Transient Refractive Index Research Articles

Transient gas-induced differential refractive index effects in as-drawn hollow core optical fibers

When a hollow core fiber is drawn, the core and cladding holes within the internal cane geometry are pressurized with an inert gas to enable precise control over the internal microstructure of the fiber and counteract surface tension forces. Primarily by considering the temperature drop as the fiber passes through the furnace and the geometrical transformation of the internal microstructure from preform-to-fiber, we recently established that the gas pressure within the final ‘as-drawn’ fiber is substantially below atmospheric pressure. We have also established that slight changes in the gas refractive index within the core and surrounding cladding holes induced by changes in gas pressure are sufficient to significantly affect both the modality and loss of the fiber. Here we demonstrate, through both simulations and experimental measurements, that the combination of these effects leads to transient changes in the fiber’s attenuation when the fibers are opened to atmosphere post-fabrication. It is important to account for this phenomenon for accurate fiber characterization, particularly when long lengths of fiber are drawn where it could take many weeks for every part of the internal microstructure to reach atmospheric pressure.

Influence of Hot Carrier Cooling and Band‐Filling Effect on Linewidth of Perovskite Microlasers

AbstractIn semiconductor‐based microlasers, the generated excited carriers can affect both the optical absorption coefficient and refractive index, influencing the lasing behavior and device performance. In this study, the lasing behavior of individual MAPbBr3 microrods pumped by femtosecond laser pulses is investigated using an ultrafast optical Kerr gating technique. By analyzing the spectral and temporal evolution of the lasing behavior, it is found that the band‐filling (BF) effect can cause a significant change in the refractive index of the material, which results in an unfavorable red‐shift and broadening of the resonant modes, deteriorating the laser linewidth and quality factor. The hot carrier cooling process can provide a buffer for alteration of the energy level occupation state, resulting in a small transient refractive index change and slight red‐shift. These results offer insights into the lasing behavior driven by photogenerated carrier dynamics and provide an optimization strategy for semiconductor‐based microlasers.

Correlation of refractive index based and THz streaking arrival time tools for a hard X-ray free-electron laser.

To fully exploit ultra-short X-ray pulse durations routinely available at X-ray free-electron lasers to follow out-of-equilibrium dynamics, inherent arrival time fluctuations of the X-ray pulse with an external perturbing laser pulse need to be measured. In this work, two methods of arrival time measurement were compared to measure the arrival time jitter of hard X-ray pulses. The methods were photoelectron streaking by a THz field and a transient refractive index change of a semiconductor. The methods were validated by shot-to-shot correction of a pump-probe transient reflectivity measurement. An ultimate shot-to-shot full width at half-maximum error between the devices of 19.2 ± 0.1 fs was measured.

Single line of sight frame camera based on the RadOptic effect of ultrafast semiconductor detector

A new optical beam splitting method is proposed, based on which the optical frame camera capable of capturing multiple frames in a single exposure is designed and experimentally verified. The operation of the frame camera is based on an ultra-fast response semiconductor detector. It is equipped with an optical beam splitter and an optical imaging module. The ultrafast semiconductor detector receives an optical pulse that produces a transient refractive index change, and ultrafast physical processes are recorded by diffracting the probe laser through the transient phase grating. The interaction of an X-ray pulse with a semiconductor detector to produce a phase grating is simulated, based on the Monte Carlo method. The optical beam splitting mode separates a laser into two optical pulses with a certain time difference in the direction of polarization perpendicular to each other. The imaging module filters the diffracted probe laser in the spectral plane and then images multiple frames. The frame camera was used to record the temporal and spatial distribution characteristics of femtosecond laser pulses with a temporal resolution of 4.1 ps. This frame camera has great potential and value for applying to experimental studies of inertial confinement fusion.

Theoretical study on time response of semiconductor photorefractive effects under subpicosecond ultra-fast X-rays.

A theoretical model that can efficiently calculate the refractive index response of semiconductors under ultrafast X-ray radiation is established based on the photorefractive effect of semiconductors. The proposed model is used to interpret X-ray diagnostics experiments, and the results are in good agreement with experiments. In the proposed model, a rate equation model of free carrier density calculation is adopted with the X-ray absorption cross-sections calculated by atomic codes. The two-temperature model is used to describe the electron-lattice equilibration and the extended Drude model is applied to calculate the transient refractive index change. It is found that faster time response can be achieved for semiconductors with shorter carrier lifetime and sub-picosecond resolution can be obtained for InP and [Formula: see text]. The material response time is not sensitive to X-ray energy and the diagnostics can be used in the 1-10 keV energy range. This article is part of the theme issue 'Dynamic and transient processes in warm dense matter'.

Direct observation of ultrafast singlet exciton fission in three dimensions

We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub-10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO2) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of maximal orbital π-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. Our methodology lays the foundation for the study of three dimensional transport on ultrafast timescales.

Electron dynamics in fused silica after strong field laser excitation detected by spectroscopic imaging pump-probe ellipsometry

Laser ablation of dielectrics by ultrashort pulsed laser radiation is induced by nonlinear photoexcitation, such as multiphoton and tunneling processes, and the subsequent impact excitation, due to electron-electron collisions. Previous theoretical and experimental studies demonstrate that these two phenomena strongly depend on the transient optical properties of the dielectric and the electron-electron collision times. To estimate these transient optical properties and the collision time, the excited electrons are usually considered as a nearly free-electron gas, being described by the Drude theory. All previous experimental studies support using the Drude theory by time-resolved measurements of the transient reflectance during and after irradiation. However, a comprehensive characterization of the transient complex refractive index, and, thus, a more complex view of the optical properties upon irradiation above the ablation threshold fluence was still missing. Therefore, this paper presents the time-, spectral-, and fluence-resolved complex refractive index of fused silica during the interaction with ultrashort pulsed laser radiation (800 nm, 40 fs, and 4.4 J/${\mathrm{cm}}^{2}$) measured by spectroscopic imaging pump-probe ellipsometry. Comparing the state-of-the-art Drude model to the experimental data reveals that, although the Drude model replicates the transient reflectance of the material, it deviates significantly from the transient refractive index in the considered temporal range up to 200 fs after irradiation. Thus, the Drude model cannot correctly describe the transient optical properties, which leads to the assumption that the electrons cannot be considered as a nearly free-electron gas at this time. Alternatively, a Lorentz model, considering the excited electrons as bound, can replicate the measured data much better, which indicates that the electrons might be excited into localized states.

Signal recovery of a Fabry–Pérot interferometric x-ray pulse detector based on the RadOptic effect

The signal recovery of a Fabry–Pérot interferometric x-ray pulse detector based on the RadOptic effect in the non-limiting case was investigated in this research. A Fe-doped InP with an invariant excess carrier recombination mechanism was used as the interference cavity material to achieve a constant temporal instrumental response function (tIRF). A linear and time-invariant detection system described by the convolution of the time-varying x-ray pulse and the constant tIRF was established based on the transient refractive index variation model determined by the three effects of band filling, band shrinkage, and free-carrier absorption. For the non-limiting case, the accumulation of excess carriers enhanced the sensitivity but altered the fluctuations of the real x-ray pulse. To realistically reconstruct the x-ray pulse, two-photon absorption of the infrared ultrashort pulse was used to simulate the ultrashort x-ray excitation to obtain the tIRF. Finally, using the conjugate gradient method, the original signal recorded by the detection system was deconvoluted to recover the signal. The success of signal recovery in the non-limiting case provided the basis for the development of detectors with adjustable sensitivity controlled by carrier lifetime.

Polaron-formation revealed by transient XUV imaginary refractive index changes in different iron compounds.

We use transient extreme ultraviolet (XUV) reflection absorption spectroscopy to study polaron formation on hematite and akaganeite surfaces. We observed a reduction in the offset of the imaginary refractive index (κoffset) that was attributed to the photoemission cross-section. The difference in the κoffset reduction indicated that deeper-trapped polarons were formed in hematite than in akaganeite.

Interannual Relationship between the Boreal Spring Arctic Oscillation and the Northern Hemisphere Hadley Circulation Extent

AbstractThis study investigates the interannual relationship and the dynamical linkage between the boreal spring Arctic Oscillation (AO) and the Northern Hemisphere Hadley circulation extent (HCE). The spring AO is positively correlated with the HCE, with one standard positive deviation of the AO index corresponding to approximately 0.42° latitude poleward shift of the HCE. The interaction between the planetary wave and the zonal winds over the subtropics results in an anomalous eddy momentum flux divergence, which shifts the HCE poleward. The AO related transient eddy momentum flux divergence makes nearly 2 times larger contributions than those of the stationary component to the HCE change. The increased equatorward transient wave flux over the subtropics is possibly related to the larger meridional gradient of the transient wave refractive index there. The AO positive phase corresponds to an enhanced planetary wave propagation from the midlatitude Atlantic Ocean to the subtropics, which resembles the North Atlantic Oscillation pattern. The autumn and winter AO–HCE relationship is similar to that during spring, while summer has the weakest relationship, which could be mainly attributed to the far poleward extension of the climatological HCE during summer.

Formation of slantwise orientated nanoscale ripple structures on a single-crystal 4H-SiC surface by time-delayed double femtosecond laser pulses

An effective control of the nanoscale periodic ripple structures on a single-crystal 4H-SiC surface is carried out by temporally delayed double collinear femtosecond laser pulse trains with different linear polarizations. The observed evolution of the ripple slant angle shows a dramatic decrease within an initial time delay range of ~10 ps between the two laser pulses, but keeps almost steady for larger time elapse. Through the detailed analyses, we propose a theoretical model to reveal the underlying physics: regarding the double femtosecond laser pulses, the first laser pulse transfers the semiconductor into nonequilibrium states associated with a transient refractive index grating on the surface, which subsequently tends to change the direction of the surface plasmon excitation of the second laser pulse. These behaviors can eventually result in the slantwise oriented laser intensity fringes and corresponding periodic ablation of grooves. Such investigations provide additional insights into the ripple formation processes, which is very important for control of nanomanufacturing materials by femtosecond lasers.

Ultrafast pump-probe ellipsometry setup for the measurement of transient optical properties during laser ablation.

Ultrashort pulsed lasers offer a high potential in precise and efficient material processing and deep understanding of the fundamental laser-material interaction aspects is of great importance. The transient pulse reflectivity in conjunction with the transient absorption influences decisively the laser-material interaction. Direct measurements of the absorption properties by ultrafast time-resolved ellipsometry are missing to date. In this work, a unique pump-probe ellipsometry microscope is presented allowing the determination of the transient complex refractive index with a sub-ps temporal resolution. Measurements on molybdenum show ultrafast optical penetration depth changes of -6% to + 77% already within the first 10ps after the laser pulse impact. This indicates a significant absorption variation of the pump pulse or subsequent pulses irradiating the sample on this timescale and paves the road towards a better understanding of pulse duration dependent laser ablation efficiency, double or burst mode laser ablation and lattice modifications in the first ps after the laser pulse impact.

Optimizing the time resolution of supercontinuum spectral interferometry

Single-shot supercontinuum spectral interferometry is a powerful technique for measuring transient refractive index changes. In principle, its time resolution is limited only by the available probe bandwidth. However, this assumes that the phase extraction has sufficient spectral resolution and that the probe spectral phase is exactly known. Using an analytical model for the spectral phase and amplitude modulation of a chirped probe pulse by a weak transient phase perturbation, we show how the probe chirp and spectral resolution determine the achievable time resolution. A simple, practical technique for precise in situ measurement of the probe spectral phase is described in detail, and the sensitivity of the extracted temporal phase profile to uncertainty in the probe spectral phase is demonstrated in numerical simulations.

Influence of temperature distribution of heat affected zone on transient refractive index modifications of transparent materials

Thermoelastic wave equations are numerically solved by using finite element method to study effect of spatial distribution of microscale heat source, created inside transparent material, on physical parameters of heat affected zone. It is observed that temperature distribution has significant effect on spatial and temporal behavior of the displacement, strain, stress and, consequently, on refractive index changes of the material. Therefore, temperature distribution, which can be caused by laser pulse, can be employed as an important parameter to control refractive index profile inside transparent materials.

Spatiotemporal pulse-splitting of a filamentary femtosecond laser pulse via multi-filament interaction

We report the spatiotemporal pulse-splitting of a filamentary pulse with splitting gaps in 102 μm magnitude via interaction with a plasma grating. The spatiotemporal pulse-splitting is interpreted by the transient refractive index change of the interaction area based on the ionization model. The laser pulse is temporally split during the filamentation process. When the precedent part of the temporal splitting probe pulse interacts with the plasma grating, the refractive index of the interaction area is significantly changed, resulting in the spatial separation of the subsequent part from its original position. The splitting gaps between the original pulse and the splitting pulse depend on the crossing angle between the filamentary pulse and the plasma grating and the relative position of the pulse interacts on the plasma grating.

Femtosecond filamentation by intensity clamping at a Freeman resonance

We demonstrate that Freeman resonances have a strong impact on the nonlinear optical response in femtosecond filaments. These resonances decrease the transient refractive index within a narrow intensity window and strongly affect the filamentation dynamics. In particular, we demonstrate that the peak intensity of the filament can be clamped at these resonances, hinting at the existence of new regimes of filamentation with electron densities considerably lower than predicted by the standard model. This sheds further light on the phenomenon of filamentary intensity clamping and the nearly plasmaless filaments predicted by the controversial higher-order Kerr model.

Si-rich SiNx based Kerr switch enables optical data conversion up to 12 Gbit/s.

Silicon photonic interconnection on chip is the emerging issue for next-generation integrated circuits. With the Si-rich SiNx micro-ring based optical Kerr switch, we demonstrate for the first time the wavelength and format conversion of optical on-off-keying data with a bit-rate of 12 Gbit/s. The field-resonant nonlinear Kerr effect enhances the transient refractive index change when coupling the optical data-stream into the micro-ring through the bus waveguide. This effectively red-shifts the notched dip wavelength to cause the format preserved or inversed conversion of data carried by the on-resonant or off-resonant probe, respectively. The Si quantum dots doped Si-rich SiNx strengthens its nonlinear Kerr coefficient by two-orders of magnitude higher than that of bulk Si or Si3N4. The wavelength-converted and cross-amplitude-modulated probe data-stream at up to 12-Gbit/s through the Si-rich SiNx micro-ring with penalty of −7 dB on transmission has shown very promising applicability to all-optical communication networks.

Nonresonant Transient Refractive Index Spectroscopy in Semiconductor Quantum Dots

We report transient refractive index change in semiconductor nanoparticles dispersed in polymethylmethacrylate matrix via pump-probe experiment. At lower pump intensities the detected signal consists of the pulse autocorrelation-shaped part and another part delayed by 300 fs. The latter’s relative intensity depends on the pump level. However in CdS monocrystal the detected signal was found to lack this second feature completely.

Efficient population transfer in modulation doped single quantum wells by intense few-cycle terahertz pulses

We demonstrate the direct observation of non-equilibrium intersubband dynamics in a modulation-doped multiple quantum well sample subject to intense few-cycle terahertz (THz) pulses. The transmission spectra show a distinct dependence on the incident THz field strength and contain signatures of a multitude of nonlinear effects that can be observed owing to the large THz-pulse bandwidth. We focus our attention on a case of transient nonlinear refractive index caused by the efficient transfer of electronic population from the ground state to higher-excited states of the quantum well sample. By comparing the experimental results with a one-dimensional finite-difference model going beyond the slowly varying envelope approximation, we prove that, depending on the pulse shape, the leading part of the intense pulse efficiently transfers electrons from the ground state to higher lying excited states. For weak electric fields and small-population transfer, the linear Lorentz model holds. For strong electric fields, up to 55 and 20% of the ground-state electrons are transferred to the first and second excited subbands, respectively, which could lead to the observation of the optical gain.

Application of inverse Abel techniques in in-line holographic microscopy

In this paper, we compare two well-established techniques, namely the BASEX and the Fourier–Hankel, as regards their efficiency of retrieving the three-dimensional distribution of cylindrically symmetric objects in the presence of noise. This situation is commonly encountered in pump–probe experiments where the refractive index profile of elongated structures, such as plasma strings or transient refractive index changes, is under study. We performed numerical experiments for a variety of objects, with respect to the spatial distribution and size, and for various statistical distributions and levels of noise. In this area of application, the BASEX can surpass the Fourier–Hankel technique in respect of reconstruction quality, spatial resolution and robustness to noise especially after proper tuning of the basis set functions width.