Session Z52: Optospintronics and Optical Probes of 2D, Topological, and Semiconductor Materials

The effect of spontaneous emission on the dynamical operation of spin lasers.

Through angular momentum conservation, injecting spin-polarized carriers in lasers offers control of the helicity of emitted light and room-temperature spintronics applications, beyond magnetoresistence [1]. The push to scale down such spin lasers is accompanied by an enhanced contribution of spontaneous emission, usually assumed to be negligibly small. With a current injection required for the onset of lasing, there is a striking change from incoherent to coherent emitted light. However, in scaled-down lasers with a large spontaneous emission, this transition is blurred and even identifying the lasing threshold is a challenge [2]. The resulting changes with spontaneous emission strongly modify not only the steady-state [2], but also the dynamical operation of spin lasers, which is crucial for their applications [1]. We explore here various trends in this dynamical operation and provide guidance for an optimized design of scaled-down spin lasers.

[1] M. Lindemann et al., Nature 568, 212 (2019).

[2] G. Xu, K. Patel, and I. Zutic, Appl. Phys. Lett. in press   

High-birefringence spin lasers as coupled harmonic oscillators

While birefringence-anisotropy of the refractive index-is usually considered detrimental in lasers, recent breakthroughs reveal that a large birefringence is responsible for an ultrafast operation of semiconductor lasers with injected spin-polarized carriers [1]. The modulation of the emitted polarization of light in such spin lasers can be realized much faster than the modulation of the light intensity. A transparent description of high-birefringence lasers is provided by recently developed intensity equations [2], which we show to be directly connected to the coupled damped and driven harmonic oscillators. An intuitive understanding of this mechanical analog allows us recover the main trends for the dynamical operation of spin lasers under both intensity and polarization modulation, including the resulting bandwidths. While the experiments on dynamical operation of spin lasers are currently limited to the regime which corresponds to the case of weakly-coupled harmonic oscillators, we also make predictions for an unexplored regime of spin lasers that are represented by a strong-coupling regime [3]

1. M. Lindemann et al., Nature 568, 212 (2019)

2. G. Xu, J. Cao, V. Labinac, and I. Žutić, PRB 103, 045306, (2021)

3. V. Labinac et al., preprint.   

Optical injection of spin current in Ge1-xSnx

Ge_1-xSn_x is a silicon-compatible semiconductor of great interest because of the possibility to engineer the bandgap energy and directness, and the potential for designing monolithic photonic and optoelectronic devices. In this work, we exploit the recent developments made in establishing high-quality, direct bandgap Ge_1-xSn_x layers, to investigate the optical injection and coherent control of spin currents in this group IV semiconductor. The study of these processes could play a key role in the development of coherent photon-to-spin interfaces, thus providing the foundation to create compact optical schemes for long distance distribution of entanglement. To this end, a full zone 30-band k·p model is applied to obtain the band structure of relaxed and strained Ge_1-xSn_x. For one- and two-photon absorption, the optical injection of carrier, spin, current, and spin current are calculated in the independent particle approximation. Critical properties such as the two-photon absorption anisotropy and linear-circular dichroism are also extracted. Finally, coherent control with a bichromatic field of frequencies ω and 2ω is investigated. In this case, with the incorporation of Sn in Ge, a significant increase in spin current injection is found for a relatively broad range of energies.   

Fe3GeTe2-based magnetic van der Waals heterostructures for valleytronics applications

The monolayer transition metal dichalcogenides (TMDs) are promising for valleytronics applications, due to their large spin-orbit coupling and loss of inversion symmetry. So far, numerous works have shown the optical control of the valley degree of freedom of TMDs. But to electrical control of this degree of freedom remains challenging. Here, we demonstrate the novel van der Waals heterostructures that incorporate a monolayer WSe₂ integrated with an ultrathin Fe₃GeTe₂ (FGT) magnetic contact. The exploited magnetic contact could inject spin-polarized carriers into a monolayer TMD, leading to valley-dependent polarization, as confirmed by our helicity-dependent electroluminescence and reflective magnetic circular dichroism measurements. In addition, our measurements indicate that injecting spin-polarized holes from FGT into WSe₂ can more effectively lead to valley polarization, compared with injecting spin-polarized electrons from FGT. Based on our density functional theory calculations, we reveal such phenomenon is related to the unique electronic structure of FGT.   

Dynamic Polarization of Resident Electrons in WSe2 and WS2 Monolayers

We demonstrate very efficient spin-valley pumping of resident electrons in both WSe2 and WS2 mono- layers using circularly polarized light [1]. We use the degree of circular polarization of the photoluminescence (PL) associated with negative trions as probes of the polarization of resident electrons : both the intervalley triplet trion and the intravalley singlet trion which consist in the binding of a photo-generated electron-hole pair with a resident electron from the opposite and same valley respectively. In a charge adjustable WSe2 monolayer [2], we measure a very large positive PL circular polarization ~ 90% for the triplet trion and a negative polarization −40% for the singlet trion. This valley pumping also manifests as a large increase of the triplet trion PL intensity with circular excitation as compared to linear excitation. Interestingly, these results demonstrate that circularly polarized excitation photo-generates electron-hole pairs in one valley and dynamically polarize resident electrons in the opposite valley.  The spatial variation of the resident electron spin/valley polarization will also be discussed.

[1] C. Robert et al, Nature Com. 12, 5455 (2021)

[2] M. Yang et al, Arxiv 2110.00887 (2021)   

Layer Dependent Spin Properties in InSe

All-optical spin control enabled by optical spin selection rules offers chances for high speed spin orientation and manipulation. Although materials such as low-dimensional semiconductors have been extensively studied for spintronic applications, layered 2D materials offer novelty such as valleytronics and layer-dependent spin properties. InSe, a van der Waals material, has high electron mobility as well as predictions for optical spin selection rules, suggesting potential for 2D spintronics. Here we present layer-dependent optical-spin properties in InSe down to a few-layers. Optical orientation and Zeeman splitting contributions to polarization were identified separately in polarized photoluminescence. Time-resolved Kerr rotation revealed spin dynamics in multilayers. These results establish potential for orientation and manipulation of spin in InSe, opening the door for combining layer-dependent spin and favorable electronic properties in 2D materials.   

Measurements of the Spin and Valley Hall Effect in Monolayer WSe2 Dependence on Voltage and Temperature for Spintronic Applications

The coupled spin and valley Hall effect (SVHE) in monolayer transition metal dichalcogenides (TMDs) offers the opportunity to manipulate the spin and valley degree of freedom in these systems for spintronics. We applied magneto-optic Kerr effect (MOKE) with a 700 nm laser to directly measure the SVHE in monolayer tungsten diselenide (WSe₂) field effect transistors. Carrier-density-dependent and temperature-dependent reflectance spectra were measured, and the dielectric function dispersion was calculated to calibrate the MOKE data. We will show the SVHE MOKE signature is strongly dependent on the source-drain current, gate induced carrier density, and temperature, and that the signal can persist towards warmer temperatures, providing critical measurements needed to make use of the SVHE in devices.   

Emergent magnetic response in correlated transition metal dichalcogenide superlattices

Moiré superlattices of stacked two-dimensional materials emerged as a promising platform for quantum engineering many-body physics. The controllable filling of moiré minibands offers a knob to tune the carrier interactions. For example, at filling factor of v = -1, one hole per moiré unit cell, local magnetic moments have been observed. In this talk, we will present our magnetic circular dichroism measurements of interaction induced magnetic responses near v = -1. Parameters such us electric fields and temperature are investigated. The nature of the magnetic interactions will be discussed.   

Ferromagnetism of itinerant electrons in monolayer MoS2

Coulomb interactions are crucial in determining the ground state of an ideal two-dimensional electron gas (2DEG) in the limit of low electron densities. In this regime, Coulomb interactions dominate over single-particle effects. In gallium arsenide, electrons are typically localized at these low densities. In contrast, in transition metal dichalcogenides, Coulomb correlations in a 2DEG can be anticipated at experimentally relevant electron densities. We focus on monolayer MoS₂. Equipping MoS₂ with gates allows electrons to be injected, creating a 2DEG. The electronic ground-state is probed optically with near-resonant photoluminescence spectroscopy as a function of the electron density. Owing to the robust optical selection, this probe is highly spin- and valley-selective. Crucially, the cross-polarized emission channels give information on intervalley exchange mechanisms. By identifying the formation of different trion states, we present experimental evidence of itinerant ferromagnetism in monolayer MoS₂. Out of the four available conduction bands, only two are occupied. These two bands have the same spin but different valley quantum numbers. This spin-polarization can be understood by strong intervalley Coulomb scattering in models including corrections to Fermi liquid theory.   

First-principles calculation of optical conductivity response in armchair graphene nanoribbon

2D materials is of great interest for making future high pase electronic devices. In this talk, I will present first-principles calculation results for conductivity measurements in pristine 7 zigzag edge of armchair graphene nanoribbons (7aGNRs), periodicity crystal structures of single boron doped 7aGNRs and periodicity crystal structures of ring-pentagon 7aGNRs.  I will report that pristine unstrained 7aGNRs is optically inactive but turns to be optically active due to the application of strain engineering. In boron doped and ring-pentagon structures of 7aGNRs, first-priciples calculation results show non-vansihing optical conductivity because doping with boron atom and creating a single carbon atom vacancy convert graphene into mettalic. I will also present the response of strain on the Berry curvature of these structures. Result suggests  that fermions are spread through out the Brillion zone in the reciprocal space for optimized 7aGNRs but localized near the Γ-point due to application of strain. I acknowledge Northwest Missouri State University for providing generous travel expesne support.    

Nonlocal Helicity Dependent Photocurrent Response in Topological Insulators

Topological insulators (TIs) are a remarkable class of materials due to their linear dispersion relation and exotic spin textures at the surface and have recently shown millimeter-long diffusion lengths with a highly tunable Fermi level. We use scanning photocurrent microscopy to study Sb doped Bi2Se3 as a function of temperature, wavelength, gate voltage, and angle to construct spatial maps that illustrate a nonlocal helicity-dependent photocurrent response. Remarkably, these maps display a spin diffusion length of more than 100 μm. We compare the polarization-independent photocurrent response to this observed helicity-dependent response to shed light on the deeper nature of the carriers responsible for these remarkable observations, which may be due to the carrier states forming an exciton condensate.   

Quantized Microwave Faraday rotation

The phenomenon of rotation of polarization in the presence of static magnetic field known as Faraday rotation [1] is very well known. Here we present the quantitative observation of microwave Faraday rotation conducted with GaAs/AlGaAs semiconductor heterostructure. The microwave Faraday rotation observed in high mobility two-dimensional electron gas arises as a result of cyclotron motion of charge carriers. The Faraday rotation induced can be understood by Fresnel analysis for the transmission of right and left-handed circularly polarized microwaves. As with the Hall effect, a continuous classical as well as quantized Faraday rotation is observed. In the quantum Hall regime, the Faraday rotation is quantized in units of fine structure constant. The dielectric response of the semiconductor host, and the modification of the wave impedance and field distribution by a wave guide [2] will lead to a modification of the quantized Faraday rotation away from the vacuum fine structure constant, α ≈ 1/137. The effect of frequency dependent electromagnetic confinement can be accounted with an effective fine structure constant α*.

[1] M. Faraday, Phil. Trans. Roy. Soc. London 136, 1 (1846).

[2] D. L. Sounas and C. Caloz, IEEE Trans. Microwave Theory Tech. 60, 901 (2012).   

Time dependent measurements of dynamic nuclear polarization growth under periodic optical electron spin pumping in gallium arsenide

Through dynamic nuclear polarization, optically pumped electron spins polarize the nuclear spin system in gallium arsenide. The effective magnetic field from the polarized nuclear spins will affect the Larmor precession frequency of the electron spin system, which will synchronize to discrete integer or half integer values; this is known as nuclear-induced frequency focusing. The rate of dynamic nuclear polarization is dependent on the coupling between the electron and nuclear spin systems. Observations from optical pump probe Kerr rotation experiments show nuclear polarization growth behavior dependence on pump wavelength and external field. These lab time dependent observations are corroborated by calculations with a model using the optical Stark effect.    

Microwave-free dynamic nuclear polarization via sudden thermal jumps

Dynamic Nuclear Polarization (DNP) presently stands as the preferred strategy to enhance the sensitivity of nuclear magnetic resonance measurements, but its application relies on the use of high-frequency microwave to manipulate electron spins, an increasingly demanding task as the applied magnetic field grows. Here we investigate the dynamics of a system hosting a polarizing agent formed by two distinct paramagnetic centers near a level anti-crossing. We theoretically show that nuclear spins polarize efficiently under a cyclic protocol that combines alternating thermal jumps and radio-frequency pulses connecting hybrid states with opposite nuclear and electronic spin alignment. Central to this process is the difference between the spin-lattice relaxation times of either electron spin species, transiently driving the electronic spin bath out of equilibrium after each thermal jump. Without the need for microwave excitation, this route to enhanced nuclear polarization may prove convenient, particularly if the polarizing agent is designed to feature electronic level anti-crossings at high magnetic fields.