Quantum Confined Stark Effect Research Articles

GaN-based parallel micro-light-emitting diode arrays with dual-wavelength InxGa1-xN/GaN MQWs for visible light communication.

The dual-wavelength InxGa1-xN/GaN micro light emitting diode (Micro-LED) arrays are fabricated by flip-chip parallel connection. It is noted that the Micro-LED arrays with smaller diameter present considerably bigger light output power density (LOPD). For all Micro-LEDs, the LOPD increases continuously with increasing injection current density until it "turns over". It also can be observed that the maximum value of LOPD is determined by the blue quantum well (QW) for the broad area LED. In comparison, the green peak intensity dominates the change of LOPD in the Micro-LEDs. In addition, the enhancement of the green peak intensity value for the Micro-LEDs are considered as a consequence of the combined effects of the reduction in the quantum-confined Stark effect (QCSE) and the crowding effect, high LEE as well as geometric shape. Moreover, -3dB modulation bandwidths of the four different kinds of Micro-LEDs increase with the decrease of the device diameter in the same injected current density, higher than that of the broad area LED. The -3dB modulation bandwidth of the 60 µm Micro-LED shows 1.4 times enhancement compared to that of the broad area LED under the current density of 300 mA/cm2. Evidently, the dual-wavelength InxGa1-xN/GaN Micro-LEDs have great potential in both solid-state lighting (SSL) and the visible light communication (VLC) in the future fabrication.

Measurement of the quantum-confined Stark effect in InAs/In(Ga)As quantum dots with p-doped quantum dot barriers.

The quantum-confined Stark effect in InAs/In(Ga)As quantum dots (QDs) using non-intentionally doped and p-doped QD barriers was investigated to compare their performance for use in optical modulators. The measurements indicate that the doped QD barriers lead to a better figure of merit (FoM), defined as the ratio of the change in absorption Δα for a reverse bias voltage swing to the loss at 1 V α(1 V), FoM=Δα/α (1 V). The improved performance is due to the absence of the ground-state absorption peak and an additional component to the Stark shift. Measurements indicate that p-doping the QD barriers can lead to more than a 3x increase in FoM modulator performance between temperatures of -73 °C to 100 °C when compared with the stack with NIDQD barriers.

The effect of dry etching condition on the performance of blue micro light-emitting diodes with reduced quantum confined Stark effect epitaxial layer

As the size of micro light-emitting diodes (μLEDs) decreases, μLEDs encounter etching damage especially at the sidewalls that critically affects their properties. In this study, we investigated the influence of etching bias power (Pbias) on the performance of μLEDs and found that the current–voltage and light output–current characteristics of μLEDs were enhanced when Pbias was reduced. It was shown that at low Pbias, the chemical reaction between etching gas and gallium nitride, rather than ion sputtering, dominated the etching process, leading to low plasma damage and rough surface morphology. Additionally, to understand the etching-induced surface roughening behaviors, various substrates with different threading dislocation densities were treated at low Pbias. It was found that for the sample (with p-contact size of 10 × 10 μm2), the efficiency droop was approximately 20%, although the current reached 10 mA due most probably to the suppressed polarization effect in the quantum well. It was further observed that the external quantum efficiency (EQE) was dependent on Pbias, where the lowest Pbias yielded the highest maximum EQE, indicating that the plasma damage was mitigated by reducing Pbias. Optimization of dry etching and polarization-suppression conditions could pave the way for realizing high-performance and brightness μLEDs for next-generation displays.

Interface Recombination in Ga- and N-Polar GaN/(Al,Ga)N Quantum Wells Grown by Molecular Beam Epitaxy

We explore and systematically compare the morphological, structural and optical properties of GaN/(Al,Ga)N multiple quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PA-MBE) on freestanding GaN$(0001)$ and GaN$(000\bar{1})$ substrates. Samples of different polarity are found to be comparable in terms of their morphological and structural perfection and exhibit essentially identical quantum well widths and Al content. Regardless of the crystal orientation, the exciton decay in the MQWs at 10 K is dominantly radiative and the photoluminescence (PL) energy follows the quantum confined Stark effect (QCSE) for different quantum well widths. A prominent free-to-bound transition involving interface shallow donors is, however, visible for the N-polar MQWs. At room-temperature, in contrast, the exciton decay in all samples is dominated by nonradiative recombination taking place at point defects, presumably Ca or V N located at the bottom QW interface. Remarkably, the N-polar MQWs exhibit a higher PL intensity and longer decay times than the Ga-polar MQWs at room-temperature. This improved internal quantum efficiency is attributed to the beneficial orientation of the internal electric field that effectively reduces the capture rate of minority carriers by interface trap states.

Optical Investigation of p-GaAs/i-GaN0.38yAs1-1.38ySby/n-GaAs Quantum Wells Emitters

We have studied the 1.55 μm optical properties of p-GaAs/i-GaN0.38yAs1-1.38ySby/n-GaAs quantum wells using a self-consistent calculation combined with the anticrossing model. We have found that the increase of injected carriers’ density induces the increase of optical gain and radiative current density. The rise of doping density causes a blue shift of the fundamental transition energy accompanied with significant increase of optical gain. The quantum-confined Stark effect on radiative current density is also studied. The variation of radiative current as function of well width and Sb composition is also examined. In order to operate the emission wavelength at the optical fiber telecommunication domain, we have adjusted the well parameters of p-GaAs/i-GaN0.38yAs1-1.38ySby/n-GaAs.

Charge and field driven integrated optical modulators: comparative analysis: opinion

Electro optic modulators being key for many signal processing systems must adhere to requirements given by both electrical and optical constraints. Distinguishing between charge driven (CD) and field driven (FD) designs, we answer the question of whether fundamental performance benefits can be claimed of modulators based on emerging electro-optic materials. Following primary metrics, we compare the performance of emerging electro-optic and electro-absorption modulators such as graphene, transparent conductive oxides, and Si, based on charge injection with that of the ‘legacy’ FD modulators, such as those based on lithium niobate and quantum confined Stark effect. We show that for rather fundamental reasons and when considering energy and speed only, FD modulators always outperform CD ones in the conventional wavelength scale photonic waveguides. However, for waveguides featuring a sub-wavelength optical mode, such as those assisted by plasmonics, the emerging CD devices are indeed highly competitive especially for applications where component-density on-chip is a factor.

Shape-Tuned Multiphoton-Emitting InP Nanotetrapods.

As the properties of a semiconductor material depend on the fate of the excitons, manipulating exciton behavior is the primary objective of nanomaterials. Although nanocrystals exhibit unusual excitonic characteristics owing to strong spatial confinement, studying the interactions between excitons in a single nanoparticle remains challenging due to the rapidly vanishing multiexciton species. Here, a platform for exciton tailoring using a straightforward strategy of shape-tuning of single-crystalline nanocrystals is presented. Spectroscopic and theoretical studies reveal a systematic transition of exciton confinement orientation from 3D to 2D, which is solely tuned by the geometric shape of material. Such a precise shape-effect triggers a multiphoton emission in single nanotetrapods with arms longer than the exciton Bohr radius of material. In consequence, the unique interplay between the multiple quantum states allows a geometric modulation of the quantum-confined Stark effect and nanocrystal memory effect in single nanotetrapods. These results provide a useful metric in designing nanomaterials for future photonic applications.

On the Two-Dimensional Quantum Confined Stark Effect in Strong Electric Fields

We consider a Stark Hamiltonian on a two-dimensional bounded domain with Dirichlet boundary conditions. In the strong electric field limit we derive, under certain local convexity conditions, a three-term asymptotic expansion of the low-lying eigenvalues. This shows that the excitation frequencies are proportional to the square root of the boundary curvature at a certain point determined by the direction of the electric field.

InGaN quantum well with gradually varying indium content for high-efficiency GaN-based green light-emitting diodes.

High-efficiency GaN-based green LEDs are of paramount importance to the development of the monolithic integration of multicolor emitters and full-color high-resolution displays. Here, the InGaN quantum well with gradually varying indium (In) content was proposed for improving the performance of GaN-based green LEDs. The InGaN quantum well with gradually varying In content not only alleviates the quantum-confined Stark effect (QCSE), but also yields a low Auger recombination rate. Consequently, the gradual In content green LEDs exhibited increased light output power (LOP) and reduced efficiency droop as compared to constant In content green LEDs. At 60 A/cm2, the LOPs of the constant In content green LEDs and the gradual In content green LEDs were 33.9 mW and 55.2 mW, respectively. At 150 A/cm2, the efficiency droops for the constant In content green LEDs and the gradual In content green LEDs were 61% and 37.6%, respectively. This work demonstrates the potential for the gradual In content InGaN to replace constant In content InGaN as quantum wells in LED devices in a technologically and commercially effective manner.

Membrane potential sensing: Material design and method development for single particle optical electrophysiology.

We review the development of "single" nanoparticle-based inorganic and organic voltage sensors, which can eventually become a viable tool for "non-genetic optogenetics." The voltage sensing is accomplished with optical imaging at the fast temporal response and high spatial resolutions in a large field of view. Inorganic voltage nanosensors utilize the Quantum Confined Stark Effect (QCSE) to sense local electric fields. Engineered nanoparticles achieve substantial single-particle voltage sensitivity (∼2% Δλ spectral Stark shift up to ∼30% ΔF/F per 160 mV) at room temperature due to enhanced charge separation. A dedicated home-built fluorescence microscope records spectrally resolved images to measure the QCSE induced spectral shift at the single-particle level. Biomaterial based surface ligands are designed and developed based on theoretical simulations. The hybrid nanobiomaterials satisfy anisotropic facet-selective coating, enabling effective compartmentalization beyond non-specific staining. Self-spiking- and patched-HEK293 cells and cortical neurons, when stained with hybrid nanobiomaterials, show clear photoluminescence intensity changes in response to membrane potential (MP) changes. Organic voltage nanosensors based on polystyrene beads and nanodisk technology utilize Fluorescence (Förster) Resonance Energy Transfer (FRET) to sense local electric fields. Voltage sensing FRET pairs achieve voltage sensitivity up to ∼35% ΔF/F per 120 mV in cultures. Non-invasive MP recording from individual targeted sites (synapses and spines) with nanodisks has been realized. However, both of these QCSE- and FRET-based voltage nanosensors yet need to reach the milestone of recording individual action potentials from individual targeted sites.

Non-Polar Gallium Nitride for Photodetection Applications: A Systematic Review

Ultraviolet photodetectors have been widely utilized in several applications, such as advanced communication, ozone sensing, air purification, flame detection, etc. Gallium nitride and its compound semiconductors have been promising candidates in photodetection applications. Unlike polar gallium nitride-based optoelectronics, non-polar gallium nitride-based optoelectronics have gained huge attention due to the piezoelectric and spontaneous polarization effect–induced quantum confined-stark effect being eliminated. In turn, non-polar gallium nitride-based photodetectors portray higher efficiency and faster response compared to the polar growth direction. To date, however, a systematic literature review of non-polar gallium nitride-based photodetectors has yet to be demonstrated. Hence, the objective of this systematic literature review is to critically analyze the data related to non-polar gallium nitride-based photodetectors. Based on the pool of literature, three categories are introduced, namely, growth and fabrication, electrical properties, and structural, morphological, and optical properties. In addition, bibliometric analysis, a precise open-source tool, was used to conduct a comprehensive science mapping analysis of non-polar gallium nitride-based photodetectors. Finally, challenges, motivations, and future opportunities of non-polar gallium nitride-based photodetectors are presented. The future opportunities of non-polar GaN-based photodetectors in terms of growth conditions, fabrication, and characterization are also presented. This systematic literature review can provide initial reading material for researchers and industries working on non-polar gallium nitride-based photodetectors.

Comparative investigation into key optoelectronic characteristics of semipolar InGaN blue laser diodes: A strategy to mitigate quantum-confine stark effect

In recent years, semipolar InGaN-based blue laser diodes (LDs) have raised indescribable appeal within solid-state lighting industry as promising alternatives for currently available polar c-plane LDs due to reduction in internal polarization field and related quantum-confined stark effect. However, any comparative study in terms of energy band gap, optical emission profile, power output (P) and forward bias response (I-V) etc. between semipolar-oriented InGaN blue LDs is inherently unattainable. This work aims to demonstrate the impact of internal polarization field on aforementioned key optoelectronic parameters of In0.17Ga0.83N/GaN double quantum well (DQW) blue LD along semipolar (101-2), (112-2), (101-1), (202-1), (303-1-), (202-1-) and (101-1-) crystal orientations by figuring out six-band k.p Hamiltonian at the brillouin zone center aided from Euler’s tensor transformation method. Three-level rate equations based LD analogous circuit model and state-space matrix are employed here to obtain P-I-V and frequency characteristics. Radiative as well as non-radiative recombination mechanisms for LD systems are portrayed by addressing modified ABC model. Our numerical investigation illustrates that best hole effective mass, optical gain, lasing power, threshold current and forward voltage can be attained from (112-2)-oriented InGaN blue LD due to insignificant presence of spontaneous plus piezoelectric (PZ) polarization charge. In addition with these, the inspection of bode plot and relevant optical power gain (dB) confirms the stability and superiority of this semipolar blue LD system for achieving high-speed visible-light communications at relatively low current densities.

Spatially indirect intervalley excitons in bilayer WSe2

Spatially indirect excitons with displaced wavefunctions of electrons and holes play a pivotal role in a large portfolio of fascinating physical phenomena and emerging optoelectronic applications, such as valleytronics, exciton spin Hall effect, excitonic integrated circuit and high-temperature superfluidity. Here, we uncover three types of spatially indirect excitons (including their phonon replicas) and their quantum-confined Stark effects in hexagonal boron nitride encapsulated bilayer WSe2, by performing electric field-tunable photoluminescence measurements. Because of different out-of-plane electric dipole moments, the energy order between the three types of spatially indirect excitons can be switched by a vertical electric field. Remarkably, we demonstrate, assisted by first-principles calculations, that the observed spatially indirect excitons in bilayer WSe2 are also momentum-indirect, involving electrons and holes from Q and K/{\Gamma} valleys in the Brillouin zone, respectively. This is in contrast to the previously reported spatially indirect excitons with electrons and holes localized in the same valley. Furthermore, we find that the spatially indirect intervalley excitons in bilayer WSe2 can exhibit considerable, doping-sensitive circular polarization. The spatially indirect excitons with momentum-dark nature and highly tunable circular polarization open new avenues for exotic valley physics and technological innovations in photonics and optoelectronics.

Luminescence measurement of band gap

Optical band gap or band gap is an important characteristic parameter of semiconductor materials. In this study, several representative InGaN/GaN multiple quantum well structures are taken as the research objects, and the test conditions that need to be met for the luminescence measurement of the optical band gap of the InGaN well layer at a certain target temperature are discussed in depth. Since the InGaN well layer is a multi-element alloy and is subjected to stress from the GaN barrier layer, there exist not only impurity/defect-related non-radiation centers in the well layer, but also localized potential fluctuation induced by composition fluctuation and quantum confinement Stark effect (QCSE) induced by polarization field. Therefore, in order to obtain a more accurate optical band gap of the InGaN well layer, we propose the following test conditions that the luminescence measurement should meet at least, that is, the influence of the non-radiation centers, the localized centers and the QCSE on the emission process at the target temperature must be eliminated. Although these test conditions need to be further improved, it is expected that this test method can provide valuable guidance or ideas for measuring the semiconductor optical band gap.

Electro-Optical Numerical Modeling for the Design of UVA Nitride-Based Vertical-Cavity Surface-Emitting Laser Diodes

Design rules are proposed for UVA vertical-cavity surface-emitting laser diodes, and the modal gain and cavity properties are analyzed using an electro-optical numerical model. To achieve a low threshold current, it is important to align the gain-peak and resonance wavelengths, but the quantum-confined Stark effect makes that difficult because the former wavelength shifts with changing current density. The relationship between the gain peak and the injection current is discussed. Also, the quantum-well design affects the gain characteristics, and the gain properties with different quantum-well number, composition, and width are discussed.

Ultrafast imaging of terahertz electric waveforms using quantum dots

Microscopic electric fields govern the majority of elementary excitations in condensed matter and drive electronics at frequencies approaching the Terahertz (THz) regime. However, only few imaging schemes are able to resolve sub-wavelength fields in the THz range, such as scanning-probe techniques, electro-optic sampling, and ultrafast electron microscopy. Still, intrinsic constraints on sample geometry, acquisition speed and field strength limit their applicability. Here, we harness the quantum-confined Stark-effect to encode ultrafast electric near-fields into colloidal quantum dot luminescence. Our approach, termed Quantum-probe Field Microscopy (QFIM), combines far-field imaging of visible photons with phase-resolved sampling of electric waveforms. By capturing ultrafast movies, we spatio-temporally resolve a Terahertz resonance inside a bowtie antenna and unveil the propagation of a Terahertz waveguide excitation deeply in the sub-wavelength regime. The demonstrated QFIM approach is compatible with strong-field excitation and sub-micrometer resolution—introducing a direct route towards ultrafast field imaging of complex nanodevices in-operando.

Electrically tunable nonlinear polaritonic metasurface

Nonlinear polaritonic metasurfaces created by the coupling of intersubband nonlinearities in semiconductor heterostructures with optical modes in nanoresonators have recently demonstrated efficient frequency mixings at very low pumping intensities of the order of a few tens of kilowatts per square centimetre. In these subwavelength structures, the efficiency, spectral bandwidth and local nonlinear phase of wave mixing do not depend on phase matching but only on the nonlinear response of the constituent meta-atoms. We exploit this property to demonstrate an electrically tunable nonlinear metasurface that combines a plasmonic nanocavity and a quantum-engineered semiconductor heterostructure, in which the magnitude and phase of the local nonlinear responses are controlled by a bias voltage through the quantum-confined Stark effect. We demonstrate spectral tuning, dynamic intensity modulation and dynamic beam manipulation for second-harmonic generation. Our work suggests a route for electrically reconfigurable flat nonlinear optical elements with versatile functionalities.

Quantum dot photoluminescence as a versatile probe to visualize the interaction between plasma and nanoparticles on a surface

We experimentally demonstrate that the interaction between plasma and nanometer-sized semiconductor quantum dots (QDs) is directly connected to a change in their photoluminescence (PL) spectrum. This is done by taking in situ, high resolution, and temporally resolved spectra of the light emitted by laser-excited QDs on an electrically floating sample exposed to a low pressure argon plasma. Our results show a fast redshift of the PL emission peak indicating the quantum-confined Stark effect due to plasma-generated excess charges on the substrate and near the QD surface, while other plasma-induced (thermal and ion) effects on longer timescales could clearly be distinguished from these charging effects. The presented results and method open up pathways to direct visualization and understanding of fundamental plasma–particle interactions on nanometer length scales.

Vertical Hybrid Integration Devices Using Selectively Defining Underneath Si Waveguide

3D hybrid integration based on III-V/Si waveguides by selective undercut wet etching (SUWE) of the lower Si layer has been demonstrated on a silicon-on-insulator (SOI) template. The process was performed on an adhesive-bonding material structure, defined by the upper InP/InAlGaAs (InAlGaAs quantum well) p-i-n heterostructure and the lower 220 nm SOI. After the top p-i-n III-V tapered waveguide was first fabricated and covered by Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , KOH was then used for SUWE of Si layer from SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , forming the bottom SOI single-mode waveguide. In comparison to monolithic III-V waveguide structure (InP based material), the simulated confinement factor of active region shows an enhancement from 10.3% to 16.7%. SOA/EAM devices were also processed and integrated in III-V layer, showing 7.2 dB optical gain at 70 mA injection current in SOA and 21.7 dB extinction ratio within 0~−2 V swing voltage in EAM. The strong quantum confined Stark effect (QCSE) in MQW confirms the improved confinement factor. It suggests a vertical self-alignment scheme could be used for realizing compact and submicron scale heterogeneous integration in a Si photonics template.

Optical Properties of GaN-Based Green Light-Emitting Diodes Influenced by Low-Temperature p-GaN Layer.

GaN-based green light-emitting diodes (LEDs) with different thicknesses of the low-temperature (LT) p-GaN layer between the last GaN barriers and p-AlGaN electron blocking layer were characterized by photoluminescence (PL) and electroluminescence (EL) spectroscopic methods in the temperature range of 6–300 K and injection current range of 0.01–350 mA. Based on the results, we suggest that a 20 nm-thick LT p-GaN layer can effectively prevent indium (In) re-evaporation, improve the quantum-confined Stark effect in the last quantum well (QW) of the active region, and finally reduce the efficiency droop by about 7%.