Mesoscopic Limit Cycles in Coupled Nanolasers.

Two coupled nanolasers exhibit a mode switching transition, theoretically described by mode beating limit cycle oscillations. Their decay rate is vanishingly small in the thermodynamic limit, i.e., when the spontaneous emission noise tends to zero. We provide experimental statistical evidence of mesoscopic limit cycles (∼10^{3} intracavity photons). Specifically, we show that the order parameter quantifying the limit cycle amplitude can be reconstructed from the mode intensity statistics. We observe a maximum of the averaged amplitude at the mode switching, accounting for limit cycle oscillations. We finally relate this maximum to a dip of mode cross-correlations, reaching a minimum of g_{ij}^{(2)}=2/3, which we show to be a mesoscopic limit. Coupled nanolasers are thus an appealing test bed for the investigation of spontaneous breaking of time translation symmetry in the presence of strong quantum fluctuations.

Improving image contrast in fluorescence microscopy with nanostructured substrates.

Metallic and dielectric nanostructures can show sharp contrasted resonances, sensitive to the environment, and high field enhancement in sub-wavelength volumes. For this reason, these structures are commonly used as molecular sensors. Only few works have focused on their application in optical microscopy, in particular in superresolution. In this work we have designed, fabricated and optically tested a nanostructured TiO(2) substrate, fabricated by direct embossing of TiO(2) derived film, as a substrate for fluorescence microscopy. Moreover, using numerical simulations, we have compared the signal to background noise with respect to other metallo-dielectric structures. We show that the TiO(2) structure is a good candidate for reducing the thickness of the fluorescence excitation down to ~100 nm. Therefore, this substrate can be used to obtain Total Internal Reflection (TIRF) axial resolution without a TIRF-Microscopy system.

Tracking exceptional points above the lasing threshold.

Recent studies on exceptional points (EPs) in non-Hermitian optical systems have revealed unique traits, including unidirectional invisibility, chiral mode switching and laser self-termination. In systems featuring gain/loss components, EPs are commonly accessed below the lasing threshold, i.e., in the linear regime. In this work, we experimentally demonstrate that EP singularities in coupled semiconductor nanolasers can be accessed above the lasing threshold, where they become branch points of a nonlinear dynamical system. Contrary to the common belief that unavoidable cavity detuning impedes the formation of EPs, here we demonstrate that such detuning is necessary for compensating the carrier-induced frequency shift, hence restoring the EP. Furthermore, we find that the pump imbalance at lasing EPs varies with the total pump power, enabling their continuous tracking. This work uncovers the unstable nature of EPs above laser threshold in coupled semiconductor lasers, offering promising opportunities for the realization of self-pulsing nanolaser devices and frequency combs.

Excitability in a semiconductor laser with saturable absorber.

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.

Metasurface-based total internal reflection microscopy.

Recent years have seen a tremendous progress in the development of dielectric metasurfaces for visible light applications. Such metasurfaces are ultra-thin optical devices that can manipulate optical wavefronts in an arbitrary manner. Here, we present a newly developed metasurface which allows for coupling light into a microscopy coverslip to achieve total internal reflection (TIR) excitation. TIR fluorescence microscopy (TIRFM) is an important bioimaging technique used specifically to image cellular membranes or surface-localized molecules with high contrast and low background. Its most commonly used modality is objective-type TIRFM where one couples a focused excitation laser beam at the edge of the back focal aperture of an oil-immersion objective with high numerical aperture (N.A.) to realize a high incident-angle plane wave excitation above the critical TIR angle in sample space. However, this requires bulky and expensive objectives with a limited field-of-view (FOV). The metasurface which we describe here represents a low cost and easy-to-use alternative for TIRFM. It consists of periodic 2D arrays of asymmetric structures fabricated in TiO2 on borosilicate glass. It couples up to 70% of the incident non-reflected light into the first diffraction order at an angle of 65° in glass, which is above the critical TIR angle for a glass-water interface. Only ∼7% of the light leaks into propagating modes traversing the glass surface, thus minimizing any spurious background fluorescence originating far outside the glass substrate. We describe in detail design and fabrication of the metasurface, and validate is applicability for TIRFM by imaging immunostained human mesenchymal stem cells over a FOV of 200 µm x 200 µm. We envision that these kinds of metasurfaces can become a valuable tool for low-cost and TIRFM, offering high contrast, low photodamage, and high surface selectivity in fluorescence excitation and detection.

Dynamics of band-edge photonic crystal lasers.

Band-edge photonic crystal lasers were fabricated and their temporal characteristics were minutely analyzed using a high resolution up-conversion system. The InGaAs/InP photonic crystal laser operates at room temperature at 1.55 microm with turn on time ranging from 17ps to 30ps.

Coupled optical excitable cells.

In this work we investigate experimentally the dynamics of two coupled optical excitable cells, namely, two semiconductor lasers with optical feedback. We analyze the dynamics observed in terms of the statistical properties of the time series and in terms of the phase space reconstruction from the data. We build a model based on a simple set of deterministic equations (on a two torus) plus noise in order to capture the essential features of the dynamics observed. We discuss the validity of our theoretical results in terms of families of excitable systems and coupling terms.