Optics (physics.optics)

Abstract: Generated in high-Q optical microresonators, dissipative Kerr soliton microcombs constitute broadband optical frequency combs with chip sizes and repetition rates in the microwave to millimeter-wave range. For frequency metrology applications such as spectroscopy, optical atomic clocks and frequency synthesizers, octave-spanning soliton microcombs generated in dispersion optimized microresonator are required, which allow self-referencing for full frequency stabilization. In addition, field-deployable applications require the generation of such soliton microcombs simple, deterministic, and reproducible. Here, we demonstrate a novel scheme to generate self-emerging solitons in integrated lithium niobate microresonators. The single soliton features a broadband spectral bandwidth with dual dispersive waves, allowing 2f-3f self-referencing. Via harnessing the photorefractive effect of lithium niobate to significantly extend the soliton existence range, we observe a spontaneous yet deterministic single-soliton formation. The soliton is immune to external perturbation and can operate continuously over 13 hours without active feedback control. Finally, via integration with a pre-programed DFB laser, we demonstrate turnkey soliton generation. With further improvement of microresonator Q and hybrid integration with chip-scale laser chips, compact soliton microcomb devices with electronic actuation can be created, which can become central elements for future LiDAR, microwave photonics and optical telecommunications.

Abstract: Pushing the limits of precision and reproducibility in ultrafast laser-based nanostructuring requires detailed control over the properties of the illumination. Most traditional methods of laser-based manufacturing rely on the simplicity of Gaussian beams for their well-understood propagation behavior and ease of generation. However, a variety of benefits can be obtained by moving beyond Gaussian beams to single or multiple tailored beams working toward optimal spatial and temporal control over the beam profiles. In this chapter, we center our attention on methods to generate and manipulate complex light beams and the resulting material interactions that occur in response to irradiations with these non-traditional sources. We begin with a discussion on the main differences between Gaussian and more complex light profiles, describing the mechanisms of phase and spatial control before narrowing the discussion to approaches for spatial structuring associated with materials processing with ultrashort laser pulses. Such structuring can occur in both far-field propagating architectures, considering rapidly varying spatial profiles generated mechanically or optically, as well as near-field, non-propagating beams associated with plasmonic and dielectric systems. This chapter emphasizes some of the unique abilities of complex light to shape materials at the nanoscale from a fundamental perspective while referencing potential applications of such methods.

Abstract: The complex range of interactions between electrons and electromagnetic fields gave rise to countless scientific and technological advances. A prime example is photon-induced nearfield electron microscopy (PINEM), enabling the detection of confined electric fields in illuminated nanostructures with unprecedented spatial resolution. However, PINEM is limited by its dependence on strong fields, making it unsuitable for sensitive samples, and its inability to resolve complex phasor information. Here, we leverage the nonlinear, over-constrained nature of PINEM to present an algorithmic microscopy approach, achieving far superior nearfield imaging capabilities. Our algorithm relies on free-electron Ramsey-type interferometry to produce orders-of-magnitude improvement in sensitivity and ambiguity-immune nearfield phase reconstruction, both of which are optimal when the electron exhibits a fully quantum behavior. Our results demonstrate the potential of combining algorithmic approaches with novel modalities in electron microscopy, and may lead to various applications from imaging sensitive biological samples to performing full-field tomography of confined light.

Abstract: The phase-coherent frequency division of a stabilized optical reference laser to the microwave domain is made possible by optical frequency combs (OFCs). Fundamentally, OFC-based clockworks rely on the ability to lock one comb tooth to this reference laser, which probes a stable atomic transition. The active feedback process associated with locking the comb tooth to the reference laser introduces complexity, bandwidth, and power requirements that, in the context of chip-scale technologies, complicate the push to fully integrate OFC photonics and electronics for fieldable clock applications. Here, we demonstrate passive, electronics-free synchronization of a microresonator-based dissipative Kerr soliton (DKS) OFC to a reference laser. We show that the Kerr nonlinearity within the same resonator in which the DKS is generated enables phase locking of the DKS to the externally injected reference. We present a theoretical model to explain this Kerr-induced synchronization (KIS), and find that its predictions for the conditions under which synchronization occur closely match experiments based on a chip-integrated, silicon nitride microring resonator. Once synchronized, the reference laser is effectively an OFC tooth, which we show, theoretically and experimentally, enables through its frequency tuning the direct external control of the OFC repetition rate. Finally, we examine the short- and long-term stability of the DKS repetition rate and show that the repetition rate stability is consistent with the frequency division of the expected optical clockwork system.

Abstract: Coherent dual-comb spectroscopy (DCS) is a form of Fourier transform spectroscopy (FTS) benefiting from advantageous properties of optical frequency combs. Unlike traditional FTS, DCS enables high-resolution measurements at high speeds because it does not face the trade-off between resolution and update rate inherent to mechanical scanning of the optical delay. However, high complexity of the optical system and limited sensitivity of the measurements remain major challenges for deploying broadband DCS in the short-wave infrared (SWIR, 1.4-3 {\mu}m) and mid-infrared (mid-IR, >3 {\mu}m) regions where there are strong ro-vibrational absorption bands of many molecules. We address these challenges via a wavelength-tunable dual-comb optical parametric oscillator (OPO) where both OPO pump beams are generated in a single laser cavity, while both signal and idler beams are generated in a single OPO cavity. These linear cavities are based on spatial multiplexing and operated in free-running mode. The near-common-path of the beams in each cavity leads to low uncorrelated noise, enabling comb-line-resolved measurements at a moderate optical comb line spacing of 250 MHz. The source is operated at an instantaneous bandwidth below 1 THz, resulting in high power per comb line of up to 70 {\mu}W (signal) and 150 {\mu}W (idler). Combined with repetition rate differences of up to 20 kHz, aliasing-free measurements are enabled. The accessible spectrum spans 1290 nm to 1670 nm (signal) and 2700 nm to 5160 nm (idler). In a proof-of-principle SWIR DCS experiment, we achieve a signal-to-noise ratio (SNR) of 34 dB for an integration time of 2 s.

Abstract: Time-resolved electron microscopy aims at tracking nanoscale excitations and dynamic states of matter with a temporal resolution ultimately reaching the attosecond regime. Periodically time-varying fields in an illuminated specimen cause free-electron inelastic scattering, which enables the spectroscopic imaging of near-field intensities. However, access to the evolution of nanoscale fields and structures within the light cycle requires a sensitivity to the optical phase. Here, we introduce Free-Electron Homodyne Detection (FREHD) as a universally applicable approach to electron microscopy of phase-resolved optical responses at high spatiotemporal resolution. In this scheme, a phase-controlled reference interaction serves as the local oscillator to extract arbitrary sample-induced modulations of a free-electron wave function. We demonstrate this principle through the phase-resolved imaging of plasmonic fields with few-nanometer spatial and sub-cycle temporal resolutions. Due to its sensitivity to both phase- and amplitude-modulated electron beams, FREHD measurements will be able to detect and amplify weak signals stemming from a wide variety of microscopic origins, including linear and nonlinear optical polarizations, atomic and molecular resonances and attosecond-modulated structure factors.

Abstract: The electron motion in atoms and molecules is at the heart of all phenomena in nature that occur outside the nucleus. Recently, ultrafast electron and X-ray imaging tools have been developed to image the ultrafast dynamics of matter in real time and space. The cutting-edge temporal resolution of these imaging tools is on the order of a few tens to a hundred femtoseconds, limiting imaging to atomic dynamics. Hence electron motion imaging remains beyond the reach. Here, we achieved attosecond electron imaging temporal resolution in a transmission electron microscope, orders of magnitude faster than the highest reported imaging resolution, to demonstrate, which we coin it as (attomicroscopy) to image the field-induced electron dynamics in neutral multilayer graphene. Our results show that the electron motion between the carbon atoms in graphene is due to the field-driven electron dynamics in the conduction band and depends on the field waveform, strength, and polarization direction. This attomicroscopy imaging provides more insights into the electron motion of neutral matter in real time and space and would have long-anticipated real-life attosecond science applications in quantum physics, chemistry, and biology.