Author: Xiao Hu  Source: Wuhan National Laboratory for Optoelectronics

Surface plasmon polariton (SPP) waveguide can offer tight light confinement in deep subwavelength scale (i.e. breaking the diffraction limit), which is regarded as one of the most promising candidates for future large-scale photonic integrated circuits. Metasurface and graphene have attracted increasing interests in the fields of optoelectronics.

Prof. Jian Wang’s group named Multi-Dimensional Photonics Laboratory (MDPL) at Wuhan National Laboratory for Optoelectronics (WNLO), has been devoted to the researches of design and fabrication of silicon photonic integrated devices and their applications in optical signal processing. Very recently, a series of research progresses have been achieved in this area.

 Master candidate Zhonglai Zhang from Jian Wang’s group designed a novel long-range hybrid wedge plasmonic (LRHWP) waveguide composed of two identical dielectric nanowires symmetrically placed on two opposed wedges of a diamond shaped metal wire. With strong coupling between the dielectric nanowire mode and long-range surface plasmon polariton (SPP) mode, both deep subwavelength mode confinement and low propagation loss are achieved. This work has been published by Scientific Reports (vol. 4, article number: 3720, 2014). PhD candidate Chengcheng Gui from Jian Wang’s group presented a novel design of wedge hybrid plasmonic terahertz (THz) waveguide consisting of a silicon (Si) nanowire cylinder above a triangular gold wedge with surrounded high-density polyethylene as cladding. It features long propagation length and ultra-small deep-subwavelength mode confinement. The work has been published by Scientific Reports (vol. 5, article number: 11457, 2015). The undergraduate candidate Chao Xiang supervised by Prof. Jian Wang, designed an ultra-compact active hybrid plasmonic ring resonator for lasing applications at deep sub-wavelength scale. The combined contributions of hybrid plasmonic mode, circular-shaped cross section of nanowire, and ring resonator structure with round-trip whispering-gallery cavity, benefit reduced metallic absorption loss, tight mode confinement, enhanced cavity feedback, achieving high quality factor (Q), small mode volume (V), high Purcell factor (Fp), low threshold gain (Gth), and ultra-small footprint with sub-micron size. This work has also been published by Scientific Reports (vol. 4, article number: 3720, 2014). In addition, PhD candidate Jing Du from Jian Wang’s group designed a V-shaped antenna array to realize on-chip multicasting from a single Gaussian beam to four orbital angular momentum (OAM) beams, which has been published by Scientific Reports (vol. 5, article number: 9662, 2015).

Figure 1. (a) The designed long-range hybrid wedge plasmonic (LRHWP) waveguide. (b) The designed wedge hybrid plasmonic terahertz (THz) waveguide. (c) The proposed active hybrid plasmonic ring resonator. (d) Concept and principle of N-fold multicasting of OAM beams using V-shaped antenna phase array. Inset: a V-shaped antenna.

 

Graphene has attracted a high level of research interest because of its linear, massless band structure E±(p)=±V|p|, where the upper (lower) sign corresponds to the electron (hole) band, p is the quasi-momentum, and V≈106 m/s is the Fermi velocity. Graphene has been suggested as a material that might have large χ(3) nonlinearities, which is due to its linear band structure allowing interband optical transitions at all photon energies. Four-wave mixing (FWM) has been observed in graphene in various configurations, e.g. slow-light graphene-silicon photonic crystal waveguide, graphene optically deposited onto fiber ferrules, and graphene based on microfiber. Recently, cooperating with PhD candidate MengQi Zeng from Prof. Lei Fu’s group in Wuhan University, PhD candidate Xiao Hu from Jian Wang’s group presented an experimental demonstration of FWM-based wavelength conversion of a 10-Gbaud quadrature phase-shift keying (QPSK) signal using graphene grown by chemical vapor deposition (CVD) method. The work has been published by Optics Express (vol. 23, no. 20, pp. 26158-26167, 2015).

Coherent perfect absorption (CPA), a new method for controlling absorption through coherent illumination, has recently been reported in nanostructured graphene film, nonresonant suspending monolayer graphene, and planar metamaterials. The coherent absorption can be tuned by changing the relative phase of two beams, which suffers from relatively low modulation speed. The Fermi level and resultant optical absorptions of graphene can be tuned by the driven voltage. In the principle of CPA, altering the intensity of one beam actually can also change the transmission of the other beam. PhD candidate Xiao Hu from Jian Wang’s group proposed and simulated high-speed gate tunable THz CPA by exploiting split-ring graphene. This work has been published by Optics Letters (vol. 40, no. 23, pp. 5538-5541, 2015).