arXiv subjects

J. K. Wahlstrand

Publications and source records attributed to J. K. Wahlstrand.

At least 19 recordsLinked to original sources

arXiv2026

Precise measurement of the Kerr coefficient using phase-sensitive pump-probe hyperspectral imaging

Phase-sensitive pump-probe hyperspectral imaging is a precise technique for absolute two-beam measurements of the optical Kerr coefficient ($n_2$). The irradiance profile is characterized and background effects are rejected by rastering the pump beam across the probe beam to yield a complex-valued hyperspectral image of the pump-induced nonlinear response. Information about the temporal irradiance profile is carried in the spectral response. The technique is demonstrated by measuring $n_2$ of a fused silica sample near 1~$μ$m wavelength and benchmarked against a measurement using Z-scan [Sheik-Bahae \emph{et al.}, IEEE J. Quantum Electron. \textbf{26}, 760--769 (1990)], the most widely used single-beam technique. The two measurements are consistent when the two-beam grating effect from the Raman contribution to the nonlinearity is considered. Uncertainty contributions are described in detail and the outlook is discussed for improvements in precision.

physics.optics↗

arXiv2026

Theory of the two-photon Franz-Keldysh effect and electric-field-induced bichromatic coherent control

The effect of a constant electric field on two-photon absorption in a semiconductor is calculated using an independent-particle theory. The theoretical framework is an extension of a theory of the one-photon Franz-Keldysh effect [Wahlstrand and Sipe, Phys. Rev. B 82, 075206 (2010)]. The theory includes the effect of the constant field, including field-induced coupling between closely spaced bands, in the electronic wavefunctions and calculates optical absorption perturbatively. Numerical calculations are performed using a 14-band $\mathbf{k} \cdot\mathbf{p}$ band structure model for GaAs. For all nonzero tensor elements, field-enabled two-photon absorption (TPA) below the band gap and Franz-Keldysh oscillations in the TPA spectrum are predicted, with a generally larger effect in tensor elements with more components parallel to the constant electric field direction. Some tensor elements that are zero in the absence of a field become nonzero in the presence of the constant electric field and depend on its sign. Notably, these elements are linear in the electric field to lowest order and may be substantial away from band structure critical points at room temperature and/or with a non-uniform field. Electric-field-induced changes in the carrier injection rate due to interference between one- and two-photon absorption are also calculated. The electric field enables this bichromatic coherent control process for polarization configurations where it is normally forbidden, and also modifies the spectrum of the process for configurations where it is allowed by crystal symmetry.

physics.optics↗

arXiv2024

Phase-sensitive pump-probe measurement of the complex nonlinear susceptibility of silicon across the direct band edge

The nonlinear response of materials, an increasingly important aspect of light-matter interaction, can be challenging to measure in highly absorbing materials. Here, we introduce an interferometric technique that enables a direct measurement of the nonlinear complex permittivity in a bulk medium from reflectivity alone. We demonstrate the utility of pump-probe supercontinuum (SC) spectral interferometry in reflection by measuring time-dependent variations in the complex dielectric function ($n$, $k$) over the visible wavelength range in bulk silicon. Transient phase shifts in the reflected SC due to a near infrared pump pulse allow us to track modifications to $k$; whereas changes in $n$ are derived from transient fluctuations in the reflected SC probe amplitude. The ultrafast response is attributed to effective two-photon absorption ($β$) and Kerr ($n_2$) coefficients. We observe the onset of strong two-photon absorption as the two-photon energy is tuned through the direct band edge of silicon ($E_1$ = 3.4 eV) for the first time to our knowledge. This technique allows straightforward spectroscopic measurements of the $χ^{(3)}$ nonlinear response at the surface of absorbing materials.

physics.optics↗

arXiv2022

Coherent Contributions to Population Dynamics in a Semiconductor Microcavity

Multidimensional coherent spectroscopy (MDCS) is used to separate coherent and incoherent many-body contributions to the population-time dynamics in a GaAs-based semiconductor microcavity encapsulating a single InGaAs quantum well. In a three-pulse four-wave-mixing scheme, the second delay time is the population time that in MDCS probes excited-state coherences and population dynamics. Nonlinear optical interactions can mix these contributions, which are isolated here for the lower- and upper-exciton-polariton through the self- and mutual-interaction features. Results show fast decays and oscillations arising from the coherent response, including a broad stripe along the absorption energy axis, and longer time mutual-interaction features that do not obey a simple population decay model. These results are qualitatively replicated by Bloch equation simulations for the 1s exciton strongly coupled to the intracavity field. The simulations allow for separation of coherent and incoherent Pauli-blocking and Coulomb interaction terms within the chi^(3)-limit, and direct comparison of each feature in one-quantum rephasing and zero-quantum spectra.

cond-mat.mes-hall↗

arXiv2020

Two-beam coupling by a hot electron nonlinearity

Transparent conductive oxides such as indium tin oxide (ITO) bear the potential to deliver efficient all-optical functionality due to their record-breaking optical nonlinearity at epsilon near zero (ENZ) wavelengths. All-optical applications generally involve more than one beam, but the coherent interaction between beams has not previously been discussed in materials with a hot electron nonlinearity. Here we study the optical nonlinearity at ENZ in ITO and show that spatial and temporal interference has important consequences in a two beam geometry. Our pump-probe results reveal a polarization-dependent transient that is explained by momentary diffraction of pump light into the probe direction by a temperature grating produced by pump-probe interference. We further show that this effect allows tailoring the nonlinearity by tuning frequency or chirp. Having fine control over the strong and ultrafast ENZ nonlinearity may enable applications in all-optical neural networks, nanophotonics, and spectroscopy.

physics.optics↗

arXiv2019

Absolute measurement of laser ionization yield in atmospheric pressure range gases over 14 decades

Strong-field ionization is central to intense laser-matter interactions. However, standard ionization measurements have been limited to extremely low density gas samples, ignoring potential high density effects. Here, we measure strong-field ionization in atmospheric pressure range air, N2 and Ar over 14 decades of absolute yield, using mid-IR picosecond avalanche multiplication of single electrons. Our results are consistent with theoretical rates for isolated atoms and molecules and quantify the ubiquitous presence of ultra-low concentration gas contaminants that can significantly affect laser-gas interactions.

physics.plasm-ph↗

arXiv2018

Coherent ultra-broadband laser-assisted injection radiation from a laser plasma accelerator

The injection of electrons into a laser wakefield accelerator (LWFA) is observed to generate an intense coherent ultra-broadband and ultrashort pulse radiation flash, consistent with the acceleration of electrons from rest to nearly the speed of light in a distance < ~ 1 $μ$m. The flash is sufficiently bright to induce large nonlinear refractive index shifts in optical materials; we estimate a source brightness temperature of ~$10^{18}$ K. We present measurements of the flash spectra, coherence, pulse duration, polarization and angular distribution, providing a detailed picture of electron injection dynamics in LWFA. These are characteristic of laser-assisted injection of off-axis electrons, which preserves wake coherence.

physics.acc-ph↗

arXiv2018

Bound electron nonlinearity beyond the ionization threshold

Although high field laser-induced ionization is a fundamental process underlying many applications, there have been no absolute measurements of the nonlinear polarizability of atoms and molecules in the presence of ionization. Such information is crucial, for example, for understanding the propagation of high intensity ultrashort pulses in matter. Here, we present absolute space- and time-resolved measurements of the ultrafast laser-driven nonlinear polarizability in argon, krypton, xenon, nitrogen, and oxygen up to an ionization fraction of a few percent. These measurements enable determination of the non-perturbative bound electron nonlinearity well beyond the ionization threshold, where it is found to be approximately linear in intensity.

physics.optics↗

arXiv2016

Two-photon vibrational excitation of air by long-wave infrared laser pulses

Ultrashort long-wave infrared (LWIR) laser pulses can resonantly excite vibrations in N2 and O2 through a two-photon transition. The absorptive, vibrational component of the ultrafast optical nonlinearity grows in time, starting smaller than, but quickly surpassing, the electronic, rotational, and vibrational refractive components. The growth of the vibrational component results in a novel mechanism of 3rd harmonic generation, providing an additional two-photon excitation channel, fundamental + 3rd harmonic. The original and emergent two-photon excitations drive the resonance exactly out of phase, causing spatial decay of the absorptive, vibrational nonlinearity. This nearly eliminates two-photon vibrational absorption. Here we present simulations and analytical calculations demonstrating how these processes modify the ultrafast optical nonlinearity in air. The results reveal nonlinear optical phenomena unique to the LWIR regime of ultrashort pulse propagation in atmosphere.

physics.optics↗

arXiv2016

Spatio-temporal optical vortices

We present the first experimental evidence, supported by theory and simulation, of spatiotemporal optical vortices (STOVs). Quantized STOVs are a fundamental element of the nonlinear collapse and subsequent propagation of short optical pulses in material media. A STOV consists of a ring-shaped null in the electromagnetic field about which the phase is spiral, forming a dynamic torus which is concentric with and tracks the propagating pulse. Depending on the sign of the material dispersion, the local electromagnetic energy flow is saddle or spiral about the STOV. STOVs are born and evolve conserving topological charge; they can be simultaneously created in pairs with opposite windings, or generated from a point null. Our results, here obtained for optical pulse collapse and filamentation in air, are generalizable to broad class of nonlinearly propagating waves.

physics.optics↗

arXiv2015

Absolute measurement of the ultrafast nonlinear electronic and rovibrational response in H$_2$ and D$_2$

The electronic, rotational, and vibrational components of the ultrafast optical nonlinearity in H$_2$ and D$_2$ are measured directly and absolutely at intensities up to the ionization threshold of $\sim$10$^{14}$ W/cm$^2$. As the most basic nonlinear interactions of the simplest molecules exposed to high fields, these results constitute a benchmark for high field laser-matter theory and simulation.

physics.optics↗

arXiv2015

Measurement of the nonlinear refractive index of air constituents at mid-infrared wavelengths

We measure the nonlinear refractive index coefficients in N$_2$, O$_2$ and Ar from visible through mid-infrared wavelengths (λ= 0.4 - 2.4 μm). The wavelengths investigated correspond to transparency windows in the atmosphere. Good agreement is found with theoretical models of $χ^{(3)}$. Our results are essential for accurately simulating the propagation of ultrashort mid-IR pulses in the atmosphere.

physics.optics↗

arXiv2014

Sensitivity of propagation and energy deposition in femtosecond filamentation to the nonlinear refractive index

The axial dependence of femtosecond filamentation in air is measured under conditions of varying laser pulsewidth, energy, and focusing f-number. Filaments are characterized by the ultrafast z-dependent absorption of energy from the laser pulse and diagnosed by measuring the local single cycle acoustic wave generated. Results are compared to 2D+1 simulations of pulse propagation, whose results are highly sensitive to the instantaneous (electronic) part of the nonlinear response of $N_2$ and $O_2$. We find that recent measurements of the nonlinear refractive index ($n_2$) in [J.K. Wahlstrand et al., Phys. Rev. A. 85, 043820 (2012)] provide the best match and an excellent fit between experiments and simulations.

physics.optics↗

arXiv2014

Collection of remote optical signals by air waveguides

Collection of weak signals from remote locations is the primary goal and the primary hurdle of optical stand-off detection schemes. Typically, the measured signal is enhanced using large numerical aperture collection optics and high gain detectors. We show that the signal in remote detection techniques can be enhanced by using a long-lived air waveguide generated by an array of femtosecond filaments. We present a proof of principle experiment using an air plasma spark source and a ~1 m air waveguide showing an increase in collected signal of ~50%. For standoff distances of 100 m, this implies that the signal-to-noise ratio can be increased by a factor ~10$^4$.

physics.optics↗

arXiv2014

Direct measurement of the acoustic waves generated by femtosecond filaments in air

We present direct measurements of the gas acoustic dynamics following interaction of spatial single- and multi-mode 50 fs, 800 nm pulses in air at 10 Hz and 1 kHz repetition rates. Results are in excellent agreement with hydrodynamic simulations. Under no conditions for single filaments do we find on-axis enhancement of gas density; this occurs only with multi-filaments. We also investigate the propagation of probe beams in the gas density profile induced by a single extended filament. We find that light trapping in the expanding annular acoustic wave can create the impression of on-axis guiding in a limited temporal window.

physics.optics↗

arXiv2013

Demonstration of long-lived high power optical waveguides in air

We demonstrate that femtosecond filaments can set up an extended and robust thermal waveguide structure in air with a lifetime of several milliseconds, making possible the very long range guiding and distant projection of high energy laser pulses and high average power beams. As a proof of principle, we demonstrate guiding of 110 mJ, 7 ns, 532 nm pulses with 90% throughput over ~15 Rayleigh lengths in a 70 cm long air waveguide generated by the long timescale thermal relaxation of an array of femtosecond filaments. The guided pulse was limited only by our available laser energy. In general, these waveguides should be robust against the effects of thermal blooming of extremely high average power laser beams.

physics.optics↗

arXiv2013

Optical beam dynamics in a gas repetitively heated by femtosecond filaments

We investigate beam pointing dynamics in filamentation in gases driven by high repetition rate femtosecond laser pulses. Upon suddenly exposing a gas to a kilohertz train of filamenting pulses, the filament is steered from its original direction to a new stable direction whose equilibrium is determined by a balance among buoyant, viscous, and diffusive processes in the gas. Results are shown for Xe and air, but are broadly applicable to all configurations employing high repetition rate femtosecond laser propagation in gases.

physics.optics↗

arXiv2013

Effect of two-beam coupling in strong-field optical pump-probe experiments

Nonlinear optics experiments measuring phase shifts induced in a weak probe pulse by a strong pump pulse must account for coherent effects that only occur when the pump and probe pulses are temporally overlapped. It is well known that a weak probe beam experiences a greater phase shift from a strong pump beam than the pump beam induces on itself. The physical mechanism behind the enhanced phase shift is diffraction of pump light into the probe direction by a nonlinear refractive index grating produced by interference between the two beams. For an instantaneous third-order response, the effect of the grating is to simply double the probe phase shift, but when delayed nonlinearities are considered, the effect is more complex. A comprehensive treatment is given for both degenerate and nondegenerate pump-probe experiments in noble and diatomic gases. Results of numerical calculations are compared to a recent transient birefringence measurement [Loriot et al., Opt. Express 17, 13429 (2009)] and a recent spectral interferometry experiment [Wahlstrand et al., Phys. Rev. A 85, 043820 (2012)]. We also present results from two new experiments using spectrally-resolved transient birefringence with 800 nm pulses in Ar and air and degenerate chirped pulse spectral interferometry in Ar. Both experiments support the interpretation of the negative birefringence at high intensity as arising from a plasma grating.

physics.optics↗