Circular Polarization Of Photoluminescence Research Articles

Optical Alignment and Optical Orientation of Excitons in CdSe/CdS Colloidal Nanoplatelets.

Optical alignment and optical orientation of excitons are studied experimentally on an ensemble of core/shell CdSe/CdS colloidal nanoplatelets. Linear and circular polarization of photoluminescence during resonant excitation of excitons is measured at cryogenic temperatures and with magnetic fields applied in the Faraday geometry. The developed theory addresses the optical alignment and optical orientation of excitons in colloidal nanocrystals, taking into account both bright and dark exciton states in the presence of strong electron-hole exchange interaction and the random in-plane orientation of nanoplatelets within the ensemble. Our theoretical analysis of the obtained experimental data allows us to evaluate the exciton fine structure parameters, the g-factors, and the spin lifetimes of the bright and dark excitons. The optical alignment effect enables the identification of the exciton and trion contributions to the emission spectrum, even in the absence of their clear separation in the spectra.

Exciton recombination and spin relaxation in strong magnetic fields in ultrathin (In,Al)As/AlAs quantum wells with indirect band gap and type-I band alignment

The exciton recombination and spin dynamics are studied in monolayer-thick (In,Al)As/AlAs quantum wells characterized by an indirect band gap and a type-I band alignment. The exciton recombination time and the photoluminescence intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to no effect of an in-plane field, at a temperature of 1.8 K a magnetic field applied parallel to the growth axis drastically slows down the recombination and reduces the intensity of photoluminescence. The magnetic-field-induced circular polarization of photoluminescence is studied as a function of the magnetic field strength and direction, as well as sample temperature. The observed nonmonotonic behavior of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. Taking into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states, we evaluate the heavy-hole longitudinal $g$ factor of 3.6, the radiative recombination time for the bright excitons of 0.13 ms, and the nonradiative recombination time of the bright and dark excitons of 0.43 ms, as well as the spin relaxation times of electron of $25\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ and heavy hole of $16\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$, bound in the exciton.

Spin relaxation of holes in In0.53Ga0.47As/InP quantum wells

Polarization-resolved photoluminescence was used to study spin relaxation of photoexcited holes in In0.53Ga0.47As/InP quantum wells in a quantizing magnetic field as a function of temperature. At a temperature below 10 K, the circular polarization of the photoluminescence due to the spin-split valence band Landau levels was found temperature-independent. In this temperature range fast hole spin relaxation as compared to their lifetime leads to the photoluminescence circular polarization determined by the ratio of these times. Increasing temperature resulted in efficient hole spin thermalization in the Zeeman split valence band Landau levels and as a consequence, in vanishing photoluminescence polarization. Fits of the experimental data by the theory allowed a determination of the hole spin relaxation times related to different Landau levels and the corresponding hole effective g-factor. Direct measurements of the hole spin relaxation times prove the obtained results.

Recombination and spin dynamics of excitons in thin (Ga,Al)(Sb,As)/AlAs quantum wells with an indirect band gap and type-I band alignment

The dynamics of exciton recombination and spin relaxation in thin (Ga,Al)(Sb,As)/AlAs quantum wells (QWs) with indirect band gap are studied. The band alignment in these QWs is identified as type I. The exciton recombination time exceeds hundreds of microseconds, while the spin relaxation times of the electron and the heavy hole in an exciton do not exceed hundreds of nanoseconds. The heavy-hole longitudinal $g$ factor is determined to be $+2.5$. Despite the long exciton lifetimes, the photoluminescence circular polarization degree induced by a magnetic field is unexpectedly small and does not exceed $25%$.

Circularly Polarized Photoluminescence from Chiral Perovskite Thin Films at Room Temperature.

Hybrid organic-inorganic perovskites allow the synthesis of high-quality, nanostructured semiconducting films via easily accessible solution-based techniques. This has allowed tremendous development in optoelectronic applications, primarily solar cells and light-emitting diodes. Allowed by the ease of access to nanostructure, chirality has recently been introduced in semiconducting perovskites as a promising way to obtain advanced control of charge and spin and for developing circularly polarized light sources. Circular polarization of photoluminescence (CPL) is a powerful tool to probe the electronic structure of materials. However, CPL in chiral perovskites has been scarcely investigated, and a study in bulk thin films and at room temperature is still missing. In this work, we fabricate bromine-based chiral perovskites by using a bulky chiral organic cation mixed with CsBr, resulting in Ruddlesden-Popper perovskite thin films. We measure CPL on these films at room temperature and, by using unpolarized photoexcitation, we record a degree of circular polarization of photoluminescence in the order of 10-3 and provide a full spectral characterization of CPL. Our results show that chirality is imparted on the electronic structure of the semiconductor; we hypothesize that the excess in polarization of emitted light originates from the charge in the photogenerated Wannier exciton describing an orbit in a symmetry-broken environment. Furthermore, our experiments allow the direct measurement of the magnetic dipole moment of the optical transition, which we estimate to be ≥0.1 μB. Finally, we discuss the implications of our findings on the development of chiral semiconducting perovskites as sources of circularly polarized light.

MnGa Acceptor Center in GaAs (Review)

The model of the MnGa acceptor in GaAs in which initial ground state of a coupled hole Γ8 is changed due to the antiferromagnetic exchange interaction with five 3d electrons of the Mn core is described. The acceptor energy spectrum and the wave functions of its states and also their changes under action of deformations, electric and magnetic fields are considered. Expressions are presented for the description of various properties of isolated MnGa acceptors in GaAs, and the data of some experiments (changes in the $${\text{Mn}}_{{{\text{Ga}}}}^{{\text{0}}}$$ luminescence and absorption spectra and polarization under uniaxial pressures and in magnetic field, EPR spectra, temperature dependence of the magnetic susceptibility, circular polarization of photoluminescence during excitation by polarized light in magnetic field, etc.) are analyzed. It is demonstrated that, in some cases, it is necessary to take into account the existence of random electric and strain fields splitting the acceptor states in the crystal. The analysis results show that this model agrees well with most of the experimental results. The exchange interaction constant is in the range of 3–5 meV.

Акцепторный центр Mn-=SUB=-Ga-=/SUB=- в GaAs ( О б з о р )

AbstractThe model of the Mn_Ga acceptor in GaAs in which initial ground state of a coupled hole Γ_8 is changed due to the antiferromagnetic exchange interaction with five 3 d electrons of the Mn core is described. The acceptor energy spectrum and the wave functions of its states and also their changes under action of deformations, electric and magnetic fields are considered. Expressions are presented for the description of various properties of isolated Mn_Ga acceptors in GaAs, and the data of some experiments (changes in the $${\text{Mn}}_{{{\text{Ga}}}}^{{\text{0}}}$$ luminescence and absorption spectra and polarization under uniaxial pressures and in magnetic field, EPR spectra, temperature dependence of the magnetic susceptibility, circular polarization of photoluminescence during excitation by polarized light in magnetic field, etc.) are analyzed. It is demonstrated that, in some cases, it is necessary to take into account the existence of random electric and strain fields splitting the acceptor states in the crystal. The analysis results show that this model agrees well with most of the experimental results. The exchange interaction constant is in the range of 3–5 meV.

Dynamics of exciton magnetic polarons in CdMnSe/CdMgSe quantum wells: Effect of self-localization

We study the exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted-magnetic-semiconductor quantum wells using time-resolved photoluminescence (PL). The magnetic field and temperature dependencies of this dynamics allow us to separate the non-magnetic and magnetic contributions to the exciton localization. We deduce the EMP energy of 14 meV, which is in agreement with time-integrated measurements based on selective excitation and the magnetic field dependence of the PL circular polarization degree. The polaron formation time of 500 ps is significantly longer than the corresponding values reported earlier. We propose that this behavior is related to strong self-localization of the EMP, accompanied with a squeezing of the heavy-hole envelope wavefunction. This conclusion is also supported by the decrease of the exciton lifetime from 600 ps to 200 - 400 ps with increasing magnetic field and temperature.

Luminescence imaging of photoelectron spin precession during drift in a p-type GaAs microfabricated Hall bar

Using a microfabricated p-type GaAs Hall bar, it is shown that the combined application of co-planar electric and magnetic fields enables the observation of spatial oscillations of the photoluminescence circular polarization due to the precession of drifting spin-polarized photoelectrons. Observation of these oscillations as a function of electric field gives a direct measurement of the minority carrier drift mobility and reveals that the spin coherence length can be tuned up to more than 10μm with electric fields below 1 kV/cm.

Growth condition dependence of photoluminescence polarization in (100) GaAs/AlGaAs quantum wells at room temperature

We conducted systematic measurements on the carrier lifetime (τc), spin relaxation time (τs), and circular polarization of photoluminescence (Pcirc) in (100) GaAs/AlGaAs multiple quantum wells grown by molecular beam epitaxy (MBE). The τc values are strongly affected by MBE growth conditions (0.4–9 ns), whereas the τs are almost constant at about 0.13 ns. The result suggests that spin detection efficiency [τs/(τc + τs)], which is expected to be proportional to a steady-state Pcirc, is largely dependent on growth condition. We confirmed that the Pcirc has similar dependence on growth condition to those of τs/(τc + τs) values. The study thus indicates that choosing the appropriate growth condition of the QW is indispensable for obtaining a high Pcirc from a spin-polarized light-emitting diode (spin-LED).

Optical orientation of electron spins and valence-band spectroscopy in germanium

We have investigated optical orientation in the vicinity of the direct gap of bulk germanium. The electron spin polarization is studied via polarization-resolved photoluminescence excitation spectroscopy unfolding the interplay between doping and ultrafast electron transfer from the center of the Brillouin zone towards its edge. As a result, the direct-gap photoluminescence circular polarization can vary from 30% to $\ensuremath{-}60%$ when the excitation laser energy increases. This study provides also simultaneous access to the resonant electronic Raman scattering due to inter-valence-band excitations of spin-polarized holes, yielding a fast spectroscopic approach for the determination of the energy spectrum of holes in semiconducting materials.

Experimental investigation of the optical spin-selection rules in bulk Si and Ge/Si quantum dots

We study the relationship between the circular polarization of photoluminescence and the magnetic field-induced spin-polarization of the recombining charge carriers in bulk Si and Ge/Si quantum dots. First, we quantitatively compare experimental results on the degree of circular polarization of photons resulting from phonon-assisted radiative transitions in intrinsic and doped bulk Si with calculations which we adapt from recently predicted spin-dependent phonon-assisted transition probabilities in Si. The excellent agreement of our experiments and calculations quantitatively verifies these spin-dependent transition probabilities and extends their validity to weak magnetic fields. Such magnetic fields can induce a luminescence polarization of up to 3%/T. We then investigate phononless transitions in Ge/Si quantum dots as well as in degenerately doped Si. Our experiments systematically show that the sign of the degree of circular polarization of luminescence resulting from phononless transitions is opposite to the one associated with phonon-assisted transitions in Si and with phononless transitions in direct band gap semiconductors. This observation implies qualitatively different spin-dependent selection rules for phononless transitions, which seem to be related to the confined character of the electron wave function.

Microsecond Lifetime of Exciton Spin Polarization in (In,Al)As/AlAs Quantum Dots

The time of spin relaxation of excitons in (In,Al)As/AlAs quantum dots with an indirect bandgap and type-I band alignment is determined by measuring the dynamics of photoluminescence circular polarization induced by a magnetic field B. The spin relaxation time τS increases with decreasing magnetic field in proportion to B−5; its value is ∼40 µs in a magnetic field of 6 T at a temperature of 1.8 K. As the temperature T increases in a magnetic field of 7 T, the value of τS decreases as T−1.1. The character of the dependences of τS on the magnetic field and temperature evidences that spin relaxation of excitons is provided by a process with participation of one acoustic phonon.

Probing the nature of carrier localization in GaInNAs epilayers by optical methods

Photoluminescence (PL), optical pumping, and reflectance studies of nominally undoped and p-type GaInNAs epilayers are presented. The PL peak energy of the nominally undoped sample exhibits an S-shaped dependence on temperature for T < 50 K. This is attributed to recombination of bound excitons localized on traps. The energy of the PL circular-polarization maximum coincides with the energy of the free-exciton related reflectance feature at all temperatures. In heavily p-type samples the S-shaped temperature-dependence of the PL energy disappears, and the PL peak and circular polarization maximum coincide with the reflectance feature at all temperatures, indicating that the PL is free-exciton-like.

Valley polarization and intervalley scattering in monolayer MoS2

We probe the degree of circular polarization of the emitted photoluminescence from a single layer of MoS2 as a function of the circularly polarized photo-excitation energy. A Single layer of MoS2 has strong emission at around 1.9 eV associated with a direct transition at the K-point of the Brillouin zone. The circular polarization of the photoluminescence is very high for excitation near the bandgap and has a power-law decrease as the excitation energy increases. We identify phonon-assisted intervalley scattering as the primary spin relaxation mechanism and present a model that explains the wide variation in values for the polarization reported in the literature.

Antiferromagnetic interaction in coupled CdSe/ZnMnSe quantum dot structures

Spin polarization of nonmagnetic CdSe quantum dots (QDs) coupled to adjacent ZnMnSe diluted magnetic semiconductor (DMS) is investigated by CW and time-resolved magneto-optical spectroscopy under tunable laser excitation. Efficient enhancement in the degree of σ− circular polarization of photoluminescence from the CdSe QDs is observed under optical excitation at the σ+-active exciton state of the DMS. The fact that the enhancement persists much longer than the exciton lifetime of the DMS rules out a role of the DMS excitons. A possible explanation is discussed in terms of antiferromagnetic coupling between the excitons in QDs and aligned Mn ions in DMS.

The influence of δ--layer's magnetization on polarization of photoluminescence of quantum well in singular and vicinal InGaAs/GaAs/δ- heterostructures

Magneto-optical properties and spin dynamics in a p-type GaAs-based heterostructures containing an InGaAs quantum well and 2 nm thick GaAs layer doped with 5 at. % Mn (GaAs:Mn) on flat and vicinal substrates were studied between 2 and 300 K. A significant enhancement of photoluminescence circular polarization Pc = 15 % was found at temperatures T < 30 K. In the non-magnetic quantum well Pc = 0 %. The polarization of the InGaAs quantum well photoluminescence follows the temperature dependence of the GaAs:Mn layer magnetization. Two types of ferromagnetism were found. In the heterostructures grown on the flat substrate parallel to the (001) GaAs plane the magnetization obeys the Bloch T 3/2 temperature dependence while for the structures grown on the vicinal surface (disoriented by 3°) the magnetization follows percolation dependence. The later is attributed to the formation of strongly disordered islands of GaAs:Mn.

Dynamics of dense spin ensemble excited in a barrier layer and detected in a well

Photoluminescence (PL) polarization of a spin ensemble was examined over a wide excitation wavelength range from 520 nm to 700 nm and a temperature range from 3.5 K to 300 K after it transfers from a (AlGa)As barrier layer and eventually quenches irradiatively in a GaAs quantum well (QW). A highest PL circular polarization of 30% can be kept at temperatures up to 120 K, while its room temperature value reaches about 17%. It is found that the main features of the optical spin orientation in bulk Al0.27Ga0.73As materials can be reproduced in terms of the wavelength dependence of PL polarization degree, as the spin polarized ensemble transfers and relaxes into GaAs QW. The transient of PL polarization degree also indicates that a dense spin ensemble collected from the barrier region is in favor of conserving its polarization in GaAs QW as evidenced by a rising temporal response.

Magnetic polaron formation and exciton spin relaxation in single Cd1−xMnxTe quantum dots

We study the formation dynamics of a spontaneous ferromagnetic order in single self-assembled CdMnTe quantum dots. By measuring time-resolved photoluminescence, we determine the formation times for QDs with Mn ion contents x varying from 0.01 to 0.2. At low x these times are orders of magnitude longer than exciton spin relaxation times evaluated from the decay of photoluminescence circular polarization. This allows us to conclude that the direction of the spontaneous magnetization is determined by a momentary Mn spin fluctuation rather than resulting from an optical orientation. At higher x, the formation times are of the same order of magnitude as found in previous studies on higher dimensional systems. We also find that the exciton spin relaxation accelerates with increasing Mn concentration.

Circular polarization of photoluminescence of GaAs/AlGaAs quantum wells as a function of their growth conditions

The degree of circular polarization of the photoluminescence of samples with GaAs/AlGaAs quantum wells grown with growth interruption at both interfaces and under conventional growth conditions has been investigated. It has been revealed that, at a low measurement temperature (4.2 K), the values of the circular polarization measured at different detection energies within the spectral profile of the luminescence band differ substantially for the sample grown with the growth interruption, whereas this effect is not observed for the sample grown without growth interruption. An increase in the sample temperature to 77 K leads to the disappearance of the effect under consideration. The observed behavior has been explained in terms of the model that accounts for a significantly different degree of localization of charge carriers in the growth islands (formed at the interfaces of the structure) with the spatial sizes determined by the specific growth conditions of the sample.