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The propagation of exciton polaritons in near-field-coupled quantum-dot (QD) chains is modeled by a density-matrix formalism. It is shown that at least for low-temperature operation it is possible using electronically controlled switching by the quantum-confined Stark effect in such QD chains to rival and outperform room-temperature CMOS electronics in footprint and switch energy, though not in speed.

Dispersion properties of nanoarray waveguides composed of near-field-coupled arrays of metal-clad quantum dots (QDs) are calculated. The high loss due to operation of the metal shells close to resonance is mitigated by using optical gain in the QDs. The conditions for achieving loss compensated operation are given, with realistic material parameters and neglecting inhomogeneous broadening.

Ten and twenty period multiple quantum well structures with 1.5-5.4 nm GaN wells and 1.2-5.1 nm AlN barriers were grown on sapphire by molecular beam epitaxy. Layer thicknesses were determined by X-ray diffraction measurements and simulations. Reciprocal space mapping showed that the relaxation of the quantum well layers was independent of the buffer layer thickness. Intersubband absorption was observed by Fourier transform infrared spectroscopy at 1.5-3.5 m. Monolayer fluctuations in the quantum well width induced multiple peaks in spectra, which were well fitted to Lorentzian peaks of only 57 meV linewidth. Samples were very homogeneous as the absorption peak energy varied less than 1% along 4 cm on 2 inch wafers. The intersubband transition energies were calculated considering the conduction-band nonparabolicity, built-in fields, strain, and many-body effects. The calculation and comparison to the fitted Lorentzian peak energies indicated a moderate blueshift due to many-body effects. It was shown by both experiments and calculations that the AlN barrier width affects the intersubband transition energy

High In content InxAl1-xN nanocolumns were successfully self- assembled on Si (111) substrates under nitrogen-rich conditions, by rf-plasma assisted molecular beam epitaxy. The InAlN nano-columns (XIn=0.87) were grown at 416 oC, and the room tempera-ture (RT) PL full-width-at-half-maximum (FWHM) was relatively broad at 164 meV. When the AlInN nanocolumns of 1.48mm wavelength (XIn=0.92) were grown at a higher temperature of 463 oC but increasing the V/III ratio, however the RT-PL- FWHM be-came narrower to be 106 meV. InN/In0.77Al0.23N multiple quantum well (MQW) structures of 100 wells were integrated into InAlN nanocolumns, where three such samples with the different InN well widths of 3, 2 and 1 nm were grown. The RT-PL peak wave-length shifted from 1.53 to 1.2 um depending on the well layer thickness. The quantum confinement shift is theoretically analyzed.

Ten period GaN/AlN multiple quantum well (MQW) structures were grown by plasma-assisted molecular beam epitaxy (MBE) on sapphire substrates and metal-organic vapour-phase epitaxy (MOVPE)-grown GaN templates. Samples were investigated by high-resolution X-ray diffraction (HR-XRD) and Fourier transform infrared (FT-IR) spectroscopy. Intersubband (IS) absorbances and FWHM of IS absorption peaks indicated that samples grown on the GaN templates had better characteristics, resulting in a FWHM as low as 93 meV at a peak energy of 700 meV.

GaN/AlN multiple quantum well structures of 1, 5 and 20 periods were grown by molecular beam epitaxy (MBE). To investigate structural parameters, symmetrical scan (0 0 0 2) and reciprocal space mapping in the vicinity of the GaN ( 1 0 1 ¯ 5 ) plane were made by X-ray diffraction (XRD). The layer thickness, composition and relaxation were determined and gave good agreement with simulated results. The 20 period multiple quantum well (MQW) sample exhibited an intersubband resonance at 360 meV, which corresponds well to the structure data determined by XRD.

All-optical modulation using intersubband transition (ISBT) resonant light (wavelength of 1.55 μm) and induced by UV interband transition (IBT) resonant light (325 nm or 213 nm) was demonstrated in a GaN/AlN multiple quantum well (MQW) waveguide device. The modulation was selective and occurred only for p-polarized ISBT resonant light in accordance with ISBT characteristics. An IBT relaxation time of 1.5 ns was obtained by fitting the experimental results with a three-level model, taking into consideration the valence band, the conduction band and the trap-state.

The room-temperature operation of a GaN/AlN quantum well infrared photodetector (QWIP) using the intersubband transition (ISBT) in a GaN/AlN multiple quantum well (MQW) was demonstrated for the first time. The GaN/AlN QWIP was operated under DC biasing with a vertically conductive geometry to the MQW layer. A clear photoinduced response was observed for P polarized 1.47µm light irradiation. Dependencies of the photoresponse on the applied DC bias voltage, and the polarization and wavelength of incident light were evaluated for the GaN/AlN QWIP. The maximum responsivity was estimated to be 0.11mA/W for a DC bias of 15V at room temperature.

Development in photonics for communications and interconnects pose increasing requirements on reduction of footprint, power dissipation and cost, as well as increased bandwidth. Nanophotonics integrated photonics has been viewed as a solution to this, capitalizing on development in nanotechnology and an increased understanding of light matter interaction on the nanoscale. The latter can be exemplified by plasmonics and low dimensional semiconductors such as quantum dots (QDs). In this scenario the development of improved electrooptic materials is of great importance, the electrooptic polymers being an example, since they potentially offer superior properties for optical phase modulators in terms of power and integratability. Phase modulators are essential for e.g. the rapidly developing advanced modulation formats, since phase modulation basically can generate any type of modulation. The electrooptic polymers, in combination with plasmonics nanoparticle array waveguides or nanostructured hybrid plasmonic media can give extremely compact and low power dissipation modulators. Low-dimensional semiconductors, e.g. in the shape of QDs, can be employed for modulation or switching functions, offering possibilities for scaling to 2 or 3 dimensions for advanced switching functions. In both the high field confinement plasmonics and QDs, the nanosizing is due to nearfield interactions, albeit being of different physical origin in the two cases. Epitaxial integration of III-V structures on Si plays an important role in developing high-performance light sources on silicon, eventually integrated with silicon electronics. A brief remark on all-optical vs. electronically controlled optical switching systems is also given.

The current development in photonics for communications and interconnects pose increasing requirements on reduction of footprint, power dissipation and cost, as well as increased bandwidth. Integrated nanophotonics has been viewed as one solution to this, capitalizing on development in nanotechnology as such as well as on increased insights into light matter interaction on the nanoscale. The latter can be exemplified by plasmonics and low-dimensional semiconductors such as quantum dots (QDs). In this scenario the development of better electrooptic materials is also of great importance, the electrooptic polymers being an example, since they potentially offer improved properties for optical phase modulators in terms of power and probably cost and general flexibility. Phase modulators are essential for e.g. the rapidly developing advanced modulation formats for telecom, since phase modulation basically can generate any type of modulation. The electrooptic polymers, e.g. in combination with plasmonics nanoparticle array waveguides or nanostructured hybrid plasmonic media can theoretically give extremely compact and low power dissipation modulators, still to be demonstrated. The low-dimensional semiconductors, e.g. in the shape of QDs, can be employed for modulation or switching functions, offering possibilities in the future for scaling to 2 or 3 dimensions for advanced switching functions. In both the plasmonics and QD cases, nanosizing and low power dissipation are generally due to near-field interactions, albeit being of different physical origin in the two cases. A comparison of all-optical and electronically controlled switching is given.

This chapter treats several approaches for employing nanophotonics or near-nanophotonics concepts to create low-power switches. The partly interrelated issues of low power dissipation and small dev ...

Nanophotonics has received much attention in recent years, fuelled by general interest and progress in nanotechnology but also by rapid advances in photonics technology. The tutorial will cover basics and applications of nanophotonics.

Integrated nanophotonics has shown a remarkable development in recent years and can find applications in virtually all fields of photonics, though its predominant focus has so far been in telecom and lately computer interconnects. This progress has primarily been based on nano- and III–V technology development, the introduction of silicon technology, with larger refractive index contrast than previously available, as well as subwavelength plasmonics structures. For the latter, a main problem, especially for ICT applications, has been the large optical loss associated with deep subwavelength confinement, as discussed below.

Integrated photonics has largely been based on silicon/silica and III–Vs in recent times. Both have their advantages and disadvantages, but neither seem capable of supporting a further significant reduction of device footprint to sustain the exponential decrease of this important parameter that we have witnessed since decades. The talk will analyze different aspects of plasmonics from this and a functionality point of view and further discuss some possible alternative materials.

Metal nanoparticle arrays offer the possibility to considerably surpass the optical field confinement of silicon waveguides. The properties of directional couplers composed of such plasmonic nanoar ...

The properties of integrated-photonics directional couplers composed of near-field-coupled arrays of metal nanoparticles are analyzed theoretically. It is found that it is possible to generate very compact, submicron length, high field-confinement and functionality devices with very low switch energies. The analysis is carried out for a hypothetical lossless silver to demonstrate the potential of this type of circuits for applications in telecom and interconnects. Employing losses of real silver, standalone devices with the above properties are still feasible in optimized metal nanoparticle structures.

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.

We review some of the issues involved in using different plasmonic guided-wave structures for modulation, switching and filtering.

Nanophotonics in general and plasmonics in particular have received much attention in recent years, fuelled by a general interest in nanotechnology but also by rapid advances in integrated photonics over the years, primarily brought about by using silicon/air or quartz interfaces, giving larger refractive index contrast than previously employed [1,2]. The minimum lateral spatial field width for a planar silicon waveguide in air is ∼300 nm, with a wavelength in the medium of ∼500 nm, at a vacuum wavelength of 1550 nm. Photonics integration density has thus shown a steady exponential growth since the 80s, but to pursue this state of affairs, it is necessary to find a successor/complement to the silicon technology. Such new (meta) materials seem to have to rely on a negative e (such as in metals) somewhere in the system [3,4] since this offers a possibility for increasing the integration density in photonic lightwave circuits in two ways: (i) By using principles other than total internal reflection, i.e. surface polaritons and (ii) Employing e.g. metamaterials to generate artificially very large effective media refractive indices. Both methods could allow denser lateral packing of waveguides as well as shorter resonators and filters. In general, operating the metal/dielectric structures close to resonance [4] gives the highest confinement but also the highest spatial losses, since in essence (i) the field is forced into the lossy metal and (ii) in general the group velocity is significantly reduced. Thus, a main problem for many (albeit not all) applications is the optical loss associated with these high confinement metal-based metamaterials. In anticipation of a possible future breakthrough in developing metamaterials with at least a factor of 10 lower losses [5, 6] we analyze in this paper different resonant and nonresonant structures and the achievable performance. One example is waveguides made from arrays of near resonantly operated near-field coupled metal nanoparticles in the shape of e.g. spheres, which have attracted some attention [3,7,8]. Nanoarray waveguides, based on silver nanoparticles are, however, very lossy, see e.g. [9].

Publisher Summary This chapter reviews the theory of high-speed modulators and practical design approaches, which include a comparison of lumped and traveling-wave designs, and experimental results. The emphasis is on electro-absorption devices based on the Franz–Keldysh effect, the quantum-confined Stark effect, and intersubband absorption. Current high-speed light wave systems make use of electro-optic modulators based on lithium niobate or electro-absorption modulators based on semiconductor materials. In commercial systems, very high-speed lithium niobate devices require a traveling wave structure, while the semiconductor devices are usually lumped. The optical single-mode waveguide-based modulator has emerged as a critical component with rapid development in high data rate communications. This is brought about by the ever-increasing demands from the Internet and requirements for very high-speed computer connections. This growing importance of the modulator has taken place because the external modulator is the only known device that can cope with time division multiplexed signals at 100 Gb/s and beyond—rates that are increasingly required.

The characteristics of intersubband transitions in III-nitride quantum wells are promising for detectors and all-optical switches through a high intrinsic speed (similar to 1 THz), and can also pro ...

We have used MBE to grow MQW structures on MOVPE GaN/sapphire templates. The MQW devices are intended for high speed intersubband electroabsorption modulator devices operating at 1.55-mu m. The GaN ...

Electroabsorption modulators (EAM) based on quantum-confined Stark effect (QCSE) in multiple-quantum wells (MQW) have been demonstrated to provide high-speed, low drive voltage, and high extinction ratio. They are compact in size and can be monolithically integrated with source lasers. In order to achieve both high speed and low drive-voltage operation, travelling-wave (TW) electrode structures can be used for EAMs. Modulation bandwidths of 100 GHz (-3 dBe) have been accomplished and transmission at 80 Gbit/s with non-return-to-zero (NRZ) code has been demonstrated for InP-based TWEAMs, indicating the possibility of reaching speeds of 100Gbit/s and beyond. In order to further increase the efficiency and reduce the drive voltage, MQW structures using intersubband (IS) instead of interband (QCSE) absorption are being investigated. Materials systems with large conduction band offset, such as GaN/Al(Ga) N or InGaAs/AlAsSb, are required for IS modulators. Critical for the performance of IS modulators is the linewidth of the absorption peak, hence IS modulators have the potential to outperform interband modulators given that a sufficiently high material quality can be achieved.

We theoretically and experimentally evaluated energy dissipation of nanophotonic devices based on energy transfer via near-field interactions and their interfaces with optical far-fields. The lower bound is about 104 times more energy-efficient than electronic devices. We also examined some fundamental differences between near-field-mediated optical energy transfer logic and electrical logic in terms of energy dissipation.

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.

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.

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 ...

We have demonstrated efficient intersubband electroabsorption in InGaAs/InAlGaAs/InAlAs step quantum wells grown by metal-organic vapor-phase epitaxy. An absorption modulation of 6 dB (Delta alpha= ...

We have demonstrated efficient intersubband (IS) electroabsorption in InGaAs/InAlGaAs/InAlAs step quantum wells grown by metal-organic vapor phase epitaxy (MOVPE). An absorption modulation of 2300 cm-1 at λ = 5.7 μm due to Stark shift of the IS resonance was achieved with a low applied voltage swing of ± 0.5 V in a multipass waveguide structure. Two useful wavelength ranges of λ≈5.4–5.8 μm and 6.3–6.6 μm were obtained by considering the two flanks of the IS resonance. Based on the experimental results it is estimated that an electroabsorption modulator with a low peak-to-peak voltage of VPP = 0.9 V can yield a modulation speed of f3dB = 120 GHz with the present material by using a strongly confining surface plasmon waveguide of 30 μm length.

Due to the large lattice constant mismatch and thermal expansion coefficient difference between GaN and AlN, large strain is generated inside the GaN/AlN multiple quantum wells, which causes cracks in the structure. We investigated such cracks by optical microscopy and AFM. The crack density was studied with buffer and cap layer thickness, the number of quantum well periods, and the temperature reduction rate after growth as parameters. It was found that the crack density increased exponentially, with the number of periods above 4. Besides, a very thin, 100 nm, GaN buffer layer and similar to 300 nm GaN cap layer greatly reduced the crack density.

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 ...

A novel hybrid plasmonic waveguide with nano-scale confinement, consisting of a metal layer separated from a SOI nano-rib by a thin silica layer has been realized. The loss of 0.01dB/µm allows for ultra-high density photonic integration.

We discuss the status of photonics technology and review its possible evolution by way of several representative examples of emerging research areas, which could have a significant impact over a long-term, say 10-year perspective. The selected areas are considered potentially capable of providing the quantum leap progress necessary to solve some of the shortcomings of current photonics technology.