arXiv subjects

Vivek Venkataraman

Publications and source records attributed to Vivek Venkataraman.

13 recordsLinked to original sources

arXiv2026

Social Network Structure, Wealth, and Wealth Inequality Across Cultures

Despite theory tying wealth inequality to social structure, empirical evidence has been limited to a few studies based on online social media data. This study uses a very different type of data, expands the global coverage to very different types of societies, and investigates new questions. In particular, we collect data from ~3500 sharing units (households) in 46 communities across the globe, representing considerable human social and cultural diversity. In each, we analyze the relationship between people's material wealth and the structure of social networks: borrowing money, sharing food, working together, socializing, etc. In almost all communities, a sharing unit's material wealth is positively associated with the number of other sharing units it both helps and is helped by. A sharing unit's wealth is also associated with the relative wealth of the sharing units to which it is linked---a form of economic homophily. Notably, communities with greater wealth inequality are also characterized by a network structure in which poorer sharing units are less well connected to wealthier ones. We augment our unique cross-cultural data with other community-level environmental, institutional, and economic attributes, opening new avenues for future research into the co-determination of wealth and social networks.

cs.SI↗

arXiv2023

FSO-QKD protocols under free space losses and device imperfections: a comparative study

Quantum key distribution (QKD) is a technique to establish a secret key between two parties through a quantum channel. Several QKD protocols have been proposed and implemented over optical fibers or free space links. The main challenge of operating QKD protocols over a free space link is atmospheric losses. In this paper, we have studied and compared the performance of single and entangled photon based QKD protocols by evaluating the quantum bit error rate and secure key rate for terrestrial free-space quantum communication by considering different free-space losses, such as geometrical losses, atmospheric losses as well as device imperfections.

quant-ph↗

arXiv2019

Supercontinuum generation in angle-etched diamond waveguides

We experimentally demonstrate on-chip supercontinuum generation in the visible region in angle etched diamond waveguides. We measure an output spectrum spanning 670 nm to 920 nm in a 5mm long waveguide using 100 fs pulses with 187 pJ of incident pulse energy. Our fabrication technique, combined with diamonds broad transparency window, offers a potential route toward broadband supercontinuum generation in the UV domain.

physics.optics↗

arXiv2017

An integrated diamond Raman laser pumped in the near-visible

Using a high-Q diamond microresonator (Q > 300,000) interfaced with high-power-handling directly-written doped-glass waveguides, we demonstrate a Raman laser in an integrated platform pumped in the near-visible. Both TM-to-TE and TE-to-TE lasing is observed, with a Raman lasing threshold as low as 20 mW and Stokes power of over 1 mW at 120 mW pump power. Stokes emission is tuned over a 150 nm (60 THz) bandwidth around 875 nm wavelength, corresponding to 17.5% of the center frequency.

physics.optics↗

arXiv2017

Mechanical and optical nanodevices in single-crystal quartz

Single-crystal α-quartz, one of the most widely used piezoelectric materials, has enabled a wide range of timing applications. Owing to the fact that integrated thin-film based quartz platform is not available, most of these applications rely on macroscopic, bulk crystal-based devices. Here we show that the Faraday cage angled-etching technique can be used to realize nanoscale electromechanical and photonic devices in quartz. Using this approach, we demonstrate quartz nanomechanical cantilevers and ring resonators featuring Qs of 4,900 and 8,900, respectively.

cond-mat.mes-hall↗

arXiv2017

Competition between Raman and Kerr effects in microresonator comb generation

We investigate the effects of Raman and Kerr gain in crystalline microresonators and determine the conditions required to generate modelocked frequency combs. We show theoretically that strong, narrowband Raman gain determines a maximum microresonator size allowable to achieve comb formation. We verify this condition experimentally in diamond and silicon microresonators and show that there exists a competition between Raman and Kerr effects that leads to the existence of two different comb states.

physics.optics↗

arXiv2017

Second harmonic generation in nano-structured thin-film lithium niobate waveguides

Integrated thin-film lithium niobate platform has recently emerged as a promising candidate for next-generation, high-efficiency wavelength conversion systems that allow dense packaging and mass-production. Here we demonstrate efficient, phase-matched second harmonic generation in lithographically-defined thin-film lithium niobate waveguides with sub-micron dimensions. Both modal phase matching in fixed-width waveguides and quasi-phase matching in periodically grooved waveguides are theoretically proposed and experimentally demonstrated. Our low-loss (~ 3.0 dB/cm) nanowaveguides possess normalized conversion efficiencies as high as 41% W-1cm-2.

physics.optics↗

arXiv2015

Waveguide-loaded silica fibers for coupling to high-index micro-resonators

Tapered silica fibers are often used to rapidly probe the optical properties of micro-resonators. However, their low refractive index precludes phase-matching when coupling to high-index micro-resonators, reducing efficiency. Here we demonstrate efficient optical coupling from tapered fibers to high-index micro-resonators by loading the fibers with an ancillary adiabatic waveguide-coupler fabricated via angled-etching. We demonstrate greatly enhanced coupling to a silicon multimode micro-resonator when compared to coupling via the bare fiber only. Signatures of resonator optical bistability are observed at high powers. This scheme can be applied to resonators of any size and material, increasing the functional scope of fiber coupling.

physics.optics↗

arXiv2015

On-Chip Diamond Raman Laser

Synthetic single-crystal diamond has recently emerged as a promising platform for Raman lasers at exotic wavelengths due to its giant Raman shift, large transparency window and excellent thermal properties yielding a greatly enhanced figure-of-merit compared to conventional materials. To date, diamond Raman lasers have been realized using bulk plates placed inside macroscopic cavities, requiring careful alignment and resulting in high threshold powers (~W-kW). Here we demonstrate an on-chip Raman laser based on fully-integrated, high quality-factor, diamond racetrack micro-resonators embedded in silica. Pumping at telecom wavelengths, we show Stokes output discretely tunable over a ~100nm bandwidth around 2-μm with output powers >250 μW, extending the functionality of diamond Raman lasers to an interesting wavelength range at the edge of the mid-infrared spectrum. Continuous-wave operation with only ~85 mW pump threshold power in the feeding waveguide is demonstrated along with continuous, mode-hop-free tuning over ~7.5 GHz in a compact, integrated-optics platform.

physics.optics↗

arXiv2014

Integrated high quality factor lithium niobate microdisk resonators

Lithium Niobate (LN) is an important nonlinear optical material. Here we demonstrate LN microdisk resonators that feature optical quality factor ~ 100,000, realized using robust and scalable fabrication techniques, that operate over a wide wavelength range spanning visible and near infrared. Using our resonators, and leveraging LN's large second order optical nonlinearity, we demonstrate on-chip second harmonic generation with a conversion efficiency of 0.109 W-1.

physics.optics↗

arXiv2010

Enhanced Two-Photon Absorption in a Hollow-Core Photonic Bandgap Fiber

We show that two-photon absorption (TPA) in Rubidium atoms can be greatly enhanced by the use of a hollow-core photonic bandgap fiber. We investigate off-resonant, degenerate Doppler-free TPA on the 5S1/2 - 5D5/2 transition and observe 1% absorption of a pump beam with a total power of only 1 mW in the fiber. These results are verified by measuring the amount of emitted blue fluorescence and are consistent with the theoretical predictions which indicate that transit time effects play an important role in determining the two-photon absorption cross-section in a confined geometry.

physics.optics↗

arXiv2010

Analytical Drain Current Model of Nanoscale Strained-Si/SiGe MOSFETs for Analog Circuit Simulation

For nanoscale CMOS applications, strained-silicon devices have been receiving considerable attention owing to their potential for achieving higher performance and compatibility with conventional silicon processing. In this work, an analytical model for the output current characteristics (I-V) of nanoscale bulk strained-Si/SiGe MOSFETs, suitable for analog circuit simulation, is developed. We demonstrate significant current enhancement due to strain, even in short channel devices, attributed to the velocity overshoot effect. The accuracy of the results obtained using our analytical model is verified using two-dimensional device simulations.

cond-mat.mes-hall↗

arXiv2010

Compact Modeling of Parasitic Internal Fringe Capacitance Effects on the Threshold Voltage of High-K Gate Dielectric Nanoscale SOI MOSFETs

A compact model for the effect of parasitic internal fringe capacitance on threshold voltage in high-K gate dielectric SOI MOSFETs is developed. Our model includes the effects of the gate dielectric permittivity, spacer oxide permittivity, spacer width, gate length and width of MOS structure. A simple expression for parasitic internal fringe capacitance from the bottom edge of the gate electrode is obtained and the charges induced in the source and drain regions due to this capacitance are considered. We demonstrate an increase in surface potential along the channel due to these charges resulting in a decrease in the threshold voltage with increase in gate dielectric permittivity. The accuracy of the results obtained using our analytical model is verified using 2-D device simulations.

cond-mat.mes-hall↗