M. Belotti

Light scattering and Fano resonances in high-Q photonic crystal nanocavities

The authors show that light scattering from high-Q planar photonic crystal nanocavities can display Fano-like resonances corresponding to the excitation of localized cavity modes. By changing the scattering conditions, we are able to tune the observed lineshapes from strongly asymmetric and dispersivelike resonances to symmetric Lorentzians. Results are interpreted according to the Fano model of quantum interference between two coupled scattering channels. Combined measurements and line shape analysis on a series of silicon L3 nanocavities as a function of nearby hole displacement demonstrate that Q factors as high as 1.1×105 can be directly measured in these structures. Furthermore, a comparison with theoretically calculated Q factors allows to extract the rms deviation of hole radii due to weak disorder of the photonic lattice.

Microfluidic tunable dye laser with integrated mixer and ring resonator

We report on results of design and fabrication of a microfluidic dye laser that consists of a ring resonator, a waveguide for laser emission output, and microfluidic elements for flow control, all integrated on a chip. The optical resonator and the waveguide were obtained by photolithography, whereas microfluidic elements such as channels, valves, and pumps were fabricated by multilayer soft lithography. As results, the prototype device worked with a few nanoliters of Rhodamine 6G dye molecules in ethanol solution and showed a laser threshold of ∼15μJ∕mm2 when optically pumped with a frequency doubled Nd:YAG laser at 532nm wavelength. The modification of the laser output intensity due to photobleaching effect was characterized by changing the dye flow velocity through the cavity. In addition, the emission wavelength of the laser could be easily tuned by changing the dye molecule concentration with the integrated microfluidic elements.

All-optical switching in 2D silicon photonic crystals with low loss waveguides and optical cavities

A study of the optical transmission of low-loss W1.5 photonic crystal waveguides built on silicon membranes and operating at telecom wavelengths is presented. The feasibility of performing all-optical switching is demonstrated for W1.5 waveguides coupled with L3 cavities, systems amenable for incorporation in on-chip devices. Switching of waveguide transmission is achieved by means of optical excitation of free carriers using a 2.5 ns pump laser. Experimental results are reproduced by finite-difference time-domain simulations which model the response of the finite system and band structure calculations describing the infinite, ideal one.

Strong enhancement of Er3+ emission at room temperature in silicon-on-insulator photonic crystal waveguides

We have realized silicon-on-insulator photonic crystal (PhC) waveguides with intense 1.54μm emission at room temperature. The slabs contain a thin layer of SiO2 with Er3+ doped silicon nanoclusters embedded at the center of the Si core and are patterned with a triangular lattice of holes. An enhancement by more than two orders of magnitude of the Er3+ near-normal emission is observed when the transition is in resonance with an appropriate mode of the PhC slab. The results are in very good agreement with calculated photonic bands and emission spectra. These findings are important for the realization of Si-compatible efficient light emitters at telecom wavelengths.

All-optical switching in silicon-on-insulator photonic wire nano–cavities

All-optical switching with a very low power is demonstrated on photonic crystal wire nano-cavities on silicon-on-insulator with large quality factors and high transmission in the telecom range.

Optical response of one-dimensional (Si/SiO2)m photonic crystals

One-dimensional photonic crystals made of (Si/SiO2)m multilayers with m=2,…8 have been grown on SiO2 4-in. wafers by repeated polysilicon low-pressure chemical vapor deposition, oxidation, and wet etching steps. The poly-Si and SiO2 layers were about 220 and 660 nm thick, respectively, thus realizing λ/4 distributed Bragg reflectors. Spectroscopic ellipsometry in the 1.4–5 eV range was used to determine the dielectric function of poly-Si and the actual layer thicknesses, as well as to check the structural and compositional homogeneity of the structures. In order to measure the photonic crystal properties, specular reflectance and transmittance measurements were performed from 0.2 to 6 eV at different angles of incidence θ⩽50° and for transverse electric and transverse magnetic polarizations. The stop-bands characteristic of Bragg reflector multilayers appear up to the fifth order and become more pronounced with increasing m, reaching almost complete rejection for m=4 periods. The experimental spectra were...

Replication of photonic crystals by soft ultraviolet-nanoimprint lithography

Nanoimprint lithography assisted by ultraviolet photopolymerization through a soft elastomer-based mold is applied to the fabrication of silicon-on-insulator slab photonic crystals for optical wavelengths. Variable angular reflectance is used to measure the dispersion of the photonic leaky modes. Experimental results are in good agreement with both theoretical calculations and previous results obtained by standard nanoimprint lithography as well as conventional nanofabrication techniques such as electron-beam lithography.

Measurement of photonic mode dispersion and linewidths in silicon-on-insulator photonic crystal slabs

The dispersion of photonic modes in one-dimensional (1-D) and two-dimensional (2-D) patterned silicon-on-insulator (SOI) waveguides, also containing line defects, is fully investigated both above and below the light line. Quasi-guided (radiative), as well as truly guided modes are probed by means of angle- and polarization-resolved microreflectance and attenuated total reflectance measurements. For the 1-D case, the sharp resonances observed in reflectance spectra are analyzed in terms of the Airy-Fano model, and the measured linewidths are shown to be very close to theoretical predictions. In the 2-D lattices containing W1 line defects the presence of a supercell repetition leads to the simultaneous excitation of defect and bulk modes which are folded in a reduced Brillouin zone. The measured dispersion is in very good agreement with full three-dimensional calculations based on expansion on the waveguide modes, indicating that a deep understanding of the propagation properties of patterned SOI waveguides is achieved.

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.

Soft nanoimprint lithography

We developed a UV assisted soft nanoimprint lithography (UV-SNIL) that can be applied for the reproduction of nanometer features over large areas. Based on a simple argument deduced from the Navier-Stokes equation, we suggest several solutions to enhance the imprinting process ability. One of the solutions is to use tri-layer soft stamps, which consists of a rigid carrier, a low Youngs module buffer and a top layer supporting nanostructure patterns to be replicated. Typically, the buffer and the top layer are made of polydimethylsiloxane (PDMS) of 5 mm thickness and polymethylmetacrylate (PMMA) of 10-50 μm thickness respectively. Patterning of the stamp top layer can be done in three different ways, i.e., spin coating, nano-compression and direct writing, all resulting in 100 nm features over a large wafer area. Another solution is to use a bilayer resist system for which imprinting is performed on the top layer while the final pattern is obtained by transferring the top layer image into the bottom layer by reactive ion etching. Comparing to other imprint techniques, UV-SNIL works at room temperature and low pressure, which is applicable for a wafer-scale replication at high throughput. For the research purpose, we also demonstrate nanostructure fabrication by lift-off techniques.