Semiconductor lasers are convenient and compact sources of light, offering highly efficient operation, direct electrical control and integration opportunities. In this review we describe how semiconductor quantum-dot structures can provide an efficient means of amplifying and generating ultrafast (of the order of 100 fs), high-power and low-noise optical pulses, with the potential to boost the repetition rate of the pulses to beyond 1 THz. Such device designs are opening up new possibilities in ultrafast science and technology.

Mode-locked quantum-dot lasers

We provide an in-depth treatment of the various mechanisms by which an incident light beam can produce an intensity- or flux-dependent change in the refractive index and absorption coefficient of different materials. Whenever possible, the mechanisms are initially traced to single-atom and -molecule effects in order to provide physical understanding. Representative values are given for the various mechanisms. Nine different mechanisms are discussed, starting with the Kerr effect due to atoms and/or molecules with discrete states, including organic materials such as molecules and conjugated polymers. Simplified two and/or three-level models provide useful information, and these are summarized. The nonlinear optics of semiconductors is reviewed for both bulk and quantum-confined semiconductors, focusing on the most common types II–VI and III–V. Also discussed in some detail are the different nonlinear mechanisms that occur in liquid crystals and photorefractive media. Additional nonlinear material systems and mechanisms such as glasses, molecular reorientation of single molecules, the electrostrictive effect, the nuclear effect (vibrational contributions), cascading, and the ever-present thermal effects are quantified, and representative tables of values are given.

Nonlinear refraction and absorption: mechanisms and magnitudes

In this paper a tutorial of optical injection locking of semiconductor lasers is given, with particular emphasis on the enhancement of system parameters. Furthermore, physical intuition of each parameter enhancement is explained and practical design rules and trends are also shown.

Enhanced Modulation Characteristics of Optical Injection-Locked Lasers: A Tutorial

Layered hybrid organic perovskites (HOPs) structures are a class of low-cost two-dimensional materials that exhibit outstanding optical properties, related to dielectric and quantum confinement effects. Whereas modeling and understanding of quantum confinement are well developed for conventional semiconductors, such knowledge is still lacking for 2D HOPs. In this work, concepts of effective mass and quantum well are carefully investigated and their applicability to 2D HOPs is discussed. For ultrathin layers, the effective-mass model fails. Absence of superlattice coupling and importance of non-parabolicity effects prevents the use of simple empirical models based on effective masses and envelope function approximations. An alternative method is suggested in which 2D HOPs are treated as composite materials, and a first-principles approach to the calculation of band offsets is introduced. These findings might also be relevant for other classes of layered 2D functional materials.

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Understanding Quantum Confinement of Charge Carriers in Layered 2D Hybrid Perovskites

We report on the experimental realization and characterization of an asynchronous heralded single-photon source based on spontaneous parametric down-conversion. Photons at 1550 nm are heralded as being inside a single-mode fibre with more than 60% probability, and the multi-photon emission probability is reduced by a factor of up to more than 500 compared to Poissonian light sources. These figures of merit, together with the choice of telecom wavelength for the heralded photons, are compatible with practical applications needing very efficient and robust single-photon sources.

https://iopscience.iop.org/article/10.1088/1367-2630/6/1/163/pdf

High-quality asynchronous heralded single-photon source at telecom wavelength

We demonstrate simultaneous lasing at two well-separated wavelengths in self-assembled InAs quantum-dot lasers, via ground-state (GS) and excited-state (ES) transitions. This effect is reproducible and strongly depends on the cavity length. By a master-equation model, we attribute it to incomplete clamping of the ES population at the GS threshold.

/pdf/simultaneous-two-state-lasing-in-quantum-dot-lasers-28l1gdkppd.pdf

Simultaneous two-state lasing in quantum-dot lasers

Measurements on 1.3-/spl mu/m quantum-dot lasers are presented that reveal a number of interesting effects. 1) At high bias, a second lasing line appears, corresponding to the excited state transition. 2) The linewidth enhancement factor increases dramatically above threshold. 3) The modulation performance is degraded when the second lasing line appears. A comprehensive numerical model is developed to explain this behavior. We attribute it to incomplete gain clamping above threshold. This is caused by a combination of the finite intraband relaxation time and the limited density of states.

Impact of intraband relaxation on the performance of a quantum-dot laser

InAs quantum-dot (QD) laser structures are grown on (113)B-oriented InP substrate by gas-source molecular-beam epitaxy. Following an optimized growth procedure, a high density of 1.1×1011cm−2 of uniformly sized QDs is achieved. Broad-area lasers containing three stacked QD layers have been realized and tested. Laser emission on the ground-state transition (λ=1.59μm) is obtained at room temperature (RT), at a threshold current density as low as 190A∕cm2. Ground-state modal gain and transparency current density is measured to be 7cm−1 and 23A∕cm2 per dot layer. Ground-state laser emission is also demonstrated from low temperature (100 K, Jth=33A∕cm2) to high temperature (350 K), exhibiting an insensitive threshold in the [100, 170] K range, and a 55 K characteristic temperature at RT.

High-gain and low-threshold InAs quantum-dot lasers on InP

We study the electronic and optical properties of $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}$ quantum dots (QDs) on (100) and $(311)B$ substrates. Atomic force microscopy (AFM) and cross-sectional scanning tunneling microscopy (X-STM) are used to define the size and shape of the quantum dots for calculations. Eight-band $\mathbf{k}\mathbf{∙}\mathbf{p}$ calculations including strain and piezoelectric effects are then performed on such a structure for two different kinds of substrate orientation $(311)B$ and (100). Results for several QD heights found on $(311)B$ substrate fit well the experimental data obtained from photoluminescence measurements. On $(311)B$ substrate, the shear and hydrostatic deformations are found to be enhanced compared to those on (100) substrate thus affecting the electronic and optical properties. The $(311)B$ QDs are found to activate second-order ($S\text{\ensuremath{-}}P$ channels) transitions resulting from the complete loss of symmetry due to the presence of the piezoelectric field.

/pdf/electronic-and-optical-properties-of-inas-inp-quantum-dots-4miyt6b4ng.pdf

Electronic and optical properties of InAs/InP quantum dots on InP(100) and InP(311)B substrates : Theory and experiment

We present a comparative study of carrier diffusion in semiconductor heterostructures with different dimensionality [InGaAs quantum wells (QWs), InAs quantum dots (QDs), and disordered InGaNAs QWs (DQWs)]. In order to evaluate the diffusion length in the active region of device structures, we introduce a method based on the measurement of the current-voltage and light-current characteristics in light-emitting diodes where current is injected in an area ,1 mm 2 . By analyzing the scaling behavior of devices with different sizes, we deduce the effective active area, and thus the diffusion length. A strong reduction in the diffusion length is observed going from QWs sLd< 2.7 mmd to QDs sLd, 100 nmd, DQWs being an intermediate case (Ldiff < 0 ‐ 200 nm depending on the carrier density). These results show that lateral composition fluctuations, either intended or unintended, produce strong carrier localization and significantly affect the carrier profile in a device even at room temperature.

/pdf/carrier-diffusion-in-low-dimensional-semiconductors-a-wmd8hl3jgh.pdf

Cyril Paranthoen

Papers

Simultaneous two-state lasing in quantum-dot lasers

Impact of intraband relaxation on the performance of a quantum-dot laser

High-gain and low-threshold InAs quantum-dot lasers on InP

Electronic and optical properties of InAs/InP quantum dots on InP(100) and InP(311)B substrates : Theory and experiment

Carrier diffusion in low-dimensional semiconductors: A comparison of quantum wells, disordered quantum wells, and quantum dots