arXiv subjects

Srini Krishnamurthy

Publications and source records attributed to Srini Krishnamurthy.

11 recordsLinked to original sources

arXiv2024

Wavelength-Agnostic Metasurface Design for Next-Generation 2D Photodetectors

We explore a versatile technique for inverse designing 2D photonic crystal metasurfaces. These surfaces, known for their ability to manipulate light-matter interactions, can be precisely controlled to achieve specific functionalities. The key lies in efficiently optimizing the geometric patterns and dimensions of the metasurface. Through a composite method which exploits two well-established paradigms - Covariance Matrix Adaptation optimization and Rigorous Coupled Wave Analysis (RCWA), we demonstrate our ability to design and optimize resonances in metaelements to achieve desired optical performance such as near-perfect absorption at chosen wavelengths/optical modes, which otherwise proves to be challenging or even impossible with conventional inverse design implementations. We apply our method to design three-layered structures involving a monolayer absorber, transparent metasubstrate, and a back mirror to get near 100% absorption at one or two chosen wavelengths. For illustration, we choose black phosphorus and silicon metasurface to predict ~100% absorption in a monolayer at 1550 nm. The versatile technique can be applied to tailor reflectance and transmittance for any optical mode and wavelength. This computationally efficient design method paves the way for creating high-performance 2D metasurface-based devices with a variety of applications, including quantum technology components such as single photon sensors and biphoton sources, communication systems, and non-linear light conversion.

physics.optics↗

arXiv2024

New design paradigm for highly efficient and low noise photodetector

Achieving high quantum efficiency (QE) with low dark count is essential for highly sensitive photodetectors (PDs), including single photon avalanche detectors (SPADs). However, high QE requires a thicker absorber region, which leads to high dark current and noise, which in turn affects the detectivity of PDs and the photodetection efficiency and dark count of SPADs.The holy grail of photodetector and avalanche photodiode designs is to achieve highest QE with thinnest absorber and still enable large avalanche to gain as needed. We have developed a new design paradigm which exploits the coupling between dielectric Mie resonance and transverse propagating waves in thin layers. The Mie resonance launches the incident light at an angle in an ultrathin absorber, and when coupled to transverse waves, the light propagates laterally and is fully absorbed owing to the longer optical path. Consequently, with appropriate choice of materials for a chosen wavelength, a high absorption(~90%) within typically <100 nm absorber thickness is possible. For illustration, we apply our approach to design Si-based detector operating at 810 nm and InGaAs-based detector operating at 1550 nm and predict that the dark current at room temperature is reduced at least by two orders of magnitude. In addition, the lateral distances are often in a few microns and hence these designs can potentially enable avalanching for a large optical gain.

physics.app-ph↗

arXiv2024

Resonant structure for improved directionality and extraction of single photons

Fluorescent atomic defects, especially in dielectric materials, such as diamond are quite promising for several emerging quantum applications. However, efficient light extraction, directional emission, and narrow spectral emission are key challenges. We have designed dielectric metasurface exploiting Mie-resonance and the Kerker condition to address these issues. Our designed diamond metasurface, tailored for nitrogen-vacancy (NV) defect centers in diamond, predicts up to 500x improvement in the collection of 637 nm (zero phonon line) photons over that from the bare diamond. Our design achieves highly directional emission, predominantly emitting in a 20 degree lobe in the forward direction. This makes light collection more efficient, including for fiber-based collection. The predicted results are stable against the position of the emitter placed in the metaelement, thus alleviating the challenging fabrication requirement of precise positioning of the defect center. Equally importantly, our design approach can be applied to enhance single photon emission also from other defects such as SiV, other materials such as hBN, and other sources such as quantum dots.

physics.optics↗

arXiv2024

Highly sensitive and efficient 1550 nm photodetector for room temperature operation

Photonic quantum technologies such as effective quantum communication require room temperature (RT) operating single- or few- photon sensors with high external quantum efficiency (EQE) at 1550 nm wavelength. The leading class of devices in this segment is avalanche photodetectors operating particularly in the Geiger mode. Often the requirements for RT operation and for a high EQE are in conflict, resulting in a compromised solution. We have developed a device which employs a two-dimensional (2D) semiconductor material on a co-optimized dielectric photonic crystal substrate to simultaneously decrease the dark current by three orders of magnitude at RT and maintain an EQE of >99%. The device is amenable to avalanching and form a basis for single photon detection with ultra-low dark current and high photodetection efficiency. Harnessing the high carrier mobility of 2D materials, the device has ~ps jitter time and can be integrated into a large 2D array camera.

physics.optics↗

arXiv2018

Nonperturbative solution to the integral equation of scattering theory

We obtain a nonperturbative, analytical solution to integral equation of scattering theory by assuming the field within the scattering object is a spherical wave with a scattering amplitude equal to that of the far field. This approximation transforms the integral equation into a simple algebraic equation which can be readily solved to obtain a closed-form expression for the scattering amplitude. We show this approximation is valid for homogeneous potentials of compact support, namely circular and square cylinders, and that the calculated scattering cross sections for spheres and square cylinders are accurate for frequencies through the fundamental resonance. Then we apply our analytical expression to the inverse scattering problem for spheres and show that accurate reconstructions are possible even under resonance conditions. The simplicity and accuracy of our method suggest it can be a reliable and efficient tool for understanding a wide range of scattering problems in optics.

physics.class-ph↗

arXiv2018

Thermal conductivity reduction by acoustic Mie resonance in nanoparticles

We evaluate the impact of acoustic Mie resonance in nanoparticles on the thermal conductivity of semiconductor and polymer composites. By appropriately choosing the bulk modulus and density, and selecting the size of the nanoparticle to align the Mie resonances with the dominant portion of the thermal conductivity spectrum, we show that large reductions in thermal conductivity are achievable with dilute concentrations of nanoparticles. In semiconductor alloys, where the spectral thermal conductivity is known, our model can explain the steep reductions in thermal conductivity observed previously. However, the results of our effort to evaluate acoustic Mie resonance in polymer composites are inconclusive due to uncertainties in the spectral thermal conductivity. Acoustic Mie resonances can be useful for maximizing ZT for thermoelectric applications, since a dilute loading of nanoparticles can reduce thermal conductivity with minimal impact on electrical conductivity.

cond-mat.mes-hall↗

arXiv2017

Nonlinear absorption in dielectric metamaterials

We solve the nonlinear Maxwell equations in an InP-based dielectric metamaterial, considering both two-photon absorption and photo-induced free-carrier absorption. We obtain the intensity-dependent reflection, absorption, and effective permittivity and permeability of the metamaterial. Our results show that nonlinear absorption dampens both the electric and magnetic Mie resonance, although the magnetic resonance is more affected because it occurs at longer wavelengths where the free-carrier absorption cross section is larger. Owing to field concentration in the metamaterial at resonance, the threshold intensity for nonlinear absorption is smaller by a factor of about 30 compared to a homogeneous layer of the same thickness. Our results have implications on the use of dielectric metamaterials for nonlinear applications such as frequency conversion and optical limiting.

physics.optics↗

arXiv2017

Transfer matrix method for four-flux radiative transfer

We develop a transfer matrix formalism for four-flux radiative transfer models, which is ideally suited for studying transport through multiple scattering layers. The model, derived for spherical particles within the diffusion approximation, predicts the specular and diffuse reflection and transmission of multilayer composite films for diffuse or collimated incidence. The model shows remarkable agreement with numerical Monte Carlo simulations for a range of absorption and film thicknesses, as well as for an example multilayer slab.

physics.optics↗

arXiv2016

Metasurface polarization splitter

Polarization beam splitters, devices that separate the two orthogonal polarizations of light into different propagation directions, are one of the most ubiquitous optical elements. However, traditionally polarization splitters rely on bulky optical materials, while emerging optoelectronic and photonic circuits require compact, chip-scale polarization splitters. Here we show that a subwavelength rectangular lattice of cylindrical silicon Mie resonators functions as a polarization splitter, efficiently reflecting one polarization while transmitting the other. We show that the polarization splitting arises from the anisotropic permittivity and permeability of the metasurface due to the two-fold rotational symmetry of the rectangular unit cell. The high polarization efficiency, low loss, and low profile make these metasurface polarization splitters ideally suited for monolithic integration with optoelectronic and photonic circuits.

physics.optics↗

arXiv2016

Thermal insulator transition induced by interface scattering

We develop an effective medium model of thermal conductivity that accounts for both percolation and interface scattering. This model accurately explains the measured increase and decrease of thermal conductivity with loading in composites dominated by percolation and interface scattering, respectively. Our model further predicts that strong interface scattering leads to a sharp decrease in thermal conductivity, or an insulator transition, at high loadings when conduction through the matrix is restricted and heat is forced to diffuse through particles with large interface resistance. The accuracy of our model and its ability to predict transitions between insulating and conducting states suggest it can be a useful tool for designing materials with low or high thermal conductivity for a variety of applications.

cond-mat.mes-hall↗

arXiv2014

Generalized effective-medium theory for metamaterials

We present an effective-medium model for calculating the frequency-dependent effective permittivity $ε(ω)$ and permeability $μ(ω)$ of metamaterial composites containing spherical particles with arbitrary permittivity and permeability. The model is derived from the zero-scattering condition within the dipole approximation, but does not invoke any additional long-wavelength approximations. As a result, it captures the effects of spatial dispersion and predicts a finite effective refractive index and antiresonances in $ε(ω)$ and $μ(ω)$, in agreement with numerical finite-element calculations.

physics.optics↗