All-optical Excitation Research Articles

Patterned magnetophotonic crystal for all-optical magnetization precession generation

A magnetophotonic crystal (MPC) with a locally etched magnetic layer is proposed for all-optical excitation of magnetization precession in the confined area. When the sample is illuminated by circularly polarized monochromatic plane wave, the optical MPC mode and the effective magnetic field excited by the inverse Faraday effect are localized inside the etched area. Therefore, the optical impact on the spin system in a magnetically ordered medium becomes confined in both lateral directions on the order of tens of nanometers and along the film thickness serving as a nanoconfined source of spin waves. The dependence of the spatial distribution of the effective magnetic field and magnetization precession amplitude on the depth and dimensions of the etched area are addressed. A proposed design of the MPC can find its applications in the field of magnetization dynamics generation as a compact and stable source of the spin waves that allows compact arrangement of the sources for some complex spin waves excitations.

A two-dimensional mid-infrared optoelectronic retina enabling simultaneous perception and encoding

Infrared machine vision system for object perception and recognition is becoming increasingly important in the Internet of Things era. However, the current system suffers from bulkiness and inefficiency as compared to the human retina with the intelligent and compact neural architecture. Here, we present a retina-inspired mid-infrared (MIR) optoelectronic device based on a two-dimensional (2D) heterostructure for simultaneous data perception and encoding. A single device can perceive the illumination intensity of a MIR stimulus signal, while encoding the intensity into a spike train based on a rate encoding algorithm for subsequent neuromorphic computing with the assistance of an all-optical excitation mechanism, a stochastic near-infrared (NIR) sampling terminal. The device features wide dynamic working range, high encoding precision, and flexible adaption ability to the MIR intensity. Moreover, an inference accuracy more than 96% to MIR MNIST data set encoded by the device is achieved using a trained spiking neural network (SNN).

Layer-dependent optically induced spin polarization in InSe

Optical control of spin in semiconductors has been pioneered using nanostructures of III-V and II-VI semiconductors, but the emergence of two-dimensional van der Waals materials offers an alternative low-dimensional platform for spintronic phenomena. Indium selenide (InSe), a group-III monochalcogenide van der Waals material, has shown promise for opto-electronics due to its high electron mobility, tunable direct bandgap, and quantum transport. There are predictions of spin-dependent optical selection rules suggesting potential for all-optical excitation and control of spin in a two-dimensional layered material. Despite these predictions, layer-dependent optical spin phenomena in InSe have yet to be explored. Here, we present measurements of layer-dependent optical spin dynamics in few-layer and bulk InSe. Polarized photoluminescence reveals layer-dependent optical orientation of spin, thereby demonstrating the optical selection rules in few-layer InSe. Spin dynamics are also studied in many-layer InSe using time-resolved Kerr rotation spectroscopy. By applying out-of-plane and in-plane static magnetic fields for polarized emission measurements and Kerr measurements, respectively, the $g$-factor for InSe was extracted. Further investigations are done by calculating precession values using a $\textbf{k} \cdot \textbf{p}$ model, which is supported by \textit{ab-initio} density functional theory. Comparison of predicted precession rates with experimental measurements highlights the importance of excitonic effects in InSe for understanding spin dynamics. Optical orientation of spin is an important prerequisite for opto-spintronic phenomena and devices, and these first demonstrations of layer-dependent optical excitation of spins in InSe lay the foundation for combining layer-dependent spin properties with advantageous electronic properties found in this material.

Foundry-processed optomechanical photonic integrated circuits

Photonic integrated circuits (PICs) are a maturing technology with foundries enabling wafer-scale PIC fabrication. At the same time, optomechanics, in which micro-/nano-optical and -mechanical structures are coupled, is well-established with many basic research and practical applications. However, optomechanical devices have so far required highly-customized fabrication that limits their inclusion in foundry-processed PICs. To address this need, we design optomechanical PICs using standard low-loss process design kit (PDK) components. Our approach ensures access to the foundry’s low-loss PDK components and enables process compatibility. As a demonstration, we design a foundry-processed optomechanical Mach-Zehnder interferometer (MZI). Measurements demonstrate that a π-phase shift can be accumulated over an optomechanical interaction length of only 60 µm and tunable phase shifting can be achieved using gradient electric force actuation. We further demonstrate all-optical excitation and readout of mechanical resonances for sensing applications. Our PDK-focused optomechanics design approach enables the co-integration of optomechanics, photonics, and electronics in a single PIC.

Ultrafast laser-induced guided elastic waves in a freestanding aluminum membrane

Ultrafast laser-induced guided acoustic waves in thin, freely suspended films are important for many applications adopting the laser-ultrasonics technique. These waves show unique dispersion relations, leading to minimal propagation losses and acoustic energy confinement. While this principle has been known, the separation of various physical effects in the formation of measured signals involving these guided acoustic waves has not been clearly elaborated. Here, we present a combined experimental and theoretical study on all-optical excitation and detection of these waves in a thin, freestanding aluminum membrane. The acoustic dynamics is excited and measured by using a femtosecond time-resolved pump-probe technique with controlled probing position, enabling spatially resolved detection. The measured signals are compared with an advanced numerical model that we developed earlier [H. Zhang et al., Phys. Rev. Appl. 13, 014010 (2020)], showing excellent agreement. The combination of experiment and simulation allows us to decode various physical effects in the signal formation, including different acoustic field components. Unknown material properties, such as acoustic attenuation coefficients, and the two complex photoelastic constants are quantitatively retrieved by fitting the measured signals. Furthermore, we provide evidence of nonlinear propagation of the excited guided acoustic waves.

Evaluation of disbonds at various interfaces of adhesively bonded aluminum plates using all-optical excitation and detection of zero-group velocity Lamb waves

Inspection of adhesively bonded metallic plates, commonly used in aircraft structures, remains challenging for modern non-destructive testing (NDT) techniques. When a probing ultrasound (US) wave interacts with the plate boundaries, it produces multiple propagating guided waves (or Lamb modes). Analysis of these waves can be complicated due to a strong geometrical dispersion. However, recent studies showed that for specific frequencies zero-group velocity (ZGV) modes exist. Any changes in a bounded structure, like delaminations, drastically alter the conditions for zero-group velocity waves, thus making this method highly sensitive for NDT applications. Laser ultrasound (LU) provides a very broad bandwidth of the generated waves, thus it is a feasible tool for a spectroscopy-based investigation of the ZGV modes. In this paper, we demonstrate an application of high-repetition rate LU with a non-contact fiber-optic Sagnac interferometer on receive for high resolution imaging of delaminations in a structure consisting of 3 epoxy-bonded aluminum plates. The investigation is supported by numerical analysis of Lamb waves existing in the structure to determine ZGV modes sensitive to delaminations at particular bonding interfaces. Tracking the selected modes permits imaging and identification of defects. We also show that mean frequency estimation of the ZGV modes can improve the contrast-to-noise ratio compared to an amplitude of the single ZGV frequency.

Optically tunable topological photonic crystal

Topological photonic insulators can enable the development of efficient integrated photonic devices with minimized scattering losses. The optical properties of the majority of topological structures proposed to date are fixed by designs such that no changes to their performance can be made once the device has been fabricated. However, tunability is important for many applications, including modulators, switches, and optical buffers. We therefore propose a straightforward way to dynamically control transmission in a silicon-based topological photonic crystal using all-optical free-carrier excitation that allows for fast refractive index modulation. The changes in both the real and imaginary parts of the refractive index cause a blue shift of the transmission spectrum by up to 20 nm and a reduction in transmission by approximately 85% . With the control mechanism used here, we can achieve switching times of the order of nanoseconds. The structures proposed here are compatible with the standard semiconductor industry fabrication process and operate at telecommunication wavelengths.

Thermophoresis in self-associating systems: probing poloxamer micellization by opto-thermal excitation.

Due to its exquisite sensitivity to interfacial properties, thermophoresis, i.e., particle motion driven by thermal gradients, can provide novel, exclusive, and often surprising information on the structural properties of colloidal or macromolecular fluids and on particle/solvent interactions at the nanoscale. Here, by using an all-optical thermal excitation technique, thermal lensing, we show that thermophoresis can be profitably exploited to investigate the self-association of an amphiphilic block copolymer, poloxamer P407, which takes place above a concentration-dependent critical micellization temperature (cmt). In particular we show that, around and above the cmt, the direction of the poloxamer thermophoretic motion displays a remarkable double sign inversion, which is fully correlated with a peak in the thermal expansivity of the solution marking the progressive dehydration of the propylene oxide groups of P407 and their incorporation into the micellar core. This rather puzzling behaviour of the thermophoretic mobility and of the Soret coefficient in the P407 micellization region can tentatively be explained by properly taking into account the temperature-dependent balance between micellized and nonassociated poloxamer chains.

Electrometry by optical charge conversion of deep defects in 4H-SiC

Optically active point defects in various host materials, such as diamond and silicon carbide (SiC), have shown significant promise as local sensors of magnetic fields, electric fields, strain, and temperature. Modern sensing techniques take advantage of the relaxation and coherence times of the spin state within these defects. Here we show that the defect charge state can also be used to sense the environment, in particular high-frequency (megahertz to gigahertz) electric fields, complementing established spin-based techniques. This is enabled by optical charge conversion of the defects between their photoluminescent and dark charge states, with conversion rate dependent on the electric field (energy density). The technique provides an all-optical high-frequency electrometer which is tested in 4H-SiC for both ensembles of divacancies and silicon vacancies, from cryogenic to room temperature, and with a measured sensitivity of [Formula: see text] Finally, due to the piezoelectric character of SiC, we obtain spatial 3D maps of surface acoustic wave modes in a mechanical resonator.

Excitation of coupled spin\u2013orbit dynamics in cobalt oxide by femtosecond laser pulses

Ultrafast control of magnets using femtosecond light pulses attracts interest regarding applications and fundamental physics of magnetism. Antiferromagnets are promising materials with magnon frequencies extending into the terahertz range. Visible or near-infrared light interacts mainly with the electronic orbital angular momentum. In many magnets, however, in particular with iron-group ions, the orbital momentum is almost quenched by the crystal field. Thus, the interaction of magnons with light is hampered, because it is only mediated by weak unquenching of the orbital momentum by spin–orbit interactions. Here we report all-optical excitation of magnons with frequencies up to 9 THz in antiferromagnetic CoO with an unquenched orbital momentum. In CoO, magnon modes are coupled oscillations of spin and orbital momenta with comparable amplitudes. We demonstrate excitations of magnon modes by directly coupling light with electronic orbital angular momentum, providing possibilities to develop magneto-optical devices operating at several terahertz with high output-to-input ratio.

Magneto-Plasmonics and Resonant Interaction of Light with Dynamic Magnetisation in Metallic and All-Magneto-Dielectric Nanostructures.

A significant interest in combining plasmonics and magnetism at the nanoscale gains momentum in both photonics and magnetism sectors that are concerned with the resonant enhancement of light-magnetic-matter interaction in nanostructures. These efforts result in a considerable amount of literature, which is difficult to collect and digest in limited time. Furthermore, there is insufficient exchange of results between the two research sectors. Consequently, the goal of this review paper is to bridge this gap by presenting an overview of recent progress in the field of magneto-plasmonics from two different points of view: magneto-plasmonics, and magnonics and magnetisation dynamics. It is expected that this presentation style will make this review paper of particular interest to both general physical audience and specialists conducting research on photonics, plasmonics, Brillouin light scattering spectroscopy of magnetic nanostructures and magneto-optical Kerr effect magnetometry, as well as ultrafast all-optical and THz-wave excitation of spin waves. Moreover, readers interested in a new, rapidly emerging field of all-dielectric nanophotonics will find a section about all-magneto-dielectric nanostructures.

All-optical investigation of tunable picosecond magnetization dynamics in ferromagnetic nanostripes with a width down to 50 nm.

Ferromagnetic nanostripes are important elements for a number of interesting technologies including magnetic racetrack memory, spin logic and magnonics. Understanding and controlling magnetization dynamics in such nanostripes are hence important problems in nanoscience and technology. Here we present an all-optical excitation and detection of ultrafast magnetization dynamics, including spin waves, in 5 μm long Ni80Fe20 nanostripes with varying stripe widths from 200 nm down to 50 nm. We observed a strong width dependent variation in the frequency, anisotropy and the spatial nature of spin waves in these systems. The effect of inter-stripe interaction is also studied and the 50 nm wide stripe is found to be nearly magnetostatically isolated, allowing us to detect the dynamics of a 50 nm wide individual stripe using an all-optical measurement technique. The tunability in magnetization dynamics with stripe widths is important for their applications in various spin based technologies.

Electrically tunable organic vertical-cavity surface-emitting laser

An electrically tunable organic vertical-cavity surface-emitting laser (VCSEL) is demonstrated and characterized. A lasing wavelength tunability of Δλ = 10 nm with 6 V actuation is shown for a red laser emission tuned between λ = 637 nm and λ = 628 nm. Wavelength tuning of the VCSEL structure is enabled by electrostatic deflection of a reflective flexible membrane that is suspended over an air gap and a dielectric mirror, forming a 3λ lasing cavity. The lasing gain medium consists of an evaporated organic thin film coated on a reflective membrane, which is then additively placed over a patterned substrate containing the dielectric mirror to fabricate an array of air-gap-VCSEL structures, each 100 μm in diameter. Beyond the electrostatic actuation of these tunable lasers, the VCSEL array geometry also has the potential to be used as pressure sensors with an all-optical remote excitation and readout and a pressure sensitivity of 64 Pa/nm in the demonstrated configuration.

Plasmon-assisted high reflectivity and strong magneto-optical Kerr effect in permalloy gratings

We demonstrate experimentally a strong plasmon-assisted enhancement of the transverse magneto-optical Kerr effect in permalloy gratings. The enhanced transverse magneto-optical Kerr effect is accompanied by an increased grating reflectivity with the maximum of enhancement being correlated with plasmonic Fano resonances. This correlation was confirmed by an intuitive Fano model and also through full-vectorial optical simulations. Simultaneously high reflectivity and transverse magneto-optical Kerr effect as well as narrowest ferromagnetic resonance linewidth and vanishing anisotropy make permalloy nanostructures attractive for applications in spintronics and nano-optics such as, for example, all-optical excitation of propagating spin waves and spectral tuning of optical nanoantennas.

MAGNETO-OPTICAL MEASUREMENTS OF COLLECTIVE SPIN DYNAMICS OF TWO-DIMENSIONAL ARRAYS OF FERROMAGNETIC NANOELEMENTS

Magnetic nanodot arrays are interesting systems for future applications in nanotechnology including patterned magnetic media, magnonic crystals, magnetic logic, sensors, STNOs and biomedical applications. All applications require the knowledge base of magnetization processes of magnetic nanodot arrays at various time and length scales. Here, we review the present status of experimental studies of picosecond precessional magnetization dynamics in magnetic nanodot arrays. We discuss the fabrication methods of magnetic nanodot arrays and excitation and detection methods of precessional dynamics by optical means. We further discuss the all-optical excitation and detection of precessional dynamics in Ni80Fe20 (permalloy) nanodot arrays with width between 200 nm and 50 nm and with interdot separation between 50 nm and 400 nm. A transition from strongly collective dynamics to completely isolated dynamics through various weakly collective regimes, variation of precession frequency and damping with the interdot separation, effects of dipolar and quadrupolar interdot interaction, effects of the variation of dot size on the dynamics of single elements and arrays, and anisotropy of collective dynamics have been thoroughly studied by experimental and micromagnetic simulation results. Finally, we discuss the future directions in the research on the dynamics of magnetic nanodot arrays.

Thermoelastic Characterization of Changing Phase Distribution in Hardened Steel by Laser Ultrasonics

The increase of hardness of steel during a heat treatment intended to give components more performance is a result of a drastic change in grain size and microstructure, which in turn can be analyzed via changes in acoustic wave scattering. The degree of scattering is related to the grain size, alloy phases, elastic anisotropy, and phonon spectra, which are connected with the structural heterogeneity. In this study, an axially oriented hardening profile in a steel rod was induced by a Jominy test. All-optical photoacoustic excitation and detection schemes were used to establish the relation between the hardness, the elastic modulus, the elastic scattering, and the thermal diffusivity on a series of eight samples cut out from the gradient part of the rod. For each sample, the scattering of the photoacoustically excited traveling surface acoustic waves detected in a heterodyne diffraction and beam deflection setup was extracted from their damping behavior at different wavelengths and frequencies. Also, the thermal diffusivity was determined by fitting the slow time evolution of the laser-induced photo-thermoelastic displacement signal, and was found to be decreasing with decreasing grain size and increasing hardness.

All-optical broadband excitation of the motional state of trapped ions

We have developed a novel all-optical broadband scheme for exciting, amplifying and measuring the secular motion of ions in a radio frequency trap. Oscillation induced by optical excitation has been coherently amplified to precisely control and measure the ion's secular motion. Requiring only laser line-of-sight, we have shown that the ion's oscillation amplitude can be precisely controlled. Our excitation scheme can generate coherent motion which is robust against variations in the secular frequency. Therefore, our scheme is ideal to excite the desired level of oscillatory motion under conditions where the secular frequency is evolving in time. Measuring the oscillation amplitude through Doppler velocimetry, we have characterized the experimental parameters and compared them with a molecular dynamics simulation which provides a complete description of the system.

Generation of inhomogeneous plane shear acoustic modes by laser-induced thermoelastic gratings at the interface of transparent and opaque solids

The detailed theoretical description of how picosecond plane shear acoustic fronts can be excited by ultrafast lasers at the interface of two isotropic media, a transparent medium and an opaque medium, is presented. The processes leading to the emission of inhomogeneous plane bulk shear acoustic modes from the interaction at the interface of plane inhomogeneous compression/dilatation modes thermoelastically generated by laser interference gratings are analyzed. The theory describes the basic features of the spectral transformation function of the laser light conversion into shear modes and predicts an interval of frequencies where it is possible to achieve the emission into the transparent medium of propagating shear inhomogeneous modes only, while the compression/dilatation inhomogeneous modes will be evanescent and will be localized at the interface. The guidelines for optimal choice of the materials, with the goal of improving the amplitude of the photoexcited picosecond shear acoustic fronts are proposed. All-optical monitoring, i.e., excitation and detection, by fs-ps laser pulses of picosecond plane inhomogeneous shear acoustic fronts propagating in thin films and substrates can be applied for the noncontact determination of shear rigidity of materials.

Solitary surface acoustic waves and bulk solitons in nanosecond and picosecond laser ultrasonics

Recent achievements of nonlinear acoustics concerning the realization of solitons and solitary waves in crystals and their surfaces attained by nanosecond and picosecond laser ultrasonics are discussed and compared. The corresponding pump–probe setups are described, which allow an all-optical contact-free excitation and detection of short strain pulses in the broad frequency range between 10 MHz and about 300 GHz. The formation of solitons in the propagating longitudinal strain pulses is investigated for nonlinear media with intrinsic lattice-based dispersion. The excitation of solitary surface acoustic waves is realized by a geometric film-based dispersion effect. Future developments and potential applications of nonlinear nanosecond and picosecond ultrasonics are discussed.

THz Generation From Resonant Excitation of Semiconductor Nanostructures: Investigation of Second-Order Nonlinear Optical Effects

This paper reviews recent experiments on the generation of free-space terahertz (THz) radiation by all-optical excitation of semiconductor nanostructures using second-order nonlinear effects. It is focused on resonant excitation schemes only, in which so-called shift and injection currents may be generated, depending on the polarization state of the excitation pulse and on the symmetry of the semiconductor structure. In particular, the generation of such currents by interband excitation of (110)-oriented GaAsZAl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> As quantum wells with a 150-fs laser pulse is discussed and it is shown that the resonant excitation of the lowest exciton transitions in these nanostructures allows for the observation of interesting and surprising effects.