Multi-cavity photonic systems, known as photonic molecules (PMs), are ideal multi-well potential building blocks for advanced quantum and nonlinear optics [1][2][3][4]. A key phenomenon arising in double well potentials is the spontaneous breaking of the inversion symmetry, i.e. a transition from a delocalized to two localized states in the wells, which are mirror images of each other. Although few theoretical studies have addressed mirror-symmetry breaking in micro and nanophotonic systems [5][6][7][8][9], no experimental evidence has been reported to date. Thanks to the potential barrier engineering implemented here, we demonstrate spontaneous mirror-symmetry breaking through a pitchfork bifurcation in a PM composed of two coupled photonic crystal nanolasers. Coexistence of localized states is shown by switching them with short pulses. This offers exciting prospects for the realization of ultra-compact, integrated, scalable optical flip-flops based on spontaneous symmetry breaking. Furthermore, we predict such transitions with few intracavity photons for future devices with strong quantum correlations.Spontaneous symmetry breaking (SSB) unifies diverse physical mechanisms through which a given symmetric system ends up in an asymmetric state [10]. It explains many central questions from particle and atomic physics to nonlinear optics (the Goldstone boson and the Higgs mechanism [11,12], phase transitions in BoseEinstein condensates -BECs- [13,14], metamaterials [15], bifurcations in lasers [16,17], photorrefractive media [18], to mention just a few). A paradigmatic symmetry in this context is given by reflection in a double-well potential (DWP), as it is the case of pyramidal molecules (e.g. ammonia) [19]: SSB dictates whether the state of a system will be delocalized or, in turn, confined within either well. In photonics, such a mechanism is possible provided the third order nonlinearities overcome photon tunneling [20]. In this work we experimentally show SSB in a photonic molecule (PM) given by two evanescently coupled photonic crystal (PhC) nanolasers. Switchable localized modes with broken mirror-symmetry will be demonstrated herein. This can be prospected as a nanoscale version of a laser flip-flop [21]; the memory is pumped incoherently, set and reset can be induced with positive pulses and there is no coherent driving beam to bias the device, as in conventional bistable cavities. This paves the way for the realization of ultra-small flip-flop optical memories based on SSB.We represent the PM as a DWP, symmetric with respect to the inversion plane. We describe the dynamics in terms of the complex amplitudes of the photonic field at the left (ψ L ) and right (ψ R ) sites, |ψ| 2 being photon number. A finite potential barrier leads to a tunneling rate K. We further consider a local (nonlinear) interaction U |ψ L,R | 2 , and a lifetime τ due to losses. SSB instabilities occur as long as K is lower than a critical value K c (|K| < |K c |), with |K c τ | ∼ |U | · |ψ| 2 [22]. In the case of our PM laser, |ψ| 2...

We show that a monolithic and compact vertical cavity laser with intracavity saturable absorber can emit short excitable pulses. These calibrated optical pulses can be excited as a response to an input perturbation whose amplitude is above a certain threshold. Subnanosecond excitable response is promising for applications to novel all-optical devices for information processing or logical gates.

We analyze multi-longitudinal-mode semiconductor lasers experimentally. We show that the intensity of each mode displays large amplitude oscillations but obeys a highly organized antiphase dynamics leading to an almost constant total intensity output. For each mode, regular switching is observed in the megahertz range, while the optical frequency as a function of time follows a well defined sequence from blue to red. Using a multimode theoretical model, we identify that four-wave mixing is the dominant mechanism at the origin of the observed dynamics. The asymmetry of the susceptibility function of semiconductor materials allows us to explain the optical frequency sequence.

We generate an observable which relates the interspike time statistics in a noise driven excitable system with its phase space global properties. Experimental results from a semiconductor laser with optical feedback are analyzed within this framework. PACS numbers: 42.65.Sf, 05.40.Ca, 42.55.Px Escape problems from metastable states are ubiquitous in nature [1]. From biology to physics, several situations are adequately modeled through noise driven equations for which the dynamical output consists of a sequence of spike responses with a more or less complex interspike time distribution [2,3]. In these cases, efforts usually are devoted to the calculation of rate coefficients. Kramers made the seminal contribution to this program. He computed escape rates from both the local properties of the deterministic part of the model in the neighborhood of the metastable state and the noise level [4]. In Ref.[5], pseudoregular oscillations were found in noise driven excitable systems for a specific case: the infinitely dissipative regime. In this work, we analyze the consequences of the global properties of a general excitable system (presenting finite dissipation) in the interspike time distribution of its response to added noise. We find that the interspike distributions present a nontrivial structure. The global properties we refer to are the stable and unstable manifolds of the fixed points of the deterministic part of the model. In particular, we analyze the results of an experiment (a semiconductor laser with optical feedback close to the onset of a regime called low frequency fluctuations) [6][7][8], in terms of a simple model [9].The experimental setup is shown in Fig. 1(a). The diode laser used in our experiment is the single transverse-mode Sharp LT030MD0 (nominal wavelength l 750 nm; solitary laser threshold I th 36.66 mA). The temperature of the laser is stabilized to better than 0.01 ± C. The beam is collimated and directed toward a high reflection mirror (R 99%) located at 50 cm from the laser, which redirects the beam back to it. An antireflection coated lens ( f 25 cm) is placed within the cavity in order to focus the beam into the mirror, which seems to improve the coupling efficiency. The optical feedback strength is controlled by an acusto-optic modulator (AOM) placed inside the cavity, in such a way that a variable amount of light can be removed from the zero order thus reducing the feedback level. The intensity output is detected by a 1 GHz bandwidth photodiode and the signal is analyzed with a HP 54616B 500 MHz digital oscilloscope. In this work, we are interested in the slow dynamics, i.e., time scales much larger than the external cavity round-trip time (t ഠ 3 ns). Actually, the short-time dynamics are washed out by the use of a 30 MHz low-pass filter. Different dynamical scenarios are observed as the parameters (current, feedback) are varied, which are extensively described in the literature (see [6], and references therein).For pump values considerably smaller than the solitary laser threshold t...

We experimentally demonstrate excitability in a semiconductor two-dimensional photonic crystal. Excitability is a nonlinear dynamical mechanism underlying pulselike responses to small perturbations in systems possessing one stable state. We show that a band-edge photonic crystal resonator exhibits class II excitability, resulting from the nonlinear coupling between the high-Q optical mode, the charge-carrier density, and the fast (sub-micros) thermal dynamics. In this context, the critical slowing down of the electro-optical dynamics close to the excitable threshold can delay the optical response by an amount comparable to the duration of the output pulse (5 ns). The latter results from a short thermal dynamical excursion along a high local intensity manifold of the phase space.

Bistability, excitability, and self-pulsing regimes in an InP-based two-dimensional (2D) photonic crystal nanocavity with quantum wells as an active medium are investigated. A resonant cw beam is evanescently coupled into the cavity through a tapered microfiber. In such conditions, we show that the cavity exhibits class II excitability, which arises from the competition between a fast electronic nonlinear effect, given by carrier-induced refractive index change, and slow thermal dynamics. Multiple perturbation-pulse experiments allow us to measure the refractory time ("dead time" between two excitable pulses) of the excitable nanocavity system.

We demonstrate a large tuning of the coupling strength in Photonic Crystal molecules without changing the inter-cavity distance. The key element for the design is the "photonic barrier engineering", where the "potential barrier" is formed by the air-holes in between the two cavities. This consists in changing the hole radius of the central row in the barrier. As a result we show, both numerically and experimentally, that the wavelength splitting in two evanescently-coupled Photonic Crystal L3 cavities (three holes missing in the ΓK direction of the underlying triangular lattice) can be continuously controlled up to 5× the initial value upon ∼ 30% of hole-size modification in the barrier. Moreover, the sign of the splitting can be reversed in such a way that the fundamental mode can be either the symmetric or the anti-symmetric one without altering neither the cavity geometry nor the inter-cavity distance. Coupling sign inversion is explained in the framework of a Fabry-Perot model with underlying propagating Bloch modes in coupled W1 waveguides.

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