Maysamreza Chamanzar

Effect of the Dielectric Constant of the Surrounding Medium and the Substrate on the Surface Plasmon Resonance Spectrum and Sensitivity Factors of Highly Symmetric Systems: Silver Nanocubes

Silver nanocubes (AgNCs), 60 nm, have four extinction surface plasmon resonance (SPR) peaks. The finite difference time domain (FDTD) simulation method is used to assign the absorption and scattering peaks and also to calculate the plasmon field intensity for AgNCs. Because AgNCs have a highly symmetric cubic shape, there is a uniform distribution of the plasmon field around them, and they are thus sensitive to asymmetric dielectric perturbations. When the dielectric medium around a nanoparticle is changed anisotropically, either by placing the particle on a substrate or by coating it asymmetrically with a solvent, the plasmon field is distorted, and the plasmonic absorption and scattering spectra could shift differently. For the 60 nm AgNC, we found that the scattering resonance peak shifted more than the absorption peak. This changes the extinction bandwidth of these overlapping absorption and scattering bands, and consequently the figure of merit of the nanoparticle, as a localized SPR sensor, no longer has a constant value.

High resolution on-chip spectroscopy based on miniaturized microdonut resonators.

We experimentally demonstrate a high resolution integrated spectrometer on silicon on insulator (SOI) substrate using a large-scale array of microdonut resonators. Through top-view imaging and processing, the measured spectral response of the spectrometer shows a linewidth of ~0.6 nm with an operating bandwidth of ~50 nm. This high resolution and bandwidth is achieved in a compact size using miniaturized microdonut resonators (radius ~2 μm) with a high quality factor, single-mode operation, and a large free spectral range. The microspectrometer is realized using silicon process compatible fabrication and has a great potential as a high-resolution, large dynamic range, light-weight, compact, high-speed, and versatile microspectrometer.

Hybrid integrated plasmonic-photonic waveguides for on-chip localized surface plasmon resonance (LSPR) sensing and spectroscopy

We experimentally demonstrate efficient extinction spectroscopy of single plasmonic gold nanorods with exquisite fidelity (SNR > 20dB) and high efficiency light coupling (e. g., 9.7%) to individual plasmonic nanoparticles in an integrated platform. We demonstrate chip-scale integration of lithographically defined plasmonic nanoparticles on silicon nitride (Si3N4) ridge waveguides for on-chip localized surface plasmon resonance (LSPR) sensing. The integration of this hybrid plasmonic-photonic platform with microfluidic sample delivery system is also discussed for on-chip LSPR sensing of D-glucose with a large sensitivity of ∼ 250 nm/RIU. The proposed architecture provides an efficient means of interrogating individual plasmonic nanoparticles with large SNR in an integrated alignment-insensitive platform, suitable for high-density on-chip sensing and spectroscopy applications.

On-chip hybrid photonic-plasmonic light concentrator for nanofocusing in an integrated silicon photonics platform.

The enhancement and confinement of electromagnetic radiation to nanometer scale have improved the performances and decreased the dimensions of optical sources and detectors for several applications including spectroscopy, medical applications, and quantum information. Realization of on-chip nanofocusing devices compatible with silicon photonics platform adds a key functionality and provides opportunities for sensing, trapping, on-chip signal processing, and communications. Here, we discuss the design, fabrication, and experimental demonstration of light nanofocusing in a hybrid plasmonic-photonic nanotaper structure. We discuss the physical mechanisms behind the operation of this device, the coupling mechanisms, and how to engineer the energy transfer from a propagating guided mode to a trapped plasmonic mode at the apex of the plasmonic nanotaper with minimal radiation loss. Optical near-field measurements and Fourier modal analysis carried out using a near-field scanning optical microscope (NSOM) show a tight nanofocusing of light in this structure to an extremely small spot of 0.00563(λ/(2n(rmax)))(3) confined in 3D and an exquisite power input conversion of 92%. Our experiments also verify the mode selectivity of the device (low transmission of a TM-like input mode and high transmission of a TE-like input mode). A large field concentration factor (FCF) of about 4.9 is estimated from our NSOM measurement with a radius of curvature of about 20 nm at the apex of the nanotaper. The agreement between our theory and experimental results reveals helpful insights about the operation mechanism of the device, the interplay of the modes, and the gradual power transfer to the nanotaper apex.

Hybrid nanoplasmonic-photonic resonators for efficient coupling of light to single plasmonic nanoresonators.

We show that efficient coupling of lightwave is possible to an individual plasmonic nanoresonator in a hybrid plasmonic-photonic resonator structure. The proposed hybrid structure consists of a photonic microresonator strongly coupled to a plasmonic nanoresonator. The theory and simulation results show that more than 73% of the input power in the waveguide can be coupled to the localized resonance mode of the plasmonic nanoresonator.

Energy-Looping Nanoparticles: Harnessing Excited-State Absorption for Deep-Tissue Imaging.

Near infrared (NIR) microscopy enables noninvasive imaging in tissue, particularly in the NIR-II spectral range (1000-1400 nm) where attenuation due to tissue scattering and absorption is minimized. Lanthanide-doped upconverting nanocrystals are promising deep-tissue imaging probes due to their photostable emission in the visible and NIR, but these materials are not efficiently excited at NIR-II wavelengths due to the dearth of lanthanide ground-state absorption transitions in this window. Here, we develop a class of lanthanide-doped imaging probes that harness an energy-looping mechanism that facilitates excitation at NIR-II wavelengths, such as 1064 nm, that are resonant with excited-state absorption transitions but not ground-state absorption. Using computational methods and combinatorial screening, we have identified Tm(3+)-doped NaYF4 nanoparticles as efficient looping systems that emit at 800 nm under continuous-wave excitation at 1064 nm. Using this benign excitation with standard confocal microscopy, energy-looping nanoparticles (ELNPs) are imaged in cultured mammalian cells and through brain tissue without autofluorescence. The 1 mm imaging depths and 2 μm feature sizes are comparable to those demonstrated by state-of-the-art multiphoton techniques, illustrating that ELNPs are a promising class of NIR probes for high-fidelity visualization in cells and tissue.

Strong angular dispersion using higher bands of planar silicon photonic crystals

We present experimental evidence for strong angular dispersion in a planar photonic crystal (PC) structure by properly engineering the modes in the second PC band. We show that by using the second photonic band of a square lattice PC, angular dispersion of 4 degrees /nm can be achieved. We also show that major challenges in designing practical PC devices using second band modes can be addressed by engineering the lattice and adding input/output buffer stages designed to eliminate unwanted effects.

Legendre polynomial expansion for analysis of linear one-dimensional inhomogeneous optical structures and photonic crystals

A Legendre polynomial expansion of electromagnetic fields for analysis of layers with an inhomogeneous refractive index profile is reported. The solution of Maxwells equations subject to boundary conditions is sought in a complete space spanned by Legendre polynomials. Also, the permittivity profile is interpolated by polynomials. Different cases including computation of reflection–transmission coefficients of inhomogeneous layers, band-structure extraction of one-dimensional photonic crystals whose unit-cell refractive index profiles are inhomogeneous, and inhomogeneous planar waveguide analysis are investigated. The presented approach can be used to obtain the transfer matrix of an arbitrary inhomogeneous monolayer holistically, and approximation of the refractive index or permittivity profile by dividing into homogeneous sublayers is not needed. Comparisons with other well-known methods such as the transfer-matrix method, WKB, and effective index method are made. The presented approach, based on a nonharmonic expansion, is efficient, shows fast convergence, is versatile, and can be easily and systematically employed to analyze different inhomogeneous structures.

Compact on-chip plasmonic light concentration based on a hybrid photonic-plasmonic structure

We present a novel approach for achieving tightly concentrated optical field by a hybrid photonic-plasmonic device in an integrated platform, which is a triangle-shaped metal taper mounted on top of a dielectric waveguide. This device, which we call a plasmomic light concentrator (PLC), can achieve vertical coupling of light energy from the dielectric waveguide to the plasmonic region and light focusing into the apex of the metal taper(at the scale ∼ 10nm) at the same time. For a demonstration of the PLCs presented in this paper, we numerically investigate the performance of a gold taper on a Si₃N₄ waveguide at working wavelengths around 800 nm. We show that three major effects (mode beat, nanofocusing, and weak resonance) interplay to generate this light concentration phenomenon and govern the performance of the device. Combining these effects, the PLC can be designed to be super compact while maintaining high efficiency over a wide band. In particular, we demonstrate that under optimized size parameters and wavelength a field concentration factor (FCF), which is the ratio of the norm of the electric field at the apex over the average norm of the electric field in the inputting waveguide, of about 13 can be achieved with the length of the device less than 1 μm for a moderate tip radius 20 nm. Moreover, we show that a FCF of 5 - 10 is achievable over a wavelength range of 700 - 1,100 nm with the length of the device further reduced (to about 400 nm).

Compact on-chip interferometers with high spectral sensitivity

We introduce on-chip interferometers in which the spatial output interference pattern is observed along a detection plane. We show that by using photonic crystals with strong dispersive properties in these devices, highly sensitive interferometers can be realized. We discuss potentials of these interferometers in spectroscopy and sensing applications using their strong wavelength sensitivity and their ability to spatially map the spectral information of an input signal.