We experimentally investigate quadrature correlations between pump, signal, and idler fields in an above-threshold optical parametric oscillator. We observe new quantum correlations among the pump and signal or idler beams, as well as among the pump and a combined quadrature of signal and idler beams. A further investigation of unforeseen classical noise observed in this system is presented, which hinders the observation of the recently predicted tripartite entanglement. In spite of this noise, current results approach the limit required to demonstrate three-color entanglement.

We report the reconstruction of the quantum state of squeezed vacuum generated by a continuouswave optical parametric amplifier. Homodyne detection and tomographic reconstruction methods were used to obtain the density matrix in the Fock (number state) representation. The photon number distribution exhibits odd-even oscillations, a manifestation of the photon pair production in the secondorder nonlinear medium.

Squeezed states of light are generated by the process of second-harmonic conversion within an optical cavity resonant at both fundamental and harmonic frequencies. Observations of squeezing are made by analyzing the spectral density of photocurrent fluctuations produced by the total field reflected from the nonlinear cavity. Reductions in photocurrent noise of 13% relative to the coherent-state or shot-noise level are achieved for frequency offsets near 4 MHz.

Conversion efficiency of 85% has been achieved in cw second-harmonic generation from 1.08 to 0.54 Am with a potassium titanyl phosphate crystal inside an external ring cavity. An absolute comparison between the experimental data and a simple theory is made and shows good agreement.Because of the nonlinear nature of the process and the generally small susceptibilities involved, harmonic generation with high conversion efficiency is most often realized only for pulsed radiation with high intensity for a short period. 1 By contrast, the intensity available for cw operation is usually limited to much smaller values than for pulsed operation, with a concomitant reduction in conversion efficiency. A possible remedy to this circumstance that was recognized long ago is to employ a highfinesse resonator in either a passive 2 or active 3 configuration to enhance the circulating power and hence the overall conversion efficiency. While the quest for efficient conversion in a cw setting might at first sight seem to be best pursued with crystals with large nonlinear coefficients, such as lithium niobate and potassium niobate, 4 ' 5 unfortunately the advantages of large nonlinear coefficient are often mitigated by deleterious effects such as linear and nonlinear absorption and subtle intensity-dependent changes in refractive index, which although small can quickly degrade the performance of a highfinesse cavity. On the other hand, crystals with greater transparency, with fewer thermal problems, and with a higher damage threshold such as the newly developed lithium triborate usually have much smaller nonlinear coefficients and therefore require correspondingly higher powers for the fundamental wave. 6 For many years, potassium titanyl phosphate (KTP) has been a well-known crystal for frequency doubling of Nd:YAG lasers (1.064 Am). 7 The large nonlinear coefficient of KTP together with the wide temperature width, large acceptance angle, and extremely low absorption loss make KTP a promising candidate for high conversion efficiency. However, because type II critical angle tuning is the only possible phase-matching scheme at 1.064 pum, the performance of KTP inside a high-finesse resonator is significantly degraded owing to the walk-off of ordinary and extraordinary beams within the crystal. To circumvent this difficulty, recently Garmash et al. 8 reported type II 90° noncritical phase matching in an a-cut KTP crystal at 1.08 Am, thus suggesting new possibilities for better performance of KTP in intracavity cw frequency doubling. Following this lead, we report in this Letter experiments for frequency doubling from 1.08 to 0.54 Am with a single KTP crystal inside an external ring cavity of extremely low passive loss. Our measurements are carried out over a range of input fundamental powers up to 700 mW, and a cw conversion efficiency of 85% is achieved. The experimental results are in good absolute agreement with a simple theory, which suggests that efficiencies greater than 90% should be obtainable in our current system once the f...

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Nonclassical correlations between the quadrature-phase amplitudes of two spatially separated optical beams are exploited to realize a two-channel quantum communication experiment with a high degree of immunity to interception. For this scheme, either channel alone can have an arbitrarily small signal-to-noise ratio ͑SNR͒ for transmission of a coherent ''message.'' However, when the transmitted beams are combined properly upon authorized detection, the encoded message can in principle be recovered with the original SNR of the source. An experimental demonstration has achieved a 3.2 dB improvement in SNR over that possible with correlated classical sources. Extensions of the protocol to improve its security against eavesdropping are discussed.

http://www.jeos.org/index.php/jeos_rp/article/view/07022We present a comparison among several fully-vectorial methods applied to a basic scattering problem governed by the physics of the electromagnetic interaction between subwavelength apertures in a metal film. The modelled structure represents a slit-groove scattering problem in a silver film deposited on a glass substrate. The benchmarked methods, all of which use in-house developed software, include a broad range of fully-vectorial approaches from finite-element methods, volume-integral methods, and finite-difference time domain methods, to various types of modal methods based on different expansion techniques

Abstract:We report a method on quantitative and simultaneous noncontact in-vivo hydration and sebum measurements of the skin using an infrared optical spectroscopic set-up. The method utilizes differential detection with three wavelengths 1720, 1750, and 1770 nm, corresponding to the lipid vibrational bands that lay "in between" the prominent water absorption bands. We have used an emulsifier containing hydro-and lipophilic components to mix water and sebum in various volume fractions which was applied to the skin to mimic different oily-dry skin conditions. We also measured the skin sebum and hydration values on the forehead under natural conditions and its variations to external stimuli. Good agreement was found between our experimental results and reference values measured using conventional biophysical methods such as Corneometer and Sebumeter. H. Behrendt, and J. Ring, "Skin Surface pH, Stratum corneum Hydration, Trans-epidermal water loss and skin roughness related to atopic eczema and skin dryness in a population of primary school children," Acta Derm. Venereol. 80(3), 188-191 (2000). 11. M. Janssens, J. van Smeden, G. J. Puppels, A. P. M. Lavrijsen, P. J. Caspers, and J. A. Bouwstra, "Lipid to protein ratio plays an important role in the skin barrier function in patients with atopic eczema," Br. J. Dermatol. 170(6), 1248-1255 (2014). 12. J. Kim, J. Jeon, and H. Song, "Evaluation of the degree of skin dryness and the effect of moisturizing therapy in scalp psoriasis", Annual Global Pharma Summit, Philadelphia, USA (2015). #261736Received 23 Dermatol. 96(6), 845-851 (1990 (InTech, Rijeka, 2012). 21. Y. Shirai, "Skin condition observation apparatus," U.S. Patent 10,642,683, Aug. 19, 2003. 22. H. Iwasaki, K. Miyazawa, and S. Nakauchi, "Visualization of the human face skin moisturizing-ability by spectroscopic imaging using two near-infrared bands," Proc. SPIE 6062, 606203 (2006). 23. D. P. Lookingbill and W. J. Cunliffe, "A direct gravimetric technique for measuring sebum excretion rate," Br.