Mohamadreza Najiminaini

Nano-hole array structure with improved surface plasmon energy matching characteristics

We present a nano-hole array structure in an opaque gold film that contains a cavity beneath each nano-hole. The cavity contributes to surface plasmon energy matching between the top and bottom surfaces of the gold and within the nano-hole structures. Based on bulk surface plasmon resonance (SPR) sensing experiments, the SP-matched structure had 2.8-fold higher differential transmission, 2-fold higher sensitivity, and a 7-fold higher ratio of extraordinary optical transmission at resonance to the nearby minimum compared to a conventional NHA. The results suggest that the structure with cavities has potential to improve performance of bulk SPR sensing applications.

Effect of surface plasmon energy matching on the sensing capability of metallic nano-hole arrays

We report on a nano-hole array structure with a single cavity beneath the perforated gold film. Structures were fabricated with a variety of cavity depths. The optical resonance of each structure as well as the surface plasmon (SP) energy matching between the top and bottom of the gold film were investigated. We also experimentally evaluated the sensitivity of the structures as surface plasmon resonance (SPR) sensors. We observed a 1.6-fold enhancement in bulk SPR sensitivity and a 3-fold improvement in figure of merit for a structure with a 350-nm cavity depth compared to a structure with a 5-nm cavity depth.We report on a nano-hole array structure with a single cavity beneath the perforated gold film. Structures were fabricated with a variety of cavity depths. The optical resonance of each structure as well as the surface plasmon (SP) energy matching between the top and bottom of the gold film were investigated. We also experimentally evaluated the sensitivity of the structures as surface plasmon resonance (SPR) sensors. We observed a 1.6-fold enhancement in bulk SPR sensitivity and a 3-fold improvement in figure of merit for a structure with a 350-nm cavity depth compared to a structure with a 5-nm cavity depth.

Optical resonance transmission properties of nano-hole arrays in a gold film: effect of adhesion layer

In this paper, we present a systematic study on the influence of composition of the adhesion layer between gold and a Pyrex substrate on the optical resonance transmission properties of nano-hole arrays in an optically thick gold film. Large nano-hole arrays with different hole periodicities in a square lattice arrangement were fabricated using Electron Beam Lithography using different adhesion layers (chromium, titanium, or etched adhesion layer). The fabricated nano-hole arrays were optically characterized using transmission spectroscopy. The optical performance of each nano-hole array was numerically simulated using a Finite Difference Time Domain (FDTD) method. The experiments and simulations revealed that the optical resonance transmission properties (i.e. the resonance wavelength, the spectral transmission modulation ratio, and the resonance bandwidth) of the nano-hole arrays depended highly on the composition and the thickness of the adhesion layer. The optical resonance bandwidths were larger for the nano-hole arrays with chromium or titanium adhesion layers. Also, a red-shift of the optical resonance peak was observed for nano-hole arrays with a metal adhesion layer compared to the corresponding nano-hole arrays with an etched adhesion layer, but the red-shift was greatest for the nano-hole array with the titanium adhesion layer. For adhesion layers of greater thickness, the optical resonance peaks were reduced in magnitude. Finally, nano-hole arrays with an etched adhesion layer had a significant blue-shift in the optical resonance peak and a narrower optical resonance bandwidth compared to nano-hole arrays with a titanium or a chromium adhesion layer. Consequently, a narrow optical resonance bandwidth characteristic of a nano-hole array with an etched adhesion layer can potentially enhance the spectral selectivity and offer improved optical performance.

Experimental and numerical analysis on the optical resonance transmission properties of nano-hole arrays

In this paper, we present experimental and numerical analysis on Extraordinary Optical Transmission (EOT) or optical resonance transmission through various nano-hole arrays constructed from an optically thick metal film within the visible and near infra-red spectrum. Nano-hole arrays with different geometrical parameters (hole size, hole shape, and hole periodicity) having their EOT properties in the visible and near-infrared regime were simulated based on Finite Difference Time Domain (FDTD). Large nano-hole arrays with geometric properties similar to the simulated arrays were fabricated using Electron Beam Lithography (EBL). The optical resonance transmission properties (resonance position, transmission efficiency, and spectral bandwidth of resonance peak) of the fabricated nano-hole arrays were characterized. Finally, the experimental and numerical results were analyzed to determine the dependencies and discrepancies between optical resonance transmission properties for various nano-hole arrays.

Nanohole-array-based device for 2D snapshot multispectral imaging

We present a two-dimensional (2D) snapshot multispectral imager that utilizes the optical transmission characteristics of nanohole arrays (NHAs) in a gold film to resolve a mixture of input colors into multiple spectral bands. The multispectral device consists of blocks of NHAs, wherein each NHA has a unique periodicity that results in transmission resonances and minima in the visible and near-infrared regions. The multispectral device was illuminated over a wide spectral range, and the transmission was spectrally unmixed using a least-squares estimation algorithm. A NHA-based multispectral imaging system was built and tested in both reflection and transmission modes. The NHA-based multispectral imager was capable of extracting 2D multispectral images representative of four independent bands within the spectral range of 662 nm to 832 nm for a variety of targets. The multispectral device can potentially be integrated into a variety of imaging sensor systems.

Analysis of Internet topologies: A historical view

Discovering properties of the Internet topology is important for evaluating performance of various network protocols and applications. The discovery of power-laws and the application of spectral analysis to the Internet topology data indicate a complex behavior of the underlying network infrastructure that carries a variety of the Internet applications. In this paper, we present analysis of datasets collected from the Route Views project. The analysis of collected data shows certain historical trends in the development of the Internet topology. While values of various power-laws exponents have not substantially changed over the recent years, spectral analysis of the normalized Laplacian matrix of the associated graphs reveals notable changes in the clustering of Autonomous System (AS) nodes and their connectivity.

Transillumination hyperspectral imaging for histopathological examination of excised tissue

Angular domain spectroscopic imaging (ADSI) is a novel technique for the detection and characterization of optical contrast in turbid media based on spectral characteristics. The imaging system employs a silicon micromachined angular filter array to reject scattered light traversing a specimen and an imaging spectrometer to capture and discriminate the largely remaining quasiballistic light based on spatial position and wavelength. The imaging modality results in hyperspectral shadowgrams containing two-dimensional (2D) spatial maps of spectral information. An ADSI system was constructed and its performance was evaluated in the near-infrared region on tissue-mimicking phantoms. Image-based spectral correlation analysis using transmission spectra and first order derivatives revealed that embedded optical targets could be resolved. The hyperspectral images obtained with ADSI were observed to depend on target concentration, target depth, and scattering level of the background medium. A similar analysis on a muscle and tumor sample dissected from a mouse resulted in spatially dependent optical transmission spectra that were distinct, which suggested that ADSI may find utility in classifying tissues in biomedical applications.

Effect of surface plasmon cross-talk on optical properties of closely packed nano-hole arrays

The integration and miniaturization of nanostructure-based optical devices based on interaction with surface plasmons requires the fabrication of patterns of multiple nanostructures with tight spacing. The effect of surface plasmon energy interchange (cross-talk) across large grids of nanostructures and its effect on the optical characteristics of individual nanostructures have not been investigated. In this paper, we experimentally fabricated a large grid of individual nano-hole arrays of various hole diameter, hole spacing, and inter-array spacing. The spectral optical transmission of each nano-hole array was measured and the effect of inter-array spacing on the transmission spectra and resonance wavelength was determined.

Angular domain transillumination imaging optimization with an ultrafast gated camera

By employing high-aspect-ratio parallel microchannels as an angular filter, quasiballistic photons sensitive to internal structures in a turbid medium can be captured. Scattered photons exiting the turbid medium typically exhibit trajectories with random angles compared to the initial trajectory and are mostly rejected by the filter. However, angular filter arrays cannot differentiate between quasiballistic photons (early arriving) and photons that happen to attain a scattered trajectory that is within the acceptance angle (late arriving). Therefore, we have two objectives: (1) to experimentally characterize the angular distribution and proportion of minimally deviated quasiballistic photons and multiply scattered photons in a turbid medium and (2) to combine time and angular gating principles so that early and late arriving photons can be distinguished. From the angular distribution data, the angular filter with angular acceptance about 0.4 deg yields the highest image contrast for transillumination images. The use of angular domain imaging(ADI) with time-gating enables visualization of submillimeter absorbing objects with approximately seven times higher image contrast compared to ADI in a turbid medium with a scattering level of six times the reduced mean free path.

Bolus tracking with nanofilter-based multispectral videography for capturing microvasculature hemodynamics

Multispectral imaging is a highly desirable modality for material-based analysis in diverse areas such as food production and processing, satellite-based reconnaissance, and biomedical imaging. Here, we present nanofilter-based multispectral videography (nMSV) in the 700 to 950 nm range made possible by the tunable extraordinary-optical-transmission properties of 3D metallic nanostructures. Measurements made with nMSV during a bolus injection of an intravascular tracer in the ear of a piglet resulted in spectral videos of the microvasculature. Analysis of the multispectral videos generated contrast measurements representative of arterial pulsation, the distribution of microvascular transit times, as well as a separation of the venous and arterial signals arising from within the tissue. Therefore, nMSV is capable of acquiring serial multispectral images relevant to tissue hemodynamics, which may have application to the detection and identification of skin cancer.