High-speed multiwavelength InGaAs/InP quantum well nanowire array micro-LEDs for next generation optical communications

Miniaturized light sources at telecommunication wavelengths are essential components for on-chip optical communication systems.Here,we report the growth and fabrication of highly uniform p-i-n core-shell InGaAs/InP single quantum well(QW)nanowire array light emitting diodes(LEDs)with multi-wavelength and high-speed operations.Two-dimensional cathodoluminescence mapping reveals that axial and radial QWs in the nanowire structure contribute to strong emission at the wavelength of~1.35 and~1.55μm,respectively,ideal for low-loss optical communications.As a result of simultaneous contributions from both axial and radial QWs,broadband electroluminescence emission with a linewidth of 286 nm is achieved with a peak power of~17μW.A large spectral blueshift is observed with the increase of applied bias,which is ascribed to the band-filling effect based on device simulation,and enables voltage tunable multi-wavelength operation at the telecommunication wavelength range.Multi-wavelength operation is also achieved by fabricating nanowire array LEDs with different pitch sizes on the same substrate,leading to QW formation with different emission wavelengths.Furthermore,high-speed GHz-level modulation and small pixel size LED are demonstrated,showing the promise for ultrafast operation and ultracompact integration.The voltage and pitch size controlled multi-wavelength highspeed nanowire array LED presents a compact and efficient scheme for developing high-performance nanoscale light sources for future optical communication applications.

Probing defects in ZnO by persistent phosphorescence

Native point defects in ZnO are so complicated that most of them are still debating issues, although they have been studied for decades. In this paper, we experimentally reveal two sub-components usually hidden in the low energy tail of the main broad green luminescence band peaking at 547 nm （-2.267 eV） in intentionally undoped ZnO single crystal by selecting the below-band-gap （BBG） optical excitations （e.g. light wavelengths of 385 nm and 450 nm）. Moreover, both sub-components are manifested as long persistent phosphorescence once the BBG excitations are removed. With the aid of a newly developed model, the energy depths of two electron traps involved within the long lived orange luminescence are determined to be 44 meV and 300 meV, respectively. The candidates of these two electron traps are argued to be most likely hydrogen and zinc interstitials in ZnO.

Influence of temperature and reverse bias on photocurrent spectrum and supra-bandgap spectral response of monolithic GalnP/GaAs double-junction solar cell

In this paper, influence of temperature and reverse bias on photocurrent spectrum and spectral response of a monolithic GalnP/GaAs double-junction solar cell was investigated in detail. Two sharp spectral response offsets, corresponding to the bandedge photo absorption of the bottom GaAs and the top GalnP subcells, respectively, show the starting response points of individual subcells. More interestingly, the cell photocurrent was found to enhance significantly with increasing the temperature. In addition, the cell photocurrent also increases obviously as the reverse bias voltage increases. The integrated photocurrent intensity of the top GalnP subcell was particularly addressed. A theoretical model was proposed to simulate the reverse bias dependence of the integrated photocurrent of the GalnP subceU at different temperatures.

Telecom-band multiwavelength vertical emitting quantum well nanowire laser arrays

Highly integrated optoelectronic and photonic systems underpin the development of next-generation advanced optical and quantum communication technologies,which require compact,multiwavelength laser sources at the telecom band.Here,we report on-substrate vertical emitting lasing from ordered InGaAs/InP multi-quantum well core–shell nanowire array epitaxially grown on InP substrate by selective area epitaxy.To reduce optical loss and tailor the cavity mode,a new nanowire facet engineering approach has been developed to achieve controlled quantum well nanowire dimensions with uniform morphology and high crystal quality.Owing to the strong quantum confinement effect of InGaAs quantum wells and the successful formation of a vertical Fabry–Pérot cavity between the top nanowire facet and bottom nanowire/SiO_(2) mask interface,stimulated emissions of the EH11a/b mode from single vertical nanowires from an on-substrate nanowire array have been demonstrated with a lasing threshold of~28.2μJ cm^(−2) per pulse and a high characteristic temperature of~128 K.By fine-tuning the In composition of the quantum wells,room temperature,single-mode lasing is achieved in the vertical direction across a broad near-infrared spectral range,spanning from 940 nm to the telecommunication O and C bands.Our research indicates that through a carefully designed facet engineering strategy,highly ordered,uniform nanowire arrays with precise dimension control can be achieved to simultaneously deliver thousands of nanolasers with multiple wavelengths on the same substrate,paving a promising and scalable pathway towards future advanced optoelectronic and photonic systems.

Extinction of the zero-phonon line and the first-order phonon sideband in excitonic luminescence of ZnO at room temperature： the self-absorption effect

It is firmly demonstrated in experiment that the self-absorption(SA) effect can lead to the extinction of the zero-phonon line and the first-order longitudinal optical phonon sideband of free excitonic luminescence of ZnO at room temperature. Moreover, effectiveness degree of SA effect is found to be dependent on both absorption coefficient and travelling distance of emitted photons, as well as even lattice temperature, which is uniquely reflected by the redshift amount in emission peak in ZnO. It is also unambiguously proved that the SA effect still strictly obeys the Beer-Lambert law of absorption. This work not only uncovers the long-term puzzle of significant redshift of emission peak of ZnO at higher temperatures, but also shows that the SA effect may have to be carefully taken into consideration in the study of spontaneous emission, laser and relevant optoelectronic processes in luminescent materials and optoelectronic devices.

Observation of two-times self-focusing of femtosecond laser beam in ZnO crystal by two-photon luminescence

By ‘‘seeing" the green two-photon luminescence, two separate focusing points are observed on the propagation axis of a converging femtosecond laser beam in a ZnO single crystal rod. It is found that the selffocusing effect makes a significant contribution to the formation of the first focusing point, while the second focusing point is caused by self-refocusing. The position of the first focusing point is in good agreement with the value predicted by a model developed by Chin and his co-workers. These experimental findings could be the unprecedented evidence for the self-focusing and refocusing effect of the femtosecond laser filament propagation in nonlinear media.

Storage and transfer of optical excitation energy in GaInP epilayer:Photoluminescence signatures

GaInP alloy could be the most trusted key material for fabricating super-high-efficiency single-and multijunction solar cells, especially for space applications. The storage and transfer of optical excitation energy in this key alloy is thus a key subject of the energy conversion from optical to electrical. In this article we present a study of the subject through investigating photoluminescence(PL) degradation in the GaInP epilayer at 4 K under the continuous optical excitations of ultraviolet(UV) 325 nm, visible 488.0 and 514.5 nm lasers. It is found that the decline of PL intensity with the irradiation time may be represented by I(t)/I0=(1 + tτ-1)-1+C, where I0 is the luminescence intensity at the beginning of irradiation, a time constant, and C a background. Moreover, the PL degradation degree reduces with increasing the excitation wavelength. In addition, some red shift of the PL peak is observed accompanying with the intensity decline under the UV laser excitation. These PL signatures indicate that the localized carriers within the local atomic ordering domains play a major role in the storage and transfer of the excitation energy via photon recycling processes.