Denis A. Kopylov

Ring-shaped spectra of parametric downconversion and entangled photons that never meet.

We report on the observation of an unusual type of parametric downconversion. In the regime where collinear degenerate emission is in the anomalous range of group-velocity dispersion, its spectrum is restricted in both angle and wavelength. Detuning from exact collinear-degenerate phase-matching leads to a ring shape of the wavelength-angular spectrum, suggesting a new type of spatiotemporal coherence and entanglement of photon pairs. By imposing a phase varying in a specific way in both angle and wavelength, one can obtain an interesting state of an entangled photon pair, with the two photons being never at the same point at the same time.

Optical harmonic generation from bright squeezed vacuum

Multi-photon processes are extremely important in optics. Optical harmonics generation, multi-photon absorption, ionization, polymerization or spectroscopy are widely used in practical applications. The efficiency of these processes depends on many factors, one of them being the statistics of the incident light. Indeed, the rate of an n-photon effect scales as the nth-order autocorrelation function g(n) of the incident light [1]. Therefore light sources with high correlation functions can enhance the efficiency of such effects by several orders in magnitude compared to coherent light with the same wavelength, pulse duration and energy per pulse. Here we demonstrate this enhancement in the generation of optical harmonics from bright squeezed vacuum (BSV), which can have thermal (g(n)=n!) or superbunched (g(n)=(2n-1)!!) statistics depending on the spectral filtering [2].

Optical harmonic generation enhanced due to ultrafast intensity fluctuations (Conference Presentation)

The effect of the quantum properties of light on nonlinear processes has been well studied theoretically. It has been shown that the efficiency of n-photon nonlinear processes in many cases scales as the normalized n-th order correlation function. For light with high intensity correlation function, the efficiency of the n-th harmonic generation will be considerably higher than for coherent light. The experimental observation of this effect remained difficult until recently, because of the absence of bright sources with strong and fast intensity fluctuations. For the experimental demonstration of statistical effects in optical harmonic generation we use as a pump the radiation of high-gain parametric down conversion. Such light shows quantum properties (e.g. quadrature or two-mode squeezing) and has large number of photons in one mode. The normalized n-th order correlation function for this light is (2n - 1)!!, which makes it more attractive for nonlinear processes than both coherent and thermal light. For the generation of optical harmonics we used broadband parametric down conversion around frequency-degeneracy (1600 nm) produced in 1cm BBO crystal from Ti:Sapphire laser (800 nm, 1.6ps, 5kHz, 3W mean intensity). Due to spectral filtering and post-selection technique we could vary the statistics of light from coherent to super-bunched, which allowed us to demonstrate the efficiency enhancement for second-, third-, and fourth-harmonic generation. The obtained experimental results show a good agreement with the theory.

Nonlinear optical effects in organic microstructures

Organic microstructures attract much attention due to their unique properties originating from the design of their shape and optical parameters. In this work we discuss the linear, second- and third-order nonlinear optical effects in arrays and in individual organic microstructures composed by self-assembling technique and formed randomly on top of a solid substrate. The structures under study consist of micro-spheres, -hemispheres or -frustums made of red laser dye and reveal an intense fluorescence (FL) in the visible spectral range. Importantly, that due to a high value of the refractive index and confined geometry, such micro-structures support the excitation of whispering gallery modes (WGM), which brings about strong and spectrally-selected light localization. We show that an amplification of the nonlinear optical effects is observed for these structures as compared to a homogeneous dye film of similar composition. The obtained data are in agreement with the results of the FDTD calculations performed for the structures of different dimensions. Perspectives of application of such type of organic nonlinear microresonators in optical devices are discussed.

Quasi-phase-matching second harmonic generation caused by pendulum effect in photonic crystals

Second harmonic generation (SHG) in the Laue scheme of the dynamical Bragg diffraction in 1D photonic crystal (PhC) is studied. The experiments are performed for partially annealed porous-silicon PhC containing 250 periods of the structure. Our measurements confirm phase-matched SHG under the Bragg condition. Possible types of phase- and quasi-phase-matching realized in the studied PhC are discussed.

Impact of thermal oxidation, surface chemistry and porous silicon morphology for sensing applications

An ideal diagnostic device should be inexpensive, easy-to-use, rapid and reliable. Nanostructured porous silicon (PSi) satisfies these criterions including label-free optical detection and high throughput detection. Pore morphology (size, porosity) must be tailored for each specific application, and for immunosensing applications PSi morphology has been optimized for maximal pore infiltration of larger proteins as immuno gamma globlulin (IgG). Sensor degradation by high salt concentration induces a baseline drift. Different thermal oxidation procedures have been studied in order to obtain a stable sensor in the 3 hour incubation period of the immunoassay with negligible drift