Magnetic Field Dependent Photoluminescence Research Articles

Low-Temperature Photoluminescence Dynamics Reveal the Mechanism of Light Emission by Colloidal CuInS2 Quantum Dots

Copper indium sulfide colloidal quantum dots are in focus of the nanocrystal community as their optical properties make them exciting candidates for applications in bioimaging and biosensing, solar cells, luminescent solar concentrators, and lighting. However, fulfilling this application potential is hampered by poor understanding of the mechanism of light emission by these dots. In this work, we study temperature and magnetic field dependent photoluminescence (PL) dynamics and spin dynamics at cryogenic temperatures. We cast our experimental observations against two prevailing models of PL in CuInS2 quantum dots: an excitonic mechanism and a free-to-bound mechanism, in which a delocalized electron recombines with a hole localized at a copper ion. We unambiguously show that the PL occurs as the result of the free-to-bound mechanism.

Magnetic field and angle-dependent photoluminescence of a fiber-coupled nitrogen vacancy rich diamond

Here, we investigate the magnetic field dependent photoluminescence (PL) of a fiber-coupled diamond single crystal with a high density of nitrogen vacancy (NV) centers. Angle-dependent magnetic field sweep measurements between 0 and 111 mT were performed using an oscillating illumination combined with lock-in techniques. Besides the expected superposed PL of differently oriented NV centers, a zoo of features in the PL are found. These features can be associated with level anti-crossings and cross relaxations. In particular, PL measurements allowed us to detect auto-cross relaxation between coupled NV centers. Moreover, the PL measurements at low magnetic fields show dips suggesting an interaction of NV centers with additional spin defects. The results presented here are not only a study for NV-based fiber-coupled sensors made of diamond, but also show a way to investigate with manageable effort and purely an optical multispin interaction with at least one NV center as a constituent.

Level-crossing induced spin phenomena in SiC: A theoretical study

A theoretical approach is proposed to describe the spin dynamics in defect color centers. The method explicitly considers the spin dynamics in the ground state and excited state of the defect center as well as spin state dependent transitions involving the ground state, excited state, and an additional intermediate state. The proposed theory is applied to treat spin-dependent phenomena in silicon carbide, namely, in spin-$\frac{3}{2}$ silicon-vacancy centers, termed ${V}_{\mathrm{Si}}$ or V2 centers. Theoretical predictions of magnetic field dependent photoluminescence intensity and optically detected magnetic resonance spectra demonstrate an important role of level-crossing phenomena in the spin dynamics of the ground state and excited state. The results are in good agreement with previously published experimental data [Phys. Rev. X 6, 031014 (2016)].

Giant Valley-Zeeman Splitting from Spin-Singlet and Spin-Triplet Interlayer Excitons in WSe2/MoSe2 Heterostructure.

Transition metal dichalcogenides (TMDCs) heterostructure with a type II alignment hosts unique interlayer excitons with the possibility of spin-triplet and spin-singlet states. However, the associated spectroscopy signatures remain elusive, strongly hindering the understanding of the Moiré potential modulation of the interlayer exciton. In this work, we unambiguously identify the spin-singlet and spin-triplet interlayer excitons in the WSe2/MoSe2 heterobilayer with a 60° twist angle through the gate- and magnetic field-dependent photoluminescence spectroscopy. Both the singlet and triplet interlayer excitons show giant valley-Zeeman splitting between the K and K' valleys, a result of the large Landé g-factor of the singlet interlayer exciton and triplet interlayer exciton, which are experimentally determined to be ∼10.7 and ∼15.2, respectively, which is in good agreement with theoretical expectation. The photoluminescence (PL) from the singlet and triplet interlayer excitons show opposite helicities, determined by the atomic registry. Helicity-resolved photoluminescence excitation (PLE) spectroscopy study shows that both singlet and triplet interlayer excitons are highly valley-polarized at the resonant excitation with the valley polarization of the singlet interlayer exciton approaching unity at ∼20 K. The highly valley-polarized singlet and triplet interlayer excitons with giant valley-Zeeman splitting inspire future applications in spintronics and valleytronics.

Bismuth-induced band-tail states in GaAsBi probed by photoluminescence

Band-tail states in semiconductors reflect the effects of material growth and/or treatment, affect the performance of optoelectronic applications, and are hence a well-concerned issue. Dilute-Bi GaAs is considered very competitive though the role of Bi is yet to be well clarified. We in this letter investigate the effect of Bi incorporation on the band-tail states in GaAs1−xBix by excitation power- and magnetic field-dependent photoluminescence (PL) measurements at low temperatures. Three PL features are identified from a broad PL peak, which blue-shift monotonically with the increase in excitation power. None of the PL features correlate with single Bi-content free-exciton recombination, and band-tail filling rather than the donor-acceptor pair process is responsible for the power-induced blueshift. The density of band-tail states gets enhanced with the increase in the Bi incorporation level and affects the determination of Bi-induced bandgap reduction. The results indicate that joint analysis of excitation- and magneto-PL may serve as a good probe for band-tail states in semiconductors.

Low temperature photo-induced carrier dynamics in the GaAs0.985N0.015 alloy

We report the exploration of photo-induced carrier dynamics in the GaAs0.985N0.015 Alloy. The time-resolved and high magnetic field-dependent photoluminescence experiments were carried out to identify the radiative transitions, and the localized and delocalized states at various excitation power and temperature. A nonmonotonic dependence of the PL energy on temperature at low laser power, and the observation of two different decay times at the temperature below 100 K indicate the free electrons undergo a delocalization to localization transition with decreasing temperature. In the low temperature region, the localization is further enhanced by an applied high magnetic field, and an unexpected high field blocking of the diamagnetic shift was observed.

GaInSb/InAs/AlSb quantum wells with InSb- and GaAs-like interfaces investigated by temperature- and magnetic field-dependent photoluminescence

GaInSb/InAs/AlSb quantum wells (QWs) with typical InSb- and GaAs-like interfaces (IFs) are investigated by temperature- and magnetic field-dependent photoluminescence (PL), respectively. The results show that (i) as temperature rises the PL energy of the QWs with either InSb- or GaAs-like IFs blueshifts slightly below 50 K but redshifts above and broadens rapidly, and the mechanism behind this is correlated to the IF roughness-related layer thickness fluctuation equivalent to a localization energy of about 9.5 meV; (ii) the PL diminishes monotonously as magnetic field rises except for the delocalized PL process of the InSb-like IF QWs, and the magnetic field-induced PL quenching is attributed to the IF roughness-induced electron-hole separation in the type-II QWs; and (iii) the magnetic field-dependent PL energy follows a typical excitonic diamagnetic shift for both located and dislocated states, and the deduced exciton binding energy, reduced effective mass, and average wavefunction extent are insensitive to the IF type. Comparison of different IF-type GaInSb/InAs QWs indicates that while the PL of the InSb-like IF sample contains type-I component as the IF confines heavy holes and acts as pseudo-barrier for electrons, leading to the coexistence of electrons and holes at the IFs, the IF-type does not affect the carrier localization and the in-plane excitonic behavior obviously.

Photoluminescence probing of interface evolution with annealing in InGa(N)As/GaAs single quantum wells

The effects of thermal annealing on the interfaces of InGa(N)As/GaAs single quantum wells (SQWs) are investigated by excitation-, temperature-, and magnetic field-dependent photoluminescence (PL). The annealing at 750 °C results in more significant blueshift and narrowing to the PL peak than that at 600 °C. Each of the PL spectra can be reproduced with two PL components: (i) the low-energy component (LE) keeps energetically unchanged, while the high-energy component (HE) moves up with excitation and shows at higher energy for the In0.375Ga0.625As/GaAs but crosses over with the LE at a medium excitation power for the In0.375Ga0.625N0.012As0.988/GaAs SQWs. The HE is broader than the corresponding LE, the annealing at 750 °C narrows the LE and HE and shrinks their energetic separation; (ii) the PL components are excitonic, and the InGaNAs shows slightly enhanced excitonic effects relative to the InGaAs SQW; (iii) no typical S-shape evolution of PL energy with temperature is detectable, and similar blueshift and narrowing are identified for the same annealing. The phenomena are mainly from the interfacial processes. Annealing improves the intralayer quality, enhances the interfacial In-Ga interdiffusion, and reduces the interfacial fluctuation. The interfacial interdiffusion does not change obviously by the small N content and hence similar PL-component narrowing and blueshift are observed for the SQWs after a nominally identical annealing. Comparison with previous studies is made and the PL measurements under different conditions are shown to be effective for probing the interfacial evolution in QWs.

Magnetic field induced great photoluminescence enhancement in an Er^3+:YVO_4 single crystal used for high magnetic field calibration

An optical technique is proposed for the accurate calibration of pulsed high magnetic fields utilizing the magnetic field dependent photoluminescence (PL) properties in an Er(3+):YVO(4) single crystal at 80 K. Bright green PL emissions are excited by a 487.5 nm laser line and can be enhanced greatly by a magnetic field at certain field values (B(c)). Since the B(c)'s under 10 T are extremely stable for a given sample at a certain temperature, and the FWHM of the enhancement peaks are less than 0.9 T, an Er(3+):YVO(4) single crystal is proven to be a good candidate for pulsed high magnetic field calibration. The detailed processes and numerous advantages of the technique are presented in this work.

Contactless electron effective mass determination in GaInNAs/GaAs quantum wells

The electron effective masses in n-type modulation doped Ga0.7In0.3NyAs1−y/GaAs quantum wells with nitrogen mole fractions of y = 0.004 and 0.010 were investigated experimentally. Two experimental techniques: magnetic field dependent photoluminescence measurements and phonon-plasmon coupled-mode line-shape analysis of vibrational spectroscopy measurements, were employed in the investigations. In the first technique, the effective masses of the electrons have been determined from the diamagnetic energy shift dependencies up to 11 T. The vibrational properties of the samples were studied using Raman scattering spectroscopy at room temperature. The effective masses obtained from both two techniques are in good agreement with the current results in the literature.

Circularly polarized photoluminescence from platinum porphyrins in organic hosts: Magnetic field and temperature dependence

We study the temperature and magnetic field-dependent photoluminescence from the metalorganic molecules octaethyl-porphine platinum (PtOEP) and porphine platinum (PtP) that are doped into organic hosts. We first consider PtOEP in the polymer host poly-dioctylfluorene (PFO), which is characteristic of the phosphorescent dopants and polymers used in organic light-emitting diodes. We observe that the intensity of the PtOEP zero-phonon emission band, which is strongly suppressed at low temperatures to 1.6 K, increases dramatically with applied magnetic field and is accompanied by a marked circular polarization. This “magnetic brightening” effect, similar to that observed in other organic systems such as carbon nanotubes, highlights the interplay between low-energy optically active and optically forbidden excited states of PtOEP, which become mixed in applied magnetic fields. To elucidate these findings, we also investigate (i) dilute PtOEP in n-octane hosts (where emission lines are much sharper), and (ii) dilute PtP in n-octane hosts, for which the emission spectra are simpler and can be directly compared with theory. Detailed electronic structure calculations of PtP were performed, and a model for the magnetic field and temperature dependence of the zero phonon emission lines is developed, which agrees quantitatively with the data for PtP and with the circular polarization of the PtOEP emission.

Suppression of Mn photoluminescence in ferromagnetic state of Mn-doped ZnS nanocrystals

Magnetic ordering can tune optical properties of photoluminescent dilute magnetic semiconductors. The results of a magnetic field dependent photoluminescence (PL) up to 50 T above and below ferromagnetic transition temperature ${T}_{C}$ in Mn-doped ZnS nanocrystals have been reported here. The PL intensity corresponding to internal transition between states of Mn significantly decreases with application of magnetic field below ${T}_{C}$ but no such suppression was observed above ${T}_{C}$ even at 50 T. The zero-field PL intensity also exhibits continuous suppression with decreasing temperature in the ferromagnetic state. The PL intensity profiles could be fitted consistently using a model of magnetic-ordering-induced spin-sensitive energy transfer to Mn states below ${T}_{C}$.

Magnetic field-dependent photoluminescence linewidths as a probe of disorder length scales in quantum wells

Photoluminescence from highly disordered GaAs quantum wells is studied in magnetic fields up to 50T. The monotonic decrease of the photoluminescence linewidth with increasing quantum well thickness indicates that interface roughness is the primary source of line broadening. The magnetic field-dependent exciton linewidth shows an unexpected behavior. We observe not only just a monotonic increase in linewidth but also a field-dependent decreasing linewidth in thicker quantum wells. These observations are understood by postulating the existence of two correlation lengths for the interface fluctuations, one much smaller than the exciton size and the other one of the order of the exciton size.

Temperature and Magnetic Field Dependent Photoluminescence from Carbon Nanotubes

Photoluminescence as a function of temperature and magnetic field from single walled carbon nanotube solutions is described. This is modelled assuming that it is dominated by the small energy splitting between the dark and bright states of the singlet excitons which are found to be in the region of 1-5 meV for nanotubes of 0.8-1.2nm. The emission energies show a large red-shift due to the introduction of an Aharanov-Bohm phase by magnetic field along the tube axis and the luminescence intensity is strongly enhanced at low temperatures due to the mixing of the different valley states of the excitons.

Transformation of the Fourier Spectrum of Modulated Photoluminescence of Rubrene Due to the Non-Linearity of the Magnetic Field Spin Effect

Fourier analysis is applied for investigation of the magnetic field dependent photoluminescence in a polycrystalline film of rubrene. The sine-shaped external magnetic field results in oscillations of a rate constant at multiple harmonics. Action of a sum of two harmonic perturbations results in numerous harmonics at difference and summary frequencies. Possible kinetic applications of the bichromatic perturbation of the rate constant and subsequent frequency transformation by the non-linear system are discussed.

Electron and hole spin lifetimes in Zn1−xMnxSe and Zn1−yFeySe based heterostructures

We have studied carrier spin relaxation in diluted magnetic semiconductor (DMS) epilayer and spin superlattice samples using polarization and magnetic field dependent photoluminescence. The giant Zeeman splitting of the DMS materials permits unambiguous identification of electron and hole excitonic spin relaxation times. We find that the heavy hole spin lifetime τhs is comparable to the radiative recombination time τr, and substantially longer than the electron spin lifetime τes, with τhs/τr≊4 and τes/ τr≊0.06. We attribute these long spin lifetimes to the strain induced splitting of the bulk degenerate light and heavy hole bands, which inhibits mixing of the hole spin states and subsequent heavy hole spin flip relaxation processes. In the regime of weak confinement, the spin relaxation times are independent of the degree of quantum confinement, the number of interfaces, the magnetic ion concentration, and the magnetic species. We conclude that spin flip via exchange scattering between the carriers and magnetic ions is not the dominant excitonic spin relaxation mechanism.