Optical Technique for Probing Defects in Thin Semiconductor Film
Collected light scattering and localization through fiber based scanning probe microscope was used to detect optical signature of crystallographic defects in nonlinear materials. III-V epitaxial films are ideal candidate for electron transport and opto-electronic semiconductors . Growth of such polar materials like GaAs and InP over the mismatch such as Si leave us with variety of crystallographic defects such as treading dislocations. These defects act as sink for charge carriers and scattering point for mobile charge carriers . A pulsed laser light penetrate through the film and get scattered and localized by these defects beneath the surface. The propagating scattered light projected back on the surface as hotspot and picked up by the 50nm fiber aperture probe of the nearfield system. To our knowledge this is the first optical technique for crystallographic defects at such a high resolution in semiconductor thin film field. Nonlinear response of broken centro symmetry of materials such as GaAs or InP leaving the optical signal clear of fundamental surface reflection.
The figure shows the different of nonlinear optical response between mismatched GaAs-Si and reference GaAs-GaAs films. Cross STEM images on the side show the presence of TD defects at mismatched interface.
Second Harmonics Optical Detection of Strain Field
We had noninvasively probed strain fields around Cu pillars in Si(001) using optical second-harmonic generation (SHG) microscopy. These Cu pillars used as TSV (through silicon vias) to electrically contact vertically stacked silicon layers. [Applied Physics Letters 108, 151602 (2016)]
Artificial Optical Magnetism
I had used Atomic force microscope (AFM) to position individual gold nanospheres into a subwavelength plasmonic metamolecule consisting of four closely spaced nanoparticles [Nature Nanotechnology 8, 95 (2013)]. This structure supported a strong magnetic response coupled to a broad electric resonance in visible range. Asymmetries in the assembled nanoring enable the interaction between electric and magnetic modes, leading to the first observation of a magnetic-based Fano scattering resonance at optical frequencies. Such a metamolecule are suitable for building block for negative-index metamaterials. Figure shows the scattering signal of the structure when particles are few hundred nm apart and scattering signal of the structure when particles are very close (few nm apart).
Plasmonic Nano-Protractor
A polarization-resolved scattering technique was used to identify the relative orientation between two nanoparticles based on deformation of Fano resonance of the plasmonic structure [Nature Photonics 7, 367 (2013)]. The Fano resonance was due to overlapping of bright dipole resoncne of nanosphere and dark quadrupole resonance of nanorod. AFM was used to bring the nanoparticles to near contact. Figure shows the manipulation sequences of nanosphere and nanorod by AFM and the plasmonic scattering difference when particles are far and close to each other.
Plasmonic Effect on Exciton Lifetime
Single gold nanoshphere was brought close to single CdSe/ZnS quntum dot in a controll way with help of AFM manipulation technique. As gold nanosphere approches the quntum dot, the gradual change of lifetime of the excitions in quantum dots were monitored and quenching of the recombination time were observed [Nano Letters 11, 1049 (2011)]. Typical lifetime of ~30 ns was reduced down to ~1 ns. This is because of modification of excitation rate and quantum yeild of quantum dot due to 6enhancment of local plasmonic field and availability of non radiative chanell (gold nanosphere).
Resonant Energy Transfer between Quantum Dots
A lab made nearfield scanning optical microscope (NSOM) was used to bring two groups of resonannt CdSe/ZnS quantum dots into close proximity of each other. For the first time, resonannt energy transfer was measured lively as function of separation between quantum dots [Physical Review Letters B 84, 075301(2011)]. As small quantum dots attached to the NSOM probe were moving toward the large quantum dots, quenching of photoluminescence signal of small quantum dots were observed during the excitation of both groups. Figure shows the interacting spectrum of resonant quantum dots attached to the the NSOM probe and deposited on solid immersion lens collected by spectrometer as function of quantum dots distance. Movement of NSOM probe changes the interacting distance. For more details please check my thesis (Farbod Shafiei-Purdue University 2008).