Single mode lasing is experimentally demonstrated in a transversely multi-moded InP-based semiconductor microring arrangement. In this system, mode discrimination is attained by judiciously utilizing the exceptional points in a parity-time (PT) symmetric double microring configuration. The proposed scheme is versatile, robust to small fabrication errors, and enables the device to operate in a stable manner considerably above threshold while maintaining spatial and spectral purity. The results presented here pave the way towards a new class of chip-scale semiconductor lasers that utilize gain/loss contrast as a primary mechanism for mode selection.Integrated photonic laser systems with larger cross sections are desirable for many applications since they allow for higher energies within the cavities while managing the thermal load and keeping the impact of optical nonlinearities under control. Unfortunately, however, merely enlarging the transverse dimensions of the waveguides inevitably gives rise to competing higher-order spatial modes. This, in turn, compromises the spectral and spatial fidelity of the laser and limits the power allocated within a specific mode [1]. These limitations exist on all scales, and may even be exacerbated in chip-scale semiconductor lasers, where the large gain bandwidths of the active media already pose a challenge in promoting single-mode operation [2].So far, utilizing intra-cavity dispersive elements has been the primary approach for longitudinal mode selection [3], while tapering along the direction of propagation, engineering the refractive index in the cross section, as well as evanescent filtering are some of the extensively explored techniques to enforce single spatial mode operation in such arrangements [4][5][6][7]. Yet, in spite of their success, they are not always compatible with on-chip microcavity structures and in most cases are quite sensitive to small fabrication tolerances. In this respect, it would be desirable to explore alternative avenues to address these issues.Lately, the selective breaking of parity-time (PT) symmetry has been proposed as a viable strategy for obtaining single transverse mode operation [8]. It is suggested that by pairing an active resonator/waveguide with a lossy but otherwise identical partner, it is possible to enforce single-spatial-mode performance even in the presence of strong mode competition in multi-moded laser/amplifier configurations. In general, a structure is considered to be PT-symmetric if it is invariant under the simultaneous action of the (space) and (time) inversion operations [9]. Despite 2 having a non-Hermitian representation, such a system may still support an entirely real spectrum. While originally developed in the context of quantum mechanics [9,10], such notions have recently attracted considerable attention in different areas of optics such as photonic lattices, microresonators, gratings, and lasers [11][12][13][14][15][16][17][18][19][20][21][22]. In optical settings, a structure is PT symmetric if the real part...

Non-Hermitian exceptional points (EPs) represent a special type of degeneracy where not only the eigenvalues coalesce, but also the eigenstates tend to collapse on each other. Recent studies have shown that, in the presence of an EP, light–matter interactions are profoundly modified, leading to a host of unexpected optical phenomena ranging from enhanced sensitivity to chiral light transport. Here we introduce a family of unidirectional resonators based on a novel type of broadband exceptional points. In active settings, the resulting unidirectionality exhibits resilience to perturbations, thus, providing a robust and tunable approach for directly generating beams with distinct orbital angular momenta (OAM). This work could open up new possibilities for manipulating OAM degrees of freedom in applications pertaining to telecommunications and quantum information sciences, while at the same time may expand the notions of non-Hermiticity in the orbital angular momentum space.

The behavior of a parity-time (PT) symmetric coupled microring system is studied when operating in the vicinity of an exceptional point. Using the abrupt phase transition around this point, stable single-mode lasing is demonstrated in spectrally multi-moded micro-ring arrangements.The notion of PT-symmetry was first introduced within the context of mathematical physics [1]. It generally implies that even nonHermitian systems can support entirely real spectra, provided their corresponding Hamiltonians commute with the parity-time (PT) operator. In recent years, several studies have shown that PT symmetric structures can be readily realized in photonic arrangements by exploiting the mathematical isomorphism between the quantum-mechanical Schrödinger equation and the paraxial wave equation of optics [2][3][4][5][6][7]. In this regard, a necessary albeit not sufficient condition for optical PT symmetry to hold is that the refractive index distribution involved should respect the following relationship * (− ) = ( ). In other words, the real part of the complex refractive index profile associated with a photonic PT symmetric structure must be an even function of position, while its imaginary component representing gain or loss must have an odd distribution.By virtue of their non-Hermiticity, PT-symmetric arrangements are capable of supporting exceptional points in their parameter space. Exceptional points, also known as non-Hermitian degeneracies, typically occur in non-conservative physical systems. At these points, the eigenvectors as well as their associated eigenvalues coalesce, resulting in an abrupt phase transition that dramatically changes the behavior of the system [8]. In optical settings, such degeneracies can arise from the interplay of gain and loss in the underlying design. A multitude of recent theoretical and experimental studies has shown different ways of how the presence of exceptional or phase-transition points can be fruitfully utilized to attain new behavior and functionality in non-Hermitian photonic systems, including those respecting PT symmetry [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Thus far, this methodology has been explored in a number of laser studies where a lower threshold and an inverse pump dependence of the output power have been investigated [18][19][20][21][22][23][24]. Lately, the selective breaking of PT symmetry has also been considered for laser mode management applications [23,24]. In particular, in [24], a PT symmetric double ring configuration was proposed as a promising avenue to enforce single longitudinal mode operation in inherently multi-moded semiconductor micro-cavities. This type of system is comprised of two identical ring resonators, where one provides gain while its counterpart is subjected to an appropriate amount of loss.In this work, our goal is to experimentally characterize the behavior of a parity-time symmetric photonic molecule around the exceptional point. We start with developing a mathematical model for a coupled cavity arrangement sh...

Due to the high spontaneous emission coupled into the resonance mode in metallic nanolasers, there has been a debate concerning the coherence properties of this family of light sources. The second-order coherence function can unambiguously determine the nature of a given radiation. In this paper, an approach to measure the second-order coherence function for broad linewidth sources in the near-infrared telecommunication band is established based on a modified Hanbury Brown and Twiss configuration. Using this set-up, it is shown that metallic coaxial and diskshaped nanolasers with InGaAsP multiple quantum well gain systems are indeed capable of generating coherent radiation.

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