Quantum-dot Optical Amplifiers Research Articles

The effect of doping on the electrons and holes in quantum dot semiconductor optical amplifiers

Abstract The doping effect on the reservoir carrier temperature of the electron and hole in quantum dot optical amplifiers (QD SOAs) has been formalized and modeling, taking into account the most of carrier heating (CH) contributions such as; free carrier absorption, carrier heating time relaxation, interdot relaxation time, occupation probability of dot level, injected current and electron-hole interaction. The theoretical simulation shows the carrier temperature increasing straight forward with increasing the surface density of the donor and accepter which had not studied earlier as the best of our knowledge. It clears that, the surface carrier concentration of donor or acceptor atoms supplying more hot carriers for wetting layer (WL) which is leading an increasing the carrier temperature. In other word, the long life time of carriers from WL to Quantum Dot (QD) states is very high. Consequently, the collision of carriers and nonradiative relaxation are increasing the CH effect. Also, the majority of carrier is responsible on the increasing of electron or hole temperature, so the variation electron temperature in N-type is higher than hole in valence band, while the reverse is done with doping with P-type.

Effect of carrier (hole) temperature on performance of optical amplifiers quantum dot structure

Abstract Carriers temperature in quantum dot (QD) optical amplifiers has been modelled theoretically taken into account hole contribution, which is not considered early. The contributions of wetting layer (WL), first excited state (ES1), and second excited state (ES2) have also been considered. Effect of WL−ES2 recombination time of both electrons and holes, carrier heating (CH) time of electrons and holes, in addition to electron hole recombination time are examined. The results show that there is a gap between electron and hole heating temperature due to the difference between there recovery times. It is found that fast hole (long electron) recombination time yield a high carrier temperature.

Wave mixing efficiency in InAs/GaAs semiconductor quantum dot optical amplifiers and lasers

The nonlinear features of both semiconductor optical amplifiers (SOAs) and semiconductor lasers, which are made from the same InAs/GaAs quantum dot (QD) wafers, are investigated in detail. By employing pump-probe driven four-wave mixing as an experimental tool, the wave conversion process shows notably different profiles for the two types of devices. Due to the contributions of ultrafast, sub-picosecond mechanisms, such as carrier heating and spectral hole burning, the pump-probe frequency can be easily tuned to the THz range. SOAs generally benefit more from sub-picosecond carrier dynamics, hence exhibiting a higher conversion efficiency (CE) in the THz range, compared to their laser diode counterparts. The discrepancy even exceeds 10 dB. In addition, laser experiments yield some differences from the amplifier ones, hence leading to a higher nonlinear CE at small detuning ranges. These results strongly improve our insight into the fundamental nonlinear properties of InAs/GaAs QD material, and contribute to the conception of novel devices for future on-chip applications in all-optical communication networks, such as signal wavelength conversion, mode-locking, and optical frequency comb generation.

Effect of time relaxations on the carrier heating of InAs/GaAs quantum dot semiconductor optical amplifier

AbstractCarrier temperature in InAs quantum dot (QD) optical amplifiers has been formalized theoretically by using the double‐excited states system. High current injection gives high carrier temperature and a punch in the curve appears at low current due to the ground state carrier contribution. Increasing carrier‐heating time increased carrier temperature. It is shown the slow relaxation for both the wetting layer to QD and the interdot relaxations are given high carrier temperature. Pulses with wide full width at half‐maximum give high carrier temperature since its recovery time is increased. Free carrier absorption gives a main contribution to the heat in the QD structure.

An analytical model for vertical dual-cavity quantum-dot optical amplifiers

Novel closed-form model for the threshold conditions and the optical gains of vertical dual-cavity semiconductor optical amplifier (VDC-SOA) is derived. Expressions for the transfer functions that relate the average photon density inside the two cavities to the incident photon density are derived. The model is simple, accurate and easy to use for designing dual-wavelength laser amplifiers. Good agreement is obtained between the analytical model and numerical calculations. The transmitted and reflective bistability characteristics of quantum dot VDC-SOA have been investigated. The threshold condition of each mode is expressed in terms of the top-cavity and the bottom-cavity currents. The dual modes $$\lambda_{o}^{ + }$$ and $$\lambda_{o}^{ - }$$ show different bistability behavior as a function of the top cavity current when the device operates at fixed operating point below the threshold curve. We find that the contrast ratio of mode $$\lambda_{o}^{ + }$$ is a weak function of the device currents, and the hysteresis width increases when the top cavity current is increased. For mode $$\lambda_{o}^{ - }$$ large contrast ratio and hysteresis width are obtained when the two cavities exhibit equal currents. Our analysis also reveals that the spectral characteristics of mode $$\lambda_{o}^{ + }$$ are different from that of mode $$\lambda_{o}^{ - }$$.

Enhancing environmental stability of a PbS quantum dot optical fiber amplifier via rational interface design

A PbS quantum dot optical fiber amplifier with enhanced processability and environmental stability is described. Modification of PbS quantum dots with multifunction copolymers was demonstrated via spectroscopy to aid dispersion of the quantum dots in a sol during the fabrication of a quantum dot optical fiber amplifier without degradation or modification of optical properties. The influence of multifunction copolymers and oleylamine-coated PbS quantum dots on the structure was studied via scanning electron microscopy. The nanoparticle shape of oleylamine-coated PbS quantum dots after polymer modification remained unchanged compared to those previously reported. With the addition of a fluorine-containing component, a PbS quantum dot optical fiber amplifier retained 96% of its initial gain after 100 days, as compared with 16% for quantum dots modified with a non-fluorine-containing polymer, it was certain that fluorine bond of multifunction copolymers had a good improvement in durability of PbS quantum dot. Simultaneously, PbS quantum dot optical amplifier gain as high as 17 dB was achieved at 1550 nm.

Static and dynamic characteristics of an InAs/InP quantum-dot optical amplifier operating at high temperatures.

We report on high quality InAs/InP quantum dot optical amplifiers for the 1550 nm wavelength range operating over a wide temperature range of 25 to 100 °C. A temperature dependent shift of the peak gain wavelength at a rate of 0.78 nm/K is observed. Consequently, two possible modes of operation are performed for a systematic device characterization over the entire temperature range. In the first mode, the signal wavelength is tuned to always match the peak gain wavelength while in the second mode, the signal wavelength is kept constant as the gain spectrum shifts with the temperature. Static characteristics, such as gain spectra and saturation levels, as well as dynamical properties, are presented. Distortion-less amplification of a single 28 Gbit/s signal and cross-talk free amplification of two channels, detuned by 2 nm, were demonstrated over the entire temperature range.

Controlling Photon Echo in a Quantum-Dot Semiconductor Optical Amplifier Using Shaped Excitation

Storage and manipulation of quantum information requires (nearly) perfect timing---or precisely timed pulse sequences, anyhow. In quantum memory based on photon echoes, for example, the appearance time of the echo must be determined with great accuracy. This study shows how to control photon echoes in a quantum-dot optical amplifier operating at room temperature. As the amplifier is a distributed device, propagation effects are dominant in the generation of the echo pulse, and shaping the excitation pulse serves to control the echo's appearance time and strength.

Heterogeneous quantum dot/silicon photonics-based wavelength-tunable laser diode with a 44 nm wavelength-tuning range

A heterogeneous wavelength-tunable laser diode combining quantum dot and silicon photonics technologies is proposed. A compact wavelength-tunable filter with two ring resonators was carefully designed and fabricated using silicon photonics technology. The tunable laser combining the wavelength-tunable filter and an optical amplifier, which includes InAs quantum dots, achieved a 44.0 nm wavelength-tuning range at around 1250 nm. The broadband optical gain of the quantum dot optical amplifier was effectively used by the optimized wavelength-tunable filter. This heterogeneous wavelength-tunable laser diode could become a breakthrough technology for high-capacity data transmission systems.

Ultra-compact wavelength-tunable quantum-dot laser with silicon-photonics double ring filter

Compact, wavelength-tunable light sources are desired for the enhancement of information communication technology and bio-imaging applications. We propose a compact, wavelength-tunable laser diode with a wide wavelength-tunable range around 1230 nm consisting of a quantum-dot optical amplifier and a silicon photonic tunable filter. High-quality InAs quantum dots grown with the sandwiched sub-nano separator technique were used as the optical gain medium. The wavelength-tunable filter was constructed with ring resonators fabricated using silicon photonics. The single-mode laser oscillation was demonstrated with a 28.5-nm wavelength-tunable range.

Nonlinear pulse propagation in InAs/InP quantum dot optical amplifiers: Rabi oscillations in the presence of nonresonant nonlinearities

We study the interplay between coherent light-matter interactions and non-resonant pulse propagation effects when ultra-short pulses propagate in room-temperature quantum-dot (QD) semiconductor optical amplifiers (SOAs). The signatures observed on a pulse envelope after propagating in a transparent SOA, when coherent Rabi-oscillations are absent, highlight the contribution of two-photon absorption (TPA), and its accompanying Kerr-like effect, as well as of linear dispersion, to the modification of the pulse complex electric field profile. These effects are incorporated into our previously developed finite-difference time-domain comprehensive model that describes the interaction between the pulses and the QD SOA. The present, generalized, model is used to investigate the combined effect of coherent and non-resonant phenomena in the gain and absorption regimes of the QD SOA. It confirms that in the QD SOA we examined, linear dispersion in the presence of the Kerr-like effect causes pulse compression, which counteracts the pulse peak suppression due to TPA, and also modifies the patterns which the coherent Rabi-oscillations imprint on the pulse envelope under both gain and absorption conditions. The inclusion of these effects leads to a better fit with experiments and to a better understanding of the interplay among the various mechanisms so as to be able to better analyze more complex future experiments of coherent light-matter interaction induced by short pulses propagating along an SOA.

Pump-probe quantum state tomography in a semiconductor optical amplifier.

Pump-probe quantum state tomography was applied to the transmission of a coherent state through an In(Ga)As based quantum dot optical amplifier during the interaction with an optical pump pulse. The Wigner function and the statistical moments of the field were extracted and used to determine the degree of population inversion and the signal-to-noise ratio in a sub-picosecond time window.

Rabi oscillations and self-induced transparency in InAs/InP quantum dot semiconductor optical amplifier operating at room temperature

We report direct observations of Rabi oscillations and self-induced transparency in a quantum dot optical amplifier operating at room temperature. The experiments make use of pulses whose durations are shorter than the coherence time which are characterized using Cross-Frequency-Resolved Optical Gating. A numerical model which solves the Maxwell and Schrödinger equations and accounts for the inhomogeneously broadened nature of the quantum dot gain medium confirms the experimental results. The model is also used to explain the relationship between the observability of Rabi oscillations, the pulse duration and the homogeneous and inhomogeneous spectral widths of the semiconductor.

Measurement and Analysis of Temperature-Dependent Optical Modal Gain in Single-Layer InAs/InP(100) Quantum-Dot Amplifiers in the 1.6- to 1.8-$\mu\hbox{m}$ Wavelength Range

In this paper, measurements and analysis of the small-signal net modal gain of single-layer InAs/InP(100) quantum-dot (QD) optical amplifiers are presented. The amplifiers use only a single layer of InAs QDs on top of a thin InAs quantum well. The devices have been fabricated using a layer stack that is compatible with active-passive integration scheme, which makes further integration possible. The measurement results show sufficient optical gain in the amplifiers and can thus be used in applications such as lasers for long-wavelength optical coherence tomography and gas detection. The temperature dependence of the modal gain is also characterized. An existing rate-equation model was adapted and has been applied to analyze the measured gain spectra. The current injection efficiency has been introduced in the model to obtain a good fit with the measurement. It is found that only a small portion ( ~ 1.7%) of the injected carriers is actually captured by the QDs. The temperature dependence of several parameters describing the QDs is also discovered. The mechanisms causing the blue shift of peak gain as the current density increases and the temperature changes are analyzed and discussed in detail.

Stable rectangular pulses supported by a quantum dot optical amplifier

AbstractWe predict the formation of a stable solitary rectangular pulse in semiconductor quantum dot media. The formation of such a pulse occurs when a quantum dot system is pumped in a way, where the s shell excitation exhibits gain while the p shell absorbs. We provide formulas to describe the rectangular pulse by the systems material parameters. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast gain dynamics in InP quantum-dot optical amplifiers

We measured the gain dynamics at the ground-state transition in an electrically pumped InP/AlGaInP quantum-dot optical amplifier at room temperature by femtosecond differential transmission. The gain shows an ultrafast recovery within 200 fs, even faster than in state-of-the-art InAs/GaAs quantum-dot amplifiers. This finding, likely to be due to the less confined and more closely spaced hole levels in InP dots, is promising for optical signal processing at high bit rates. We furthermore measured the pump-induced refractive index changes and deduced a linewidth enhancement factor similar to the one in InAs/GaAs quantum dots.

Analytical description of gain depletion and recovery in quantum dot optical amplifiers

We present an analytical description of ultrashort femtosecond pump–probe experiments and investigate the gain response of quantum dot (QD) semiconductor optical amplifiers. The calculation provides a full analytical solution of numerical studies to recent experiments in such structures (Gomis-Bresco et al 2008 Phys. Rev. Lett.101 256803). Our approach is based on QD Bloch equations, which are analytically evaluated within the third-order temporal perturbation theory (χ(3) level). In particular, we study the influence of the coherence and the population dynamics of two confined QD levels on the gain as a function of the delay time between the pump and the probe pulse. We discuss how to engineer optimal conditions for high-performance QD amplifiers, which are characterized by an ultrafast gain recovery and a pronounced gain depletion.

Ultrafast gain dynamics in 1.3μm InAs∕GaAs quantum-dot optical amplifiers: The effect of p doping

Ultrafast gain dynamics of the ground-state transition are measured in electrically pumped InAs∕GaAs quantum-dot amplifiers emitting near 1.3μm at room temperature. Gain recovery on a subpicosecond time scale occurs at high electrical injection. However, when comparing p-doped and undoped devices fabricated under identical conditions and operating at the same gain, faster absorption recovery but slower gain dynamics are observed in p-doped amplifiers. The slower gain dynamics is attributed to a reduced reservoir of excited-state electrons in p-doped quantum-dot devices, which limits the recovery of the electron ground-state occupation mediated by intradot carrier-carrier scattering.

Artificial control of optical gain polarization by stacking quantum dot layers

Polarization insensitivity of InAs∕GaAs quantum dot (QD) optical amplifier has been demonstrated by controlling the dot shape. The height of the QD has been controlled by stacking closely InAs islands to form a columnar QD. Room-temperature polarized amplified spontaneous emission from the columnar QDs has been investigated by using variable stripe-length method. With increasing the aspect ratio, transverse-magnetic-mode-dominant optical gain has been achieved. We obtained almost polarization insensitive optical gain for QDs with seven stacking layers.

Two-photon-absorption-assisted Tera Hertz optical gain modulation in quantum-dot optical amplifiers

Ultrafast carrier dynamics is examined in a quantum-dot (QD) semiconductor optical amplifier with a rate equation model which includes two-photon absorption (TPA) and gain bleaching during the propagation of 120 fs optical pulses at the wavelength near telecommunication wavebands. The TPA-generated carriers in the bulk region of the waveguide relax to the QDs via the wetting layer, which occurs synchronously in time and space with QD carrier depletion. This leads to efficient enhancement of the rate of carrier capture into the QDs, reducing the pattern effects for ultra high-speed optical signal processing.