High-Efficiency Coupling of Free Electrons to Sub-λ3 Modal Volume, High-Q Photonic Cavities.

We report on the design, realization, and experimental investigation by spatially resolved monochromated electron energy loss spectroscopy (EELS) of high-quality-factor cavities with modal volumes smaller than λ3, with λ being the free-space wavelength of light. The cavities are based on a slot defect in a 2D photonic crystal slab made up of silicon. They are optimized for high coupling of electrons accelerated to 100 kV to quasi-transverse electrical modes polarized along the slot direction. We studied the cavities in two geometries and took advantage of the deep sub-optical wavelength spatial resolution of the electron microscope and high spectral resolution of the monochromator to comprehensively describe the optical excitations of the slab. The first geometry, for which the cavities have been designed, corresponds to an electron beam traveling along the slot direction. The second consists of the electron beam traveling perpendicular to the slab. In both cases, a large series of modes is identified. The dielectric slot mode energies are measured to be in the 0.8-0.85 eV range, as per design, and surrounded by two bands of dielectric and air modes of the photonic structure. The dielectric even slot modes, to which the cavity mode belongs, are highly coupled to the electrons with up to 3.2% probability of creating a slot photon per incident electron. Although the experimental spectral resolution (around 30 meV) alone does not allow to disentangle cavity photons from other slot photons, the excellent agreement between the experiments and finite-difference time-domain simulations allows us to deduce that among the photons created in the slot, around 30% are stored in the cavity mode. A systematic study of the energy and coupling strength as a function of the photonic band gap parameters permits us to foresee an increase of coupling strength by fine-tuning phase-matching. Our work demonstrates free electron coupling to high-quality-factor cavities with low mode densities and sub-λ3 modal volumes, making it an excellent candidate for applications such as quantum nano-optics with free electrons.

Möbius Strip Microlasers: A Testbed for Non-Euclidean Photonics.

We report on experiments with Möbius strip microlasers, which were fabricated with high optical quality by direct laser writing. A Möbius strip, i.e., a band with a half twist, exhibits the fascinating property that it has a single nonorientable surface and a single boundary. We provide evidence that, in contrast to conventional ring or disk resonators, a Möbius strip cavity cannot sustain whispering gallery modes (WGM). Comparison between experiments and 3D finite difference time domain (FDTD) simulations reveals that the resonances are localized on periodic geodesics.

Monolithic integration of ultraviolet microdisk lasers into photonic circuits in a III-nitride-on-silicon platform.

Ultraviolet microdisk lasers are integrated monolithically into photonic circuits using a III-nitride-on-silicon platform with gallium nitride (GaN) as the main waveguide layer. The photonic circuits consist of a microdisk and a pulley waveguide, terminated by out-coupling gratings. In this Letter, we measure quality factors up to 3500 under continuous-wave excitation. Lasing is observed from 374 to 399 nm under pulsed excitation, achieving low-threshold energies of 0.14mJ/cm2 per pulse (threshold peak powers of 35kW/cm2). A large peak-to-background dynamic of around 200 is observed at the out-coupling grating for small gaps of 50 nm between the disk and the waveguide. These devices operate at the limit of what can be achieved with GaN in terms of operation wavelength.

Demonstration of critical coupling in an active III-nitride microdisk photonic circuit on silicon.

On-chip microlaser sources in the blue constitute an important building block for complex integrated photonic circuits on silicon. We have developed photonic circuits operating in the blue spectral range based on microdisks and bus waveguides in III-nitride on silicon. We report on the interplay between microdisk-waveguide coupling and its optical properties. We observe critical coupling and phase matching, i.e. the most efficient energy transfer scheme, for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the spontaneous emission regime. Whispering gallery mode lasing is demonstrated for a wide range of parameters with a strong dependence of the threshold on the loaded quality factor. We show the dependence and high sensitivity of the output signal on the coupling. Lastly, we observe the impact of processing on the tuning of mode resonances due to the very short coupling distances. Such small footprint on-chip integrated microlasers providing maximum energy transfer into a photonic circuit have important potential applications for visible-light communication and lab-on-chip bio-sensors.

Silicon-Phosphorene Nanocavity-Enhanced Optical Emission at Telecommunications Wavelengths.

Generating and amplifying light in silicon (Si) continues to attract significant attention due to the possibility of integrating optical and electronic components in a single material platform. Unfortunately, silicon is an indirect band gap material and therefore an inefficient emitter of light. With the rise of integrated photonics, the search for silicon-based light sources has evolved from a scientific quest to a major technological bottleneck for scalable, CMOS-compatible, light sources. Recently, emerging two-dimensional materials have opened the prospect of tailoring material properties based on atomic layers. Few-layer phosphorene, which is isolated through exfoliation from black phosphorus (BP), is a great candidate to partner with silicon due to its layer-tunable direct band gap in the near-infrared where silicon is transparent. Here we demonstrate a hybrid silicon optical emitter composed of few-layer phosphorene nanomaterial flakes coupled to silicon photonic crystal resonators. We show single-mode emission in the telecommunications band of 1.55 µm ( Eg = 0.8 eV) under continuous wave optical excitation at room temperature. The solution-processed few-layer BP flakes enable tunable emission across a broad range of wavelengths and the simultaneous creation of multiple devices. Our work highlights the versatility of the Si-BP material platform for creating optically active devices in integrated silicon chips.

Nonlinearities in GaAs cavities with high CW input powers enabled by photo-oxidation quenching through ALD encapsulation.

We demonstrate that conformal encapsulation using atomic layer deposition of GaAs nano-cavity resonator made of photonic crystal cavity prevents photo-induced oxidation. This improvement allows injecting a large quantity of energy in the resonator without any degradation of the material, thus enabling spectral stability of the resonance. We prove second harmonic and third harmonic generation over more than one decade of pump power variation, thanks to this encapsulation, with a total efficiency (ηSHG = 8.3 × 10-5 W-1 and ηTHG = 1.2 × 10-3 W-2 ) and a large net output energy for both operations (PSHGout=0.2nW and PTHGout=8pW).

High-performance and power-efficient 2×2 optical switch on Silicon-on-Insulator.

A compact (15µm × 15µm) and highly-optimized 2×2 optical switch is demonstrated on a CMOS-compatible photonic crystal technology. On-chip insertion loss are below 1 dB, static and dynamic contrast are 40 dB and >20 dB respectively. Owing to efficient thermo-optic design, the power consumption is below 3 mW while the switching time is 1 µs.

High-frequency self-induced oscillations in a silicon nanocavity.

We show that self-induced oscillations at frequencies above GHz and with a high spectral purity can be obtained in a silicon photonic crystal nanocavity under optical pumping. This self-pulsing results from the interplay between the nonlinear response of the cavity and the photon cavity lifetime. We provide a model to analyze the mechanisms governing the onset of self-pulsing, the amplitudes of both fundamental and harmonic oscillations and their dependences versus input power and oscillation frequency. Theoretically, oscillations at frequencies higher than 50 GHz could be achieved in this system.

Schottky MSM junctions for carrier depletion in silicon photonic crystal microcavities.

Collection of free carriers is a key issue in silicon photonics devices. We show that a lateral metal-semiconductor-metal Schottky junction is an efficient and simple way of dealing with that issue in a photonic crystal microcavity. Using a simple electrode design, and taking into account the optical mode profile, the resulting carrier distribution in the structure is calculated. We show that the corresponding effective free carrier lifetime can be reduced by 50 times when the bias is tuned. This allows one to maintain a high cavity quality factor under strong optical injection. In the fabricated structures, carrier depletion is correlated with transmission spectra and directly visualized by Electron Beam Induced Current pictures. These measurements demonstrate the validity of this carrier extraction principle. The design can still be optimized in order to obtain full carrier depletion at a smaller energy cost.

High quality factor in a two-dimensional photonic crystal cavity on silicon-on-insulator.

We propose a design for high quality factor two-dimensional (2D) photonic crystal cavities on silicon-on-insulator (SOI). A quality factor of up to 1.2×10(7) with a modal volume of 2.35(λ/n)(3) is simulated. A very high quality factor of 200,000 is experimentally demonstrated for a 2D cavity fabricated on SOI.

All-silicon photonic crystal photoconductor on silicon-on-insulator at telecom wavelength.

We demonstrate an all-silicon photodetector working at telecom wavelength. The device is a simple metal-semiconductor-metal detector fabricated on silicon-on-insulator. A two-dimensional photonic crystal nanocavity (Q=60,000) is used to increase the response that arises from the linear and two-photon absorption of silicon. The responsivity of the detector is about 20 mA/W and its bandwidth is larger than 1 GHz.

Enhanced spontaneous Raman scattering in silicon photonic crystal waveguides on insulator.

We study the spontaneous Raman scattering in a W1 photonic crystal waveguide on silicon-on-insulator where the lower silica cladding remains. Despite the vertical asymmetry that exists in such a waveguide, we numerically and experimentally show that the propagation losses at the pump and the Stokes wavelengths remain low enough to allow a significant exaltation of the spontaneous Raman scattering. In particular, we observe a reshaping of the Raman spectrum and a more than ten-fold enhancement of the Raman scattering efficiency in a W1 photonic crystal waveguide as compared to a single-mode ridge waveguide.

Two-dimensional photonic crystals with large complete photonic band gaps in both TE and TM polarizations.

Photonic crystals exhibiting a photonic band gap in both TE and TM polarizations are particularly interesting for a better control of light confinement. The simultaneous achievement of large band gaps in both polarizations requires to reduce the symmetry properties of the photonic crystal lattice. In this letter, we propose two different designs of two-dimensional photonic crystals patterned in high refractive index thin silicon slabs. These slabs are known to limit the opening of photonic band gaps for both polarizations. The proposed designs exhibit large complete photonic band gaps: the first photonic crystal structure is based on the honey-comb lattice with two different hole radii and the second structure is based on a "tri-ellipse" pattern in a triangular lattice. Photonic band gap calculations show that these structures offer large complete photonic band gaps deltaomega/omega larger than 10% between first and second photonic bands. This figure of merit is obtained with single-mode slab waveguides and is not restricted to modes below light cone.

Discontinuous wavelength super-refraction in photonic crystal superprism.

Experimental results on wavelength-dependent angular dispersion in InGaAsP triangular lattice planar photonic crystals are presented. An abrupt variation of the angular dispersion is observed for TM-polarized waves whose frequencies are comprised between those of the fourth and sixth allowed bands. According to the crystal period, the measured angle of refraction is found to either decrease or increase by 30 degrees within a wavelength range smaller than 30 nm. Experimental results are reproduced well from 2D finite difference time domain calculations. The observed phenomena are interpreted from the coupling of the incident light to different modes of the photonic crystal that travel with different group velocities and propagate in different directions within the crystal. Mode dispersion curves and mode patterns are calculated along with isofrequency curves to support this explanation. The observed discontinuous wavelength super-refraction opens a new approach to the application of superprisms.