299 research outputs found
Degradation studies of InAs/GaAs QD lasers grown on Si
Lowering the threshold gain of InAs quantum dot lasers grown on Silicon, significantly extends device lifetime. Measurements on degraded devices show increased optical mode loss is responsible for degradation and a consequent shortening of lasing wavelength
Low-Noise GaAs Quantum Dots for Quantum Photonics
Quantum dots are both excellent single-photon sources and hosts for single
spins. This combination enables the deterministic generation of Raman-photons
-- bandwidth-matched to an atomic quantum-memory -- and the generation of
photon cluster states, a resource in quantum communication and
measurement-based quantum computing. GaAs quantum dots in AlGaAs can be matched
in frequency to a rubidium-based photon memory, and have potentially improved
electron spin coherence compared to the widely used InGaAs quantum dots.
However, their charge stability and optical linewidths are typically much worse
than for their InGaAs counterparts. Here, we embed GaAs quantum dots into an
---diode specially designed for low-temperature operation. We
demonstrate ultra-low noise behaviour: charge control via Coulomb blockade,
close-to lifetime-limited linewidths, and no blinking. We observe high-fidelity
optical electron-spin initialisation and long electron-spin lifetimes for these
quantum dots. Our work establishes a materials platform for low-noise quantum
photonics close to the red part of the spectrum.Comment: (19 pages, 12 figures, 1 table
Optically driving the radiative Auger transition
In a radiative Auger process, optical decay is accompanied by simultaneous
excitation of other carriers. The radiative Auger process gives rise to weak
red-shifted satellite peaks in the optical emission spectrum. These satellite
peaks have been observed over a large spectral range: in the X-ray emission of
atoms; close to visible frequencies on donors in semiconductors and quantum
emitters; and at infrared frequencies as shake-up lines in two-dimensional
systems. So far, all the work on the radiative Auger process has focussed on
detecting the spontaneous emission. However, the fact that the radiative Auger
process leads to photon emission suggests that the transition can also be
optically excited. In such an inverted radiative Auger process, excitation
would correspond to simultaneous photon absorption and electronic
de-excitation. Here, we demonstrate optical driving of the radiative Auger
transition on a trion in a semiconductor quantum dot. The radiative Auger and
the fundamental transition together form a -system. On driving both
transitions of this -system simultaneously, we observe a reduction of
the fluorescence signal by up to . Our results demonstrate a type of
optically addressable transition connecting few-body Coulomb interactions to
quantum optics. The results open up the possibility of carrying out THz
spectroscopy on single quantum emitters with all the benefits of optics:
coherent laser sources, efficient and fast single-photon detectors. In analogy
to optical control of an electron spin, the -system between the
radiative Auger and the fundamental transitions allows optical control of the
emitters' orbital degree of freedom.Comment: 8 pages, 6 figure
Degradation of IIIâV Quantum Dot Lasers Grown Directly on Silicon Substrates
Initial age-related degradation mechanisms for InAs quantum dot lasers grown on silicon substrates emitting at 1.3 ÎŒm are investigated. The rate of degradation is observed to increase for devices operated at higher carrier densities and is therefore dependent on gain requirement or cavity length. While carrier localization in quantum dots minimizes degradation, an increase in the number of defects in the early stages of aging can increase the internal optical-loss that can initiate rapid degradation of laser performance due to the rise in threshold carrier density. Population of the two-dimensional states is considered the major factor for determining the rate of degradation, which can be significant for lasers requiring high threshold carrier densities. This is demonstrated by operating lasers of different cavity lengths with a constant current and measuring the change in threshold current at regular intervals. A segmented-contact device, which can be used to measure the modal absorption and also operate as a laser, is used to determine how the internal optical-loss changes in the early stages of degradation. Structures grown on silicon show an increase in internal optical loss, whereas the same structure grown on GaAs shows no signs of increase in internal optical loss when operated under the same conditions
Neuropsychiatric symptoms in 921 elderly subjects with dementia: a comparison between vascular and neurodegenerative types.
Objective:â i) to describe the neuropsychiatric profile of elderly subjects with dementia by comparing vascular (VaD) and degenerative dementias, i.e. dementia with Lewy bodies (DLB) and Alzheimerâs disease (AD); ii) to assess whether the severity and type of dementia are associated with clinically relevant neuropsychiatric symptoms (CRâNPS).
Method:â One hundred and thirtyâone outâpatients with VaD, 100 with DLB and 690 with AD were studied. NPS were evaluated by the neuropsychiatric inventory (NPI).
Results:â Vascular dementia had lower total and domainâspecific NPI scores and a lower frequency of CRâNPS than AD and DLB, for which frequency of CRâNPS increased significantly with disease severity, particularly in AD. Logistic regression analysis showed that a higher CDR score and a diagnosis of degenerative dementia were independently associated with CRâNPS.
Conclusion:â Vascular dementia is associated less with CRâNPS than AD and DLB. Frequency of CRâNPS increases with disease severity in AD and, to a lesser extent, in DLB
Charge Tunable GaAs Quantum Dots in a Photonic n-i-p Diode
In this submission, we discuss the growth of charge-controllable GaAs quantum dots embedded in an n-i-p diode structure, from the perspective of a molecular beam epitaxy grower. The QDs show no blinking and narrow linewidths. We show that the parameters used led to a bimodal growth mode of QDs resulting from low arsenic surface coverage. We identify one of the modes as that showing good properties found in previous work. As the morphology of the fabricated QDs does not hint at outstanding properties, we attribute the good performance of this sample to the low impurity levels in the matrix material and the ability of n- and p-doped contact regions to stabilize the charge state. We present the challenges met in characterizing the sample with ensemble photoluminescence spectroscopy caused by the photonic structure used. We show two straightforward methods to overcome this hurdle and gain insight into QD emission properties
Increasing Maximum Gain in InAs Quantum Dot Lasers on GaAs and Si
InAs quantum-dot (QD) lasers emitting at 1300nm with nominally undoped and modulated p-type doping are studied. Modal-gain measurements indicate a higher gain can be achieved from the ground-state for a given Fermi-level separation with p-doping and a reduced temperature-dependence of threshold current for short-cavity lasers
Optical performance monitoring from FIR filter coefficients in coherent receivers
Abstract: We present a robust and precise optical performance monitoring technique from FIR filter coefficients in coherent receivers with digital equalization. Residual chromatic dispersion, DGD and OSNR are simultaneously estimated from measured 111 Gbit/s data
Development of a psychiatric disorder linked to cerebellar lesions
Cerebellar dysfunction plays a critical role in neurodevelopmental disorders with long-term behavioral and neuropsychiatric symptoms. A 43-year-old woman with a cerebellum arteriovenous malformation and history of behavioral dysregulation since childhood is described. After the rupture of the cerebellar malformation in adulthood, her behavior morphed into specific psychiatric symptoms and cognitive deficits occurred. The neuropsychological assessment evidenced impaired performance in attention, visuospatial, memory, and language domains. Moreover, psychiatric assessment indicated a borderline personality disorder. Brain MRI examination detected macroscopic abnormalities in the cerebellar posterior lobules VI, VIIa (Crus I), and IX, and in the posterior area of the vermis, regions usually involved in cognitive and emotional processing. The described patient suffered from cognitive and behavioral symptoms that are part of the cerebellar cognitive affective syndrome. This case supports the hypothesis of a cerebellar role in personality disorders emphasizing the importance of also examining the cerebellum in the presence of behavioral disturbances in children and adults
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