Session U65: Superlattices and Nanostructures III: Optical Phenomena

Stable White Light-Emitting Diodes using Metal Halide Perovskites

Inorganic lead halide perovskite nanocrystals (NCs) are emerging as attractive materials for their applications in modern lighting technologies due to their excellent optoelectronic properties. However, the inclusion of toxic element (lead) and stability issues in the perovskite NCs hinder their practical applications in the devices. Here, we report on the transformation of lead halide perovskite NCs to lead depleted (by 20 %) perovskite NCs and incorporation of lead depleted perovskite NCs in the commercially available polymers. As a result, the composites retain the excellent optical properties without degrading photoluminescence quantum yield. Furthermore, we have fabricated white light-emitting diodes (WLEDs) using these composites. Bright white light is achieved with superior color quality. Thus, this work demonstrates that the halide perovskite NCs are promising alternatives to conventional phosphors for the fabrication of efficient, less-toxic, and stable WLEDs.   

Switchable Excitonic Circular Polarization in CdSe/CdMnS Nanoplatelets with Bilayer Core and Magnetically Doped Shell

We have utilized time-resolved photoluminescence (TRPL) to study the excitonic circular polarization from CdSe/CdMnS core/shell nanoplatelets (NPLs) with a bilayer core. This technique allows detailed study of the emission dynamics as a function of magnetic field, temperature, doping concentration, and excitation wavelength.
In the presence of an external magnetic field, pulsed excitation below the shell gap results in near-zero circular polarization at all times. In contrast, pulsed excitation with a photon energy larger than the shell gap results in a rapid (100 ps) build-up of the excitonic circular polarization which subsequently remains constant at a level of up to 40%.
We explore a model that describes these results; we also discuss possible applications.   

Controlling Linear and Nonlinear Optical Properties of Boron Nitride with Moiré Superlattices

Van der Waals materials enable the creation of artificial heterostructures with exciting new emergent properties. Recently, the twist angle between adjacent layers has proved to be an important control parameter in these stacks. In this work, we use conventional dry-transfer techniques to fabricate small-twist-angle few-layer hexagonal boron nitride (hBN) heterostructures. We study the resulting moiré superlattice with a variety of linear and nonlinear atomic force microscopy probes. These techniques yield a local variation of the transverse-optical-phonon oscillator strength, near-field second harmonic generation, and the piezoelectric response. These combined demonstrations of hBN moiré physics implicate local strain as an important tuning parameter for the linear and nonlinear optical properties of Van der Waals materials.   

Photoluminescence study of non-polar m-plane InGaN and near strain-balanced AlGaN/InGaN superlattices

We present the first detailed study of the temperature-dependence of photoluminescence (PL) from m-plane bulk In_xGa_1-xN (x<0.25) layers and near strain-balanced AlGaN/InGaN superlattices grown by plasma-assisted molecular-beam epitaxy on free standing m-plane GaN substrates. The experimental PL peak positions were found to deviate from inter-band transition energies calculated using structural parameters obtained from high-resolution x-ray diffraction. The effect is stronger for superlattices than for bulk InGaN films. The temperature dependence of PL suggests the discrepancy is due to localization centers in InGaN. The low-temperature PL linewidths are narrower than some published results for both In_xGa_1-xN (x<0.1) and Al_0.2Ga_0.8N/In_0.09Ga_0.91N superlattices indicating lower alloy inhomogeneity than previously reported. We also observed the PL intensity drops dramatically as temperature increases. This suggests the presence of a high-density of non-radiative defects. We will describe our efforts to optimize growth conditions to increase PL efficiency and reduce PL linewidth at room temperature.   

Backward output-side-grooves regulated light wave resonance in a subwavelength metallic slit and an ultrahigh energy accumulator

Featuring conceptual breakthroughs in subwavelength wave mechanics, our work studies the output-side groove effect and finds a backward coupling mechanism. The transmitted light is scattered by the grooves and then re-enters into the slit. In one case study, this causes a phase delay of the reflected wave and is considered a reduction in the resonant film thickness. The traveling wave in the slit can be more in phase to significantly enhance the transmission. For another arrangement of the grooves, we find that the wave reflection inside the slit is raised up to 100% due to backward coupling. As the same arrangement of the grooves are patterned at the entrance side of the slit, an incident wave on the slit will be trapped in roundtrips after the transmission through the entrance. At resonance, the roundtrip wave is superposed constructively. Our analysis yields that the intensity of the time-averaged energy can be five million times that of the incident wave. This finding is groundbreaking in subwavelength optics and should have many applications in nonlinear optics, particle trapping, biomedicine, etc.   

Nonreciprocal spatio-temporal modulated metasurfaces

Spatio-temporal modulated metasurfaces (STMMs) are dynamic two-dimensional arrays of sub-wavelength resonators with arbitrary reflection amplitude and phase reconfigurability, and have the potential to revolutionize fundamental and applied photonics. Just as their static counterparts, STMMs can also enable arbitrary wave-front engineering with the key advantage of on-demand reconfigurability and control of the frequency and momentum harmonic contents of scattered waves. In this talk I will report our recent theory and experimental advances in STMMs for dynamical wave-front shaping and nonreciprocal propagation of free space electromagnetic waves. Our experimental measurements reveal on-demand wave-front control of frequency conversion processes. We also demonstrate maximum violation of Lorentz reciprocity in both beam steering and focusing due to nonreciprocal excitation of surface waves. We develop an analytical generalized Bloch-Floquet theory valid for arbitrary modulations, providing excellent agreement with the experiments.   

Bright magnetic dipole radiation from two-dimensional lead halide perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) comprise a versatile class of solution-processable materials with outstanding optoelectronic properties. Analogous to conventional semiconductors, light-matter interactions in 2D HOIPs have been completely described within the electric dipole (ED) approximation. Here, using energy-momentum spectroscopies, we demonstrate that the ED approximation insufficiently describes light-matter interactions in 2D HOIPs. We identify an optical transition in 2D HOIPs that exhibits striking multipolar radiation patterns at an energy 90 meV below the primary exciton emission. The transition is evidently intrinsic in origin and arises from an unconventionally fast magnetic dipole (MD) transition from a polaron-like excited state. These results suggest the presence of even-parity excited state symmetries that have not yet been established, and represent the first observation of MD optical interactions in a bulk crystal. In addition to being of fundamental interest, this demonstration may be crucial for resolving open questions regarding the previously observed bounty of complex optical signatures in 2D HOIPs, and may have implications for quantum information applications based on “dark” excitons in 2D materials.   

Photon circulation in photonic structures integrated with quantum dots - a consequence of multi-path geometry

The typical route to achieve circulation of photons is via artificial gauge fields. We show that even without any artificial gauge field, by using multiple path geometry such as, that of a triangle and the quantum nature of bosonic particles, the steady state photonic current can circulate in a photonic structure integrated with quantum dots. Our system is made up of three quantum dots which are connected in a triangular geometry, this geometry is embedded in photonic crystal and waveguide. The photon-exciton interaction limits the number of photons that can be transferred between the quantum dots, a phenomenon also called photon blockade. We use the Lindblad form of the quantum optical master equation to calculate the steady state currents with the maximal photon number per site set by the photon blockade. Our results show that the photon circulations have a non-trivial dependence not only on the internal system parameters such as the tunneling coefficient, or the effective photon-photon interaction but also on external parameters such as the system-reservoir coupling. This system can be tailored to function as a density independent local pump or memory device.   

Temperature-dependent photoluminescence of carbon quantum dots

Temperature-dependent photoluminescence (PL) and time-resolved PL (TRPL) of carbon quantum dots (CQD) were investigated to explain the possible source of luminescence. XPS, PL, and UV-Vis absorption measurements suggest that the nitrogen-related functional groups, emission, and absorption depend on the amount of the CQD in the film. The energy loss between the absorption and emission spectra indicates the presence of vibrational relaxation in CQD films. The 0-0 transition peaks of the CQD films exhibit blueshift with faster decay time as temperature increases. This temperature-dependent blueshift and lifetimes possibly indicate the presence of thermally activated hopping sites which was also seen in another organic semiconductors. These hopping sites are related to nitrogen-based functional groups which are confirmed by the presence of additional deconvoluted peaks in the C1s high resolution XPS measurements. These measurements show that the presence of the conjugated functional groups is the primary reason for the exciton diffusion dynamics in the CQD. However, the excessive amount of CQD in the film results increases reabsorption and re-emission process. Thus, limiting the temperature-dependent PL and TRPL behaviors of films with larger amount of CQD.   

Ultra-thin film nanophotonics device on various optics application

The vast majority of optical metamaterial are fabricated on rigid substrates. Ultra-thin thickness and resulting mechanical flexibility in nanophotonics is the key attribute of various optics device including flexible imaging/display arrays, wearable photonics device, spectrometer, mechanics drive holography, multi-layer stacking Photonic device. Our fabrication techniques majorly depend on stamp transfer of dielectric material or direct patterning on flexible substrate. The significant difference is that fabricated device on past researches is all on thick polymer substrate. When direct bond such kind of flexible chip to any other devices, the thick substrate will damage the optical properties because of the scattering and reflection. Our flexible substrate is 200nm - 3 um PMMA, the thickness of which can be precisely controlled. The subwavelength thickness of this PMMA film will primarily reduce scattering and reflection optical reflection, which it will also protect the device surface. Furthermore, the ultra-thin PMMA layer is easy to remove after sticking the metasurface + PMMA to devices by special removing tape. Our ultra-thin PMMA substrate flexible device enables direct integration of optical metasurface to various optical devices like another metasuraface.   

Extraordinary Transmission in Coupled Nano-Bridged Nanosphere Quasi-Complimentary Arrays

Ultra-sensitive structures can be based on percolation, which at the threshold radically change their response (e.g. conduction, transmission, etc.). However, due to the critical nature, such structures are very unstable at the threshold. Here, we demonstrate that the stability can be restored, with minimal loss of sensitivity, by engineering weak links between the units of a percolation system. Our system consists of a pair of strongly coupled complementary arrays made of metallic films, obtained by depositing plasmonic metal on a periodic array of spheres with submicron dimensions through a modified standard nanosphere lithography (NSL). In an earlier work we demonstrated, that the modification of NSL leads to the formation inter-sphere nano-bridges, which after metallic deposition work as the weak links. We simulate plasmonics of these arrays, demonstrating ultrahigh sensitivity and stability of the extraordinary optical transmission (EOT) of this structure near the percolation threshold.   

Exciton–phonon coupling in resonant Raman scattering: intrinsic attributes and extrinsic tunability in ZnTe of different dimensionalities

The dependence of electron-phonon coupling (EPC) on nanostructure size has been controversial over three decades. Often, the EPC was probed by resonant Raman scattering (RRS) using the 2LO to 1LO intensity ratio R21 to extract the Huang-Rhys factor (S) by applying Albrecht’s theory, where the bulk reference S was calculated using a theoretical model developed for a bound exciton with Fröhlich interaction. We show that in ZnTe, in contrast to the previous reports, R21 exhibits a much larger intrinsic value and minimal change from bulk to 30 nm nanowire [1], indicating previously reported size dependences were likely affected by extrinsic mechanisms. Indeed, the ratio can be tuned extrinsically over one order in magnitude controllably either during or post growth, allowing for programing EPC in nanoscale devices. We point out that R21 is not directly related to Huang-Rhys factor, lattice relaxation is minimal for bulk and moderately small nanostructures, and Albrecht’s theory is not applicable to RRS [2]. This work provides unambiguous experimental results for validating EPC theories with reduced dimensionality.
[1] Y. Yi et al., Phys. Rev. Appl.
[2] Y. Zhang, Journal of Semiconductors 40, 091102 (2019).   

Analysis of χ(2) of III-V quantum-well structures using transfer matrix techniques

Nano-layered quantum wells (QWs) composed of III-V semiconductors provide unexplored opportunities to engineer χ⁽²⁾ (e.g. for electro-optic and quantum information applications) by optimizing thickness, separation, and shape of individual QW layers as well as the number N of repeated layers. A digital alloys growth technique was used, which avoids phase segregation that often plagues III-V alloy growth, and also lends itself readily to nano-structuring to enhance χ⁽²⁾. Second harmonic generation (SHG) was used to probe the total nonlinear χ⁽²⁾ response of a series of N multiple-QW (MQW) layers made up of InAs QWs and AlSb barriers, sandwiched in between a GaSb oxidation cap and GaSb buffer layer, grown on GaSb substrate. The measured SHG signal is composed of a contribution from each of the layers that interfere with each other. To isolate the N-dependent SHG polarization of the MQW layer of interest, we implemented a SHG transfer matrix formalism to model the SHG signal as a coherent superposition of a variable-N MQW layer with fixed substrate and cap layer SH polarizations. Experimental results show up to 25x stronger SHG from MQW structures than from GaSb substrate. The model attributes this enhancement partly to geometrical effects and to enhanced χ⁽²⁾ of the MQW structures.   

Optically-induced dressed states in αα materials: electronic and transport properties

We have obtained the energy dispersion relations and their wave functions for the so-called interacting Floquet states in pseudospin-1 α-T₃ materials for various value of the parameter α in the presence of an external off-resonant dressing field. The obtained dressed states and their basic electronic properties significantly depend on the polarization of the applied radiation, while our derived results now depend on both the parameter and the light intensity. The energy subbands become directly dependent on the valley index τ once elliptically polarized irradiation is applied. We have calculated optical and transport conductivities for an irradiated dice lattice, which represents a limiting case of an α-T₃ structure for α equal to 1.