All-optical adaptive control of quantum cascade random lasers

In-plane emission manipulation of random optical modes by using a zero-index material

In this work, we have proposed to implement a zero-index material (ZIM) to control the in-plane emission of planar random optical modes while maintaining the intrinsic disordered features. Light propagating through a medium with near-zero effective refractive index accumulates little phase change and is guided to the direction determined by the conservation law of momentum. By enclosing a disordered structure with a ZIM based on all-dielectric photonic crystal (PhC), broadband emission directionality improvement can be obtained. We find the maximum output directionality enhancement factor reaches 30, around 6-fold increase compared to that of the random mode without ZIM. The minimum divergence angle is ∼6° for single random optical mode and can be further reduced to ∼3.5° for incoherent multimode superposition in the far field. Despite the significant directionality enhancement, the random properties are well preserved, and the Q factors are even slightly improved. The method is robust and can be effectively applied to the disordered medium with different structural parameters, e.g., the filling fraction of scatterers, and different disordered structure designs with extended or strongly localized modes. The output direction of random optical modes can also be altered by further tailoring the boundary of ZIM. This work provides a novel and universal method to manipulate the in-plane emission direction as well as the directionality of disordered medium like random lasers, which might enable its on-chip integration with other functional devices.

2D tunable all-solid-state random laser in the visible

A two-dimensional (2D) solid-state random laser emitting in the visible is demonstrated, in which optical feedback is provided by a controlled disordered arrangement of air-holes in a dye-doped polymer film. We find an optimal scatterer density for which threshold is minimum and scattering is the strongest. We show that the laser emission can be red-shifted by either decreasing scatterer density or increasing pump area. We show that spatial coherence is easily controlled by varying pump area. Such a 2D random laser provides with a compact on-chip tunable laser source and a unique platform to explore non-Hermitian photonics in the visible.

Self-Induced Mode-Locking in Electrically Pumped Far-Infrared Random Lasers

Mode locking, the self-starting synchronous oscillation of electromagnetic modes in a laser cavity, is the primary way to generate ultrashort light pulses. In random lasers, without a cavity, mode-locking, the nonlinear coupling amongst low spatially coherent random modes, can be activated via optical pumping, even without the emission of short pulses. Here, by exploiting the combination of the inherently giant third-order χ⁽³⁾ nonlinearity of semiconductor heterostructure lasers and the nonlinear properties of graphene, the authors demonstrate mode-locking in surface-emitting electrically pumped random quantum cascade lasers at terahertz frequencies. This is achieved by either lithographically patterning a multilayer graphene film to define a surface random pattern of light scatterers, or by coupling on chip a saturable absorber graphene reflector. Intermode beatnote mapping unveils self-induced phase-coherence between naturally incoherent random modes. Self-mixing intermode spectroscopy reveals phase-locked random modes. This is an important milestone in the physics of disordered systems. It paves the way to the development of a new generation of miniaturized, electrically pumped mode-locked light sources, ideal for broadband spectroscopy, multicolor speckle-free imaging applications, and reservoir quantum computing.

Coalescence of Anderson-localized modes at an exceptional point in 2D random media

In non-Hermitian settings, the particular position at which two eigenstates coalesce in the complex plane under a variation of a physical parameter is called an exceptional point. An open disordered system is a special class of non-Hermitian system, where the degree of scattering directly controls the confinement of the modes. Herein a non-perturbative theory is proposed which describes the evolution of modes when the permittivity distribution of a 2D open dielectric system is modified, thereby facilitating to steer individual eigenstates to such a non-Hermitian degeneracy. The method is used to predict the position of such an exceptional point between two Anderson-localized states in a disordered scattering medium. We observe that the accuracy of the prediction depends on the number of localized states accounted for. Such an exceptional point is experimentally accessible in practically relevant disordered photonic systems.

Ultrafast Generation of Coherent Phonons in Two-Dimensional Bismuthene

Coherent phonons generated through regulation of lattice oscillation via ultrafast laser pulses or X-rays have been desired in various fields, including optoelectronics, thermal and quantum information, and communications. Phonon coherence of two-dimensional (2D) materials is particularly attractive as it enables controllable information transmission but is challenging as the weak interplanar coupling makes phonon excitation extremely difficult. Herein we managed to generate size-dependent phonon coherence from bulk Bi to few-layer bismuthene by an ultrafast femtosecond laser pulse and made a systematic comparison thorough a combination of computation, transient absorption, and reflectance spectroscopic methods. The results witnessed the A_1g phonon excitation in all of the three Bi materials with distinct thicknesses, and the quantum size effect of 2D materials caused phonon confinement and eventual bond softening manifested as a red-shifted vibration frequency and shortened decoherence time compared with those of their bulk counterpart. This study offers new perspectives for tailoring coherent phonons in 2D materials for quantum science and technology including quantum communication, computing, and design of novel quantum devices, etc.

Deep learning control of THz QCLs

Artificial neural networks are capable of fitting highly non-linear and complex systems. Such complicated systems can be found everywhere in nature, including the non-linear interaction between optical modes in laser resonators. In this work, we demonstrate artificial neural networks trained to model these complex interactions in the cavity of a Quantum Cascade Random Laser. The neural networks are able to predict modulation schemes for desired laser spectra in real-time. This radically novel approach makes it possible to adapt spectra to individual requirements without the need for lengthy and costly simulation and fabrication iterations.

Non-Hermiticity-Governed Active Photonic Resonances

Photonic resonances play an essential role in the generation and propagation of light in optical and photonic devices, as well as in light-matter interaction, including nonlinear optical responses. Previous studies in lasers and other open systems have shown exotic roles played by non-Hermiticity on modifying passive resonances, defined in the absence of optical gain and loss. Here we report a new type of resonances in non-Hermitian photonic systems that does not originate from a passive resonance, identified by analyzing a unique quantization condition in the non-Hermitian extension of the Wentzel-Kramers-Brillouin method. Termed active photonic resonances, these unique resonances are found in non-Hermitian systems with a spatially correlated complex dielectric function, which is related to supersymmetry quantum mechanics after a Wick rotation. Remarkably, such an active photonic resonance shifts continuously on the real frequency axis as optical gain increases, suggesting the possibility of a tunable on-chip laser that can span a wavelength range over 100 nm.