Bala Krishna Juluri - Academia.edu (original) (raw)

Papers by Bala Krishna Juluri

A tunable opto-fluidic microlens configuration named liquid gradient refractive index (L-GRIN) le... more A tunable opto-fluidic microlens configuration named liquid gradient refractive index (L-GRIN) lens is described. The variable light focusing is achieved through the diffusion based refractive index gradient within a microfluidic device.

We have designed and simulated a parabolic gradient- index (GRIN) photonic crystal (PC) based bea... more We have designed and simulated a parabolic gradient- index (GRIN) photonic crystal (PC) based beam aperture modifier and a deflector. The parabolic GRIN PC is achieved by using free standing dielectric columns with graded radii transverse to the light propagation direction. The performance of the beam aperture modifier and the deflector can be tuned by changing the gradient coefficient of the GRIN PCs. Finite- difference time-domain (FDTD) methods were used to characterize changes in beam width and propagation direction. Compared with conventional approaches, the GRIN PC-based beam aperture modifier and beam deflector described here can be conveniently fabricated down to the micrometer scale. Keywords-Nanophotonics; Microfabrication

Lab on A Chip, 2009

We present ultra-fast homogeneous mixing inside a microfluidic channel via single-bubble-based ac... more We present ultra-fast homogeneous mixing inside a microfluidic channel via single-bubble-based acoustic streaming. The device operates by trapping an air bubble within a ''horse-shoe'' structure located between two laminar flows inside a microchannel. Acoustic waves excite the trapped air bubble at its resonance frequency, resulting in acoustic streaming, which disrupts the laminar flows and triggers the two fluids to mix. Due to this technique's simple design, excellent mixing performance, and fast mixing speed (a few milliseconds), our single-bubble-based acoustic micromixer may prove useful for many biochemical studies and applications.

Lab on A Chip, 2009

We report a tunable optofluidic microlens configuration named the Liquid Gradient Refractive Inde... more We report a tunable optofluidic microlens configuration named the Liquid Gradient Refractive Index (L-GRIN) lens for focusing light within a microfluidic device. The focusing of light was achieved through the gradient refractive index (GRIN) within the liquid medium, rather than via curved refractive lens surfaces. The diffusion of solute (CaCl(2)) between side-by-side co-injected microfluidic laminar flows was utilized to establish a hyperbolic secant (HS) refractive index profile to focus light. Tailoring the refractive index profile by adjusting the flow conditions enables not only tuning of the focal distance (translation mode), but also shifting of the output light direction (swing mode), a second degree of freedom that to our knowledge has yet to be accomplished for in-plane tunable microlenses. Advantages of the L-GRIN lens also include a low fluid consumption rate, competitive focusing performance, and high compatibility with existing microfluidic devices. This work provides a new strategy for developing integrative tunable microlenses for a variety of lab-on-a-chip applications.

Journal of Applied Physics, 2010

Photonic crystal (PC) composites are sequenced series of PCs that feature the same periods but di... more Photonic crystal (PC) composites are sequenced series of PCs that feature the same periods but different filling fractions. By properly tuning the filling fractions of the individual PCs and merging the working band of each PC into a continuous frequency range, wide-band self-collimation of optical signals can be realized. The band diagrams and the equal-frequency contours of the PC structures were calculated through the plane wave expansion method and the finite-difference time-domain method was employed to simulate the propagation of electromagnetic waves through the PC structures. Our results show that while a single PC can only collimate optical waves over a narrow frequency range, a PC composite exhibits a much wider collimation band. Such a wide-band optical collimation lens can be useful in applications that demand directional optical energy flow over a long distance, such as optical imaging and biosensing.

Abstract Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithogra... more Abstract Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combined with a two-step reactive ion etching technique. The optical properties of these arrays were investigated both experimentally and theoretically. Specifically, the effects of disk diameter on localized surface plasmon resonance (LSPR) were characterized and compared with results from discrete dipole approximation (DDA) calculations. The effects of glass substrate, Cr interfacial layer, and Au thickness on LSPR were investigated ...

Journal of Applied Physics, 2011

Frequency-addressed tunable transmission is demonstrated in optically thin metallic nanohole arra... more Frequency-addressed tunable transmission is demonstrated in optically thin metallic nanohole arrays embedded in dual-frequency liquid crystals (DFLCs). The optical properties of the composite system are characterized by the transmission spectra of the nanoholes, and a prominent transmission peak is shown to originate from the resonance of localized surface plasmons at the edges of the nanoholes. An ~17 nm shift in the transmission peak is observed between the two alignment configurations of the liquid crystals. This DFLC-based active plasmonic system demonstrates excellent frequency-dependent switching behavior and could be useful in future nanophotonic applications.

Optics Express, 2009

In this work, we investigate the propagation of designer surface plasmons in planar perfect elect... more In this work, we investigate the propagation of designer surface plasmons in planar perfect electric conductor structures that are subject to a parabolic graded-index distribution. A three-dimensional, fully vectorial finitedifference time-domain method was used to engineer a structure with a parabolic effective group index by modulating the dielectric constant of the structure's square holes. Using this structure in our simulations, the lateral confinement of propagating designer surface plasmons is demonstrated. Focusing, collimation and waveguiding of designer plasmons in the lateral direction is realized by changing the width of the source beam. Our findings contribute to applications of designer surface plasmons that require energy concentration, diffusion, guiding, and beam aperture modification within planar perfect electric conductors.

Journal of Applied Physics, 2008

Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combi... more Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combined with a two-step reactive ion etching technique. The optical properties of these arrays were investigated both experimentally and theoretically. Specifically, the effects of disk diameter on localized surface plasmon resonance (LSPR) were characterized and compared with results from discrete dipole approximation (DDA) calculations. The effects of glass substrate, Cr interfacial layer, and Au thickness on LSPR were investigated computationally. Furthermore, thermal treatment was found to be essential in improving the nanodisk arrays' LSPR properties. Using atomic force microscopy and DDA calculations, it was established that the improvements in LSPR properties were due to thermally induced morphologic changes. Finally, microfluidic channels were integrated with the annealed disk arrays to study the sensitivity of LSPR to the change in surroundings' refractive index. The dependence of LSPR on surroundings' refractive index was measured and compared with calculated results.

Acs Nano, 2009

A microcantilever, coated with a monolayer of redox-controllable, bistable [3]rotaxane molecules ... more A microcantilever, coated with a monolayer of redox-controllable, bistable [3]rotaxane molecules (artificial molecular muscles), undergoes reversible deflections when subjected to alternating oxidizing and reducing electrochemical potentials. The microcantilever devices were prepared by precoating one surface with a gold film and allowing the palindromic [3]rotaxane molecules to adsorb selectively onto one side of the microcantilevers, utilizing thiol-gold chemistry. An electrochemical cell was employed in the experiments, and deflections were monitored both as a function of (i) the scan rate (<20 mV s ؊1 ) and (ii) the time for potential step experiments at oxidizing (>؉0.4 V) and reducing (<؉0.2 V) potentials. The different directions and magnitudes of the deflections for the microcantilevers, which were coated with artificial molecular muscles, were compared with (i) data from nominally bare microcantilevers precoated with gold and (ii) those coated with two types of control compounds, namely, dumbbell molecules to simulate the redox activity of the palindromic bistable [3]rotaxane molecules and inactive 1-dodecanethiol molecules. The comparisons demonstrate that the artificial molecular muscles are responsible for the deflections, which can be repeated over many cycles. The microcantilevers deflect in one direction following oxidation and in the opposite direction upon reduction. The ϳ550 nm deflections were calculated to be commensurate with forces per molecule of ϳ650 pN.

Journal of Physical Chemistry C, 2009

We report on chemical etching of ordered Au nanostructure arrays to continuously tune their local... more We report on chemical etching of ordered Au nanostructure arrays to continuously tune their localized surface plasmon resonances (LSPR). Real-time extinction spectra were recorded from both Au nanodisks and nanospheres immobilized on glass substrates when immersed in Au etchant. The time-dependent LSPR frequencies, intensities, and bandwidths were studied theoretically with discrete dipole approximations and the Mie solution, and they were correlated with the evolution of the etched Au nanostructures' morphology (as examined by atomic force microscopy). Since this chemical etching method can conveniently and accurately tune LSPR, it offers precise control of plasmonic properties and can be useful in applications such as surfaceenhanced Raman spectroscopy and molecular resonance spectroscopy.

Journal of Applied Physics, 2010

We designed and simulated a beam aperture modifier and a beam deflector using two-dimensional par... more We designed and simulated a beam aperture modifier and a beam deflector using two-dimensional parabolic gradient-index (GRIN) photonic crystals (PCs). The GRIN PCs are composed of dielectric columns with graded radii along the direction transverse to propagation. Both finite-difference time-domain methods and gradient optics analytical solutions were used to characterize the change in beam width and propagation direction. Multifunctional GRIN

Microfluidics and Nanofluidics, 2010

In this study, we report a rapid microfluidic mixing device based on chaotic advection induced by... more In this study, we report a rapid microfluidic mixing device based on chaotic advection induced by microbubble–fluid interactions. The device includes inlets for to-be-mixed fluids and nitrogen gas. A side-by-side laminar flow segmented by monodisperse microbubbles is generated when the fluids and the nitrogen are co-injected through a flow focusing micro-orifice. The flow subsequently enters a series of hexagonal expansion chambers, in which the hydrodynamic interaction among the microbubbles results in the stretch and fold of segmented fluid volumes and rapid mixing and homogenization. We characterize the performance of the microfluidic mixer and demonstrate rapid mixing within 20 ms. We further show that bubbles can be conveniently removed from the mixed fluids using a microfluidic comb structure on completion of the mixing.

Journal of Nanotechnology in Engineering and Medicine, 2010

We have advanced nanosphere lithography for fabricating ordered Au nanodisk and nanohole arrays o... more We have advanced nanosphere lithography for fabricating ordered Au nanodisk and nanohole arrays on glass substrates. Further, we demonstrate the applications of the nanodisks and nanoholes in understanding exciton-plasmon couplings and patterning micro-/nano-particles, respectively.

Abstract Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithogra... more Abstract Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combined with a two-step reactive ion etching technique. The optical properties of these arrays were investigated both experimentally and theoretically. Specifically, the effects of disk diameter on localized surface plasmon resonance (LSPR) were characterized and compared with results from discrete dipole approximation (DDA) calculations. The effects of glass substrate, Cr interfacial layer, and Au thickness on LSPR were investigated ...

Optics Express, 2009

In this work, we investigate the propagation of designer surface plasmons in planar perfect elect... more In this work, we investigate the propagation of designer surface plasmons in planar perfect electric conductor structures that are subject to a parabolic graded-index distribution. A three-dimensional, fully vectorial finitedifference time-domain method was used to engineer a structure with a parabolic effective group index by modulating the dielectric constant of the structure's square holes. Using this structure in our simulations, the lateral confinement of propagating designer surface plasmons is demonstrated. Focusing, collimation and waveguiding of designer plasmons in the lateral direction is realized by changing the width of the source beam. Our findings contribute to applications of designer surface plasmons that require energy concentration, diffusion, guiding, and beam aperture modification within planar perfect electric conductors.

Journal of Physical Chemistry C, 2009

In this work, we show using both experiments and classical electrodynamic simulations that plasmo... more In this work, we show using both experiments and classical electrodynamic simulations that plasmon splitting in resonant molecule-coated nanoparticles increases linearly as the square root of absorbance of the molecular layer. This linear relationship shows the same universal behavior established in analogous systems such as cavity-polariton and surface plasmon polariton systems. To explain this behavior, a simple physical mechanism based on linear dispersion and absorption is proposed. The insights obtained in this study can ...

A tunable opto-fluidic microlens configuration named liquid gradient refractive index (L-GRIN) le... more A tunable opto-fluidic microlens configuration named liquid gradient refractive index (L-GRIN) lens is described. The variable light focusing is achieved through the diffusion based refractive index gradient within a microfluidic device.

We have designed and simulated a parabolic gradient- index (GRIN) photonic crystal (PC) based bea... more We have designed and simulated a parabolic gradient- index (GRIN) photonic crystal (PC) based beam aperture modifier and a deflector. The parabolic GRIN PC is achieved by using free standing dielectric columns with graded radii transverse to the light propagation direction. The performance of the beam aperture modifier and the deflector can be tuned by changing the gradient coefficient of the GRIN PCs. Finite- difference time-domain (FDTD) methods were used to characterize changes in beam width and propagation direction. Compared with conventional approaches, the GRIN PC-based beam aperture modifier and beam deflector described here can be conveniently fabricated down to the micrometer scale. Keywords-Nanophotonics; Microfabrication

Lab on A Chip, 2009

We present ultra-fast homogeneous mixing inside a microfluidic channel via single-bubble-based ac... more We present ultra-fast homogeneous mixing inside a microfluidic channel via single-bubble-based acoustic streaming. The device operates by trapping an air bubble within a ''horse-shoe'' structure located between two laminar flows inside a microchannel. Acoustic waves excite the trapped air bubble at its resonance frequency, resulting in acoustic streaming, which disrupts the laminar flows and triggers the two fluids to mix. Due to this technique's simple design, excellent mixing performance, and fast mixing speed (a few milliseconds), our single-bubble-based acoustic micromixer may prove useful for many biochemical studies and applications.

Lab on A Chip, 2009

We report a tunable optofluidic microlens configuration named the Liquid Gradient Refractive Inde... more We report a tunable optofluidic microlens configuration named the Liquid Gradient Refractive Index (L-GRIN) lens for focusing light within a microfluidic device. The focusing of light was achieved through the gradient refractive index (GRIN) within the liquid medium, rather than via curved refractive lens surfaces. The diffusion of solute (CaCl(2)) between side-by-side co-injected microfluidic laminar flows was utilized to establish a hyperbolic secant (HS) refractive index profile to focus light. Tailoring the refractive index profile by adjusting the flow conditions enables not only tuning of the focal distance (translation mode), but also shifting of the output light direction (swing mode), a second degree of freedom that to our knowledge has yet to be accomplished for in-plane tunable microlenses. Advantages of the L-GRIN lens also include a low fluid consumption rate, competitive focusing performance, and high compatibility with existing microfluidic devices. This work provides a new strategy for developing integrative tunable microlenses for a variety of lab-on-a-chip applications.

Journal of Applied Physics, 2010

Photonic crystal (PC) composites are sequenced series of PCs that feature the same periods but di... more Photonic crystal (PC) composites are sequenced series of PCs that feature the same periods but different filling fractions. By properly tuning the filling fractions of the individual PCs and merging the working band of each PC into a continuous frequency range, wide-band self-collimation of optical signals can be realized. The band diagrams and the equal-frequency contours of the PC structures were calculated through the plane wave expansion method and the finite-difference time-domain method was employed to simulate the propagation of electromagnetic waves through the PC structures. Our results show that while a single PC can only collimate optical waves over a narrow frequency range, a PC composite exhibits a much wider collimation band. Such a wide-band optical collimation lens can be useful in applications that demand directional optical energy flow over a long distance, such as optical imaging and biosensing.

Abstract Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithogra... more Abstract Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combined with a two-step reactive ion etching technique. The optical properties of these arrays were investigated both experimentally and theoretically. Specifically, the effects of disk diameter on localized surface plasmon resonance (LSPR) were characterized and compared with results from discrete dipole approximation (DDA) calculations. The effects of glass substrate, Cr interfacial layer, and Au thickness on LSPR were investigated ...

Journal of Applied Physics, 2011

Frequency-addressed tunable transmission is demonstrated in optically thin metallic nanohole arra... more Frequency-addressed tunable transmission is demonstrated in optically thin metallic nanohole arrays embedded in dual-frequency liquid crystals (DFLCs). The optical properties of the composite system are characterized by the transmission spectra of the nanoholes, and a prominent transmission peak is shown to originate from the resonance of localized surface plasmons at the edges of the nanoholes. An ~17 nm shift in the transmission peak is observed between the two alignment configurations of the liquid crystals. This DFLC-based active plasmonic system demonstrates excellent frequency-dependent switching behavior and could be useful in future nanophotonic applications.

Optics Express, 2009

In this work, we investigate the propagation of designer surface plasmons in planar perfect elect... more In this work, we investigate the propagation of designer surface plasmons in planar perfect electric conductor structures that are subject to a parabolic graded-index distribution. A three-dimensional, fully vectorial finitedifference time-domain method was used to engineer a structure with a parabolic effective group index by modulating the dielectric constant of the structure's square holes. Using this structure in our simulations, the lateral confinement of propagating designer surface plasmons is demonstrated. Focusing, collimation and waveguiding of designer plasmons in the lateral direction is realized by changing the width of the source beam. Our findings contribute to applications of designer surface plasmons that require energy concentration, diffusion, guiding, and beam aperture modification within planar perfect electric conductors.

Journal of Applied Physics, 2008

Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combi... more Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combined with a two-step reactive ion etching technique. The optical properties of these arrays were investigated both experimentally and theoretically. Specifically, the effects of disk diameter on localized surface plasmon resonance (LSPR) were characterized and compared with results from discrete dipole approximation (DDA) calculations. The effects of glass substrate, Cr interfacial layer, and Au thickness on LSPR were investigated computationally. Furthermore, thermal treatment was found to be essential in improving the nanodisk arrays' LSPR properties. Using atomic force microscopy and DDA calculations, it was established that the improvements in LSPR properties were due to thermally induced morphologic changes. Finally, microfluidic channels were integrated with the annealed disk arrays to study the sensitivity of LSPR to the change in surroundings' refractive index. The dependence of LSPR on surroundings' refractive index was measured and compared with calculated results.

Acs Nano, 2009

A microcantilever, coated with a monolayer of redox-controllable, bistable [3]rotaxane molecules ... more A microcantilever, coated with a monolayer of redox-controllable, bistable [3]rotaxane molecules (artificial molecular muscles), undergoes reversible deflections when subjected to alternating oxidizing and reducing electrochemical potentials. The microcantilever devices were prepared by precoating one surface with a gold film and allowing the palindromic [3]rotaxane molecules to adsorb selectively onto one side of the microcantilevers, utilizing thiol-gold chemistry. An electrochemical cell was employed in the experiments, and deflections were monitored both as a function of (i) the scan rate (<20 mV s ؊1 ) and (ii) the time for potential step experiments at oxidizing (>؉0.4 V) and reducing (<؉0.2 V) potentials. The different directions and magnitudes of the deflections for the microcantilevers, which were coated with artificial molecular muscles, were compared with (i) data from nominally bare microcantilevers precoated with gold and (ii) those coated with two types of control compounds, namely, dumbbell molecules to simulate the redox activity of the palindromic bistable [3]rotaxane molecules and inactive 1-dodecanethiol molecules. The comparisons demonstrate that the artificial molecular muscles are responsible for the deflections, which can be repeated over many cycles. The microcantilevers deflect in one direction following oxidation and in the opposite direction upon reduction. The ϳ550 nm deflections were calculated to be commensurate with forces per molecule of ϳ650 pN.

Journal of Physical Chemistry C, 2009

We report on chemical etching of ordered Au nanostructure arrays to continuously tune their local... more We report on chemical etching of ordered Au nanostructure arrays to continuously tune their localized surface plasmon resonances (LSPR). Real-time extinction spectra were recorded from both Au nanodisks and nanospheres immobilized on glass substrates when immersed in Au etchant. The time-dependent LSPR frequencies, intensities, and bandwidths were studied theoretically with discrete dipole approximations and the Mie solution, and they were correlated with the evolution of the etched Au nanostructures' morphology (as examined by atomic force microscopy). Since this chemical etching method can conveniently and accurately tune LSPR, it offers precise control of plasmonic properties and can be useful in applications such as surfaceenhanced Raman spectroscopy and molecular resonance spectroscopy.

Journal of Applied Physics, 2010

We designed and simulated a beam aperture modifier and a beam deflector using two-dimensional par... more We designed and simulated a beam aperture modifier and a beam deflector using two-dimensional parabolic gradient-index (GRIN) photonic crystals (PCs). The GRIN PCs are composed of dielectric columns with graded radii along the direction transverse to propagation. Both finite-difference time-domain methods and gradient optics analytical solutions were used to characterize the change in beam width and propagation direction. Multifunctional GRIN

Microfluidics and Nanofluidics, 2010

In this study, we report a rapid microfluidic mixing device based on chaotic advection induced by... more In this study, we report a rapid microfluidic mixing device based on chaotic advection induced by microbubble–fluid interactions. The device includes inlets for to-be-mixed fluids and nitrogen gas. A side-by-side laminar flow segmented by monodisperse microbubbles is generated when the fluids and the nitrogen are co-injected through a flow focusing micro-orifice. The flow subsequently enters a series of hexagonal expansion chambers, in which the hydrodynamic interaction among the microbubbles results in the stretch and fold of segmented fluid volumes and rapid mixing and homogenization. We characterize the performance of the microfluidic mixer and demonstrate rapid mixing within 20 ms. We further show that bubbles can be conveniently removed from the mixed fluids using a microfluidic comb structure on completion of the mixing.

Journal of Nanotechnology in Engineering and Medicine, 2010

We have advanced nanosphere lithography for fabricating ordered Au nanodisk and nanohole arrays o... more We have advanced nanosphere lithography for fabricating ordered Au nanodisk and nanohole arrays on glass substrates. Further, we demonstrate the applications of the nanodisks and nanoholes in understanding exciton-plasmon couplings and patterning micro-/nano-particles, respectively.

Abstract Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithogra... more Abstract Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combined with a two-step reactive ion etching technique. The optical properties of these arrays were investigated both experimentally and theoretically. Specifically, the effects of disk diameter on localized surface plasmon resonance (LSPR) were characterized and compared with results from discrete dipole approximation (DDA) calculations. The effects of glass substrate, Cr interfacial layer, and Au thickness on LSPR were investigated ...

Optics Express, 2009

In this work, we investigate the propagation of designer surface plasmons in planar perfect elect... more In this work, we investigate the propagation of designer surface plasmons in planar perfect electric conductor structures that are subject to a parabolic graded-index distribution. A three-dimensional, fully vectorial finitedifference time-domain method was used to engineer a structure with a parabolic effective group index by modulating the dielectric constant of the structure's square holes. Using this structure in our simulations, the lateral confinement of propagating designer surface plasmons is demonstrated. Focusing, collimation and waveguiding of designer plasmons in the lateral direction is realized by changing the width of the source beam. Our findings contribute to applications of designer surface plasmons that require energy concentration, diffusion, guiding, and beam aperture modification within planar perfect electric conductors.

Journal of Physical Chemistry C, 2009

In this work, we show using both experiments and classical electrodynamic simulations that plasmo... more In this work, we show using both experiments and classical electrodynamic simulations that plasmon splitting in resonant molecule-coated nanoparticles increases linearly as the square root of absorbance of the molecular layer. This linear relationship shows the same universal behavior established in analogous systems such as cavity-polariton and surface plasmon polariton systems. To explain this behavior, a simple physical mechanism based on linear dispersion and absorption is proposed. The insights obtained in this study can ...