Scientific research
Integrating Nanowires into Silicon Photonics

Silicon photonics has been developed successfully with a top-down fabrication technique to enable large-scale photonic integrated circuits with high reproducibility, but is limited intrinsically by the material capability for active or nonlinear applications. On the other hand, free-standing nanowires synthesized via a bottom-up growth present great material diversity and structural uniformity, but precisely assembling free-standing nanowires for on-demand photonic functionality remains a great challenge. Here we report hybrid integration of free-standing nanowires into silicon photonics with high flexibility by coupling free-standing nanowires onto target silicon waveguides that are simultaneously used for precise positioning. Coupling efficiency between a free-standing nanowire and a silicon waveguide is up to ~97% in the telecommunication band. A hybrid nonlinear-free-standing nanowires–silicon waveguides Mach–Zehnder interferometer and a racetrack resonator for significantly enhanced optical modulation are experimentally demonstrated, as well as hybrid active-free-standing nanowires–silicon waveguides circuits for light generation. These results suggest an alternative approach to flexible multifunctional on-chip nanophotonic devices.
— B. G. Chen et al., Nat. Commun. 8, 20 (2017).

2D Materials for Optical Modulation: Challenges and Opportunities

Owing to their atomic layer thickness, strong light-material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D materialbased optical modulation in three categories is reviewed: free-space, fiberbased, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.
— S. L. Yu et al., Adv. Mater. 29, 1606128 (2017).

Single CdTe Nanowire Optical Correlator for Femtojoule Pulses

Relying on the transverse second harmonic (TSH) generation in a highly nonlinear CdTe nanowire, we demonstrate a single nanowire optical correlator. Pulses to be measured were equally split and coupled into two ends of a suspending nanowire via tapered optical fibers. By transferring the spatial intensity profile of the TSH image into the time-domain temporal profile of the input pulses, we operate the nanowire as a miniaturized optical correlator with input energy goes down to 2 fJ/pulse for 1064 nm 200 fs pulses. The miniature fJ-pulse correlator may find applications from low power on-chip optical communication, biophotonics to ultracompact laser spectroscopy.
— C. G. Xin et al., Nano Lett. 16, 4807-4810 (2016).

All-optical graphene modulator based on optical Kerr phase shift

Graphene-based optical modulators have recently attracted much attention because of their characteristic ultrafast and broadband response. Their modulation depth (MD) and overall transmittance (OT), however, are often limited by optical loss arising from interband transitions. We report here an all-optical, all-fiber optical modulator with a Mach-Zehnder interferometer structure that has significantly higher MD and OT than graphene-based loss modulators. It is based on the idea of converting optically induced phase modulation in the graphene-cladded arm of the interferometer to intensity modulation at the output of the interferometer. The device has the potential to be integrable into a photonic system in real applications.
— S. L. Yu et al., Optica 3, 541-544 (2016).

Single-band 2-nm-linewidth plasmon resonance in a strongly coupled Au nanorod

This paper reports a dramatic reduction in plasmon resonance line width of a single Au nanorod by coupling it to a whispering gallery cavity of a silica microfiber. With fiber diameter below 6 μm, strong coupling between the nanorod and the cavity occurs, leading to evident mode splitting and spectral narrowing. Using a 1.46-μm-diameter microfiber, we obtained single-band 2-nm-line-width plasmon resonance in an Au nanorod around a 655-nm-wavelength, with a quality factor up to 330 and extinction ratio of 30 dB. Compared to an uncoupled Au nanorod, the strongly coupled nanorod offers a 30-fold enhancement in the peak intensity of plasmonic resonant scattering.
— P. Wang et al., Nano Lett. 15, 7581–7586 (2015).

Graphene-doped polymer optical nanofibers

Graphene-doped polymer nanofibers are fabricated by taper drawing solvated polyvinyl alcohol doped with liquid-phase exfoliated graphene flakes. As-drawn nanofibers, with typically diameter of hundreds of nanometers and length up to tens of millimeters, show excellent uniformity and surface smoothness for optical waveguiding. Owing to their tightly confined waveguiding behavior, light-matter interaction in these subwavelength-diameter nanofibers is significantly enhanced. Using femto-second pulses around 1350-nm wavelength, we demonstrate saturable absorption behaviors of such nanofibers with a saturation threshold down to 0.25 pJ/pulse (peak power ~1.3 W). Additionally, using 1064-nm-wavelength naosecond pulses as switching light, we show all-optical modulation of a 1550-nm-wavelength signal light guided along a single nanofiber with switching peak power of ~3.2 W.
— C. Meng et al., Light: Science & Applications 4, e348 (2015).

Single nanowire optical correlator

Integration of miniaturized elements has been a major driving force behind modern photonics. Nanowires have emerged as potential building blocks for compact photonic circuits and devices in nanophotonics. We demonstrate here a single nanowire optical correlator (SNOC) for ultrafast pulse characterization based on imaging of the second harmonic (SH) generated from a cadmium sulfide (CdS) nanowire by counterpropagating guided pulses. The SH spatial image can be readily converted to the temporal profile of the pulses, and only an overall pulse energy of 8 μJ is needed to acquire a clear image of 200 fs pulses. Such a correlator should be easily incorporated into a photonic circuit for future use of onchip ultrafast optical technology.
— H. K. Yu et al., Nano Lett. 14, 3487-3490 (2014).

Ultrafast (200-GHz) graphene all-optical modulator

Graphene offers broadband light-matter interactions with ultrafast responses. The bandwidth of previous graphene-based optical modulators were usually limited to ~1 GHz by electric parasite response. Now by using an all-optical scheme, Li et al. show that a graphene-clad microfiber all-optical modulator can achieve a modulation depth of 38% and a response time of ∼2.2 ps, corresponding to a bandwidth of ~200 GHz. This modulator is compatible with current high-speed fiber-optic communication networks and may open the door to meet future demand of ultrafast optical signal processing.
— W. Li et al., Nano Lett. 14, 955-959 (2014). ACS Editor's Choice

Subtle balance in nanowaveguides: loss v.s. confinement

In deep-subwavelength optical nanowires, the waveguiding loss increases with increasing optical confinement, which may also lead to long-wavelength cutoff. Guo et al. pictured subtle balance between loss, confinement and more in a recent article.
— X. Guo et al., Acc. Chem. Res. 47, 656-666 (2014).

Photon-plasmon hybrid nanowire (NW) laser

By near-field coupling a CdSe and a Ag nanowires, Wu et al. demonstrated a hybrid photon-plasmon laser operating at 723 nm wavelength at room temperature, which offers subdiffraction-limited beam size and pure plasmon modes with mode area of 0.008λ2.
— X. Q. Wu et al., Nano Lett. 13, 5654-5659 (2013).
See also highlight in Phys.ORG
"Photon-plasmon nanowire laser offers new opportunities in light manipulation" by Lisa Zyga

70% photon-to-plasmon conversion efficiency in a single Au nanorod

When a Au nanorod is placed inside an optical nanofiber, it can be efficiently excited by the waveguiding mode with photon-to-plasmon conversion efficiency as high as 70%, and is highly potential for realizing ultra-low power nanoparticle plasmonic devices. Check out more details of our Au-nanorod-doped polymer nanofibers.
— P. Wang et al., Nano Lett. 12, 3145-3150 (2012).