Tuning the exciton binding energies in single self-assembled InGaAs/GaAs quantum dots by piezoelectric-induced biaxial stress

Quantum dots get a bright upgrade

Developing a bright, deterministic source of entangled photon pairs has been an outstanding scientific and technological challenge. Semiconductor quantum dots are a promising candidate for this task. A new device combining a circular Bragg resonator and a piezoelectric actuator achieves high brightness and entanglement fidelity simultaneously, overcoming previous limitations. This breakthrough enhances quantum dot applications in entanglement-based quantum communication protocols.

Surface Doping and Dual Nature of the Band Gap in Excitonic Insulator Ta2NiSe5

Excitons in semiconductors and molecules are widely utilized in photovoltaics and optoelectronics, and high-temperature coherent quantum states of excitons can be realized in artificial electron-hole bilayers and an exotic material of an excitonic insulator (EI). Here, we investigate the band gap evolution of a putative high-temperature EI Ta2NiSe5 upon surface doing by alkali adsorbates with angle-resolved photoemission and density functional theory (DFT) calculations. The conduction band of Ta2NiSe5 is filled by the charge transfer from alkali adsorbates, and the band gap decreases drastically upon the increase of metallic electron density. Our DFT calculation, however, reveals that there exist both structural and excitonic contributions to the band gap tuned. While electron doping reduces the band gap substantially, it alone is not enough to close the band gap. In contrast, the structural distortion induced by the alkali adsorbate plays a critical role in the gap closure. This work indicates a combined electronic and structural nature for the EI phase of the present system and the complexity of surface doping beyond charge transfer.

Optical gain and entanglement through dielectric confinement and electric field in InP quantum dots

Quantum dots are widely recognized for their advantageous light-emitting properties. Their excitonic fine structure along with the high quantum yields offers a wide range of possibilities for technological applications. However, especially for the case of colloidal QDs, there are still characteristics and properties which are not adequately controlled and downgrade their performance for applications which go far beyond the simple light emission. Such a challenging task is the ability to manipulate the energetic ordering of exciton and biexciton emission and subsequently control phenomena such as Auger recombination, optical gain and photon entanglement. In the present work we attempt to engineer this ordering for the case of InP QDs embedded in polymer matrix, by means of their size, the dielectric confinement and external electric fields. We employ well tested, state of the art theoretical methods, in order to explore the conditions under which the exciton-biexciton configuration creates the desired conditions either for optical gain or photon entanglement. Indeed, this appears to be feasible for QDs with small diameters (1 nm, 1.5 nm) embedded in a host material with high dielectric constant and additional external electric fields. These findings offer a new design principle which might be complementary to the well-established type II core-shell QDs approach for achieving electron-hole separation.

Biexciton dynamics in halide perovskite nanocrystals

Lead halide perovskite nanocrystals are attracting considerable interest as next-generation optoelectronic materials. Optical responses of nanocrystals are determined by excitons and exciton complexes such as trions and biexcitons. Understanding of their dynamics is indispensable for the optimal design of optoelectronic devices and the development of new functional properties. Here, we summarize the recent advances on the exciton and biexciton photophysics in lead halide perovskite nanocrystals revealed by femtosecond time-resolved spectroscopy and single-dot spectroscopy. We discuss the impact of the biexciton dynamics on controlling and improving the optical gain.

Double-Pulse Generation of Indistinguishable Single Photons with Optically Controlled Polarization

Single-photon sources play a key role in photonic quantum technologies. Semiconductor quantum dots can emit indistinguishable single photons under resonant excitation. However, the resonance fluorescence technique typically requires cross-polarization filtering, which causes a loss of the unpolarized quantum dot emission by 50%. To solve this problem, we demonstrate a method for generating indistinguishable single photons with optically controlled polarization by two laser pulses off-resonant with neutral exciton states. This scheme is realized by exciting the quantum dot to the biexciton state and subsequently driving the quantum dot to an exciton eigenstate. By combining with a magnetic field, we demonstrated the generation of photons with optically controlled polarization (the degree of polarization is 101(2)%), laser-neutral exciton detuning up to 0.81 meV, high single-photon purity (99.6(1)%), and indistinguishability (85(4)%). Laser pulses can be blocked using polarization and spectral filtering. Our work makes an important step toward indistinguishable single-photon sources with near-unity collection efficiency.

Vanishing fine structure splitting in highly asymmetric InAs/InP quantum dots without wetting layer

Contrary to simplified theoretical models, atomistic calculations presented here reveal that sufficiently large in-plane shape elongation of quantum dots can not only decrease, but even reverse the splitting of the two lowest optically active excitonic states. Such a surprising cancellation of bright-exciton splitting occurs for shape-anisotropic nanostructures with realistic elongation ratios, yet without a wetting layer, which plays here a vital role. However, this non-trivial effect due to shape-elongation is strongly diminished by alloy randomness resulting from intermixing of InAs quantum-dot material with the surrounding InP matrix. Alloying randomizes, and to some degree flattens the shape dependence of fine-structure splitting giving a practical justification for the application of simplified theories. Finally, we find that the dark-exciton spectra are rather weakly affected by alloying and are dominated by the effects of lateral elongation.

Advanced technologies for quantum photonic devices based on epitaxial quantum dots

Quantum photonic devices are candidates for realizing practical quantum computers and networks. The development of integrated quantum photonic devices can greatly benefit from the ability to incorporate different types of materials with complementary, superior optical or electrical properties on a single chip. Semiconductor quantum dots (QDs) serve as a core element in the emerging modern photonic quantum technologies by allowing on-demand generation of single-photons and entangled photon pairs. During each excitation cycle, there is one and only one emitted photon or photon pair. QD photonic devices are on the verge of unfolding for advanced quantum technology applications. In this review, we focus on the latest significant progress of QD photonic devices. We first discuss advanced technologies in QD growth, with special attention to droplet epitaxy and site-controlled QDs. Then we overview the wavelength engineering of QDs via strain tuning and quantum frequency conversion techniques. We extend our discussion to advanced optical excitation techniques recently developed for achieving the desired emission properties of QDs. Finally, the advances in heterogeneous integration of active quantum light-emitting devices and passive integrated photonic circuits are reviewed, in the context of realizing scalable quantum information processing chips.

Phonon-Induced Enhancement of Photon Entanglement in Quantum Dot-Cavity Systems

We report on simulations of the degree of polarization entanglement of photon pairs simultaneously emitted from a quantum dot-cavity system that demand revisiting the role of phonons. Since coherence is a fundamental precondition for entanglement and phonons are known to be a major source of decoherence, it seems unavoidable that phonons can only degrade entanglement. In contrast, we demonstrate that phonons can cause a degree of entanglement that even surpasses the corresponding value for the phonon-free case. In particular, we consider the situation of comparatively small biexciton binding energies and either finite exciton or cavity mode splitting. In both cases, combinations of the splitting and the dot-cavity coupling strength are found where the entanglement exhibits a nonmonotonic temperature dependence which enables entanglement above the phonon-free level in a finite parameter range. This unusual behavior can be explained by phonon-induced renormalizations of the dot-cavity coupling g in combination with a nonmonotonic dependence of the entanglement on g that is present already without phonons.

Slow and fast single photons from a quantum dot interacting with the excited state hyperfine structure of the Cesium D1-line

Hybrid interfaces between distinct quantum systems play a major role in the implementation of quantum networks. Quantum states have to be stored in memories to synchronize the photon arrival times for entanglement swapping by projective measurements in quantum repeaters or for entanglement purification. Here, we analyze the distortion of a single-photon wave packet propagating through a dispersive and absorptive medium with high spectral resolution. Single photons are generated from a single In(Ga)As quantum dot with its excitonic transition precisely set relative to the Cesium D₁ transition. The delay of spectral components of the single-photon wave packet with almost Fourier-limited width is investigated in detail with a 200 MHz narrow-band monolithic Fabry-Pérot resonator. Reflecting the excited state hyperfine structure of Cesium, “slow light” and “fast light” behavior is observed. As a step towards room-temperature alkali vapor memories, quantum dot photons are delayed for 5 ns by strong dispersion between the two 1.17 GHz hyperfine-split excited state transitions. Based on optical pumping on the hyperfine-split ground states, we propose a simple, all-optically controllable delay for synchronization of heralded narrow-band photons in a quantum network.

Scalable in operando strain tuning in nanophotonic waveguides enabling three-quantum-dot superradiance

The quest for an integrated quantum optics platform has motivated the field of semiconductor quantum dot research for two decades. Demonstrations of quantum light sources, single photon switches, transistors and spin-photon interfaces have become very advanced. Yet the fundamental problem that every quantum dot is different prevents integration and scaling beyond a few quantum dots. Here, we address this challenge by patterning strain via local phase transitions to selectively tune individual quantum dots that are embedded in a photonic architecture. The patterning is implemented with in operando laser crystallization of a thin HfO₂ film 'sheath' on the surface of a GaAs waveguide. Using this approach, we tune InAs quantum dot emission energies over the full inhomogeneous distribution with a step size down to the homogeneous linewidth and a spatial resolution better than 1 µm. Using these capabilities, we tune multiple quantum dots into resonance within the same waveguide and demonstrate a quantum interaction via superradiant emission from three quantum dots.

Hubbard excitons in two-dimensional nanomaterials

Excitons in two-dimensional nanomaterials are studied by solving the many-electron Hamiltonian with a configuration-interaction approach. It is shown that graphene or phosphorene nanoflakes can not accommodate any excitonic bound states if the long-range Coulomb interaction is suppressed when the systems are placed in a high-k dielectric environment or on a metal substrate. Hence it is revealed that an electron-hole pair created by an optical excitation does not always form an exciton even in a confined nanostructure. The negative exciton binding energy is found to exhibit distinct dependence on the strength of short-range Coulomb interaction as the system undergoes a phase transition from non-magnetic to anti-ferromagnetic. It is further shown that the electron-hole pair may form an exciton state only when the long-range Coulomb interaction is recovered in the nanoflakes.

Spin-Mechanical Coupling of an InAs Quantum Dot Embedded in a Mechanical Resonator

We demonstrate strain-induced coupling between a hole spin in a quantum dot and mechanical motion of a cantilever. The optical transitions of quantum dots integrated into GaAs mechanical resonators are measured synchronously with the motion of the driven resonators. In a Voigt magnetic field, both electron and hole spin splittings are measured, showing negligible change for the electron spin but a large change for the hole spin of up to 36%. This large effect is attributed to the stronger spin orbit interaction of holes compared to electrons.

Ferroelectric and Piezoelectric Effects on the Optical Process in Advanced Materials and Devices

Piezoelectric and ferroelectric materials have shown great potential for control of the optical process in emerging materials. There are three ways for them to impact on the optical process in various materials. They can act as external perturbations, such as ferroelectric gating and piezoelectric strain, to tune the optical properties of the materials and devices. Second, ferroelectricity and piezoelectricity as innate attributes may exist in some optoelectronic materials, which can couple with other functional features (e.g., semiconductor transport, photoexcitation, and photovoltaics) in the materials giving rise to unprecedented device characteristics. The last way is artificially introducing optical functionalities into ferroelectric and piezoelectric materials and devices, which provides an opportunity for investigating the intriguing interplay between the parameters (e.g., electric field, temperature, and strain) and the introduced optical properties. Here, the tuning strategies, fundamental mechanisms, and recent progress in ferroelectric and piezoelectric effects modulating the optical properties of a wide spectrum of materials, including lanthanide-doped phosphors, quantum dots, 2D materials, wurtzite-type semiconductors, and hybrid perovskites, are presented. Finally, the future outlook and challenges of this exciting field are suggested.

Temperature-Dependent Coercive Field Measured by a Quantum Dot Strain Gauge

Coercive fields of piezoelectric materials can be strongly influenced by environmental temperature. We investigate this influence using a heterostructure consisting of a single crystal piezoelectric film and a quantum dots containing membrane. Applying electric field leads to a physical deformation of the piezoelectric film, thereby inducing strain in the quantum dots and thus modifying their optical properties. The wavelength of the quantum dot emission shows butterfly-like loops, from which the coercive fields are directly derived. The results suggest that coercive fields at cryogenic temperatures are strongly increased, yielding values several tens of times larger than those at room temperature. We adapt a theoretical model to fit the measured data with very high agreement. Our work provides an efficient framework for predicting the properties of ferroelectric materials and advocating their practical applications, especially at low temperatures.

Semiconductor devices for entangled photon pair generation: a review

Entanglement is one of the most fascinating properties of quantum mechanical systems; when two particles are entangled the measurement of the properties of one of the two allows the properties of the other to be instantaneously known, whatever the distance separating them. In parallel with fundamental research on the foundations of quantum mechanics performed on complex experimental set-ups, we assist today with bourgeoning of quantum information technologies bound to exploit entanglement for a large variety of applications such as secure communications, metrology and computation. Among the different physical systems under investigation, those involving photonic components are likely to play a central role and in this context semiconductor materials exhibit a huge potential in terms of integration of several quantum components in miniature chips. In this article we review the recent progress in the development of semiconductor devices emitting entangled photons. We will present the physical processes allowing the generation of entanglement and the tools to characterize it; we will give an overview of major recent results of the last few years and highlight perspectives for future developments.

A bright triggered twin-photon source in the solid state

A non-classical light source emitting pairs of identical photons represents a versatile resource of interdisciplinary importance with applications in quantum optics and quantum biology. To date, photon twins have mostly been generated using parametric downconversion sources, relying on Poissonian number distributions, or atoms, exhibiting low emission rates. Here we propose and experimentally demonstrate the efficient, triggered generation of photon twins using the energy-degenerate biexciton-exciton radiative cascade of a single semiconductor quantum dot. Deterministically integrated within a microlens, this nanostructure emits highly correlated photon pairs, degenerate in energy and polarization, at a rate of up to (234±4) kHz. Furthermore, we verify a significant degree of photon indistinguishability and directly observe twin-photon emission by employing photon-number-resolving detectors, which enables the reconstruction of the emitted photon number distribution. Our work represents an important step towards the realization of efficient sources of twin-photon states on a fully scalable technology platform.

Excitonic complexes in GaN/(Al,Ga)N quantum dots

Here we report a theoretical investigation of excitonic complexes in polar GaN/(Al,Ga)N quantum dots (QDs). A sum rule between the binding energies of charged excitons is used to calculate the biexciton binding energy. The binding energies of excitonic complexes in GaN/AlN are shown to be strongly correlated to the QD size. Due to the large hole localization, the positively charged exciton energy is found to be always blueshifted compared to the exciton one. The negatively charged exciton and the biexciton energy can be blueshifted or redshifted according to the QD size. Increasing the size of GaN/AlN QDs makes the identification of charged excitons difficult, and the use of an Al_0.5Ga_0.5N barrier can be advantageous for clear identification. Our theoretical results for the binding energy of exciton complexes are also confronted with values deduced experimentally for InAs/GaAs QDs, confirming our theoretical prediction for charged excitonic complexes in GaN/(Al,Ga)N QDs. Finally, we realize that the trends of excitonic complexes in QDs are significantly related to competition between the local charge separation (whatever its origin) and the correlation effect. Following our findings, entangled photons pairs can be produced in QDs with careful control of their size in order to obtain zero exciton-biexciton energy separation.

Combining in-situ lithography with 3D printed solid immersion lenses for single quantum dot spectroscopy

In the current study, we report on the deterministic fabrication of solid immersion lenses (SILs) on lithographically pre-selected semiconductor quantum dots (QDs). We demonstrate the combination of state-of-the-art low-temperature in-situ photolithography and femtosecond 3D direct laser writing. Several QDs are pre-selected with a localization accuracy of less than 2 nm with low-temperature lithography and three-dimensional laser writing is then used to deterministically fabricate hemispherical lenses on top of the quantum emitter with a submicrometric precision. Due to the printed lenses, the QD light extraction efficiency is enhanced by a factor of 2, the pumping laser is focused more, and the signal-to-noise ratio is increased, leading to an improved localization accuracy of the QD to well below 1 nm. Furthermore, modifications of the QD properties, i.e. strain and variation of internal quantum efficiency induced by the printed lenses, are also reported.

Monolithically Integrated Microelectromechanical Systems for On-Chip Strain Engineering of Quantum Dots

Elastic strain fields based on single crystal piezoelectric elements represent an effective way for engineering the quantum dot (QD) emission with unrivaled precision and technological relevance. However, pioneering researches in this direction were mainly based on bulk piezoelectric substrates, which prevent the development of chip-scale devices. Here, we present a monolithically integrated Microelectromechanical systems (MEMS) device with great potential for on-chip quantum photonic applications. High-quality epitaxial PMN-PT thin films have been grown on SrTiO3 buffered Si and show excellent piezoelectric responses. Dense arrays of MEMS with small footprints are then fabricated based on these films, forming an on-chip strain tuning platform. After transferring the QD-containing nanomembranes onto these MEMS, the nonclassical emissions (e.g., single photons) from single QDs can be engineered by the strain fields. We envision that the strain tunable QD sources on the individually addressable and monolithically integrated MEMS pave the way toward complex quantum photonic applications on chip.

Experimental Demonstration of a Hybrid-Quantum-Emitter Producing Individual Entangled Photon Pairs in the Telecom Band

Quantum emitters generating individual entangled photon pairs (IEPP) have significant fundamental advantages over schemes that suffer from multiple photon emission, or schemes that require post-selection techniques or the use of photon-number discriminating detectors. Quantum dots embedded within nanowires (QD-NWs) represent one of the most promising candidate for quantum emitters that provide a high collection efficiency of photons. However, a quantum emitter that generates IEPP in the telecom band is still an issue demanding a prompt solution. Here, we demonstrate in principle that IEPPs in the telecom band can be created by combining a single QD-NW and a nonlinear crystal waveguide. The QD-NW system serves as the single photon source, and the emitted visible single photons are split into IEPPs at approximately 1.55 μm through the process of spontaneous parametric down conversion (SPDC) in a periodically poled lithium niobate (PPLN) waveguide. The compatibility of the QD-PPLN interface is the determinant factor in constructing this novel hybrid-quantum-emitter (HQE). Benefiting from the desirable optical properties of QD-NWs and the extremely high nonlinear conversion efficiency of PPLN waveguides, we successfully generate IEPPs in the telecom band with the polarization degree of freedom. The entanglement of the generated photon pairs is confirmed by the entanglement witness. Our experiment paves the way to producing HQEs inheriting the advantages of multiple systems.

Large and Uniform Optical Emission Shifts in Quantum Dots Strained along Their Growth Axis

We introduce a calibration method to quantify the impact of external mechanical stress on the emission wavelength of distinct quantum dots (QDs). Specifically, these emitters are integrated in a cross-section of a semiconductor core wire and experience a longitudinal strain that is induced by an amorphous capping shell. Detailed numerical simulations show that, thanks to the shell mechanical isotropy, the strain in the core is uniform, which enables a direct comparison of the QD responses. Moreover, the core strain is determined in situ by an optical measurement, yielding reliable values for the QD emission tuning slope. This calibration technique is applied to self-assembled InAs QDs submitted to incremental elongation along their growth axis. In contrast to recent studies conducted on similar QDs submitted to a uniaxial stress perpendicular to the growth direction, optical spectroscopy reveals up to ten times larger tuning slopes, with a moderate dispersion. These results highlight the importance of the stress direction to optimize the QD optical shift, with general implications, both in static and dynamic regimes. As such, they are in particular relevant for the development of wavelength-tunable single-photon sources or hybrid QD opto-mechanical systems.

Wavelength-tunable entangled photons from silicon-integrated III-V quantum dots

Many of the quantum information applications rely on indistinguishable sources of polarization-entangled photons. Semiconductor quantum dots are among the leading candidates for a deterministic entangled photon source; however, due to their random growth nature, it is impossible to find different quantum dots emitting entangled photons with identical wavelengths. The wavelength tunability has therefore become a fundamental requirement for a number of envisioned applications, for example, nesting different dots via the entanglement swapping and interfacing dots with cavities/atoms. Here we report the generation of wavelength-tunable entangled photons from on-chip integrated InAs/GaAs quantum dots. With a novel anisotropic strain engineering technique based on PMN-PT/silicon micro-electromechanical system, we can recover the quantum dot electronic symmetry at different exciton emission wavelengths. Together with a footprint of several hundred microns, our device facilitates the scalable integration of indistinguishable entangled photon sources on-chip, and therefore removes a major stumbling block to the quantum-dot-based solid-state quantum information platforms.

Optical response of a quantum dot–epoxy resin composite: effect of tensile strain

The structural applications of quantum dots (QDs) can be artificially realized through the preparation of QDs-based structural materials, which exhibit unique characteristics of photoluminescence (PL) in response to mechanical deformation.

Magnetic ordering induced giant optical property change in tetragonal BiFeO3

Magnetic ordering could have significant influence on band structures, spin-dependent transport, and other important properties of materials. Its measurement, especially for the case of antiferromagnetic (AFM) ordering, however, is generally difficult to be achieved. Here we demonstrate the feasibility of magnetic ordering detection using a noncontact and nondestructive optical method. Taking the tetragonal BiFeO3 (BFO) as an example and combining density functional theory calculations with tight-binding models, we find that when BFO changes from C1-type to G-type AFM phase, the top of valance band shifts from the Z point to Γ point, which makes the original direct band gap become indirect. This can be explained by Slater-Koster parameters using the Harrison approach. The impact of magnetic ordering on band dispersion dramatically changes the optical properties. For the linear ones, the energy shift of the optical band gap could be as large as 0.4 eV. As for the nonlinear ones, the change is even larger. The second-harmonic generation coefficient d33 of G-AFM becomes more than 13 times smaller than that of C1-AFM case. Finally, we propose a practical way to distinguish the two AFM phases of BFO using the optical method, which is of great importance in next-generation information storage technologies.

Strain-Induced Spatial and Spectral Isolation of Quantum Emitters in Mono- and Bilayer WSe2

Two-dimensional transition metal dichalcogenide semiconductors are intriguing hosts for quantum light sources due to their unique optoelectronic properties. Here, we report that strain gradients, either unintentionally induced or generated by substrate patterning, result in spatially and spectrally isolated quantum emitters in mono- and bilayer WSe2. By correlating localized excitons with localized strain variations, we show that the quantum emitter emission energy can be red-tuned up to a remarkable ∼170 meV. We probe the fine-structure, magneto-optics, and second-order coherence of a strained emitter. These results raise the prospect of strain-engineering quantum emitter properties and deterministically creating arrays of quantum emitters in two-dimensional semiconductors.

Towards Scalable Entangled Photon Sources with Self-Assembled InAs/GaAs Quantum Dots

The biexciton cascade process in self-assembled quantum dots (QDs) provides an ideal system for realizing deterministic entangled photon-pair sources, which are essential to quantum information science. The entangled photon pairs have recently been generated in experiments after eliminating the fine-structure splitting (FSS) of excitons using a number of different methods. Thus far, however, QD-based sources of entangled photons have not been scalable because the wavelengths of QDs differ from dot to dot. Here, we propose a wavelength-tunable entangled photon emitter mounted on a three-dimensional stressor, in which the FSS and exciton energy can be tuned independently, thereby enabling photon entanglement between dissimilar QDs. We confirm these results via atomistic pseudopotential calculations. This provides a first step towards future realization of scalable entangled photon generators for quantum information applications.

Energy-tunable sources of entangled photons: a viable concept for solid-state-based quantum relays

We propose a new method of generating triggered entangled photon pairs with wavelength on demand. The method uses a microstructured semiconductor-piezoelectric device capable of dynamically reshaping the electronic properties of self-assembled quantum dots (QDs) via anisotropic strain engineering. Theoretical models based on k·p theory in combination with finite-element calculations show that the energy of the polarization-entangled photons emitted by QDs can be tuned in a range larger than 100 meV without affecting the degree of entanglement of the quantum source. These results pave the way towards the deterministic implementation of QD entanglement resources in all-electrically-controlled solid-state-based quantum relays.

Single photons on-demand from light-hole excitons in strain-engineered quantum dots

We demonstrate for the first time on-demand and wavelength-tunable single-photon emission from light-hole (LH) excitons in strain engineered GaAs quantum dots (QDs). The LH photon emission from tensile-strained GaAs QDs is systematically investigated with polarization-resolved, power-dependent photoluminescence spectroscopy, and photon-correlation measurements. By integrating QD-containing nanomembranes onto a piezo-actuator and driving single QDs with picosecond laser pulses, we achieve triggered and wavelength-tunable LH single-photon emission. Fourier transform spectroscopy is also performed, from which the coherence time of the LH single-photon emission is studied. We envision that this new type of LH exciton-based single-photon source (SPS) can be applied to realize an all-semiconductor based quantum interface in distributed quantum networks [Phys. Rev. Lett. 2008, 100, 096602].

Theory of strain tuning fine structure splitting in self-assembled InAs/GaAs quantum dots

We analytically derive the change in exciton fine structure splitting (FSS) under external stresses in self-assembled InAs/GaAs quantum dots, using the Bir-Pikus model. We find that the FSS change is mainly due to the strain-induced valence band mixing and valence-conduction band coupling. The exciton polarization angle under strain is determined by the argument of the electron-hole off-diagonal exchange integrals. The theory agrees well with the empirical pseudopotential calculations.

Tailoring the exciton fine structure of cadmium selenide nanocrystals with shape anisotropy and magnetic field

We use nominally spheroidal CdSe nanocrystals with a zinc blende crystal structure to study how shape perturbations lift the energy degeneracies of the band-edge exciton. Nanocrystals with a low degree of symmetry exhibit splitting of both upper and lower bright state degeneracies due to valence band mixing combined with the isotropic exchange interaction, allowing active control of the level splitting with a magnetic field. Asymmetry-induced splitting of the bright states is used to reveal the entire 8-state band-edge fine structure, enabling complete comparison with band-edge exciton models.

Single-photon property characterization of 1.3 μm emissions from InAs/GaAs quantum dots using silicon avalanche photodiodes

We developed a new approach to test the single-photon emissions of semiconductor quantum dots (QDs) in the optical communication band. A diamond-anvil cell pressure device was used for blue-shifting the 1.3 μm emissions of InAs/GaAs QDs to 0.9 μm for detection by silicon avalanche photodiodes. The obtained g⁽²⁾(0) values from the second-order autocorrelation function measurements of several QD emissions at 6.58 GPa were less than 0.3, indicating that this approach provides a convenient and efficient method of characterizing 1.3 μm single-photon source based on semiconductor materials.

Valence band offset, strain and shape effects on confined states in self-assembled InAs/InP and InAs/GaAs quantum dots

I present a systematic study of self-assembled InAs/InP and InAs/GaAs quantum dot single-particle and many-body properties as a function of the quantum dot-surrounding matrix valence band offset. I use an atomistic, empirical tight-binding approach and perform numerically demanding calculations for half-million-atom nanosystems. I demonstrate that the overall confinement in quantum dots is a non-trivial interplay of two key factors: strain effects and the valence band offset. I show that strain effects determine both the peculiar structure of confined hole states of lens type InAs/GaAs quantum dots and the characteristic 'shell-like' structure of confined hole states in the commonly considered 'low-strain' lens type InAs/InP quantum dot. I also demonstrate that strain leads to single-band-like behavior of hole states of disk type ('indium flushed') InAs/GaAs and InAs/InP quantum dots. I show how strain and valence band offset affect quantum dot many-body properties: the excitonic fine structure, an important factor for efficient entangled photon pair generation, and the biexciton and charged exciton binding energies.

Coherent piezoelectric strain transfer to thick epitaxial ferromagnetic films with large lattice mismatch

Strain control of epitaxial films using piezoelectric substrates has recently attracted significant scientific interest. Despite its potential as a powerful test bed for strain-related physical phenomena and strain-driven electronic, magnetic, and optical technologies, detailed studies on the efficiency and uniformity of piezoelectric strain transfer are scarce. Here, we demonstrate that full and uniform piezoelectric strain transfer to epitaxial films is not limited to systems with small lattice mismatch or limited film thickness. Detailed transmission electron microscopy (TEM) and x-ray diffraction (XRD) measurements of 100 nm thick CoFe(2)O(4) and La(2/3)Sr(1/3)MnO(3) epitaxial films on piezoelectric 0.72Pb(Mg(1/3)Nb(2/3))O(3)-0.28PbTiO(3) substrates (+4.3% and -3.8% lattice mismatch) indicate that misfit dislocations near the interface do not hamper the transfer of piezoelectric strain. Instead, the epitaxial magnetic oxide films and PMN-PT substrates are strained coherently and their lattice parameters change linearly as a function of applied electric field when their remnant growth-induced strain state is negligible. As a result, ferromagnetic properties such as the coercive field, saturation magnetization, and Curie temperature can be reversibly tuned by electrical means. The observation of efficient piezoelectric strain transfer in large-mismatch heteroepitaxial structures opens up new possibilities for the engineering of strain-controlled physical properties in a broad class of hybrid material systems.

A nanomembrane-based wavelength-tunable high-speed single-photon-emitting diode

We demonstrate an all-electrically operated wavelength-tunable on demand single-photon source for the first time. The device consists of a light-emitting diode in the form of a semiconductor nanomembrane containing self-assembled quantum dots integrated onto a piezoelectric crystal. Triggered single photons are generated via injection of ultrashort electrical pulses into the diode, while their energy can be precisely tuned over a broad range by varying the voltage applied to the piezoelectric crystal. High speed operation of this single-photon-emitting diode up to 0.8 GHz is demonstrated. These results represent an important step toward the realization of electrically driven sources of indistinguishable photons on demand.

Controlling a nanowire quantum dot band gap using a straining dielectric envelope

We tune the emission wavelength of an InAsP quantum dot in an InP nanowire over 200 meV by depositing a SiO(2) envelope using plasma-enhanced chemical vapor deposition without deterioration of the optical quality. This SiO(2) envelope generates a controlled static strain field. Both red and blue shift can be easily achieved by controlling the deposition conditions of the SiO(2). Using atomistic empirical tight-binding calculations, we investigate the effect of strain on a quantum dot band structure for different compositions, shape, and crystal orientations. From the calculations, we estimate the applied strain in our experiment. This enables engineering of the band gap in nanowires with unprecedented possibilities to extend the application range of nanowire devices.

Temperature dependent empirical pseudopotential theory for self-assembled quantum dots

We develop a temperature dependent empirical pseudopotential theory to study the electronic and optical properties of self-assembled quantum dots (QDs) at finite temperature. The theory takes the effects of both lattice expansion and lattice vibration into account. We apply the theory to InAs/GaAs QDs. For the unstrained InAs/GaAs heterostructure, the conduction band offset increases whereas the valence band offset decreases with increasing temperature, and there is a type-I to type-II transition at approximately 135 K. Yet, for InAs/GaAs QDs, the holes are still localized in the QDs even at room temperature, because the large lattice mismatch between InAs and GaAs greatly enhances the valence band offset. The single-particle energy levels in the QDs show a strong temperature dependence due to the change of confinement potentials. Because of the changes of the band offsets, the electron wavefunctions confined in QDs increase by about 1-5%, whereas the hole wavefunctions decrease by about 30-40% when the temperature increases from 0 to 300 K. The calculated recombination energies of excitons, biexcitons and charged excitons show red shifts with increasing temperature which are in excellent agreement with available experimental data.

Universal recovery of the energy-level degeneracy of bright excitons in InGaAs quantum dots without a structure symmetry

The lack of structural symmetry which usually characterizes semiconductor quantum dots lifts the energetic degeneracy of the bright excitonic states and hampers severely their use as high-fidelity sources of entangled photons. We demonstrate experimentally and theoretically that it is always possible to restore the excitonic degeneracy by the simultaneous application of large strain and electric fields. This is achieved by using one external perturbation to align the polarization of the exciton emission along the axis of the second perturbation, which then erases completely the energy splitting of the states. This result, which holds for any quantum dot structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum technology.

Experimental methods of post-growth-tuning of the excitonic fine structure splitting in semiconductor quantum dots

Deterministic sources of polarization entangled photon pairs on demand are considered as important building block for quantum communication technology. It has been demonstrated that semiconductor quantum dots (QDs), exhibiting a sufficiently small excitonic fine structure splitting (FSS) can be used as triggered, on-chip sources of polarization entangled photon pairs. As-grown QDs usually do not exhibit the required values of the FSS, making the availability of post-growth tuning techniques highly desired. This article reviews the effect of different post-growth treatments and external fields on the FSS such as thermal annealing, magnetic fields, the optical Stark effect, electric fields and anisotropic stress. As a consequence of the tuning of the FSS for some tuning techniques a rotation of the polarization of the emitted light is observed. The joint modification of polarization orientation and FSS can be described by an anticrossing of the bright excitonic states.

Nanomembrane quantum-light-emitting diodes integrated onto piezoelectric actuators

We integrate resonant-cavity light-emitting diodes containing quantum dots onto substrates with giant piezoelectric response. Via strain, the energy of the photons emitted by the diode can be precisely controlled during electrical injection over a spectral range larger than 20 meV. Simultaneously, the exciton fine-structure-splitting and the biexciton binding energy can be tuned to the values required for entangled photon generation.

Influence of substrate orientation on exciton fine structure splitting of InAs/InP nanowire quantum dots

: In this paper, we use an atomistic approach to investigate strain distributions, single particle and many body electronic properties of InAs/InP nanowire quantum dots with substrate orientation varying from [111] to high-index [119], and compared with [001] case. We show that single particle gap for high-index [11k] substrates is increased with respect to [111] and [001] cases, and oscillates with the substrate index due to faceting effects. Surprisingly, the overall shell-like structure of single particle states is preserved even for highly facetted, high-index substrates. On the contrary, we demonstrate that besides two limiting high-symmetry cases, [001] and [111], the bright exciton splitting varies strongly with substrate orientation. For [112]-oriented substrate, the fine structure splitting reaches maximum due to crystal lattice anisotropy despite fully cylindrical isotropic shape of nanowire quantum dot.

Thinning and shaping solid films into functional and integrative nanomembranes

Conventional solid films on certain substrates play a crucial role in various applications, for example in flat panel displays, silicon technology, and protective coatings. Recently, tremendous attention has been directed toward the thinning and shaping of solids into so-called nanomembranes, offering a unique and fantastic platform for research in nanoscience and nanotechnology. In this Review, a conceptual description of nanomembranes is introduced and a series of examples demonstrate their great potential for future applications. The thinning of nanomembranes indeed offers another strategy to fabricate nanomaterials, which can be integrated onto a chip and exhibit valuable properties (e.g. giant persistent photoconductivity and thermoelectric property). Furthermore, the stretching of nanomembranes enables a macroscale route for tuning the physical properties of the membranes at the nanoscale. The process by which nanomembranes release from a substrate presents several approaches to shaping nanomembranes into three-dimensional architectures, such as rolled-up tubes, wrinkles, and the resulting channels, which can provide fascinating applications in electronics, mechanics, fluidics, and photonics. Nanomembranes as a new type of nanomaterial promise to be an attractive direction for nanoresearch.

Laser location and manipulation of a single quantum tunneling channel in an InAs quantum dot

We use a femtowatt focused laser beam to locate and manipulate a single quantum tunneling channel associated with an individual InAs quantum dot within an ensemble of dots. The intensity of the directed laser beam tunes the tunneling current through the targeted dot with an effective optical gain of 10(7) and modifies the curvature of the dot's confining potential and the spatial extent of its ground state electron eigenfunction. These observations are explained by the effect of photocreated hole charges which become bound close to the targeted dot, thus acting as an optically induced gate electrode.

Dependence of the redshifted and blueshifted photoluminescence spectra of single In(x)Ga(1-x)As/GaAs quantum dots on the applied uniaxial stress

We apply external uniaxial stress to tailor the optical properties of In(x)Ga(1-x)As/GaAs quantum dots. Unexpectedly, the emission energy of single quantum dots controllably shifts to both higher and lower energies under tensile strain. Theoretical calculations using a million atom empirical pseudopotential many-body method indicate that the shifting direction and magnitude depend on the lateral extension and more interestingly on the gallium content of the quantum dots. Our experimental results are in good agreement with the underlying theory.

Stretchable graphene: a close look at fundamental parameters through biaxial straining

Tunable biaxial stresses, both tensile and compressive, are applied to a single layer graphene by utilizing piezoelectric actuators. The Gruneisen parameters for the phonons responsible for the D, G, 2D and 2D' peaks are studied. The results show that the D peak is composed of two peaks, unambiguously revealing that the 2D peak frequency (omega(2D)) is not exactly twice that of the D peak (omega(D)). This finding is confirmed by varying the biaxial strain of the graphene, from which we observe that the shift of omega(2D)/2 and omega(D) are different. The employed technique allows a detailed study of the interplay between the graphene geometrical structures and its electronic properties.

Effect of mechanical strain on the optical properties of quantum dots: controlling exciton shape, orientation, and phase with a mechanical strain

We show how a nanomechanical strain can be used to dynamically reengineer the optics of quantum dots, giving a tool to manipulate mechanoexciton shape, orientation, fine structure splitting, and optical transitions, transfer carriers between dots, and interact qubits for quantum processing. Most importantly, a nanomechanical strain reengineers both the magnitude and phase of the exciton exchange coupling to tune exchange splittings, change the phase of spin mixing, and rotate the polarization of mechanoexcitons, providing phase and energy control of excitons.

Resolved sideband emission of InAs/GaAs quantum dots strained by surface acoustic waves

The dynamic response of InAs/GaAs self-assembled quantum dots (QDs) to strain is studied experimentally by periodically modulating the QDs with a surface acoustic wave and measuring the QD fluorescence with photoluminescence and resonant spectroscopy. When the acoustic frequency is larger than the QD linewidth, we resolve phonon sidebands in the QD fluorescence spectrum. Using a resonant pump laser, we have demonstrated optical frequency conversion via the dynamically modulated QD, which is the physical mechanism underlying laser sideband cooling a nanomechanical resonator by means of an embedded QD.