369 research outputs found
Randomization of gold nano-brick arrays: a tool for SERS enhancement
Get PDFSurface enhanced Raman scattering (SERS) was measured on periodic and randomly arranged patterns of Au nano-bricks (rectangular parallelepipeds). Resonant SERS conditions were investigated of a near-IR dye deposited on nanoparticles. Random mixtures of Au nano-bricks with different aspect ratio R showed stronger SERS enhancement as compared to periodic patterns with constant aspect ratio (R varies from 1 to 4). SERS mapping revealed up to ∼ 4 times signal increase at the hot-spots. Experimental observation is verified by numerical modeling and is qualitatively consistent with generic scaling arguments of interaction between plasmonic nanoparticles. The effect of randomization on the polarization selectivity for the transverse and longitudinal modes of nano-bricks is shown
Tunable Raman Selectivity via Randomization of a Rectangular Pattern of Nanodisks
Get PDFWe show that randomization of a nanodisk array on a rectangular lattice can be used to spectrally tune the surface-enhanced Raman scattering (SERS) intensity and to control the field enhancement. The spectral selectivity of the Raman modes exhibits a nonlinear dependence on the extinction. Randomization of the rectangular pattern brings a favorable increase of SERS intensity via a reduced selectivity and extinction changes at the Stokes band. The extinction dependence for two polarizations on the spacing in the rectangular array is demonstrated. For a given polarizations on the extinction strength depends primarily on the spacing between the nanodisks in the orthogonal direction. Such randomization and polarization control of SERS increases the versatility, e.g., all-optical control, of this important sensing tool
SERS scaling rules
No full textWe demonstrate the qualitative analysis of surface-enhanced Raman scattering (SERS) intensity and optical extinction by experimentally and numerically. This analytical methods are well matched not only the simple square lattice array of nanostructures, but also the rectangular lattices. We also demonstrate SERS selectivity of modes controlling the optical extinction of excitation and scattering wavelength. Both square lattice and rectangular lattice have similar tendency, but the rectangular lattice structures have much higher selectivity of SERS modes
Scaling Rules of SERS Intensity
No full textAn intricate relationship is revealed between the intensity of surface-enhanced Raman scattering (SERS) and the optical extinction. The unusual trends of SERS intensity decrease while the extinction increases observed in experiments is fully explained for the first time. The SERS intensity is well known to be strongly correlated with the extinction at excitation and Stokes wavelengths, but the dependence is more involved than the simple product of the two extinction enhancements. The radiative and nonradiative losses, different for different Stokes wavelengths, strongly affect the relationship between SERS and extinction. Dye molecules are shown experimentally and numerically to experience different local field enhancements for the different Raman modes, and a simple qualitative analytical description of this universal phenomenon is provided here
Comment on electro-optic effect and valence-band mixing in zinc-blende semiconductors
No full textA Comment on the Letter by Jacob B. Khurgin, and Paul Voisin, Phys. Rev. Lett. 81, 3777 (1998). The authors of the Letter offer a Reply
EXPERIMENTS IN OPTIMIZATION OF FREE SPACE OPTICAL COMMUNICATION LINKS FOR APPLICATIONS IN A MARITIME ENVIRONMENT
Get PDFThe United States Navy relies heavily on radio frequency communication networks and this reliance generates two major operational limitations: bandwidth, and lack of contingency capability in the event of jamming or detection by adversaries. One possible complementary solution to current radio frequency systems is through the use of free-space optical communication links. Free-space optical communication links are inherently high-bandwidth and highly directional, making them hard to detect or jam. These links have drawbacks as well. A laser beam propagating in a maritime environment can experience significant random intensity fluctuations due to optical turbulence, which can lead to power loss at the receiver and degraded performance. Understanding the effects of the maritime environment on the propagating laser beam is critical to the improvement of laser communication in this environment. For example, the probability density function of the intensity for a given detector is vital for estimating the fade statistics of an optical signal and its effect on the bit-error rate of a communication system. Understanding the evolution and form of the probability density function as it relates to distance, turbulence level, and detector type holds great benefit for optimizing the maritime communication link in a given optical channel.
Our research focuses on how to modify the transmit characteristics of the laser beam in order to minimize the intensity fluctuations and time and depth of fades – which can be on the order of milliseconds and tens of decibels. Specifically, modifying the partial spatial coherence properties of the beam at the transmitter offers significant potential in minimizing the deep fades at the receiver.
Also, field experimentation is critical. To that end, two field tests off the Atlantic Coast and five field experiments at the United States Naval Academy were performed. Additionally, working in a controlled laboratory setting capable of simulating some of the scaled effects of the environment holds great advantages in cost, testing methods, and optimization. We built an in-laboratory hot-air turbulence emulator with a modular and extendable design, allowing us to research comparisons between the field and laboratory experiments. This has greatly enhanced our ability to project theory into practice.
Thesis Adviser: Dr. Frederic M. Davidson
Thesis Readers: Dr. Frederic M. Davidson and Dr. Jacob Khurgi
Scaling rules for surface enhanced Raman scattering
No full textAn intricate relationship between the intensity of surface-enhanced Raman scattering (SERS) and the optical extinction are revealed. The observed unusual trend of SERS intensity decrease with the increase of extinction is explained analytically and numerically
Relative merits of phononics vs. plasmonics: the energy balance approach
No full textThe common feature of various plasmonic schemes is their ability to confine optical fields of surface plasmon polaritons (SPPs) into subwavelength volumes and thus achieve a large enhancement of linear and nonlinear optical properties. This ability, however, is severely limited by the large ohmic loss inherent to even the best of metals. However, in the mid- and far-infrared ranges of the spectrum, there exists a viable alternative to metals – polar dielectrics and semiconductors, in which dielectric permittivity (the real part) turns negative in the Reststrahlen region. This feature engenders the so-called surface phonon polaritons, capable of confining the field in a way akin to their plasmonic analogs, the SPPs. Since the damping rate of polar phonons is substantially less than that of free electrons, it is not unreasonable to expect that phononic devices may outperform their plasmonic counterparts. Yet a more rigorous analysis of the comparative merits of phononics and plasmonics reveals a more nuanced answer, namely, that while phononic schemes do exhibit narrower resonances and can achieve a very high degree of energy concentration, most of the energy is contained in the form of lattice vibrations so that enhancement of the electric field and, hence, the Purcell factor is rather small compared to what can be achieved with metal nanoantennas. Still, the sheer narrowness of phononic resonances is expected to make phononics viable in applications where frequency selectivity is important
Optical isolation by temporal modulation: size, frequency, and power constraints
Get PDFOptical isolators are indispensable components of optical networks.
Magneto-optic isolators have excellent operating characteristics, including
low-to-no power consumption, but are not well suited for on-chip integration.
The technique of temporal modulation of dielectric constant offers an
alternative way to achieve isolation without magnetic field but is not without
its own drawbacks. In this work I examine diverse methods of optical isolation
via temporal modulation and show that independent on whether modulation is
achieved by carrier injection, Pockels and acousto-optic effects, or any other
conceivable method, there is essentially the same set of constraints on
footprint, modulation frequency, and, most important, on power consumption
required to achieve full isolation without excessive insertion loss. This power
is estimated to be on the order of at least a hundred of milliwatts and whether
this requirement is acceptable will depend on ongoing progress of both
magneto-optic and time modulated integrated technologies
Structural and Spectral Control in Solar Nanophotonics
No full textThis dissertation examines the structural control of light absorption in nanophotonic systems, presenting both conventional and innovative strategies to enhance optical performance. The research is structured into four key sections, each addressing critical aspects of photovoltaic systems and plasmonic tuning of optical spectra.
The first section focuses on material utilization in traditional photovoltaic systems, specifically in the context of Maryland's climatic and operational conditions. It investigates challenges related to energy generation and waste management, providing insights into the implications of long-term waste accumulation. This section also highlights the importance of recycling and circular economy policies at the state level. It sets the stage for exploring thin-film and third-generation photovoltaics, emphasizing the need for innovative approaches to improve efficiency and sustainability.
The second section delves into novel photovoltaic materials, particularly lead sulfide colloidal quantum dots (PbS-CQDs), which represent a promising third-generation photovoltaic material. These nanoscale materials offer exceptional tunability in optical and electronic properties, making them ideal for near-infrared (NIR) photovoltaics. The research explores their material characteristics, device design, and operating principles, incorporating machine learning techniques to enhance characterization of CQD solar cells. Additionally, this section presents a comparative study of vertical and lateral CQD field-effect transistors, demonstrating applications in photosensing and advanced material characterization.
In the third section, the focus shifts to photonic band engineering and advanced patterning techniques for CQD films to achieve spectral tuning beyond their intrinsic properties. Through simulation methodologies and modern data-driven techniques—collectively known as inverse design—the research explores novel tandem solar cell architectures. Computational design studies are presented, addressing metamaterial optical parameter extraction, near-perfect absorbance in plasmonically enhanced 2D materials, and spectral tuning using aperiodic plasmonic arrays.
The final section investigates the optical properties of core-shell plasmonic nanoparticles. It introduces the spectral tuning capabilities enabled by manipulating nanoparticle shapes, with an emphasis on aluminum as an abundant plasmonic material. Additionally, it examines gold nanoshells and the impact of ligand and solvent modifications in aerosolized phases on their optical behavior. These findings demonstrate strategies for fine-tuning plasmonic aerosols for applications ranging from atmospheric sensing to energy harvesting
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