III-V Semiconductor Quantum Research Articles

Efficient adaptive Bayesian estimation of a slowly fluctuating Overhauser field gradient

Slow fluctuations of Overhauser fields are an important source for decoherence in spin qubits hosted in III-V semiconductor quantum dots. Focusing on the effect of the field gradient on double-dot singlet-triplet qubits, we present two adaptive Bayesian schemes to estimate the magnitude of the gradient by a series of free induction decay experiments. We concentrate on reducing the computational overhead, with a real-time implementation of the schemes in mind. We show how it is possible to achieve a significant improvement of estimation accuracy compared to more traditional estimation methods. We include an analysis of the effects of dephasing and the drift of the gradient itself.

Heterogeneous integration of single InAs/InP quantum dots with the SOI chip using direct bonding.

Quantum information processing with photons in small-footprint and highly integrated silicon-based photonic chips requires incorporating non-classical light sources. In this respect, self-assembled III-V semiconductor quantum dots (QDs) are an attractive solution, however, they must be combined with the silicon platform. Here, by utilizing the large-area direct bonding technique, we demonstrate the hybridization of InP and SOI chips, which allows for coupling single photons to the SOI chip interior, offering cost-effective scalability in setting up a multi-source environment for quantum photonic chips. We fabricate devices consisting of self-assembled InAs QDs embedded in the tapered InP waveguide (WG) positioned over the SOI-defined Si WG. Focusing on devices generating light in the telecom C-band compatible with the low-loss optical fiber networks, we demonstrate the light coupling between InP and SOI platforms by observing photons outcoupled at the InP-made circular Bragg grating outcoupler fabricated at the end of an 80 µm-long Si WG, and at the cleaved edge of the Si WG. Finally, for a device with suppressed multi-photon generation events exhibiting 80% single photon generation purity, we measure the photon number outcoupled at the cleaved facet of the Si WG. We estimate the directional on-chip photon coupling between the source and the Si WG to 5.1%.

Enhanced Spin Polarization from Biaxially Strained Colloidal Quantum Dots.

Electron and hole spin polarization is crucial for quantum dots to be used in spin lasers and quantum information processing. However, the degree of spin polarization in II-VI and III-V semiconductor quantum dots is low because of the degenerated valence band. Here, we increase the light and heavy hole degeneracy by introducing biaxial strain into CdSe-based quantum dots, enabling the degree of spin polarization to be increased from 20% to 50% under photoexcitation. The optical gain threshold measurement further reveals that the increase in polarization helps to reduce the gain threshold.

Rational ligand design for enhanced carrier mobility in self-powered SWIR photodiodes based on colloidal InSb quantum dots.

Solution-processed colloidal III-V semiconductor quantum dot photodiodes (QPDs) have potential applications in short-wavelength infrared (SWIR) imaging due to their tunable spectral response range, possible multiple-exciton generation, operation at 0-V bias voltage and low-cost fabrication and are also expected to replace lead- and mercury-based counterparts that are hampered by reliance on restricted elements (RoHS). However, the use of III-V CQDs as photoactive layers in SWIR optoelectronic applications is still a challenge because of underdeveloped ligand engineering for improving the in-plane conductivity of the QD assembled films. Here, we report on ligand engineering of InSb CQDs to enhance the optical response performance of self-powered SWIR QPDs. Specifically, by replacing the conventional ligand (i.e., oleylamine) with sulfide, the interparticle distance between the CQDs was shortened from 5.0 ± 0.5 nm to 1.5 ± 0.5 nm, leading to improved carrier mobility for high photoresponse speed to SWIR light. Furthermore, the use of sulfide ligands resulted in a low dark current density (∼nA cm-2) with an improved EQE of 18.5%, suggesting their potential use in toxic-based infrared image sensors.

Monolithic Polarizing Circular Dielectric Gratings on Bulk Substrates for Improved Photon Collection from InAs Quantum Dots.

III-V semiconductor quantum dots (QDs) are near-ideal and versatile single-photon sources. Because of the capacity for monolithic integration with photonic structures as well as optoelectronic and optomechanical systems, they are proving useful in an increasingly broad application space. Here, we develop monolithic circular dielectric gratings on bulk substrates - as opposed to suspended or wafer-bonded substrates - for greatly improved photon collection from InAs quantum dots. The structures utilize a unique two-tiered distributed Bragg reflector (DBR) structure for vertical electric field confinement over a broad angular range. Opposing "openings" in the cavities induce strongly polarized QD luminescence without harming collection efficiencies. We describe how measured enhancements depend on the choice of collection optics. This is important to consider when evaluating the performance of any photonic structure that concentrates farfield emission intensity. Our cavity designs are useful for integrating QDs with other quantum systems that require bulk substrates, such as surface acoustic wave phonons.

Room-temperature electric field control of spin filtering efficiency for enhanced modulation of optical spin polarization in a defect-functional 0D-2D hybrid nanostructure.

In order to accomplish spin-based photoelectric information processing, it is necessary to modulate electron spin polarization in III-V semiconductor quantum dots (QDs) using an electric field. However, there is a principal limitation to the spin polarization degree and its control range, as the electron spin polarization is rapidly lost during injection into the QDs at room temperature (RT). Here, electric field control of optical spin polarization in the range of 15-40% is demonstrated at RT using InAs QDs tunnel-coupled with a defect-functional GaNAs quantum well (QW) spin filter. This compares with an electric field control of 1-4% for InAs QDs tunnel-coupled with an InGaAs QW. Transient polarization in the range of 30-60% is also obtained in the ultrafast time domain of less than 100 ps, the degree of polarization depending on the electric field. The enhanced polarization control is achieved by tuning the amplified spin polarization of electrons tunnel-injected from the GaNAs QW into QDs via the electric-field-dependent spin-filtering efficiency of GaNAs. These findings will provide a new way to extensively modulate the electron spin polarization in opto-semiconductors, by electric-field-induced on/off switching of spin amplification.

Two-Photon Interference of Single Photons from Dissimilar Sources

Entanglement swapping and heralding are at the heart of many protocols for distributed quantum information. For photons, this typically involves Bell-state measurements based on two-photon interference effects. In this context, hybrid systems that combine high rate, ultrastable, and pure quantum sources with long-lived quantum memories are particularly interesting. Here, we develop a theoretical description of pulsed two-photon interference of photons from dissimilar sources to predict the outcomes of second-order cross-correlation measurements. These are directly related to, and hence used to quantify, photon indistinguishability. We study their dependence on critical system parameters such as quantum state lifetime and emission frequency, and quantify the impact of time jitter, pure dephasing, and spectral wandering. We show that for a fixed lifetime of one of the two emitters, for each frequency detuning there is an optimal lifetime of the second emitter that leads to the highest photon indistinguishability. Expectations for different hybrid combinations involving III-V semiconductor quantum dots, color centers in diamond, atom-scale defects in two-dimensional materials and neutral atoms are quantitatively compared for real-world system parameters. Our work provides a theoretical basis for the treatment of dissimilar emitters and enables assessment of which imperfections can be tolerated in hybrid photonic quantum networks.

Directional radiation enhancement of nanowire quantum dots based on line-array plasmonic antenna coupling

The integration of a single III-V semiconductor quantum dot with a plasmonic nanoantenna as a means toward efficient single-photon sources (SPEs) is limited due to its weak, wide-angle emission, and low emission rate. These limitations can be overcome by designing a unique linear array of plasmonic antenna structures coupled to nanowire-based quantum dot (NWQD) emitters. A linear array of a coupled device composed of multiple plasmonic antennas at an optimum distance from the quantum dot emitter can be designed to enhance the directionality and the spontaneous emission rate of an integrated single-photon emitter. Finite element modeling has been used to design these compact structures with high quantum efficiencies and directionality of single-photon emission while retaining the advantages of NWQDs. The Purcell enhancement factor of these structures approaches 66.1 and 145.8, respectively. Compared to a single NWQD of the same diameter, the fluorescence was enhanced by 1054 and 2916 times. The predicted collection efficiencies approach 85% (numerical aperture, NA = 0.5 ) and 80% ( NA = 0.5 ), respectively. Unlike single-photon emitters based on bulky conventional optics, this is a unique nanophotonic single-emission photon source based on a line-array configuration that uses a surface plasmon-enhanced design with minimum dissipation. The designs presented in this work will facilitate the development of SPEs with potential integration with semiconductor optoelectronics.

Universality of optical absorptance quantization in two-dimensional group-IV, III-V, II-VI, and IV-VI semiconductors

The optical absorptance of a single graphene layer over a wide range of wavelengths is known to be remarkably constant at the universal value $\ensuremath{\pi}\ensuremath{\alpha}$ where $\ensuremath{\alpha}$ is the fine structure constant. Using atomistic tight-binding calculations, we show that the absorptance spectra of nanometer-thin layers (quantum wells) of group-IV, III-V, II-VI, or IV-VI semiconductors are characterized by marked plateaus at integer values of $\ensuremath{\pi}\ensuremath{\alpha}$, in the absence of excitonic effects. In the case of InAs, the results obtained are in excellent agreement with the currently available experimental data. By revisiting experimental data on semiconductor superlattices, we show that $\ensuremath{\pi}\ensuremath{\alpha}$ is also a metric of their absorption when normalized to a single period. We conclude that the $\ensuremath{\pi}\ensuremath{\alpha}$ quantization is universal in semiconductor quantum wells provided that excitonic effects are weak and is therefore not specific to the zero-gap graphene case. The physical origin of this universality and its limits are discussed using analytical models that capture the main underlying physics of the lowest optical transitions in III-V and II-VI semiconductor quantum wells. These models show that the absorptance is ruled by $\ensuremath{\pi}\ensuremath{\alpha}$ independent of the material characteristics because of the presence of a dominant term in the Hamiltonian, linear in the wave vector $\mathbf{k}\phantom{\rule{4pt}{0ex}}(\ensuremath{\sim}\mathbf{V}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{k})$, which couples the conduction band to the valence bands. However, the prefactor in front of $\ensuremath{\pi}\ensuremath{\alpha}$ is not unity as in graphene due to the different nature of the electronic states. In particular, the spin-orbit coupling plays an important role in bringing the absorptance plateaus closer to integer values of $\ensuremath{\pi}\ensuremath{\alpha}$. The case of IV-VI semiconductor (PbSe) quantum wells characterized by a rocksalt lattice and multivalley physics is very similar to that of graphene, with the specification that a ``massful gap'' is formed around the Dirac points.

Bandgap Engineering of the II-IV, III-V and IV-VI Semiconductor Quantum Dots for Technological Applications

Bandgap Engineering of the II-IV, III-V and IV-VI Semiconductor Quantum Dots for Technological Applications

(Invited) Synthesis and Surface Engineering of III-V Semiconductor Quantum Dots for Infrared Optoelectronic Applications

Colloidal quantum dots (QDs) have widely been considered as emerging candidates for solution-processable optoelectronic devices. Especially, QDs that are optically active in NIR and SWIR spectral windows, offer a great promise for infrared emitters, infrared photodetectors, photovoltaics, and biological imaging applications. It is thus essential to develop high-quality infrared QDs without the use of heavy metals such as cadmium, lead, or mercury. Among various semiconducting materials, III-V compound semiconductors show great potential in practical optoelectronic applications not only due to their wide tunability in the infrared region, but also due to their excellent optical and electronic properties. However, the synthesis of III-V semiconductor-based QDs has been limited by the lack of understanding about the precursors of group V elements such as P, As and Sb. In this presentation, we would like to discuss about the pnictogen precursors to synthesize various III-V QDs and also the surface properties of the resultant QDs. By controlling the reaction process with the pnictogen precursors, we achieve size and stoichiometry-tunable III-V QDs including InP, InAs, InSb and alloy QDs. Based on structural, analytical, optical, and electrical characterization of the QDs, we study the effects of surface treatments on their optoelectronic properties. We demonstrate that III-V QD thin films are promising candidate materials for infrared optoelectronic devices. Figure 1

Sterically Encumbered Tris(trialkylsilyl) Phosphine Precursors for Quantum Dot Synthesis.

The synthesis of nanomaterials with a narrow size distribution is challenging, especially for III-V semiconductor nanoparticles (also known as quantum dots). Concerning phosphides, this issue has been largely attributed the use of overly reactive precursors. The problem is exacerbated due to the narrow range of competent reagents for III-V semiconductor syntheses. We report the use of sterically encumbered tris(triethylsilyl) phosphine and tris(tributylsilyl) phosphine for InP quantum dot (QD) synthesis among others. The hypothesis was that these reagents are less reactive than the near-ubiquitous precursor tris(trimethylsilyl) phosphine and can be used to create more homogeneous materials. It was found that the InP products' quantum yields and emission color saturation (fwhm) were improved, but not to the levels realized in CdSe QDs. Regardless, these reagents have other positive attributes; they are less pyrophoric and can be applied toward the synthesis of II-V semiconductors and organophosphorus compounds. Concerning safe practices, we demonstrate that ammonium bifluoride is an effective replacement for highly toxic HF for the post-treatment of III-V semiconductor quantum dots.

Berry phase and spin precession without magnetic fields in semiconductor quantum dots

We investigate electric field control of spin manipulation through Berry phase in III-V semiconductor quantum dots. By utilizing degenerate and non-degenerate perturbation theories, we diagonalize the total Hamiltonian of a semiconductor quantum dot and express the solution of time dependent Schrodinger equation in terms of complete and incomplete elliptic integrals of the second kind, respectively. This allows us to investigate the interplay between the Rashba and Dresselhaus spin-orbit couplings. In particular, we provide theoretical descriptions of several novel properties focusing on spin manipulation through (a) Berry phase, (b) geometric phase and (c) spin echo phenomenon followed by a strong beating patterns during the adiabatic transport of the quantum dots.

Direct Measurement of Hyperfine Shifts and Radio Frequency Manipulation of Nuclear Spins in Individual CdTe/ZnTe Quantum Dots.

We achieve direct detection of electron hyperfine shifts in individual CdTe/ZnTe quantum dots. For the previously inaccessible regime of strong magnetic fields B_{z}≳0.1 T, we demonstrate robust polarization of a few-hundred-particle nuclear spin bath, with an optical initialization time of ∼1 ms and polarization lifetime exceeding ∼1 s. Nuclear magnetic resonance spectroscopy of individual dots reveals strong electron-nuclear interactions characterized by Knight fields |B_{e}|≳50 mT, an order of magnitude stronger than in III-V semiconductor quantum dots. Our studies confirm II-VI semiconductor quantum dots as a promising platform for hybrid electron-nuclear spin qubit registers, combining the excellent optical properties comparable to III-V dots and the dilute nuclear spin environment similar to group-IV semiconductors.

Magnetogyrotropic reflection from quantum wells induced by bulk inversion asymmetry

Bulk inversion asymmetry (BIA) of III-V and II-VI semiconductor quantum wells is demonstrated by reflection experiments in magnetic field oriented in the structure plane. The linear in the magnetic field contribution to the reflection coefficients is measured at oblique incidence of $s$ and $p$ polarized light in vicinity of exciton resonances. We demonstrate that this contribution to the reflection is caused by magnetogyrotropy of quantum wells, i.e. by the terms in the optical response which are linear in both the magnetic field strength and light wavevector. Theory of magnetogyrotropic effects in light reflection is developed with account for linear in momentum BIA induced terms in the electron and hole effective Hamiltonians. Theoretical estimates agree with the experimental findings. We have found the electron BIA splitting constant in both GaAs and CdTe based quantum wells is about three times smaller than that for heavy holes.

Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance.

This Perspective outlines basic structural and optical properties of lead halide perovskite colloidal nanocrystals, highlighting differences and similarities between them and conventional II–VI and III–V semiconductor quantum dots. A detailed insight into two important issues inherent to lead halide perovskite nanocrystals then follows, namely, the advantages of defect tolerance and the necessity to improve their stability in environmental conditions. The defect tolerance of lead halide perovskites offers an impetus to search for similar attributes in other related heavy metal-free compounds. We discuss the origins of the significantly blue-shifted emission from CsPbBr3 nanocrystals and the synthetic strategies toward fabrication of stable perovskite nanocrystal materials with emission in the red and infrared parts of the optical spectrum, which are related to fabrication of mixed cation compounds guided by Goldschmidt tolerance factor considerations. We conclude with the view on perspectives of use of the colloidal perovskite nanocrystals for applications in backlighting of liquid-crystal TV displays.

Defect characterization of III-V semiconductor quantum dot devices for enhancing the radiation tolerance of space solar cells

Defect characterization of III-V semiconductor quantum dot devices for enhancing the radiation tolerance of space solar cells

Hyperfine coupling of hole and nuclear spins in symmetric (111)-grown GaAs quantum dots

In self assembled III-V semiconductor quantum dots, valence holes have longer spin coherence times than the conduction electrons, due to their weaker coupling to nuclear spin bath fluctuations. Prolonging hole spin stability relies on a better understanding of the hole to nuclear spin hyperfine coupling which we address both in experiment and theory in the symmetric (111) GaAs/AlGaAs droplet dots. In magnetic fields applied along the growth axis, we create a strong nuclear spin polarization detected through the positively charged trion X$^+$ Zeeman and Overhauser splittings. The observation of four clearly resolved photoluminescence lines - a unique property of the (111) nanosystems - allows us to measure separately the electron and hole contribution to the Overhauser shift. The hyperfine interaction for holes is found to be about five times weaker than that for electrons. Our theory shows that this ratio depends not only on intrinsic material properties but also on the dot shape and carrier confinement through the heavy-hole mixing, an opportunity for engineering the hole-nuclear spin interaction by tuning dot size and shape.

Photoluminescence monitoring of oxide formation and surface state passivation on InAs quantum dots exposed to water vapor

The room-temperature light emission of uncapped III-V semiconductor quantum dots is used to investigate the properties and evolution of the surface under exposure to a humid environment. Enhanced photoluminescence intensity resulting from exposure to polar molecules has already been reported; here we demonstrate that the external environment also has a relevant effect on the emission energy of quantum dots. Experimental results are interpreted on the basis of a model of the quantum system that takes into account the formation of oxide on pristine III-V surfaces and the presence of surface states. As a result of our study, we can clearly distinguish the effect of surface oxidation from that of surface state passivation on the emission of InAs surface quantum dots. This work sheds new light on the properties of semiconductor surface quantum dots as building blocks of novel and highly efficient sensing devices based on optical transduction.

Optical control of the spin of a magnetic atom in a semiconductor quantum dot

Abstract:The control of single spins in solids is a key but challenging step for any spin-based solid-state quantumcomputing device. Thanks to their expected long coherence time, localized spins on magnetic atoms in a semiconductor host could be an interesting media to store quantum information in the solid state. Optical probing and control of the spin of individual or pairs of Manganese (Mn) atoms (S = 5/2) have been obtained in II-VI and IIIV semiconductor quantum dots during the last years. In this paper, we review recently developed optical control experiments of the spin of an individual Mn atoms in II-VI semiconductor self-assembled or strain-free quantum dots (QDs).We first show that the fine structure of the Mn atom and especially a strained induced magnetic anisotropy is the main parameter controlling the spin memory of the magnetic atom at zero magnetic field. We then demonstrate that the energy of any spin state of a Mn atom or pairs of Mn atom can be independently tuned by using the optical Stark effect induced by a resonant laser field. The strong coupling with the resonant laser field modifies the Mn fine structure and consequently its dynamics.We then describe the spin dynamics of a Mn atom under this strong resonant optical excitation. In addition to standard optical pumping expected for a resonant excitation, we show that the Mn spin population can be trapped in the state which is resonantly excited. This effect is modeled considering the coherent spin dynamics of the coupled electronic and nuclear spin of the Mn atom optically dressed by a resonant laser field. Finally, we discuss the spin dynamics of a Mn atom in strain-free QDs and show that these structures should permit a fast optical coherent control of an individual Mn spin.