Nanophotonics

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“Nanophotonics or Nano-optics is the study of the behavior of light on the nanometer scale. It is considered as a branch of optical engineering which deals with optics, or the interaction of light with particles or substances, at deeply subwavelength length scales. Technologies in the realm of nano-optics include near-field scanning optical microscopy (NSOM), photoassisted scanning tunnelling microscopy, and surface plasmon optics. Traditional microscopy makes use of diffractive elements to focus light tightly in order to increase resolution. But because of the diffraction limit (also known as the Rayleigh Criterion), propagating light may be focused to a spot with a minimum diameter of roughly half the wavelength of the light. Thus, even with diffraction-limited confocal microscopy, the maximum resolution obtainable is on the order of a couple of hundred nanometers. The scientific and industrial communities are becoming more interested in the characterization of materials and phenomena on the scale of a few nanometers, so alternative techniques must be utilized. Scanning Probe Microscopy (SPM) makes use of a “probe”, (usually either a tiny aperture or super-sharp tip), which either locally excites a sample or transmits local information from a sample to be collected and analyzed. The ability to fabricate devices in nanoscale that has been developed recently provided the catalyst for this area of study. The study has the potential to revolutionize the telecommunications industry by providing low power, high speed, interference-free devices such as electrooptic and all-optical switches on a chip.” ——Wikipedia

In the Penn State BioNEMS laboratory, we aim to (1) investigate interactions of light and metal at nanoscale to address fundamental issues of nanophotonics, (2) develop cost-effective and high-throughput nanofabrication techniques and nanoengineering methods to produce plasmonic nanostructures for different applications, and (3) design, model and fabricate active plasmonic devices to benefit information technology, and medical diagnosis and therapy.

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Go to Top | References

YJ Liu et al., Journal of Physical Chemistry C, 2011, DOI:10.1021/jp111256u.

We conduct a real-time study of all-optical modulation of localized surface plasmon resonance (LSPR) coupling in a hybrid system that integrates a photoswitchable optical grating with a gold nanodisk array. This hybrid system enables us to investigate two important interactions: (1) LSPR enhanced grating diffraction and (2) diffraction-mediated LSPR in the Au nanodisk array. The physical mechanism underlying these interactions was analyzed and experimentally confirmed. With its advantages in cost-effective fabrication, easy integration, and all-optical control, the hybrid system described in this work could be valuable in many nanophotonic applications..

YJ Liu and Q Hao et al., Applied Physics Letters, Vol. 97, pp. 091101, 2010.

A frequency-addressed plasmonic switch was demonstrated by embedding a uniform gold nanodisk array into dual-frequency liquid crystals DFLCs. The optical properties of the hybrid system were characterized by extinction spectra of localized surface plasmon resonances LSPRs. The LSPR peak was tuned using a frequency-dependent electric field. A 4 nm blueshift was observed for frequencies below 15 kHz, and a 23 nm redshift was observed for frequencies above 15 kHz. The switching time for the system was 40 ms. This DFLC-based active plasmonic system demonstrates an excellent, reversible, frequency-dependent switching behavior and could be used in future integrated nanophotonic circuits.

VKS Hsiao et al., Advanced Materials, Vol. 20, pp. 3528-3522, 2008.

An optically controlled plasmonic switch that uses Au nanodisk arrays embedded in azobenzene-doped, photoresponsive liquid crystals (LCs) is reported. The switch utilizes the photo-induced phase transition of azobenzene-doped LCs to alter the localized surface plasmon resonance (LSPR) of the Au nanodisks. The experimental modulation of the LSPR matches well with theoretical calculations. The influence of the angle of the incident light, power of the pump light, and wavelength of the probe light are also investigated.

Go to Top | References

M Lu et al., Journal Of Applied Physics vol. 108, pp. 103505 2010.

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 PCs combining both beam aperture modification and beam deflection were also designed and simulated. These GRIN PC based designs can be used as optical connectors and bidirectional waveguide couplers in applications such as miniaturized photonic integrated circuits.

Figure: Schematic of (a) a beam aperture modifier and (b) a beam deflector based on GRIN PCs. The radii of the dielectric columns change transversely to the light propagation direction, forming parabolic GRIN PCs.

J Shi et al., Journal Of Applied Physics vol. 108, pp. 043514 2010.

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.

Figure: (a) Single PC structure PC1 or PC2 that had homogenous periodicity and homogenous filling fractions. (b) A PC composite consisting of two PCs of the same periodicity but different filling fractions. A point source was placed on the left side of each PC structure. Arrows indicate the direction of energy transportation.

BK Juluri et al., Optics Express vol. 17, pp. 2997 2009.

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.

Figure: (a) Principle of a gradient index based lens. A parabolic gradient (green line) of the group index (N) along the transverse direction of the propagation (Y-axis) enables the focusing or collimation of incoming beam (red arrows). A PEC structure with periodic array of square holes with varying (b) size of square holes and © dielectric constant, along the transverse direction of propagation (Y-axis) enables the focusing or collimation of DSPs (red arrows)

Go to Top | References

YB Zheng*, BK Juluri* et al., Advanced Materials, Vol. 22, pp. 3603-3607, 2010. (*equal contributions) (featured as front cover image)

Dynamic tuning of plasmon–exciton resonant coupling in arrays of nanodisk–J-aggregate complexes is demonstrated. The angle-resolved spectra of an array of bare gold nanodisks exhibit continuous shifting of localized surface plasmon resonance. This characteristic enables the production of real-time, controllable spectral overlap between molecular resonance and plasmonic resonance. The resonant interaction strength as a function of spectral overlap is explored and the coupling strength changes with the incident angle of a probe light, in accord with simulations based on coupled dipole approximation method.

Q Hao et al., Journal of Physical Chemistry C, Vol. 114, PP. 18059–18066, 2010. (featured as front cover image)

Using pyridine as a probe molecule, we performed surface enhanced Raman spectroscopy (SERS) studies on platinum and gold nanodisk arrays at both plasmon resonant and off-plasmon resonant excitation wavelengths. A large Raman cross-section enhancement factor (EF) of ∼10^6 was obtained with plasmon resonant excitation on the Au array, and the EF decreases with off-resonant excitations. However, for Pt nanodisks the experimental EF is much smaller (∼10^2) and not sensitive to excitation wavelength. Electric field intensities calculated in Au and Pt nanoparticles using the discrete dipole approximation (DDA) with a dielectric function including or excluding interband transitions allowed us to explain the SERS EF differences at different excitation wavelengths. The observed SERS insensitivity to excitation wavelength in Pt was explained using Fano interference between the free plasmon electrons and continuum interband transitions. The importance of Fano interference was explored analytically in the electrostatic limit by varying the contribution from the interband transitions.

BK Juluri et al., J. Phys. Chem. C, vol. 113, pp. 18499, 2009.

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 be used as a general principle for designing resonant molecule-coated nanoparticles for realizing tunable nanophotonic devices and molecular sensing.

Figure: Extinction spectra of gold nanorods and absorption spectra of J-aggregates. The inset shows a schematic of a resonant moleculecoated plasmonic nanostructure consisting of gold nanorod and Jaggregate molecules.

YB Zheng et al., J. Phys. Chem. C, Vol. 113, pp. 7021, 2009.

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.

Figure: (a) Real-time extinction spectra recorded from Au nanodisk arrays immobilized on a glass substrate and immersed in a Au etchant solution (Etchant 1). (b) AFM image of a Au nanodisk array before chemical etching. (c and d) Three-dimensional AFM images of the Au nanodisks after 0.5 and 1.5 min chemical etching, respectively.

YB Zheng et al., Journal of the Association for Laboratory Automation, Vol. 13, pp. 215-226, 2008.

We developed cost-effective and high-throughput techniques to fabricate metal nanostructure arrays of various geometries on solid substrates. Surface plasmons of these nanostructure arrays were investigated both experimentally and theoretically. We systematically studied the effects of different parameters on the localized surface plasmon resonance of the nanostructure arrays. We further developed a few approaches to tailor surface plasmons for different applications. As an example of the applications of these nanostructure arrays, we demonstrated all-optical plasmonic switches/modulators based on long-range ordered Au nanodisk arrays and photoresponsive liquid crystals. The advantages of such arrays include low-cost, high-throughput, and tailorable plasmonic properties. These arrays can serve as a platform that will stimulate further progress in both fundamental research and engineering applications of plasmonics.

Figure: (a) Schematic of fabrication of nanohole arrays in a Au thin film on a silicon substrate; (bee) scanning electron microscopy images of the hexagonally arranged nanohole arrays in Au thin films, with O2 reactive ion etching times of 3, 6, 8, and 11 min, respectively.

Go to Top | References

TJ Huang et al., Applied Physics Letters, Vol. 85, pp. 5391-5393, 2004. (featured as front cover image)

An array of microcantilever beams, coated with a self-assembled monolayer of bistable, redox-controllable rotaxane molecules, undergoes controllable and reversible bending when it is exposed to chemical oxidants and reductants. Conversely, beams that are coated with a redox-active but mechanically inert control compound do not display the same bending. A series of control experiments and rational assessments preclude the influence of heat, photothermal effects, and pH variation as potential mechanisms of beam bending. Along with a simple calculation from a force balance diagram, these observations support the hypothesis that the cumulative nanoscale movements within surface-bound “molecular muscles” can be harnessed to perform larger-scale mechanical work.

BK Juluri et al., ACS Nano, Vol. 3, pp. 291-300, 2009.

A microcantilever, coated with a monolayer of redox-controllable, bistable 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 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 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 550 nm deflections were calculated to be commensurate with forces per molecule of 650 pN. The thermal relaxation that characterizes the device’s deflection is consistent with the double bistability associated with the palindromic rotaxane and reflects a metastable contracted state. The use of the cooperative forces generated by these self-assembled, nanometer-scale artificial molecular muscles that are electrically wired to an external power supply constitutes a seminal step toward molecular-machine-based nanoelectromechanical systems (NEMS).

Figure: Schematic of the experimental setup used for in situ electrochemical activation of the palindromic bistable rotaxane molecules. The inset shows the reversible electrochemical oxidation and reduction of R^8 to produce the microcantilever deflection (WE: working electrode, CE: counter electrode, RE: reference electrode).

Go to Top | Optical Modulation Based On Liquid Crystal | Optical Metamaterial | Plasmonics on Molecules and Nanoparticles

1. Yan Jun Liu, Yue Bing Zheng, Justin Liou, I-Kao Chiang, Iam Choon Khoo, and Tony Jun Huang, All-Optical Modulation of Localized Surface Plasmon Coupling in a Hybrid System Composed of Photo-Switchable Gratings and Au Nanodisk Arrays, Journal of Physical Chemistry C, 2011, DOI:10.1021/jp111256u. (featured as front cover image) [PDF]
2. Shuai Gao, Chunfeng Zhang, Yanjun Liu, Huaipeng Su, Lai Wei, Tony Huang, Nicholas Dellas, Shuzhen Shang, Suzanne E. Mohney, Jingkang Wang, and Jian Xu, Lasing From Colloidal InP/ZnS Quantum Dots, Optics Express, Vol. 19, pp. 5528-5535, 2011. [PDF]
3. Mengqian Lu, Bala Krishna Juluri, Sz-Chin Steven Lin, Brian Kiraly, Tieyu Gao, and Tony Jun Huang, Beam Aperture Modification and Beam Deflection Using Gradient-Index Photonic Crystals, Journal of Applied Physics, Vol. 108, pp. 103505, 2010. [PDF]
4. Yanhui Zhao, Sz-Chin Steven Lin, Ahmad Ahsan Nawaz, Brian Kiraly, Qingzhen Hao, Yanjun Liu and Tony Jun Huang, Beam bending via plasmonic lenses, Optics Express, Vol. 18, pp. 23458-23465, 2010. [PDF]
5. Jinjie Shi, Bala Krishna Juluri, Sz-Chin Steven Lin, Mengqian Lu, Tieyu Gao, and Tony Jun Huang, Photonic Crystal Composites-Based Wide-Band Optical Collimator, Journal of Applied Physics, Vol. 108, pp. 043514, 2010. [PDF]
6. Yan Jun Liu*, Qingzhen Hao*, Joseph S. T. Smalley, Justin Liou, Iam Choon Khoo, and Tony Jun Huang, A Frequency-Addressed Plasmonic Switch Based on Dual-Frequency Liquid Crystal, Applied Physics Letters, Vol. 97, pp. 091101, 2010. (*equal contributions) (featured as front cover image) [PDF]
7. Bala Krishna Juluri, Mengqian Lu, Yue Bing Zheng, Lasse Jensen, Tony Jun Huang, Coupling between Molecular and Plasmonic Resonances: Effect of Molecular Absorbance, Journal of Physical Chemistry C, Vol. 113, pp 18499–18503, 2009. [PDF]
8. Yan Jun Liu, Hai Tao Dai, Xiao Wei Sun, and Tony Jun Huang, Electrically Switchable Phase-Type Fractal Zone Plates and Fractal Photon Sieves, Optics Express, Vol. 17, pp. 12418-12423, 2009. [PDF]
9. Yue Bing Zheng, Linlin Jensen, Wei Yan, Thomas R. Walker, Bala Krishna Juluri, Lasse Jensen, Tony Jun Huang, Chemically Tuning the Localized Surface Plasmon Resonances of Gold Nanostructure Arrays, Journal of Physical Chemistry C, Vol. 113, pp. 7019-7024, 2009. [PDF]
10. Bala Krishna Juluri, Sz-Chin Steven Lin, Thomas R. Walker, Lasse Jensen and Tony Jun Huang, Propagation of Designer Surface Plasmons in Structured Conductor Surfaces with Parabolic Gradient Index, Optics Express, Vol. 17, pp. 2997-3006, 2009. [PDF]
11. Vincent K. S. Hsiao, Yue Bing Zheng, Bala Krishna Juluri, Tony Jun Huang, Light-Driven Plasmonic Switches Based on Au Nanodisk Arrays and Photoresponsive Liquid Crystals, Advanced Materials, Vol. 20, pp. 3528-3522, 2008. (featured as front cover image) [PDF]
12. Yue Bing Zheng, Tony Jun Huang, Surface Plasmons of Metal Nanostructure Arrays: from Nanoengineering to Active Plasmonics, Journal of the Association for Laboratory Automation, Vol. 13, pp. 215-226, 2008. [PDF]
13. Bala Krishna Juluri, Yue Bing Zheng, Daniel Ahmed, Lasse Jensen, Tony Jun Huang, Effects of Geometry and Composition on Charge-Induced Plasmonic Shifts in Gold Nanoparticles, Journal of Physical Chemistry C, Vol. 112, pp. 7309-7312, 2008. [PDF]
14. Yuebing Zheng, Bala Krishna Juluri, Xiaole Mao, Thomas R. Walker, Tony Jun Huang, Systematic Investigation of Localized Surface Plasmon Resonance of Long-Range Ordered Au Nanodisk Arrays, Journal of Applied Physics, Vol. 103, pp. 014308, 2008. [PDF]
15. Yuebing Zheng, Tony Jun Huang, Amit Yogesh Desai, Shi Jie Wang, Lee Kheng Tan, Han Gao, Alfred Cheng Hon Huan, Thermal Behavior of Localized Surface Plasmon Resonance of Au/TiO2 Core/Shell Nanoparticle Arrays, Applied Physics Letters, Vol. 90, pp. 183117, 2007. [PDF]