Using three-dimensional kinetic simulations, we examine the emission of collimated $\gamma$-ray beams from structured laser-irradiated targets with a prefilled cylindrical channel and its scaling with laser power (in the multi-PW range). The laser power is increased by increasing the laser energy and the size of the focal spot while keeping the peak intensity fixed at $5\times {10}^{22}\mathrm{W}/{\mathrm{cm}}^2$. The channel radius is increased proportionally to accommodate the change in laser spot size. The efficiency of conversion of the laser energy into a beam of MeV-level $\gamma$ rays (with a ${10}^{\circ }$ opening angle) increases rapidly with the incident laser power $P$ before it roughly saturates above $P\approx 4\mathrm{PW}$. Detailed particle tracking reveals that the power scaling is a result of enhanced electron acceleration at higher laser powers. One application that directly benefits from such a strong scaling is pair production via two-photon collisions. We investigate two schemes for generating pairs through the linear Breit-Wheeler process: colliding two $\gamma$-ray beams and colliding one $\gamma$-ray beam with black-body radiation. The two scenarios project up to ${10}^4$ and ${10}^5$ pairs, respectively, for the $\gamma$-ray beams generated at $P=4\mathrm{PW}$. A comparison with a regime of laser-irradiated hollow channels corroborates the robustness of the setup with prefilled channels.

Silicon-vacancy qubits in silicon carbide ($\mathrm{Si}\mathrm{C}$) are emerging tools in quantum-technology applications due to their excellent optical and spin properties. In this paper, we explore the effect of temperature and strain on these properties by focusing on the two silicon-vacancy qubits, V1 and V2, in $4H$-$\mathrm{Si}\mathrm{C}$. We apply density-functional theory beyond the Born-Oppenheimer approximation to describe the temperature-dependent mixing of electronic excited states assisted by phonons. We obtain a polaronic gap of around 5 and 22 meV for the V1 and V2 centers, respectively, which results in a significant difference in the temperature-dependent dephasing and zero-field splitting of the excited states, which explains recent experimental findings. We also compute how crystal deformations affect the zero-phonon line of these emitters. Our predictions are important ingredients in any quantum applications of these qubits sensitive to these effects.

Magnetic tunnel junctions are nanoscale spintronic devices with microwave-generation and -detection capabilities. Here, we use the rectification effect called a “spin diode” in a magnetic tunnel junction to wirelessly detect the microwave emission of another junction in the auto-oscillatory regime. We show that the rectified spin-diode voltage measured at the receiving junction end can be reconstructed from the independently measured auto-oscillation and spin-diode spectra in each junction. Finally, we adapt the auto-oscillator model to the case of the spin-torque oscillator and the spin diode and we show that it accurately reproduces the experimentally observed features. These results will be useful for the design of circuits and chips based on spintronic nanodevices communicating through microwaves.

By identifying the similarities between the coupled-wave equations and the parametrically driven nonlinear Schrödinger equation, we show the existence conditions of quadratic solitons in continuous-wave pumped singly resonant degenerate optical parametric oscillators (SRDOPOs). Compared to the previously explored doubly resonant DOPOs, quadratic solitons in SRDOPOs are advantageous in their robustness against perturbations induced by dispersion of the effective third-order nonlinearity and temporal walk-off between the signal and the pump. Terahertz comb bandwidth and femtosecond pulse duration are attainable in an example periodically poled Lithium niobate waveguide resonator in the short-wave infrared. The working principle can be extended to other material platforms, making it a competitive ultrashort pulse and broadband comb source architecture at the mid-infrared spectral range.

From fundamental discovery to practical application, advances in the optical and quantum sciences rely upon precise control of light-matter interactions. Systems of coupled optical cavities are ubiquitous in these efforts, yet the design and active modification of the hybridized mode properties remains challenging. Here, we demonstrate the design, fabrication, and analysis of a tunable heterogeneous photonic molecule consisting of a ring resonator strongly coupled to a nanobeam photonic crystal cavity. Leveraging the disparity in mode volume between these two strongly coupled cavities, we combine theory and experiment to establish the ability to actively tune the mode volume of the resulting supermodes over a full order of magnitude. As the mode volume determines the strength of light-matter interactions, this work illustrates the potential for strongly coupled cavities with dissimilar mode volumes in applications requiring designer photonic properties and tunable light-matter coupling, such as photonics-based quantum simulation.

Integrating multiple functionalities into a single metasurface is becoming of great interest for future intelligent communication systems. While such devices have been extensively explored for reciprocal functionalities, in this work, we integrate a wide variety of nonreciprocal applications into a single platform. The proposed structure is based on spatiotemporally modulated impedance sheets supported by a grounded dielectric substrate. We show that, by engineering the excitation of evanescent modes, nonreciprocal interactions with impinging waves can be configured at will. We demonstrate a plethora of nonreciprocal components, such as wave isolators, phase shifters, and circulators, on the same metasurface. This platform allows switching between different functionalities by modifying only the pumping signals (harmonic or nonharmonic), without changing the main body of the metasurface structure. This solution opens the door for future real-time reconfigurable and environment-adaptive nonreciprocal wave controllers.

The principle of reciprocity underpins one of the fundamental effects in optics that signifies symmetric transmission with respect to the interchange of the source and observation points. Breaking reciprocity, however, enables additional functionalities and greatly enriches the applications of photonics in terms of nonreciprocal devices. Here, a realistic nanoscale cascaded-cavity system that envisages nonreciprocal optical isolation and efficient wavelength conversion based on spatiotemporally modulated index of refraction are proposed and numerically demonstrated. The on-chip, linear, magnetic-free nonreciprocal isolation and dynamically controllable wavelength conversion can be a promising candidate for silicon nanophotonic and optoelectronic devices.

Optical resonant cavities play an important role in electromagnetic wave control; they can confine electromagnetic waves and improve the interaction between light and matter. However, because of the limitations of the standing-wave formation conditions for Fabry-Perot-type resonance, the miniaturization of optical resonant cavities formed using traditional materials is difficult. Recently, the miniaturization of three-dimensional optical resonant cavities has been demonstrated based on electric hyperbolic metamaterials (HMMs); their isofrequency contour takes the form of an open hyperboloid because the principal components of the permittivity tensor have opposite signs. Here, based on the permeability tensor, we propose theoretically and verify experimentally a planar magnetic hyperbolic cavity with a subwavelength scale, $(\lambda /12)\times (\lambda /17)$, using a circuit-based HMM in the microwave regime. Furthermore, the anomalous scaling laws in the circuit-based magnetic hyperbolic cavities are studied. As the frequency increases, the mode order decreases, which is markedly different from traditional cavities. It is also possible to realize size-independent cavity modes based on the HMMs by design. In addition, by considering a composite structure that contains two hyperbolic cavities, the coupling of two hyperbolic cavity modes in the near-field regime is demonstrated. The circuit-based hyperbolic cavities not only extend previous research work on hyperbolic cavities to magnetic HMMs, but they also have a planar structure that is easier to integrate and has a smaller loss. Finally, the hyperbolic cavities may enable their use in some microwave-related applications, such as in high-sensitivity sensors, resonance imaging, and miniaturized narrowband filters.

Band engineering near Dirac points is an emerging topic in condensed matter physics and photonics, enabling multifaceted devices of record-high conductivity and zero refractive index in both electronic and photonic structures. Recently, an extended class of Dirac cone, type I, II, or III, has attracted much attention with its controlled directionality of singular wave behaviors near the Dirac points. However, despite the significance of the discovery and applications of each type of Dirac cone, the theoretical framework and unified design structure bridging different types of Dirac cones have not been clearly elucidated. Here, we propose a universal design platform for all types of photonic Dirac cones and reveal the key parameter determining their types and transformations. Starting from the accidentally induced photonic Dirac cone, we show that the inverse design of an optical potential landscape enables the deterministic control of the spectral ordering, which leads to designer construction of each type of Dirac cone. This approach will pave the way for the design of artificial degeneracies with tunable anisotropy for general wave systems.

Acousto-optical devices, such as modulators, filters, or deflectors, implement a simple and effective way of light modulation and signal-processing techniques. However, their operation wavelengths are restricted to the visible and near-infrared frequency regions due to a quadratic decrease of the efficiency of acousto-optical interactions with increasing wavelength. At the same time, almost all materials with a high value of the acousto-optic figure of merit are nontransparent at wavelengths longer than 5 µm, while the transparent materials possess a significantly lower acousto-optic figure of merit. Here, we propose and demonstrate by calculations how these limitations can be overcome using specially designed planar semiconductor structures that support electromagnetic modes strongly coupled to the incident light in the Otto configuration. Such an approach can be used for an efficient acousto-optical device operating in the mid-infrared range of 8–14 µm. An acoustic wave excited by a piezoelectric transducer in a semiconductor prism is utilized to modulate the coupling coefficient of the incident light to the guided mode of the semiconductor structure, which results in up to 100% modulation of the transmitted light at a spatial scale less than the ultrasound wavelength. It allows the utilization of acoustic waves with a short decay distance, and therefore, it provides a unique possibility to achieve an efficient acousto-optical modulation at frequencies about 1 GHz, which are unreachable for traditional acousto-optics.

Photonic resonators that allow an electromagnetic field to be confined in an ultrasmall volume with a long decay time are crucial to a number of applications requiring enhanced nonlinear effects. For applications to integrated photonic devices on chip, compactness and optimized in-plane transmission become relevant figures of merit as well. Here we optimize an encapsulated $\mathrm{Si}/{\mathrm{Si}\mathrm{O}}_2$ photonic-crystal nanobeam cavity at telecom wavelengths by means of a global optimization procedure, where only the first few holes surrounding the cavity are varied to decrease its radiative losses. This strategy allows us to theoretically achieve intrinsic quality factors close to 10 million, sub-diffraction-limited mode volumes, and in-plane transmission above 65%, in a structure with a very small footprint of about $8\mu {\mathrm{m}}^2$. We address and quantitatively assess the dependence of the main figures of merit on the nanobeam length and on fabrication disorder. Finally, we study a system of two optimized and laterally coupled nanobeam cavities with the goal of demonstrating an unconventional photon blockade at room temperature in a monolithic passive device. We estimate the single-photon nonlinearity of this device and discuss the relevant figures of merit, which lead to sub-Poissonian photon statistics of the transmitted signal. Our results hold promise for prospective experiments in low-power nonlinear and quantum photonics.

There is an increasing and demanding interest in the realization of secure communication schemes. Here, we conceive and realize an approach for a message-encoding scheme between two parties, Alice and Bob, by exploiting the infinite number of polarization states of unpolarized thermal light on the Poincaré sphere together with their correlation properties to camouflage and recover a message, thus realizing ghost polarization communication. At first, we investigate the second-order correlation coefficient $g^{(2)}(\tau )$ of classical unpolarized broadband amplified-spontaneous-emission light emitted by an erbium-doped fiber amplifier at 1550 nm by manipulating its instantaneous polarization state utilizing various polarization optics in the beam paths of a Hanbury-Brown and Twiss like ghost polarimetry setup using ultra-fast two-photon absorption in a photomultiplier tube. The observed polarization state modifications are in excellent agreement with our developed model based on the Stokes vector dynamics and a Glauber protocol for $g^{(2)}(\tau )$. On the basis of these results we then proceed towards the realization of a message-encoding scheme. By changing the instantaneous polarization state using a half-wave plate, Alice encodes and subsequently transmits a message. The camouflaged message can be recovered uniquely by Bob measuring the second-order correlations of the modified instantaneous polarization state with his reference beam. By using the agreed keypad of the communication scheme, he is able to retrieve the bit values of the encoded message. Finally, we address real-world implementation of this first proof-of-principle demonstration and discuss its security issues. This scheme demonstrates a method of establishing a secure communication link between two parties directly on the physical layer based on polarization correlations of classical light. It is expected that by the realization of this correlated photon modality not only avenues for ghost modalities, in general, are opened but also insight into polarization, even more than 175 years after Stokes will be achieved.

Attosecond pulses in the soft x-ray (SXR) to water-window energy region offer the tools for creating and studying target-specific localized inner-shell electrons or holes in materials, enabling monitoring or controlling charge and energy flows in a dynamic system on attosecond timescales. Recently, a number of laboratories have reported generation of continuum harmonics in the hundred-electron-volt to kilovolt region with few-cycle long-wavelength mid-infrared lasers. These harmonics have the bandwidth to support pulses with durations of a few to a few-ten attoseconds. But harmonics generated in a gas medium have attochirps that cannot be fully compensated by materials over a broad spectral range; thus, realistically what are the typical shortest attosecond pulses that one can generate? To answer this question, it is essential that the temporal attosecond pulses be accurately characterized. By reanalyzing the soft x-ray attosecond metrology reported in three recent experiments [Li et al. 53-attosecond x-ray pulses reach the carbon K-edge, Nat. Commun. 8, 186 (2017); Gaumnitz et al. Streaking of 43-attosecond soft x-ray pulses generated by a passively CEP-stable mid-infrared driver, Opt. Exp. 25, 27506 (2017); Cousin et al. Attosecond Streaking in the Water Window: A New Regime of Attosecond Pulse Characterization, Phys. Rev. X 7, 041030 (2017)] using a newly developed broadband phase retrieval algorithm, we demonstrate that the generated attosecond pulses in the first two papers have durations of about 60 as, longer than what they have reported. Similarly, the duration from the third experiment is retrieved to be about half of the 322-as upper limit cited in that work. We also introduce the autocorrelation (AC) of the streaking spectrogram. By comparing the ACs from the experiments and from the retrieved SXR pulses, the accuracy of the retrieved results can be directly visualized. Our retrieval method is fast and accurate, and it shall provide a powerful tool for the metrology of the emerging few-ten-attosecond pulses.

Terahertz technology has attracted wide attention because of its potential applications in diverse fields, such as spectroscopy, imaging, and nondestructive evaluation, but the lack of functional devices, including polarization convertors, is still a major hurdle for these applications. In previous studies on metamaterial-based polarization convertors, only one single polarization conversion is generally achieved. This paper presents a transmissive dual-functional terahertz waveplate based on an all-dielectric metamaterial composed of periodic subwavelength pillars made of pure silicon that can achieve two different polarization conversions (i.e., a combination of quarter- and half-waveplates) at the same operation frequency and for x- and y-polarized incidences, respectively. Theoretical and numerical analyses are provided to illustrate the design principle. A sample is fabricated and characterized, and the experimental results show that the two polarization conversions are obtained at 1.01 THz, which is in good agreement with the corresponding simulations. Such a dual-functional terahertz waveplate will find applications in terahertz systems and the design strategy can be extended to other frequency ranges as well.

Many quantum key distribution systems employ a laser followed by an optical attenuator to prepare weak coherent states in the source. Their mean photon number must be precalibrated to guarantee the security of key distribution. Here we experimentally show that this calibration can be broken with a high-power laser attack. We test four fiber-optic attenuator types used in quantum key distribution systems, and find that two of them exhibit a permanent decrease in attenuation after laser damage. This results in higher mean photon numbers in the prepared states and may allow an eavesdropper to compromise the key.

Single-photon sources are of great interest because they are key elements in different promising applications of quantum technologies. Here we demonstrate a highly efficient tunable on-demand microwave single-photon source based on a transmon qubit with an intrinsic emission efficiency above $98\mathrm{\%}$. The high efficiency ensures a negligible pure dephasing rate and the necessary conditions for generation of indistinguishable photons. We provide an extended discussion and analysis of the efficiency of the photon generation. To further experimentally confirm the single-photon property of the source, correlation functions of the emission field are also measured using linear detectors with a GPU-enhanced signal-processing technique. Our results experimentally demonstrate that frequency tunability and negligible pure dephasing rate can be achieved simultaneously and show that such a tunable single-photon source can be good for various practical applications in quantum communication, simulations and information processing in the microwave regime.