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Metasurfaces are arrays of subwavelength spaced nanostructures that can manipulate the amplitude, phase, and polarization of light to achieve a variety of optical functions beyond the capabilities of 3D bulk optics. However, they suffer from limited performance and efficiency when multiple functions with large deflection angles are required because the non-local interactions due to optical coupling between nanostructures are not fully considered. Here we introduce a method based on supercell metasurfaces to demonstrate multiple independent optical functions at arbitrary large deflection angles with high efficiency. In one implementation the incident laser is simultaneously diffracted into Gaussian, helical and Bessel beams over a large angular range. We then demonstrate a compact wavelength-tunable external cavity laser with arbitrary beam control capabilities – including beam shaping operations and the generation of freeform holograms. Our approach paves the way to novel methods to engineer the emission of optical sources. The angular dependence is a well-known issue in metasurface engineering. Here the authors introduce a supercell metasurface able to implement multiple independent functions under large deflection angles with high efficiency, leading to a wavelength tunable laser with arbitrary wavefront control.

The terahertz region of the electromagnetic spectrum has been the least utilized owing to inadequacies of available sources. We introduce a compact, widely frequency-tunable, extremely bright source of terahertz radiation: a gas-phase molecular laser based on rotational population inversions optically pumped by a quantum cascade laser. By identifying the essential parameters that determine the suitability of a molecule for a terahertz laser, almost any rotational transition of almost any molecular gas can be made to lase. Nitrous oxide is used to illustrate the broad tunability over 37 lines spanning 0.251 to 0.955 terahertz, each with kilohertz linewidths. Our analysis shows that laser lines spanning more than 1 terahertz with powers greater than 1 milliwatt are possible from many molecular gases pumped by quantum cascade lasers.

High-quality optical ring resonators can confine light in a small volume and store it for millions of roundtrips. They have enabled the dramatic size reduction from laboratory scale to chip level of optical filters, modulators, frequency converters, and frequency comb generators in the visible and the near-infrared. The mid-infrared spectral region (3−12 μm), as important as it is for molecular gas sensing and spectroscopy, lags behind in development of integrated photonic components. Here we demonstrate the integration of mid-infrared ring resonators and directional couplers, incorporating a quantum cascade active region in the waveguide core. It enables electrical control of the resonant frequency, its quality factor, the coupling regime and the coupling coefficient. We show that one device, depending on its operating point, can act as a tunable filter, a nonlinear frequency converter, or a frequency comb generator. These concepts extend to the integration of multiple active resonators and waveguides in arbitrary configurations, thus allowing the implementation of purpose-specific mid-infrared active photonic integrated circuits for spectroscopy, communication, and microwave generation. Multifunctional active mid-infrared ring resonators and directional couplers with quantum cascade laser cores allow electrical control of resonant frequency and quality factors, tunable filtering and frequency comb generation.

Optical resonators enable the generation, manipulation, and storage of electromagnetic waves. The physics underlying their operation is determined by the interference of electromagnetic waves, giving rise to the resonance spectrum. This mechanism causes the limitations and trade-offs of resonator design, such as the fixed relationship between free spectral range, modal linewidth, and the resonator’s refractive index and size. Here, we introduce a new class of optical resonators, generating resonances by designing the optical path through transverse mode coupling in a cascaded process created by mode-converting mirrors. The generalized round-trip phase condition leads to resonator characteristics that are markedly different from Fabry-Perot resonators and can be tailored over a wide range. We confirm the existence of these modes experimentally in an integrated waveguide cavity with mode converters coupling transverse modes into one supermode. We also demonstrate a transverse mode-independent transmission and show that its engineered spectral properties agree with theoretical predictions. Resonators are key components in optics. In this work, the authors introduce a class of optical resonators with distinctly different properties from conventional resonators, allowing fundamental design trade-offs to be circumvented.

Quantum cascade lasers (QCL) have revolutionized the generation of mid-infrared light. Yet, the ultrafast carrier transport in mid-infrared QCLs has so far constituted a seemingly insurmountable obstacle for the formation of ultrashort light pulses. Here, we demonstrate that careful quantum design of the gain medium and control over the intermode beat synchronization enable transform-limited picosecond pulses from QCL frequency combs. Both an interferometric radio-frequency technique and second-order autocorrelation shed light on the pulse dynamics and confirm that mode-locked operation is achieved from threshold to rollover current. Furthermore, we show that both anti-phase and in-phase synchronized states exist in QCLs. Being electrically pumped and compact, mode-locked QCLs pave the way towards monolithically integrated non-linear photonics in the molecular fingerprint region beyond 6 μm wavelength. Producing pulses in the mid-IR often requires bulky sources and has been inaccessible with compact and versatile quantum cascade lasers (QCLs). Here, the authors demonstrate actively mode-locked, mid-IR QCL operation at room temperature.

The purity of an optical vortex beam depends on the spread of its energy among different azimuthal and radial modes, also known as - and -modes. The smaller the spread, the higher the vortex purity and more efficient its creation and detection. There are several methods to generate vortex beams with well-defined orbital angular momentum, but only few exist allowing selection of a pure radial mode. These typically consist of many optical elements with rather complex arrangements, including active cavity resonators. Here, we show that it is possible to generate pure vortex beams using a single metasurface plate—called -plate as it controls radial modes—in combination with a polarizer. We generalize an existing theory of independent phase and amplitude control with birefringent nanopillars considering arbitrary input polarization states. The high purity, sizeable creation efficiency, and impassable compactness make the presented approach a powerful complex amplitude modulation tool for pure vortex generation, even in the case of large topological charges.

Nonlinear interactions in many physical systems lead to symmetry breaking phenomena in which an initial spatially homogeneous stationary solution becomes modulated. Modulation instabilities have been widely studied since the 1960s in different branches of nonlinear physics. In optics, they may result in the formation of optical solitons, localized structures that maintain their shape as they propagate, which have been investigated in systems ranging from optical fibres to passive microresonators. Recently, a generalized version of the Lugiato–Lefever equation predicted their existence in ring quantum cascade lasers with an external driving field, a configuration that enables the bistability mechanism at the basis of the formation of optical solitons. Here, we consider this driven emitter and extensively study the structures emerging therein. The most promising regimes for localized structure formation are assessed by means of a linear stability analysis of the homogeneous stationary solution (or continuous-wave solution). In particular, we show the existence of phase solitons – chiral structures excited by phase jumps in the cavity – and cavity solitons. The latter can be deterministically excited by means of writing pulses and manipulated by the application of intensity gradients, making them promising as frequency combs (in the spectral domain) or reconfigurable bit sequences that can encode information inside the ring cavity.

Geometric arrays of vortices found in various systems owe their regular structure to mutual interactions within a confined system. In optics, such vortex crystals may form spontaneously within a resonator. Their crystallization is relevant in many areas of physics, although their usefulness is limited by the lack of control over their topology. On the other hand, programmable devices like spatial light modulators allow the design of nearly arbitrary vortex distributions but without any intrinsic evolution. By combining non-Hermitian optics with on-demand topological transformations enabled by metasurfaces, we report a solid-state laser that generates 10 × 10 vortex laser arrays with actively tunable topologies and non-local coupling dictated by the array’s topology. The vortex arrays exhibit sharp Bragg diffraction peaks, witnessing their coherence and topological charge purity, which we spatially resolve over the whole lattice by introducing a parallelized analysis technique. By structuring light at the source, we enable complex transformations that allow to arbitrarily partition orbital angular momentum within the cavity and to heal topological charge defects, thus realizing robust and versatile resonators for applications in topological optics.A solid-state laser that generates 10 × 10 vortex laser arrays is demonstrated. The topologies are actively tunable.

Wave instability—the process that gives rise to turbulence in hydrodynamics 1 —represents the mechanism by which a small disturbance in a wave grows in amplitude owing to nonlinear interactions. In photonics, wave instabilities result in modulated light waveforms that can become periodic in the presence of coherent locking mechanisms. These periodic optical waveforms are known as optical frequency combs 2 – 4 . In ring microresonator combs 5 , 6 , an injected monochromatic wave becomes destabilized by the interplay between the resonator dispersion and the Kerr nonlinearity of the constituent crystal. By contrast, in ring lasers instabilities are considered to occur only under extreme pumping conditions 7 , 8 . Here we show that, despite this notion, semiconductor ring lasers with ultrafast gain recovery 9 , 10 can enter frequency comb regimes at low pumping levels owing to phase turbulence 11 —an instability known to occur in hydrodynamics, superconductors and Bose–Einstein condensates. This instability arises from the phase–amplitude coupling of the laser field provided by linewidth enhancement 12 , which produces the needed interplay of dispersive and nonlinear effects. We formulate the instability condition in the framework of the Ginzburg–Landau formalism 11 . The localized structures that we observe share several properties with dissipative Kerr solitons, providing a first step towards connecting semiconductor ring lasers and microresonator frequency combs 13 . Wave destabilization is demonstrated in semiconductor ring lasers operating at low pumping levels, where ultrafast gain recovery leads to the emergence of a frequency comb regime owing to phase turbulence.