skip to main content

Attention:

The NSF Public Access Repository (PAR) system and access will be unavailable from 11:00 PM ET on Thursday, February 13 until 2:00 AM ET on Friday, February 14 due to maintenance. We apologize for the inconvenience.


Search for: All records

Creators/Authors contains: "Sifat, Abid Anjum"

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. We theoretically show that the optical chiral properties of tightly focused laser beams can be characterized by means of force detection. To measure the chiral properties of a beam of given handedness in the microscopic focal volume, we determine the photoinduced force exerted on a sharp tip, which is illuminated first by the beam of interest and second by an auxiliary beam of opposite handedness, in a sequential manner. We show that the difference between the force measurements is directly proportional to the chiral properties of the beam of interest. In particular, the gradient force difference Δ⟨Fgrad ,z⟩ is found to have exclusive correspondence to the time-averaged helicity density of the incident light, whereas the differential scattering force provides information about the spin angular momentum density of light. We further characterize and quantify the helicity-dependent Δ⟨Fgrad ,z⟩ using a Mie scattering formalism complemented with full wave simulations, underlining that the magnitude of the difference force is within an experimentally detectable range. 
    more » « less
  2. Photo-induced force microscopy (PiFM) is a scan probe technique that offers images with spectroscopic contrast at a spatial resolution in the nanometer range. PiFM utilizes the non-propagating, enhanced near field at the apex of a sharp tip to locally induce a polarization in the sample, which in turn produces an additional force acting on the cantilevered tip. This photo-induced force, though in the pN range or less, can be extracted from the oscillation properties of the cantilever, thus enabling the generation of photo-induced force maps. Since its inception in 2010, the PiFM technique has grown into a useful nano-spectrocopic tool that has expanded its reach in terms of imaging capabilities and applications. In this review, we present various technical implementations of the PiFM approach. In addition, we discuss the physical origin of the PiFM signal, highlighting the contributions from dipole–dipole forces as well as forces that derive from photo-thermal processes. 
    more » « less
  3. Abstract

    The process of tip‐enhanced Raman scattering (TERS) depends critically on the morphology near the apex of the tip used in the experiment. Many tip designs have focused on optimization of electromagnetic enhancement in the near‐field, which is controlled to a large extent by subtle details at the nanoscale that remain difficult to reproduce in the tip fabrication process. The use of focused ion beams (FIB) permit modification of larger features on the tip in a reproducible manner, yet this approach cannot produce sub‐20‐nm structures important for optimum near‐field enhancement. Nonetheless, FIB milling offers excellent opportunities for improving the far‐field radiation properties of the tip‐antenna, a feature that has received relatively little attention in the TERS research community. In this work, we use finite‐difference time‐domain (FDTD) simulations to study both the near‐field and far‐field radiation efficiency of several tip‐antenna systems that can be constructed with FIB techniques in a feasible manner. Starting from blunt etched tips, we find that excellent overall enhancement of the TERS signal can be obtained with pillar‐type tips. Furthermore, by applying vertical grooves on the tip's shaft, the overall efficiency can be improved even more, producing TERS signals that are up to 10‐fold stronger than signals obtained from an ideal (unmodified) sharp tip of 10‐nm radius. The proposed designs constitute a feasible route toward a tip fabrication process that not only yields more reproducible tips but also promises much stronger TERS signals.

     
    more » « less