All-optical Measurement Research Articles

Mesoscopic ultrafast nonlinear optics—the emergence of multimode quantum non-Gaussian physics

Over the last few decades, nonlinear optics has become significantly more nonlinear, traversing nearly a billionfold improvement in energy efficiency, with ultrafast nonlinear nanophotonics in particular emerging as a frontier for combining both spatial and temporal engineering. At present, cutting-edge experiments in nonlinear nanophotonics place us just above the mesoscopic regime, where a few hundred photons suffice to trigger highly nonlinear dynamics. In contrast to classical or deep-quantum optics, the mesoscale is characterized by dynamical interactions between mean-field, Gaussian, and non-Gaussian quantum features, all within a close hierarchy of scales. When combined with the inherent multimode complexity of optical fields, such hybrid quantum-classical dynamics present theoretical, experimental, and engineering challenges to the contemporary framework of quantum optics. In this review, we highlight the unique physics that emerges in multimode nonlinear optics at the mesoscale and outline key principles for exploiting both classical and quantum features to engineer novel functionalities. We briefly survey the experimental landscape and draw attention to outstanding technical challenges in materials, dispersion engineering, and device design for accessing mesoscopic operation. Finally, we speculate on how these capabilities might usher in some new paradigms in quantum photonics, from quantum-augmented information processing to nonclassical-light-driven dynamics and phenomena to all-optical non-Gaussian measurement and sensing. The physics unlocked at the mesoscale present significant challenges and opportunities in theory and experiment alike, and this review is intended to serve as a guide to navigating this new frontier in ultrafast quantum nonlinear optics.

Optical Absorption in Tilted Geometries as an Indirect Measurement of Longitudinal Plasma Waves in Layered Cuprates.

Electromagnetic waves propagating in a layered superconductor with arbitrary momentum, with respect to the main crystallographic directions, exhibit an unavoidable mixing between longitudinal and transverse degrees of freedom. Here we show that this basic physical mechanism explains the emergence of a well-defined absorption peak in the in-plane optical conductivity when light propagates at small tilting angles relative to the stacking direction in layered cuprates. More specifically, we show that this peak, often interpreted as a spurious leakage of the c-axis Josephson plasmon, is instead a signature of the true longitudinal plasma mode occurring at larger momenta. By combining a classical approach based on Maxwell's equations with a full quantum derivation of the plasma modes based on modeling the superconducting phase degrees of freedom, we provide an analytical expression for the absorption peak as a function of the tilting angle and light polarization. We suggest that an all-optical measurement in tilted geometry can be used as an alternative way to access plasma-wave dispersion, usually measured by means of large-momenta scattering techniques like resonant inelastic X-ray scattering (RIXS) or electron energy loss spectroscopy (EELS).

All-optical instantaneous RF signal frequency measurement system based on a linear ACF with a steep slope

A new microwave photonic structure for measuring the frequency of an RF signal, to the best of our knowledge, is presented. The frequency of an unknown RF signal can be determined by simply measuring the system output optical powers. The proposed frequency measurement system can be designed so that the ratio of the two system output optical powers as a function of the RF signal frequency or the amplitude comparison function (ACF) has a steep linear slope over a wide frequency range. This enables the RF signal frequency to be measured in high resolution and high accuracy. The proposed frequency measurement system has a simple and compact structure, and is free of high-speed photodetectors as well as RF components and instruments. It also has a fast response time compared to many reported photonics-based frequency measurement systems. A proof-of-concept experiment is carried out. Experimental results show a linear ACF with a slope of more than 4.4 dB/GHz over a frequency measurement range of 5–26 GHz and a frequency measurement accuracy of better than ±0.1GHz.

All-optical measurement of magnetic fields for quantum gas experiments

We present an all-optical method to measure and compensate for residual magnetic fields present in a cloud of ultracold atoms trapped in an optical dipole trap. Our approach leverages the increased loss from the trapped atomic sample through electromagnetically induced absorption. Modulating the excitation laser provides coherent sidebands, resulting in a Λ-type pump–probe scheme. Scanning an additional magnetic offset field leads to pairs of sub-natural linewidth resonances, whose positions encode the magnetic field in all three spatial directions. Our measurement scheme is readily implemented in typical quantum gas experiments and has no particular hardware requirements.

Glioblastoma Margin as a Diffusion Barrier Revealed by Photoactivation of Plasmonic Nanovesicles.

Glioblastoma (GBM) is the most complex and lethal primary brain cancer. Adequate drug diffusion and penetration are essential for treating GBM, but how the spatial heterogeneity in GBM impacts drug diffusion and transport is poorly understood. Herein, we report a new method, photoactivation of plasmonic nanovesicles (PANO), to measure molecular diffusion in the extracellular space of GBM. By examining three genetically engineered GBM mouse models that recapitulate key clinical features including the angiogenic core and diffuse infiltration, we found that the tumor margin has the lowest diffusion coefficient (highest tortuosity) compared with the tumor core and surrounding brain tissue. Analysis of the cellular composition shows that tortuosity in the GBM is strongly correlated with neuronal loss and astrocyte activation. Our all-optical measurement reveals the heterogeneous GBM microenvironment and highlights the tumor margin as a diffusion barrier for drug transport in the brain, with implications for therapeutic delivery.

Advancing thermochemical diagnostics in kilogram-scale explosive fireballs via laser absorption spectroscopy

This article presents methodological advances in the state-of-the-art for making time-dependent, thermochemical measurements within kilogram-scale explosive post-detonation fireballs utilizing tunable laser absorption spectroscopy. This measurement capability is critical for validating multi-scale, multi-physics models of post-detonation dynamics. The technique is based on hardened gauges built around rapidly-tunable lasers and custom post-processing algorithms that provide quantitative thermochemical data interior to large and opaque explosive fireballs. The authors present a holistic overview of the technique including gauge design, the laser absorption diagnostic, and the custom data processing algorithms. Additionally, fielding high-bandwidth laser absorption probes at stand-off ranges presents new challenges in data processing that must compensate for long distance signal transmission effects. We highlight representative data from a hardened gauge measurement at 0.81 m stand-off from a 2.78 kg LX-14 explosive charge detonated in an outdoor test arena. We discuss progress in all-optical measurement of temperature, pressure, and water vapor number density at a 100 kHz repetition rate during the first 10 ms of the fireball evolution. We conclude the article with a brief discussion on our current approach for comparing hardened gauge measurements with computational fluid dynamic simulations.

Strain-modulated band structure and high harmonic generations in two-dimensional MoS2

Two-dimensional (2D) condensed matter is a material that is restricted in one direction while being periodic in the other. Since the restricted size of 2D materials is comparable to the wavelength of electrons, a quantum confinement effect may occur. Moreover, the absence of periodicity provides weak screening in 2D materials, which brings novel physical properties such as the quantum well, which is widely applied in quantum information, and the fine absorption structures in graphene. Among the 2D materials, the monolayer transition metal chalcogenides represented by MoS2 have attracted wide attention due to the direct band gap in the visible light region (1.8 eV) and valley polarizations, which are prospective for solar cells as well as photoelectric devices. High-harmonic generation (HHG) is a strong non-linear process during which a high-energy laser impulse is applied to materials and high-harmonic radiations are yielded. As a typical ultrafast dynamic, HHG has important applications in laser generation, such as EUV lithographic metrology and high-resolution coherent imaging. According to the Bloch oscillation model, HHG is highly dependent on band structure. Here, we report the strain-dependent HHG dynamics in MoS2. Further investigation reveals that the strain-dependence of HHG is caused by band modulation under different strains, which is dominant during HHG. Our research sheds light on ways to achieve effective modulations in ultrafast dynamics, implying an all-optical measurement band structure in strained materials.

All-optical birth time measurement of circularly polarized attosecond pulses

The advancement of attosecond science has made circularly polarized attosecond pulses increasingly essential. Despite ongoing research on their generation, few studies have explored methods of characterizing these pulses. To address this issue, we propose an all-optical technique for measuring the birth time of circularly polarized attosecond pulses generated by crossing two counter-rotating circularly polarized pulses in a noncollinear geometry. By introducing a weak second-harmonic to alter the phase of the electron trajectories, the even and odd high harmonics (HHs) are separated spatially, allowing one to determine the birth time of the attosecond pulses from the modulation of the even harmonics as a function of delay between the perturbing and driving fields. The spatial isolation of the even harmonic allows extending the method to the characterization of isolated attosecond pulse, which is challenging for existing in-situ methods. It provides deeper insight into the control and generation of circularly polarized HHs in both time and space domains, which will benefit circular dichroism attosecond metrology.

Measuring Near-Field Radiative Heat Transfer in a Graphene- SiC Heterostructure

Near-field radiative heat transfer between closely placed objects may exceed the far-field black-body radiation limit by orders of magnitude, especially between polaritonic materials. Great efforts have been made to experimentally measure this fundamental effect in various systems. In this work, we manage to experimentally characterize the near-field thermal emission between less explored plasmon-phonon hybrid material systems made of a graphene-$\mathrm{Si}\mathrm{C}$ heterostructure. The experiment is carried out using a custom-designed all-optical measurement setup. A heat flux enhancement factor of 26 over the black-body radiation limit is obtained at a 150-nm vacuum gap. Three fundamental modes, i.e., surface plasmon and phonon polaritons with frustrated modes, are identified to contribute the near-field heat flux in this hybrid system. The measured results are well reproduced from the analytical calculations, indicating the robustness of the measurement. Deeper experimental explorations for near-field heat transfer between more complicated material systems could be encouraged in the future.

Multi-frame multi-exposure shock wave imaging and pressure measurements.

Shock wave visual detection was traditionally performed using streak cameras, limited to homogeneous shock wave emission, with the corresponding shock wave pressure measurements available at rather large distances or numerically estimated through equation of state for water. We demonstrate a multi-frame multi-exposure shock wave velocity measurement technique for all in-plane directions of propagation, based on custom-built illumination system allowing multiple illumination pulses within each frame at multi-MHz frame rates and at up to 200 MHz illumination pulse repetition frequency at sub-nanosecond pulse durations. The measurements are combined and verified using a fiber-optic probe hydrophone, providing independent shock wave pressure and time-of-flight measurements, creating a novel all-optical measurement setup. The measured pressures at distances around 100 µm from the plasma center exceed 500 MPa, while camera-based measurements at even shorter distances indicate pressures above 1 GPa.

Few-cycle vacuum squeezing in nanophotonics.

One of the most fundamental quantum states of light is the squeezed vacuum, in which noise in one of the quadratures is less than the standard quantum noise limit. In nanophotonics, it remains challenging to generate, manipulate, and measure such a quantum state with the performance required for a wide range of scalable quantum information systems. Here, we report the development of a lithium niobate-based nanophotonic platform to demonstrate the generation and all-optical measurement of squeezed states on the same chip. The generated squeezed states span more than 25 terahertz of bandwidth supporting just a few optical cycles. The measured 4.9 decibels of squeezing surpass the requirements for a wide range of quantum information systems, demonstrating a practical path toward scalable ultrafast quantum nanophotonics.

Attosecond measurement via high-order harmonic generation in low-frequency fields

The spectral phase of high-order harmonic and attosecond pulses is typically shaped by the interaction of the recollision electron with the strong field in the continuum. However, the phase of the transition-moment coupling bound and continuum states can be significant in shaping the emitted radiation. It has commonly been assumed that the propagation and recombination steps of the recollision process can be described independently. Here, we investigate the effect that the transition moment has recollision trajectories by incorporating the transition-moment phase into the Lewenstein model of recollision. We then use our model to investigate the all-optical measurement of the transition-moment phase around the Cooper minimum in argon and the spectral minimum due to two-center interference in a diatomic molecular system. Our results indicate that, while all-optical methods are generally sensitive to the transition-moment phase, they are insensitive to the phase shifts due to two-center interference and ionic structure. Thus, we have resolved the apparent discrepancy between studies with conflicting conclusions regarding the sensitivity of all-optical approaches to the transition-moment phase. Our work demonstrates that all-optical measurements focus on photorecombination time delays attributable to electronic structure and dynamics. Our method will allow any laboratory capable of generating attosecond pulses to perform these measurements, even at wavelengths at which the single-photoionization cross section becomes small.

Near-field imaging of dipole emission modulated by an optical grating

Attosecond measurements have been achieved in technically demanding pump-probe experiments by photoelectron streaking with stable infrared lasers and extreme-ultraviolet (XUV) instruments. Here, we demonstrate an efficient single-image all-optical measurement of an isolated attosecond pulse for its complete temporal characterization. We create the attosecond pulse with a 0.1-mJ, few-cycle, infrared pump beam and modulate it with an obliquely incident same-frequency weak beam. By refocusing the XUV beams, we obtain a spectrally resolved XUV image, showing the spectral phase of the attosecond pulse. Near-field imaging allows us to measure our pulse in 150 shots. This efficiency will be important for attosecond pulses in the water-window region. For complex systems, multi-electron dynamics is encoded in the temporal structure of attosecond pulses.

Spatio-temporal characterization of attosecond pulses from plasma mirrors.

Reaching light intensities above 1025 W/cm2 and up to the Schwinger limit of the order of 1029 W/cm2 would enable testing fundamental predictions of quantum electrodynamics. A promising — yet challenging — approach to achieve such extreme fields consists in reflecting a high-power femtosecond laser pulse off a curved relativistic mirror. This enhances the intensity of the reflected beam by simultaneously compressing it in time down to the attosecond range, and focusing it to sub-micrometre focal spots. Here we show that such curved relativistic mirrors can be produced when an ultra-intense laser pulse ionizes a solid target and creates a dense plasma that specularly reflects the incident light. This is evidenced by measuring the temporal and spatial effects induced on the reflected beam by this so-called ʼplasma mirrorʼ. The all-optical measurement technique demonstrated here will be instrumental for the use of relativistic plasma mirrors with the upcoming generation of Petawatt lasers that recently reached intensities of 5 × 1022 W/cm2, and therefore constitutes a viable experimental path to the Schwinger limit.

All-optical single-shot complete electric field measurement of extreme ultraviolet free electron laser pulses

Recent advances in ultrafast extreme ultraviolet (EUV) and x-ray light sources provide direct access to fundamental time and length scales for biology, chemistry, and materials physics. However, such light pulses are challenging to measure due to the need for femtosecond time resolution at difficult-to-detect wavelengths. Also, single-shot measurements are needed because severe pulse-to-pulse fluctuations are common. Here we demonstrate single-shot, complete field measurements by applying a novel version of frequency resolved optical gating. An EUV free electron laser beam creates a transient grating containing the pulse’s electric field information, which is read out with a 400 nm probe pulse. By varying the time delay between two copies of the EUV pump, rather than between the pump and the probe, we separate the needed coherent wave mixing from the slow incoherent response. Because this approach uses photoionization, it should be applicable from the vacuum ultraviolet to hard x rays.

A Simple and All-Optical Microwave Doppler Frequency Shift and Phase Measurement System Based on Sagnac Loop and I/Q Detection

Microwave measurement system, which acquires the microwave parameters including frequency, phase, and amplitude information, is one of the backbone parts in modern radio frequency (RF) systems. In this article, a simple and novel optical approach is proposed to perform broadband in-phase/quadrature (I/Q) downconverter and is applied in Doppler frequency shift (DFS) and phase measurement. In the proposed approach, a Mach–Zehnder modulator (MZM) is used to modulate both RF and local oscillator (LO) signals. A Sagnac loop structure and optical coupler are used to suppress the optical carrier. A dense wavelength division multiplexer (DWDM) and two photodiodes (PDs) are used to build I/Q channels. The proposed approach has an all-optical structure and no frequency-dependent electronic devices are needed. In the experimental demonstration, the I/Q downconverter achieves low phase imbalance ( $\text {dB}\cdot \text {Hz}^{2/3}$ ) over an ultrawide operating frequency. When DFS measurement is implemented using the I/Q downconverter, the microwave DFS is measured with a maximum measurement error of 8 Hz, and a clear direction can be obtained. When the system is applied for phase detection, the phase shift is successfully estimated with a measurement error of less than 7°.

Interferometric and fluorescence analysis of shock wave effects on cell membrane

Shock waves generated by laser pulses have been gaining attention for biological and medical applications in which shock-induced cell membrane deformation influences cell permeation. However, the mechanisms through which the deformation of cell membranes affects permeability remain mostly unknown because of the difficulty of observing in real time the transient and dynamic behaviors of the shock waves and the cells. Here we present an all-optical measurement method that can quantitatively capture the pressure distribution of the propagating shock wave and simultaneously monitor the dynamic behavior of cell membranes. Using this method, we find that the profile of the shock wave dictates the cell membrane permeation. The results suggest a possible mechanism of membrane permeation where sharp pressure gradients create pores on the membrane. Our measurement will foster further understanding of the interaction of shock waves with cells, while the proposed mechanism advances biological and medical applications of shock waves.

Optical Fiber Ultrasound Probe for Radiofrequency Ablation Temperature Monitoring: In-Vitro Results

Radiofrequency ablation (RFA) is a clinical technique which inactivates targeted biological tissues by heating to remove tumors or to cure diseases like atrial fibrillation. Temperature is a fundamental parameter for ablation dose control and treatment effect evaluation. This paper presents the design, fabrication and characterization of an all-optical fiber ultrasound temperature measurement probe. The novelty of this probe is to both generate and receive ultrasound optically through fibers. The ultrasound generation was achieved with a 425 μm multi-mode fiber based on photoacoustic (PA) principle and the receiving part applied a 125 μm single mode fiber with a Fabry-Perot (FP) interferometer on its tip. The temperature was measured through detecting the time-of-flight (TOF) of the reflected ultrasound pulses. The probe's feasibility of measuring temperature during ablation had been verified in vitro while excised bovine liver tissue was used as the ultrasound reflector.

Graphene Oxide Functionalized Optical Planar Waveguide for Water Content Measurement in Alcohol

An abrupt change in optical transmission characteristic of a graphene oxide (GO) coated optical planar waveguide was observed. This observation was based on the peculiar characteristics of the graphene oxide film, namely its high transverse-electric polarized light propagation loss, highly selective permeability of water, and change in optical propagation characteristic in the presence of water. The as-fabricated GO-coated optical waveguide showed a large polarization dependent loss of ~32 dB in the C-band optical fiber communication window (1550 nm). The response of the proposed sensor was first tested by using water. When a drop of water was applied onto the GO coating, the large polarization dependent loss was fully suppressed almost instantaneously. This effect was reversible as the polarization dependent loss was restored after complete water evaporation from the GO coating. All-optical measurement of water content in alcohol was then demonstrated by using the GO-coated optical waveguide. By analyzing the drying profile of the water-alcohol mixture, water content in the range of 0.2 volume % – 100 volume % could be measured. These measurements were carried out by using solution volume of 1.0 μL only. The all-optical sensing nature of the proposed sensor has potential applications in in-situ monitoring of water content in alcohol.

All-optical measurement of high-order fractional molecular echoes by high-order harmonic generation.

An all-optical measurement of high-order fractional molecular echoes is demonstrated by using high-order harmonic generation (HHG). Excited by a pair of time-delayed short laser pulses, the signatures of full and high order fractional (1/2 and 1/3) alignment echoes are observed in the HHG signals measured from CO 2 molecules at various time delays of the probe pulse. By increasing the time delay of the pump pulses, much higher order fractional (1/4) alignment echo is also observed in N 2O molecules. With an analytic model based on the impulsive approximation, the spatiotemporal dynamics of the echo process are retrieved from the experiment. Compared to the typical molecular alignment revivals, high-order fractional molecular echoes are demonstrated to dephase more rapidly, which will open a new route towards the ultrashort-time measurement. The proposed HHG method paves an efficient way for accessing the high-order fractional echoes in molecules.