Surface plasmon polariton nanolasers
Quest of searching for small coherent light sources has never been stopped for these micro-to-nano scale light sources are not only practically essential for small footprint, low power consumption, high density, and parallel signal processing applications but also provide insightful way to investigate the interaction between light and matter. Substantial efforts have been made to scale down the optical cavity volume to contain only several photon modes, such as photonic crystal defect type lasers, microdisk lasers and nanowire lasers. These lasers, however, require the cavity size in the order of few (lamda/n)3 to sustain a proper mode profile with a reasonable cavity Q value. Recently, optical cavities surrounded with metal claddings have been realized to reduce the cavity volume since the optical field penetrating into the metal claddings would decay so rapidly that the optical mode could be further shrunk at the cost of a lower cavity Q value due to the strong absorption of metal. On the other hand, metal can provide an intriguing way to extremely diminish the cavity mode beyond the diffraction limit by forming the surface plasmon at the interface of metal and dielectric layers. The ultra-small electromagnetic field distribution of the surface plasmon mode would significantly facilitate the interaction between light and matter by enhancing the Purcell factor, which is beneficial to the laser operation in a nano-scale cavity size. Furthermore, a novel type of the surface plasmon based amplification of stimulated emission of radiation (spaser) has been proposed and demonstrated. A group of Au nano-spheres and Au nanorods coated with the gain medium and a two dimensional array of protruding Au bow-tie nano-structures sitting on the active medium manifested the spasing action particularly based on the localized surface plasmon mode.
Meanwhile, unambiguous demonstration of a single nanolaser came along with the nanowire gain medium lying on the metal separated by a thin dielectric layer to form a Fabry-Perot type surface plasmon (or surface plasmon polariton) cavity. The detected photons generated from the coherent surface plasmons escaping at the end facet of the nanowire Fabry-Perot cavity. Since the emitted photons have one-to-one corresponding characteristics to the cavity surface plasmons, these photons also show the coherent signature. A higher cavity Q value can be realized by using a nanosquare to form a whispery gallery surface plasmon mode facilitated by the total internal reflection7or by using a coaxial structure to reduce the end facet output coupling loss of the surface plasmon9. With the improvement of cavity Q value, the nanolaser operation can be elevated to room temperature. Furthermore, the near thresholdless signature had been observed in the nanoscale coaxial laser.
Successful demonstration of a plasmon nanolaser typically relies on its enhanced Purcell factor, which is inversely proportional to the mode volume. In addition, slower propagating speed of plasmon could raise the Purcell factor by adding more chance for interaction between the gain medium and plasmon. Slow group velocity can be achieved at the band edge provided by the distributed feedback mechanism in the two dimensional periodic structure. However, the distributed feedback mechanism needs a relatively large area which is against the small dimension requirement of nanolasers. It has been known that the refractive index of silver (Ag) has a giant variation at the ultraviolet (UV) wavelength range due to the interband absorption. This index variation can directly influence the dispersion of the surface plasmon to achieve a large group index, resulting a small mode volume and a large Purcell factor. To demonstrate this effect, we use the zinc oxide (ZnO) nanowire as the gain medium to match the UV wavelength range. The large exciton binding energy and oscillator strength of ZnO are also beneficial to the coupling between the ZnO exciton and surface plasmon. In this study, we are first to report on the UV nanolaser operation based on the exciton-surface-plasmon coupling at room temperature. A large group index, and small mode volume, accompanying with an enhanced Purcell factor, a reasonable Q value and a large confinement factor ensure our ZnO surface plasmon nanolasers to be operated below the exciton Mott density and at room temperature. Such UV plasmon nanolasers shall have great potential in many applications, such as biosensing, optical storage, sub-wavelength imaging and photolithography.