Design and modelling of dispersion-engineered rib waveguide for ultra broadband mid-infrared supercontinuum generation

We report a dispersion-engineered As₂Se₃ chalcogenide glass rib waveguide structure for ultra broadband mid-infrared supercontinuum generation across molecular ‘fingerprint region’. The proposed rib waveguide structure offers non-linear coefficient as high as 18,250 W^?1 km^?1 at 2.5 μm. Supercontinuum spectrum spanning 2–15 μm, which not only covers the both atmospheric transparent windows (3–5 μm and 8–13 μm) in the mid-infrared domain but also covers the important molecular ‘fingerprint domain’, is obtained using only 4 mm-long rib waveguide structure. To the best of our knowledge, such broadband mid-infrared supercontinuum spectrum in As₂Se₃-based chalcogenide waveguide geometry is reported for the first time. The proposed design of rib waveguide has potential for robust, integrated and low-cost supercontinuum sources in various applications including frequency comb generation, chemical sensing, food quality control and early cancer diagnostics.     

Phase-locked ultra-broadband supercontinuum generation in a mid-infrared polarization gating

We theoretically investigate the UV-assisted high-order harmonic generation (HHG) process in a mid-infrared polarization gating (PG) field. The ionization and recombination steps of the HHG process can be controlled simultaneously by the UV and PG pulses. The influences of the delay between the UV and PG pulses on the phase-locking degree and the intensity of the harmonics are presented quantitatively. The results show that the harmonics are phase-locked as well as the intensity is significantly enhanced by 3 or 4 orders at a proper delay. Then a phase-locked ultra-broadband supercontinuum with a bandwidth of 240?eV is produced efficiently, supporting the generation of a Fourier-transform-limited pulse duration below one atomic unit of time. In addition, efficient isolated 110 as pulses with tunable central wavelengths can be obtained directly by selecting a different range of the supercontinuum.     

Highly efficient terahertz pulse generation by optical rectification in stoichiometric and cryo-cooled congruent lithium niobate

We report, and review in detail, experiments resulting in a record 3.7% optical-to-terahertz (THz) conversion efficiency by optical rectification (OR) in cryogenically cooled congruent lithium niobate (cLN) using a near-optimal 680 fs pump pulse at 1030 nm. In addition, we report a record conversion efficiency of 1.7% at room temperature using stoichiometric lithium niobate (sLN) which results in 21.8 μJ of THz energy from a 1.2 mJ optical pulse. Electro-optical sampling measurements reveal the THz pulses to be single-cycle and centered at 0.45?THz. The experimentally measured efficiency, THz waveform, and THz spectrum are in good agreement with theoretical calculations. Finally, spatial beam profile measurements are also provided. To our knowledge, these results represent an order of magnitude improvement in efficiency of THz generation by OR in lithium niobate over previous results.     

Mid-infrared supercontinuum generation in tapered As2S3 chalcogenide planar waveguide

Abstract

We numerically demonstrate mid-infrared supercontinuum generation in a non-uniformly tapered chalcogenide planar waveguide. This planar rib waveguide of As₂S₃ glass on MgF₂ is 2 cm long with increasing etch depth longitudinally to manage the total dispersion. This waveguide has zero dispersion at two wavelengths. The dispersion profile varies along the propagation distance, leading to continuous modification of the phase-matching condition for dispersive wave emission and enhancement of energy transfer efficiency between solitons and dispersive waves. Numerical simulations are conducted for secant input pulses at a wavelength of 1.55 μm with a width of 50 fs and peak power of 2 kW. Results show this proposed scheme significantly broadens the generated continuum, extending from ~1 to ~7 μm.     

CMOS compatible on-chip telecom-band to mid-infrared supercontinuum generation in dispersion-engineered reverse strip/slot hybrid Si3N4 waveguide

A silicon nitride (Si₃N₄)-based reverse strip/slot hybrid waveguide with single vertical silica slot is proposed to acquire extremely low and flat chromatic dispersion profile. This is achieved by design and optimization of the geometrical structural parameters of the reverse hybrid waveguide. The flat dispersion varying between ±10 ps/(nm·km) is obtained over 610 nm bandwidth. Both the effective area and nonlinear coefficient of the waveguide across the entire spectral range of interest are investigated. This led to design of an on-chip supercontinuum (SC) source with ?30 dB bandwidth of 2996 nm covering from 1.209 to 4.205 μm. Furthermore, we discuss the output signal spectral and temporal characteristic as a function of the pump power. Our waveguide design offers a CMOS compatible, low-cost/high yield (no photolithography or lift-off processes are necessary) on-chip SC source for near- and mid-infrared nonlinear applications.     

Ultrabroad supercontinuum generation in tellurite equiangular spiral photonic crystal fiber

Simulations are presented of a very broad and flat supercontinuum (SC) in both the normal and anomalous group velocity dispersion regimes of the same equiangular spiral photonic crystal fiber at low pumping powers. For a pump wavelength at 1557?nm and average pump power of 11.2?mW, we obtained a bandwidth >3?μm (970?nm–4100?nm) at 40 dB below the peak spectral power with fiber dispersion ～2.1?ps/km nm at 1557?nm. In the same fiber, at pump wavelength 1930?nm and average pump power of 12?mW the SC bandwidth was more than two octaves (1300?nm–3700?nm) and dispersion was ～1.3?ps/km nm at 1930?nm. This demonstrates the potential use of the fiber for multi-wavelength pumping with commercially available sources at fairly low power.     

High field broadband THz generation in organic materials

Organic salt crystals, e.g. DAST, OH1, and DSTMS, pumped by ultra-short infrared laser are efficient THz emitters. We review our latest results on the generation in organic crystals of THz single-cycle transients with field strength of 1.5?MV/cm. The energy conversion reaches 2% with photon conversion efficiency up to 200%. THz radiation produced in such crystals offers excellent beam propagation properties and can be focused down to diffraction limited spot size in order to realize the highest field. This source covers the full spectral range between 0.1 and 10?THz. Further, we discuss the possibility to control the absolute phase and the polarity of the THz field.     

Visible to mid-infrared supercontinuum generated in novel GeS2–Ga2S3–CsI step-index fibre

In this work, two kinds of GeS₂–Ga₂S₃–CsI chalcogenide glasses had been fabricated in conventional meltquenching method, and these glasses express excellent transmittance (0.52–10.5?µm), low refractive index difference, high Raman gain and good thermal stability against crystallization. We designed a highly nonlinear step-index fibre consisting of a 72GeS₂–18Ga₂S₃–10CsI core and a 73GeS₂–15Ga₂S₃–12CsI cladding with the zero dispersion wavelength in near-infrared (<1?µm), and theoretically analysed the optical and nonlinear characteristics of this fibre in detail. Substantial simulation results revealed that wide ultra-flat supercontinuum broadening from the visible to the mid-infrared range can be achieved by pumping this fibre at 1064?nm.     

Incoherent triggering of picosecond pulse pumped supercontinuum

We numerically study spectral and noise properties of picosecond pulse pumped supercontinuum using an incoherent continuous wave trigger with 1% pump intensity. We have swept the trigger central wavelength from 1404 to 1724?nm, and observe obvious improvement in the SC bandwidth and moderate enhancement of SNR when the ASE-based CW-trigger is located at a proper wavelength range. We also varied the trigger bandwidth from 0.5 to 6?nm and find a reduced effect of the trigger when its associated coherence time decreases below the duration of the temporal pump pulse. Finally, we have varied the pump pulse peak intensity from 20 W to 120 W to demonstrate the power dependency of the suggested CW triggering technique.     

Maximizing the bandwidth of coherent,mid-IR supercontinuum using highly nonlinear aperiodic nanofibers

We describe in detail a new procedure of maximizing the bandwidth of mid-infrared (mid-IR) supercontinuum (SC) in highly nonlinear microstructured As₂Se₃ and tellurite aperiodic nanofibers. By introducing aperiodic rings of first and secondary air holes into the cross-sections of our microstructured fiber designs, we achieve flattened and all-normal dispersion profiles over much broader bandwidths than would be possible with simple periodic designs. These fiber designs are optimized for efficient, broadband, and coherent SC generation in the mid-IR spectral region. Numerical simulations show that these designs enable the generation of a SC spanning over 2290?nm extending from 1140 to 3430?nm in 8?cm length of tellurite nanofiber with input energy of E?=?200?pJ and a SC bandwidth of over 4700?nm extending from 1795 to 6525?nm generated in only 8?mm-length of As₂Se₃-based nanofiber with input energy as low as E?=?100?pJ. This work provides a new type of broadband mid-IR SC source with flat spectral shape as well as excellent coherence and temporal properties by using aperiodic nanofibers with all-normal dispersion suitable for applications in ultrafast science, metrology, coherent control, non-destructive testing, spectroscopy, and optical coherence tomography in the mid-IR region.     

Spectrum modification of XUV radiation in the presence of a delayed weak control field

We use a delayed weak laser beam to control the spectral features of extreme ultraviolet (XUV) pulses generated by a strong femtosecond laser beam through phase-matched high harmonic generation (HHG) in an atomic medium, e.g. krypton. The variation of the HHG spectrum reveals the influence of the free electrons on the propagation of the XUV field in the medium. In addition, a signature of the autoionization process is visible. Our findings provide a promising technique to study ultrafast dynamics of atomic and molecular gases.     

Generation of an extreme ultraviolet supercontinuum in a two-color spatially inhomogeneous field

We theoretically investigate the high-order harmonic generation from the helium ion in a two-color spatially inhomogeneous laser field. The numerical calculations show that this spatiotemporally synthesized laser field can not only significantly extend the harmonic cut-off, but also generate an ultrabroad extreme ultraviolet supercontinuum with a 1737 eV bandwidth. In addition, the spatiotemporal combination can eliminate the long quantum path and select the short one, and then an ultrashort isolated 12.3 as pulse with a bandwidth of 310.2 eV is obtained directly. Further, the dependences of the harmonic spectrum on the parameters including spatial inhomogeneity, time delay, carrier-envelope phase, and intensity are further discussed.     

Many-Body Spin Correlations in Nonlinear Optical Spectroscopy

We study the role of spin correlations in nonlinear absorption due to optical transitions from a deep impurity level to states above a Fermi sea. We demonstrate that the Hubbard repulsion between two electrons occupying the impurity state leads to a logarithmic divergence of the third-order nonlinear optical susceptibility ⁽³⁾ at the absorption threshold. This divergence is a manifestation of the Kondo physics in the nonlinear optical response of Fermi sea systems. Remarkably, the light-induced Kondo temperature, which governs the shape of the Kondo-absorption spectrum, can be tuned by varying the intensity and frequency of the pump optical field. We also show that, for off-resonant pump excitation, the pump–probe spectrum exhibits a narrow peak below the linear absorption onset.     

Ultrafast nonlinear optical properties of TiO2 nanoclusters at 850 nm

We discuss nanoclusters of titanium dioxide, bulk CdTe and CS₂ as characteristic materials to illustrate two main ideas; firstly, it is possible to argue about the origin of the nonlinearity even with a high repetition laser, provided that you have as large a peak power (50 kW) as the one found in femtosecond lasers. Secondly, the search for dimensional confinement to enhance nonlinear optical properties has experienced some difficulties with conflicting results. We show that electronic nonlinear behavior increases from TiO₂, CS₂ to CdTe and the nonlinear thermal effect increases from CS₂, CdTe to TiO₂.     

Photocarrier‐Induced Active Control of Second‐Order Optical Nonlinearity in Monolayer MoS2

Atomically thin transition metal dichalcogenides (TMDs) in their excited states can serve as exceptionally small building blocks for active optical platforms. In this scheme, optical excitation provides a practical approach to control light‐TMD interactions via the photocarrier generation, in an ultrafast manner. Here, it is demonstrated that via a controlled generation of photocarriers the second‐harmonic generation (SHG) from a monolayer MoS₂ crystal can be substantially modulated up to ≈55% within a timeframe of ≈250 fs, a set of performance characteristics that showcases the promise of low‐dimensional materials for all‐optical nonlinear data processing. The combined experimental and theoretical study suggests that the large SHG modulation stems from the correlation between the second‐order dielectric susceptibility χ⁽²⁾ and the density of photoexcited carriers in MoS₂. Indeed, the depopulation of the conduction band electrons, at the vicinity of the high‐symmetry K/K′ points of MoS₂, suppresses the contribution of interband electronic transitions in the effective χ⁽²⁾ of the monolayer crystal, enabling the all‐optical modulation of the SHG signal. The strong dependence of the second‐order optical response on the density of photocarriers reveals the promise of time‐resolved nonlinear characterization as an alternative route to monitoring carrier dynamics in excited states of TMDs.     

Second harmonic generation from a ferroelectric film

We derive an expression for transmittivity (T_SHG) of second harmonic generation (SHG) signals from a ferroelectric (FE) film. Intensities of up and down fields in the medium are investigated in relation to T_SHG. The derivations are made based on undepletion of input fields and nonlinear wave equation derived from the Maxwell equations. We present two cases: film without mirrors and with partial mirrors. Expressions for the newly derived nonlinear susceptibility coefficients of SHG for real crystal symmetry [J. Opt. Soc. Am. B 19 (2002) 2007] are used to get more realistic results. Variations in T_SHG with respect to film thickness are illustrated.     

Hot‐Electron‐Assisted Femtosecond All‐Optical Modulation in Plasmonics

The optical Kerr nonlinearity of plasmonic metals provides enticing prospects for developing reconfigurable and ultracompact all‐optical modulators. In nanostructured metals, the coherent coupling of light energy to plasmon resonances creates a nonequilibrium electron distribution at an elevated electron temperature that gives rise to significant Kerr optical nonlinearities. Although enhanced nonlinear responses of metals facilitate the realization of efficient modulation devices, the intrinsically slow relaxation dynamics of the photoexcited carriers, primarily governed by electron–phonon interactions, impedes ultrafast all‐optical modulation. Here, femtosecond (≈190 fs) all‐optical modulation in plasmonic systems via the activation of relaxation pathways for hot electrons at the interface of metals and electron acceptor materials, following an on‐resonance excitation of subradiant lattice plasmon modes, is demonstrated. Both the relaxation kinetics and the optical nonlinearity can be actively tuned by leveraging the spectral response of the plasmonic design in the linear regime. The findings offer an opportunity to exploit hot‐electron‐induced nonlinearities for design of self‐contained, ultrafast, and low‐power all‐optical modulators based on plasmonic platforms.     

Flat broadband wavelength conversion based on cascaded second-harmonic generation and difference frequency generation in segmented quasi-phase matched gratings

Bandwidth enhancement and response flattening of wavelength conversion based on single-pass and double-pass cascaded second harmonic generation and difference frequency generation were investigated in segmented quasi-phase matched (QPM) gratings. It is shown that the signal and pump bandwidths are both efficiently widened by increasing the segment number of the QPM grating and optimising the poling period of each segment. The ripple on the matching response is also very small. The conversion bandwidth in a 3-cm-long three-segment waveguide reaches 150–160?nm, which is over the whole conventional band and long-wavelength band. Larger signal bandwidth can be obtained with a little response flatness penalty and conversion efficiency penalty, which can be compensated by increasing the input pump power. Compared with a sinusoidally chirped optical superlattice device, a wavelength converter based on the segmented gratings has higher conversion efficiency, broader bandwidth and better pump-wavelength tolerance, and is easier to fabricate in practice.     

Spatial chirp in the focusing of few-optical-cycle pulses by a mirror

We calculate the electric field of few-optical-cycle pulses at the focus of a perfectly conducting mirror by adding coherently the Airy diffraction patterns for each pulse frequency. We will show that the pulse suffers temporal spreading generated by a change in the spectrum of the pulse as a function of position in the focal plane, which introduces spatial chirp to the pulse. A double pulse appears near to the diffraction minima of the carrier frequency due to the variations in the spectra.