Zhaoyi Li

Modulation of mid-infrared light using graphene-metal plasmonic antennas

We show that large modulation of the amplitude and phase of mid-infrared light can be achieved by dynamically shifting the resonance of graphene-metal plasmonic antennas via electrical tuning of the optical conductivity of graphene. Intensity modulation with on-off extinction ratio exceeding 100 and phase modulation over a range of 240° are demonstrated by simulations of scattered light from arrays of such antennas. The modulation rate is estimated to be on the order of a few GHz. These properties are promising for creating reconfigurable flat optical components such as spatial light modulators in the mid-infrared spectral range.

Controlling propagation and coupling of waveguide modes using phase-gradient metasurfaces

Research on two-dimensional designer optical structures, or metasurfaces, has mainly focused on controlling the wavefronts of light propagating in free space. Here, we show that gradient metasurface structures consisting of phased arrays of plasmonic or dielectric nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Based on this design principle, we experimentally demonstrate waveguide mode converters, polarization rotators and waveguide devices supporting asymmetric optical power transmission. We also demonstrate all-dielectric on-chip polarization rotators based on phased arrays of Mie resonators with negligible insertion losses. Our gradient metasurfaces can enable small-footprint, broadband and low-loss photonic integrated devices.

Correlated Perovskites as a New Platform for Super-Broadband-Tunable Photonics

We report strong and non-volatile optical modulation utilizing electron-doping induced phase change of a perovskite, SmNiO<inf>3</inf>. Broadband modulation (λ=400nm–17μm) is demonstrated using thin-film SmNiO<inf>3</inf>, and narrowband modulation is realized with metasurfaces integrated with SmNiO<inf>3</inf>.

Metasurface-assisted phase-matching-free second harmonic generation in lithium niobate waveguides

The phase-matching condition is a key aspect in nonlinear wavelength conversion processes, which requires the momenta of the photons involved in the processes to be conserved. Conventionally, nonlinear phase matching is achieved using either birefringent or periodically poled nonlinear crystals, which requires careful dispersion engineering and is usually narrowband. In recent years, metasurfaces consisting of densely packed arrays of optical antennas have been demonstrated to provide an effective optical momentum to bend light in arbitrary ways. Here, we demonstrate that gradient metasurface structures consisting of phased array antennas are able to circumvent the phase-matching requirement in on-chip nonlinear wavelength conversion. We experimentally demonstrate phase-matching-free second harmonic generation over many coherent lengths in thin film lithium niobate waveguides patterned with the gradient metasurfaces. Efficient second harmonic generation in the metasurface-based devices is observed over a wide range of pump wavelengths (λ = 1580–1650 nm).Phase matching is a crucial condition for nonlinear optical processes. Here, Wang et al. demonstrate that a gradient metasurface composed of phased array antennas allows phase-matching-free frequency conversion over a pump wavelength range of almost 100 nm.

Controlling light propagation in optical waveguides using one dimensional phased antenna arrays

We demonstrated using full-wave simulations that phased array antennas patterned on optical waveguides can strongly affect mode coupling and propagation in the waveguides. We designed broadband small-footprint integrated photonic devices based on the concept.

Optical conductivity-based ultrasensitive mid-infrared biosensing on a hybrid metasurface

Optical devices are highly attractive for biosensing as they can not only enable quantitative measurements of analytes but also provide information on molecular structures. Unfortunately, typical refractive index-based optical sensors do not have sufficient sensitivity to probe the binding of low-molecular-weight analytes. Non-optical devices such as field-effect transistors can be more sensitive but do not offer some of the significant features of optical devices, particularly molecular fingerprinting. We present optical conductivity-based mid-infrared (mid-IR) biosensors that allow for sensitive and quantitative measurements of low-molecular-weight analytes as well as the enhancement of spectral fingerprints. The sensors employ a hybrid metasurface consisting of monolayer graphene and metallic nano-antennas and combine individual advantages of plasmonic, electronic and spectroscopic approaches. First, the hybrid metasurface sensors can optically detect target molecule-induced carrier doping to graphene, allowing highly sensitive detection of low-molecular-weight analytes despite their small sizes. Second, the resonance shifts caused by changes in graphene optical conductivity is a well-defined function of graphene carrier density, thereby allowing for quantification of the binding of molecules. Third, the sensor performance is highly stable and consistent thanks to its insensitivity to graphene carrier mobility degradation. Finally, the sensors can also act as substrates for surface-enhanced infrared spectroscopy. We demonstrated the measurement of monolayers of sub-nanometer-sized molecules or particles and affinity binding-based quantitative detection of glucose down to 200 pM (36 pg/mL). We also demonstrated enhanced fingerprinting of minute quantities of glucose and polymer molecules.Metasurfaces: mid-infrared biosensorsA highly sensitive glucose sensor has been constructed from a functionalized graphene-metallic hybrid metasurface. Pioneered by a US-Chinese collaboration from Columbia University, University of South Carolina, Brookhaven National Laboratory and Nanjing University the device by detecting the changes in graphene optical conductivity, enables ultrasensitive detection of glucose concentrations as small as 200 pM via a small shift in its plasmonic resonant wavelength which is then measured. Importantly, the new sensing principle overcomes the detection limit defined by the molecular weight or the changes in local refractive index, which for a long time has impeded the development of more sensitive optical biosensors. The device consists of a monolayer of graphene covering an array of gold nanorods, atop a platinum-silicon dioxide-platinum sandwich that serves as an optical cavity. When the graphene is functionalized with boronic acid which serves as a glucose binding agent, the device’s wavelength response was seen to clearly red shift with increasingly glucose concentration. Experiments indicate a dynamic range of measurement of over 6 orders of magnitude from 2nM to 10mM.

Tunable mid-infrared biosensors based on graphene metasurfaces

This paper presents actively tunable mid-infrared plasmonic biosensors that allow for detection of both concentrations and fingerprints of biomolecules. Human immunoglobulin G (IgG) concentrations down to 30 pM was resolved from the plasmonic resonance shift, corroborated by the increase in the intensities of protein amide I and amide II bands. Thanks to the enhancement of the interactions between microscale infrared light and nanoscale molecules, vibrational fingerprints of protein molecules (amide I and II) were achieved at the level of monolayer protein. By employing the graphene-metallic hybrid structure, variation of the graphene optical conductivity by a bias voltage actively tuned the plasmonic resonance toward the protein vibrational resonance frequency. This further improved the enhancement factor of amide I from 13 to 20, at a rate of ∼0.07/cm−1, exceeding most of reported tunable biosensors at the mid-infrared range.

Active metasurface devices based on correlated perovskites

There has been persistent exploration of new active materials and novel device architectures to dynamically control light with larger modulation depth and increased spectral range, at faster speed, and using less power. In this presentation, I will present experimental results showing that samarium nickelate (SmNiO3), a prototypical phase-change perovskite nickelate, exhibits reversible large refractive index changes over an ultra-broad spectral range, from the visible to the long-wavelength mid-infrared (λ = 400 nm-17 μm). The super broadband performance is due to strong electron correlation effects that allow extraordinarily large bandgap tuning of the order of 3 eV, and this new mechanism can be exploited to create active photonic devices.

Correlated perovskites as a new platform for super broadband tunable photonics

We report strong and non-volatile optical modulation utilizing electron-doping induced phase change of a perovskite, SmNiO 3 . Broadband modulation (λ=400nm–17μm) is demonstrated using thin-film SmNiO 3 , and narrowband modulation is realized with metasurfaces integrated with SmNiO 3 .

Experimental demonstration of optical waveguide mode converters using one-dimensional phased antenna arrays

We created optical waveguide mode converters by patterning one-dimensional phased antenna arrays on optical waveguides. We experimentally characterized waveguide mode conversion by measuring far-field emission patterns of the converted modes at λ =4 μm.