Pump-probe phase spectroscopy with submilliradian sensitivity and nanosecond time delay using Michelson interferometers.

Using two Michelson interferometers, we describe an experimental scheme for sensitive pump-probe spectral interferometry measurements at long time delays. It has practical advantages over the Sagnac interferometer method typically used when long-time delays are required. First, with the Sagnac interferometer, achieving many nanosecond delays requires expanding the size of the interferometer so that the reference pulse arrives before the probe pulse.

Analysis of complex multidimensional optical spectra by linear prediction.

We apply Linear Prediction from Singular Value Decomposition (LPSVD) to two-dimensional complex optical data in the time-domain to generate spectra with advantages over discrete Fourier transformation (DFT). LPSVD is a non-iterative procedure that fits time-domain complex data to the sum of damped sinusoids, or Lorentzian peaks in the spectral domain. Because the fitting is linear, it is not necessary to give initial guess parameters as in nonlinear fits.

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, to our knowledge, the coherent interaction between beams has not previously been discussed in these materials, which have 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.

Nonlinearity and ionization in Xe: experiment-based calibration of a numerical model.

Recently proposed universality of the nonlinear response is put to the test and used to improve a previously designed model for xenon. Utilizing accurate measurements resolving the nonlinear polarization and ionization in time and space, we calibrate the scaling parameters of the model and demonstrate agreement with several experiments spanning the intensity range relevant for applications in nonlinear optics at near-infrared and mid-infrared wavelengths. Applications to other species including small molecules are discussed, suggesting a self-consistent way to calibrate light-matter interaction models.

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, N_{2}, and Ar over 14 decades of absolute yield, using mid-IR picosecond avalanche multiplication of single electrons.

Polarization Dependence of Charge Conduction in Conjugated Polymer Films Investigated with Time-Resolved Terahertz Spectroscopy.

Room temperature Time-Domain Terahertz (TDS) and Time-Resolved Terahertz (TRTS) spectroscopic methods are employed to measure carrier mobility and charge generation efficiency in thin-film semiconductor polymers. Interrogation of the dependence on excitation and probe polarizations yields insight into the underlying material properties that guide charge transport. We apply THz polarization anisotropy probes to analyze charge conduction in preparations of the copolymer PCDTPT, consisting of alternating cyclopenta-dithiophene (donor) and thiadiazolo-pyridine (acceptor) units.

Automated polarization-dependent multidimensional coherent spectroscopy phased using transient absorption.

An experimental apparatus is described for multidimensional optical spectroscopy with fully automated polarization control, based on liquid crystal variable retarders. Polarization dependence of rephasing two-dimensional coherent spectra are measured in a single scan, with absolute phasing performed for all polarization configurations through a single automated auxiliary measurement at the beginning of the scan. A factor of three improvement in acquisition time is demonstrated, compared to the apparatus without automated polarization control.

Controlling femtosecond filament propagation using externally driven gas motion.

The thermal density depression (or "density hole") produced by a high-repetition-rate femtosecond filament in air acts as a negative lens, altering the propagation of the filament. We demonstrate the effects of externally driven gas motion on these density holes and the resulting filament steering, and we derive an expression for the gas velocity that maximizes the effect. At gas velocities more than ∼3 times this value, the density hole is displaced from the filament, and it no longer affects filament propagation.

Contactless THz-based bulk semiconductor mobility measurements using two-photon excitation.

We perform contactless bulk mobility measurements for ZnSe, ZnTe, GaP, CdS, and GaSe in an optical pump THz probe experiment. As opposed to above-gap excitation or contact methods, two-photon absorption excites the entire sample thickness producing measurable signals with 10 carriers/cm and higher density. For ZnTe and GaSe samples, the measured mobility using two-photon excitation is higher than that measured with one-photon excitation.

Bound-Electron Nonlinearity Beyond the Ionization Threshold.

We present absolute space- and time-resolved measurements of the ultrafast laser-driven nonlinear polarizability in argon, krypton, xenon, nitrogen, and oxygen up to ionization fractions of a few percent. These measurements enable determination of the strongly nonperturbative bound-electron nonlinear polarizability well beyond the ionization threshold, where it is found to remain approximately quadratic in the laser field, a result normally expected at much lower intensities where perturbation theory applies.

Energy deposition of single femtosecond filaments in the atmosphere: erratum.

In this erratum the funding section of Opt. Lett.41, 3908 (2016)OPLEDP0146-959210.

Energy deposition of single femtosecond filaments in the atmosphere.

We present spatially resolved measurements of energy deposition into atmospheric air by femtosecond laser filaments. Single filaments formed with varying laser pulse energy and pulsewidth were examined using longitudinal interferometry, sonographic probing, and direct energy loss measurements. We measure peak and average energy absorption of ∼4  μJ/cm and ∼1  μJ/cm for input pulse powers up to ∼6 times the critical power for self-focusing.

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

We measure the nonlinear refractive index coefficients in N₂, O₂, and Ar from visible through mid-infrared wavelengths (λ=0.4-2.4  μm).

Multi-MeV Electron Acceleration by Subterawatt Laser Pulses.

We demonstrate laser-plasma acceleration of high charge electron beams to the ∼10  MeV scale using ultrashort laser pulses with as little energy as 10 mJ. This result is made possible by an extremely dense and thin hydrogen gas jet. Total charge up to ∼0.

Optical mode structure of the air waveguide.

Analysis is performed on propagation of light in long-lived optical waveguides in air generated by arrays of femtosecond filaments. Mode structure, leakage losses, and coupling efficiency are studied analytically and numerically as a function of wavelength and time delay after the waveguide-initiating filaments.

Quantum control of molecular gas hydrodynamics.

We demonstrate that strong impulsive gas heating or heating suppression at standard temperature and pressure can occur from coherent rotational excitation or deexcitation of molecular gases using a sequence of nonionizing laser pulses. For the case of excitation, subsequent collisional decoherence of the ensemble leads to gas heating significantly exceeding that from plasma absorption under the same laser focusing conditions. In both cases, the macroscopic hydrodynamics of the gas can be finely controlled with ∼40  fs temporal sensitivity.

Direct imaging of the acoustic waves generated by femtosecond filaments in air.

We present time-resolved measurements of the gas acoustic dynamics following interaction of spatial single- and higher-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 multifilaments.

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 sudden exposure of a gas to a kilohertz train of filamenting pulses, successive filaments are steered from their original direction to a new stable direction whose equilibrium is determined by a balance among buoyant, viscous, and diffusive processes in the gas. The beam mode is preserved.

The effect of long timescale gas dynamics on femtosecond filamentation.

Femtosecond laser pulses filamenting in various gases are shown to generate long- lived quasi-stationary cylindrical depressions or 'holes' in the gas density. For our experimental conditions, these holes range up to several hundred microns in diameter with gas density depressions up to ~20%. The holes decay by thermal diffusion on millisecond timescales.

Ionization-grating-induced nonlinear phase accumulation in spectrally resolved transient birefringence measurements at 400 nm.

We report experimental confirmation of the ionization-grating-induced transient birefringence predicted by Wahlstrand and Milchberg [Opt. Lett. 36, 3822 (2011)] and discuss its impact on the higher-order Kerr effect interpretation by Loriot et al.

High field optical nonlinearity and the Kramers-Kronig relations.

The nonlinear optical response to high fields is absolutely measured for the noble gas atoms He, Ne, Ar, Kr, and Xe. We find that the response is quadratic in the laser field magnitude up to the ionization threshold of each gas. Its size and quadratic dependence are well predicted by a Kramers-Kronig analysis employing known ionization probabilities, and the results are consistent with calculations using the time-dependent Schrödinger equation.

Electric field-induced coherent control in GaAs: polarization dependence and electrical measurement [Invited].

A static electric field enables coherent control of the photoexcited carrier density in a semiconductor through the interference of one- and two-photon absorption. An experiment using optical detection is described. The polarization dependence of the signal is consistent with a calculation using a 14-band k · p model for GaAs.

Optical nonlinearity in Ar and N2 near the ionization threshold.

We directly measure the nonlinear optical response in argon and nitrogen in a thin gas target to laser intensities near the ionization threshold. No instantaneous negative nonlinear refractive index is observed, nor is saturation, in contrast with a previous measurement [Opt. Express 17, 13429 (2009)] and calculations [Phys.

Effect of a plasma grating on pump-probe experiments near the ionization threshold in gases.

Calculations are performed of the phase shift caused by the spatial modulation in the plasma density due to interference between a strong pump pulse and a weak probe pulse. It is suggested that a recent experiment [Opt. Express 17, 13429 (2009)] observed an effective birefringence from this plasma grating rather than from the higher-order Kerr effect.

Optical coherent control induced by an electric field in a semiconductor: a new manifestation of the Franz-Keldysh effect.

In (100)-oriented GaAs illuminated at normal incidence by a laser and its second harmonic, interference between one- and two-photon absorption results in ballistic current injection, but not modulation of the overall carrier injection rate. Results from a pump-probe experiment on a transversely biased sample show that a constant electric field enables coherent control of the carrier injection rate. We ascribe this to the nonlinear optical Franz-Keldysh effect and calculate it for a two-band parabolic model.