Volker J. Sorger

Plasmon lasers at deep subwavelength scale

Laser science has been successful in producing increasingly high-powered, faster and smaller coherent light sources. Examples of recent advances are microscopic lasers that can reach the diffraction limit, based on photonic crystals, metal-clad cavities and nanowires. However, such lasers are restricted, both in optical mode size and physical device dimension, to being larger than half the wavelength of the optical field, and it remains a key fundamental challenge to realize ultracompact lasers that can directly generate coherent optical fields at the nanometre scale, far beyond the diffraction limit. A way of addressing this issue is to make use of surface plasmons, which are capable of tightly localizing light, but so far ohmic losses at optical frequencies have inhibited the realization of truly nanometre-scale lasers based on such approaches. A recent theoretical work predicted that such losses could be significantly reduced while maintaining ultrasmall modes in a hybrid plasmonic waveguide. Here we report the experimental demonstration of nanometre-scale plasmonic lasers, generating optical modes a hundred times smaller than the diffraction limit. We realize such lasers using a hybrid plasmonic waveguide consisting of a high-gain cadmium sulphide semiconductor nanowire, separated from a silver surface by a 5-nm-thick insulating gap. Direct measurements of the emission lifetime reveal a broad-band enhancement of the nanowire’s exciton spontaneous emission rate by up to six times owing to the strong mode confinement and the signature of apparently threshold-less lasing. Because plasmonic modes have no cutoff, we are able to demonstrate downscaling of the lateral dimensions of both the device and the optical mode. Plasmonic lasers thus offer the possibility of exploring extreme interactions between light and matter, opening up new avenues in the fields of active photonic circuits, bio-sensing and quantum information technology.

Room-temperature sub-diffraction-limited plasmon laser by total internal reflection

Plasmon lasers are a new class of coherent optical amplifiers that generate and sustain light well below its diffraction limit. Their intense, coherent and confined optical fields can enhance significantly light-matter interactions and bring fundamentally new capabilities to bio-sensing, data storage, photolithography and optical communications. However, metallic plasmon laser cavities generally exhibit both high metal and radiation losses, limiting the operation of plasmon lasers to cryogenic temperatures, where sufficient gain can be attained. Here, we present a room-temperature semiconductor sub-diffraction-limited laser by adopting total internal reflection of surface plasmons to mitigate the radiation loss, while using hybrid semiconductor-insulator-metal nanosquares for strong confinement with low metal loss. High cavity quality factors, approaching 100, along with strong λ/20 mode confinement, lead to enhancements of spontaneous emission rate by up to 18-fold. By controlling the structural geometry we reduce the number of cavity modes to achieve single-mode lasing.

High-Q surface-plasmon-polariton whispering-gallery microcavity

Surface plasmon polaritons (SPPs) are electron density waves excited at the interfaces between metals and dielectric materials. Owing to their highly localized electromagnetic fields, they may be used for the transport and manipulation of photons on subwavelength scales. In particular, plasmonic resonant cavities represent an application that could exploit this field compression to create ultrasmall-mode-volume devices. A key figure of merit in this regard is the ratio of cavity quality factor, Q (related to the dissipation rate of photons confined to the cavity), to cavity mode volume, V (refs 10, 11). However, plasmonic cavity Q factors have so far been limited to values less than 100 both for visible and near-infrared wavelengths. Significantly, such values are far below the theoretically achievable Q factors for plasmonic resonant structures. Here we demonstrate a high-Q SPP whispering-gallery microcavity that is made by coating the surface of a high-Q silica microresonator with a thin layer of a noble metal. Using this structure, Q factors of 1,376 ± 65 can be achieved in the near infrared for surface-plasmonic whispering-gallery modes at room temperature. This nearly ideal value, which is close to the theoretical metal-loss-limited Q factor, is attributed to the suppression and minimization of radiation and scattering losses that are made possible by the geometrical structure and the fabrication method. The SPP eigenmodes, as well as the dielectric eigenmodes, are confined within the whispering-gallery microcavity and accessed evanescently using a single strand of low-loss, tapered optical waveguide. This coupling scheme provides a convenient way of selectively exciting and probing confined SPP eigenmodes. Up to 49.7 per cent of input power is coupled by phase-matching control between the microcavity SPP and the tapered fibre eigenmodes.

Ultra-compact silicon nanophotonic modulator with broadband response

Abstract Electro-optic modulators have been identified as the key drivers for optical communication and signal processing. With an ongoing miniaturization of photonic circuitries, an outstanding aim is to demonstrate an on-chip, ultra-compact, electro-optic modulator without sacrificing bandwidth and modulation strength. While silicon-based electro-optic modulators have been demonstrated, they require large device footprints of the order of millimeters as a result of weak non-linear electro-optical properties. The modulation strength can be increased by deploying a high-Q resonator, however with the trade-off of significantly sacrificing bandwidth. Furthermore, design challenges and temperature tuning limit the deployment of such resonance-based modulators. Recently, novel materials like graphene have been investigated for electro-optic modulation applications with a 0.1 dB per micrometer modulation strength, while showing an improvement over pure silicon devices, this design still requires device lengths of tens of micrometers due to the inefficient overlap between the thin graphene layer, and the optical mode of the silicon waveguide. Here we experimentally demonstrate an ultra-compact, silicon-based, electro-optic modulator with a record-high 1 dB per micrometer extinction ratio over a wide bandwidth range of 1 μm in ambient conditions. The device is based on a plasmonic metal-oxide-semiconductor (MOS) waveguide, which efficiently concentrates the optical modes’ electric field into a nanometer thin region comprised of an absorption coefficient-tuneable indium-tin-oxide (ITO) layer. The modulation mechanism originates from electrically changing the free carrier concentration of the ITO layer which dramatically increases the loss of this MOS mode. The seamless integration of such a strong optical beam modulation into an existing silicon-on-insulator platform bears significant potential towards broadband, compact and efficient communication links and circuits.

Plasmonic Fabry-Perot Nanocavity

We experimentally demonstrate a novel, all-plasmonic nanoscopic cavity exhibiting Q-factors up to 200 at visible frequencies. The Fabry-Pérot type resonator uses tall metallic fins that reflect up to 98% of incident surface plasmon to concentrate light within a subwavelength cavity mode. High aspect ratio metal fins, constructed using lithography and electroplating, reduce surface plasmon scattering out of the surface, while a short cavity length reduces the propagation loss. A simple Fabry-Pérot cavity model adapted for surface plasmon dispersion and reflection describes the underlying physics of the nanocavities and the results agree well with Johnsons and Christies permittivity data. The occurrence of an optimum wavelength for plasmon storage in these cavities allows us to clearly visualize the fundamental trade-off between propagation loss and the spatial extent of surface plasmon polaritons. The subwavelength optical mode area within these cavities enables the enhancement of weak optical processes such as spontaneous emission and nonlinear optics at nanoscale dimensions.

Spotlight on Plasmon Lasers

A plasmonics-based design approach is enabling coherent light sources to be built at the nanometer scale. Lasers are the workhorse of the information age, sending massive amounts of light packets through vast networks of optic fibers. Demands for ever-increasing speed and functionalities call for scaling down of photonic devices, similar to the trend in electronics. However, photonic devices face the fundamental challenge of the diffraction limit of light—a limitation that prevents squeezing light into spaces smaller than half of its wavelength. This barrier limits traditional optical components to sizes that are hundreds of times larger than that of their electronic counterparts. Surface plasmons are collective electronic oscillations on a metal-dielectric interface with a much smaller wavelength than the excitation or emitted photons, and have emerged as a promising solution to overcome such a barrier (1). In 2003, the surface plasmon laser or “spaser” was theoretically proposed. The idea was to tightly confine light in the form of localized plasmons into deep subwavelength dimensions overlapping with a gain medium to achieve stimulated emission and light amplification or lasing, creating a coherent light source at the nanometer scale (2). That proposal is now being realized with several plasmonics-based design approaches being used to fabricate nanometer-scale coherent light sources.

Strongly enhanced molecular fluorescence inside a nanoscale waveguide gap.

We experimentally demonstrate dramatically enhanced light-matter interaction for molecules placed inside the nanometer scale gap of a plasmonic waveguide. We observe spontaneous emission rate enhancements of up to about 60 times due to strong optical localization in two dimensions. This rate enhancement is a nonresonant nature of the plasmonic waveguide under study overcoming the fundamental bandwidth limitation of conventional devices. Moreover, we show that about 85% of molecular emission couples into the waveguide highlighting the dominance of the nanoscale optical mode in competing with quenching processes. Such optics at molecular length scales paves the way toward integrated on-chip photon source, rapid transfer of quantum information, and efficient light extraction for solid-state-lighting devices.

Multiplexed and Electrically Modulated Plasmon Laser Circuit

With unprecedented ability to localize electromagnetic field in time and space, the nanometer scale laser promises exceptionally broad scientific and technological innovation. However, as the laser cavity becomes subwavelength, the diffraction of light prohibits the directional emission, so-called the directionality, one of the fundamental attributes of the laser. Here, we have demonstrated a deep subwavelength waveguide embedded (WEB) plasmon laser that directs more than 70% of its radiation into an embedded semiconductor nanobelt waveguide with dramatically enhanced radiation efficiency. The unique configuration of WEB plasmon laser naturally integrates photonic and electronic functionality allowing both efficient electrical modulation and wavelength multiplexing. We have demonstrated a plasmonic circuit integrating five independently modulated multicolored plasmon laser sources multiplexed onto a single semiconductor nanobelt waveguide, illustrating the potential of plasmon lasers for large scale, ultradense photonic integration.

λ-Size ITO and Graphene-Based Electro-Optic Modulators on SOI

One of the key devices that convert electronic signals into high bit-rate photonic data is the electro-optic modulator (EOM). Its on-chip design plays an important role for the integration of electronic and photonic devices for various types of applications including photonic computing and telecommunication. Recently, indium tin oxide (ITO) and graphene have attracted significant attention primarily due to their extraordinary electro-optic properties for the design of ultra-compact EOMs to handle bandwidth and modulation strength trade-off. Here we show design details of a high-performance ITO-EOM in a plasmonic silicon-on-insulator hybrid structure. Results show that ITO is capable of changing its extinction coefficient by a factor of 136 leading to 3 λ-short devices with an extinction ratio of about 1dB/μm. Further numerical device optimizations demonstrate the feasibility for an extinction ratio and on-chip insertion loss of about 6 dB/μm and 0.25 dB, respectively, for a sub-wavelength compact (0.78 λ) EOM design using ITO. Utilizing graphene as an active switching material in a similar ultra-compact plasmonic hybrid EOM design yields enhanced light-matter interaction, in which extinction-ratio is 9 times larger than the insertion-loss for a 0.78 λ short device. Both ITO and graphene EOMs are capable of broadband operations (>500 nm) since no resonator is deployed.

Roadmap on optical energy conversion

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with a better understanding of the thermodynamics of the photon energy-conversion processes reshaped the landscape of energy-conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon-recycling schemes reduce the entropy production in the optical energy-conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, to reduce the thermal emission losses, and to achieve noncontact radiative cooling of solar cells as well as of optical and electronic circuitries. Light–matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third- and fourth-generation energy-conversion devices, including up- and down-conversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy-conversion technologies amplifies the role of cost-driven and environmental considerations. This roadmap on optical energy conversion provides a snapshot of the state of the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts, and investors.