One-Shot Phase Image Distinction of Plasmonic and Dielectric Nanoparticles

Abstract

Nanoscale phase-control is one of the most powerful approaches to specifically tailor electrical fields in modern nanophotonics. Especially the precise sub-wavelength assembly of many individual nano-building-blocks has given rise to exciting new materials as diverse as metamaterials, for miniaturizing optics, or 3D assembled plasmonic structures for biosensing applications. Despite its fundamental importance, the phase-response of individual nanostructures is experimentally extremely challenging to visualize. Here, we address this shortcoming and measure the quantitative scattering phase of different nanomaterials such as gold nanorods and spheres as well as dielectric nanoparticles. Beyond reporting spectrally resolved responses, with phase-changes close to π when passing the particles' plasmon resonance, we devise a simple method for distinguishing different plasmonic and dielectric particles purely based on their phase behavior. Finally, we integrate this novel approach in a single-shot two-color scheme, capable of directly identifying different types of nanoparticles on one sample, from a single widefield image.Phase, an elusive quantity in day-to-day life, is one of the most crucial parameters in physics and the lifesciences. Historically, measurements based on phase-contrast have proven to be extremely powerful and highly sensitive to tiny changes in the morphology of structures 1 even if the underlying absolute phasevalue remained unobserved. Since these pioneering works, technology has advanced rapidly and different phase-detecting techniques are of crucial importance across many scientific fields. In nanoscale physics, the phase of a nanoparticle (NP) accesses its nanoscale response 2-5 and is a crucial parameter for tailoring complex photonic structures such as metasurfaces which manipulate the amplitude, direction and polarization of light at nano-to micro-scales 6,7 . In the biomedical sciences, phase-based measurements are often key enablers for ultrasensitive measurements 8 such as protein-binding to plasmonic structures [9][10][11] and the emerging field of quantitative phase imaging is expected to dramatically contribute to future diagnostics 12,13 . However, even though macro-scale phase measurements are relatively easy to implement, determining the absolute scattering phase of individual nano-objects is far from trivial as one has to consider the phase of the observation wave and account for the sub-diffraction limited nature of the particle. A versatile experimental approach to directly and reproducibly determine the absolute phase of such objects would both advance the observation-based development of nanophotonic materials and allow implementing novel imaging methodologies that exploit absolute phase measurements as a robust contrast mechanism for distinguishing and identifying different materials in a complex environment.

Wavefront Microscopy Using Quadriwave Lateral Shearing Interferometry: From Bioimaging to Nanophotonics

“…Let us mention that off-axis DHM was also used recently to characterize single nanoparticles, − with very similar sensitivity and spatial resolution as CGM.…”

Amplitude and Phase Spectra of Light Scattered from a Single Nanoparticle

“…There have been a few studies ,,− that have measured the scattering phase of a NP. Stoller et al, Hauler et al, and Saemisch et al measured the ϕ scat of a single metallic NP using an interferometer added to an optical microscope. They extracted the complex dielectric function of the material from the ϕ scat spectra and identified the metallic and nonmetallic NPs from the mixture .…”

Widefield phototransient imaging for visualizing 3D motion of resonant particles in scattering environments

“…The system outlined above is similar to a conventional multiplexed off-axis holographic microscope 13 which allows using arbitrarily broadband pulses, as the grating ensures interference across the entire camera chip. 12,[14][15][16][17] To record photoinduced signals, an additional pump pulse illuminates the sample at a variable, but precisely controlled, pump-probe time-delay. Rather than recording separate sample images in the presence ( pump ON ) or absence ( pump OFF ) of the pump pulse, the UHT microscope simultaneously records the pump ON and pump OFF images in a single camera exposure via a multiplexed holographic demodulation scheme.…”

“…Fig. 1a shows a schematic of an UHT microscope, 10 a multiplexed off-axis holographic imaging system 12 which is modified to allow generating, demodulating and, ultimately, detecting photoinduced signals with ultrafast pulses (Methods). Briefly, a probe beam illuminates the sample which is imaged onto a conventional CMOS camera in either a dark-field or a bright-field configuration.…”

“…3b), thus making them essentially indistinguishable when measured in a linear colour-blind fashion. 12 Their pump–probe time-delay dependent UHT response, however, differs considerably as can be seen by evaluating the scattering signal change computed as the difference between images acquired in the presence and absence of the pump pulse (Fig. 3c).…”