Low-temperature polarized and time-resolved photoluminescence spectroscopy in high magnetic fields (up to 30 T) has been used to study the spin-polarization, spin-relaxation, and radiative lifetimes of excitons in wurtzite semiconductor (e.g., CdSe) colloidal nanocrystals. The applied magnetic field leads to a significant degree of circular polarization of the exciton photoluminescence, accompanied by a reduction in the photoluminescence decay time. The circular polarization arises from the Zeeman splitting of exciton levels, whereas lifetime reduction results from a polarization-preserving field-induced mixing of exciton levels. We analyze these experimental findings in terms of a simple model that combines both Zeeman effect and exciton-level mixing, as a function of the relative orientation of the nanocrystal c-axis and the magnetic field. This model is able to simultaneously describe the degree of circular polarization and lifetime reduction of the exciton photoluminescence, permitting us to quantify the exciton, electron, and hole g-factors.
In dilute nitrides [e.g., Ga(AsN), (InGa)(AsN)] the formation of stable N-2H-H complexes following H irradiation removes the effects nitrogen has on the optical (i.e., refractive index [1]), structural [2], and electronic [3] properties of the material. In particular, H binding to N atoms in GaAs 1−x N x leads to an increase in the band gap energy of the N-containing material (∼1.33 eV for x = 1% at T = 5 K) up to the value it has in GaAs (1.52 eV at 5 K). Therefore, by allowing H incorporation only in selected regions of the sample - e.g., by deposition of H-opaque masks prior to the hydrogenation - it is possible to attain a spatially controlled modulation of the band gap energy in the growth plane. This technique, referred to as in-plane Band Gap Engineering, can be employed to tailor the carrier-confining potential down to a nm scale, resulting in the fabrication of site-controlled, dilute nitride-based quantum dots (QDs). We demonstrate here that such QDs emit single photons on demand, as revealed by measuring the second-order correlation function of the single-exciton emission [4].Coupled to the possibility of erasing/rewriting the fabricated patterns through multiple annealing/hydrogenation procedures, the inherently precise control over the position of the nanostructures fabricated with this method renders them naturally suited for the integration with photonic crystal nanocavities.
We present two different methods to pattern the band gap of dilute nitrides in their growth plane by exploiting the unique capability of H to passivate N in these materials. By deposition of metallic masks on and subsequent H irradiation of GaAs1−xNx, we artificially create zones of the crystal having the band gap of untreated GaAs1−xNx well surrounded by GaAs‐like barriers. Alternatively, by focusing an energetic e−‐beam on the surface of hydrogenated GaAs1−xNx we displace H atoms from their N passivation sites, thus leading to a controlled decrease in the crystal band gap in the spatial region where the e−‐beam is steered.
The effects of hydrogen incorporation on the electronic properties of Ga(AsBi) alloys are investigated in a wide range of Bi-concentration (0.6% ≤ x ≤ 10.6%) by Hall effect measurements in magnetic fields up to 14 T and by photoluminescence spectroscopy. For all the investigated Bi-concentrations, we report the passivation of Bi-induced shallow acceptor levels—responsible for the native p-type conductivity in Ga(AsBi)—and a tenfold increase of the hole mobility upon hydrogen incorporation in the host lattice. The emission energy is, instead, negligibly affected by hydrogenation, indicating that the narrowing of the band-gap energy with Bi and the native p-type conductivity are two uncorrelated effects arising from different Bi-induced electronic levels. Passivation by hydrogen of the shallow Bi-acceptor levels makes also possible to identify deep Bi-acceptor states.
Abstract In dilute nitride In y Ga 1− y As 1− x N x alloys, a spatially controlled tuning of the energy gap can be realized by combining the introduction of N atoms—inducing a significant reduction of this parameter—with that of hydrogen atoms, which neutralize the effect of N. In these alloys, hydrogen forms N–H complexes in both Ga‐rich and In‐rich N environments. Here, photoluminescence measurements and thermal annealing treatments show that, surprisingly, N neutralization by H is significantly inhibited when the number of In‐N bonds increases. Density functional theory calculations account for this result and reveal an original, physical phenomenon: only in the In‐rich N environment, the In y Ga 1− y As host matrix exerts a selective action on the N–H complexes by hindering the formation of the complexes more effective in the N passivation. This thoroughly overturns the usual perspective of defect‐engineering by proposing a novel paradigm where a major role pertains to the defect‐surrounding matrix.
Abstract We present the design, manufacturing, and testing of a 37-element array of corrugated feedhorns for Cosmic Microwave Background CMB) measurements between 140 and 170 GHz. The array was designed to be coupled to Kinetic Inductance Detector arrays, either directly (for total power measurements) or through an orthomode transducer (for polarization measurements). We manufactured the array in platelets by chemically etching aluminum plates of 0.3 mm and 0.4 mm thickness. The process is fast, low-cost, scalable, and yields high-performance antennas compared to other techniques in the same frequency range. Room temperature electromagnetic measurements show excellent repeatability with an average cross polarization level about − 20 dB, return loss about − 25 dB, first sidelobes below − 25 dB and far sidelobes below − 35 dB. Our results qualify this process as a valid candidate for state-of-the-art CMB experiments, where large detector arrays with high sensitivity and polarization purity are of paramount importance in the quest for the discovery of CMB polarization B -modes.
The Millimeter Sardinia radio Telescope Receiver based on Array of Lumped elements KIDs (MISTRAL) is a new high resolution, wide field-of-view camera that was successfully installed in May 2023 at the Sardinia Radio Telescope (SRT). SRT is a 64m fully steerable gregorian radio telescope, and it underwent an upgrade funded by a National Operational Program (PON) with the aim to expand the fleet of receivers of the radio telescope in order to cover frequency up to the W–band. The W-band sky has been extensively studied by Cosmic Microwave Background experiments, both ground-based (ACT, SPT) and satellite-based (WMAP, Planck). However, their resolution is limited to ≈1′ from ground telescopes and ≈10′ from satellite at best. With this new instrument, we aim to map the microwave sky at a resolution of ≈12′′, a capability only shared by few instruments in the world, unlocking the exploration of a plethora of science cases from the recently upgraded SRT. The heart of MISTRAL is a ≈90mm silicon focal plane populated with 415 cryogenic Lumped Elements Kinetic Inductance Detectors (LEKIDs). These detectors are copuled with the telescope using a cold (4K) re-imaging optical system, producing a diffraction limited field-of-view of 4 ′. The system is enclosed in a custom, four stage cryostat, built with strict requirements on its size, in order to fit on the rotating turret that allows to switch the receivers to be quickly moved in and out of the gregorian focus position. The sub-K stage cools the detectors down to 200-240 mK. MISTRAL is now installed on the gregorian focus of SRT and is undergoing the technical commissioning, and will soon enter the scientific commissioning phase. In this contribution we will survey the subsystems of MISTRAL and their performance at the focus of the radio telescope, and report the current status of the technical commissioning.
