Measurements of the nonlinear refractive index of air, N2, and O2 at 10 μm using four-wave mixing

Refractiveindex.info database of optical constants

We introduce the refractiveindex.info database, a comprehensive open-source repository containing optical constants for a wide array of materials, and describe in detail the underlying dataset. This collection, derived from a meticulous compilation of data sourced from peer-reviewed publications, manufacturers' datasheets, and authoritative texts, aims to advance research in optics and photonics. The data is stored using a YAML-based format, ensuring integrity, consistency, and ease of access. Each record is accompanied by detailed metadata, facilitating a comprehensive understanding and efficient utilization of the data. In this descriptor, we outline the data curation protocols and the file format used for data records, and briefly demonstrate how the data can be organized in a user-friendly fashion akin to the books in a traditional library.

Self-channeling of a multi-Joule 10 µm picosecond pulse train through long distances in air

In the long-wave infrared (LWIR) range, where, due to wavelength scaling, the critical power of Kerr self-focusing Pcr in air increases to 300-400 GW, we demonstrate that without external focusing a train of picosecond CO2 laser pulses can propagate in the form of a single several-centimeter diameter channel over hundreds of meters. The train of 10 µm pulses, for which the total energy ≥20 J is distributed over several near-terawatt picosecond pulses with a maximum power ≤2Pcr, is generated naturally during short pulse amplification in a CO2 laser. It is observed that the high-power 10 µm beam forms a large diameter "hot gas" channel in the ambient air with a ≥ 50 ms lifetime. Simulations of the experiment show that such filamentation-free self-channeling regime has low propagation losses and can deliver multi-Joule/TW-power LWIR pulses over km-scale distances.

Sensitivity enhancement of nonlinear refractive index measurement by Gaussian-Bessel beam assisted z-scan method

Characterizing the nonlinear optical properties of numerous materials plays a prerequisite role in nonlinear imaging and quantum sensing. Here, we present the evaluation of the nonlinear optical properties of Rb vapor by the Gaussian-Bessel beam assisted z-scan method. Owed to the concentrated energy in the central waist spot and the constant intensity of the beam distribution, the Gaussian-Bessel beam enables enhanced sensitivity for nonlinear refractive index measurement. The nonlinear self-focusing and self-defocusing effects of the Rb vapor are illustrated in the case of blue and red frequency detunings from 5S_1/2 - 5P_3/2 transition, respectively. The complete images of the evolution of nonlinear optical properties with laser power and frequency detuning are acquired. Furthermore, the nonlinear refractive index n₂ with a large scale of 10^-6 cm²/W is determined from the measured transmittance peak-to-valley difference of z-scan curves, which is enhanced by a factor of ∼ 1.73 compared to the result of a equivalent Gaussian beam. Our research provides an effective method for measuring nonlinear refractive index, which will considerably enrich the application range of nonlinear material.

Generation of long homogeneous plasma channels with high power long-wave IR pulsed Bessel beams

Long-wave multi-joule ultrashort laser pulses are predicted to confine highly uniform electromagnetic energy and field intensities while sustaining high density uniform plasmas within nonlinear Bessel zones under extreme driving conditions in contrast to near-IR sources. This opens up novel applications in laser wakefield generation, radiofrequency/microwave guiding, and lightning control.

Single-shot measurement of the nonlinear refractive index of air at 9.2 µm with a picosecond terawatt CO2 laser

We developed a simple, accurate single-shot method to determine the nonlinear refractive index of air by measuring the evolution of the spatial shape of a laser beam propagating through the atmosphere. A distinctive feature of this new method, which relies on a modified Fresnel propagation model for data analysis, is the use of a hard aperture for producing a well-defined, high-quality beam from a comparatively non-uniform quasi-flat-top beam, which is typical for high-peak-power lasers. The nonlinear refractive index of air for a very short (2 ps) long-wave infrared (LWIR) laser pulse was measured for the first time, to the best of our knowledge, yielding n₂=3.0×10^-23m²/W at 9.2 µm. This result is 40% lower than a corresponding measurement with longer (200 ps) LWIR pulses at a similar wavelength.

Nonlinear refractive index of solids in mid-infrared

We measure the third-order nonlinear optical response of various dielectrics and semiconductors using the spectrally resolved two-beam coupling method at 2.3 µm, 3.5 µm, 4.5 µm, and 8.3 µm. These materials include fused silica, sapphire, calcium fluoride, magnesium fluoride, zinc sulphide, and zinc selenide. We compare our results with previous literature results and theoretically expected values using two-band model theory. The dispersion of the nonlinear refractive index n₂ over this wavelength range is found to be negligible.

Transient mid-IR nonlinear refraction in air

We use the polarization-sensitive, time-resolved Beam-Deflection technique to measure the nonlinear refraction of air, exciting in both the near and mid-IR and probing in the mid-IR. This gives us the first measurements for air using both excitation and probe in the mid-IR, and we find no dispersion of the bound-electronic nonlinear refractive index, n_2,el(λ_p;λ_e), assuming, as has been shown earlier, that the nuclear rotational nonlinear refraction is nearly dispersionless. From these data, we can model the pulsewidth dependence of the effective nonlinear refractive index, n_2,eff, i.e., as would be measured by a single beam. Interestingly, n_2,eff is maximized for a pulsewidth of approximately 0.5 ps. The position of this maximum is nearly independent of pressure while its magnitude decreases with increasing pressure and temperature. From the measurements and modeling, we predict the nonlinear refraction in the atmosphere at different altitudes.

Optical visualization and imaging of nanomaterials

Direct visualization and imaging of nanomaterials under ambient conditions is of great significance for their characterization and application. In most cases, the observation of individual nanomaterials usually requires high-resolution electron microscopes under high vacuum. In comparison, an optical microscope is much more convenient due to its facile operation and open space. However, the resolution of optical microscopes is much lower than that of electron microscope-based tools. Therefore, effective visualization and imaging strategies for nanomaterials are required to realize their direct observation, accurate location and controllable manipulation. In this review, we summarized the progress of optical visualization and imaging strategies for nanomaterials in recent years, including vapor-condensation-assisted optical visualization, nanoparticle-assisted optical visualization, substrate-assisted optical visualization and fluorescence visualization, and the applications of these techniques were also introduced. We believe that this review will inspire further improvement in optical visualization of nanomaterials and drive the application of nanomaterials in a broader domain.

Measurement of the Kerr nonlinear refractive index of the Rb vapor based on an optical frequency comb using the z-scan method

We report the measurement of the Kerr nonlinear refractive index of the rubidium vapor via the high sensitivity z-scan method by using an optical frequency comb. The novel self-focusing and self-defocusing effects of the vapor are presented with red and blue detunings of the laser frequency. The optical nonlinear characteristics of the rubidium vapor are clearly interpreted under different experimental parameters. Furthermore, the Kerr nonlinear refractive index n₂ is obtained from the measured dispersion curve, and it basically occurs on the order of 10^-6 cm²/W. The evolutions of the Kerr nonlinear coefficient n₂ with the laser power and frequency detuning, respectively, are studied. To the best of our knowledge, the use of pulsed lasers to measure the Kerr nonlinear refractive index n₂ of atomic vapor has not been reported yet. The direct measurement of the Kerr nonlinear coefficient will greatly help us understand and optimize nonlinear optical processes and find its more potential applications in quantum optics.

Powerful terahertz waves from long-wavelength infrared laser filaments

Strong terahertz (THz) electric and magnetic transients open up new horizons in science and applications. We review the most promising way of achieving sub-cycle THz pulses with extreme field strengths. During the nonlinear propagation of two-color mid-infrared and far-infrared ultrashort laser pulses, long, and thick plasma strings are produced, where strong photocurrents result in intense THz transients. The corresponding THz electric and magnetic field strengths can potentially reach the gigavolt per centimeter and kilotesla levels, respectively. The intensities of these THz fields enable extreme nonlinear optics and relativistic physics. We offer a comprehensive review, starting from the microscopic physical processes of light-matter interactions with mid-infrared and far-infrared ultrashort laser pulses, the theoretical and numerical advances in the nonlinear propagation of these laser fields, and the most important experimental demonstrations to date.

Self-Guiding of Long-Wave Infrared Laser Pulses Mediated by Avalanche Ionization

Nonlinear self-guided propagation of intense long-wave infrared (LWIR) laser pulses is of significant recent interest, as it promises high power transmission without beam breakup and multifilamentation. Central to self-guiding is the mechanism for the arrest of self-focusing collapse. Here, we show that discrete avalanche sites centered on submicron aerosols can arrest self-focusing, providing a new mechanism for self-guided propagation of moderate intensity LWIR pulses in outdoor environments. Our conclusions are supported by simulations of LWIR pulse propagation using an effective index approach that incorporates the time-resolved plasma dynamics of discrete avalanche breakdown sites.

Hygroscopic Micro/Nanolenses along Carbon Nanotube Ion Channels

Nanolenses of alkali metal halides can be a unique optical element due to their hygroscopicity, optical transparency, and high mobility of constituent ions. It has been challenging, however, to form and place such lenses in a controlled manner. Here, we report micro/nanolenses of various alkali metal halides arranged as a one-dimensional (1D) array, using the exterior of single-walled carbon nanotubes (SWNTs) as a template for forming the lenses. Applying an electrical bias to an aqueous solution of alkali metal halides placed at the end of an SWNT array causes ionic transport along the exterior of SWNTs and the subsequent formation of salt micro/nanocrystals. The crystals serve as micro/nanolenses that optically visualize individual SWNTs and amplify their Raman scattering by orders of magnitude. Molecules dissolved in the ionic solution can be electrokinetically transported along the nanotubes, captured within the lenses, and analyzed by Raman spectroscopy, which we demonstrate by detecting ∼12 attomoles of glucose and 2 femtomoles of urea. The hygroscopic salt nanolenses are robust under various ambient conditions indefinitely, by transitioning to liquid droplets above their deliquescence relative humidity, yet can be removed nondestructively by water. Our approach could have broad implications in the optical visualization of 1D nanostructures, molecular transport or chemical reactions in 1D space, and molecular spectroscopy in salty environments.

Ultrashort infrared 2.5-11 μm pulses: spatiotemporal profiles and absolute nonlinear response of air constituents

We measure the detailed spatiotemporal profiles of femtosecond laser pulses in the infrared wavelength range of λ=2.5-11  μm and the absolute nonlinear response of major air constituents (N₂, O₂, and Ar) over this range. The spatiotemporal measurements reveal wavelength-dependent pulse front tilt and temporal stretching in the infrared pulses.

Multi-terawatt 10 μm pulse atmospheric delivery over multiple Rayleigh ranges

We predict that long wavelength self-trapped multi-terawatt pulses can be sustained over multiple kilometers in the atmosphere. Unlike filaments, these pulses exhibit low loss propagation and retain most of their launch power at range. A novel mechanism involving an aggregation of weakly linear and nonlinear cumulative optical responses is shown to be responsible and is dominated by an ultrafast dynamical lensing resulting from a field intensity driven many-body Coulomb mediated free electron polarization associated with spatially separated species in the gas. An initial few picosecond pulse can compress down to 140 fs over multiple kilometers.