Circularly polarized light sources with free-space directional emission play a key role in chiroptics(1), spintronics(2), valleytronics(3) and asymmetric photocatalysis(4). However, conventional approaches fail to simultaneously realize pure circular polarization, high directionality and large emission angles in a compact emitter. Metal-halide perovskite semiconductors are promising light emitters(5-8), but the absence of an intrinsic spin-locking mechanism results in poor emission chirality. Further, device integration has undermined the efficiency and directionality of perovskite chiral emitters. Here we realize compact spin-valley-locked perovskite emitting metasurfaces where spin-dependent geometric phases are imparted into bound states in the continuum via Brillouin zone folding, and thus, photons with different spins are selectively addressed to opposite valleys. Employing this approach, chiral purity of 0.91 and emission angle of 41.0 degrees are simultaneously achieved, with a beam divergence angle of 1.6 degrees. With this approach, we envisage the realization of chiral light- emitting diodes, as well as the on-chip generation of entangled photon pairs.

The ultimate goal for singlet fission is that each photo-excited singlet exciton, S1, will result in two triplet excitons with unity yield. However, the singlet fission is now recognized to be complicated, involving bright/dark excited states of different spin multiplicity. Identifying the role of such states is vital to optimize singlet fission yield but difficult due to their elusive spectral signature. Here, we develop an experimental protocol based on a refined magneto-optical probe to access the fast time evolution of various excited states. In diphenylhexatriene crystal, the S1 is found to undergo two competing processes?to form one of the two dark triplet pair intermediates having different exchange energies or to form a bright state, Sx, exhibiting excimer-like delayed photoluminescence. Our result provides a clear picture of a competition event in singlet fission, which is beneficial for the development and tailoring of singlet fission materials with high yield.

We report on the first successful growth of wurtzite (WZ) GaBiAs nanowires (NWs) and reveal the effects of Bi incorporation on the electronic band structure by using polarization-resolved optical spectroscopies performed on individual NWs. Experimental evidence of a decrease in the band-gap energy and an upward shift of the topmost three valence subbands upon the incorporation of Bi atoms is provided, whereas the symmetry and ordering of the valence band states remain unchanged, that is, Γ9, Γ7, and Γ7 within the current range of Bi compositions. The extraordinary valence band structure of WZ GaBiAs NWs is explained by anisotropic hybridization and anticrossing between p-like Bi states and the extended valence band states of host WZ GaAs. Moreover, the incorporation of Bi into GaAs is found to significantly reduce the temperature sensitivity of the band-gap energy in WZ GaBiAs NWs. Our work therefore demonstrates that utilizing dilute bismide alloys provides new avenues for band-gap engineering and thus photonic engineering with NWs.

We report on experimental determination of the strain and bandgap of InAsP in epitaxially grown InAsP-InP core-shell nanowires. The core-shell nanowires are grown via metal-organic vapor phase epitaxy. The as-grown nanowires are characterized by transmission electron microscopy, X-ray diffraction, micro-photoluminescence (mu PL) spectroscopy, and micro-Raman (mu-Raman) spectroscopy measurements. We observe that the core-shell nanowires are of wurtzite (WZ) crystal phase and are coherently strained with the core and the shell having the same number of atomic planes in each nanowire. We determine the predominantly uniaxial strains formed in the core-shell nanowires along the nanowire growth axis and demonstrate that the strains can be described using an analytical expression. The bandgap energies in the strained WZ InAsP core materials are extracted from the mu PL measurements of individual core-shell nanowires. The coherently strained core-shell nanowires demonstrated in this work offer the potentials for use in constructing novel optoelectronic devices and for development of piezoelectric photovoltaic devices.

Semiconductor spintronics and opto-spintronics have intrigued intense attention as they promise great advance of contemporary semiconductor information technology with integrated spin functionalities. Over the last few decades, the development of growth techniques and discovery of topological band structures have led to the explosion of a wide range of low-dimensional semiconductor materials, many of which have superior properties compared to their bulk ancestors. The limited dimension of materials imposes constraint on the motion of charge carriers and causes spin interactions of various forms, which have profound influence on the spin properties that are important for various spintronic and/or opto-spintronic applications.

In this context, semiconductor quantum dot (QD) structures (QDS) and 3D topological insulator (TI) have emerged as promising material systems that exhibit distinct spin properties: In QDS, carriers are restricted in all three dimensions. The 3D confinement quenches the spin-orbit interaction (SOI) mediated spin depolarization/dephasing processes and, as a consequence, leads to a prolonged spin relaxation time, which can be used for non-volatile storage or quantum bits in quantum information technology; Whereas, the surface state of 3D TI, on the contrary, has the electronic structure that is dominated by SOI such that the orientation of the electron spin is tied to its momentum. The strong SOI limits the spin relaxation time but can be utilized to generate spin polarized current that is free from backscattering. This thesis work focuses on these two prototypical materials to provide an in-depth understanding of the spin phenomena as well as to tailor their spin properties such that novel spintronic and/or opto-spintronic devices can be built on.

To employ QDS for storage of spin information, first and foremost is to be able to generate and detect spin polarization effectively and efficiently. For this purpose, we have carefully inspected both the spin injection and spin detection processes in various QDS. In this thesis work, spin polarized carriers or excitons are generated via optical orientation that converts the angular momentum of the absorbed photons to the photo-generated carriers or excitons. The as-generated spin polarized carriers/excitons then need to relax their energy before getting injected to the QDS. We have found that the spin injection process is influenced by the interactions with phonons (Paper 1) and disordered environment associated with the injection path (Paper 2). In the former case, we show that the longitudinal optical (LO) phonon contributes to accelerated relaxation of the carrier/exciton energy to the QDS ground state, which preserves the spin polarization. By engineering the energy of the QDS, we can take advantage of such LO-phonon assisted process and can avoid the spin injection loss due to the commonly observed phonon bottleneck effect. In the latter case, we discover that the surrounding media of the QDS is generally disordered, distributed by potential fluctuations caused by alloying or strain randomness. Exciton injection via such localized potential undergoes spin relaxation caused by an anisotropic exchange interaction (AEI), which leads to appreciable spin injection loss at low temperatures.

The AEI is also found to be responsible for the low spin detection efficiency observed in the undoped QDS reported in Paper 3. The AEI causes mixing and splitting of exciton spin states, which leads to not only a low PL polarization degree of the QDS but also a serious issue in generation of entangled photon pairs utilizing QDS. We show that the aforementioned spin injection (Paper 2) and spin detection (Paper 3) loss associated with the AEI can be effectively tuned in the QDS by the arrangement of the constituting QDs. The effect originates from the modification of the strain and shape anisotropy both inside and outside the QDS due to the collective interaction with the neighboring QDs, which introduces a new degree of freedom in electronic-structure engineering of the QDS.

In the doped QDS, we have found that the spin detection efficiency can additionally be affected by the exciton charge states and a hyperfine interaction (HFI) with the nuclear-spin bath. In Paper 4, we discover a dynamic charging process that the charged states of an InGaAs QD ensemble are altered with different excitation power densities and excitation photon energies. The charging effect leads to an anomalous spectral dependence of PL polarization such that the copolarized emission can be dynamically converted to the counter-polarized one. This finding thus calls for caution in the correlation between the optical and spin polarization in QDS with a complex charging environment. The effect of the HFI depends on the condition of nuclear spin polarization. In QDS with an unpolarized nuclear-spin bath, the HFI is a primary electron/exciton spin depolarization/dephasing source in QDS at low temperatures. In Paper 5, we show that the ensemble spin dephasing time of QDs at a cryogenic temperature correlates with the averaged size of QDs. The behavior can be accounted for by electron spin dephasing in a fluctuating nuclear field, which is experimentally verified for the first time. The results thus highlight the important role of the HFI in the electron spin dephasing in the QDs. On the other hand, finite nuclear spin polarization can be achieved through the dynamic nuclear polarization (DNP) process that transfers the angular momentum from the spin-polarized electron to nuclei. DNP is recognized to be important for spintronics and quantum information in nuclear spin-rich nanostructures. This is not only because of its role in suppressing the aforementioned electron spin dephasing, but also because it is behind the idea of exploring the long coherent nuclear spin as a quantum computing qubit. In Paper 6, we have investigated the effect of DNP in a series of QDS, where the strength and orientation of the nuclear field resulted from the DNP are identified and measured. We find that the DNP is built along a tilted axis that deviates from the commonly observed orientation along the QD growth axis and the nuclear field develops a substantial transverse component. This anomalous behavior of the DNP is found to arise from the nuclear quadrupole interaction with an oblique principal axis. The resulting tilting nuclear field can further lead to dephasing and depolarization of the electron spin that has previously been overlooked. The results uncover the detrimental effect rooted in the complex electrostatic environment of the nuclei inside the QDS and call for special care of the strain and alloying engineering of the nanostructures.

In the case of 3D TI, we aim at providing both the experimental and theoretical understanding of the surface spin photocurrent as well as innovations in future opto-spintronic applications utilizing the semiconductor-TI interface. As has been shown earlier, the circular polarized excitation light creates a spin photocurrent that is resistant to moderate scattering. In Paper 7, we present detailed studies of the dependence of the spin photocurrent on the incident angle of the excitation light in a prototypical 3D TI, Bi2Te3. We point out that the spin photocurrent, as a result of spin-selective optical transitions, is associated with both the in-plane and out-of-plane spin texture of the topological surface states. We focus on the contribution of the out-of-plane spin texture, which is less explored, and demonstrate, for the first time, spin injection from a conventional semiconductor, GaAs, to a 3D TI. In favor of this hybrid system, we show that the spin photocurrent contributed by the spin injection exceeds that from the TI alone and the magnitude and direction of the current can be controlled by applying a transverse magnetic field. In Paper 8, we give a tight-binding description of the microscopic origin of the spin photocurrent in Bi2Te3, where we have provided theoretical calculations of the spin photocurrent as a function of the excitation incidence angle, Fermi energy and different scattering potentials. The results explain the observation of the out-of-plane spin texture contribution reported in Paper 7, which should have been forbidden by symmetry, and provide a pathway for opto-spintronic applications based on a TI-semiconductor hybrid system.

Semiconductor quantum-dot (QD) structures are promising for spintronic applications owing to strong quenching of spin relaxation processes promoted by carrier and excitons motions. Unfortunately, spin injection efficiency in such nanostructures remains very low and the exact physical mechanism for the spin loss is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QDs and quantum-dot molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer immediately surrounding QDs and QMSs that in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We identify the microscopic origin of the observed severe spin loss during spin injection as being due to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We find that the AEI of the injected excitons and thus the efficiency of the spin injection processes are correlated with the overall geometric symmetry of the QMSs, as the latter largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier. These results pave the way for a better understanding of spin injection processes and the microscopic origin of spin loss in QD structures, which in turn provides a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of the QMSs thereby overcoming a major bottleneck in spintronic device applications utilizing semiconductor QDs.

By employing photoluminescence(PL) spectroscopy under dual-wavelength optical excitation, we uncover the presence of deep-level hole traps in Ga(In)NP alloys grown by molecular beam epitaxy(MBE). The energy level positions of the traps are determined to be at 0.56 eV and 0.78 eV above the top of the valance band. We show that photo-excitation of the holes from the traps, by a secondary light source with a photonenergy below the bandgapenergy, can lead to a strong enhancement (up to 25%) of the PL emissions from the alloys under a primary optical excitation above the bandgapenergy. We further demonstrate that the same hole traps can be found in various MBE-grown Ga(In)NP alloys, regardless of their growth temperatures, chemical compositions, and strain. The extent of the PL enhancement induced by the hole de-trapping is shown to vary between different alloys, however, likely reflecting their different trap concentrations. The absence of theses traps in the GaNP alloy grown by vapor phase epitaxy suggests that their incorporation could be associated with a contaminant accompanied by the N plasma source employed in the MBEgrowth, possibly a Cu impurity.

Fine-structure splitting (FSS) of excitons in semiconductor nanostructures is a key parameter that has significant implications in photon entanglement and polarization conversion between electron spins and photons, relevant to quantum information technology and spintronics. Here, we investigate exciton FSS in self-organized lateral InAs/GaAs quantum-dot molecular structures (QMSs) including laterally aligned double quantum dots (DQDs), quantum-dot clusters (QCs), and quantum rings (QRs), by employing polarization-resolved microphotoluminescence (μPL) spectroscopy. We find a clear trend in FSS between the studied QMSs depending on their geometric arrangements, from a large FSS in the DQDs to a smaller FSS in the QCs and QRs. This trend is accompanied by a corresponding difference in the optical polarization directions of the excitons between these QMSs, namely, the bright-exciton lines are linearly polarized preferably along or perpendicular to the [11̅0] crystallographic axis in the DQDs that also defines the alignment direction of the two constituting QDs, whereas in the QCs and QRs, the polarization directions are randomly oriented. We attribute the observed trend in the FSS to a significant reduction of the asymmetry in the lateral confinement potential of the excitons in the QRs and QCs as compared with the DQDs, as a result of a compensation between the effects of lateral shape anisotropy and piezoelectric field. Our work demonstrates that FSS strongly depends on the geometric arrangements of the QMSs, which effectively tune the degree of the compensation effects and are capable of reducing FSS even in a strained QD system to a limit similar to strain-free QDs. This approach provides a pathway in obtaining high-symmetry quantum emitters desirable for realizing photon entanglement and spintronic devices based on such nanostructures, utilizing an uninterrupted epitaxial growth procedure without special requirements for lattice-matched materials combinations, specific substrate orientations, and nanolithography.