Optics (physics.optics)

Abstract: Virtual perfect absorption (VPA) is an effect simulating real absorption of light by using excitation at a complex frequency corresponding to a scattering zero. We theoretically study VPA in resonantly absorbing and amplifying media irradiated by two counterpropagating waves with exponentially growing amplitudes. We show that VPA critically depends on the medium density (i.e., level of loss or gain) deteriorating in the high-density limit. In contrast, almost ideal VPA persists in the PT-symmetric loss-gain bilayer. For high enough gain, the powerful quasilasing pulses are observed at later times symmetrically (single amplifying layer) or asymmetrically (PT-symmetric structure) generated in both propagation directions.

Abstract: Structured illumination enables the tailoring of an imaging device's optical transfer function to enhance resolution. We propose the incorporation of a temporal periodic modulation, specifically a rotating mask, to encode multiple transfer functions in the temporal domain. This approach is demonstrated using a confocal microscope configuration. At each scanning position, a temporal periodic signal is recorded. By filtering around each harmonic of the rotation frequency, multiple images of the same object can be constructed. The image carried by the $n{\mathrm{th}}$ harmonic is a convolution of the object with a phase vortex of topological charge $n$, similar to the outcome when using a vortex phase plate as an illumination. This enables the collection of chosen high spatial frequencies from the sample, thereby enhancing the spatial resolution of the confocal microscope.

Abstract: With the characteristics of ultrasmall, ultrafast and topological protection, optical skyrmions has great prospects in application of high intensity data stroage, high resolution microscopic imaging and polarization sensing. The flexible control of the optical skyrmions is the premise of practical application. At present, the manipulation of optical skyrmions usually relies upon the change of spatial structure, which results in a limited-tuning range and a discontinuous control in the parameter space. Here, we propose continuous manipulation of the graphene plasmons skyrmions based on the electrotunable properties of graphene. By changing the Fermi energy of one pair of the standing waves and the phase of the incident light can achieve the transformation of the topological state of the graphene plasmons skyrmions, which can be illustrated by the change of the skyrmion number from 1 to 0.5. The direc manipulation of the graphene plasmons skyrmions is demonstrated by the simulation results based on the finite element method. Our work suggests a feasible way to flexibly control the optical skyrmions topological field, which can be used for novel integrated photonics devices in the future.

Abstract: Soap bubbles are simple, yet very unique and marvelous objects. They exhibit a number of interesting properties such as beautiful interference colors and the formation of minimal surfaces. Various optical phenomena have been studied in soap films and bubbles, but so far they were not employed as optical cavities. Here we demonstrate, that dye doped soap or smectic liquid crystal bubbles can support whispering gallery mode lasing, which is observed in the spectrum as hundreds of regularly spaced peaks, resembling a frequency comb. The lasing enabled the measurement of size changes as small as 10 nm in a millimeter-sized, ~100 nm thick bubble. Bubble lasers were used as extremely sensitive electric field sensors with a sensitivity of 11 Vm$^{-1}$Hz$^{-1/2}$. They also enable the measurement of pressures up to a 100 bar with a resolution of 1.5 Pa, resulting in a dynamic range of almost 10$^7$. By connecting the bubble to a reservoir of air, almost arbitrarily low pressure changes can be measured while maintaining an outstanding dynamic range. The demonstrated soap bubble lasers are a very unique type of microcavities which are one of the best electric field and pressure microsensors to date and could in future also be employed to study thin films and cavity optomechanics.

Abstract: The production of brighter coherent XUV radiation by intense laser pulses through the process of high-harmonic generation (HHG) is a central challenge in contemporary nonlinear optics. We study the generation and spatial propagation of high harmonics analytically and via ab initio simulations. We focus on the length scales defining the growth of the harmonic signal with propagation distance and show that the well-known coherence length limits HHG only for relatively low driving intensities. For higher intensities, the photoionisation of the medium, naturally accompanying HHG, leads to essentially transient phase matching and laser frequency blue shift. By systematically taking both of these factors into account, we demonstrate that the behaviour of the harmonic signal at higher intensities is defined by another length scale -- the blue-shift length. In this generation regime the XUV intensity at a given frequency first grows quadratically and then saturates passing the blue-shift length, but the total harmonic efficiency continues growing linearly due to the linear increase of the harmonic line bandwidth. The changeover to this generation regime takes place for all harmonic orders roughly simultaneously. The rate of the efficiency growth is maximal if the static dispersion is compensated by photoelectrons near the centre of the laser pulse. Our theory offers a robust way to choose the generation conditions that optimise the growth of the harmonic signal with propagation.

Abstract: Radio-frequency interference is a growing concern as wireless technology advances, with potentially life-threatening consequences like interference between radar altimeters and 5G cellular networks. Mobile transceivers mix signals with varying ratios over time, posing challenges for conventional digital signal processing (DSP) due to its high latency. These challenges will worsen as future wireless technologies adopt higher carrier frequencies and data rates. However, conventional DSPs, already on the brink of their clock frequency limit, are expected to offer only marginal speed advancements. This paper introduces a photonic processor to address dynamic interference through blind source separation (BSS). Our system-on-chip processor employs a fully integrated photonic signal pathway in the analogue domain, enabling rapid demixing of received mixtures and recovering the signal-of-interest in under 15 picoseconds. This reduction in latency surpasses electronic counterparts by more than three orders of magnitude. To complement the photonic processor, electronic peripherals based on field-programmable gate array (FPGA) assess the effectiveness of demixing and continuously update demixing weights at a rate of up to 305 Hz. This compact setup features precise dithering weight control, impedance-controlled circuit board and optical fibre packaging, suitable for handheld and mobile scenarios. We experimentally demonstrate the processor's ability to suppress transmission errors and maintain signal-to-noise ratios in two scenarios, radar altimeters and mobile communications. This work pioneers the real-time adaptability of integrated silicon photonics, enabling online learning and weight adjustments, and showcasing practical operational applications for photonic processing.

Abstract: We show that in optical clocks based on Rabi interrogation, both laser-frequency and magnetic-field flicker ($1/f$) noise with zero mean can lead to servo errors at the $10^{-18}$ level if the negative-detuning (red) and positive-detuning (blue) sides of the transition are always probed in the same order. This is due to the strong correlations of flicker noise in combination with an imbalance in the response of the servo discriminator to positive and negative differential frequency noise between the red- and blue-side probing. This imbalance is particularly large for a normalized discriminator. We derive an analytical expression for the servo error based on the correlation function of the laser-frequency or magnetic-field noise and compare it to numerical servo simulations to demonstrate how the error depends on the noise level, servo parameters, and probing sequence. We also show that the servo error can be avoided by normalizing the discriminator with a moving mean or by reversing the red/blue probing order for every second servo cycle.

Abstract: Bragg gratings offer high-performance filtering and routing of light on-chip through a periodic modulation of a waveguide's effective refractive index. Here, we model and experimentally demonstrate the use of Sb2Se3, a nonvolatile and transparent phase-change material, to tune the resonance conditions in two devices which leverage periodic Bragg gratings: a stopband filter and Fabry-Perot cavity. Through simulations, we show that similar refractive indices between silicon and amorphous Sb2Se3 can be used to induce broadband transparency, while the crystalline state can enhance the index contrast in these Bragg devices. Our experimental results show the promise and limitations of this design approach and highlight specific fabrication challenges which need to be addressed in future implementations.