Ali K. Jahromi

Observation of Poynting’s vector reversal in an active photonic cavity

Optical power flow along an active cavity having net gain can differ dramatically from that in a passive cavity. We report the first observation of Poynting’s vector reversal in an active cavity under scattering boundary conditions. At a sub-lasing gain, which we call Poynting’s threshold, a null develops in Poynting’s vector, which divides the cavity into sections with oppositely directed energy flows. Furthermore, we demonstrate that the direction of Poynting’s vector at a fixed point in an active cavity can be controllably reversed without changing its magnitude via an intra-cavity passive element, thereby suggesting a potential methodology for optical switching.

Basis-neutral Hilbert-space analyzers

Interferometry is one of the central organizing principles of optics. Key to interferometry is the concept of optical delay, which facilitates spectral analysis in terms of time-harmonics. In contrast, when analyzing a beam in a Hilbert space spanned by spatial modes – a critical task for spatial-mode multiplexing and quantum communication – basis-specific principles are invoked that are altogether distinct from that of ‘delay’. Here, we extend the traditional concept of temporal delay to the spatial domain, thereby enabling the analysis of a beam in an arbitrary spatial-mode basis – exemplified using Hermite-Gaussian and radial Laguerre-Gaussian modes. Such generalized delays correspond to optical implementations of fractional transforms; for example, the fractional Hankel transform is the generalized delay associated with the space of Laguerre-Gaussian modes, and an interferometer incorporating such a ‘delay’ obtains modal weights in the associated Hilbert space. By implementing an inherently stable, reconfigurable spatial-light-modulator-based polarization-interferometer, we have constructed a ‘Hilbert-space analyzer’ capable of projecting optical beams onto any modal basis.

Transparent Perfect Mirror

A mirror that reflects light fully and yet is transparent appears paradoxical. Current so-called transparent or “one-way” mirrors are not perfectly reflective and thus can be distinguished from a standard mirror. Constructing a transparent “perfect” mirror has profound implications for security, privacy, and camouflage. However, such a hypothetical device cannot be implemented in a passive structure. We demonstrate here a transparent perfect mirror in a non-Hermitian configuration: an active optical cavity where a certain prelasing gain extinguishes Poynting’s vector at the device entrance. At this threshold, all vestiges of the cavity’s structural resonances are eliminated and the device presents spectrally flat unity-reflectivity, thus, becoming indistinguishable from a perfect mirror when probed optically across the gain bandwidth. Nevertheless, the device is rendered transparent by virtue of persisting amplified transmission resonances. We confirm these predictions in two photonic realizations: a comp...

Statistical parity-time-symmetric lasing in an optical fibre network

Parity-time (PT)-symmetry in optics is a condition whereby the real and imaginary parts of the refractive index across a photonic structure are deliberately balanced. This balance can lead to interesting optical phenomena, such as unidirectional invisibility, loss-induced lasing, single-mode lasing from multimode resonators, and non-reciprocal effects in conjunction with nonlinearities. Because PT-symmetry has been thought of as fragile, experimental realisations to date have been usually restricted to on-chip micro-devices. Here, we demonstrate that certain features of PT-symmetry are sufficiently robust to survive the statistical fluctuations associated with a macroscopic optical cavity. We examine the lasing dynamics in optical fibre-based coupled cavities more than a kilometre in length with balanced gain and loss. Although fluctuations can detune the cavity by more than the free spectral range, the behaviour of the lasing threshold and the laser power is that expected from a PT-stable system. Furthermore, we observe a statistical symmetry breaking upon varying the cavity loss.Parity-time-symmetric optical systems have so far only been realized using microscopic cavities because the necessary gain-loss balance is thought to be fragile with regard to statistical fluctuations. Here, Jahromi et al.. demonstrate PT-symmetric lasing using kilometre-long fibre cavities.

Coherent Perfect Absorption in a Weakly Absorbing Fiber

Coherent perfect absorption refers to the interferometric enhancement of absorption in a partially lossy medium upto 100%. This can be achieved without modifying the absorbing medium itself, instead of engineering its photonic environment. Ion-doped fibers are one of the most technologically relevant absorbing materials in optics, which are widely employed in fiber amplifiers and lasers. Realizing complete optical absorption of an incident field in short-length moderately-doped fibers remains a challenge for the cost-effective design of compact fiber lasers. Here, we exploit the concept of coherent perfect absorption to overcome this challenge, whereby two appropriately designed fiber Bragg gratings define a short-length erbium-doped-fiber cavity that enforces complete absorption of an incident field on resonance—-independently of the doped-fiber intrinsic absorption. This approach applies to any spectral window and guarantees the efficient utilization of the fiber dopants along its length, thus, suggesting the possibility of next-generation efficient single-longitudinal-mode fiber lasers for applications in optical communication, sensing, and metrology.

Observation of Poynting’s Vector Reversal in a Cavity with Net Gain

We report the first observation of Poynting’s vector reversal in an active optical cavity, which results in a counter-intuitive drop in the transmission and reflection when the total gain exceeds the lasing threshold.