Petter Holmström

Sub-μm2 power splitters by using silicon hybrid plasmonic waveguides

Nano-scale power splitters based on Si hybrid plasmonic waveguides are designed by utilizing the multimode interference (MMI) effect as well as Y-branch structure. A three-dimensional finite-difference time-domain method is used for simulating the light propagation and optimizing the structural parameters. The designed 1 × 2 50:50 MMI power splitter has a nano-scale size of only 650 nm × 530 nm. The designed Y-branch power splitter is also very small, i.e., about 900 nm × 600 nm. The fabrication tolerance is also analyzed and it is shown that the tolerance of the waveguide width is much larger than±50 nm. The power splitter has a very broad band of over 500 nm. In order to achieve a variable power splitting ratio, a 2×2 two-mode interference coupler and an asymmetric Y-branch are used and the corresponding power splitting ratio can be tuned in the range of 97.1%:2.9%-1.7%:98.3% and 84%:16%-16%:84%, respectively. Finally a 1×4 power splitter with a device footprint of 1.9 μm × 2.6 μm is also presented using cascaded Y-branches.

A metal-wire/quantum-dot composite metamaterial with negative ε and compensated optical loss

Numerical simulations of a binary mixture of quantum dots exhibiting gain with silver nanorods are performed, showing the feasibility of lossless negative epsilon operation for realistic material s ...

Composite metal/quantum-dot nanoparticle-array waveguides with compensated loss

We calculate the dispersion properties of waveguides composed of near-field-coupled arrays of metal-clad quantum dots (QDs). The high optical loss incurred by operating the metal shells close to resonance is mitigated by using optical gain in the QDs. A condition for achieving loss compensated operation is given based on realistic material parameters and neglecting inhomogeneous broadening.

Dielectric function of quantum dots in the strong confinement regime

The complex dielectric function of quantum dots (QDs) with a core-shell structure is modeled in the strong confinement regime. These results should be useful for the design of negative epsilon optical metamaterials, where the gain due to QDs could be an essential ingredient. Using the dielectric function it is also shown that conventional expressions for the gain substantially overestimate it for narrow linewidths.

High structural quality InN∕In0.75Ga0.25N multiple quantum wells grown by molecular beam epitaxy

InN/In0.75Ga0.25N multiple quantum wells (MQWs) were grown by rf plasma-assisted molecular beam epitaxy. The high-resolution transmission electron microscope and x-ray diffraction measurements show ...

Directional coupler wavelength filters based on waveguides exhibiting electromagnetically induced transparency

We describe the principle and analyze the operation of an integrated optics directional coupler filter based on coupling between a regular waveguide and one that exhibits electromagnetically induced transparency. Bandwidth length products on the order of 2 pm /spl times/ mm are obtainable, as an example, using this approach.

Lower bound of energy dissipation in optical excitation transfer via optical near-field interactions.

We theoretically analyzed the lower bound of energy dissipation required for optical excitation transfer from smaller quantum dots to larger ones via optical near-field interactions. The coherent interaction between two quantum dots via optical near-fields results in unidirectional excitation transfer by an energy dissipation process occurring in the larger dot. We investigated the lower bound of this energy dissipation, or the intersublevel energy difference at the larger dot, when the excitation appearing in the larger dot originated from the excitation transfer via optical near-field interactions. We demonstrate that the energy dissipation could be as low as 25 μeV. Compared with the bit flip energy of an electrically wired device, this is about 10⁴ times more energy efficient. The achievable integration density of nanophotonic devices is also analyzed based on the energy dissipation and the error ratio while assuming a Yukawa-type potential for the optical near-field interactions.

Electroabsorption modulator using intersubband transitions in GaN-AlGaN-AlN step quantum wells

We calculate the high-speed modulation properties of an electroabsorption modulator for lambda=1.55 mum based on Stark shifting an intersubband resonance in GaN-AlGaN-AlN step quantum wells. In a realistic simulation assuming an absorption linewidth Gamma=100 meV we obtain an RC-limited electrical f3dB~60 GHz at an applied voltage swing Vpp=2.8 V. We also show that a small negative effective chirp parameter suitable for standard single-mode fiber is obtained and that the absorption is virtually unsaturable. The waveguide is proposed to be based on the plasma effect in order to simultaneously achieve a strong confinement of the optical mode, a low series resistance, and lattice-matched cladding and core waveguide layers. Extrapolated results reflecting the decisive dependence of the high-speed performance on the intersubband absorption linewidth Gamma are also given. At the assumed linewidth the modulation speed versus signal power ratio is on a par with existing lumped interband modulators based on the quantum confined Stark effect

Limits on Integration as Determined by Power Dissipation and Signal-to-Noise Ratio in Loss-Compensated Photonic Integrated Circuits Based on Metal/Quantum-Dot Materials

We analyze the power dissipation that is associated with using the gain of an embedded medium (quantum dots) to overcome the losses inherent in plasmonics systems employed to produce a negative dielectric constant for nanophotonics circuits. This power dissipation is primarily due to the dissipative losses in the metal structures and Auger recombination in the quantum dots. The impact of amplifier mediated signal-to-noise ratio (SNR) degradation and its effect on integration is analyzed, and a tradeoff between low power dissipation and SNR is quantified.

A high-speed intersubband modulator based on quantum interference in double quantum wells

Calculations on a modulator based on quantum interference in AlGaAs/GaAs asymmetric double quantum wells (QWs) are performed. The modulation of the absorption is based on the anti-crossing behavior of the two lowest states in the coupled wells. At anti-crossing, the oscillator strengths of the transitions from these two lowest states to a higher state are changed in opposite directions. The width of the barrier between the wells should be thick enough to allow a large change in oscillator strength with applied field, yet thin enough so that the absorption peaks of the transitions are resolved. The QWs are designed so that one absorption peak has only a small energy shift for the transition used for modulation while the absorption varies rapidly with the applied voltage. A complete structure including a surface plasmon waveguide is proposed enabling calculations of modal absorption. Parameters important for the performance of the modulator are then determined. An extinction ratio of 10 dB at a wavelength of 8.4 /spl mu/m is predicted for a device length of 18 /spl mu/m and a peak-to-peak voltage of 0.9 V. The resistance-capacitance-limited 3-dB bandwidth is 130 GHz. The predicted performance compares very favorably with present interband modulators based on the quantum-confined Stark effect.