(Received 6 January 2012; accepted 14 January 2012; published online 20 February 2012.)
Abstract: We report on a study of terahertz (THz) generation using implanted InGaAs photomixers and
multi-wavelengthquantumdot lasers. We carry out InGaAs materials growth, optical characterization, device design and fabrication, and photomixing experiments. This approach is capable of generating a comb of electromagnetic radiation from microwave to terahertz. For shortening photomixer carrier lifetime, we employ proton implantation into an epitaxial layer of lattice matched InGaAs grown on InP. Under a 1.55 μm multi- mode InGaAs/InGaAsP quantumdotlaser excitation, a frequency comb with a constant frequency spacing of 50 GHz generated on the photomixer is measured, which corresponds to the beats of the laser longitudinal modes. The measurement is performed with a Fourier transform infrared spectrometer. This approach aﬀords a convenient method to achieve a broadband multi-peak coherent THz source.
in a hybrid quantumdot comb laser. Results show that both the optical bandwidth and the flatness of the optical frequency comb can be improved.
High performance optical interconnects with large bandwidth play an important role in the next generation of data centers and supercomputers , and wavelength-division multiplexing (WDM) becomes promising in using the vast amount of bandwidth offered by optical fibers. Multiple single-wavelengthlaser is currently applied in achieving WDM. Nevertheless, multi-wavelengthlaser such as frequency comb laser become more attractive owing to its free spectral range (FSR) that is determined and fixed by the length of laser cavity. On the other hand, Si-based quantumdot (QD) lasers is a promising WDM light source, as they have shown high tolerance to both temperature  and external optical feedback , as well as low relative intensity noise (RIN). In the particular case of QD materials, increasing the inhomogeneous gain bandwidth above a certain value does not systematically bring an increase of a frequency comb bandwidth, and in some cases it can even lead only to unlocked regimes (no frequency combs). One explanation might be that for larger gain linewidth the four wave mixing intended as self-injection locking mechanism , in providing equally spaced and phase-locked modes becomes less efficient since dispersion for high frequency active modes is bigger. Last but not the least, the inhomogeneous broadening is an incoherent mechanism that one might expect to be detrimental to some extent for a coherence phenomenon as self-mode locking. In this work, we point out ways to improve the comb dynamics of a hybrid-silicon QD comb laser through the linewidth enhancement factor (LEF) and optical injection. It is shown that the frequency comb dynamics is enhanced at larger LEFs whereas the optical injection can further improve both the bandwidth and the flatness of the entire comb spectrum.
In recent years we have reported on InAs/InP quantumdot (QD) multi-wavelength lasers emitting light over a large wavelength range covering the C- and L-bands [12–17]. In addition to emitting a stable comb of wavelengths distributed over a 10-20 nm wide band, these Fabry- Perot lasers self-mode-lock with no need for special mode-locking structures. Therefore they provide a simple multi-wavelength coherent comb source (CCS) with channel spacing determined by the laser cavity length. The unique properties of these lasers arise from the gain medium which is composed of millions of InAs semiconductor dots less than 50 nm in diameter. Each QD acts like an isolated light source interacting independently of its neighbours, and emits light at its own unique wavelength. In other words the InAs QD gain medium is inhomogeneously broadened, unlike the uniform semiconductor layers in quantum well (QW) lasers that are deployed in telecommunications today. The combination of this inhomogeneous broadening and mode-locking results in a coherent multi-wavelengthlaser source where each channel is inherently stable with lower intensity noise than comparable quantum well (QW) based semiconductor lasers [16-17].
Keywords: frequency comb, quantumdot, silicon photonics, optical injection, linewidth enhancement factor. 1 INTRODUCTION
Broadband optical light sources play a key role in the rapid development of wavelength-division multiplexing (WDM) technologies, which are solutions of high transmission capacity to meet the huge demand in the upcoming 5G telecommunication industry, next generation of data centers and LiDAR system applied to self- driving cars [ 1 ]. In contrast to the multiple single-wavelength lasers configuration, the multi-wavelength light source such as the optical frequency combs (OFCs) is therefore a competitive candidate for performing WDM functions, owning to the possibility to achieve a large number channels with equidistant free spectral range (FSR), which is able to support the huge demand in transmission capacity [ 2 ]; on the other hand, its reduced device footprint is also advantageous for photonic integrated circuits (PICs) applications. Quantumdot (QD) lasers have been found to be efficient solution to OFCs owning to straightforward comb generation, large gain bandwidth, narrow spectral linewidth, low relative intensity noise (RIN) and high temperature stability [ 3 ], [ 4 ]; hybrid semiconductor comb lasers fabricated on silicon substrate are developed as well to meet the requirement of low-cost and energy-efficient integrated photonic component for PICs. In this paper, we report on the improved performance of 1.3 µ m hybrid QD comb lasers. To do so, we analyze the linewidth enhancement factor ( α H -
DOI: 10.1103/PhysRevLett.102.047401 PACS numbers: 78.67.Hc, 78.20.Ls, 78.47. p, 78.55.Cr
Quantum dots have confined energy levels analogous to ordinary atoms. Two quantum dots in close proximity can be viewed as an artificial diatomic molecule when coherent tunnel coupling leads to the formation of delocalized states. The properties of such quantum-dot molecules (QDMs) have been the focus of much research because of potential applications in novel optoelectronic devices or quantum information processing. In analogy with natural diatomic molecules, one expects the lowest energy delo- calized molecular state to have bonding orbital character. However, recent theoretical studies have predicted that the molecular ground state for a hole in an InAs QDM can have antibonding character [ 1 – 3 ]. If verified by experiment, an antibonding molecular ground state would provide a strik- ing example of a novel property of artificial atoms that cannot simply be explained as a rescaled version of the physics of real atoms.
De nos jours, les lasers à semi-conducteurs jouent un rôle crucial dans le domaine dans le développement des communications optiques modernes. L’accroissement exponentiel du trafic de données dans les réseaux optiques est un moteur pour le développement de nouvelles technologies de lasers à semi-conducteurs. Depuis la première démonstration de laser à semi- conducteurs en 1962, les développements ont successivement porté sur les lasers à semi- conducteurs massifs (3D), sur les lasers à puits quantiques (Qwell, 2D) et à fils quantiques (Qwire, 1D), ou sur les lasers à nanostructures quantiques plus avancés tels que les îlots quantiques (Qdot ou Qdash suivant l’anisotropie de la géométrie). Les lasers à îlots quantiques dans la filière InAs/GaAs sont parfaitement adaptés pour la bande O (1260-1360 nm) des communications sur fibres optiques, alors que la filière InAs/InP développée plus récemment, est plus adaptée à la bande C (1530-1565 nm) et aux communications très longues distances. Par comparaison avec les lasers à puits quantiques, les lasers à îlots quantiques présentent des caractéristiques statiques potentiellement supérieures telles que, de faibles courants de seuil [ Liu99 ], des stabilités en température plus élevées [ Mikhrin05 ], des gains matériaux importants [ Maximov04 ], et des gains différentiels élevés [ Bhattacharyya99 ]. Les îlots quantiques possèdent par ailleurs des niveaux d’énergie quantifiés, que l’on peut représenter de manière simplifiée au niveau composant par un état fondamental (GS) de paires électrons-trous, mais aussi des niveaux excités (ES) et un quasi- continuum d’états qui forme un réservoir de porteurs (RS). Les populations de ces états ainsi que les différents phénomènes de relaxation de porteurs, rendent l’analyse de la dynamique des composants lasers à îlots quantiques plus complexes que dans le cas des lasers à puits quantiques. Par ailleurs, les îlots quantiques obtenus par les approches « bottom-up » et le mode de croissance Stranski-Krastanow, présentent des élargissements inhomogènes des pics d’émission (30-80 meV) liés aux fluctuations de taille des îlots [ Bhattacharya00 ], qui peuvent avoir un impact négatif sur les performances des lasers. Si les lasers à îlots quantiques peuvent potentiellement atteindre des coefficients de couplage phase-amplitude quasi- nul (facteur α) [ Saito00 ], les valeurs expérimentales sont assez dispersées et dépassent même parfois 10 [ Dagens05 ]. Par ailleurs, la bande passante des lasers à îlots modulés reste typiquement inférieure à 10 GHz (si on exclut le cas de l’injection tunnel ou des lasers à dopage p), et donc plus faible que celle des meilleurs lasers à puits quantiques [ Bhattacharya00a ].
hybrid molecules. Here we report the realization of resonance-hybrid states in a few-electron triple quantumdot
(TQD) obseved by excitation spectroscopy. The stabilization of the resonance-hybrid state and the bond between contributing states are achieved through access to the intermediate states with double occupancy of the dots. This explains why the energy of the hybridized singlet state is significantly lower than that of the triplet state. The properties of the three-electron doublet states can also be understood with the resonance-hybrid picture and geometrical phase. As well as for fundamental TQD physics, our results are useful for the investigation of materials such as quantumdot arrays, quantum information processors, and chemical reaction and quantum simulators.
It is evident that exciton emission along the wire axis (direction ) is linearly polarized. This is sur- prising, since the growth axis of wires corresponds to the crystallographic three-fold axis with high symme- try (at least C 3 for both possible crystalline phases: wurtzite and sphalerite). Therefore, linear polariza- tion is definitely not related to NW anisotropy in the plane perpendicular to axis . Two important fac- tors need to be mentioned: (1) the polarization degree varies strongly from one sample to the other; (2) the polarization direction is tied to crystallographic axes of the substrate. The detected luminescence line is specific in that its polarization is not associated with line splitting, which was often the case in anisotropic quantum dots .
In this Letter, a 42.7 Gbit/s RZ transmitter using the QD FP MLLD module is assessed thanks to Bit Error Rate (BER) measurements, and its chirp is evaluated through chromatic dispersion tolerance investigations.
Source description and characterisation: The chip and the module were fabricated at III-V lab . The QD-based hetero-structures were grown by GSMBE on a S-doped (100) InP wafer. The active core consists in 6 InAs QDs layers and a GaInAsP injector quantum well which is separated by a thin InP barrier designed for phonon-assisted tunnel injection (TI) lasers emitting at 1.55 µm. The TI layers are enclosed within barriers and two separate confinement hetero-structure (SCH) layers. Along with the FP chip, a temperature probe, a Peltier cooler and a micro-wave V-type connector have been integrated into a butterfly module.
The laser epitaxial structure of the MLL device is a multi-stack "Dots-in-a-WELL" (DWELL) structure that is composed of an optimized six-stack InAs QD active region grown by elemental source molecular beam epitaxy (MBE) on an n+-doped, <100>-oriented GaAs substrate. 12 The 3-µm-wide optical ridge-waveguide devices are fabricated following standard dry-etch, planarization, and metallization processing. The two-section QD passive MLLs are made with a total cavity length of 7.8-mm and a saturable absorber (SA) length of 1.1-mm. A highly reflective coating (R ≈ 95%) is applied to the mirror facet next to the SA and the output facet is cleaved (R ≈ 32%).
The proposed UMWS PON is believed to be a promising candidate for the next generation access network thanks to the following reasons. First, the PON provides an advantage of simply upgrading the present TDM-PON in the upstream capacity by introducing multiple wavelengths (to avoid higher burst mode data speed at ONUs) and keeping the fiber transmission link intact. Moreover, all ONUs share the all upstream wavelengths to transfer upstream data with a wavelength or finer sub-wavelength granularity, which improves significantly bandwidth utilization with inter-channel statistical multiplexing (via wavelength channel switch). Last but not least, the UMWS PON also presents a cautious upgrade path in that wavelength channels can be added on the user demand. More precisely, only ONUs with higher traffic demands may be upgraded by deploying proposed self-seeding laser module, while ONUs with lower traffic demands remain unaffected. Thus, a single-channel TDM- PON can be upgraded alternatively into a heterogeneous WDM (wavelength division multiplexed) /TDM PON in which the ONUs differ in upstream capabilities.
Wavelength-division multiplexing (WDM) solutions can be strongly supported by a variety of photonics integrated technolo- gies that can be reused as what was done in telecommunication industry in using the vast amount of bandwidth offered by opti- cal fibers which contributed to enhance the data transmission. Therefore, the largest future cost reduction due to integration will then likely be enjoyed by more feature-rich blocks such as WDM versus single-wavelength interface[ 1 ]. The realiza- tion of WDM functions is mainly based on the multiple single- wavelengthlaser sources, however, the massive laser bar could be disadvantage in several applications such as photonic inte- grated circuits (PICs). Optical frequency combs (OFCs) are thus a competitive candidate for WDM, considering that the past laser bar could be easily replaced by a single laser. Various researches on OFCs were deployed in the past decade for optical com- munications in particular with quantumdot (QD) lasers which were found to be efficient comb light sources owing to the very large gain bandwidth[ 2 ] as well as to the narrow linewidth[ 2 , 3 ], low relative intensity noise[ 4 ] and higher resistance against ex- ternal reflections[ 5 , 6 ] and temperature[ 7 ]. With the view of developing low-cost and energy-efficient integrated photonic components for PIC technologies, hybrid semiconductor comb lasers fabricated on silicon substrate have already shown high transmission efficiency[ 8 ]. Further improvements need to be
External cavity lasers show a variety of uses, for which quantum well semiconductor lasers are already com- mercially used. Due to the atom-like discrete energy levels, quantum dots exhibit various properties resulting from the three-dimensional confinement of carriers, like high stability against temperature variation, large gain bandwidth, and low-threshold lasing operation. Quantum dots seem to be ideal to address the challenges in the further development of various semiconductor applications, such as high-resolution spectroscopy or broad- band optical communication networks, for which a range of spectral and temporal characteristics is required, for instance a narrow spectral linewidth, low intensity noise or wide wavelength tunability. In this view, exter- nal cavity quantumdot gain chips can be envisoned to replace the current quantum well technology. Using a semi-analytical rate equation model, we successfully analyze both dynamical and noise properties of an external cavity laser made with quantumdot gain medium, operating under strong optical feedback. This paper inves- tigates the turn-on delay, the relative intensity noise, and the frequency noise and compares them to the case without optical feedback. These numerical investigations of an external cavity quantumdot gain chip provide meaningful building blocks for future fabrication research or for developing high performance device such as wavelength-selective components.
We have studied the linear and differential conductances as well as the charge susceptibility of a noninteracting quantumdot by using the Keldysh nonequilibrium Green function technique. The obtained expressions are exact and allows one to study the variation of the conductances and charge susceptibility with temperature and any parameters of the double quantumdot model, energy levels ε 1 , ε 2 of the dots, Γ L , Γ R and interdot hopping t. We have then discussed the evolution of the stability
CA 93106, USA;
5 Center for High Technology Materials, University of New-Mexico, New Mexico 87106, USA
Abstract—This work experimentally investigates the relative intensity noise (RIN) of semiconductor quantumdot (QD) lasers epitaxially grown on silicon. Owing to the low threading dislocations density and the p-modulation doped GaAs barrier layer in the active region, a RIN level as low as -150 dB/Hz at 9 GHz is demonstrated. The results show that the p-doping decreases the high-frequency RIN and the damping factor. In the latter, a damping factor up to 30 GHz at three times the threshold is extracted from the RIN spectrum along with a K-factor of 1.7 ns. These results pave the way for high speed and low noise QD devices for future integrated photonics technologies.
[ http://dx.doi.org/10.1063/1.4906921 ]
Semiconductor optical waveguides that embed individ- ual quantum dots (QDs) feature appealing assets to realize integrated quantum photonic circuits for quantum informa- tion processing or quantum simulation. 1 In most implemen- tations, achieving a large coupling between the emitter and a single guided mode is of critical importance. In this perspec- tive, the fraction b of spontaneous emission (SE) funnelled into the guided mode of interest constitutes a very important figure of merit. Nowadays, state-of-the-art photonic crystal waveguides offer close to unity b-factors. 2 – 4 However, the large transmission losses of this waveguide technology 5 pre- vents a straightforward scaling up to large photonic chips. One way to get around this difficulty is to couple a short pho- tonic crystal section to a ridge waveguide, 6 which features excellent optical transmission. 7 Alternatively, in a much sim- pler approach, the whole photonic circuit—including the QD section—can be built with ridge waveguides. Using this strategy, an on-chip QD-beam splitter and QD-spin interface have been recently demonstrated. 8 , 9 On the detection side, a superconducting nanowire located in the evanescent field of a ridge waveguide can be used to detect guided photons with low noise and high efficiency. 10 Despite these promising advances, the tailoring of QD spontaneous emission by ridge waveguides has not yet been investigated experimentally.
The InAs QD structure was grown by gas source molecular beam epitaxy (MBE) on 2-inch InP(311)B substrates. SK InAs QDs were formed at 480° C after the deposition of few monolayers (ML) of InAs onto InP-lattice matched Ga 0.2 In 0.8 As 0.435 P 0.565 alloy with a typical bandgap of 1.18
µm (addressed as Q1.18 hereinafter). This alloy is used as the absorbing layer for optical injection in VECSEL operation. When dealing with QDs as the active region of a VECSEL, the most difficult part is to get high enough gain at the operating wavelength. This fact implies a challenge of achieving high QDs density per QD layer, as well as stacking of a large number of QD layers.
applicable in extending the coherence of the GHZ state. This problem will be investigated in the future.
IV. SUMMARY AND CONCLUSION
In summary, we designed and analyzed theoretically a lateral quantumdot molecule combined with a micromagnet generating a maximally entangled three particle GHZ GS. The quantumdot molecule consists of three quantum dots with one electron spin each forming a central equilateral triangle. The antiferromagnetic spin-spin interaction is changed to the ferromagnetic interaction by additional doubly occupied quantum dots, one dot near each side of a triangle. Exact diagonalization studies of the Hubbard model of the molecule determine the phase diagram in parameter space and a set of parameters is established for which the GS of the molecule in a radial magnetic field is well approximated by a GHZ state.
from 50 to 5 cm. Our initial results show that both the p-doped and undoped QD lasers are free of chaos as the feedback ratio is increased up to ∼ 20%. Despite that, experiments conducted in the short-cavity region raise period-one oscillation for the undoped QD laser. In this study, the boundaries associated to the appearance of the periodic oscillation are well identified and are shown to depend on the external cavity length. As counterpart, the p-doped QD laser is always free of any dynamics up to the maximum feedback strength in the short delay region. Last but not the least, whatever the external cavity length, the p-doped QD laser always exhibits a higher tolerance for the external feedback over the undoped one owning to the lower α-factor. Overall, these results show for the first time the p-modulation doping effect on the enhancement of feedback insensitivity in both short- and long-delay configurations, which is of paramount importance for the development of ultra-stable silicon transmitters for photonic technologies.
Wavelength Tunability Assessment of a 170 Gbit/s transmitter using a Quantum Dash Fabry Perot mode-locked laser
M. Costa e Silva (1) , H. Ramanitra (2) , M. Gay (1) , L. Bramerie (1) , S. Lobo (1) , M. Joindot (1) , J.C. Simon (1) , A. Shen (3) , G- H. Duan (3)