Prof. Tapas Kumar Chini

Senior Professor
Room No	:	Isotope bldg.
Ext.	:	4214
Email id	:	tapask.chini[AT]saha.ac.in
Division	:	Surface Physics and Material Science Division
Centre	:	Centre for Nanomaterials and Surface Physics

Research

Theme of our current research

Cathodoluminescence (CL) in scanning electron microscope (SEM) is a powerful tool that has been traditionally used for the optical characterization of bulk semiconductors and geological rocks. However, in recent years CL-SEM is being utilized for spectro-microscopy study of nanostructured semiconductors, such as quantum wells or quantum dots and localized surface plasmon resonance (LSPR) modes from metallic nanostructures. While photon emission from semiconductor materials on interaction with an electron beam is well-understood, CL from plasmonic metal nanostructures is a relatively new field and deserves more attention.

♣ Science :

Surface plasmons (SP)—or more specifically surface plasmon polaritons (SPP) are propagating, transverse electromagnetic (EM) waves coupled to the electron plasma of a conductor at a dielectric interface (such as a metal sheet in air). The electric field intensity associated to a SPP in the perpendicular direction at both sides of the interface is evanescent. Thus SPPs are basically bound non-radiative 2D surface modes whose existence is characterized by opposite signs of the real part of the dielectric functions of the materials separating the interfaces. More interesting are the localized surface plasmons (LSP), which are non-propagating excitations of the conduction electrons in metallic nanoparticles.



LSP when excited resonantly with a particular wavelength of the exciting light or evanescent wave associated with fast-moving electrons within electron microscope, can decay into radiative photons that can be collected into far-field. LSP resonances can show highly localized enhancement of near-field amplitude at the nanostructured metal surface and similar enhancement of the far-field intensity. Exploiting EM field enhancement from such nano-structures has led to several interesting effects, such as, enhanced fluorescence, enhanced photocarrier generation, surface enhanced Raman scattering (SERS) that can have potential applications in bio sensing, photovoltaics and single molecule detection. So,surface plasmons form the foundation of a rapidly growing new field of photonic research called plasmonics that exploits the unique optical properties of metallic nanostructures to route and manipulate light at nanometer length scales.

♣ Our object :

Mapping the spatial variation of the photon emission from light or elctron beam excited metal nanoparticle is a direct probe of resonant modes of plasmonic nanostructures and, consequently, provides a direct way to map the local electric fields. Often the local EM field enhancement in the plasmonic structures is confined spatially on length scales of ∼ 10−50 nm and varies strongly with the morphology and composition of nanoparticles, meaning, ensemble measurement of nanoparticle's optical properties is not expected to reflect the spectrum of an individual entity. Consequently, single nanoparticle spectroscopy with high spatial resolution imaging capability is an essential tool to understand the plasmonic properties of nanoparticles. A subdiffraction-limited optical imaging technique, such as near-field scanning optical microscopy (NSOM), can achieve a resolution of ∼20 nm or slightly better, but (in the case of NSOM) is constrained by the requirement of fabricating very sharp tips and undesirable effects originating from tip-sample interactions.



Alternatively, detection of electron beam induced radiation emission (EIRE) also called as cathodoluminescence (CL) in a transmission electron microscope (TEM) / scanning electron microscope (SEM) or detection of energy loss suffered by the inelastically scattered transmitted electrons in TEM are shown to constitute an excellent probe of plasmons that allows capturing highest (1-40 nm) resolution information in the spatial and spectral domains. Our main object is to directly visualize the spatial profile of the localized surface plasmon resonance modes of anisotropic noble metal (as for example Au) nanoparticles with special emphasis on the higher order modes. This is becasue the identification of higher order plasmonic modes provides a deeper understanding of the physical properties and spectroscopic fingerprint necessary for assessing the quality of the concerned metal nano structures. We use CL-SEM technique as it is advantageous over electron energy loss spectroscopy (EELS) or CL-TEM because a large sample area on a thick substrate can be accessed without any stringent requirement of sample preparation, such as maintaining electron transparency (<100 nm) for use in TEM. CL is in fact different from EELS, because CL probes the scattering physics, while EELS probes the extinction one. The image contrast in CL is a measure of the ability of the e-beam to locally excite the modes. Consequently, the CL image allows us to construct excitability maps and spectra which are proportional to the local density of optical states (LDOS) of the structure. We analyse our experimental radiation spectra and photon maps with the simulated radiation spectra and near-field maps obtained using a commecially available finite-difference time-domain (FDTD) numerical simulation package.

♣ FDTD approach in a nutshell

Mie theory is a classic example of an early technique for modeling plasmonic response of spherically symmetric structures, the scattering and absorption from which can be solved analytically using Maxwell's equations. However, for complex shaped anisotropic nanoparticle, Mie theory is significantly limited in its applicability. Recent advances in numerical simulation methods such as the Finite-difference time-domain (FDTD) has allowed for electrodynamic simulations of almost arbitrarily complex geometries. In FDTD, the macroscopic Maxwell’s equations are solved in discretized space and discretized time to follow the response of a material to any applied electromagnetic (EM) field. The system is typically excited with either light (such as a plane wave source) or an electron beam (created from a current source). For numerical investigation of the electron beam excited photon emission in a CL setup, the electron beam can be modeled as a line current density source represented as follows: J(t,2r⃗) = −evûz δ(z − νt)δ(x − x0)δ(y − y0), where e is the electronic charge, and v is the velocity of electron, (x0,y0) represents the position of the electron beam, z is the direction of electron velocity, and ûz is the unit vector along the z direction. In the frequency domain, this corresponds to a current density, J(ω,r⃗) = evûz exp(iωz/ν)δ(x − x0)δ(y − y0). In the simulation, this current density is modeled as a series of dipoles with temporal phase delay (z/ν) that is related to the electron velocity, ν (here ν = 0.32c corresponding to the 30 keV electron energy used in the experiment with c being the velocity of light in free space).

Last Updated on Friday, 11 April 2014 19:21

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Research
*Theme of our current research* Cathodoluminescence (CL) in scanning electron microscope (SEM) is a powerful tool that has been traditionally used for the optical characterization of bulk semiconductors and geological rocks. However, in recent years CL-SEM is being utilized for spectro-microscopy study of nanostructured semiconductors, such as quantum wells or quantum dots and localized surface plasmon resonance (LSPR) modes from metallic nanostructures. While photon emission from semiconductor materials on interaction with an electron beam is well-understood, CL from plasmonic metal nanostructures is a relatively new field and deserves more attention. ♣ Science : Surface plasmons (SP)—or more specifically surface plasmon polaritons (SPP) are propagating, transverse electromagnetic (EM) waves coupled to the electron plasma of a conductor at a dielectric interface (such as a metal sheet in air). The electric field intensity associated to a SPP in the perpendicular direction at both sides of the interface is evanescent. Thus SPPs are basically bound non-radiative 2D surface modes whose existence is characterized by opposite signs of the real part of the dielectric functions of the materials separating the interfaces. More interesting are the localized surface plasmons (LSP), which are non-propagating excitations of the conduction electrons in metallic nanoparticles. LSP when excited resonantly with a particular wavelength of the exciting light or evanescent wave associated with fast-moving electrons within electron microscope, can decay into radiative photons that can be collected into far-field. LSP resonances can show highly localized enhancement of near-field amplitude at the nanostructured metal surface and similar enhancement of the far-field intensity. Exploiting EM field enhancement from such nano-structures has led to several interesting effects, such as, enhanced fluorescence, enhanced photocarrier generation, surface enhanced Raman scattering (SERS) that can have potential applications in bio sensing, photovoltaics and single molecule detection. So,surface plasmons form the foundation of a rapidly growing new field of photonic research called plasmonics that exploits the unique optical properties of metallic nanostructures to route and manipulate light at nanometer length scales. ♣ Our object : Mapping the spatial variation of the photon emission from light or elctron beam excited metal nanoparticle is a direct probe of resonant modes of plasmonic nanostructures and, consequently, provides a direct way to map the local electric fields. Often the local EM field enhancement in the plasmonic structures is confined spatially on length scales of ∼ 10−50 nm and varies strongly with the morphology and composition of nanoparticles, meaning, ensemble measurement of nanoparticle's optical properties is not expected to reflect the spectrum of an individual entity. Consequently, single nanoparticle spectroscopy with high spatial resolution imaging capability is an essential tool to understand the plasmonic properties of nanoparticles. A subdiffraction-limited optical imaging technique, such as near-field scanning optical microscopy (NSOM), can achieve a resolution of ∼20 nm or slightly better, but (in the case of NSOM) is constrained by the requirement of fabricating very sharp tips and undesirable effects originating from tip-sample interactions. Alternatively, detection of electron beam induced radiation emission (EIRE) also called as cathodoluminescence (CL) in a transmission electron microscope (TEM) / scanning electron microscope (SEM) or detection of energy loss suffered by the inelastically scattered transmitted electrons in TEM are shown to constitute an excellent probe of plasmons that allows capturing highest (1-40 nm) resolution information in the spatial and spectral domains. Our main object is to directly visualize the spatial profile of the localized surface plasmon resonance modes of anisotropic noble metal (as for example Au) nanoparticles with special emphasis on the higher order modes. This is becasue the identification of higher order plasmonic modes provides a deeper understanding of the physical properties and spectroscopic fingerprint necessary for assessing the quality of the concerned metal nano structures. We use CL-SEM technique as it is advantageous over electron energy loss spectroscopy (EELS) or CL-TEM because a large sample area on a thick substrate can be accessed without any stringent requirement of sample preparation, such as maintaining electron transparency (<100 nm) for use in TEM. CL is in fact different from EELS, because CL probes the scattering physics, while EELS probes the extinction one. The image contrast in CL is a measure of the ability of the e-beam to locally excite the modes. Consequently, the CL image allows us to construct excitability maps and spectra which are proportional to the local density of optical states (LDOS) of the structure. We analyse our experimental radiation spectra and photon maps with the simulated radiation spectra and near-field maps obtained using a commecially available finite-difference time-domain (FDTD) numerical simulation package. ♣ FDTD approach in a nutshell Mie theory is a classic example of an early technique for modeling plasmonic response of spherically symmetric structures, the scattering and absorption from which can be solved analytically using Maxwell's equations. However, for complex shaped anisotropic nanoparticle, Mie theory is significantly limited in its applicability. Recent advances in numerical simulation methods such as the Finite-difference time-domain (FDTD) has allowed for electrodynamic simulations of almost arbitrarily complex geometries. In FDTD, the macroscopic Maxwell’s equations are solved in discretized space and discretized time to follow the response of a material to any applied electromagnetic (EM) field. The system is typically excited with either light (such as a plane wave source) or an electron beam (created from a current source). For numerical investigation of the electron beam excited photon emission in a CL setup, the electron beam can be modeled as a line current density source represented as follows: J(t,2r⃗) = −evûz δ(z − νt)δ(x − x0)δ(y − y0), where e is the electronic charge, and v is the velocity of electron, (x0,y0) represents the position of the electron beam, z is the direction of electron velocity, and ûz is the unit vector along the z direction. In the frequency domain, this corresponds to a current density, J(ω,r⃗) = evûz exp(iωz/ν)δ(x − x0)δ(y − y0). In the simulation, this current density is modeled as a series of dipoles with temporal phase delay (z/ν) that is related to the electron velocity, ν (here ν = 0.32c corresponding to the 30 keV electron energy used in the experiment with c being the velocity of light in free space).