R Prasanth

All-optical switching due to state filling in quantum dots

We report all-optical switching due to state filling in quantum dots (QDs) within a Mach–Zehnder interferometric switch (MZI). The MZI was fabricated using InGaAsP/InP waveguides containing a single layer of InAs/InP QDs. A 1530–1570 nm probe beam is switched by optical excitation of one MZI arm. By exciting below the InGaAsP band gap, we prove that the refractive index nonlinearity is entirely due to the QDs. The switching efficiency is 5 rad/(μW absorbed power), corresponding to a 6 fJ switching energy. Probe wavelength insensitivity was obtained using a broad size distribution of QDs.

Compact polarization-independent Mach-Zehnder space switch combining carrier depletion and the quantum confined Stark effect

We have developed an n-doped InGaAs-InAsP quantum well between InP, which is suited for a polarization-independent Mach-Zender interferometric (MZI) space switch operating at 1.55 /spl mu/m. The InAsP is compressively strained and the InGaAs is tensile strained for polarization independence and for strain balancing. The important boundary condition for the design of this structure is the waveguide loss, which we limit to 0.6 dB/cm, and the crosstalk due to imbalance in the MZI, which we limit to <-30 dB. To reduce the size of the phase shifting region, while imposing this boundary condition, we combine the quantum confined Stark effect (QCSE) effect and the carrier-depletion effect by using an n-doped quantum well. The QCSE was first optimized for an undoped InGaAs-InAsP quantum well. A polarization independent /spl Delta/n of 7.8/spl middot/10/sup -4/ at 100 kV/cm was obtained at the expense of 0.2-dB/cm excess waveguide loss and 0.1-dB/mm electroabsorption loss. The carrier-depletion effect in a 2/spl middot/10/sup 11/cm/sup -2/-doped QW increases /spl Delta/n with a factor 2.6 to 2/spl middot/10/sup -3/, at the expense of 0.4-dB/cm free-carrier absorption-induced waveguide loss. The combination of the QCSE and carrier depletion results in a phase-shifter length of 0.46 mm for an MZI in push-pull configuration.

Low-crosstalk penalty MZI space switch with a 0.64-mm phase shifter using quantum-well electrorefraction

We investigate Mach-Zehnder interferometric (MZI) space switches with quantum-well phase shifters. We find that the minimum phase shifter length is limited by additional crosstalk due to electroabsorption-induced imbalance in the MZI. This criterion also provides an optimal detuning between the bandgap and the operating wavelength of the MZI. Finally, we present a novel MZI with an ultrashort 0.64-mm phase shifter.

Electro-refraction in quantum dots: dependence on lateral size and shape

Photonic switches require low-loss polarization-independent phase-shifting elements. In a composite quantum well, a 0.46-mm phase shifter provides a /spl pi//4 phase shift by combining the quantum confined Stark effect (QCSE) and the carrier depletion effect. We investigate whether the discrete energy levels and the high peak absorption in quantum dots (QDs) provide an opportunity for increasing the electro-refraction. The electro-refraction in strained cylindrical InAs-GaAs QDs is explored using a numerical model based on the 4 /spl times/ 4 Luttinger-Kohn Hamiltonian. The excitonic states are calculated by matrix diagonalization with plane-wave basis states. We observe that the QCSE sharply increases with the height of the QD and is also optimized for small-radius QDs. The QCSE in pyramidal QDs is considerably larger than in a box or cylinders. We find a peak electro-refraction of /spl Delta/n=0.35 in cone-shaped pyramidal QDs, which is a factor of 35 larger than in the quantum-well case. Finally, in the waveguide geometry, we find an electro-refraction of 1.3/spl times/10/sup -2/ at a residual QD absorption of 0.15 dB/cm.

Electrorefraction in quantum dots: dependence on lateral size and shape

Photonic switches require low loss polarization independent phase shifting elements. In a composite quantum well, a 0.46 mm phase shifter provides a /spl pi//4 phase shift by combining the Quantum Confined Stark Effect (QCSE) and carrier depletion effect. We investigate whether the discrete energy levels and the high peak absorption in quantum dots (QDs) provide an opportunity for increasing the electro-refraction. The electro-refraction in strained cylindrical InAs/GaAs QDs is explored using a numerical model based on the 4/spl times/4 Luttinger-Kohn Hamiltonian. The excitonic states are calculated by matrix diagonalization with plane-wave basis states. We observe that the QCSE sharply increases with the height of the QD and is also optimized for small radius QDs. The QCSE in pyramidal QDs is considerably larger than in squares or cylinders. We finally present large electro-refraction in, cone shaped pyramidal QDs.

InAsP∕InGaAs composite quantum well for separate TE and TM gain

Composite InAsP∕InGaAs quantum wells are a promising candidate for realizing polarization-independent semiconductor optical amplifiers at 1.55μm. We investigated the possibility of 8nm tensile-strained InGaAs well surrounded by two compressively-strained InAsP layers, for achieving separate gain for TE and TM polarized light. The InAsP layers provide strain compensation while simultaneously shifting the band gap to 1.55μm. The edge photoluminescence spectra shows that the gain for TE and TM polarized light is in the order of (3:1).