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Physics Physics Optics

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Wednesday 30 September

Optics

Judd-Ofelt analysis of holmium-doped alumino-silicate optical glass prepared by MCVD combined with nanoparticle doping

Petr Varak, Michal Kamradek, Pavla Nekvindova et al. (+2 more)

We present a detailed Judd–Ofelt (JO) analysis of Ho$^{3+}$-doped alumino-silicate optical fiber preforms in a wide range of compositions and prepared by both the standard solution doping method as well as the more advanced nanoparticle doping. The absorption spectra were measured, the absorption cross sections were calculated for each observed transition, and the Judd–Ofelt analysis was conducted. The preforms exhibited the values of JO parameters of $\Omega_2 = (7.2--10.6) \times 10^{-20}\,\mathrm{cm}^2$, depending on the Al$_2$O$_3$ content, and $\Omega_4$ and $\Omega_6$ around $2.5$ and $1.2 \times 10^{-20}\,\mathrm{cm}^2$, respectively. The radiative transition parameters, such as transition probabilities, branching ratios, and radiative lifetimes, were calculated. The radiative lifetime of the ${}^5I_7 \rightarrow {}^5I_8$ transition, in the $16.1--17.2\,\mathrm{ms}$ range, combined with a measured value in the $0.9--1.9\,\mathrm{ms}$ range, was used to calculate the quantum efficiency, which was in the $5--11\,\%$ range. The preforms prepared by nanoparticle doping containing high Al$_2$O$_3$ contents above $8\,\mathrm{mol.}\,\%$ exhibited superior values of measured lifetime (above $1.5\,\mathrm{ms}$) and quantum efficiency (above $8\,\%$) compared to the standard solution-doped samples. To the best of our knowledge, this work is the first comprehensive report on the JO analysis of Ho$^{3+}$-doped alumino-silicate glass. The calculated parameters may be used in various calculations, simulations, and modelling of holmium-doped fiber lasers and other devices.
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All authors Petr Varak, Michal Kamradek, Pavla Nekvindova, Jan Hrabovsky, Pavel Peterka

Optics

Driving Spintronic Terahertz Emitters at GHz Repetition Rates

Thomas A. Gething, Henry De Libero, Morgan T. Hibberd et al. (+3 more)

Spintronic terahertz emitters (STEs) are versatile sources of terahertz (THz) frequency radiation that offer high electric field strengths and gap-free bandwidths covering the THz (0.1-30 THz) band. In addition, they are low-cost to produce and easy to use in a variety of optical setups. However, an obstacle to their wider applicability is the recently reported decrease in optical fluence damage threshold when driven with high-repetition-rate lasers. Here, we use an asynchronous optical sampling (ASOPS) THz time-domain spectrometer (THz-TDS) to observe the degradation of the THz signal in real-time when driven by a 1 GHz repetition-rate laser. We demonstrate that the loss of THz emission is permanent and is likely the result of thermal damage to the STE structure. However, by utilizing substrates with high thermal conductivities, an increase of the damage threshold of over three orders of magnitude is observed enabling stable emission from a STE for the first time when driven by a high-repetition-rate laser.
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All authors Thomas A. Gething, Henry De Libero, Morgan T. Hibberd, Thomas Thomson, Paul W. Nutter, Darren M. Graham

Optics

Bias-resolved comb-formation regimes in a simple two-section quantum-dot mode-locked laser for isolator-free, terabit-scale O-band interconnects

Ying Shi, William He, Xiangpeng Ou et al. (+4 more)

Practical O-band quantum-dot (QD) mode-locked comb sources must combine wide flat-top bandwidth, low radio-frequency (RF) timing noise, error-correctable bare-line modulation, and feedback tolerance, yet these functions are usually optimized separately and their relation to comb-formation dynamics remains unclear. Here we map the gain-current/saturable-absorber-bias plane of a simple two-section InAs/GaAs QD mode-locked laser to resolve distinct application-specific regimes. A low-injection, high-reverse-bias regime produces 0.80 ps pulses, consistent with amplitude-modulated operation, whereas a high-injection, intermediate-bias regime yields a 16.16 nm (2.75 THz) flat-top comb with 110 lines and a 14.98 ps extended waveform, consistent with a stronger frequency-modulated contribution. To our knowledge, this is the broadest reported 3 dB bandwidth among QD mode-locked comb sources. At a separate low-noise bias, the beatnote exhibits a 0.58 kHz Lorentzian linewidth and a 41.6 fs integrated timing jitter over the 4 to 80 MHz range, to our knowledge the lowest reported for a high-channel-count O-band QD passive comb. In isolator-free 25 Gb/s NRZ transmission, all 94 carriers across a 13.73 nm 3 dB band remain below the 7 percent HD-FEC threshold. Using all 110 carriers at the maximum-bandwidth bias gives a projected aggregate rate of 2.75 Tb/s. At a separate feedback-test bias, broadband mode locking is preserved and, beyond approximately -28 dB feedback, the RF beatnote enters a feedback-stabilized regime whose linewidth is reduced 42-fold. Thus, short-pulse, broadband, low-noise, transmission-ready, and feedback-stabilized operation occupy distinct selectable regions rather than a single universal optimum. This regime-resolved map links QD comb-formation physics to isolator-free, terabit-scale O-band interconnects.
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All authors Ying Shi, William He, Xiangpeng Ou, Defan Sun, Weiping Li, Xin Yao, Yating Wan

Optics · Cross-listed from quant-ph

Fixed-Point Bifurcations and Path-Information Bounds in Nonlinear Exceptional-Point Sensing

Xue-Hao Yu and Cong-Feng Qiao

Nonlinear exceptional points (NEPs) were proposed to circumvent the noise–responsivity cancellation that limits linear exceptional-point sensors. At an NEP, the stationary frequency response can be singular while the Hamiltonian Petermann factor remains finite, suggesting a genuine signal-to-noise ratio (SNR) enhancement. Yet recent model-specific analyses of nonlinear fluctuations have reached conflicting conclusions. We prove that any singular stationary frequency response requires a fixed-point bifurcation of the nonlinear dynamics. In local fluctuation regimes, the singularities in responsivity and long-time frequency uncertainty are both set by the critical relaxation rate, leading to exact cancellation in the SNR and precluding any divergent enhancement. Furthermore, we derive a path-information bound that applies across both local and nonlinear fluctuation regimes and places a finite ceiling on the long-time SNR enhancement. Notably, this upper bound is attainable in the local fluctuation regime, implying that frequency readout under such parameter settings is information-optimal.
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Optics

Frequency comb generation in active optomechanical cavities

Madhurendra Mishra and Adarsh Ganesan

We numerically investigate frequency-comb generation in an electrically pumped vertical-cavity surface-emitting laser coupled to two mechanical modes of a suspended mirror. Displacement-dependent cavity loss and radiation-pressure feedback couple the mechanical motion to the carrier and photon densities, producing harmonics and combination-frequency components in the photon-density spectrum. Mechanical quality factors primarily control the intensity of comb lines, whereas resonance frequencies determine their spectral distribution. The initial mechanical amplitudes influence the threshold current and relative spectral intensities, with the simultaneous excitation of both modes yielding a broader set of observable components. These results establish the dependence of the calculated comb structure on mechanical dissipation, resonance frequencies, and initial conditions in an active optomechanical cavity.
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Optics

Surface quantum response of plasmonic heterodimers: revealing the hidden nonlocality

Ida Juliane Bundgaard, Christos Mystilidis, Frederik Wulff Christensen et al. (+7 more)

We probe the quantum response of plasmonic heterodimers with extended (plane waves) and localized (point dipoles and electron beams) sources. Implementing the surface-response formalism based on Feibelman parameters, we explore nanosphere heterodimers where one particle is characterized by electron spill-in, while the other by spill-out. We identify situations where the two quantum surface corrections that act in opposite spectral-shift directions can cancel out, producing mesoscopic spectra that do not differ from the predictions of the local-response approximation. This picture changes dramatically once localized sources are implemented, allowing one to selectively probe the quantum response of the particles independently. Such probing occurs in terms of higher-order modes in the case of electron beams, or for the complete modal landscape in the case of point dipole sources. This description offers a new insight into the mechanisms governing light–matter interactions in extreme plasmonic architectures, and opens new perspectives for the understanding and tailoring of plasmonic–quantum emitter architectures.
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All authors Ida Juliane Bundgaard, Christos Mystilidis, Frederik Wulff Christensen, Lorenz Huber, P. Elli Stamatopoulou, Carsten Rockstuhl, P. A. D. Gonçalves, Ulrich Hohenester, N. Asger Mortensen, Christos Tserkezis

Optics

Experimental Demonstration of Chiral Bound States in the Continuum Enabled by Time-reversal Symmetry Breaking

Hao-Chang Mo, Wen-Jin Zhang, Ze-Yu Wu et al. (+3 more)

Bound states in the continuum (BICs) provide a powerful route to optical resonances with vanishing radiation loss and enhanced light-matter interactions. Of particular interest are chiral BICs, whose resonant states exhibit intrinsic circular polarization and spin-selective radiation. Magneto-optical photonic crystals have recently been predicted to host such states through time-reversal-symmetry breaking. Here, we experimentally demonstrate chiral BICs induced by time-reversal symmetry breaking in a magneto-optical photonic crystal, where an out-of-plane magnetic field lifts a degenerate BIC pair into two nondegenerate resonances with opposite circular polarizations. Numerical simulations reveal the associated chiral phase vortices and strong spin-selective radiation, while microwave measurements confirm the predicted behavior through angle-resolved transmission spectra and pronounced broadband circular dichroism. Our results establish an experimental platform for exploring chiral BIC physics in magneto-optical photonic structures, opening new opportunities for spin-selective photonics.
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All authors Hao-Chang Mo, Wen-Jin Zhang, Ze-Yu Wu, Xin-Tao He, Xiao-Dong Chen, Jian-Wen Dong

Optics · Cross-listed from cond-mat.mes-hall

Optical observation of interlayer spin correlation

Akiyoshi Park, Pranshoo Upadhyay, Andrey Grankin et al. (+16 more)

Spin correlations govern numerous collective behaviors of quantum materials, underpinning exotic phenomena such as unconventional superconductivity and topological magnetism. In layered materials, the interlayer spin correlation is important because it characterizes the magnetic ground state and determines spin transport across the interface. Yet interlayer spin correlations have remained hard to measure directly, leaving one of the most basic quantities of two-dimensional magnetism out of experimental reach. Here we provide the first direct optical probe of interlayer spin correlations, using two-magnon Raman scattering in the van der Waals antiferromagnet (AFM) CrSBr, in remarkable agreement with a microscopic spin-wave model without any fitting parameter. Moreover, the two-magnon channel switches on only in the AFM state and vanishes when a magnetic field takes the crystal to a ferromagnetic state. We furthermore establish an exciton-mediated variant of the technique, where tuning the laser near the exciton resonance enhances the signal roughly tenfold due to the exciton's large oscillator strength. Magnon-pair Raman spectroscopy thus opens a direct optical window into interlayer spin correlations in van der Waals magnets, extendable to twisted bilayers and proximity-coupled heterostructures.
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All authors Akiyoshi Park, Pranshoo Upadhyay, Andrey Grankin, Emil Viñas Boström, Mahdi Ghafariasl, Hassan Alnatah, Masoud Mohammadi-Arzanagh, Gautam Nambiar, Beini Gao, Alireza Alvandi, Sakthi Rajmano Madhan Kumar, Ghadah Alshalan, Isaac Sherwood, Mahmoud Jalali Mehrabad, You Zhou, Arun Ramanathan, Xavier Roy, Angel Rubio, Mohammad Hafezi

Optics · Cross-listed from quant-ph

Rayleigh-Wood Threshold Controls Nonlinear Photon Correlations in Atomic Arrays

Zai-Cheng Xiao and Li-Ping Yang

Two-dimensional atomic arrays provide versatile free-space quantum optical interfaces by coupling photons to collective lattice modes. Here we show that the Rayleigh–Wood anomaly provides a sharp control mechanism for both linear and nonlinear photon scattering from a single atomic layer. As the lattice spacing crosses the diffraction threshold, newly opened radiative channels rapidly broaden the dominant collective modes, converting specular reflection into diffuse off-axis scattering. In transmission, momentum-space Fano interference between the incident and collectively scattered fields produces single-photon intensity zeros that strongly reshape the momentum-space correlation pattern. An integrated nonlinear contrast reveals a pronounced channel asymmetry: upon opening the Rayleigh channels, the connected two-photon contribution drops by several orders of magnitude relative to the factorized background in transmission, while remaining close to unity in reflection. These results establish radiative diffraction thresholds as a control principle for quantum nonlinear optics in single-layer atomic arrays.
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Optics

Metaphotonics for High-Harmonic Generation

Pavel Tonkaev, Anton Rudenko, Ziwen Wang et al. (+2 more)

We summarise the recent advances on the generation of high-order harmonics in optical metaphotonic structures such as isolated resonators and metasurfaces. In such subwavelength-patterned structures, extreme nonlinear effects are expected from the interaction between ultrafast laser pulses and structured planar surfaces which support various resonances. High-harmonic generation (HHG) is considered as an effective tool for realising extreme ultraviolet light sources and attosecond pulses, and it was observed previously in gases, liquids, and solids. Resonant metaphotonics can offer a novel sub-wavelength platform for the HHG effects, and it can provide new strategies for the design of efficient integrated light sources. We start our discussion from a brief overview of HHG in gases, liquids, and unstructured solids, and then move to summarising the recent experimental observations of HHG in individual resonant nanoparticles and resonant dielectric metasurfaces. We focus on different types of resonances (plasmonic vs. Mie resonances vs. bound states in the continuum), and also present the observation of non-integer power dependencies of the generated harmonics driven by strong resonances. We also mention current unsolved problems and identify new promising research directions involving HHG in metasurfaces.
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All authors Pavel Tonkaev, Anton Rudenko, Ziwen Wang, Tran Trung Luu, Yuri Kivshar

Optics · Cross-listed from quant-ph

Polarisation-resolved identification of spontaneous four-wave mixing processes in a multimode fused tapered fibre coupler

Jefferson Flórez, Chams Baker, Benjamin J. Sussman et al. (+3 more)

Integrated quantum photonics benefits from photon-pair sources that generate photons directly in waveguides, where they can be efficiently collected, routed, and manipulated. Here, we investigate spontaneous four-wave mixing (SFWM) in a fused tapered-fibre microcoupler formed from two single-mode fibres, and report four contributions. First, we observe SFWM photon pairs in this multimode device: its elliptical central region supports three spatial mode profiles, each with two polarisations, giving six guided modes, and pumping it near 800 nm yields two photon pairs at 648/1048 nm and 660/1021 nm. Second, because several allowed SFWM processes produce similar wavelengths and wavelength alone does not identify their origin, we distinguish the processes by combining phase-matching calculations and selection rules with two independent polarisation measurements: the dependence of the coincidence rate on pump polarisation and polarisation tomography of the generated photons. Third, within the present geometrical model, we find that the 660/1021 nm pair is generated by co-polarised pump photons in a process occurring entirely within a single higher-order mode, whereas the 648/1048 nm pair is generated by orthogonally polarised pump photons in an intermodal process coupling a fundamental and a higher-order mode; the signal photons from the two processes have similar output polarisations, while the idler photons are nearly orthogonal. Fourth, extending the analysis beyond the measured operating point, we theoretically identify pairs of simultaneous SFWM processes driven by a common pump that could generate either polarisation entanglement or composite spatial-polarisation entanglement, depending on the pump wavelength. More generally, this work provides a practical strategy for identifying intermodal SFWM processes in multimode waveguides when spectral information alone is insufficient.
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All authors Jefferson Flórez, Chams Baker, Benjamin J. Sussman, Xiaoyi Bao, Jeff S. Lundeen, Lambert Giner

Tuesday 29 September

Optics · Cross-listed from eess.IV

LensBridge: Frequency-Guided Compound Degradation Adaptation for Lens Aberration Correction and Veiling Glare Removal

Xiaolong Qian, Zhonghua Yi, Qi Jiang et al. (+4 more)

Simplified optical systems often exhibit residual lens aberrations and Veiling Glare (VG), resulting in spatially varying blur and contrast reduction. Large-scale Lens Libraries (LensLib) enable reusable aberration correction models by covering diverse Point Spread Functions (PSFs), but their aberration-only training distribution does not include target-specific veiling glare. Extending such foundations to compound degradation is challenging because realistic target-system compound pairs are difficult to obtain. To address this challenge, we propose LensBridge, a two-stage framework that first establishes a reusable aberration correction foundation and then adapts it to compound optical degradation using only a few unpaired target observations. In Stage I, we build a PSF-aware one-step diffusion foundation by constructing discrete degradation priors from LensLib PSFs and learning to retrieve them directly from aberrated images, enabling PSF-aware correction without requiring explicit PSF at inference. In Stage II, we adapt this foundation to compound degradation through frequency-domain guidance. At the data level, Frequency-guided Degradation Completion (FDC) transfers target low-frequency characteristics to LensLib aberrated images while preserving aberration structures to synthesize compound training pairs; at the model level, Frequency-guided Pseudo Decomposition (FPD) forms aberration- and VG-dominant pseudo observations to condition separate adaptation branches. Extensive experiments across multiple optical systems demonstrate that LensBridge effectively extends reusable aberration correction foundations to joint aberration correction and veiling glare removal without target-system paired supervision. All code will be available at https://github.com/XiaolongQian/LensBridge.
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All authors Xiaolong Qian, Zhonghua Yi, Qi Jiang, Kailun Yang, Shuhang Xie, Shaohua Gao, Kaiwei Wang

Optics · Cross-listed from quant-ph

Normal-ordered perturbative expansion for matter systems interacting with continuous-mode quantum photon fields

Liwen Ko, Robert L. Cook and K. Birgitta Whaley

We develop a normal-ordered perturbative expansion method to compute the reduced dynamics of a matter system interacting with a general continuous-mode photon field state. In this expansion, all field correlation functions are automatically normal-ordered, while the free evolution is modified by spontaneous emission. Our normal-ordered expansion elucidates the physical meaning of non-normal-ordered correlation terms in the conventional expansion. Furthermore, under intermediate coupling strengths, the normal-ordered expansion converges significantly faster than the conventional expansion, allowing one to extend the applicability of perturbative expansion to much larger matter-light coupling strengths. Interestingly, given an $m$-photon Fock state input, the normal-ordered expansion truncates exactly at the $2m$-th order. For special classes of input states, we show the normal-ordered expansion is consistent with the corresponding master equations. We demonstrate the normal-ordered expansion numerically for a two-level system interacting with coherent state, $m$-photon Fock state, and cat state inputs.
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Optics

Nonreciprocal Polychromatic Radiation from a Space-Time-Modulated Omega-Topology Meta-Transceiver

Sajjad Taravati and Ya-Adama Kabia

Time-modulated metasurfaces offer a magnet-free route to nonreciprocal wave control, but existing realizations modulate only the electric response of the structure, and proposals for simultaneous electric-magnetic modulation have so far remained theoretical. We experimentally demonstrate a space-time-periodic meta-transceiver whose omega-topology unit cell integrates varactor-loaded electric (dipole) and magnetic (ring) resonators, both modulated by a common unidirectional traveling-wave bias. This modulation gives rise to two independent, experimentally resolved manifestations of nonreciprocity: on transmit, the incident field is efficiently converted into a polychromatic comb of harmonics that radiate at distinct, bias-reconfigurable angles, while on receive, the same modulation instead suppresses frequency conversion and routes the incoming wave preferentially toward one port over the other. These two effects - nonreciprocal polychromatic conversion and unidirectional wave routing - arise from a single unidirectional space-time modulation but act on distinct physical observables, frequency content and spatial direction, respectively. Measured near the electric resonance with a shared bias, these effects characterize the complete cell and do not separate its electric and magnetic contributions. Our results establish the space-time-modulated omega-topology cell as an experimentally viable platform for reconfigurable, full-duplex nonreciprocal wave transformation.
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Optics · Cross-listed from quant-ph

Input-output formulation of quantum light spectroscopy

Liwen Ko, Robert L. Cook and K. Birgitta Whaley

We develop an input-output formulation of quantum light spectroscopy (QLS) by combining the input-output theory, commonly used in quantum optics, with the perturbative expansion method for nonlinear spectroscopy, commonly used in chemical physics. The use of the input-output relation provides physical intuition for how matter systems alter the photon input field. As examples, we consider linear spectroscopy, fluorescence, intra-beam $G^{(2)}$, and Hong-Ou-Mandel correlation. For certain initial photon states, we show that signal expectation values can be evaluated non-perturbatively. For general initial photon states, the signals can be computed via a perturbative expansion of the input-output relation. The input-output relation reduces the work needed to perturbatively expand the optical signal, allowing for cleaner derivations and clearer physical interpretations.
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Optics · Cross-listed from cond-mat.mtrl-sci

Energy-Efficient Non-Volatile Photonic Switching via Composition-Engineered Sn-Doped GST

Xingyu Zhang, Daniel Vaz, Chi-Yi Kao et al. (+5 more)

Integrated phase-change photonics allows non-volatile optical switching with no static power draw, but the electrical energy needed to reversibly switch the material's state limits practical scaling. Efforts to improve energy efficiency have mostly focused on device optimization, such as redesigning the integrated microheater or waveguide. Here, we take a different, materials-centered approach by alloying Sn to Ge$_2$Sb$_2$Te$_5$ (GST) without altering the device structure or CMOS compatibility. We explore Sn concentrations ranging from 0 at.% to 20 at.% on waveguide-integrated PN and PIN heaters and observe a reduction in both the amorphization and crystallization energies in the 5–10 at.% range. At higher Sn content this trend reverses and the switching energy increases, which is suggestive of phase segregation when combined with TEM compositional analysis. A 10 at.% Sn device continues to switch over 1000 cycles, with observable degradation in the extinction ratio from 4.5 $\pm$ 0.3 dB to 2.4 $\pm$ 0.3 dB. Post-cycling imaging shows PCM migration and void formation which we believe can be further improved by enhanced encapsulation. Overall, Sn alloying offers a route to lower programming energy in integrated phase-change photonics without negatively affecting the switching speed relative to un-doped GST.
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All authors Xingyu Zhang, Daniel Vaz, Chi-Yi Kao, Marcus Tamura, Xiong Feng, Carlos A. Rios Ocampo, Bhavin J. Shastri, Nathan Youngblood

Optics · Cross-listed from quant-ph

Interferometric Readout of Momentum-Space Topology in a Programmable Dissipative Photonic Circuit

Andrea Cataldo, Emil J. Bergholtz, Daniel Leykam et al. (+2 more)

Topology under non-Hermitian dynamics is encoded in the phase of the bulk evolution, yet strong dissipation suppresses the amplitudes carrying it. We resolve this using a programmable photonic integrated circuit that implements such dynamics in synthetic momentum space through unitary dilation, with the phase recovered by phase-shifted interferometry. For the non-Hermitian Su-Schrieffer-Heeger model, the method distinguishes trivial and non-trivial Zak phases and yields a coherence winding $q=\pm 1$ induced by an exceptional point. Extending to a synthetic torus via a Rice-Mele pump gives the first Chern number $\mathrm{Ch}_1=0,1$ for trivial and non-trivial cycles, showing a programmable route to momentum-space topology under strongly dissipative non-Hermitian dynamics.
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All authors Andrea Cataldo, Emil J. Bergholtz, Daniel Leykam, Jun Gao, Ali W. Elshaari

Optics

Multimode radiation pressure actuation of a mechanical resonator using a spatial mode sorter

Morgan E. Choi, Daniel Allepuz-Requena, Diego Torres-Barajas et al. (+2 more)

Multimode optomechanical scattering is ubiquitous in free-space imaging protocols, but its radiation pressure counterpart is challenging to detect with extended objects. Here we demonstrate radiation pressure actuation of a nanomechanical membrane using dynamically structured light from a spatial mode demultiplexer (SPADE). Using single- and two-mode illumination, we directly image the position-dependent intermodal coupling between Hermite-Gauss (HG) modes as mediated by the membrane's motion. As an application, we use multimode radiation pressure feedback to simultaneously cool two nearly degenerate membrane modes. Our results are applicable to diverse mechanical systems and serve as a stepping stone to SPADE-based quantum optomechanics.
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All authors Morgan E. Choi, Daniel Allepuz-Requena, Diego Torres-Barajas, Neshat Sadafi, Dalziel J. Wilson

Optics

Receiver-Informed Transmitter for 200Gbps C-Band Direct-Detection PON With FFE-Free ONU

Safiya Dabwan, Geraldo Gomes and Stylianos Sygletos

We introduce and numerically demonstrate receiver-informed transmitter (RITX) waveform design for 200 Gb/s PAM-4 C-band direct-detection PON. Using 50 GHz optoelectronics, RITX achieves N1/E1 loss classes with FFE-Free ONU, enabling 40 km reach and 48.1%/66.7% network-wide complexity savings at 1:64/1:128 splits
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Optics · Cross-listed from cond-mat.mes-hall

Interlayer dark excitons in a van der Waals heterostructure

Rundong Ma, Konstantin Davydov, Liuxin Gu et al. (+9 more)

Interlayer excitons in transition metal dichalcogenide (TMD) heterostructures exhibit long lifetimes and long-range transport, making them promising for excitonic devices and quantum many-body phases, such as Bose-Einstein condensates. Achieving these goals requires a precise understanding of spin-allowed bright and nominally spin-forbidden dark excitons, because the lowest-energy exciton species governs population, transport, and condensation. Despite substantial progress, unambiguously distinguishing singlet and triplet interlayer excitons has been challenging, as moiré excitons in these heterostructures can mimic their optical signatures. Here, we report the direct spectroscopic identification of bright (singlet) and dark (triplet) interlayer excitons in high-quality, dual-gated WSe$_2$/hBN/WSe$_2$ homobilayers. Electric-field-dependent photoluminescence and reflectance reveal two momentum-direct interlayer transitions with distinct spin configurations. The interlayer dark excitons obey selection rules that differ from those of bright excitons. Strikingly, interlayer dark excitons retain strong valley polarization, even with their ultralong lifetime exceeding microseconds. Finally, we demonstrate twist-angle control, wherein twist-induced electron-hole momentum mismatch modulates interlayer exciton emission. These results provide critical insights into the electronic and excitonic structure of TMD heterostructures, opening new avenues for excitonic many-body physics and optoelectronic devices.
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All authors Rundong Ma, Konstantin Davydov, Liuxin Gu, Lifu Zhang, Hassan Alnatah, Beini Gao, Ruihao Ni, Suji Park, Houk Jang, Takashi Taniguchi, Kenji Watanabe, You Zhou

Optics · Cross-listed from quant-ph

Density Matrices of Pulsed Quantum Light

Joscelyn van der Veen and Daniel F. V. James

Pulsed light is represented by density matrices whose correlation functions have different properties of stationarity and coherence. We show that these density matrices yield equivalent measurement results after accounting for finite detection times. This in turn reveals a physical meaning to the length of a quantization cavity and explains the origin of the physical spectrum for pulsed light. By generalizing the density matrices for pulsed Gaussian states of light, we can describe pulsed light of varying statistics in the same basis and relate the photocount rate to the pulse spectrum.
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Optics

Nonlinear Excitation of Ideal Anapoles

Radoslaw Kolkowski, Matti Kaivola and Andriy Shevchenko

Optical anapoles have attracted significant interest as a promising platform for enhancing light-matter interactions at the nanoscale. An ideal anapole is a localized excitation of electromagnetic fields that does not produce any far-field radiation. In practice, anapoles are not ideal due to the parasitic scattered fields associated with higher-order multipoles. Here, we use the recently established exact current multipole expansion and the exact anapole condition to design an ideal anapole in which radiation due to the multipole orders up to the electric and magnetic octupoles is either eliminated or strongly suppressed, reducing the total radiated power by orders of magnitude. We also propose and study realistic photonic structures in which such anapoles can be excited by a uniform linearly polarized plane wave through second-harmonic generation that involves the off-diagonal elements of the second-order susceptibility tensor. In this case, the anapole condition is reached off-resonantly by adjusting the polarization angle of the incident field, which allows for the anapoles to be excited over a wide spectral range in scatterers of different sizes. Using this approach, we achieve scattering suppression on the order of $10^{-4}$ in photonic structures that are half a wavelength in size. The method offers the possibility to realize localized electromagnetic excitations that are truly isolated from the surroundings.
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Optics

Attosecond correlation interferometry

Assaf Shonfeld, Keren Deutsch, Noa Yaffe et al. (+6 more)

Correlations between optical modes, ranging from classical fluctuations to quantum entanglement, are a cornerstone of modern optics and emerging quantum technologies. However, probing these correlations on the natural timescale of electronic motion - the attosecond regime - remains a major challenge. Here, we generate and characterize correlated attosecond extreme-ultraviolet (XUV) emission by perturbing gas-phase high-harmonic generation with two-mode bright squeezed vacuum. The signal and idler fields imprint their correlated amplitude and phase fluctuations onto two families of harmonics, thereby transferring these correlations from the infrared to broadband XUV modes. Even harmonics serve as an intrinsic attosecond phase-sensitive probe of these correlations through interference between indistinguishable XUV pathways. Shot-resolved covariance measurements reveal the amplitude correlations, while even-harmonics resolve the phase correlations with sub-cycle precision. Time-domain reconstruction of the correlated emission reveals a transition from maximal fluctuations to near-silence within only 30 as. Whereas the signal and idler driven attosecond pulse trains exhibit excess fluctuations individually, their joint reconstruction shows strongly suppressed fluctuations, reflecting their shared correlations. These results establish a framework for generating and probing quantum correlations on attosecond timescales, opening the door to quantum attosecond science.
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All authors Assaf Shonfeld, Keren Deutsch, Noa Yaffe, Michael Birk, Matan Even Tzur, Barak Dayan, Oren Cohen, Nirit Dudovich, Chen Mor

Optics · Cross-listed from cond-mat.mes-hall

Multistate Torsion in Zitterbewegung

Wojciech J. Jankowski

We derive nonlinear Zitterbewegung velocity of electrons augmented by band torsion in crystals. The equation of motion with multistate quantum geometric tensors and torsion culminates in transient nonlinear photocurrent responses distinct from the recently measured resonant steady-state photovoltaic effects induced by circularly polarized light. Tomography protocols for measuring the multistate quantum geometric torsion tensors are proposed.
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Optics

Optical Fourier Architecture for Universal Nonlinear Functions

Martin F. X. Mauser, Joshua Morris, Sara Galatro et al. (+2 more)

We introduce an exact algebraic architecture that evaluates an arbitrary finite Fourier series using a two-mode ($2 \times 2$) linear optical circuit, with the only tunable components being single-mode phase shifters encoding the function argument. We prove that such a circuit must exist for every Fourier series and derive an analytical method for its construction based on spectral factorisation. The resulting optical system exhibits an $\mathcal{O}(N)$ depth for an $N$-harmonic expansion, executing function evaluations in the passive optical time-of-flight. Finally, we validate our claims numerically, demonstrating that even for sequences with thousands of Fourier terms, our proposed circuit construction correctly synthesises continuous and discontinuous nonlinear functions. Our architecture thus provides a universal, deterministic foundation for single-variable nonlinear optical computing on integrated photonic platforms.
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All authors Martin F. X. Mauser, Joshua Morris, Sara Galatro, Philip Walther, Borivoje Dakic

Optics · Cross-listed from quant-ph

Operational theory for photonic circuits: generalizing linear optics beyond quantum theory

Ismaël Septembre, Matthias Kleinmann and Martin Plávala

We use the analogy between the quadrature operators in quantum optics and the phase-space coordinates in a mechanical harmonic oscillator to generalise the theory of linear optics beyond quantum theory. For this, we introduce a framework for analysing photonic circuits within generalised probabilistic theories based on quadrature operators. Using this framework, we ask whether phenomena like the Hong-Ou-Mandel effect, Mach-Zehnder interferometry are specific to quantum theory or also occur across a broader class of theories. We find the latter: the vanishing photon coincidences at the output of a beam splitter also occur for classical light and for a modified version of quantised light. Similarly, for Mach-Zehnder interferometry we find that there is no qualitative difference between quantum theory and these alternative theories. Our framework can serve as a foundation to study generalised theories of complex photonic set-ups and photonic quantum computers.
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Optics

Optical enhancement cavities operated at GHz repetition rate in burst mode regime

Kevin Dupraz, Frédéric Blanc, Ronic Chiche et al. (+7 more)

The design and experimental validation of a burst-mode optical cavity tailored for inverse Compton scattering X-ray sources is presented. The system is tailored for a room temperature linear accelerator operating at 880 MHz and utilizes a common-path injection scheme for both locking and burst laser pulses to optimize stability and simplicity. We detail the cavity construction, the laser system, and performance under two enhancement cavity finesse regimes (2000 and 7300), highlighting the sensitivity of cavity filling to carrier-envelope offset parameters. A theoretical model is developed to explain observed power dynamics and to guide optimization for high-energy storage. Our findings demonstrate reliable storage exceeding 300 mJ over 20 minutes, paving the way toward compact, high-flux inverse Compton scattering X-ray sources.
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All authors Kevin Dupraz, Frédéric Blanc, Ronic Chiche, Aurélien Martens, Antoine Courjaud, Loïc Amoudry, Titouan Le Barillec, Daniele Nutarelli, Yann Peinaud, Fabian Zomer

Optics

Multi-Parameter Error Mitigation in Parametric Down-Conversion via SU(1, 1) Composite Segmentation

Nastaran Ragerdi Kashani and Rouhollah Karimzadeh

Spontaneous parametric down-conversion is a primary resource for generating non-classical light, yet phase mismatch fluctuations from temperature drift, angular misalignment, and domain fabrication errors reduce pair-generation efficiency and phase stability. Here, we analyze error mitigation in spontaneous parametric down-conversion using composite segmentation governed by SU(1, 1) spatial dynamics. We show that odd spatial Fourier harmonics preserve error-cancellation flatness with pair-generation rates scaling asymptotically as $m^{-2}$, while a coordinate-dilation symmetry provides a design prescription for translating configurations across distinct operating wavelengths (405, 532, and 775 nm). Furthermore, an open-loop thermo-optic bias counteracts off-axis angular drift to broaden the 90%-efficiency angular acceptance half-width by 3.46x, and up to two missing poling domains are tolerated with no numerically resolved loss in relative conversion efficiency. Across a 5$^\circ$C thermal window, the generated pairs sustain a relative conversion efficiency of $\eta \ge 0.954$ with the modeled phase-coherence metric remaining above 0.95, providing a structurally robust approach for entangled photon generation without requiring continuous closed-loop feedback stabilization.
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Optics

Scanless quantum Fourier-transform mid-infrared spectroscopy for solids and surface analysis

Paul Gattinger, Michela Conti, Franziska Dietz et al. (+4 more)

Quantum infrared (IR) spectroscopy is a novel technique based on nonlinear interferometry and quantum sensing with undetected photons. In this approach, spectrally far-separated correlated photon pairs are used for detection and probing, respectively. Hence, the probing wavelength domain (mid-IR) is substantially different from the detection spectral range (near-IR), which enables shot-noise-limited and cost-effective sensing schemes attractive for applied metrology. In this paper, we implement and employ a custom-built scanless quantum Fourier-transform mid-IR (sQFTIR) microspectrometer for chemical characterization and analysis of solids and interfaces. The sQFTIR system employs spectral-domain acquisition, without the need to scan the optical delay. This results in enhanced sensitivity and short measurement times and therefore enables rapid spectral acquisitions in 100~ms spanning a spectral band from 3000 cm$^{-1}$ - 2400 cm$^{-1}$. The capabilities of our approach are demonstrated for hyperspectral mid-IR imaging of dried pharmaceuticals (insulin on a gold surface) and investigation of a security feature on a banknote. Furthermore, we demonstrate hyperspectral imaging and spectroscopy through a turbid ceramic medium.
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All authors Paul Gattinger, Michela Conti, Franziska Dietz, Andrei N. Halangescu Moldovan, Andreas W. Schell, Markus Brandstetter, Ivan Zorin

Optics

Silicon nitride nanophotonics for low-redundancy 3D convolution

Shuai Hu, Jingcheng Li, Qichao Ding et al. (+5 more)

Three-dimensional (3D) convolution extracts correlations in high-dimensional data, but overlapping receptive fields introduce substantial redundant data movement. Here, we demonstrate a silicon nitride 3D optical convolution accelerator (3D-OCA) that reconstructs receptive fields through coordinate-aware wavelength-to-group-delay mapping. A deterministically serialized input tensor is broadcast onto multiple wavelength channels, and a chirped waveguide Bragg grating (CWBG) compensates the temporal offsets associated with 3D kernel coordinates. This arrangement continuously forms neighboring receptive fields without repeatedly rearranging and loading their shared input samples. The integrated CWBG provides a differential group delay of 2691 ps and a dispersion of 124.8 ps/nm. At 20 Gbaud, spatial-spectral processing of Indian Pines data yields convolution agreement with a coefficient of determination up to 0.997 and 97.9% classification accuracy, compared with 98.9% digitally. At 10 Gbaud, the optical convolution layer preserves spatiotemporal features and achieves 92.5% accuracy on a four-class KTH video-recognition task. These results establish low-redundancy streaming 3D convolution across spectral and temporal data dimensions using the same optical delay architecture.
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All authors Shuai Hu, Jingcheng Li, Qichao Ding, Hongli Wang, Hailong Zhou, Chi Zhang, Jianji Dong, Xinliang Zhang