Optical coherence is of fundamental importance for both classical and quantum applications. This motivates the development of approaches for increasing the degree of coherence, which can be quantified by a measure of purity. The purity is preserved in linear conservative systems, and accordingly the manipulation of coherence was realized with specially introduced loss in bulk optical setups or diffraction on metal films involving optical absorption and plasmon coupling. Here we suggest and show experimentally for the first time that manipulation and measurement of optical coherence and state purification can be efficiently realized in integrated non-Hermitian parity-time (PT) symmetric photonic structures composed of elements with different loss or gain. Specifically, we design and fabricate laser-written waveguide directional couplers that contain two sections. The first section realizes a PT-like coupler, where one of the two waveguides features extra radiative losses via modulation. The second section consists of straight coupled waveguides with specially detuned propagation constants, which are optimized to enable a full reconstruction of the purity and optical coherence by measuring the interference pattern in both waveguides through fluorescence imaging. In PT symmetric regime, we observe that the purity of an initially fully incoherent (mixed) state is increased followed by a revival of the input state. This constitutes an important experimental evidence of reversible manipulation of light coherence in PT coupled waveguides. We anticipate that this method can facilitate a wide range of applications from classical to quantum optics, including filtering out noise and optimizing the visibility of interferometric measurements.
In this work, we propose a concept of a coupled fiber laser exhibiting PT-symmetry properties. We consider a system operated via Raman gain. The scheme comprises two identical fiber loops (ring cavities) connected by means of two fiber couplers with variable phase shift between them. We show that by changing the phase shift one can switch between generation regimes, realizing either PT-symmetric or PT-broken solution. Furthermore, the paper investigates some peculiarities of the system such as power oscillations and the role of nonlinear phase shift in fiber rings.
We demonstrate that a gold split-ball resonator (SBR) in the form of a spherical nanoparticle with a cut supports both optical magnetic and acoustic modes, which have strong field confinement around the cut. Such localization away from the bottom is expected to lead to an immunity to anchor loss and thus potentially high quality factors of acoustic oscillations when positioned on a substrate. As a result, when a planewave pulse excites the optical resonance, it can then efficiently drive the acoustic vibration through laser heating and/or optical forces. We estimate the overall heat variation by modelling the optical energy dissipation inside the SBR due to the dispersive and absorbing nature of gold at optical wavelengths. The optically induced force is given by the time averaged Lorentz force density. We simulate the mechanical vibrations under the optical excitation through time-dependent simulations using solid mechanics module of COMSOL software. Assuming a moderate quality factor of 10, under a plane wave pulsed laser pump which gives 100K temperature change to the SBR, both the laser heating and optical forces lead to the excitation of the acoustic mode at the same frequency with different magnitudes of 200pm and 10pm, resulting 10% and 0.5% modification of the total optical scattering, respectively. These results show that the SBRs support strong opto-mechanical coupling and are promising in applications such as surface-enhanced Raman spectroscopy and detection of localised strain.
We reveal a number of fundamentally important effects which underpin the key aspects of light propagation in
photonic structures composed of coupled waveguides with loss and gain regions, which are designed as optical
analogues of complex parity-time (or <i>PT</i>) symmetric potentials. We identify a generic nature of time-reversals
in <i>PT</i>-symmetric optical couplers, which enables flexible control of all-optical switching and a realization of
logic operations. We also show that light propagation in <i>PT</i>-symmetric structures can exhibit strongly nonlocal
sensitivity to topology of a photonic structure. These results suggest new possibilities for shaping optical beams
and pulses compared to conservative structures.