Near-infrared Pulses Research Articles

Femtosecond Single Cycle Pulses Enhanced the Efficiency of High Order Harmonic Generation.

High-order harmonic generation is a nonlinear process that converts the gained energy during light-matter interaction into high-frequency radiation, thus resulting in the generation of coherent attosecond pulses in the XUV and soft x-ray regions. Here, we propose a control scheme for enhancing the efficiency of HHG process induced by an intense near-infrared (NIR) multi-cycle laser pulse. The scheme is based on introducing an infrared (IR) single-cycle pulse and exploiting its characteristic feature that manifests by a non-zero displacement effect to generate high-photon energy. The proposed scenario is numerically implemented on the basis of the time-dependent Schrödinger equation. In particular, we show that the combined pulses allow one to produce high-energy plateaus and that the harmonic cutoff is extended by a factor of 3 compared to the case with the NIR pulse alone. The emerged high-energy plateaus is understood as a result of a vast momentum transfer from the single-cycle field to the ionized electrons while travelling in the NIR field, thus leading to high-momentum electron recollisions. We also identify the role of the IR single-cycle field for controlling the directionality of the emitted electrons via the IR-field induced electron displacement effect. We further show that the emerged plateaus can be controlled by varying the relative carrier-envelope phase between the two pulses as well as the wavelengths. Our findings pave the way for an efficient control of light-matter interaction with the use of assisting femtosecond single-cycle fields.

Advances in Two-Photon Imaging in Plants.

Live and deep imaging play a significant role in the physiological and biological study of organisms. Two-photon excitation microscopy (2PEM), also known as multiphoton excitation microscopy, is a fluorescent imaging technique that allows deep imaging of living tissues. Two-photon lasers use near-infrared (NIR) pulse lasers that are less invasive and permit deep tissue penetration. In this review, recent advances in two-photon imaging and their applications in plant studies are discussed. Compared to confocal microscopy, NIR 2PEM exhibits reduced plant-specific autofluorescence, thereby achieving greater depth and high-resolution imaging in plant tissues. Fluorescent proteins with long emission wavelengths, such as orange–red fluorescent proteins, are particularly suitable for two-photon live imaging in plants. Furthermore, deep- and high-resolution imaging was achieved using plant-specific clearing methods. In addition to imaging, optical cell manipulations can be performed using femtosecond pulsed lasers at the single cell or organelle level. Optical surgery and manipulation can reveal cellular communication during development. Advances in in vivo imaging using 2PEM will greatly benefit biological studies in plant sciences.

Efficient and precise delivery of microRNA by photoacoustic force generated from semiconducting polymer-based nanocarriers

One major challenge in miRNA-based therapy is to explore facile delivery strategies, which can facilitate the efficient and precise accumulation of intrinsically instable microRNAs (miRNAs) at targeted tumor sites. To address this critical issue, for the first time we demonstrate that a near-infrared (NIR) pulse laser can guide efficient delivery of miRNAs mediated by a NIR-absorbing and photoacoustic active semiconducting polymer (SP) nanocarrier, which can generate photoacoustic radiation force to intravascularly overcome the endothelial barriers. Importantly, we demonstrate an ultrafast delivery of miRNA (miR-7) to tumor tissues under the irradiation of pulse laser in 20 min, showing a 5-fold boosted efficiency in comparison to the traditional passive targeting strategy. The delivered miR-7 acts as a sensitizer of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and synergizes with TRAIL-inducing compound (TIC), leading to sustained TRAIL upregulation for effective tumor suppression in mice. As such, our results indicate that the NIR-absorbing semiconducting polymer-mediated nanocarrier platform can significantly enhance the targeted delivery efficiency of therapeutic miRNAs to tumors, resulting in potent tumor growth inhibition.

Hot-carrier dynamics in InAs/AlAsSb multiple-quantum wells

A type-II InAs/AlAs_{0.16}Sb_{0.84} multiple-quantum well sample is investigated for the photoexcited carrier dynamics as a function of excitation photon energy and lattice temperature. Time-resolved measurements are performed using a near-infrared pump pulse, with photon energies near to and above the band gap, probed with a terahertz probe pulse. The transient terahertz absorption is characterized by a multi-rise, multi-decay function that captures long-lived decay times and a metastable state for an excess-photon energy of >100 meV. For sufficient excess-photon energy, excitation of the metastable state is followed by a transition to the long-lived states. Excitation dependence of the long-lived states map onto a nearly-direct band gap (E{_g}) density of states with an Urbach tail below E{_g}. As temperature increases, the long-lived decay times increase <E{_g}, due to the increased phonon interaction of the unintentional defect states, and by phonon stabilization of the hot carriers >E{_g}. Additionally, Auger (and/or trap-assisted Auger) scattering above the onset of the plateau may also contribute to longer hot-carrier lifetimes. Meanwhile, the initial decay component shows strong dependence on excitation energy and temperature, reflecting the complicated initial transfer of energy between valence-band and defect states, indicating methods to further prolong hot carriers for technological applications.

Two-color two-dimensional terahertz spectroscopy: A new approach for exploring even-order nonlinearities in the nonperturbative regime.

Nonlinear two-dimensional terahertz (2D-THz) spectroscopy at frequencies of the emitted THz signal different from the driving frequencies allows for exploring the regime of (off-)resonant even-order nonlinearities in condensed matter. To demonstrate the potential of this method, we study two phenomena in the nonlinear THz response of bulk GaAs: (i) The nonlinear THz response to a pair of femtosecond near-infrared pulses unravels novel fourth- and sixth-order contributions involving interband shift currents, Raman-like excitations of transverse-optical phonon and intervalence-band coherences. (ii) Transient interband tunneling of electrons driven by ultrashort mid-infrared pulses can be effectively controlled by a low-frequency THz field with amplitudes below 50 kV/cm. The THz field controls the electron-hole separation modifying decoherence and the irreversibility of carrier generation.

A Multi-Band Photonic Source by Means of Phase-Matched Nonlinear Generation Processes

A technique for multi-band generation is investigated to simultaneously achieve phase-matching in multiple second-order nonlinear processes. By coupling a near-infrared excitation pulse into a LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> planar waveguiding platform, modal phase-matched second harmonic radiation is generated at wavelengths of 403 ± 0.6 nm and Cherenkov-emitted terahertz radiation is generated at frequencies up to ~3.5 THz. For the second harmonic generation process and the terahertz radiation generation process, the conversion efficiencies are measured as 1.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> %/(GW-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) and 1.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> %/(GW-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ), respectively. The investigated multi-band generation technique permits a single structure to produce radiation within multiple spectral bands, such that this technique could be used to increase the density of photonic sources on a photonic integrated chip.

Reexamining Different Factors of the Resonance-Enhanced High-Order Harmonic Generation in Atomic and Nanoparticle Laser-Induced Tin Plasmas

We reexamine the resonance enhancement of a single harmonic emission during the propagation of ultrafast pulses through atomic and nanoparticle tin-containing laser-induced plasma (LIP). We compare the single atomic Sn and Sn nanoparticle plasmas to demonstrate a distinction in the enhancement factor of the single harmonic in the case of fixed and tunable near-infrared pulses. The analysis of the dynamics of Sn LIP shows the range of optimal delays between heating and driving pulses (130–180 ns), at which the maximal harmonic yield can be achieved. The enhancements of the 17th and 18th harmonics of 806 nm pulses were analyzed in the case of single-color and two-color pumps of LIP, showing up to a 12-fold enhancement of even harmonics in the two-color pump case. We show the enhancement of a single harmonic in the vicinity of the 4d105s25p2P3/2→4d95s25p2 transitions of Sn II ions and demonstrate how this process depends on the constituency of the plasma components at different conditions of target ablation. The application of tunable (1280–1440 nm) radiation allows for demonstrating the variations of single harmonic enhancement using a two-color pump of Sn-containing LIP.

Ultrasensitive Vibrational Imaging of Retinoids by Visible Preresonance Stimulated Raman Scattering Microscopy.

High‐sensitivity chemical imaging offers a window to decipher the molecular orchestra inside a living system. Based on vibrational fingerprint signatures, coherent Raman scattering microscopy provides a label‐free approach to map biomolecules and drug molecules inside a cell. Yet, by near‐infrared (NIR) pulse excitation, the sensitivity is limited to millimolar concentration for endogenous biomolecules. Here, the imaging sensitivity of stimulated Raman scattering (SRS) is significantly boosted for retinoid molecules to 34 micromolar via electronic preresonance in the visible wavelength regime. Retinoids play critical roles in development, immunity, stem cell differentiation, and lipid metabolism. By visible preresonance SRS (VP‐SRS) imaging, retinoid distribution in single embryonic neurons and mouse brain tissues is mapped, retinoid storage in chemoresistant pancreatic and ovarian cancers is revealed, and retinoids stored in protein network and lipid droplets of Caenorahbditis elegans are identified. These results demonstrate VP‐SRS microscopy as an ultrasensitive label‐free chemical imaging tool and collectively open new opportunities of understanding the function of retinoids in biological systems.

Extreme-ultraviolet spectral compression by four-wave mixing

Extreme-ultraviolet (XUV) sources including high-harmonic generation (HHG), free-electron lasers (FELs), soft-X-ray lasers and laser-driven plasmas are widely used for applications ranging from femtochemistry and attosecond science to coherent diffractive imaging and EUV (or XUV) lithography. The bandwidth of the XUV light emitted by these sources reflects the XUV generation process used. Whereas light from soft-X-ray lasers1 and seeded XUV FELs2 typically has a relatively narrow bandwidth, plasma sources and HHG sources often emit broadband XUV pulses3. Since these characteristic properties of a given source impose limitations on applications, techniques enabling modification of the bandwidth are highly desirable. Here we introduce a concept for efficient spectral compression by four-wave mixing (FWM), exploiting a phase-matching scheme based on closely-spaced resonances. We demonstrate the compression of broadband radiation in the 145–130 nm wavelength range into a narrow-bandwidth XUV pulse at 100.3 nm wavelength in the presence of a broadband near-infrared (NIR) pulse in a krypton gas jet. Our concept provides new possibilities for tailoring the spectral bandwidth of XUV beams.

Higher pulse frequency of near-infrared laser irradiation increases penetration depth for novel biomedical applications.

BackgroundThe clinical efficiency of laser treatments is limited by the low penetration of visible light used in certain procedures like photodynamic therapy (PDT). Second Harmonic Generation (SHG) PDT is an innovative technique to overcome this limitation that enables the use of Near Infrared (NIR) light instead of visible light. NIR frequency bands present an optical window for deeper penetration into biological tissue. In this research, we compare the penetration depths of 405 and 808 nm continuous wave (CW) lasers and 808 nm pulsed wave (PW) laser in two different modes (high and low frequency).MethodsIncreasing thicknesses of beef and chicken tissue samples were irradiated under CW and PW lasers to determine penetration depths.ResultsThe 808 nm CW laser penetrates 2.3 and 2.4 times deeper than the 405 nm CW laser in beef and chicken samples, respectively. 808 nm PW (pulse frequency—500 Hz) penetrates deeper than CW laser at the same wavelength. Further, increasing the pulse frequency achieves higher penetration depths. High frequency 808 nm PW (pulse frequency—71.4 MHz) penetrates 7.4- and 6.0-times deeper than 405 nm CW laser in chicken and beef, respectively.ConclusionsThe results demonstrate the higher penetration depths of high frequency PW laser compared to low frequency PW laser, CW laser of the same wavelength and CW laser with half the wavelength. The results indicate that integrating SHG in the PDT process along with pulsed NIR light may allow the treatment of 6–7 times bigger tumours than conventional PDT using blue light.

Nonlinear optical responses of α-Fe2O3 nanosheets and application as a saturable absorber in the wide near-infrared region

In the present work, we report a strategy to synthesize α-Fe2O3 nanoflakes as a nonlinear optical (NLO) material for the optical pulse generation in the near-infrared (NIR) band. With the excitation wavelength of 1 µm, the effective nonlinear absorption coefficients βeff was −0.501 cm/GW and the nonlinear refractive index n2 was 9.33 × 10-4 cm2/GW. While at 1.34 µm, βeff was −0.713 cm/GW and n2 was 5.01 × 10-4 cm2/GW. The two-photon absorption cross sections at 1 and 1.34 µm were as high as 9.6 × 105 and 5.7 × 105 GM, respectively, indicating that the hematite nanosheets could be used as the nonlinear optical devices. The third-order NLO susceptibilities were 6.3 × 10-11 and 3.75 × 10-11 esu at 1 and 1.3 µm, respectively. Using the ultrafast spectroscopy, the excited state lifetime was ~2.62 ps, suitable for the (ultra)short pulse generation. Based on the marvelous nonlinear absorption properties of the α-Fe2O3 nanoflakes saturable absorber, passively Q-switched Nd:GdVO4 lasers at 4F3/2 → 2I11/2 and 4F3/2 → 2I13/2 lasing transitions were demonstrated, producing the shortest pulse duration of 135 ns. The experimental results confirmed that α-Fe2O3 nanoflakes were an excellent candidate for the short NIR pulse generation in a broad band.

Programmable generation of terahertz bursts in chirped-pulse laser amplification

Amplified bursts of laser pulses are sought for various machining, deposition, spectroscopic, and strong-field applications. Standard frequency- and time-domain techniques for pulse division become inadequate when intraburst repetition rates reach the terahertz (THz) range as a consequence of inaccessible spectral resolution, requirement for interferometric stability, and collapse of the chirped-pulse amplification (CPA) concept due to the loss of usable bandwidth needed for safe temporal stretching. Avoiding the burst amplification challenge and resorting to lossy post-division of an isolated laser pulse after CPA leaves the limitations of frequency- and time-domain techniques unsolved. In this Letter, we demonstrate an approach that successfully combines amplitude and phase shaping of THz bursts, formed using the Vernier effect, with active stabilization of spectral modes and efficient energy extraction from a CPA regenerative amplifier. As proof of concept, the amplified bursts of femtosecond near-infrared pulses are down-converted into tunable THz-frequency pulses via optical rectification.

Nonlinear Plasmonic Metasurface Terahertz Emitters for Compact Terahertz Spectroscopy Systems

Nonlinear plasmonic metasurfaces provide new and promising means to produce broadband terahertz (THz) radiation, due to their compact size and functionalities beyond those achievable with conventional THz emitters. However, they were driven to date only by amplified laser systems, which are expensive and have a large footprint, thus limiting the range of their potential applications. Here we study for the first time the possibility to drive metasurface emitters by low-energy near-infrared femtosecond pulses. We observe broadband THz emission from 40 nm thick metasurfaces and achieve near-infrared to THz conversion efficiencies as high as those of 2500-fold thicker ZnTe crystals. We characterize the THz emission properties and use the metasurface emitter to perform a spectroscopic measurement of α-lactose monohydrate. These results show that nonlinear plasmonic metasurfaces are suitable for integration as emitters in existing compact THz spectroscopy and imaging systems, enhancing their functionalities, and opening the door for a variety of new applications.

Time-resolved dissociative ionization and double photoionization of CO2.

CO2 single-photon double photoionization, Coulomb explosion, and dissociative ionization are studied with ultrafast extreme-ultraviolet pump and time-delayed near-infrared probe pulses. Kinetic energy release and momentum correlations for the two-body CO+ + O+ and three-body O+ + C+ + O fragmentation products are determined by 3D coincidence fragment imaging. The transient enhancement of the ratio of two-body vs three-body Coulomb explosion events and the time dependence of low and high kinetic energy release dissociation events are discussed in terms of dissociative ionization and Coulomb explosion dynamics.

THz generation by optical rectification of intense near-infrared pulses in organic crystal BNA.

Generation of terahertz radiation by optical rectification of intense near-infrared laser pulses in N-benzyl-2-methyl-4-nitroaniline (BNA) is investigated in detail by carrying out a complete characterization of the terahertz radiation. We studied the scaling of THz yield with pump pulse repetition rate and fluence which enabled us to predict the optimal operating conditions for BNA crystals at room temperature for 800 nm pump wavelength. Furthermore, recording the transmitted laser spectrum allowed us to calculate the nonlinear refractive index of BNA at 800 nm.

Resonant dual-pulse laser ignition technique based on oxygen REMPI pre-ionization

This contribution investigates a novel laser ignition method based on a dual-pulse resonant pre-ionization scheme. The first laser pulse efficiently creates initial gas ionization (seed electrons) through a 2 + 1 resonantly-enhanced multiphoton ionization (REMPI) scheme targeting molecular oxygen (λ ~ 287.6 nm). This pulse is followed by a second non-resonant near-infrared pulse (λ = 1064 nm) for energy addition into the gas via inverse bremsstrahlung absorption. The sequence of two pulses creates a laser induced plasma that exhibits high peak electron number density and temperature (ne ~ 8 × 1017 cm-3 at t = 100 ns and T ~ 8000 K at t = 10 μs, respectively). These plasma parameters are similar to those attained for typical single-pulse near-infrared laser plasmas but with the advantage of substantially lower pulse energy (by factor of ~ 2.5) in the dual-pulse REMPI case. A combustion study focusing on ignition of propane/air mixtures shows that the dual-pulse REMPI method leads to an extension of the lean flammability limit, and an increase in combustion efficiency near the lean limit, as compared to laser ignition with a single NIR pulse. The measurement results and observed gas dynamics are discussed in the context of their impact on combustion applications.

Water vapor based dispersion management in a high-pressure gas cell

We use a mixture of a high pressure gas with a minor (about 1 percent) addition of variable density water vapor to control group velocity dispersion of a near-infrared femtosecond pulse and switch it from normal to anomalous regime in the short-wavelength vicinity of the absorption band.We have shown numerically the formation of typical for anomalous dispersion O-wave frequency-angular spectrum with full suppression of conical emission on the short-wavelength side for 1.3-μm femtosecond pulse filamenting in the mixture of 30-bar nitrogen and 0.4-bar water vapor.

Generation of Intense Sub-10 fs Pulses at 385nm* *Supported by the National Key Research and Development Program of China (Grant No. 2019YFA0307703), the Major Research Plan of NSF of China (Grant No. 91850201), and the National Natural Science Foundation of China (Grant Nos. 11974426, 11974425, U1830206 and 11604386).

We demonstrated the generation and characterization of 9.7 fs, 180 μJ pulses centered at 385 nm via the frequency doubling of few-cycle near-infrared pulses. Both moderate conversion efficiency (9.5%) and broad phase matching bandwidth (20 nm) were achieved by shaping the spectra of the fundamental pulses. The strong intensity dependence of second-order harmonic generation and well controlled material dispersion ensured the inexistence of satellite pulses, which was confirmed by the self-diffraction frequency resolved optical gating measurement.

High-energy multidimensional solitary states in hollow-core fibres

Multidimensional solitary states (MDSS)—self-sustained wavepackets—have attracted renewed interest in many different fields of physics. They are of particular importance in nonlinear optics, especially for the nonlinear propagation of ultrashort pulses in multimode fibres, which contain rich spatiotemporal intermodal interactions and dynamics, albeit often in an unstable manner. Here, we report the observation of the formation of highly stable multidimensional solitary states in a molecular gas-filled large-core hollow-core fibre. We experimentally and numerically demonstrate the creation of MDSS by multimillijoule, subpicosecond near-infrared pulses and the underlying physics. We find that the MDSS have a broadband redshifted spectra with an uncommon negative quadratic spectral phase at the output of the hollow-core fibre, originating from Raman enhancement due to the strong intermodal nonlinear interactions. The spatial and temporal localization of MDSS enables the compression of the broadened pulses at the output to 10.8 fs by simple linear propagation in a piece of fused silica. The high spatiotemporal quality of MDSS is further verified by high-harmonic generation. Our results present new opportunities for studying multimodal spatiotemporal dynamics in the high-energy regime. This work also presents a route toward a new class of compact, tunable and high-energy spatiotemporally engineered coherent light sources based on picosecond ytterbium technology. The formation of multidimensional solitary states through the nonlinear propagation of high-energy pulses in a molecular gas-filled large-core hollow-core fibre is demonstrated, offering new opportunities for studying multimodal spatiotemporal dynamics in the high-energy regime.

High-power few-cycle THz generation at MHz repetition rates in an organic crystal

Ultrafast terahertz (THz) spectroscopy is a potent tool for studying the fundamental properties of matter. Limitations of current THz sources, however, preclude the technique being applied in certain advanced configurations or in the measurement of, e.g., strongly absorbing samples. In response to this problem, here we demonstrate the generation of 1.38 mW broadband THz radiation at 10 MHz repetition rate by combining the highly efficient nonlinear organic crystal HMQ-TMS with ultrafast pump pulses generated using a simple and stable external pulse compression of a high power, near-infrared (NIR) femtosecond ytterbium-doped fiber (Yb:fiber) laser. Utilizing spectral broadening in a large core, polarization maintaining photonic crystal fiber and a pair of SF11 prisms, we achieve a tenfold pulse compression of the Yb:fiber laser, yielding compressed 0.35 µJ pulses with a full-width at half maximum pulse duration of 22 fs, exerting a peak power of 13.8 MW at a repetition rate of 10 MHz. THz generation through optical rectification of the NIR pulses is explored in two distinct thicknesses of the organic crystal, leading to a maximum conversion efficiency of ∼5.5 · 10−4, an order of magnitude higher than that achieved with inorganic nonlinear crystals, e.g., gallium phosphide, for similar pump parameters. The focused THz beam has a peak on-axis field strength greater than 6.4 kV cm−1 in unpurged atmosphere. We believe that our moderately strong-field THz source is well suited to a variety of applications in ultrafast THz spectroscopy, in particular THz-enabled scattering-type near-field, and scanning tunneling spectroscopy, where multi-MHz repetition rate sources are required.