We report epitaxially regrown Photonic Crystal Surface-Emitting Lasers (PCSELs) utilizing self-assembled InAs quantum dots (QDs) exhibiting lasing at room temperature. The ability to utilize both the ground-state (GS) and excited-state (ES) of the QDs allows multiple emission wavelengths from one heterostructure. The choice of the grating periods of the photonic crystal allows lasing from neighbouring devices at the GS (~1230 nm) or ES (~1140 nm) of the QDs, 90 nm apart in wavelength. The threshold current densities are 0.69 kA/cm2 and 1.05 kA/cm2 for GS and ES respectively. The effect of PC structures, specifically etch depth of the PC on lasing performance is also discussed.
We propose a self-evaluation of visual field using free-focus retinal scanning laser display technology based on Maxwellian-viewing optics. The developed device was the combination of commercially available smartphone with built-in laser pico-projector and newly developed additional optics to realize Maxwellian-view and eye-safety. Movie with flying and fixed gazing spot was displayed on the subject’s retina for evaluation. The non-recognition areas were found by the self-evaluation not only at the Marriott blind spot but at other area corresponding to the abnormal visual field in the Amsler chart test related to the inflated upstream blood vessel due to the retinal vein occlusion.
GaAs-based quantum dot (QD) semiconductor lasers at 1300 nm exhibit a number of benefits for use in metropolitan and local area networks. Low modulation-induced chirp attributed to small linewidth enhancement factor (LEF) in QDs overcomes some limitations in a directly-modulated laser compared to quantum well counterparts. Along with the temperature-insensitive low threshold current, these QD lasers demonstrate considerable prospects towards Peltier-free direct-modulation operation.
In this work, we experimentally investigate the temperature-dependence of the LEF as a function of injection current and wavelength in state-of-the-art 1300 nm In(Ga)As/GaAs QD lasers grown by molecular beam epitaxy. These structures demonstrate a large state separation and a comparatively small inhomogeneous linewidth. Gain is measured using the Hakki-Paoli method and carrier related mode shift is extracted by correcting for thermal effects.
Previous reports have described LEF values of 1-5 over smaller temperature ranges (20-85°C) than the GPON commercial specifications in 1300 nm In(Ga)As/GaAs QD structures.
Here, at temperature range of -10 to 85°C extending beyond GPON commercial window, it is shown that even at higher temperatures where contribution from the excited state transition is expected to increase the LEF, the measured LEF remains essentially zero. This is ascribed to the large gain from the ground state transition. Such a low LEF at elevated temperatures bodes well for the use of such structures in applications where feedback insensitivity may be important. By analyzing individual modes as a function of their individual lasing thresholds, we also describe an interplay of mode position and lasing linewidth.
KEYWORDS: Retina, RGB color model, Laser safety, Eye, Visualization, Retinal scanning, Eyewear, Mirrors, FDA class I medical device development, Image resolution
Retinal imaging laser eyewear has a miniature laser projector inside the frame which provides the wearer with digital image information through the pupil using the retina as a screen. Its principle is based on the geometric optics of the Maxwellian view combined with a parallel and narrow RGB laser beam. A prototype with the trademark of RETISSA® was invented with a miniature laser projector inside the glasses frame using a non-axisymmetric free-surface reflecting mirror. The image resolution was measured based on the visual acuity testing using a retinal projected image of Landolt ring for four subjects with the different naked visual acuity of 0.04, 0.5, 0.9 and 1.2. Also, the theoretical image resolution was studied based on the beam propagation simulation under the eyeball model. The results show how to achieve high resolution and free focus in proper balance by adjusting the laser beam characteristics of the beam diameter and divergence. On the laser safety, RETISSA® was found to be in the Class I category, which has the safety factor of over 700 in the RGB radiation intensity under the international standard of IEC60825-1. RETISSA® also met the thousand times more strict Class I criteria of FDA/CDRH 21CFR1040.10 with the total RGB radiation intensity of less than 0.37 μW, indicating its laser radiation is not considered to be hazardous as stated in the definition of FDA Class I. The experimental evidence that the radiation of RETISSA® is equal to or weaker than displays of conventional digital devices also provide its proof of safety, including its long-term use as one of daily digital devices. The potential of retinal laser imaging is mentioned for use in ophthalmology medicine. The current activity on the medical welfare applications as low vision aids and ophthalmic testing equipment is reviewed including clinical research and trials in Japan and Europe.
We simulate the shape of the density of states (DoS) of the quantum dot (QD) ensemble based upon size information provided by high angle annular dark field scanning transmission electron microscopy (HAADF STEM). We discuss how the capability to determined the QD DoS from micro-structural data allows a MonteCarlo model to be developed to accurately describe the QD gain and spontaneous emission spectra. The QD DoS shape is then studied, with recommendations made via the effect of removing, and enhancing this size inhomogeneity on various QD based devices is explored.
High temperature stability and high feedback-noise tolerance of the quantum dot lasers are advantageous features for application to silicon photonics. A silicon optical interposer with the bandwidth-density of 15Tbps/cm2 at 125 °C was demonstrated using flip-chip bonding method. Moreover, we report the first demonstration of a hybrid silicon quantum dot (QD) laser, evanescently coupled to a silicon waveguide. InAs/GaAs QD laser structures with thin AlGaAs lower cladding layers were transferred, by means of direct wafer bonding, onto silicon waveguides defining cavities with adiabatic taper structures and distributed Bragg reflectors (DBRs). The laser operates at temperatures up to 115 °C under pulsed current conditions, with a characteristic temperature T0 of 303 K near room temperature. Furthermore, by reducing the width of GaAs/AlGaAs mesa down to 8 μm, continuous-wave operation is realized at 25 °C.
We describe a study of electro-absorption effects in high quality 1300nm InAs/GaAs quantum dot (QD) material grown by molecular beam epitaxy. The photocurrent spectra as a function of electric field is investigated and the quantum confined Stark shift of the QD states is compared to reports for various quantum well (QW) systems (GaAs/AlGaAs, InGaAs/GaAs, InGaAsP/InP). We show that the rate of shift of the QD absorption peak is smaller than that of the reported QW systems (~0.1 meV/kVcm-1 c.f. 0.15-0.2 meV/kVcm- 1) and that the QD ground-state absorption is comparatively insensitive to the applied electric field. We observe a strong QD absorption peak at all biases up to avalanche breakdown, which is not observed in previous reports for these QW systems.
We describe the development of hybrid quantum well (QW)/quantum dot (QD) active elements to achieve broad spectral bandwidth spontaneous emission and gain. We have previously reported that the placement of the QW within the active element is a critical factor in obtaining broad spectral bandwidth emission. We now present new designs to further broaden the spontaneous emission from hybrid structures by increasing the number of QD layers and dot density, and by using QDs with wider state-separation. Introducing chirped QD layers reduced the modulation in the spontaneous emission spectra, and by utilising self-heating effects and state-filling, a spontaneous emission with 3dB line-width of 350nm is obtained.
We have investigated the temperature dependence of InAs columnar quantum dots (CQDs) surrounded by InGaAsP
barriers with different bandgap energies toward high-temperature performance for semiconductor optical amplifiers. It
was found that larger bandgap energy in InGaAsP side barriers enabled to increase the quasi-Fermi level (F) separation
between the conduction and valence bands from theory. We have fabricated two types of CQD-SOAs with different side
barrier energies and compared temperature characteristics. Decrease in the material gains for CQD with a larger side
barrier bandgap was suppressed by 20% with increasing temperature from 25 °C to 85 °C.
High temperature (>125°C) resistant long-wavelength semiconductor lasers are attractive as light sources in a variety of
harsh environments. Here, we report extremely high temperature continuous-wave (CW) operation of QD lasers on
GaAs substrate emitted at 1300-nm-range by enhancing gain and increasing the quantized-energy separation of the QD
active layers. A suppression of the In out-diffusion during MBE from self-assembled InAs QDs significantly reduced
inhomogeneous broadening with high QD sheet density maintained. QD-FP laser exhibited record high CW-lasing
temperature for long-wavelength laser of 220°C and QD-DFB laser also exhibited high CW-lasing temperature of 150°C
by employing high gain QD active media.
We present a general method that improves the emission efficiency of InAs quantum dots (QDs) fabricated by antimony
surfactant-mediated growth. Unlike conventional InAs/GaAs QDs, we show that the control of the interface properties of
the InAs/Sb:GaAs QDs is crucial. Our method consists in growing InAs QDs on an antimony-irradiated GaAs surface, in
order to exploit the surfactant properties of antimony, and then removing the excess segregated antimony by applying a
high arsenic pressure before capping. In such a way, one benefits from the advantages of the antimony-surfactant
mediated growth (high density QDs, no coalescence, no emission blueshift after annealing), without the detrimental
formation of antimony-induced non-radiative defects. We show that the lasing characteristics of InAs/Sb:GaAs QD
lasers grown by metal organic chemical vapor deposition in the 1.3 μm band are drastically improved, with a reduced
threshold current density and higher internal quantum efficiency. These studies advance the understanding of key
processes in antimony-mediated growth of InAs QDs and will allow full utilization of its advantages for integration in
opto-electronic devices.
We demonstrate temperature-insensitive eye-opening under 10-Gb/s direct modulation of 1.3-μm p-doped quantum-dot lasers without current adjustments, which show 6.5-dB extinction ratio between 20 and 70°C. The active region consisting of ten quantum-dot layers with p-type doping enabled this highly temperature-stable dynamic performance, much superior to conventional 1.3-μm quantum-well lasers. This result opens a way to uncooled 1.3-μm quantum-dot lasers without current adjustments.
This paper reviews the recent progress of quantum-dot semiconductor optical amplifiers developed as ultrawide-band high-power amplifiers, high-speed signal regenerators, and wideband wavelength converters.
This paper provides current status and prospects of quantum-dot semiconductor optical amplifiers, based on our pioneering
work covering the proposal of their promising features, the quantum-dot optical device theory, experimental demonstrations,
and the design and assembly of all-optical switching modules.
This paper provides prospects and current status of quantum-dot semiconductor optical amplifiers (SOAs) and their comparison with bulk and quantum-well technology, based on our pioneering work covering the proposal of their promising features, subsequent experimental demonstrations, and the design of all-optical quantum-dot switching modules. The proposed promising features are diverse; high-saturation power, high-speed amplification up to 160 Gb/s under gain saturation without pattern effect, high-speed cross-gain modulation up to 160 Gb/s without pattern effect, multiple-wavelength processing over broad gain spectra, and symmetric wavelength conversion by four-wave mixing. The operation theory of quantum-dot SOAs is provided in order to treat various aspects unique to quantum dots such as spatial localization, retarded carrier relaxation, and inhomogeneous and homogeneous broadening. Pattern-effect-free amplification at 10-40Gb/s, wavelength conversion by the cross-gain modulation at 10-40Gb/s, and symmetric wavelength conversion by four-wave mixing are experimentally demonstrated in 1.3-micron InGaAs/GaAs quantum-dot SOAs. All-optical quantum-dot switching modules are proposed, which we expect to work in the next-generation flexible all-optical photonic networks.
We examined the temperature characteristics of light emission spectra and threshold currents for columnar-shaped self- assembled InGaAs quantum dot lasers which show lasing from the ground-state quantized level. While lasing occurred with one line including a series of longitudinal modes at room temperature, spectra at 80 K showed broad lasing emission over a range of 50 - 60 meV. We explain this result by a model that dots with different energies start lasing independently at low temperatures due to their spatial localization, while at room temperature the dots contribute to one-line lasing collectively via homogeneous broadening of optical gain. We attribute the origin of homogeneous broadening to scattering processes to exchange carriers between the ground state and excited states. The characteristic temperature of threshold currents was T0 equals 60 - 90 K between 287 K and 353 K. By evaluating the current dependence of the spontaneous emission spectra up to lasing thresholds at various temperatures, we found that this low T0 is due to the increase in the nonradiative current component, presumably caused by nonradiative carrier recombination through defects in the wetting layer. These results show the importance of controlling carrier distribution in the excited states including the wetting layer to achieve high-performance quantum-dot lasers.
The purpose of this work is to study the effect of carrier dynamics on quantum-dot laser performance and to clarify how the carrier relaxation lifetime and the bi-exciton effect manifest themselves in the quantum-dot laser operation. We derive carrier-photon rate equations for quantum-dot lasers and simulate the output power vs. injected current relationship and the small-signal modulation response. We clarify criteria on the carrier relaxation lifetime as well as the inhomogeneous broadening linewidth, the dot density and the crystal quality to achieve high-performance. Then, we develop an optical gain formula for the bi-exciton- exciton transition and modify rate equations to describe bi- exciton lasing in quantum dots. We show spontaneous emission and lasing properties of bi-excitons, and discuss the effect of carrier relaxation lifetime and oscillator strength on the laser operations.
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