Spiking neural networks are a class of artificial neural networks maintaining a strict analogy to brain-like processing. I’ll show a new hardware approach in which semiconductor microcavities in strong light-matter coupling regime can operate as optical spiking neurons. We demonstrated the intrinsic property of exciton-polaritons to resemble the Leaky Integrate-and-Fire spiking mechanism. Polaritons when pumped with a pulsed laser exhibit leaky-integration due to relaxation of the excitonic reservoir, threshold-and-fire mechanism due to transition to polariton condensate, and resetting due to stimulated emission of photons. Our approach provides means for energy-efficient ultrafast processing of spike-like laser pulses at the level below 1 pJ/spike.
In recent decades, GaN and related compounds gained prominence as top semiconductor materials for high-power, high-temperature optoelectronics, electronics, and power conversion. However, better substrate materials are still sought after despite extensive nitride research. Commonly used substrates like sapphire and silicon have high lattice mismatches with GaN, leading to challenges in heteroepitaxial growth.
A promising solution emerged with ScAlMgO4 (SCAM) proposed as a GaN growth substrate. SCAM offers several advantages:
i) Smaller lattice mismatch with GaN than sapphire, reducing dislocation density in grown structures.
ii) Matching thermal expansion coefficient with GaN along the a-axis, reducing residual strain.
iii) Easy cleavage along the c-plane, yielding atomically flat substrates without polishing.
iv) Ability to grow large SCAM crystals via the Chochralski method.
In this study1, we present experimental and theoretical investigations on the optical, electronic, and structural properties of ScAlMgO4. Our experimental techniques include variable angle spectroscopic ellipsometry, optical transmission, X-ray diffraction, scanning electron microscopy, and Raman spectrosco
We introduce an inverted refractive-index-contrast grating (ICG) that is a compact alternative to DBRs. In ICG a subwavelength grating made of a low refractive index material is implemented on a high refractive index cladding. We experimentally demonstrate high reflectivity of proof-of-concept ICG fabricated by 3D microprinting, in which IP-Dip photoresist grating is deposited on silicon cladding. We also show that the ICG provides nearly total optical power reflectance whenever the refractive index of the grating exceeds 1.75, irrespective of the refractive index of the cladding.
Recently, oxide semiconductors have attracted tremendous attention due to their applications in various fields from UV emitters, power electronics, nano-sensors and optoelectronics. Furthermore, global warming forces new solutions to limit greenhouse gas emissions and mitigate the negative effects of climate change. By performing thermodynamic calculations based on free-Gibbs energy and equilibrium constants of seven theoretical reactions for two oxides systems ZnO and Ga2O3, we confirm that such semiconductors could be grown by decomposition of carbon dioxide. In addition, we present a modified carbothermal method of growing ZnO nanowires. Nanowires obtained by us display excellent structural and optical properties with FWHM of photoluminescence at T = 8 K from donor bound exciton equal to 0.5 meV. We strongly believe that our modified carbothermal method is an analog of the growth process of large ZnO bulk crystals, which occurred previously in the Olawa foundry in Poland.
We show that time-delayed nonlinear effects observed between exciton-polariton condensates can be used to create neural networks in which information is encoded in time. We form condensates on semiconductor microcavity using optical pulses that reach the sample at different times. Strongly nonlinear effects are induced by time-dependent interactions with a long-lived excitonic reservoir. Such nonlinearities make it possible to create a nonlinear XOR logic gate that performs operations with a picosecond time scale. A neural network based on such a logic gate performs a speech recognition tasks with high accuracy.
The concept of Neuromorphic Photonics introduces advantages of optical information processing into the neuromorphic engineering domain. Most of the current efforts in the field are focused on identifying the potential mechanisms for useful and flexible spiking neuron implementation. We propose a new approach in which microcavities exhibiting strong exciton-photon interaction may serve as building blocks of optical spiking neurons. Our experiments prove similarities between polariton in-out pulse characteristics and the fundamental spiking behavior of a biological neuron. These effects, evidenced in photoluminescence characteristics, arise within sub-ns timescales. The presented approach provides means for energy-efficient ultrafast processing of spike-like laser pulses.
We report on the binding energy of a charged exciton (trion) confined in a single, epitaxially grown 1.7nm thick ZnO/(Zn,Mg)O quantum well as large as 22 meV or 27.6 meV when determined in micro-photoluminescence or transmission measurements, respectively. The attribution of the lines is further confirmed by the magnetic field dependences of the emission intensity. Charged exciton emission is found to persist up to near room temperature. The QW width was determined from high resolution transmission electron microscopy measurements. The binding energy comparable to thermal energy at room temperature is promising for trion based spintronic and optoelectronic applications.
We propose a grating that design is inverted with respect to a design of conventional high-contrast grating (HCG) in which low refractive index grating is implemented on high refractive index cladding. We show that inverted HCG can achieve power reflectance of nearly 100% even if the refractive index of the grating is as low as 1.8. Inverted HCG facilitates implementation of highly reflecting mirrors composed of etched SiN, HfO or 3D micro-printed IP-Dip on semiconductors such as GaAs that processing is less technologically demanding with respect to processing of HCGs or monolithic HCGs.
We demonstrate the fabrication of highly efficient sources of entangled photon pairs by inserting a semiconductor
quantum dot in an optical cavity. Two electron-hole pairs trapped in a quantum dot (QD) radiatively recombine emitting
a cascade of two polarization entangled photons. To extract both photons, we use a photonic molecule consisting of two
identical micropillars, one empty, the other embedding a chosen QD. By adjusting the diameter of the pillars and their
relative distance, we ensure that both optical transitions of the QD are simultaneously resonant to cavity modes. The
emitted photon pairs are efficiently extracted thanks to Purcell effect. Doing so, we obtain the brightest sources of
entangled photon pairs to date. We further show that the implementation of Purcell effect allows increasing the fidelity
of the two photon state by reducing spin induced phase shift during the radiative cascade.
The talk summarizes results of our recent optical studies related to spin states in II-VI and III-V semiconductor quantum dot (QD) systems. First the influence of in-plane anisotropy on the QD excitonic spin states is recalled. Then various ways of circumventing, compensating, or exploiting this influence are discussed. Short lifetime of neutral excitons (governed by inter-dot tunneling) allowed us to transfer their spin polarization to another QD before its destruction by the anisotropic exchange interaction. This spin polarization, as well as single carrier spin memory effects in quantum dots are demonstrated using trion states, negligibly perturbed by the anisotropy. Modification of the anisotropy by external perturbations (electric and magnetic field) is shown. In particular, full compensation of the anisotropy by in-plane electric field is demonstrated using optical orientation of neutral excitons. Finally, the influence of the anisotropy will be exploited to achieve circular-to-linear and linear-to-circular polarization conversion in single QDs and in coupled QD pairs.
J. Suffczyński, T. Kazimierczuk, M. Goryca, B. Piechal, A. Trajnerowicz, K. Kowalik, P. Kossacki, A. Golnik, K. Korona, M. Nawrocki, J. Gaj, G. Karczewski
This work is devoted to correlation spectroscopy of individual II-VI CdTe/ZnTe QDs in view to determine non-resonant
excitation mechanisms and provide information on spin relaxation of QD states. Second order photon autocorrelations
and cross-correlations were measured in a Hanbury-Brown and Twiss setup for neutral and charged exciton and
biexciton transitions, excited by pulses of a frequency-doubled femtosecond Ti:Sapphire laser. Some of the
measurements were circular- or linear polarization resolved and performed in magnetic field. Besides, measurements of
photoluminescence excited by pairs of laser pulses revealed fast excitation phenomena in the range of tens of ps. The
results of measurements without polarization resolution were interpreted using a simple rate equation model and allowed
us to establish the dominant role of single carrier capture in the non-resonant excitation of the QD. Polarization-dependent
correlation measurements were used to study the magnetic field controlled transition between anisotropic QD
exciton eigenstates active in linear polarization and those active in circular polarization. The same measurements
provided information on spin relaxation of the carriers left in the dot after charged exciton recombination.
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