Structural dependency of optical excitation transfer via optical near-field interactions between semiconductor quantum dots

The distribution dependency of quantum dots was theoretically and experimentally investigated with respect to the basic properties optical excitation transfer via optical near-field interactions between quantum dots. The effects of three-dimensional structure and arraying precision of quantum dots on the signal transfer performance were analyzed. In addition, the quantum dot distribution dependency of the signal transfer performance was experimentally evaluated by using stacked CdSe quantum dots and an optical near-field fiber probe tip laminated with quantum dots serving as an output terminal, showing good agreement with theory. These results demonstrate the basic properties of signal transfer via optical near-field interactions and serve as guidelines for a nanostructure design optimized to attain the desired signal transfer performances.     

金属衬底上石墨烯的红外近场光学

对石墨烯/铜体系开展了系统性的近场光学实验研究,成功观测到了区别于铜衬底的、来自石墨烯的近场光学响应信号,发现在表面台阶几何参数相同的铜衬底上的不同石墨烯样品表现出了截然不同的近场光学响应.     

Tamper resistance in optical excitation transfer based on optical near-field interactions

We present tamper resistance in optical excitation transfer via optical near-field interactions based on the energy dissipation process occurring locally in nanometric devices such as quantum dots. A theoretical comparison with electrical systems is also shown, focusing on the required environmental conditions. Numerical simulations based on virtual photon models demonstrate high tamper resistance.     

Nanometric summation architecture based on optical near-field interaction between quantum dots

A nanoscale data summation architecture is proposed and experimentally demonstrated based on the optical near-field interaction between quantum dots. Based on local electromagnetic interactions between a few nanometric elements via optical near fields, we can combine multiple excitations at a certain quantum dot, which allows construction of a summation architecture. Summation plays a key role for content-addressable memory, which is one of the most important functions in optical networks.     

Magnetic-field-dependent excitation transfer in quantum wells of diluted magnetic semiconductor

We studied the excitation transfer in double quantum wells of a diluted magnetic semiconductor using a scanning near-field optical microscope at 7 K in external magnetic fields up to 9 T. In each quantum well, local energy minima are generated by local fluctuation of layer thickness and doping concentration of magnetic components. Excitons relax into the local energy minima and transfer between the minima via near-field optical interactions even across quantum wells toward stable sites at which to localize. We measured the intensity maps of near-field photoluminescence with spatial resolution estimated to be 30 nm under varying external magnetic fields. The measurement position reproducibility was confirmed by scanning tunneling microscope images. Analysis of the maps derived the magnetic-field dependence of the typical size of exciton-localization sites for each quantum well. Based on these results, we investigated the excitation transfer between the two quantum wells lying in different layers of the double quantum well system, and showed that the exciton transfer takes place at the two specific applied magnetic-field intensities that result in the crossing of Zeeman-split energy levels of the two different wells. We concluded that both the localization and the inter-quantum-well transfer of excitons are able to be controlled by an external magnetic field. This provides the basis for functional devices operating without any wiring.     

Nanophotonic droplet: a nanometric optical device consisting of size- and number-selective coupled quantum dots

Although recent advances in fabrication technologies have allowed the realization of highly accurate nanometric devices and systems, most approaches still lack uniformity and mass-production capability sufficient for practical use. We have previously demonstrated a novel technique for autonomously coupling heterogeneous quantum dots to induce particular optical responses based on a simple phonon-assisted photocuring method in which a mixture of quantum dots and photocurable polymer is irradiated with light. The cured polymer sequentially encapsulates coupled quantum dots, forming what we call a nanophotonic droplet. Recently, we found that each quantum dot in the mixture is preferably coupled with other quantum dots of similar size due to a size resonance effect of the optical near-field interactions between them. Moreover, every nanophotonic droplet is likely to contain the same number of coupled quantum dots. In this paper, we describe the basic mechanisms of autonomously fabricating nanophotonic droplets, and we examine the size- and number-selectivity of the quantum dots during their coupling process. The results from experiments show the uniformity of the optical properties of mass-produced nanophotonic droplets, revealed by emission from the contained coupled quantum dots, due to the fundamental characteristics of our method.     

Fixed-distance coupling and encapsulation of heterogeneous quantum dots using phonon-assisted photo-curing

We propose a novel method of coupling heterogeneous quantum dots at fixed distances and capsulating the coupled quantum dots by utilizing nanometric local curing of a photo-curable polymer caused by multistep electronic transitions based on a phonon-assisted optical near-field process between quantum dots. Because the coupling and the capsulating processes are triggered only when heterogeneous quantum dots floating in a solution closely approach each other to induce optical near-field interactions between them, the distances between the coupled quantum dots are physically guaranteed to be equal to the scale of the optical near fields. To experimentally verify our idea, we fabricated coupled quantum dots, consisting of CdSe and ZnO quantum dots and a UV-curable polymer. We also measured the photoluminescence properties due to the quantum-dot coupling and showed that the individual photoluminescences from the CdSe and ZnO quantum dots exhibited a trade-off relationship.     

Accordion lattice based on the Talbot effect

We introduce an idea of producing an optical lattice relied on the Talbot effect. Our alternative scheme is based on the interference of light behind a diffraction grating in the near-field regime. We demonstrate 1D and 2D optical lattices with the simulations and experiments. This Talbot optical lattice can be broadly used from quantum simulations to quantum information. The Talbot effect is usually used in lensless optical systems, therefore it provides small aberrations.     

Direct observation of optically forbidden energy transfer between CuCl quantum cubes via near-field optical spectroscopy

We report, for the first time, evidence of near-field energy transfer among CuCl quantum cubes using an ultrahigh-resolution near-field optical microscopy and spectroscopy in the near UV region at 15 K. The sample was high-density CuCl quantum cubes embedded in a NaCl matrix. Measured spatial distributions of the luminescence intensities from 4.6-nm and 6.3-nm quantum cubes clearly established anticorrelation features. This is thought to be a manifestation of the energy transfer from the lowest state of exciton in 4.6-nm quantum cubes to the first dipole-forbidden excited state of exciton in 6.3-nm quantum cubes, which is attributed to the resonant optical near-field interaction.     

Near-field infrared response of graphene on copper substrate

The electronic properties of graphene are very sensitive to its dielectric environment. The coupling to a metal substrate can give rise to many novel quantum effects in graphene, such as band renormalization and plasmons with unusual properties, which are of high technological interest. Infrared nanoimaging are very suitable for exploring these effects considering their energy and length scales. Here, we report near-field infrared nanoimaging studies of graphene on copper synthesized by chemical vapor deposition. Remarkably, our measurements reveal three different types of near-field optical responses of graphene, which are very distinct from the near-field edge fringes observed in the substrate. These results can be understood from the modification of optical conductivity of graphene due to its coupling with the substrate. Our work provides a framework for identifying the near-field response of graphene in graphene/metal systems and paves the way for studying their novel physics and potential applications.     

Two-Dimensional Near-Field Optical Spectroscopy in Magnetic Fields up to 4 T

We have developed a magnetic-field-type scanning near-field optical microscopy (SNOM) system, which enables us to obtain two-dimensional near-field images under magnetic fields up to 4 T. We have performed two-dimensional near-field scanning optical spectroscopy on (Cd,Mn) Te self-assembled quantum dots with this system.     

Coherent nonlinear optical response of single quantum dots studied by ultrafast near-field spectroscopy

The nonlinear response of single GaAs quantum dots is studied in femtosecond near-field pump-probe experiments. At negative time delays, transient reflectivity spectra show pronounced oscillatory structure around the quantum dot exciton line, providing the first evidence for a perturbed free induction decay of the excitonic polarization. Phase-disturbing Coulomb interactions between the excitonic polarization and continuum excitations dominate the optical nonlinearity on ultrafast time scales. A theoretical analysis based on the semiconductor Bloch equations accounts for this behavior.     

Characterisation of the influence of multi-quantum barrier reflectors within GaInP/AlGaInP quantum well lasers using near-field imaging techniques

Using a near-field scanning optical microscope, near-field photocurrent and topographic imaging has measured the effect on intrinsic electric fields and photocurrent propagation resulting from inserting multi-quantum barrier (MQB) super-lattices into quantum well lasers. Measurements on devices at two different excitation wavelengths have highlighted the sensitivity of the near-field optical technique. Strong correlations were seen in the photocurrent response of the multi-quantum barrier regions when compared with simulations made on the electric field generated within the structure. As a result, photocurrent attenuation was attributed to carrier confinement in these barrier regions when compared to a control sample. The measurements illustrate the effectiveness of the MQB, in addition to the sensitivity and power of the near-field photocurrent technique.     

Direct observation of optical excitation transfer based on resonant optical near-field interaction

This article reports the direct observation of long-distance optical excitation transfer based on resonant optical near-field interactions in randomly distributed quantum dots (QDs). We fabricated optical excitation transfer paths based on randomly distributed QDs by using CdSe/ZnS core?Cshell QDs and succeeded for the first time in obtaining output signals resulting from a unidirectional optical excitation transfer length of 2.4???m. Furthermore, we demonstrate that the optical excitation transfer occurs via the resonant excited levels of the QDs with a comparative experiment using non-resonant QDs. This excitation-transfer mechanism allows for intersecting, non-interacting nano-optical wires.     

Hierarchy in optical near-fields based on compositions of nanomaterials

Optical near-field interactions exhibit hierarchical responses in the nanometer scale allowing unique functions in nanophotonic systems. Such hierarchical properties in optical near-fields originate various physical entities in the nanometer scale. Engineering nanomaterial compositions, while maintaining geometrically equivalent conditions, leads to characteristic hierarchical responses. We experimentally demonstrate such material-dependent optical near-field hierarchy using core–shell-type nanostructures composed of gold and silver.     

Near-field scanning optical microscopy based on surface-enhanced Raman scattering

We describe a near-field optical microscopy technique based on the interaction of a probe molecule with the sample surface (e.g., with a flat metal surface) in the field of external optical radiation and consider the spontaneous Raman scattering characterized, in the presence of a metal surface, by the effective polarizability of the probe molecule, depending on the frequency and the distance to the sample surface. At certain distances from the probe molecule to the surface, the effective polarizability of this molecule (determined with allowance for the polarizing influence of the surface of a semi-infinite medium) at the Stokes frequency sharply increases in comparison to the quantum polarizability of an isolated molecule, which is indicative of the formation of optical near-field resonances. It is shown that the proposed method of near-field optical microscopy is characterized by high sensitivity and high spatial resolution (on the order of 1 Å).     

Superexchange-mediated magnetization dynamics with ultracold alkaline-earth atoms in an optical lattice

Superexchange and inter-orbital spin-exchange interactions are key ingredients for understanding(orbital) quantum magnetism in strongly correlated systems and have been realized in ultracold atomic gases.Here we study the spin dynamics of ultracold alkaline-earth atoms in an optical lattice when the two exchange interactions coexist.In the superexchange interaction dominating regime,we find that the time-resolved spin imbalance shows a remarkable modulated oscillation,which can be attributed to the interplay between local and nonlocal quantum mechanical exchange mechanisms.Moreover,the filling of the long-lived excited atoms affects the collapse and revival of the magnetization dynamics.These observations can be realized in state-dependent optical lattices combined with the state-of-the-art advances in optical lattice clock spectroscopy.     

A polarizing situation: Taking an in-plane perspective for next-generation near-field studies

By enabling the probing of light–matter interactions at the functionally relevant length scales of most materials, near-field optical imaging and spectroscopy accesses information that is unobtainable with other methods. The advent of apertureless techniques, which exploit the ultralocalized and enhanced near-fields created by sharp metallic tips or plasmonic nanoparticles, has resulted in rapid adoption of near-field approaches for studying novel materials and phenomena, with spatial resolution approaching sub-molecular levels. However, these approaches are generally limited by the dominant out-of-plane polarization response of apertureless tips, restricting the exploration and discovery of many material properties. This has led to recent design and fabrication breakthroughs in near-field tips engineered specifically for enhancing in-plane interactions with near-field light components. This mini-review provides a perspective on recent progress and emerging directions aimed at utilizing and controlling in-plane optical polarization, highlighting key application spaces where in-plane near-field tip responses have enabled recent advancements in the understanding and development of new nanostructured materials and devices.     

近场光学虚拟光探针的数值分析

虚拟光探针是基于近场光学隐失场干涉原理产生的一种非实体探针，可以应用于近场光学超高密度存储、纳米光刻、近场光学成像、光谱探测、纳米样品的近场光学操作等领域。本研究采用三维时间域有限差分（FDTD）方法对近场光学虚拟光探针的光场分布特性进行了数值模拟计算和比较，分析了孔的形状、大小及偏振态等因素对虚拟光探针光场分布的影响，研究结果表明虚拟光探针的通光效率较普通的纳米孔径光纤探针提高10^2-10^4倍；其光场分布的中间峰的半峰全宽（即虚拟光探针的尺寸）在一定距离范围内基本保持不变，从而可以解决近场光学系统中纳米间距控制的难题，避免光学头与介质的磁撞。优化虚拟光探针的设计参量能有效的抑制虚拟光探针中的旁瓣。文章还给出了应用虚拟探针实现高密度光存储的原理方案。     

Pressure-induced modulation of the confinement in self-organized quantum dots produced and detected by a near-field optical probe

Near-field optical probing, or nanoprobing, achieves spatial resolution that surpasses the diffraction limit of light and makes possible the luminescence imaging and spectroscopy of single quantum dots in dense arrays of dots. We use optical nanoprobing to study self-organized InGaAs quantum dots grown on (3 1 1)B oriented GaAs substrates. Here, we emphasize a new feature of nanoprobing: pressure-induced strain modulation near the surface. Operating in near-field optical excitation–collection mode, the probe makes contact with the surface and exerts direct pressure whose main effect is a compressive uniaxial strain under the probe. By adjusting the applied pressure, we modulate the local strain environment in and around a quantum dot, but still preserve the capability to capture its near-field luminescence. Nanoprobe pressure effects modify the confinement potential and radiative emission of single quantum dots, and the coupling strength between dots. This opens new possibilities for the study and control of the optical and electronic properties of single- and coupled-quantum dots.