Matteo Cherchi

Integrated Optic Surface Plasmon Resonance Measurements in a Borosilicate Glass Substrate

The surface plasmon resonance (SPR) technique is a well-known optical method that can be used to measure the refractive index of organic nano-layers adsorbed on a thin metal film. Although there are many configurations for measuring biomolecular interactions, SPR-based techniques play a central role in many current biosensing experiments, since they are the most suited for sensitive and quantitative kinetic measurements. Here we give some results from the analysis and numerical elaboration of SPR data from integrated optics experiments in a particular borosilicate glass, chosen for its composition offering the rather low refractive index of 1.4701 at 633 nm wavelength. These data regard the flow over the sensing region (metal window) of different solutions with refractive indexes in the range of interest (1.3÷1.5) for the detection of contaminants in aqueous solutions. After a discussion of the principles of SPR, of the metal window design optimization by means of optical interaction numerical modeling, and of waveguide fabrication techniques, we give a description of system setup and experimental results. Optimum gold film window thickness and width in this guided-wave configuration has been for the first time derived and implemented on an integrated optic prototype device. Its characterization is given by means of the real time waveguide output intensity measurements, which correspond to the interaction between the sensing gold thin film window and the flowing analyte. The SPR curve was subsequently inferred. Finally, a modified version of the device is reported, with channel waveguides arranged in a Y-junction optical circuit, so that laser source stability requirements are lowered by a factor of 85 dB, making possible the use of low cost sources in practical applications.

Exploiting the Optical Quadratic Nonlinearity of Zinc-Blende Semiconductors for Guided-Wave Terahertz Generation: A Material Comparison

We present a detailed analysis and comparison of dielectric waveguides made of CdTe, GaP, GaAs and InP for modal phase matched optical difference frequency generation (DFG) in the terahertz domain. From the form of the DFG equations, we derived the definition of a very general figure of merit (FOM). In turn, this FOM enabled us to compare different configurations, by taking into account linear and nonlinear susceptibility dispersion, terahertz absorption, and a rigorous evaluation of the waveguide modes properties. The most efficient waveguides found with this procedure are predicted to approach the quantum efficiency limit with input optical power in the order of kWs.

Guided-wave frequency doubling in surface periodically poled lithium niobate: competing effects

We carried out second-harmonic generation in quasi-phase-matched α-phase lithium niobate channel waveguides realized by proton exchange and surface periodic poling. Owing to a limited ferroelectric domain depth, we could observe the interplay between second-harmonic generation and self-phase modulation due to cascading and cubic effects, resulting in a nonlinear resonance shift. Data reduction allowed us to evaluate both the quadratic nonlinearity in the near infrared as well as the depth of the uninverted domains.

Universal Charts for Optical Difference Frequency Generation in the Terahertz Domain

We present a universal and rigorous approach to study difference frequency generation in the terahertz domain, especially when reaching for the quantum efficiency limit. Through the definition of a suitable figure of merit, we have been able to keep the number of degrees of freedom to a minimum, in order to draw up suitably normalized charts, that enable to predict the optical-to-terahertz conversion efficiency of any efficient system based on wave propagation in quadratic nonlinear materials. The predictions of our approach take into account the effects of both terahertz absorption and optical pump depletion, and are found to be in good agreement with the best experimental results reported to date. This enabled also to estimate the d 22 nonlinear coefficient of high quality GaSe.

Second-harmonic generation in surface periodically poled lithium niobate waveguides: on the role of multiphoton absorption

Second-harmonic generation is investigated in lithium niobate channels realized by proton exchange and quasi-phase matched by surface periodic poling. The reduction in conversion efficiency at high powers is interpreted in terms of multiphoton absorption via two-color terms, yielding an estimate of the dominating three-photon process.

Design scheme for Mach-Zehnder interferometric coarse wavelength division multiplexing splitters and combiners

I propose an analytical approach to design flattened wavelength splitters with cascaded Mach-Zehnder interferometers when wavelength dependence of the directional couplers cannot be neglected. I start from a geometrical representation of the action of a doubly point-symmetrical filter, assuming no wavelength dependence of the couplers. Next I derive the analytical formulas behind its working principle and extend them to the wavelength-dependent case. I also show how the geometrical representation allows one to broaden the class of working structures.

Optical Bloch-mode-induced quasi phase matching of quadratic interactions in one-dimensional photonic crystals

We examine in detail the quasi-phase-matching process obtained as a stationary modulation of the fundamental field at the band edge of a finite one-dimensional photonic crystal. The treatment is carried out in terms of the structure Bloch waves and fully explains the behavior of second-harmonic generation in the grating. An integrated microstructured AlGaAs mesa waveguide is proposed that gives efficient second-harmonic and difference-frequency generation in virtue of the combined presence of a periodic modulation of the fundamental-field amplitude and of the photonic bandgap edge.

Frequency doubling in surface periodically poled lithium niobate waveguides: competing effects

We fabricated alpha-phase proton-exchanged (PE) lithium niobate (LN) channel waveguides quasi phase-matched (QPM) via surface periodic poling (SPP) and carried out the first experimental demonstration of second harmonic generation (SHG) in such devices. In order to quantitatively interpret the experimental data, we took into account not only the periodically-poled transverse region providing 1st-order QPM, but also the homogeneously-inverted portion of the waveguides extending below the shallow domains. By a numerical fit of the data we found that, because of the shallow non-inverted domains with depth of the order of 430nm (to be compared with a waveguide depth of 2.6mm), a cascaded process in phase-mismatch gives rise to an effective self-defocusing response which combines with the cubic nonlinearity of the material to provide the measured SHG resonance shift with power.

The role of nonlinear optical absorption in narrowband difference-frequency terahertz-wave generation

We present a general analysis of the influence of nonlinear optical absorption on terahertz generation via optical-difference frequency generation when reaching for the quantum conversion efficiency limit. By casting the equations governing the process in a suitably normalized form, including either two-photon- or three-photon absorption terms, we have been able to plot universal charts for phase-matched optical-to-terahertz conversion for different values of the nonlinear absorption coefficients. We apply our analysis to some experiments reported to date in order to understand to what extent multiphoton absorption could have played a role and also to predict the maximum achievable conversion efficiency at higher peak pump intensities.

Nanopatterned Ferroelectric Crystals for Parametric Generation

We report on recent results by surface periodic poling on lithium niobate and lithium tantalate. Such approach allows periodic inversion of the second order susceptibility with nanoscale features using insulating masks. We achieved a world-best 200 nm feature size, as well as good compatibility with alpha-phase proton exchanged channel waveguides in lithium niobate. Preliminary results of surface periodic poling in lithium tantalate also show similar characteristics. Surface poling is best suited for integrated optics devices in technologically-demanding configurations such as backward second harmonic generation and counter propagating optical parametric amplification