Integrated nanolasers via complex engineering of radiationless states

The development of compact and energy-efficient miniaturised lasers is a critical challenge in integrated non-linear photonics. Current research focuses on the integration of subwavelength all-dielectric lasers in CMOS compatible platforms. These systems provide a viable alternative to state-of-the-art nanoplasmonic sources, whose practicality is often hindered by high metal losses. The efficiency of dielectric nanolasers, however, is affected by the diffraction limit of light, which restricts the degree of localisation achievable with standard resonator modes. The recent development of new types of radiationless states has brought a sharp innovation in the field of subwavelength dielectric lasers. Radiationless states are exotic electromagnetic solutions that originate from the complex superposition and interaction of several resonator modes. They are associated with a high degree of near-field localisation which makes them particularly advantageous for non-linear photonics applications. In this work, we provide an overview of the most recent theoretical and experimental efforts toward the development of integrated lasers and ultrafast sources based on the amplification of exotic radiationless states. In particular, we focus our attention on two specific types of radiationless states: optical anapoles and Bound States in the Continuum (BIC). By discussing their differences and similarities, we provide a unifying view of these distinct research areas and outline possible future directions for these innovative platforms

Time-resolved nonlinear ghost imaging: route to hyperspectral single-pixel reconstruction of complex samples at THz frequencies

Terahertz (THz) is an innovative form of electromagnetic radiation providing unique spectroscopy capabilities in critical fields, ranging from biology to material science and security. The limited availability of high-resolution imaging devices, however, constitutes a major limitation in this field. In this work, we tackle this challenge by proposing an innovative type of time-space nonlinear Ghost-Imaging (GI) methodology that conceptually outperforms established single-pixel imaging protocols. Our methodology combines nonlinear pattern generation with time-resolved single-pixel measurements, as enabled by the state-of-the-art Time-Domain Spectroscopy (TDS) technique. This approach is potentially applicable to any wave-domain in which the field is a measurable quantity. The full knowledge of the temporal evolution of the transmitted field enables devising a new form of full-wave reconstruction process. This gives access not only to the morphological features of the sample with deeply subwavelength resolution but also to its local spectrum (hyperspectral imaging). As a target application, we consider hyperspectral THz imaging of a disordered inhomogeneous sample

Hyperspectral terahertz microscopy via nonlinear ghost-imaging

Ghost-imaging, based on single-pixel detection and multiple pattern illumination, is a crucial investigation tool in difficult-to-access wavelength regions. In the terahertz domain, where high-resolution imagers are mostly unavailable, Ghost-imaging is an optimal approach to embed the temporal dimension, creating a ‘hyperspectral’ imager. In this framework high-resolution is mostly out-of-reach. Hence, it is particularly critical to developing practical approaches for microscopy. Here we experimentally demonstrate Time-Resolved Nonlinear Ghost-Imaging, a technique based on near-field, optical-to-terahertz nonlinear conversion and detection of illumination patterns. We show how space-time coupling affects near-field time-domain imaging and we develop a complete methodology that overcomes fundamental systematic reconstruction issues. Our theoretical-experimental platform enables high-fidelity subwavelength imaging and carries relaxed constrains on the nonlinear generation crystal thickness. Our work establishes a rigorous framework to reconstruct hyperspectral images of complex samples inaccessible through standard fixed-time methods

Route to intelligent imaging reconstruction via terahertz nonlinear ghost imaging

Terahertz (THz) imaging is a rapidly emerging field, thanks to many potential applications in diagnostics, manufacturing, medicine and material characterisation. However, the relatively coarse resolution stemming from the large wavelength limits the deployment of THz imaging in micro- and nano-technologies, keeping its potential benefits out-of-reach in many practical scenarios and devices. In this context, single-pixel techniques are a promising alternative to imaging arrays, in particular when targeting subwavelength resolutions. In this work, we discuss the key advantages and practical challenges in the implementation of time-resolved nonlinear ghost imaging (TIMING), an imaging technique combining nonlinear THz generation with time-resolved time-domain spectroscopy detection. We numerically demonstrate the high-resolution reconstruction of semi-transparent samples, and we show how the Walsh–Hadamard reconstruction scheme can be optimised to significantly reduce the reconstruction time. We also discuss how, in sharp contrast with traditional intensity-based ghost imaging, the field detection at the heart of TIMING enables high-fidelity image reconstruction via low numerical-aperture detection. Even more striking—and to the best of our knowledge, an issue never tackled before—the general concept of “resolution” of the imaging system as the “smallest feature discernible” appears to be not well suited to describing the fidelity limits of nonlinear ghost-imaging systems. Our results suggest that the drop in reconstruction accuracy stemming from non-ideal detection conditions is complex and not driven by the attenuation of high-frequency spatial components (i.e., blurring) as in standard imaging. On the technological side, we further show how achieving efficient optical-to-terahertz conversion in extremely short propagation lengths is crucial regarding imaging performance, and we propose low-bandgap semiconductors as a practical framework to obtain THz emission from quasi-2D structures, i.e., structure in which the interaction occurs on a deeply subwavelength scale. Our results establish a comprehensive theoretical and experimental framework for the development of a new generation of terahertz hyperspectral imaging devices

Thermo-optical pulsing in a microresonator filtered fiber-laser: a route towards all-optical control and synchronization

We report on 'slow' pulsing dynamics in a silica resonator-based laser system: by nesting a high-Q rod-resonator inside an amplifying fiber cavity, we demonstrate that trains of microsecond pulses can be generated with repetition rates in the hundreds of kilohertz. We show that such pulses are produced with a period equivalent to several hundreds of laser cavity roundtrips via the interaction between the gain dynamics in the fiber cavity and the thermo-optical effects in the high-Q resonator. Experiments reveal that the pulsing properties can be controlled by adjusting the amplifying fiber cavity parameters. Our results, confirmed by numerical simulations, provide useful insights on the dynamical onset of complex self-organization phenomena in resonator-based laser systems where thermo-optical effects play an active role. In addition, we show how the thermal state of the resonator can be probed and even modified by an external, counter-propagating optical field, thus hinting towards novel approaches for all-optical control and sensing applications

Nonlocal bonding of a soliton and a blue-detuned state in a microcomb laser

Abstract Laser cavity-solitons can appear in a microresonator-filtered laser when judiciously balancing the slow nonlinearities of the system. Under certain conditions, such optical states can be made to self-emerge and recover spontaneously, and the understanding of their robustness is critical for practical applications. Here, we study the formation of a bonded state comprising a soliton and a blue-detuned continuous wave, whose coexistence is mediated by dispersion in the nonlinear refractive index. Our real-time dispersive Fourier transform measurements, supported by comprehensive theoretical analysis, reveal the presence of an elastic bonding between the two states, resulting in an enhancement of the soliton’s robustness