Andrew Sarangan

Spectral properties of angled-grating high-power semiconductor lasers

We study the spectral properties of angled-grating high-power semiconductor lasers, also known as /spl alpha/ distributed feedback (DFB) lasers. We have derived a closedform expression to describe the cavity resonance. The results of this model are shown to compare favorably with experimental data. Intrinsic device parameters such as coupling coefficient and grating period are shown to be correlated to spectral and nearfield characteristics. The formulations and insights developed in this paper allow one to calculate these critical design parameters for optimum performance.

Monoclinic optical constants, birefringence, and dichroism of slanted titanium nanocolumns determined by generalized ellipsometry

rection of the incoming vapor. 1 The physical properties of such thin films differ drastically from their bulk material. 2 Intriguing mechanical, magnetic, and optical properties, for example, can be obtained and may be explored in future applications. Form-induced polarization current confinement, crosscoupling between individual nanoelements, and quantization effects will lead to entirely different optical properties unknown from their bulk counterpart. Hodgkinson and Wu 1 predicted, and partially determined, strong nanostructure form-induced optical orthorhombic birefringence in slanted nanostructure thin films, also termed columnar thin film CTF. Deposition conditions such as the angle of the incoming vapor have strong influence on shape and arrangement of the nanostructures. 3 Controlled CTF growth together with accurate measurement of their anisotropic optical properties will allow for tailoring materials at the nanometer scale to achieve desired optical applications such as omnidirectional mirrors and thin film polarization filters. In this letter we report accurate and complete determination of the intrinsic optical properties of slanted metal nanocolumn thin films. We extend the prediction of Hodgkinson and Wu 1 by dem

Generalized ellipsometry for monoclinic absorbing materials: determination of optical constants of Cr columnar thin films.

Generalized spectroscopic ellipsometry is used to determine the form-induced birefringence and monoclinic optical constants of chromium columnar thin films. The slanted nanocolumns were deposited by glancing angle deposition under 85 degrees incidence and are tilted from the surface normal. Dichroism measured for wavelengths from 400 to 1000 nm renders the Cr nanocolumns monoclinic absorbing crystals with c axis along the nanocolumns axis, b axis parallel to the film interface, and 74.8 degrees monoclinic angle between a and c axes. The columnar thin film reveals anomalous optical dispersion, extreme birefringence, and strong dichroism and differs entirely from bulk chromium.

Chiral light intrinsically couples to extrinsic/pseudo-chiral metasurfaces made of tilted gold nanowires

Extrinsic or pseudo-chiral (meta)surfaces have an achiral structure, yet they can give rise to circular dichroism when the experiment itself becomes chiral. Although these surfaces are known to yield differences in reflected and transmitted circularly polarized light, the exact mechanism of the interaction has never been directly demonstrated. Here we present a comprehensive linear and nonlinear optical investigation of a metasurface composed of tilted gold nanowires. In the linear regime, we directly demonstrate the selective absorption of circularly polarised light depending on the orientation of the metasurface. In the nonlinear regime, we demonstrate for the first time how second harmonic generation circular dichroism in such extrinsic/pseudo-chiral materials can be understood in terms of effective nonlinear susceptibility tensor elements that switch sign depending on the orientation of the metasurface. By providing fundamental understanding of the chiroptical interactions in achiral metasurfaces, our work opens up new perspectives for the optimisation of their properties.

Temperature-dependent Sellmeier equation for the refractive index of Al(x)Ga(1-x)As.

We derived a temperature-dependent Sellmeier equation for Al(x)Ga(1-x)As material by measuring the refractive index of GaAs and AlAs with temperature dependence in a resonant cavity enhanced structure. The equation is applicable in the range of 1460-1580 nm and 26-86 degrees C and can be extrapolated to the other wavelengths and temperature ranges as well.

Co-sputtered SiC + Ag nanomixtures as visible wavelength negative index metamaterials

The fabrication and characterization of a novel metamaterial that shows negative index in the visible (blue) is reported. The real part of the negative index of this metamaterial at 405 nm, comprising co-sputtered SiC + Ag nanoparticle mixture on a glass substrate, is deduced from results of double Michelson interferometry setup which shows a negative phase delay. It is numerically verified that this metamaterial can yield near-field super-resolution imaging for both TE and TM polarizations.

Optical characterization of silver nanorod thin films grown using oblique angle deposition

Nanorods are metamaterial structures that have been shown to have wide application, ranging from biomedical uses to photovoltaic materials. These materials have unique optical characteristics. In this paper, two silver (Ag) nanorod thin-film samples are created using Glancing Angle Deposition (GLAD) at both near-room temperature (∼300 K) and cryogenic temperature (∼100 K). Generalized ellipsometry is used to measure the optical constants of the samples. The strong difference between the optical constants of the constituent materials and those of these thin films shows that the characteristics of the samples are due to how their metamaterial structures are defined.The principle optical axes of the films align well with the morphological characteristics of the nanostructures. The axis with the greatest index of refraction remains aligned to the principle axes but shifts orientation with respect to morphological characteristics between samples. Experimental results show differences in both magnitude and characteristics of the nanorod indexes. Reflectance and transmittance measurements are performed to extract absorptance data. The room temperature deposited sample shows a higher overall absorptance, while the cryogenic sample shows a clear orientation-dependent absorptance. Polarization data is analyzed to show that the 100 K thin film exhibits polarization-dependent absorptance, while the 300 K samples absorptance has a strong orientation dependence.

Polarization multiplexed fluorescence enhancer using a pixelated one-dimensional photonic band gap structure

Fluorescence enhancement using photonic crystals can produce a significant improvement in the signal-to-noise ratio for single molecule and low molecule-concentration fluorescence imaging in biological and biochemical studies. In this Letter, a pixelated one-dimensional photonic band gap structure was designed to enhance both transverse electric and transverse magnetic polarizations through a spatially multiplexed photonic crystal resonance. The average enhancement of 15.6 and 17.9 fold were experimentally verified for the transverse and longitudinal fields on the same substrate. This device may be used as an optical platform for molecular orientation determination.

TiO 2 memristor devices

This paper presents the fabrication technique and results obtained for a titanium oxide based memristor design. A memristor wafer was developed that contains isolated devices as well as small memristor crossbar arrays (36 devices). The current-voltage relationship of the devices shows a memristive switching effect that is more apparent when the devices are negatively biased.

Simulation of resonant cavity enhanced (RCE) photodetectors using the finite difference time domain (FDTD) method

The resonant cavity enhanced (RCE) photodetectors is analyzed using the finite difference time domain (FDTD) method. Unlike the analytical models, FDTD includes all of the essential considerations such as the cavity build-up time, standing wave effect and the refractive index profiles across every layer. The fully numerical implementation allows it to be used as a verification of the analytical models. The simulation is demonstrated in terms of time and space enabling one to visualize how the field inside the cavity builds up. The results are compared with the analytical models to point out the subtle differences and assumptions made in the analytical models.