G. Vecchi

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.

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.

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.

Enhancement of second harmonic in one-dimensional and two-dimensional epitaxial GaN-based photonic crystals

The second-harmonic field generated has been measured in reflection from the surface of one-dimensional and two-dimensional photonic crystals etched into a GaN layer. A very large second-harmonic enhancement is observed when simultaneously the incident beam at the fundamental frequency w excites a resonant Bloch mode and the second-harmonic field generated is coupled into a resonant Bloch mode at 2w. A smaller, but still substantially enhanced, second-harmonic generation level was also observed when the fundamental field was coupled into a resonant mode, while the second-harmonic field was not. By using calculated and experimental equifrequency surfaces, it is possible to identify the geometrical configurations that will allow quasi-phase matching to be satisfied - and observed experimentally in the available wavelength tuning range of the laser. The extended transparency window of III-nitride wide-bandgap semiconductors, coupled with large non linearities, is an appealing feature pointing towards the control and manipulation of light in photonic structures.

Nonlinear optical studies of photonic crystal waveguides

This paper reports on nonlinear optical investigations of 2D photonic crystals etched into GaAs/AlGaAs waveguides by means of second-harmonic generation (SHG) from the surface of the samples. 130-fs laser pulses from a tunable Ti:sapphire source at 800 nm and from an optical parametric oscillator in the range 1.4 to 1.6 micron have been chosen to excite SH signals in reflection and diffraction directions. The measured angles of diffracted second-harmonic beams agree with those predicted from nonlinear diffraction conditions. The intensities of the reflected and diffracted SH signals are measured by varying the angle of incidence, input polarization and wavelength. Their observed behavior reflects the GaAs crystalline symmetry and the photonic crystal structure, respectively. Moreover, the measured high diffraction-to-reflection intensity ratio suggests the possibility of tailoring the nonlinear response using diffraction modes.

Nonlinear optical response in patterned planar waveguides

Summary form only given. Second and third harmonic signals generated in reflection and diffraction from the surface of photonic crystal (PC) waveguides are considered as a spectroscopic diagnostic tool in two leading material systems: GaAs and Silicon. Measurements of second harmonic generation (SHG) in reflection from GaAs-based 2D patterned planar waveguides reveal resonant interaction of the impinging radiation with the quasi-guided modes of the photonic structure. Spectral and angular dependence of these effects reflects the structure of the sample and may be used for an accurate energy and momentum positioning of the photonic bands. Preliminary results of third harmonic generation (THG) from Si-based 1D patterned planar waveguides in the visible spectral range are also presented, indicating harmonic generation as a possible route towards systematic characterization and description of photonic structures.

Second-harmonic resonances in GaAs/AlGaAs photonic crystal slabs

Two-dimensional photonic crystals etched into GaAs/AlGaAs waveguides have been investigated by means of second-harmonic generation (SHG) from the surface of the samples. 130-fs laser pulses from a tunable Ti:sapphire source at 800 nm and from an optical parametric oscillator in the range 1.4-1.6 micron have been chosen to excite SH signals in reflection and diffraction directions. Their observed behavior reflects the GaAs crystalline symmetry and the photonic crystal structure, respectively.