Andrea Marco Malvezzi

Enhanced second- and third-harmonic generation and induced photoluminescence in a two-dimensional GaN photonic crystal

We observed visible second-harmonic and ultraviolet third-harmonic fields generated in reflection from the surface of a two-dimensional triangular GaN/sapphire photonic crystal. When the pump radiation resonates with a photonic mode, enhancement factors as high as 250 and 3500 occurred for the second- and third-harmonic signals, respectively, as compared to the unpatterned GaN slab. The very large increase of third-harmonic field, with a photon energy exceeding that of the electronic band gap, was used as an efficient mechanism to induce GaN photoluminescence.We observed visible second-harmonic and ultraviolet third-harmonic fields generated in reflection from the surface of a two-dimensional triangular GaN/sapphire photonic crystal. When the pump radiation resonates with a photonic mode, enhancement factors as high as 250 and 3500 occurred for the second- and third-harmonic signals, respectively, as compared to the unpatterned GaN slab. The very large increase of third-harmonic field, with a photon energy exceeding that of the electronic band gap, was used as an efficient mechanism to induce GaN photoluminescence.

Enhancement of visible second-harmonic generation in epitaxial GaN-based two-dimensional photonic crystal structures

Second-harmonic (SH) radiation generated in reflection is measured from the surface of a two-dimensional triangular photonic crystal in a GaN layer. A very large SH enhancement is observed when the incident radiation is resonant with a leaky photonic mode. The potential of second-harmonic generation as a tool for photonic band mapping is also envisaged. The extended transparency window of III-nitride wide band gap semiconductors coupled with large nonlinearities is an appealing feature pointing toward light control and manipulation in photonic structures.

Second-harmonic generation in reflection and diffraction by a GaAs photonic-crystal waveguide

Nonlinear reflection and diffraction measurements have been performed on a GaAs/AlGaAs photonic-crystal waveguide patterned with a square lattice: The basis in the two-dimensional unit cell consists of rings of air in the dielectric matrix. The measured angles of diffracted second-harmonic beams agree with those predicted for nonlinear diffraction conditions. Results for second-harmonic intensities as a function of incidence angle, polarization, and pump wavelength show that the reflected second-harmonic signal is dominated by the crystalline symmetry of GaAs, whereas nonlinear diffraction is determined by the photonic-crystal structure.

Optical properties and photonic mode dispersion in two-dimensional and waveguide-embedded photonic crystals

Recent experimental and theoretical work on two-dimensional (2D) and waveguide-embedded photonic crystals is reviewed. The investigated systems are 2D macroporous silicon and photonic crystal slabs based on silicon-on-insulator as well as GaAs/AlGaAs. In all these structures, reflectance at varying angles of incidence allows to determine the dispersion of photonic modes above the light line. For macroporous silicon, reflectance from the side yields a complementary measurement of the photonic gaps. In the GaAs-based system, second-harmonic generation in reflection shows a resonant enhancement when the pump beam is frequency- and momentum-matched to a photonic mode in the slab. A theory of photonic states in waveguide-embedded photonic crystals leads to a determination of mode dispersion and diffraction losses for leaky photonic modes.

Sensing by Means of Nonlinear Optics with Functionalized GaAs/AlGaAs Photonic Crystals

We report on specific functionalization of GaAs/AlGaAs photonic structures for molecular sensing via the optical second harmonic generation signal in the visible range exhibited by these nanostructures. Functionalization has been achieved by peptides selected by the phage display technology, revealing specific recognition for semiconducting surfaces. These small peptides when biotinylated serve for controlled placement of biotin onto the substrate to capture then streptavidin. Functionalization (with biotinylated peptide) and molecular recognition (of streptavidin) events both result in enhancing the nonlinear optical response of the samples. Adsorption and infiltration of biomolecules into the GaAs/AlGaAs photonic structure were monitored by atomic force and scanning electron microscopy combined with Energy Dispersive X-ray spectroscopy. We demonstrate that once functionalized with specific peptides, photonic structures could be used as miniature biosensors down to femtomolar detection sensitivity, by monitoring changes in the second harmonic signal when molecules are captured. Our results prove the outstanding sensitivity of the nonlinear approach in biosensing with photonic crystal waveguides as compared to linear absorption techniques on the same samples. The present work is expected to pioneer development of a new class of extremely small affinity-based biosensors with high sensitivity and demonstrates that photonic structures support device functionality that includes strongly confined and localized nonlinear radiation emission and detection processes.

Laser–Matter Interaction in LIBS Experiments

Some of the physical processes occurring during the formation and expansion of the laser induced plasma are briefly reviewed. Their knowledge elucidates the role of the various effects in the energy flow from the laser pulse to the observed spectroscopic quantities, the parameters of interest in LIBS measurements, via all the intermediate mechanisms such as electron and ion heating, thermal diffusion, plasma formation, expansion, and particle ablation and kinetics.

Second-harmonic generation measured on a GaAs photonic crystal planar waveguide

Second-harmonic (SH) reflection and diffraction measurements are performed, at the wavelengths of a Ti:sapphire laser on a GaAs/AlGaAs photonic crystal waveguide patterned with a square lattice, the basis consisting of rings of air in the dielectric matrix. The measured angles of diffracted SH beams agree with those predicted from nonlinear diffraction conditions. Results for reflected and diffracted SH intensities as a function of incidence angle, polarization, and pump wavelength show that, due to the low air fraction of the photonic crystal, the reflected one is dominated by the crystalline symmetry of GaAs, while the diffracted one is related to the photonic crystal structure. The large diffraction-to-reflection ratio points to the importance of nonlinear diffraction in photonic crystals. Preliminary measurements in the 1500nm range reveal explicit features related to photonic modes.

Linear and nonlinear optical characterization of Ga nanoparticle monolayers

Abstract Single layers of quasi-spherical metal nanoparticles of uniform and controlled size were fabricated by evaporation–condensation in ultrahigh vacuum, exploiting the partial wetting of Ga with respect to the underlying SiO x matrix. The net linear and nonlinear optical response of such self-assembled liquid Ga nanoparticles was determined using CW optical methods as well as pulsed laser excitation. Second, harmonic generation was observed in transmission using femtosecond laser pulses. The fluence, polarization and azimuth angle dependence as well as the second-harmonic (SH) signal dependence on particle size are presented and discussed.

Thermal effects on optical performances of a prefocussing mirror for high-resolution soft-X-ray beamlines

Abstract Heating of grazing incidence mirror surfaces by absorbed soft X-rays from a synchrotron beamline has been studied numerically to evaluate resultant displacements of the thermally loaded surface. Subsequent ray-tracing analysis has provided the corresponding change in optical performances. This procedure has been applied in particular to a thermally loaded prefocussing ellipsoidal mirror to be used in a high-spectral-resolution soft-X-ray beamline in the U2 undulator beamline of ELETTRA, and ultrahigh brightness synchrotron radiation facility under construction in Trieste. Various cooling geometries have been investigated. A detailed analysis of the results indicates deterioration of imaging properties beyond the present fabrication tolerances only at extreme irradiation levels.

Nonlinear optical studies of one-dimensional SOI photonic crystal slabs

We report on third-harmonic (TH) generation emitted from 1D photonic slabs etched into Silicon-on-Insulator (SOI) planar waveguides, as compared to the bare waveguide and (100) Silicon bulk responses. 130-fs laser pulses at ~ 810 nm and ~1550 nm have been chosen as a pump to excite TH signals in reflection and diffraction directions. The measured angles of in-plane diffracted third-harmonic beams agree with those predicted by nonlinear diffraction equations. The nonlinear reflectance as a function of the angle of incidence and azimuthal orientation of the structure has been measured. The near-infrared measurements have revealed that, whenever the pump frequency is resonant with a photonic mode, a substantial enhancement of the harmonic signal occurs. This nonlinear mechanism is in principle a very sensitive spectroscopic tool in determining and mapping the photonic band diagram of the system above the light line. The agreement between experimental data and ad hoc simulations of the nonlinear behavior of the system sheds new light on the nonlinear optical response of these nanostructured materials.