Five-layer Asymmetric Coupled Quantum Well Research Articles

Tunable Vernier Series-Coupled Microring Resonator Filters Based on InGaAs/InAlAs Multiple Quantum-Well Waveguide

We demonstrate a fourth-order Vernier serious-coupled microring resonator (MRR) filter based on the InGaAs/InAlAs five-layer asymmetric coupled quantum-well (FACQW) waveguide. As the tuning of the main resonance wavelength is driven by the electrorefractive index change in the FACQW layer caused by the quantum-confined Stark effect (QCSE), the MRR filter has lower tuning power consumption and high-speed operation. The free spectral range (FSR) of the MRR filter is extended to 9.4 nm. The main resonant peak is discretely tuned within one FSR, leading to an increase in the wavelength channel.

Hitless wavelength-selective switch with quadruple series-coupled microring resonators using multiple-quantum-well waveguides

We demonstrate a hitless wavelength-selective switch (WSS) based on InGaAs/InAlAs five-layer asymmetric coupled quantum well (FACQW) quadruple series-coupled microring resonators. The WSS is driven by the electric-field-induced change in refractive index in the FACQW core layer caused by the quantum-confined Stark effect (QCSE) for high-speed operation. The WSS with high-mesa waveguides is fabricated on a molecular beam epitaxy-grown wafer by dry etching. The fabricated WSS consists of four microrings, each with a round-trip length of 350 μm and five directional couplers with shallow grooves. A boxlike spectral response and hitless switching with higher extinction ratios than a double series-coupled microring resonator are successfully demonstrated. In addition, we propose the improvement of switching characteristics by controlling the coupling efficiencies at the directional couplers.

Low-voltage quantum well microring-enhanced Mach-Zehnder modulator

Modulation characteristics of a novel InGaAs/InAlAs multiple quantum well (MQW) microring-enhanced Mach-Zehnder modulator (MRE-MZM) is investigated in detail and its low-voltage operation with high extinction ratio is demonstrated. The MZM has a single microring resonator in one arm and is driven by the change in electrorefractive index induced by the quantum-confined Stark effect in the MQW core layer. As the MQW, a multiple five-layer asymmetric coupled quantum well (FACQW) is used to obtain a large electrorefractive index change. The driving voltage of the proposed MZM is significantly reduced owing to the enhanced phase shift in the microring resonator. High-mesa waveguide structures are grown by solid-source molecular beam epitaxy and fabricated by inductively coupled plasma etching. A directional coupler with an asymmetric branching ratio is used as an input coupler to prevent the degradation of the extinction ratio of the MZM. The extinction ratio of the fabricated MRE-MZM is approximately 27 dB. The product of the half-wave voltage and phase shifter length, V(π) · L, is 1.7 Vmm in static modulation. This value is one-quarter that of a conventional MZM with the same waveguide structure.

Hitless wavelength-selective switch based on quantum well second-order series-coupled microring resonators

A hitless wavelength-selective switch (WSS) based on InGaAs/InAlAs multiple quantum well (MQW) second-order series-coupled microring resonators is proposed and fabricated. In the core layer, a five-layer asymmetric coupled quantum well (FACQW) structure is employed. The WSS is driven by the electrorefractive index change in the FACQW core layer caused by the quantum-confined Stark effect (QCSE). The wafer for the WSS is grown by molecular beam epitaxy and waveguide structures are formed by dry etching. Boxlike spectrum responses and hitless switching characteristics of the WSS are successfully demonstrated for the first time. The change in coupling efficiency at a coupler between a ring and a busline and between rings and its effect on the switching characteristics are also discussed.

Proposal of Compact Tunable 1×2 Multimode Interference Splitter Based on Multiple Quantum Well

We propose a compact tunable 1×2 multimode interference (MMI) splitter based on an InGaAs/InAlAs multiple quantum well for an active trimmer for Mach–Zehnder modulator with a very high extinction ratio. The light power splitting characteristics of the splitter are theoretically investigated by a beam propagation method and a three dimensional finite-difference time-domain method. It is assumed that the core layer is composed of a multiple five-layer asymmetric coupled quantum well (FACQW) that is expected to exhibit a large electrorefractive index change owing to its unique quantum confined Stark effect. By applying reverse voltages and changing slightly the refractive indices of the localized regions surrounded by trenches for electrical isolation, the power splitting ratio can be tuned over a wide range. The length and width of the proposed MMI with four refractive index modulation regions are 192 and 6 µm, respectively. The length can be reduced to 115 µm if the number of index modulation regions is reduced to two, though the tuning range of the splitting ratio is narrowed. The simulation results show that the proposed compact MMI splitter has a low insertion loss and a low power consumption, and is promising for Mach–Zehnder modulators with a very high extinction ratio.

Low-Voltage Mach–Zehnder Modulator with InGaAs/InAlAs Five-Layer Asymmetric Coupled Quantum Well

We have demonstrated a low-voltage Mach–Zehnder (MZ) modulator with a multiple InGaAs/InAlAs five-layer asymmetric coupled quantum well (FACQW) for 1.55 µm wavelength. The FACQW is expected to exhibit a large electrorefractive index change owing to its unique behavior of the quantum-confined Stark effect. The MZ modulator with ridge waveguides was fabricated by molecular beam epitaxy and wet etching. The product of a half-wave voltage and a phase shifter length (VπL) was as low as 1.2 V·mm in DC modulation. In addition, we discussed the degradation of the electrorefractive index change due to the nonuniformity of an electric field in the core layer and proposed a novel combined multiple FACQW structure to prevent the degradation.

InGaAs/InAlAs Five-Layer Asymmetric Coupled Quantum Well Exhibiting Giant Electrorefractive Index Change

An InGaAs/InAlAs five-layer asymmetric coupled quantum well (FACQW) exhibiting a giant electrorefractive index change was proposed and has been studied theoretically and experimentally. Giant electrorefractive sensitivity |dn/dF| (4.4×10-4 cm/kV) at a wavelength range with a width of over 100 nm can be expected at an electric field of approximately F=-30 to -60 kV/cm. The FACQW structure was successfully fabricated using molecular beam epitaxy (MBE). The results of photoabsorption current measurements are consistent with the theory. The giant electrorefractive index change of the FACQW is very promising for realizing low-voltage and high-speed compact Mach–Zehnder modulators and switches.

Influence of Heterointerface Abruptness on Electrorefractive Effect in InGaAs/InAlAs Five-Layer Asymmetric Coupled Quantum Well

An InGaAs/InAlAs five layer asymmetric coupled quantum well (FACQW) is expected to produce a large electrorefractive index change and is a promising structure for high-performance phase modulators. However, because an FACQW is composed of ultrathin layers, poor abruptness in heterointerface may deteriorate the electrorefractive effect. We therefore investigated theoretically the influence of abruptness of heterointerfaces in an FACQW on the electrorefractive effect of the FACQW. In the analysis, it is assumed that the composition profile in a heterointerface is exponential, and the thickness of the transition layer L is defined. With the increase in L, the absorption edge is blue-shifted, which deteriorates the electrorefractive index change of the FACQW especially in the case of L≥3 monolayer (ML). In addition, we propose an improved FACQW structure, which is expected to exhibit a large electrorefractive index even in the case of L=3 ML.

Electrorefractive Effect in GaInNAs/GaAs Five-Layer Asymmetric Coupled Quantum Well

A GaInNAs/GaAs five-layer asymmetric coupled quantum well (FACQW) in the 1.3-µm-wavelength region was proposed and theoretically analyzed. The valence band structures of the FACQW were analyzed using the Luttinger–Kohn Hamiltonian based on the k·p perturbation theory and a nonvariational method. The GaInNAs/GaAs FACQW can exhibit a unique behavior of the quantum confined Stark effect, leading to a large electrorefractive index change (Δn/ΔF = 3.9 ×10-4 cm/kV) at a small electric field in a wide-transparency-wavelength region far from the absorption edge. This characteristic is comparable to InGaAs/InAlAs and InGaAsP FACQWs operated in the 1.55-µm-wavelength region. The GaInNAs/GaAs FACQW is expected to realize ultrawide-band, ultrafast optical modulators and switches and other functional devices based on the phase shift in the 1.3-µm-wavelength region. The driving voltage of a 1 mm-phase modulator is estimated to be 0.4 V.

Migration-enhanced epitaxy (MEE) growth of five-layer asymmetric coupled quantum well (FACQW) and its cross-sectional STM observation

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well that is promising for ultra-fast and ultra-low-voltage optical modulators and switches. FACQW samples were grown by the migration-enhanced epitaxy (MEE) and the conventional molecular beam epitaxy methods with steep and flat heterointerfaces in the monolayer accuracy. They were characterized with the cross-sectional scanning tunneling microscopy (STM). In the cross-sectional STM image, double-stripe structures with different contrast were observed. The stripe area corresponds to the FACQW (about 10 nm wide), sandwiched with the AlGaAs barrier layers ( 15 nm wide). A dark line observed at the middle of the FACQW stripe area corresponds to the 3-monolayer-thick AlAs layer. The cross-sectional STM images of the high-quality heterointerface FACQW structures were successfully observed for the samples grown by the MEE method. More detailed studies of this kind of cross-sectional STM observations will be very effective to obtain the optimized growth conditions for fine and complicated ultra-thin structures.

Electrorefractive properties of modified five-layer asymmetric coupled quantum well (FACQW)

We study in detail a modified structure of the five-layer asymmetric coupled quantum well (FACQW) for a large negative electrorefractive index change Δn. The influence of the layer thickness fluctuation on Δn of the modified FACQW is also discussed. Even with layer thickness fluctuation, the modified FACQW still maintains a much larger Δn than a conventional rectangular quantum well without thickness fluctuation. The results of photocurrent measurements of the modified FACQW qualitatively are in agreement with the theoretical analysis.

Fabrication and Optical Characterization of Five-Layer Asymmetric Coupled Quantum Well (FACQW)

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) that is promising for ultrafast and ultralow-voltage optical modulators and switches. We succeeded in fabricating GaAs/AlGaAs FACQW with monolayer accuracy by the molecular beam epitaxy (MBE) method by monitoring reflection high-energy electron diffraction (RHEED) specular beam intensity oscillation. Photoabsorption current measurements of the FACQW sample showed good agreement with theoretical results, and a potential for much lower voltage operation. In addition, we studied the growth sequences of GaAs/AlGaAs QWs in the migration-enhanced epitaxy (MEE) method in order to fabricate the FACQW with steeper and flatter heterointerfaces. The sequence of supplying materials for Al0.3Ga0.7As growth, on which there is no report, was modified and optimized, and the QWs of higher quality were obtained at a growth temperature of 490°C using the optimized sequence. The results of photoluminescence measurements show that the MEE method modified as mentioned above is a promising growth technique for the fabrication of FACQWs of higher quality.

Influence of One Monolayer Thickness Variation in GaAs/AlGaAs Five-Layer Asymmetric Coupled Quantum Well upon Electrorefractive Index Change

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) for ultrafast and low-voltage optical modulators and switches. First, the influence of one monolayer (ML) thickness variation of a single layer in the GaAs/AlGaAs FACQW on the electrorefractive index change Δn is theoretically studied. The thickness variation of two thicker GaAs layers has a considerable influence on Δn of the FACQW, while the thickness variation of thin AlAs and AlGaAs barrier layers has a smaller influence on Δn. The ratio of the thicknesses of the two GaAs well layers significantly affects the Δn characteristics of the FACQW. The change Δn does not vary appreciably as long as the ratio is kept constant. Second, the influence of the statistical fluctuation of the layer thickness by 1 ML in all of the layers on the Δn characteristics of the FACQW is also discussed. Even when Δn decreases with the increase of the occurrence probability of a layer being thicker or thinner by 1 ML, the FACQW still has a much larger Δn than conventional rectangular quantum wells do.

Anomalous Sharp Dip of Large Field-Induced Refractive Index Change in GaAs/AlGaAs Five-Layer Asymmetric Coupled Quantum Well

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well for ultrafast and low-voltage optical modulators and switches. Almost linear and large electrorefractive index change can be obtained in the transparency wavelength regions. In the GaAs/AlGaAs FACQW, an abrupt change in refractive index change Δn due to an applied electric field F occurs at a certain electric field range, which results in an anomalous sharp dip of Δn versus F. The physical origin and the elimination of the dip are discussed in detail. The abrupt change of refractive index is caused by significant changes of the wavefunction overlap integrals (and exciton binding energies) of transitions between the ground states for an electron (e1) and a heavy hole (hh1), and transitions between e1 and the first excited state for a heavy hole (hh2). The overlap changes are mainly due to shifts of the wavefunction distribution of hh1 and hh2, respectively. The dip can be eliminated by changing the position or Al content of the AlGaAs barrier layer in the FACQW. In addition, the larger negative index change in a modified FACQW structure is demonstrated.