Cavity-enhanced sum-frequency generation of blue light with near-unity conversion efficiency

We report on double-resonant highly efficient sum-frequency generation in the blue range. The system consists of a 10-mm-long periodically poled KTP crystal placed in a double-resonant bow-tie cavity and pumped by a fiber laser at 1064.5 nm and a Ti:sapphire laser at 849.2 nm. An optical power of 375 mW at 472.4 nm in a TEM$_{00}$ mode was generated with pump powers of 250 mW at 849.2 nm and 200 mW at 1064.5 nm coupled into the double-resonant ring resonator with 88$\%$ mode-matching. The resulting internal conversion efficiency of 95($\pm 3$)$\%$ of the photons mode-matched to the cavity constitutes, to the best of our knowledge, the highest overall achieved quantum conversion efficiency using continuous-wave pumping. Very high conversion efficiency is rendered possible due to very low intracavity loss on the level of 0.3$\%$ and high nonlinear conversion coefficient up to 0.045(0.015) W$^{-1}$. Power stability measurements performed over one hour show a stability of 0.8$\%$. The generated blue light can be tuned within 5 nm around the center wavelength of 472.4 nm, limited by the phase-matching of our nonlinear crystal. This can however be expanded to cover the entire blue spectrum (420 nm to 510 nm) by proper choice of nonlinear crystals and pump lasers. Our experimental results agree very well with analytical and numerical simulations taking into account cavity impedance matching and depletion of the pump fields.Comment: 9 pages, 5 figure

Quantum-enhanced micromechanical displacement sensitivity

We report on a hitherto unexplored application of squeezed light: for quantum-enhancement of mechanical transduction sensitivity in microcavity optomechanics. Using a toroidal silica microcavity, we experimentally demonstrate measurement of the transduced phase modulation signal with a sensitivity $-0.72(\pm 0.01)$\,dB below the shot noise level. This is achieved for resonant probing in the highly under-coupled regime, by preparing the probe in a weak coherent state with phase squeezed vacuum states at sideband frequencies

Quantitative Fiber-Enhanced Raman Sensing of Inorganic Nitrogen Species in Water

Fast and efficient water quality monitoring is essential in the pursuit of reducing the impact of human activities on the environment. We address this issue by presenting a sensing system and method based on Raman spectroscopy in liquid-filled capillaries, that enables quantitative measurement of polyatomic anions in solution. We demonstrate quantitative measurement of nitrate concentrations in water via multivariate analysis with partial least squares regression. We achieve a limit of detection of 0.13 millimolar for a measurement time of 30 s. Our Raman method is compared with gravimetrically measured concentration with good agreement and reproducibility. The Raman monitoring method can be performed in a continuous manner, thus suitable for fast continuous monitoring of water and wastewater quality

Quantitative Fiber-Enhanced Raman Sensing of Inorganic Nitrogen Species in Water

Fast and efficient water quality monitoring is essential in the pursuit of reducing the impact of human activities on the environment. We address this issue by presenting a sensing system and method based on Raman spectroscopy in liquid-filled capillaries, that enables quantitative measurement of polyatomic anions in solution. We demonstrate quantitative measurement of nitrate concentrations in water via multivariate analysis with partial least squares regression. We achieve a limit of detection of 0.13 millimolar for a measurement time of 30 s. Our Raman method is compared with gravimetrically measured concentration with good agreement and reproducibility. The Raman monitoring method can be performed in a continuous manner, thus suitable for fast continuous monitoring of water and wastewater quality

Polymer Nanoparticle Identification and Concentration Measurement Using Fiber-Enhanced Raman Spectroscopy

We present a measurement technique for chemical identification and concentration measurement of polymer nanoparticles in aqueous solution, which is achieved using Raman spectroscopy. This work delivers an improvement in measurement sensitivity of 40 times over conventional Raman measurements in cuvettes by loading polymer nanoparticles into the hollow core of a microstructured optical fiber. We apply this “fiber-enhanced” system to measure the concentration of two separate samples of polystyrene particles (diameters of 60 nm and 120 nm respectively) with concentrations in the range from 0.07 to 0.5 mg/mL. The nanoliter volume formed by the fiber presents unique experimental conditions where nanoparticles are confined within the fiber core and prevented from diffusing outside the incident electromagnetic field, thereby enhancing their interaction. Our results suggest an upper limit on the size of particle that can be measured using the hollow-core photonic crystal fiber, as the increasing angular distribution of scattered light with particle size exceeds the acceptance angle of the liquid-filled fiber. We investigate parameters such as the fiber filling rate and optical properties of the filled fiber, with the aim to deliver repeatable and quantifiable measurements. This study thereby aids the on-going process to create compact systems that can be integrated into nanoparticle production settings for in-line measurements

Squeezing-enhanced measurement sensitivity in a cavity optomechanical system

We determine the theoretical limits to squeezing-enhanced measurement sensitivity of mechanical motion in a cavity optomechanical system. The motion of a mechanical resonator is transduced onto quadrature fluctuations of a cavity optical field and a measurement is performed on the optical field exiting the cavity. We compare measurement sensitivities obtained with coherent probing and quantum-enhanced probing of the mechanical motion, i.e. the coherent probe field carries vacuum states and quadrature squeezed vacuum states at sideband frequencies, respectively. We find that quantum-enhanced probing provides little to no improvement in motion sensing for resonators in the unresolved sideband regime but may significantly increase measurement sensitivities for resonators in the resolved sideband regime