Probe Laser Beam Research Articles

Noncollinear electro-optic detection of terahertz waves: Advantages and limitations

Electro-optic sampling of terahertz waves by noncollinearly propagating femtosecond laser pulses in electro-optic crystals can provide high efficiency and high spectral resolution of terahertz detection with various types of crystals and laser wavelengths, unlike the conventional collinear scheme. We develop an analytical theory of noncollinear electro-optic sampling detection technique that describes the modulation of the probe laser beam polarization as a result of nonlinear interaction between the optical and terahertz fields. The theory accounts for finite widths of the terahertz and probe beams. It is found that noncollinear scheme operates as a low-pass terahertz filter with the frequency cut-off determined by the width of the probe beam and the crossing angle of the terahertz and probe beams. We apply the theory to two practical situations: sampling of terahertz waves by fiber laser pulses (1.55 μm wavelength) in a GaAs crystal and sampling by Ti:sapphire laser pulses (800 nm wavelength) in a LiNbO3 crystal.

Magnetic splitting of electromagnetically induced transparency and absorption spectra in a standing-wave coupled lambda-type system

We study the splitting of traveling wave and standing wave coupled Electromagnetically Induced Transparency (EIT) and Electromagnetically Induced enhanced Absorption (EIA) lines in the presence of a uniform magnetic field in an Electromagnetically Induced Grating (EIG). We realize a Λ-type system in a room-temperature Doppler-broadened buffer-gas-free, unshielded vapor cell of pure 133Cs atoms. Different orientations of the magnetic field are studied while the polarizations of the coupling and probe laser beams are arranged in the lin⊥lin configuration. The effects of the different field directions on the EIT and the EIG-induced EIA are explained in light of the existing theory. The magnetically induced high-contrast features in the EIG scheme are also discussed in comparison to the low-contrast EIT features observed for the same strengths of magnetic field.

Balanced-detection interferometric cavity-assisted photothermal spectroscopy via collimating fiber-array integration

This work reports on the implementation of a fiber-array with attached microlenses in the balanced-detection Interferometric Cavity-Assisted Photothermal Spectroscopy (ICAPS) scheme, enabling a highly compact probe laser beam path. The sample probe and the reference beam were coupled into a 1-mm spaced Fabry-Perot interferometer with a parallel beam separation of only 1.25 mm. A quantum cascade laser was used to induce photothermal waves, which were detected via a sample probe beam. A reference probe beam was used to realize balanced detection, allowing for significant noise reduction. The sensor’s metrological figures of merit were evaluated by detection of nitric oxide. For the targeted absorption band centered at 1900.08 cm−1 a 1 σ minimum detection limit of 64 ppbv was achieved with a 1-second integration time. This performance corresponds to a normalized noise equivalent absorption of 5.7 ×10−8 cm−1 W Hz−1/2.

Enhanced microwave-atom coupling via quadrupole transition-dressed Rydberg atoms

The power broadening of a coupling laser can be converted into two-photon detuning by electromagnetically induced transparency (EIT), resulting in a residual Doppler effect. The residual Doppler effect in a ladder-type EIT in a room-temperature atom ensemble is further amplified through a wavelength mismatch effect between the probe and coupling laser beams, which reduces the atomic coupling of light or microwaves. We measured the Rydberg spectra of the electric dipole (E1) and electric quadrupole (E2) microwave transitions, demonstrating that the reduction in the Rydberg EIT signal can be recovered through far-off-resonance E2 microwave transition dressing and achieving an 8-dB enhancement in the Rydberg EIT signal. The frequency-dependent dressing of the E2 transition enables the shift of the dressed Rydberg states to be tuned, thereby providing a scalable approach to optimize the interaction between the Rydberg state and microwave field.

Temperature Dependence of the Linewidths and Amplitudes of the Sub-Doppler Resonances of 87Rb D2 Line Detected with σ(σ)-Polarized Pumping and Probe Laser Beams

Temperature Dependence of the Linewidths and Amplitudes of the Sub-Doppler Resonances of 87Rb D2 Line Detected with σ(σ)-Polarized Pumping and Probe Laser Beams

Photon Acceleration by Superluminal Ionization Fronts

This paper explores the use of superluminal ionization fronts to accelerate and amplify electromagnetic radiation. These fronts are defined as optical boundaries between two regions of a gas, the neutral region and the plasma region, characterized by two different values of the refractive index. For that reason, the front velocity is not necessarily related to the motion of material particles, such as neutral atoms, ions and electrons, which can stay at rest. The fronts can therefore become superluminal without violating causality. In recent years, different experimental configurations, such as the flying focus, showed that it is possible to create superluminal fronts in the laboratory. These fronts can easily be described theoretically in a special reference frame, called the time frame, which is used here. In this frame, superluminal fronts reduce to time refraction, a process that is symmetrical to the well-known optical refraction. It is shown that propagation through such fronts can lead to considerable frequency shifts and energy amplification of probe laser beams. This could eventually be used to develop new sources of tunable radiation.

Influence of the atmosphere on the spread of laser radiation

The article is devoted to the problem of protecting information from being read by an optoelectronic channel by studying the influence of atmospheric parameters on the spread of laser radiation. The paper considers key aspects of radiation attenuation due to scattering and absorption by air gases and aerosol particles. It is shown that the efficiency of the probing laser beam depends on extreme climatic conditions, and due to scattering and absorption, the power of the laser beam will decrease as it spreads through the atmosphere. Taking into account the peculiarities of the passage of a laser beam through the atmosphere and the influence of various types of interference on the probing signal, it is possible to predict its attenuation and blocking, namely, for reading confidential speech information, lasers with radiation located in the parts of the atmospheric spectrum occupied by wide windows of transparency should be taken for the appropriate weather conditions; to protect information from being read by an optoelectronic channel, it is necessary to use parameters of the external environment that prevent direct access of the laser beam to the object of information. The presented author's vision of research into the influence of the atmosphere on the spread of laser radiation will allow us to apply the results, ideas, and proposals summarized in this article to solve current problems in the field of information protection.

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

Optoacoustic detection of nanosecond time scale photoinduced lensing effects in liquids

An all-optical photoinduced lensing method is used to excite and monitor acoustic waves in liquids. Following optical absorption, the laser pulse induces a localized temperature gradient that launches pressure waves in the excited region at the nanosecond time scale. This generates a lens-like optical element in the sample. A probe laser beam senses the refractive index change due to the acoustic and thermal effects. Piezo-optic and thermo-optic coefficients govern how the refractive index of a material changes in response to mechanical stress and temperature variations, respectively. These effects are connected to the physical properties of the liquids and can be accessed by theoretically describing the intensity signal. A complete set of physical properties of ten liquids are quantitatively described in this work. These effects find applications in a wide range of fields, from optical communication, ultrasonic imaging, and sensing to adaptive optics and fundamental research.

Probe Beam Dichroism and Birefringence in Stimulated Raman Scattering in Polyatomic Molecules.

Dichroism and birefringence in Stimulated Raman Scattering (SRS) in polyatomic molecules were studied theoretically. General expressions describing the change of the polarization matrix of the probe laser beam transmitted through initially isotropic molecular sample excited by the pump laser beam have been derived. Arbitrary polarization states and propagation directions of the incoming pump and probe beams were considered. The expressions were written in terms of spherical tensor operators that allowed for separation of the field polarization tensor and the molecular part containing three scalar values of nonlinear optical susceptibility with =0,1,2. The geometry of almost collinear propagation of the pump and probe beams through the molecular sample was considered in greater details. It was shown that the dichroism and birefringence refer to the nonlinear optical susceptibility element and that their contributions to the SRS signal can be separated experimentally by using an appropriate probe beam polarization analyzer installed in front of the photodetector. Particular cases of the off-resonant SRS and resonant SRS have been considered. The results obtained were expressed in terms of the Stokes polarization parameters of the pump and probe beams.

Operando measurements of top-emission organic light-emitting diode using electric-field-induced doubly resonant sum-frequency generation vibrational spectroscopy

We performed sum-frequency generation (SFG) measurements on top-emission (TE) organic light-emitting diodes (OLEDs) during operation. These measurements require the detection of SFG light using probe laser beams transmitted through a 9 nm thick semitransparent Mg:Ag metal electrode. Only the SFG signals from the hole transport material (4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl, NPD) are observed for the device configuration of the OLEDs in this study. The SFG peak (at approximately 1603 cm−1) shows a linear dependence on the applied voltage. This indicates that the NPD-derived SFG is enhanced by an electric-field-induced (EFI) effect owing to injected charges. In addition, the SFG efficiency of the TE OLED is considerably higher than that of a bottom-emission (BE) OLED manufactured in the same batch using the same material and film thickness. The difference in the SFG responses of the BE and TE OLEDs is caused by structural differences rather than the layer composition. The results of this study indicate that EFI-doubly resonant-SFG operando measurements can be applied to analyze the carrier behavior and charge transport of not only BE OLEDs but also TE OLEDs, the latter of which are used more commonly as commercial products.

Fabrication of Surface Acoustic wave resonator as Acousto-optic Modulator

Acousto-optic modulation techniques are gaining edge in diverse applications in the area of agriculture, entertainment and defense. Present work focuses on indigenously developed pump probe technique to study the acousto-optic modulation using surface acoustic wave (SAW) device. For this study, SAW device has been fabricated on ST-cut quartz that is an optically nonlinear piezoelectric material. Coupling of acoustic wave with optical wave has been monitored and a visible modulation in the intensity of probe laser beam has been obtained. A clear shift in the value of probe intensity (∼12 μW) is observed when surface acoustic waves are excited. Angular rotation of the fabricated SAW device under pump probe microscopy shows the periodicity of the nonlinearity in substrate while acoustic wave is propagating in it. The effect of variation in the intensity of pump laser beam on the acousto-optic modulation is also studied. The acousto-optic modulation behavior observed in surface acoustic devices in the present study demonstrates the possible application in telecommunication for signal modulation and in spectroscopy for frequency control.

Weak-measurement-induced heating in Bose-Einstein condensates.

Ultracold atoms are an ideal platform for understanding system-reservoir dynamics of many-body systems. Here, we study quantum back-action in atomic Bose-Einstein condensates, weakly interacting with a far-from resonant, i.e., dispersively interacting, probe laser beam. The light scattered by the atoms can be considered as a part of quantum measurement process, whereby the change in the system state derives from measurement back-action. We experimentally quantify the resulting back-action in terms of the deposited energy. We model the interaction of the system and environment with a generalized measurement process, leading to a Markovian reservoir. Further, we identify two systematic sources of heating and loss: a stray optical lattice and probe-induced light-assisted collisions (an intrinsic atomic process). The observed heating and loss rates are larger for blue detuning than for red detuning, where they are oscillatory functions of detuning with increased loss at molecular resonances and reduced loss between molecular resonances.

Phase jump detection and correction based on the support vector machine

In general, interferometers are used to perform electron density measurements in magnetically confined plasma, where the electron density is dependent on the refractive index of the plasma. Measurements can be made through comparisons of the phase shift variation between the probe and reference laser beam. The plasma electron density should vary continuously during discharge; however, the fringe jump is a step-like change of the apparent electron density caused by a sudden jump of the measured phase shift. The appearance of fringe jump will degrade the interferometric measurements accuracy. This study attempted to solve the fringe jump problem on the polarimeter-interferometer (POINT) diagnostics system of the Experiment Advanced Superconducting Tokamak (EAST) by proposing a support vector machine model for electron density fringe jump detection and correction. The established model can efficiently classify the fringe jump data from the raw measurement data in a manner robust to noise and interference, and subsequently correct the jump. This model greatly improves the correction efficiency and precision of electron density data from the POINT system, and is expected to be embedded into the plasma control system to perform more accurate real-time electron density feedback control. Moreover, the algorithm is not limited to specific fusion devices or interferometer diagnostics, and is applicable to other interferometric measurement systems.

Thermometry utilizing stored short-wavelength spin waves in cold atomic ensembles

Temperature, as a measure of thermal motion, is a significant parameter characterizing a cold atomic ensemble optical quantum memory. In a cold gas, storage lifetime strongly depends on its temperature and is associated with the spin wave decoherence. Here we experimentally demonstrate a new spin wave thermometry method relying on this direct dependence. The short-wavelength spin waves resulting from the counter-propagating configuration of the control and the probe laser beams make this thermometry highly suitable for probing in situ the atomic motion in elongated clouds as the ones used in quantum memories. Our technique is realized with comparable precision for memories that rely on electromagnetically induced transparency as well as far-detuned Raman storage.

Toward streaked collective Thomson scattering measurements on an extreme ultraviolet plasma light source

We show through forward modeling calculations that streaked collective Thomson scattering measurements are feasible on laser-produced tin plasmas generated under conditions relevant for extreme ultraviolet lithography. Using a 532 nm probe laser beam, the feasibility of simultaneous measurements of electron plasma wave (EPW) and ion acoustic wave (IAW) spectra is investigated. Absolute photon counts for laser scattering off both waves are calculated. Probe laser electron heating and bremsstrahlung background radiation effects are accounted for. While a large spatiotemporal region can be successfully probed based on the IAW feature, only one measurement location can be accessed through the EPW as a result of the low signal to noise ratio. A portable/traveling tabletop system is proposed.

Ultrafast 2D-IR spectroscopy of intensely optically scattering pelleted solid catalysts.

Solid, powdered samples are often prepared for infrared (IR) spectroscopy analysis in the form of compressed pellets. The intense scattering of incident light by such samples inhibits applications of more advanced IR spectroscopic techniques, such as two-dimensional (2D)-IR spectroscopy. We describe here an experimental approach that enables the measurement of high-quality 2D-IR spectra from scattering pellets of zeolites, titania, and fumed silica in the OD-stretching region of the spectrum under flowing gas and variable temperature up to ∼500 ◦C. In addition to known scatter suppression techniques, such as phase cycling and polarization control, we demonstrate how a bright probe laser beam comparable in strength with the pump beam provides effective scatter suppression. The possible nonlinear signals arising from this approach are discussed and shown to be limited in consequence. In the intense focus of 2D-IR laser beams, a free-standing solid pellet may become elevated in temperature compared with its surroundings. The effects of steady state and transient laser heating effects on practical applications are discussed.

Dynamic Optical Grating Based on a Photomechanical Molecular Crystal

AbstractOrganic photomechanical crystals transform molecular‐scale photoisomerization events into largescale crystal shape changes. The results in this paper demonstrate that this photomechanical motion can be harnessed to reconfigure reflective elements on the crystal surface to actively control light propagation. This is accomplished by using a single organic photomechanical crystal composed of 1,2‐bis(2,4‐dimethyl‐5‐phenyl‐3‐thienyl)‐3,3,4,4,5,5‐hexafluoro‐1‐cyclopentene, which undergoes a reversible ring‐open to ring‐closed photoisomerization. Electron beam lithography patterning followed by a sublimation development step leaves periodically positioned, 120 nm deep grooves that are coated with gold metal to form a reflective grating on the crystal surface. Under UV irradiation, expansion of the bulk crystal increases the grating period by ≈5% and changes the diffraction angle of a 633 nm probe laser beam by 0.7° (first order) and 1.3° (second order). The grating period change can be reversed by visible light exposure and repeated for multiple cycles. The crystal expansion and thus the diffraction angle can also be continuously tuned by controlling the duration of light exposure. Anisotropic crystal expansion and contraction due to molecular photoisomerization enables low‐power light sources to steer light beams continuously over a range of diffraction angles.

Optimization of beam shaping and error quantification of calibration approach using E-FISHG based electric field measurements

Electric-field measurement based on the electric-field-induced second-harmonic generation (E-FISHG) method is a promising tool for a noncontact field measurement in plasmas and gases. For the E-FISHG method, a probing laser beam is focused at the measurement target by a lens, and the signals integrated along the laser path are acquired. Although the signal is frequently calibrated under uniform electric fields, the yielded value is erroneous if one does not consider the difference in the electric-field-profiles between the calibration and measurement. In this paper, we review the calibration and measurement targets of relevant studies, assess the error in the conventional method for the streamer discharge measurement, and give guidelines on which calibration approach to use depending on the electric field profile to be measured. Our approach uses cylindrical-to-cylindrical electrodes and multipoint measurement corresponding to the target length along the optical path, gas, and pressure.

STUDY OF NEGATIVE REFRACTIVE INDEX IN Rb FOUR-LEVEL N-TYPE ATOMIC GAS MEDIUM

In this work, we study the generation of a negative refractive index based on electromagnetically induced transparency (EIT) in a Rb four-level N-type atomic gas medium. We derive analytic expressions for the relative permittivity and relative permeability of the medium according to the parameters of the probe, pump, and signal laser fields. We then investigate the variation of the real parts of the relative permittivity and relative permeability with respect to the intensity and frequency of the pump and signal laser fields. In the presence of the pump laser beam, the medium becomes transparent to the probe laser beam even in the resonant region. At the same time, the real parts of the relative permittivity and relative permeability are simultaneously negative (i.e., the medium exhibits a negative refractive index) in the EIT spectral domain. In the presence of the signal laser beam, the EIT effect occurs over two different frequency domains of the probe beam, so a negative refractive index is also generated in these two frequency domains. The investigation of the real parts of the relative permittivity and relative permeability with intensity and frequency of the pump and signal laser fields allowed us to find the laser parameters for the appearance of the negative refractive index, which can be useful for experimental observations.