Guillaume Aoust

Flat Optics: Controlling Wavefronts With Optical Antenna Metasurfaces

Conventional optical components rely on the propagation effect to control the phase and polarization of light beams. One can instead exploit abrupt phase and polarization changes associated with scattered light from optical resonators to control light propagation. In this paper, we discuss the optical responses of anisotropic plasmonic antennas and a new class of planar optical components (“metasurfaces”) based on arrays of these antennas. To demonstrate the versatility of metasurfaces, we show the design and experimental realization of a number of flat optical components: 1) metasurfaces with a constant interfacial phase gradient that deflect light into arbitrary directions; 2) metasurfaces with anisotropic optical responses that create light beams of arbitrary polarization over a wide wavelength range; 3) planar lenses and axicons that generate spherical wavefronts and nondiffracting Bessel beams, respectively; and 4) metasurfaces with spiral phase distributions that create optical vortex beams of well-defined orbital angular momentum.

Giant birefringence in optical antenna arrays with widely tailorable optical anisotropy

The manipulation of light by conventional optical components such as lenses, prisms, and waveplates involves engineering of the wavefront as it propagates through an optically thick medium. A unique class of flat optical components with high functionality can be designed by introducing abrupt phase shifts into the optical path, utilizing the resonant response of arrays of scatterers with deeply subwavelength thickness. As an application of this concept, we report a theoretical and experimental study of birefringent arrays of two-dimensional (V- and Y-shaped) optical antennas which support two orthogonal charge-oscillation modes and serve as broadband, anisotropic optical elements that can be used to locally tailor the amplitude, phase, and polarization of light. The degree of optical anisotropy can be designed by controlling the interference between the waves scattered by the antenna modes; in particular, we observe a striking effect in which the anisotropy disappears as a result of destructive interference. These properties are captured by a simple, physical model in which the antenna modes are treated as independent, orthogonally oriented harmonic oscillators.

Single-mode instability in standing-wave lasers: The quantum cascade laser as a self-pumped parametric oscillator

We report the observation of a clear single-mode instability threshold in continuous-wave Fabry-Perot quantum cascade lasers (QCLs). The instability is characterized by the appearance of sidebands separated by tens of free spectral ranges (FSR) from the first lasing mode, at a pump current not much higher than the lasing threshold. As the current is increased, higher-order sidebands appear that preserve the initial spacing, and the spectra are suggestive of harmonically phase-locked waveforms. We present a theory of the instability that applies to all homogeneously broadened standing-wave lasers. The low instability threshold and the large sideband spacing can be explained by the combination of an unclamped, incoherent Lorentzian gain due to the population grating, and a coherent parametric gain caused by temporal population pulsations that changes the spectral gain line shape. The parametric term suppresses the gain of sidebands whose separation is much smaller than the reciprocal gain recovery time, while enhancing the gain of more distant sidebands. The large gain recovery frequency of the QCL compared to the FSR is essential to observe this parametric effect, which is responsible for the multiple-FSR sideband separation. We predict that by tuning the strength of the incoherent gain contribution, for example by engineering the modal overlap factors and the carrier diffusion, both amplitude-modulated (AM) or frequency-modulated emission can be achieved from QCLs. We provide initial evidence of an AM waveform emitted by a QCL with highly asymmetric facet reflectivities, thereby opening a promising route to ultrashort pulse generation in the mid-infrared. Together, the experiments and theory clarify a deep connection between parametric oscillation in optically pumped microresonators and the single-mode instability of lasers, tying together literature from the last 60 years.

Pump duration optimization for optical parametric oscillators

In this paper we theoretically and experimentally investigate the optimal pulse settings to pump optical parametric oscillators (OPOs) in the transient regime. The theoretical analysis is carried out with an extended OPO rate-equation formalism that yields simple expressions when applied to rectangular pump pulses while fully taking into account pump depletion and back-conversion effects. The OPO buildup time is investigated in detail, leading to a very simple analytical formula. The study then focuses on engineering rules for the choice of the optimal rectangular-pump-pulse power and duration giving the best conversion efficiency for given input pump energy and OPO features. Making the most of a master oscillator-fiber power amplifier’s pulse-tailoring opportunities, experimental investigations covering pulsed operation ranging from nanosecond to microsecond pump duration are carried out in the case of a doubly-resonant OPO. Very good agreement with the theoretical analysis is obtained.

Optimal pump pulse shapes for optical parametric oscillators

In this paper we theoretically investigate the most efficient pump temporal shape for pulsed operation of optical parametric oscillators (OPOs). The theoretical analysis consists of solving the optimal control problem using the rate equation formalism. The OPO optimal buildup behavior is derived for any OPO configuration, and we show that the optimal pump pulse shape is a double rectangle. The first rectangle is optimized for the buildup process, while the second one is optimized for the steady state, and both depend differently on the OPO mirror reflectivities as well as the available pump energy and maximum peak power. This double rectangle is quantitatively described and is consistent with previous results reported in the literature. Also, numerical simulations are used to validate our analytical model on a doubly resonant OPO and show consistent results. The numerical simulations also show that an optimal top-hat temporal profile could provide performances very close to the ones achieved with the optimal double rectangle.

Optical parametric sources for gas sensing applications

We present our activities on the development of narrow linewidth tunable optical parametric sources and their integration in gas sensing instruments. In particular, we have introduced the nested cavity optical parametric oscillator (NesCOPO) scheme that enables to implement very compact devices. The NesCOPO was successfully demonstrated in the microsecond to nanosecond regime and in spectral ranges from short- to long-wave infrared. Its high potential was demonstrated both for local photoacoustic spectroscopy and standoff detection using lidar instruments. We also present our recent advances on rapidly tunable picosecond OPOs based on aperiodic quasi-phase matching and their application to gas detection.

Shape optimization for high quality factor flexural mechanical resonators operated in air

Resonator based sensing devices operated within a fluid medium often require maximal quality factors in order to enhance their performances. We present a tuning fork shape optimization based on an analytical approach, and identify the geometry with the lowest possible losses. We also report on the fabrication of a homemade tuning fork based on this optimization process, and experimental measurements show a quality factor of Q = 40800 in air at atmospheric pressure.

Singly Resonant OPO Pumped by a Pulsewidth-Tunable Hybrid MOPA Source

A singly resonant optical parametric oscillator is pumped by a hybrid master oscillator-power amplifier laser whose pulsewidth is varied from 4 ns to 4 μs, enabling to optimize the pumping parameters for each pulse energy.

Influence of Pump Pulse Duration on Doubly Resonant Optical Parametric Oscillators Build-up Time

A single-frequency doubly resonant optical parametric oscillator is pumped by a master oscillator-fiber power amplifier whose pulse duration is varied for 40 ns to 10 μs, enabling to optimize the pumping parameters.

Nested Cavity Optical Parametric Oscillator (NesCOPO) - A unique approach for gas sensing

Gas sensing applications have promoted huge developments of tunable lasers in the mid-infrared. This presentation will brighten the high potential of NesCOPO devices both for local photoacoustic spectroscopy and for remote detection using lidar instruments.