nm-thick ZnS films.

Exciton radiative recombination time in a group III-V/II-VI core/shell quantum dot: Simultaneous effects of pressure and temperature

Excitonic properties of a compound semiconductor from group III-V and group II-VI materials are studied applying external perturbations. The simultaneous effects of pressure and temperature on exciton energies in an InP/ZnS quantum dot (core/shell) are found taking into account the ratio of radius of core/shell dot. The results bring out that the radiative lifetime enhances monotonically when the ratio of radius of core to shell quantum dot increases and it reaches a saturation value when Rc/Rs → 1 and the radiative recombination lifetime enhances upto 9 ns. The results can be applied for tailoring the excitonic optical transitions in nano-optical devices.

Transparent All-Oxide Photovoltaics and Invisible Photodetectors

Heterostructure of wide-bandgap materials have great potential for use in transparent optoelectronics for examples ultraviolet photodetectors, transparent solar cells, and transparent electronics. This study demonstrates the exciton, band-to-band and intermediate-band optical transitions in a ZnO/NiO heterostructure at room temperature. This heterostructure exhibits Ohmic current-voltage characteristics close to that of various metal contacts such as Ag, C, Ni, and Au. Temperature-dependence studies revealed that the open-circuit voltage (VOC) of ZnO/NiO heterostructure is limited by the charge-transfer potential, analogous to excitonic solar cells. A negligibly small dark current of 1.6 × 10-8 A, a large VOC of 675 mV and a photoresponse speeds of 9.4 ms make it promising for high-speed energy-efficient optoelectronics. The optoelectronic performances of the ZnO/NiO/Ag microink suggest that broadband photons can be utilized with incident photons to current conversion efficiencies (IPCEs) of 2% and 39% in the visible and ultraviolet regions, respectively which demonstrate that the ZnO/NiO heterostructure can acts as a broadband quantum photoelectric device.

Transparent All-Oxide Photovoltaics and Invisible Photodetectors

Could transparent solar cells work as the invisible power generator? Is it possible, in order to satisfy the on-site energy production, to install the transparent solar cell into the window of buildings and vehicles without recognizing the existence of energy harvesting entities? Here we propose and demonstrate the wide energy bandgap materials for visible light transmittance and UV photon absorption for power generation. All-transparent solar cell was achieved by the heterojunction of metal oxide layers. Heterostructure of wide-bandgap materials have great potential for use in transparent optoelectronics for examples ultraviolet photodetectors, transparent solar cells, and transparent electronics. This study demonstrates the exciton, band-to-band and intermediate-band optical transitions in a ZnO/NiO heterostructure at room temperature. This heterostructure exhibits Ohmic current voltage characteristics close to that of various metal contacts such as Ag, C, Ni, and Au. Temperature-dependence studies revealed that the open-circuit voltage (VOC) of ZnO/NiO heterostructure is limited by the charge-transfer potential, analogous to excitonic solar cells. A negligibly small dark current of 1.6 × 10−8 A, a large VOC of 675 mV and a photoresponse speeds of 9.4 μs make it promising for high-speed energy-efficient optoelectronics. The optoelectronic performances of the ZnO/NiO/Ag microink suggest that broadband photons can be utilized with incident photons to current conversion efficiencies (IPCEs) of 2% and 39% in the visible and ultraviolet regions, respectively which demonstrate that the ZnO/NiO heterostructure can acts as a broadband quantum optoelectronic device.

Polarimetry of photon echo on charged and neutral excitons in semiconductor quantum wells

Coherent optical spectroscopy such as four-wave mixing and photon echo generation deliver rich information on the energy levels involved in optical transitions through the analysis of polarization of the coherent response. In semiconductors, it can be applied to distinguish between different exciton complexes, which is a highly non-trivial problem in optical spectroscopy. We develop a simple approach based on photon echo polarimetry, in which polar plots of the photon echo amplitude are measured as function of the angle φ between the linear polarizations of the two exciting pulses. The rosette-like polar plots reveal a distinct difference between the neutral and charged exciton (trion) optical transitions in semiconductor nanostructures. We demonstrate this experimentally by photon echo polarimetry of a CdTe/(Cd, Mg)Te quantum well. The echoes of the trion and donor-bound exciton are linearly polarized at the angle 2φ with respect to the first pulse polarization and their amplitudes are weakly dependent on φ. While on the exciton the photon echo is co-polarized with the second exciting pulse and its amplitude scales as cosφ.

Transparent all-oxide photovoltaics and broadband high-speed energy-efficient optoelectronics

Heterostructure of wide-bandgap materials have great potential for use in transparent optoelectronics for examples ultraviolet photodetectors, transparent solar cells, and transparent electronics. This study demonstrates the exciton, band-to-band and intermediate-band optical transitions in a ZnO/NiO heterostructure at room temperature. This heterostructure exhibits Ohmic current-voltage characteristics close to that of various metal contacts such as Ag, C, Ni, and Au. Temperature-dependence studies revealed that the open-circuit voltage (VOC) of ZnO/NiO heterostructure is limited by the charge-transfer potential, analogous to excitonic solar cells. A negligibly small dark current of 1.6 × 10−8 A, a large VOC of 675 mV and a photoresponse speeds of 9.4 μs make it promising for high-speed energy-efficient optoelectronics. The optoelectronic performances of the ZnO/NiO/Ag microink suggest that broadband photons can be utilized with incident photons to current conversion efficiencies (IPCEs) of 2% and 39% in the visible and ultraviolet regions, respectively which demonstrate that the ZnO/NiO heterostructure can acts as a broadband quantum optoelectronic device.

Band gap temperature-dependence and exciton-like state in copper antimony sulphide, CuSbS2

The temperature-dependence of the band gap of the proposed photovoltaic absorber copper antimony sulphide (CuSbS2) has been studied by Fourier-transform infrared spectroscopy. The direct gap rises from 1.608 to 1.694 eV between 300 and 4.2 K. Below 200 K an exciton-like feature develops above the absorption edge at 1.82 eV. First-principles calculations evaluate band structure, band symmetries, and dipole selection rules, suggesting distinctly enhanced absorption for certain excitonic optical transitions. Striking consistency is seen between predicted dielectric and absorption spectra and those determined by ellipsometry, which reveal rapidly strengthening absorption passing 105 cm−1 at 2.2 eV. These results suggest beneficial photovoltaic performance due to strong optical absorption arising from unusually strong electron–hole interactions in polycrystalline CuSbS2 material.

Unifying Optical Selection Rules for Excitons in Two Dimensions: Band Topology and Winding Numbers.

We show that band topology can dramatically change the photophysics of two-dimensional semiconductors. For systems in which states near the band extrema are of multicomponent character, the spinors describing these components (pseudospins) can pick up nonzero winding numbers around the extremal k point. In these systems, we find that the strength and required light polarization of an excitonic optical transition are dictated by the optical matrix element winding number, a unique and heretofore unrecognized topological characteristic. We illustrate these findings in three gapped graphene systems-monolayer graphene with inequivalent sublattices and biased bi- and trilayer graphene, where the pseudospin textures manifest into nontrivial optical matrix element winding numbers associated with different valley and photon circular polarization. This winding-number physics leads to novel exciton series and optical selection rules, with each valley hosting multiple bright excitons coupled to light of different circular polarization. This valley-exciton selective circular dichroism can be unambiguously detected using optical spectroscopy.

Excitonic nature of optical transitions in electroabsorption spectra of perovskite solar cells

We investigate the electronic structure of solution-processed perovskite solar cells using temperature-dependent electroabsorption (EA) spectroscopy. Simultaneous measurements of absorption and electromodulated spectra of semitransparent methylammonium lead iodide solar cells facilitate a direct comparison of the specific features. The EA spectra can be transformed to peak-like line shapes utilizing an approach based on the Kramers–Kronig relations. The resulting peak positions correspond well to the discrete excitonic—rather than the continuum—contribution of the absorption spectra derived from generalized Elliott fits. This indicates the excitonic nature of the observed EA resonance and is found to be consistent over the whole temperature range investigated (from T = 10 K up to room temperature). To further confirm these findings, a line shape analysis of the measured EA spectra was performed. The best agreement was achieved using a first-derivative-like functional form which is expected for excitonic systems and supports the conclusion of an excitonic optical transition. Exciton binding energies EB are estimated for the orthorhombic and tetragonal phases as 26 meV and 19 meV, respectively. Nevertheless, power-conversion efficiencies η up to 13% (11.5% stabilized) demonstrate good charge-carrier separation in the devices due to sufficient thermal dissociation and Sommerfeld-enhanced absorption.

Defects Enable Dark Exciton Photoluminescence in Single-Walled Carbon Nanotubes

Variable temperature photoluminescence excitation spectroscopy of three (n,m) species of single-walled carbon nanotubes revealed that at resonant S22 excitation, in addition to allowed excitonic optical transitions, several sidebands that should be forbidden based on selection rules were observed and appeared to have a strong temperature dependence. In particular, we found that a sideband located approximately 130 meV away from the bright S11 exciton peak relating to the K-momentum dark exciton state, called X1, decreased in intensity 5-fold as the samples were cooled. Direct optical excitation of this dark state is nominally forbidden, thus calling into question how the state is populated and why it is so prominent in the photoluminescence spectrum. Interestingly, the ratio of the integrated photoluminescence intensities of X1 to S11 scales with a Boltzmann factor unrelated to the phonon that is thought to be responsible for depopulating the K-momentum dark exciton state: an in-plane transverse optical p...

Excitons and trions in single and vertically coupled quantum dots under an electric field

We present a theoretical study of the exciton (X0), the positive and negative trions (X+ and X−) in single and vertically coupled configurations of self-assembled InGaAs quantum dots under an electric field. The quantum states of X0, X+ and X− have been investigated using a quasi-one-dimensional (Q1D) model within the effective-mass approximation. For the single quantum dots, the electric-field dependent energy levels and the average inter-particle distances for the exciton and trions have been calculated. For the coupled quantum dots, the ground and the excited states for X0, X+ and X− have also been calculated and discussed. It is found that either the hole or the electron can be tuned into resonance states by the electric field and that the transition energy spectra for both trions consequently show crossing and anticrossing patterns. The recombination probabilities of the exciton and trion optical transitions are also calculated. The theoretical results have been compared with previously reported photoluminescence data and qualitative agreement is obtained. The trion conditional wave functions are also plotted under different electric field intensities, and it is found that a molecular orbital can be formed at a critical electric field intensity. The evolution of the energy levels of the trions in coupled quantum dots can be explained by the interplay of particle transfer and the electric field.

Diameter-Dependent Excitation Energy Transfer for Enhanced Semiconducting Single-Walled Carbon Nanotube Solar Photoconversion

Tremendous progress and innovation has ushered the incorporation of semiconducting single-walled carbon nanotubes (s-SWCNTs) as active components into organic/inorganic/hybrid solar cells and thermoelectrics. For solar photoconversion, strong and tunable absorption is necessitated, suggesting that s-SWCNTs can play a role as light-harvesting components, however the narrow and distinct excitonic optical transitions found in s-SWCNT systems limit broad-band absorption. As such, we introduce encapsulation of small molecules within the endohedral volume of s-SWCNTs as a route to increase and extend light absorption in regions outside s-SWCNT excitonic transitions. Creation of this self-assembled supra-molecular system can strongly modify the opto-electronic properties of both the s-SWCNT and encapsulated molecule. We investigate excitation energy transfer occurring from encapsulated dye molecules to s-SWCNTs, which results in luminescent SWCNT excitons. We exploit the high-throughput and chiral selectivity of polyfluorene wrapping methods to generate dispersions of s-SWCNTs with dye encapsulation. We demonstrate that the selectivity of the polyfluorene polymer for semiconducting chiral distributions is not affected by the presence of encapsulated molecules. Tracking the excited state transient absorption signatures of excitation energy transfer provides detailed information regarding dynamics of both the encapsulated dye and s-SWCNTs. We observe sub-picosecond excitation energy transfer of excitons generated in dye molecules to the surrounding s-SWCNTs. Photoluminescence excitation maps reveal that the absorption of encapsulated dye molecules depends sensitively on the diameter of the s-SWCNT in which they are encapsulated. We consider a simple molecular exciton model to describe aggregate formation within the s-SWCNT endohedral volume, which reveals that the dye molecules adopt unique aggregate structures that are defined both by a competition between intermolecular interactions between the dye molecules and confinement effects due to the s-SWCNT diameter. Small molecule encapsulation in s-SWCNTs serves as a strong platform to control intermolecular interactions in small molecule systems through confinement, which opens up the possibility of enhancing photo-induced extraction of energy and charge in s-SWCNTs solar photoconversion systems.

Optical properties of lithium gallium oxide

The optical dielectric function tensor of orthorhombic single-crystal LiGaO2 was determined for polarizations along a, b, and c crystal-axis in the photon energy range from 0.04eV to 6.5eV by the generalized spectroscopic ellipsometry. In the far-infrared spectral range from 12.4meV to 40meV, the dielectric function was determined from conventional polarized transmittance and polarized reflectance measurements. Lineshape analysis of the dielectric function tensor major components allowed for a determination of the long-wavelength optical phonon characteristics, refractive indices dispersion, and parameters of interband and excitonic optical transitions.

Excitonic optical transitions characterized by Raman excitation profiles in single-walled carbon nanotubes

We examine the excitonic nature of the ${E}_{33}$ optical transition of the individual free-standing index-identified $(23,7)$ single-walled carbon nanotube by means of the measurements of its radial-breathing-mode and $G$-mode Raman excitation profiles. We confirm that it is impossible to determine unambiguously the nature of its ${E}_{33}$ optical transition (excitonic vs band to band) based only on the excitation profiles. Nevertheless, by combining Raman scattering, Rayleigh scattering, and optical absorption measurements on strictly the same individual $(23,7)$ single-walled carbon nanotube, we show that the absorption, Rayleigh spectra, and Raman excitation profiles of the longitudinal and transverse $G$ modes are best fitted by considering the nature of the ${E}_{33}$ transition as excitonic. The fit of the three sets of data gives close values of the transition energy ${E}_{33}$ and damping parameter ${\mathrm{\ensuremath{\Gamma}}}_{33}$. This comparison shows that the fit of the Raman excitation profiles provides with good accuracy the energy and damping parameter of the excitonic optical transitions in single-walled carbon nanotubes.