Optical modulators encode electrical signals to the optical domain and thus constitute a key element in high-capacity communication links 1,2 . Ideally, they should feature operation at the highest speed with the least power consumption on the smallest footprint, and at low cost 3 . Unfortunately, current technologies fall short of these criteria 4 . Recently, plasmonics has emerged as a solution offering compact and fast devices 5-7 . Yet, practical implementations have turned out to be rather elusive.Here, we introduce a 70 GHz all-plasmonic Mach-Zehnder modulator that fits into a silicon waveguide of 10 μm length. This dramatic reduction in size by more than two orders of magnitude compared with photonic Mach-Zehnder modulators results in a low energy consumption of 25 fJ per bit up to the highest speeds. The technology suggests a cheap co-integration with electronics.Mach-Zehnder modulators (MZMs) are the most versatile electro-optical converters in high-end communication systems. MZMs are unique, as they can be used to encode multiple bits within one symbol with the highest quality. They are thus instrumental in increasing the capacity of modern communication links 1 . Until now, MZMs have mostly been based on the lithium niobate material system, which requires footprints on the order of cm 2 . Recently, more compact silicon-based modulators have emerged. These devices have already shown operation at bandwidths up to 55 GHz (ref. 8), they are cost-effective, and they feature lengths on the order of hundreds of micrometres to millimetres 2,3,8-12 . Yet, complementary metal-oxide semiconductor electronics (CMOS) house hundreds of transistors on a single μm 2 , making a co-integration of today's silicon MZMs with CMOS electronics impractical 4 . In pursuit of more compact silicon modulators, various approaches have been demonstrated, such as resonant silicon ring modulators 13,14 or germanium-based electro-absorption modulators 15,16 . However, encoding advanced modulation formats is challenging 17 , and high-capacity transmission has, so far, only been achieved with MZMs 2,12 . Instead, plasmonics has drawn significant interest as an alternative solution 6,7 . In plasmonics, optical signals are converted to surface plasmon polaritons (SPPs) propagating at metal-dielectric interfaces, where they can be confined below the diffraction limit of optics 18 . This means that plasmonic devices require only a few µm 2 of footprint 19,20 . With such reduced dimensions, the technology is much closer to bridging the size gap with respect to CMOS electronics. Furthermore, there are various theoretical studies indicating that plasmonic MZMs should offer hundreds of gigahertz of bandwidth 5,21 . To date, however, there is very little experimental evidence to support this claim. Recently, a plasmonic phase modulator demonstrated operation at 40 Gbit s −1 (ref. 22). One could now envision integrating such plasmonic phase modulators into a silicon waveguide MZM configuration. However, by combining plasmonics and silicon photoni...

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Plasmonics provides a possible route to overcome both the speed limitations of electronics and the critical dimensions of photonics. We present an all-plasmonic 116-gigabits per second electro-optical modulator in which all the elements-the vertical grating couplers, splitters, polarization rotators, and active section with phase shifters-are included in a single metal layer. The device can be realized on any smooth substrate surface and operates with low energy consumption. Our results show that plasmonics is indeed a viable path to an ultracompact, highest-speed, and low-cost technology that might find many applications in a wide range of fields of sensing and communications because it is compatible with and can be placed on a wide variety of materials.

The performance of highly nonlinear organic electro-optic (EO) materials incorporated into nanoscale slots is examined. It is shown that EO coefficients as large as 190 pm/V can be obtained in 150 nm wide plasmonic slot waveguides but that the coefficients decrease for narrower slots. Possible mechanism that lead to such a decrease are discussed. Monte-Carlo computer simulations are performed, confirming that chromophore-surface interactions are one important factor influencing the EO coefficient in narrow plasmonic slots. These highly nonlinear materials are of particular interest for applications in optical modulators. However, in modulators the key parameters are the voltage-length product U_πL and the insertion loss rather than the linear EO coefficients. We show record-low voltage-length products of 70 Vµm and 50 Vµm for slot widths in the order of 50 nm for the materials JRD1 and DLD164, respectively. This is because the nonlinear interaction is enhanced in narrow slot and thereby compensates for the reduced EO coefficient. Likewise, it is found that lowest insertion losses are observed for slot widths in the range 60 to 100 nm.

Extremely small CdS particles were prepared in propanol−2 solution at − 78°C and in aqueous solution in the presence of sodium hexametaphosphate at room temperature. These colloids are colorless. Their UV absorption spectra exhibit several maxima. Aging of the colloids is accompanied by intensity variations in the absorption maxima and by a shift of the onset of absorption to longer wavelengths. These small CdS particles hardly possess semiconductor properties (Q‐state CdS). A semi‐classical treatment of the energies of an electron‐hole pair in these particles yielded the wavelengths of their absorption spectra. At the small particle sizes used, the first excited state was reached by photon‐absorption in the UV, and the second excited state was generally not reached at all. The various maxima in the absorption spectra are explained in terms of a size distribution of the colloids with preferential agglomeration numbers. Reasons for the formation of such a structured size distribution are given. The fluorescence and fluorescence excitation spectra of the small particles were also investigated. Particles below a certain size have only one broad fluorescence band at a much longer wavelengths than the onset of absorption. As the particle size increases, this band is shifted towards longer wavelengths, and finally an additional rather sharp band appears at the threshold of absorption. CdS colloids in the Q‐state can be made which fluoresce as desired anywhere between the red and the blue. Also reported are the first experiments in which the preparation of Q‐type CdS in the solid state is achieved by evaporating the solvent from the colloidal solutions.