Crina Cojocaru

Nondiffractive propagation of light in photonic crystals

We show that the diffraction of electromagnetic radiation (in particular of a visible light) can disappear in propagation through materials with periodically in space modulated refraction index, i.e. through photonic crystals.

Ultrafast dynamics of the third-order nonlinear response in a two-dimensional InP-based photonic crystal

We report experimental demonstration of very fast nonlinear response around 1.5μm in an InP-based two-dimensional photonic crystal. The nonlinearity produced by low pump powers via carrier induced nonlinear refractive index, leads to an efficient wavelength shift of a photonic crystal resonance observed in reflectivity. Thus we show that it is possible to obtain round the clock (rise and recovery) switching times shorter than 10ps with contrast ratio higher than 80%.

Observation of azimuthal modulational instability and formation of patterns of optical solitons in a quadratic nonlinear crystal

We report what is believed to be the first experimental demonstration of the azimuthal self-breaking of intense beams containing a vortex phase dislocation into sets of optical spatial solitons in a quadratic nonlinear material. The observations were performed in a KTP crystal.

Flat lensing in the visible frequency range by woodpile photonic crystals

We experimentally demonstrate full two-dimensional focalization of light beams at visible frequencies by a three-dimensional woodpile photonic crystal. The focalization (the flat lensing) with focal distances of the order of 50-70 μm is experimentally demonstrated. Experimental results are compared with numerical calculations and interpreted by harmonic expansion studies.

Second-harmonic parametric scattering in ferroelectric crystals with disordered nonlinear domain structures.

We study the second-harmonic (SH) parametric processes in unpoled crystals of Strontium Barium Niobate (SBN) with disordered structures of ferroelectric domains. Such crystals allow for the simultaneous phase matching of several second-order nonlinear processes. We analyze the polarization properties of these parametric processes using two types of generation schemes: quasi-collinear SH generation and transverse SH generation. From our experimental data we determine the ratio of d(32) and d(33) components of the second order susceptibility tensor and also the statistical properties of the random structure of the SBN crystal.

Tuning a two-dimensional photonic crystal resonance via optical carrier injection

We report on wide wavelength tuning through optical injection of carriers of a photonic resonance observed in reflectivity at 1543 nm in an InP-based two-dimensional photonic crystal slab. An 8-nm blueshift, which represents 20 times the resonance linewidth, is observed when a 4-kW/cm2 intense optical pump is incident on the sample. An analytical model that we developed, based on a coupled-mode nonlinear approach, allows us to describe this phenomenon in detail.

Planar second-harmonic generation with noncollinear pumps in disordered media

We study experimentally the process of the second harmonic generation by two noncollinear beams in quadratic nonlinear crystals with a disordered structure of ferroelectric domains. We show that the second-harmonic radiation is emitted in the form of two cones as well as in a plane representing the cross-correlation of the two fundamental pulses. We demonstrate the implementation of this parametric process for characterisation of femtosecond pulses, enabling the estimation of pulse width, chirp, and front tilt. This is achieved through monitoring the evolution of the autocorrelation trace inside the nonlinear crystal.

Third-harmonic generation via broadband cascading in disordered quadratic nonlinear media

We study parametric frequency conversion in quadratic nonlinear media with disordered ferroelectric domains. We demonstrate that disorder allows realizing broadband third-harmonic generation via cascading of two second-order quasi-phase matched nonlinear processes. We analyze both spatial and polarization properties of the emitted radiation and find the results in agreement with our theoretical predictions.

Active reflection via a phase-insensitive quadratic nonlinear interaction within a microcavity

The reflectivity of a microcavity filled with a quadratic nonlinear material is shown to be actively changed by the interaction of two waves. Within this microcavity, the reflection coefficient of a weak wave at the fundamental frequency is changed from almost 0% to a value in the vicinity of 100% by the simultaneous incidence of an intense wave at the second-harmonic frequency. This change in reflectivity is shown to be in a large degree insensitive to the input phase difference between the two waves.

Theoretical study of Čerenkov-type second-harmonic generation in periodically poled ferroelectric crystals

We study theoretically the Cerenkov-type second-harmonic generation by a Gaussian beam in quadratic nonlinear media with periodically reverted ferroelectric domains. In particular, we consider the role of fundamental beam width on the harmonic emission process. We show that varying the beam width of the fundamental wave not only affects the strength of the emitted Cerenkov harmonic signal but also changes the sensitivity of the emission process to wavelength tuning.