Mohsen Rajaei

Measurement of laterally induced optical forces at the nanoscale

We demonstrate the measurement of laterally induced optical forces using an Atomic Force Microscope (AFM). The lateral electric field distribution between a gold coated AFM probe and a single nano-aperture in a gold film is mapped by measuring the lateral optical force between the apex of the AFM probe and the nano-aperture. The fundamental torsional eigen-mode of an AFM cantilever probe was used to detect the laterally induced optical forces. We engineered the cantilever shape using focused ion beam milling to improve the detected signal to noise ratio. The measured distributions of lateral optical force agree well with electromagnetic simulations of the metal coated AFM probe interacting with the nano-aperture. This technique can be extended to simultaneously detect both lateral and longitudinal optical forces at the nanoscale by using an AFM cantilever as a multi-channel detector. This will enable simultaneous Photon Induced Force Microscopy detection of molecular responses with different incident fiel...

Photo-induced force vs power in chiral scatterrers

We study isotropic chiral polarizable particles in the interaction with circularly polarized travelling electromagnetic waves. We derive the expressions for the scattered and extinct power of such particles in terms of their polarizabilities for incident waves with opposite handedness i.e., right or left handed circular polarization. Moreover, we extract similar expressions for the exerted force on such particles induced by the proposed incidences. Based on our theoretical results, we present the link between the calculated power and force and demonstrate how these two different physical observables give rise to equivalent understanding of properties of chiral inclusions. This paves the way to formulate two alternatives to perform either the photo-induced force microscopy or power spectroscopy based on the merits of each. We also provide an original method to measure the magnetoelectric polarizability of isotropic chiral particles.

Silicon nitride on silicon-on-insulator: A platform for integration active control over passive components

We propose the use of SOI with silicon nitride layers for electronic control. A phase modulator with a V_π·L of 1.02 V·mm and a compact vertical coupling taper are demonstrated in an easy-to-fabricate planar structure.

Probing magnetic nanoprobe in structured light by a subtle soft touch (Conference Presentation)

Optical magnetism has long been the elusive, missing component in light-matter interaction. Interesting applications may emerge if optical magnetism is effectively harnessed and exploited. Of particular interest is the possible manipulation of the optical magnetic force, in the form of photo-induced magnetic force microscopy. We propose an optical system for inducing magnetic forces in an axis-aligned Si disk under azimuthally polarized beam illumination. The designed Si disk can support a magnetic resonance in the visible range under azimuthal polarization by interacting with the longitudinal magnetic field at the overlapping axis. Such structure can serve as the unique magnetic probe to “feel” the magnetic force of light. In our current step, we use photo-induced force microscopy to characterize the near-field electric field distribution of this system. Measurements show a stronger electric field enhancement near the edge of the Si disk which indicates a longitudinal magnetic field enhancement at the overlapping axis. This measurement is in accordance with theoretical modeling, confirming the observed magnetic enhancement. This indirect measurement on the magnetic response of the Si disk defines an important step towards our final goal of achieving direct mapping of the local magnetic field with photo-induced magnetic force microscopy. Also, our methodology can be extended to the characterization of arbitrary nanostructures, including metamaterials and metasurfaces, under structured light illumination.

Unscrambling Structured Chirality with Structured Light at the Nanoscale Using Photoinduced Force

We show that the gradient force generated by the near field of a chiral nanoparticle carries information about its chirality. On the basis of this physical phenomenon we propose a new microscopy technique that enables the prediction of spatial features of chirality of nanoscale samples by exploiting the photoinduced optical force exerted on an achiral tip in the vicinity of the test specimen. The tip–sample interactive system is illuminated by structured light to probe both the transverse and longitudinal (with respect to the beam propagation direction) components of the sample’s magnetoelectric polarizability as the manifestation of its sense of handedness, i.e., chirality. We specifically prove that although circularly polarized waves are adequate to detect the transverse polarizability components of the sample, they are unable to probe the longitudinal component. To overcome this inadequacy and probe the longitudinal chirality, we propose a judiciously engineered combination of radially and azimuthally...

Characterization of Si-disk magnetic nanoprobe by photoinduced force microscopy (Conference Presentation)

Due to the weak magnetic responsibility of natural existing materials at optical frequency, optical magnetism remains a “dark state” of light which is largely unexplored. However, optical magnetism is also very desirable because of the many splendid possibilities it may lead to, including ultra-compact opto-magnetic storage devices, high speed magnetic imaging, magnetic tweezers etc. Here we design a Si nano-disk structure as the magnetic nanoprobe which supports magnetic resonance in visible range with the incident azimuthally polarized beam (APB). APB features a donut shape beam profile, with a strong longitudinal magnetic field and a vanishing electric field at the beam axis. Therefore, on the magnetic resonance while the probe is aligned to the APB axis, a longitudinal magnetic dipole is excited in the probe, and interacts with the incident APB inducing an exclusive magnetic force. Making such magnetic nanoprobe under APB illumination serves as an important first step to realize the proposed photoinduced magnetic force microscopy (PIMFM), which selectively exploits the interaction between matter and the magnetic field of light to characterize the optical magnetism in nanoscale. Such investigation of the optical magnetism in samples is dearly needed in many mechanical, chemical, and life-science applications.