Metamateriales plasmónicos

Optomechanics

The simultaneous confinement and enhanced interaction of photons and phonons in periodic nanostructures has become a hot-topic in recent years giving rise to the field known as optomechanics. We employ photonic crystals(PCSs) built on suspended-silicon slabs as periodic optomechanical nanostructures to check the interaction between light and sound at the nanoscale.

PCSs consist of a periodic lattice of holes perforating a high-index semiconductor film so that light confinement in the film is achieved and the periodicity gives rise to bandgaps to forbid guided-light propagation. The introduction of line defects in PCSs has become a powerful way to create light waveguides at the nanoscale with some special properties such as lossless sharp bending or slow-light propagation. Some studies show that square- and honeycomb-lattice suspended PCSs possess bandgaps as much for photons as for acoustic waves. If such PCSs are designed to present bandgaps for guided photons at optical communication wavelengths, bandgaps for phonons appear at frequencies of some gigahertzes. Therefore, these structures can become an important platform for optomechanics as well as for demonstration of other interesting acousto-optical effects such as stimulated Brillouin scattering at the nanoscale.

Figure 1 Figure 2

SEM image of a suspended silicon photonic crystal (left), SEM image of a cavity created in a 2D square-lattice silicon phoxonic crystal slab (right)

Results

First demonstration of a silicon optomechanical crystal with a full phononic bandgap

The field of cavity optomechanics has recently attracted a lot of interest. In an optomechanical cavity, photons and phonons are strongly confined so their interaction is extremely enhanced. To avoid phonon leakage, it would be desirable to prevent any phonon loss pathways, which can be done by engineering an optomechanical crystal displaying a full phononic bandgap for all mode symmetries. In a collaboration with our colleagues at ICN (Barcelona, Spain) and at IEMN (Lille, France), the first experimental demonstration of an optomechanical cavity supporting a full phononic bandgap has been reported [1]. The figures below show a SEM image of a fabricated sample and a RF sectrum of the transduced mechanical modes. Such cavities could play a key role in cavity optomechanics, as they could improve the mechanical Q factor of mechanical modes.

SEM image

SEM image of a 1D optomechanical crystal with a full phononic bandgap at frequencies around 4 GHz

Transduced mechanical modes and Theoretically calculated OM

(Top) Transduced mechanical modes observed in the detected RF spectrum. (Bottom) Theoretically calculated OM coupling for all the observed modes

References

[1] "A 1D Optomechanical crystal with a complete phononic band gap", J. Gomis-Bresco, D. Navarro-Urrios, M. Oudich, S. El-Jallal, A. Griol, D. Puerto, E. Chavez, Y. Pennec, B. Djafari-Rouhani, F. Alzina, A. Martínez, C. M. Sotomayor-Torres. Pre-print at http://arxiv.org/abs/1401.1691. Accepted in Nature Communications.

Towards the silicon laser via phonon confinement

Recent theoretical studies have shown that the free-carrier absorption is much higher than the optical gain at ambient temperature in silicon, even if a high-Q optical cavity is formed. We have theoretically shown that the optical gain can surpass the free-carrier absorption if the cavity is designed to localize the phonons involved in the transition process [1]. The figure below sketches the acousto-optical cavity that we have considered in our study. The acousto-optical cavity confinement effect on the light emission properties has been characterized by a compound Purcell factor which includes both the optical as well as the acoustic Purcell factor (APF). A theoretical expression for the APF has also been introduced. Our theoretical results suggest that creating an acousto-optical cavity the optical gain can overcome the photon loss due to free carriers as a consequence of the localization of phonons even at room temperature, paving the way towards the pursued silicon laser.

Figure 5 Figure 6

References

[1] "Optical gain by simultaneous photon and phonon confinement in indirect bandgap semiconductor acousto-optical cavities", J. M. Escalante and A. Martínez, OPTICAL AND QUANTUM ELECTRONICS, Vol. 45, Issue 10, pp. 1045-1056, October 2013.

Demonstration of light confinement in waveguides and cavities created on silicon photonic crystal membranes with a honeycomb and square lattices

We report on the experimental measurements of light guiding through waveguides created square and honeycomb lattices photonic crystal membranes. The dimensions of the fabricated structures are chosen to provide a "phoxonic" bandgap with a photonic gap around 1550 nm. For both kinds of lattice, we observe a high-transmission band when introducing a linear defect, although it is observed for TM polarization in the honeycomb lattice and for TE in the square. Using the plane-wave expansion and the finite element methods we demonstrate that the guided modes are below the light line and, therefore, without additional losses beside fabrication imperfections [1].

Figure 7

Light transmission along waveguides in honeycomb photonic crystals. (a) Calculated photonic bands; Measured normalized spectra for TM-polarized light and for different values of d and PCS waveguide lengths. (b) d=A (black), d=1.2A (red), d=1.4A (green), d=1.6A (blue). The grey curve corresponds to a perfect crystal but with incidence along the ΓX direction. The total length of the PCS region (with or without waveguide) is 10 µm; (c) Waveguides with d=1.6A and total length of 10 µm (red), 30 µm (green), 50 µm (blue), 100 µm (brown), 150 µm (orange), and 200 µm (violet). The arrows highlight the observed transmission peaks in the 50 µm waveguide response.

We also report on the experimental measurements of light confinement in cavities created on two-dimensional square-lattice silicon "phoxonic" (or optomechanical) crystal membranes [2]. Cavities are created by removing N holes in the transversal direction. In the transmission results appear a peak inside the PBG corresponding to the excitation of cavity modes. Computations of the N=3 cavity show three localized photonic modes around 1550 nm and four localized phononic modes around 6 GHz.

Figure 8

Our results lead us to conclude that waveguides and cavities implemented in honeycomb and square lattice "phoxonic" crystals and are a very suitable platform to observe an enhanced interaction between propagating photons and phonons.

References

[1] "Honeycomb Photonic Crystal Waveguides in a Suspended Silicon Slab", D. Puerto; A. Griol; J. M. Escalante; Y. Pennec; B. Djafari Rouhani; J.-C. Beugnot; V. Laude; A. Martínez, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 24, Issue 22, pp. 2056-2059, November 2012.

[12] "Experimental demonstration of photonic confinement suspended square-lattice silicon photonic crystal cavities", D. Puerto; A. Griol; J. M. Escalante; B. Djafari Rouhani; Y. Pennec; V. Laude; J.-C. Beugnot; A. Martínez;, III Conferencia Española de Nanofótonica - CEN2012.