Optical Probe Beam Research Articles

Diffractionless transmission of optical beams through a four-level atomic system affected by a plasmonic nanostructure

A nondiffracting propagation of an optical beam through a four-level double-V-type quantum system near a plasmonic nanostructure is investigated. We study the linear absorption and dispersion properties of the quantum system as it interacts with two laser fields. We discuss the effect of the control beam with a Laguerre-Gaussian (LG) profile on the focusing of the probe beam in the presence of a plasmonic nanostructure. An appropriately selected control beam excites one transition of the atomic system and generates a spatially varying refraction index modulation for a weak probe beam that couples to the other transition. We demonstrate that placing a plasmonic nanostructure at a nanometer distance from the atomic system and using a control field with the spatial structure leads to the diffraction-less propagation of the probe beam through the atomic system. Also, it is shown that the optical properties and probe beam focusing can be controlled by adjusting the distance of the plasmonic nanostructure from the atomic system. The proposed all-optical waveguide with high contrast and transmission can be used to implement applications such as image transfer through the medium and image processing.

Modeling and correction of image drift in dynamic shadowgraphy experiments

The study of phoretic transport phenomena under non-stationary conditions presents several challenges, mostly related to the stability of the experimental apparatus. This is particularly true when investigating with optical means the subtle temperature and concentration fluctuations that arise during diffusion processes, superimposed to the macroscopic state of the system. Under these conditions, the tenuous signal from fluctuations is easily altered by the presence of artifacts. Here, we address an experimental issue frequently reported in the investigation by means of dynamic shadowgraphy of the non-equilibrium fluctuations arising in liquid mixtures under non-stationary conditions, such as those arising after the imposition or removal of a thermal stress, where experiments show systematically the presence of a spurious contribution in the reconstructed structure function of the fluctuations, which depends quadratically from the time delay. We clarify the mechanisms responsible for this artifact, showing that it is caused by the imperfect alignment of the sample cell with respect to gravity, which couples the temporal evolution of the concentration profile within the sample with the optical signal collected by the shadowgraph diagnostics. We propose a data analysis protocol that enables disentangling the spurious contributions and the genuine dynamics of the fluctuations, which can be thus reliably reconstructed.Graphic The imposition of a thermal gradient across a liquid mixture results in a time-dependent refractive index distribution. In the presence of a misalignment of the confining cell with respect to gravity, this leads to a deflection of the optical probe beam used to monitor concentration fluctuations within the sample in quantitative shadowgraphy experiments. If not properly accounted for, this effect can introduce a significant bias in the optical signal.

Local measurement of terahertz field-induced second harmonic generation in plasma filaments

The concept of Terahertz Field-Induced Second Harmonic (TFISH) Generation is revisited to introduce a single-shot detection scheme based on third order nonlinearities. Focused specifically on the further development of THz plasma-based sources, we begin our research by reimagining the TFISH system to serve as a direct plasma diagnostic. In this work, an optical probe beam is used to mix directly with the strong ponderomotive current associated with laser-induced ionization. A four-wave mixing (FWM) process then generates a strong second-harmonic optical wave because of the mixing of the probe beam with the nonlinear current components oscillating at THz frequencies. The observed conversion efficiency is high enough that for the first time, the TFISH signal appears visible to the human eye. We perform spectral, spatial, and temporal analysis on the detected second-harmonic frequency and show its direct relationship to the nonlinear current. Further, a method to detect incoherent and coherent THz inside plasma filaments is devised using spatio-temporal couplings. The single-shot detection configurations are theoretically described using a combination of expanded FWM models with Kostenbauder and Gaussian Q-matrices. We show that the retrieved temporal traces for THz radiation from single- and two-color laser-induced air-plasma sources match theoretical descriptions very well. High temporal resolution is shown with a detection bandwidth limited only by the spatial extent of the probe laser beam. Large detection bandwidth and temporal characterization is shown for THz radiation confined to under-dense plasma filaments induced by < 100 fs lasers below the relativistic intensity limit.Graphical

Vacuum Birefringence Measurement via All-Optical Interferometric Schemes

All-optical vacuum birefringence experiments will get increasingly closer to feasibility as multi-petawatt laser facilities become operational around the World. Thus, the availability of focused laser intensities in the order of IL ∼ 1022 −1024 W/cm2 are to be expected in a focused spot size ∼ 3 − 5 μm. With these values, vacuum refraction indices in the order of Δn ∼ 10−11 − 10−9 are possible with an induced phase delay on a counterpropagating optical probe beam in the order of Δφ ∼ 10−9 −10−7 radians. We discuss two all-optical interferometric schemes and detail the Mach-Zender interferometric proposal. We consider this interferometric scheme fed by both classical and non-classical input light and with two detection schemes. We outline scenarios that are likely to lead to a feasible experimental implementation.

Single-shot ultrafast terahertz photography

Multidimensional imaging of transient events has proven pivotal in unveiling many fundamental mechanisms in physics, chemistry, and biology. In particular, real-time imaging modalities with ultrahigh temporal resolutions are required for capturing ultrashort events on picosecond timescales. Despite recent approaches witnessing a dramatic boost in high-speed photography, current single-shot ultrafast imaging schemes operate only at conventional optical wavelengths, being suitable solely within an optically-transparent framework. Here, leveraging on the unique penetration capability of terahertz radiation, we demonstrate a single-shot ultrafast terahertz photography system that can capture multiple frames of a complex ultrafast scene in non-transparent media with sub-picosecond temporal resolution. By multiplexing an optical probe beam in both the time and spatial-frequency domains, we encode the terahertz-captured three-dimensional dynamics into distinct spatial-frequency regions of a superimposed optical image, which is then computationally decoded and reconstructed. Our approach opens up the investigation of non-repeatable or destructive events that occur in optically-opaque scenarios.

Observation of strong terahertz field-induced second harmonic generation in plasma filaments.

We report the observation of terahertz field-induced second harmonic (TFISH) generation produced by directly mixing an optical probe beam onto femtosecond plasma filaments. The produced TFISH signal is spatially separated from the laser-induced supercontinuum by impinging on the plasma at a noncollinear angle. The conversion efficiency of the fundamental probe beam to its second harmonic (SH) beam is greater than 0.02%, which represents a record in optical probe to TFISH conversion efficiency that is nearly five orders of magnitude larger than previous experiments. We also present the terahertz (THz) spectral buildup of the source along the plasma filament and retrieve coherent terahertz signal measurements. This method of analysis has the potential to provide local electric field strength measurements inside of the filament.

Thermally controlled optical resonator for vacuum squeezed states separation.

Future gravitational-wave detectors will use frequency-dependent squeezed vacuum states to obtain broadband reduction of quantum noise. Quantum noise is one of the major limitations to the sensitivity of these detectors. Advanced LIGO+, Advanced Virgo+, and KAGRA plan to generate frequency-dependent squeezed states by coupling a frequency-independent squeezed light state with a filter cavity. An alternative technique is under consideration, based on conditional squeezing with quantum entanglement: Einstein-Podolsky-Rosen (EPR) squeezing. In the EPR scheme, two vacuum entangled states, the signal field at ω0 and the idler field at ω0+Δ, must be spatially separated with an optical resonator and sent to two separate homodyne detectors. In this framework, we have designed and tested a solid Fabry-Perot etalon, to be used in an EPR table-top experiment prototype, thermally controlled without the use of a control probe optical beam. This device can also be used in optical experiments where the use of a bright beam to control an optical resonator is not possible, or where a simpler optical device is preferred.

Noncollinear electro-optic sampling detection of terahertz pulses in a LiNbO3 crystal while avoiding the effect of intrinsic birefringence.

We propose and experimentally prove efficient high-resolution electro-optic sampling measurement of broadband terahertz waveforms in a LiNbO3 crystal in the configuration with the probe laser beam propagating along the optical axis of the crystal. This configuration allows one to avoid the detrimental effect of strong intrinsic birefringence of LiNbO3 without any additional optical elements. To achieve velocity matching of the terahertz wave and the probe beam, the terahertz wave is introduced into the crystal through a Si prism at the Cherenkov angle to the probe beam. The workability of the scheme at different wavelengths of the probe optical beam (800 and 1550 nm) is demonstrated.

Nematic response revealed by coherent phonon oscillations in BaFe2As2

We investigate coherent phonon oscillations of BaFe$_2$As$_2$ using optical pump-probe spectroscopy. Time-resolved optical reflectivity shows periodic modulations due to $A_{1g}$ coherent phonon of $c$-axis arsenic vibrations. Optical probe beams polarized along the orthorhombic $a$- and $b$-axes reveal that the initial phase of coherent oscillations shows a systematic deviation as a function of temperature, although these oscillations arise from the same $c$-axis arsenic vibrations. The oscillation-phase remains anisotropic even in the tetragonal structure, reflecting a nematic response of BaFe$_2$As$_2$. Our study suggests that investigation on the phase of coherent phonon oscillations in optical reflectivity can offer unique evidence of a nematic order strongly coupled to a lattice instability.

Probing nonaffine expansion with light scattering.

In disordered materials under mechanical stress, the induced deformation can deviate from the affine one, even in the elastic regime. The nonaffine contribution was observed and characterized in numerical simulations for various systems and reported experimentally in colloidal gels. However, low amplitude of nonaffinity and its local character makes the experimental study challenging. We present a method based on the phase compensation of the wave scattered from a thermally dilated amorphous material using fine wavelength tuning of the optical probe beam. Using a glass frit as a sample, we ensure complete reversibility of the material deformation, while experimental observations enable us to confirm the occurrence of nonaffinity in the elastic regime. We develop a model for the coupled effect of the thermal expansion or contraction of the material and the dilatation of the incident wavelength, which allows us to estimate the magnitude of the nonaffine displacement and the spatial extent of its correlation domain.

Measurement of a129Xe transverse relaxation rate without the influence of Rb polarization-induced magnetic gradient.

We demonstrate the measurement of the transverse spin relaxation rate of Xe without the influence of the Rb polarization-induced magnetic field gradient. The optical pumping beam and probe beam turn on and off repeatedly during the measurement to reduce the relaxation originating from the Rb polarization-induced magnetic field gradient. During the absence of the optical pumping beam, the nuclear spin of the noble gas atom does not experience the alkali-polarization-induced magnetic field gradient so that the transverse spin relaxation rate (1/T2) decreases. When the duty cycle reaches zero, the measured transverse relaxation time T2 approaches the longitudinal spin relaxation time T1. Our method helps in roughly estimating the longitudinal spin relaxation time with a single measurement of the free induction decay.

Two-membrane cavity optomechanics: non-linear dynamics

We study the non-linear dynamics of a multimode optomechanical system constituted of a driven high-finesse Fabry–Pérot cavity containing two vibrating dielectric membranes. The analytical study allows to derive a full and consistent description of the displacement detection by a probe beam in the non-linear regime, enabling the faithful detection of membrane displacements well above the usual sensing limit corresponding to the cavity linewidth. In the weak driving regime where the system is in a pre-synchronized situation, the unexcited oscillator has a small, synchronized component at the frequency of the excited one; both large and small amplitude resonator motions are transduced in a nontrivial way by the non-linear response of the optical probe beam. We find perfect agreement between the experimental results, the numerical simulations, and an analytical approach based on slowly-varying amplitude equations.

Simulation of multiwavelength conditions in laser picosecond ultrasonics

Complete numerical simulations are given under SciLab® and MATLAB® coding environments, concerning propagative acoustic wavefronts, for laser picosecond ultrasonics under multiwavelength conditions. Simulations of the deformation field and its propagation into bulk material are given under different wavelength configurations for optical pump and probe beams, which are used to generate and to detect the acoustic signal. Complete insights concerning the dynamics of the acoustic waves are given, considering the absence of carrier diffusions into the material. Several numerical approaches are proposed concerning both the functions introduced to simulate the wavefront ( Heaviside or error) and the coding approach (linear/vectorized/ Oriented Object Programming), under the pure thermo-elastic approach.

Dependence on sensitivity patterns of bias fields for vector measurements using optically pumped vapor magnetometers

The optical light shift effect is an all-optical technique to create magnetic bias fields to determine the vector components of the magnetic field using a scalar optically pumped magnetometer. We show that the measurement of the induced bias fields is affected by the response pattern of the scalar magnetometer. The sensitivity of the magnetometer affects the minimum total field resolution as well as the angular resolution of the vector measurement. Three configurations of multiple-laser, optically pumped cesium vapor vector magnetometers are characterized for their spatial sensitivity patterns. Each configuration has a unique response level as a function of the relative magnetic field orientation. Thus, the formulations used for angular determination are dependent on the magnetometer configuration, and the angular resolution of the vector measurement is dependent on the relative angle between the magnetic field and the optical probe beam. The spatial dependence of the light shift signals is measured, and the equations describing their responses are presented for each configuration. Equations for determining the field angles from the bias field responses are derived.

Packaged Negative Axicon Optical Fiber Probe and Bessel Beam Interferometry for Refractive Index Measurement of Hazardous Liquid Samples

In this article, we demonstrate the packaging of an optical fiber negative axicon probe generating Bessel beam and its application in the measurement of refractive index (RI) of hazardous liquids. The negative axicon probe is placed inside cascaded capillary tube capped with a glass coverslip of RI ∼1.52 and thickness ∼0.15 mm. The packaged probe, connected to a broadband source through an optical circulator, is dipped in different hazardous liquid samples. The reflected light from the air-glass as well as the glass-liquid interface couples to the negative axicon probe and interferes with the reference beam generated at the air-negative axicon interface. The interference spectra recorded in a spectrometer through the circulator are analyzed applying Fast Fourier Transform (FFT). The FFT helps to separate and measure powers reflected from the different interfaces which are used to solve the Fresnel equation for estimating the RI of hazardous liquid samples, such as HCl, H2SO4 solutions. The resolution achieved is 1*10−4 ± 0.0005. The probe can be useful to measure RI of liquid samples remotely and in hazardous environmental condition.

Spatial confinement of laser-induced plasma by laser-induced and obstacle-reflected shock wave and its effect on optical emission of laser-induced plasma

The laser-induced plasma (LIP) and the shock wave generated by pulsed laser ablation of a graphite target in air and reflected by a flat obstacle were examined by optical emission spectroscopy and probe beam deflection measurements. The interaction between the LIP and the shock wave and its effects on the expansion of the LIP as well as on the optical emission of carbon atoms were studied. The carbon atomic emission can be enhanced or reduced in the situation with a flat obstacle standing in the propagation path of the shock wave. The enhancement or reduction of the carbon atomic emission has a close connection with the shock wave generated by graphite ablation and reflected by the obstacle. The reflected shock wave confines the expansion of the LIP and impedes the travelling of the plasma species. The enhancement was observed at the detection position close to the target and with a short block-target distance. The shock wave thus reflected encounters the luminous LIP at its early expanding stage and confines the expansion of the LIP, resulting in the enhancement in the optical emission of carbon atoms. But at the detection position far from the target and with a longer block-target distance, a reduction in the optical emission due to spatial confinement was observed. The possible mechanisms responsible for the effects of spatial confinement on the optical emission were discussed.

Imaging gigahertz zero-group-velocity Lamb waves

Zero-group-velocity (ZGV) waves have the peculiarity of being stationary, and thus locally confining energy. Although they are particularly useful in evaluation applications, they have not yet been tracked in two dimensions. Here we image gigahertz zero-group-velocity Lamb waves in the time domain by means of an ultrafast optical technique, revealing their stationary nature and their acoustic energy localization. The acoustic field is imaged to micron resolution on a nanoscale bilayer consisting of a silicon-nitride plate coated with a titanium film. Temporal and spatiotemporal Fourier transforms combined with a technique involving the intensity modulation of the optical pump and probe beams gives access to arbitrary acoustic frequencies, allowing ZGV modes to be isolated. The dispersion curves of the bilayer system are extracted together with the quality factor Q and lifetime of the first ZGV mode. Applications include the testing of bonded nanostructures.

Generation of coherent phonons by coherent extreme ultraviolet radiation in a transient grating experiment

We investigate the excitation of coherent acoustic and optical phonons by ultrashort extreme ultraviolet (EUV) pulses produced by a free electron laser. Two crossed femtosecond EUV (wavelength 12.7 nm) pulses are used to excite coherent phonons at a wavelength of 280 nm, which are detected via diffraction of an optical probe beam. Longitudinal and surface acoustic waves are measured in BK-7 glass, diamond, and Bi4Ge3O12; in the latter material, the excitation of a coherent optical phonon mode is also observed. We discuss probing different acoustic modes in reflection and transmission geometries and frequency mixing of surface and bulk acoustic waves in the signal. The use of extreme ultraviolet radiation will allow the creation of tunable GHz to THz acoustic sources in any material without the need to fabricate transducer structures.

Microwave field controlled electromagnetically induced focusing

We theoretically show the effect of a microwave field on the induced focusing of an optical weak probe beam by a copropagating nonuniform control beam in an electromagnetically induced transparency medium. Our numerical simulation shows that, in the presence of a microwave field, probe beam focusing is phase-sensitive and the focused beam size is reduced by 70% of its initial width. In contrast, in the absence of a microwave field, the reduction in the size of the transmitted probe beam is only about 30%. This approach is also successfully applied to the focusing of the diffraction-limited structure of the probe beam.

Gaussian probe beam with high spherical aberration for glucose concentration measurement.

We demonstrate that an optical probe beam with high spherical aberration used for glucose concentration measurements gives better sensitivity compared to a probe beam free of aberrations, under similar conditions. We place a singlet focusing lens at a large distance from a laser source with a Gaussian intensity profile to obtain a spherically aberrated probe beam with negligible truncation. The aberrated probe beam propagates through a transparent liquid sample. Intensity profiles of the transmitted beam are recorded by means of a homodyne profiler to perform the glucose concentration measurements accurately.