Feng Lu

Infrared absorption nano-spectroscopy using sample photoexpansion induced by tunable quantum cascade lasers

We report a simple technique that allows obtaining mid-infrared absorption spectra with nanoscale spatial resolution under low-power illumination from tunable quantum cascade lasers. Light absorption is detected by measuring associated sample thermal expansion with an atomic force microscope. To detect minute thermal expansion we tune the repetition frequency of laser pulses in resonance with the mechanical frequency of the atomic force microscope cantilever. Spatial resolution of better than 50 nm is experimentally demonstrated.

Experimental demonstration of the microscopic origin of circular dichroism in two-dimensional metamaterials

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space. While chiral optical properties are weak in most of naturally occurring materials, they can be engineered and significantly enhanced in synthetic optical media known as chiral metamaterials, where the spatial symmetry of their building blocks is broken on a nanoscale. Although originally discovered in 3D structures, circular dichroism can also emerge in a two-dimensional (2D) metasurface. The origin of the resulting circular dichroism is rather subtle, and is related to non-radiative (Ohmic) dissipation of the constituent metamolecules. Because such dissipation occurs on a nanoscale, this effect has never been experimentally probed and visualized. Using a suite of recently developed nanoscale-measurement tools, we establish that the circular dichroism in a nanostructured metasurface occurs due to handedness-dependent Ohmic heating.

Experimental Demonstration of Phase Modulation and Motion Sensing Using Graphene-Integrated Metasurfaces

Strong interaction of graphene with light accounts for one of its most remarkable properties: the ability to absorb 2.3% of the incident lights energy within a single atomic layer. Free carrier injection via field-effect gating can dramatically vary the optical properties of graphene, thereby enabling fast graphene-based modulators of the light intensity. However, the very thinness of graphene makes it difficult to modulate the other fundamental property of the light wave: its optical phase. Here we demonstrate that considerable phase control can be achieved by integrating a single-layer graphene (SLG) with a resonant plasmonic metasurface that contains nanoscale gaps. By concentrating the light intensity inside of the nanogaps, the metasurface dramatically increases the coupling of light to the SLG and enables control of the phase of the reflected mid-infrared light by as much as 55° via field-effect gating. We experimentally demonstrate graphene-based phase modulators that maintain the amplitude of the reflected light essentially constant over most of the phase tuning range. Rapid nonmechanical phase modulation enables a new experimental technique, graphene-based laser interferometry, which we use to demonstrate motion detection with nanoscale precision. We also demonstrate that by the judicious choice of a strongly anisotropic metasurface the graphene-controlled phase shift of light can be rendered polarization-dependent. Using the experimentally measured phases for the two orthogonal polarizations, we demonstrate that the polarization state of the reflected light can be by modulated by carrier injection into the SLG. These results pave the way for novel high-speed graphene-based optical devices and sensors such as polarimeters, ellipsometers, and frequency modulators.

High-sensitivity infrared vibrational nanospectroscopy in water

Mid-infrared vibrational spectroscopy is a universal label-free tool for identifying molecular compounds in chemical and biological samples on the basis of their ‘fingerprint’ vibrational absorption lines. Vibrational spectroscopy with nanometer spatial resolution can reveal the chemical composition of samples at the nanoscale, and several scanning-probe techniques have been developed to address this need1–22. It is also highly desirable to study biological and chemical samples in their native aqueous environments rather than in air. In aqueous environments, however, the sensitivity of the current vibrational nanospectroscopy techniques deteriorates dramatically7,23. Here, we report the first mid-infrared nanospectroscopy technique that retains nanoscale sensitivity and spatial resolution when the sample and the scanning probe are completely immersed in water. This method overcomes challenges including water absorption and scattering and the mechanical damping of cantilever vibrations. We further demonstrate spectroscopy and imaging of 20to 50-nmthick polymer samples with a 25-nm spatial resolution in the biologically relevant Amide I and II spectral regions. The diffraction limit restricts the spatial resolution of far-field midinfrared (λ≈3–15 μm) microscopy to the micrometer scale. To perform mid-infrared spectroscopy at the nanoscale, three major techniques have been developed: near-field scanning optical microscopy (NSOM1–9,21), infrared photoexpansion nanospectroscopy (AFM-IR10–18,22) and photoinduced force microscopy (PiFM)19,20. For operation in air, all these techniques have demonstrated a sensitivity of or close to a molecular monolayer and a spatial resolution of 10–30 nm, which is principally limited by the apex radius of the atomic force microscope (AFM) tip. However, the performance changes when a sample is immersed in water. The scattering and absorption of infrared light and the mechanical damping of cantilever oscillations by water lead to a severe degradation of the performance of all current vibrational nanospectroscopy techniques. Current state-of-the-art results include imaging of a 6-μm-diameter Melamine beads in water with ~ 1 μm spatial resolution using aperture-type NSOM7 and a photoexpansion nanospectroscopy of ~ 1-μm-thick Candida albicans fungi23 in 850–1250 cm− 1 spectral range. Scattering-type NSOM imaging of samples in ultrathin graphene-covered liquid cells has also recently been demonstrated3. In this case, the sample was covered by an ~ 10nm layer of water below a graphene sheet, and the AFM probe was operated in air. However, ultrathin graphene liquid cells are difficult to implement and are known to subject samples to very high-hydrostatic pressure (ca. 1 GPa)24. Here, we demonstrate that high-sensitivity infrared nanospectroscopy can be performed in bulk liquid cells, thus avoiding the complications associated with nanoscale liquid cells. Our method is based on the resonantly enhanced infrared photoexpansion nanospectroscopy (REINS) technique, which we have recently developed18. The schematic of our experimental setup is shown in Figure 1a. We illuminate the sample with a train of optical pulses from a quantum cascade laser. The sample is positioned on top of the prism for evanescent illumination, and a gold-coated silicon tip is placed in contact with the sample. A cover glass slide, attached at the back of the cantilever chip holder, holds a water droplet between the glass slide and prism. REINS operates by detecting the cantilever oscillation amplitude produced by the photoexpansion of a sample. The sample absorbs laser light and undergoes rapid thermal expansion. The expansion produces a force on the AFM tip that leads to a small cantilever deflection detectable by the AFM position-sensitive photodetector (PSPD); see Figure 1a. The laser pulses are sent in resonance with the cantilever oscillation, and the induced cantilever oscillation amplitude Δz is given by:

Broadly wavelength tunable bandpass filters based on long-range surface plasmon polaritons

We report a new kind of broadly tunable optical bandpass filters based on unusual properties of long-range surface plasmon polaritons. A 0.004 variation in the refractive index of the dielectric medium translates into 210 nm of bandpass tuning at telecom wavelengths. The tuning mechanism reported here may be used to create compact and widely tunable optical systems and other plasmonic components with broadly tunable optical response.

Widely-tunable optical bandpass filter based on long-range surface plasmon polaritons

We report that unique properties of long-range surface plasmon polaritons (LR SPP) allow one to produce optical components with very wide tuning range using small variations in the refractive index of the dielectric layer. Our filter is based on integration of a thin metal film between two dielectrics with dissimilar refractive index dispersion. In this configuration, the filter only has low insertion loss at a wavelength for which the refractive indices of the top and bottom dielectrics are the same, leading to a bandpass filter. As a proof-of-principle demonstration, we present operation of LR-SPP- based bandpass optical filters with refractive index matching fluids on an Au/SiO2 surface in which a 0.004 variation in the refractive index of the top dielectric translates into 210nm of bandpass tuning at telecom wavelengths. To make a more practical solid-state device, thermo-optic polymer can be used as a top dielectric and we expect that only 8°C of temperature variation translates into 200nm. The tuning mechanism proposed here may be used to create monolithic filters with tuning range spanning over more than an optical octave, compact and widely-tunable laser systems, multi-spectral imagers, and other plasmonic components with broadly-tunable optical response.

Monolithic bipolar thermopile detector sensitive to light ellipticity

We demonstrated a bipolar thermopile detector for measuring light ellipticity. Nanowire thermocouple was connected to non-chiral dimer antennas which produced opposite Ohmic heating pattern under LCP and RCP illumination.

Thermopile detector of light ellipticity

Polarimetric imaging is widely used in applications from material analysis to biomedical diagnostics, vision and astronomy. The degree of circular polarization, or light ellipticity, is associated with the S3 Stokes parameter which is defined as the difference in the intensities of the left- and right-circularly polarized components of light. Traditional way of determining this parameter relies on using several external optical elements, such as polarizers and wave plates, along with conventional photodetectors, and performing at least two measurements to distinguish left- and right-circularly polarized light components. Here we theoretically propose and experimentally demonstrate a thermopile photodetector element that provides bipolar voltage output directly proportional to the S3 Stokes parameter of the incident light.

Background-free heterodyne photoexpansion infrared nanospectroscopy

The unwanted photoacoustic pressure force in photoexpansion spectroscopy, which comes from light absorption of the sample not below the AFM tip, was suppressed at the heterodyne frequency of laser pulses and piezo-driven cantilever oscillation.

Ultra-Sensitive Mid-Infrared Photoexpansion Nanospectroscopy with Background Suppression

The ultimate sensitivity of mid-infrared photoexpansion nanospectroscopy is limited by the background signal from photoexpansion of the sample substrate and the probe tip. Here we demonstrate suppression of this signal using a second mid-infrared laser.