Ultrafast low-pump fluence all-optical modulation based on graphene-metal hybrid metasurfaces

Graphene is an attractive material for all-optical modulation because of its ultrafast optical response and broad spectral coverage.However,all-optical graphene modulators reported so far require high pump fluence due to the ultrashort photo-carrier lifetime and limited absorption in graphene.We present modulator designs based on graphene-metal hybrid plasmonic metasurfaces with highly enhanced light-graphene interaction in the nanoscale hot spots at pump and probe(signal)wavelengths.Based on this design concept,we have demonstrated high-speed all-optical modulators at near and mid-infrared wavelengths(1.56μm and above 6μm)with significantly reduced pump fluence(1–2 orders of magnitude)and enhanced optical modulation.Ultrafast near-infrared pump-probe measurement results suggest that the modulators’response times are ultimately determined by graphene’s ultrafast photocarrier relaxation times on the picosecond scale.The proposed designs hold the promise to address the challenges in the realization of ultrafast all-optical modulators for mid-and far-infrared wavelengths.

Chip-integrated metasurface full-Stokes polarimetric imaging sensor

Polarimetric imaging has a wide range of applications for uncovering features invisible to human eyes and conventional imaging sensors.Chip-integrated,fast,cost-effective,and accurate full-Stokes polarimetric imaging sensors are highly desirable in many applications,which,however,remain elusive due to fundamental material limitations.Here we present a chip-integrated Metasurface-based Full-Stokes Polarimetric Imaging sensor(MetaPolarIm)realized by integrating an ultrathin(~600 nm)metasurface polarization filter array(MPFA)onto a visible imaging sensor with CMOS compatible fabrication processes.The MPFA is featured with broadband dielectric-metal hybrid chiral metasurfaces and double-layer nanograting polarizers.This chip-integrated polarimetric imaging sensor enables single-shot full-Stokes imaging(speed limited by the CMOS imager)with the most compact form factor,records high measurement accuracy,dual-color operation(green and red)and a field of view up to 40 degrees.MetaPolarIm holds great promise to enable transformative applications in autonomous vision,industry inspection,space exploration,medical imaging and diagnosis.

Deterministic assembly of single optical cavity formed by gold dimensional DNA origami

Controllable strong interactions between a nanocavity and a single emitter is important to manipulating optical emission in a nanophotonic system but challenging to achieve.Herein a three-dimensional DNA origami,named as DNA rack(DR)is proposed and demonstrated to deterministically and precisely assemble single emitters within ultra-small plasmonic nanocavities formed by closely coupled gold nanorods(AuNRs).Uniquely,the DR is in a saddle shape,with two tubular grooves that geometrically allow a snug fit and linearly align two AuNRs with a bending angle <10°.It also includes a spacer at the saddle point to maintain the gap between AuNRs as small as 2-3 nm,forming a nanocavity estimated to be 20 nm^(3) and an experimentally measured O factor of 7.3.A DNA docking strand is designed at the spacer to position a single fluorescent emitter at nanometer accuracy within the cavity.Using Cy5 as a model emitter,a -30-fold fluorescence enhancement and a significantly reduced emission lifetime(from 1.6 ns to 670 ps)were experimentally verified,confirming significant emitter-cavity interactions.This DR-templated assembly method is capable of fitting AuNRs of variable length-to-width aspect ratios to form anisotropic nanocavities and deterministically incorporate different single emitters,thus enabling flexible design of both cavity resonance and emission wavelengths to tailor light-matter interactions at nanometer scale.

Structural color printing via polymer-assisted photochemical deposition

Structural color printings have broad applications due to their advantages of long-term sustainability,eco-friendly manufacturing,and ultra-high resolution.However,most of them require costly and time-consuming fabrication processes from nanolithography to vacuum deposition and etching.Here,we demonstrate a new color printing technology based on polymer-assisted photochemical metal deposition(PPD),a room temperature,ambient,and additive manufacturing process without requiring heating,vacuum deposition or etching.The PPD-printed silver films comprise densely aggregated silver nanoparticles filled with a small amount(estimated<20%volume)of polymers,producing a smooth surface(roughness 2.5 nm)even better than vacuum-deposited silver films(roughness 2.8 nm)at~4 nm thickness.Further,the printed composite films have a much larger effective refractive index n(~1.90)and a smaller extinction coefficient k(~0.92)than PVD ones in the visible wavelength range(400 to 800 nm),therefore modulating the surface reflection and the phase accumulation.The capability of PPD in printing both ultra-thin(~5 nm)composite films and highly reflective thicker film greatly benefit the design and construction of multilayered Fabry–Perot(FP)cavity structures to exhibit vivid and saturated colors.We demonstrated programmed printing of complex pictures of different color schemes at a high spatial resolution of~6.5μm by three-dimensionally modulating the top composite film geometries and dielectric spacer thicknesses(75 to 200 nm).Finally,PPD-based color picture printing is demonstrated on a wide range of substrates,including glass,PDMS,and plastic,proving its broad potential in future applications from security labeling to color displays.