RESUMEN
Mid-infrared laser excitation of molecules into high-lying vibrational states offers a novel route to realize controlled ground-state chemistry. Here we successfully demonstrate vibrational ladder climbing in the antisymmetric stretch of CO2 in the condensed phase by using intense down-chirped mid-infrared pulses. Spectrally resolved pump-probe measurements directly observe excited-state absorptions attributed to vibrational populations up to the v = 9 state, whose corresponding energy of 2.5 eV is 46% of the dissociation energy. By the use of global fitting analysis, important spectroscopic parameters in the high-lying vibrational states, such as transition frequencies and relaxation times, are quantitatively characterized. Remarkably, our analysis shows that 40% of the molecules are excited above the typical activation barriers in the metal-catalyzed CO2 conversions. These results not only demonstrate the promising ability of infrared excitation to produce elevated vibrational states but also represent a significant step toward accelerating CO2 conversions and other chemical processes via mode-specific vibrational excitation.
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Motor nerve organoids could be generated by culturing a spheroid of motor neurons differentiated from human induced pluripotent stem (iPS) cells within a polydimethylsiloxane (PDMS) chip which guides direction and fasciculation of axons extended from the spheroid. To isolate axon bundles from motor nerve organoids, we developed a rapid laser dissection method based on localized photothermal combustion. By illuminating a blue laser on a black mark on the culture device using a dry-erase marker, we induced highly localized heating near the axon bundles. Moving the laser enabled spatial control over the local heating and severing of axon bundles. This laser dissection requires a black mark, as other colors did not produce the same localized heating effect. A CO2 laser destroyed the tissue and the device and could not be used. With this simple, economical laser dissection technique, we could rapidly collect abundant pure axon samples from motor nerve organoids for biochemical analysis. Extracted axonal proteins and RNA were indistinguishable from manual dissection. This method facilitates efficient axon isolation for further analyses.
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This Letter presents a dispersion spectroscopy method that achieves simultaneous detection of molecular vibrational dispersion over a broad spectral range. The method is implemented with an infrared mode-locked laser, a dispersion-compensated Michelson interferometer, and a multichannel detector. Synchronous detection under interferometric phase modulation near the destructive interference condition is employed to achieve a high signal-to-noise ratio. We successfully demonstrate the method by measuring the dispersion of carbon monoxide gas, achieving a noise-equivalent dispersion of 1.3 × 10-8 cm and a corresponding noise-equivalent absorbance of 6.5 × 10-4 with a measurement time of 2.2 s.
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We demonstrate high-sensitivity vibrational absorption spectroscopy in the 2-micron wavelength range by using a mode-locked Cr:ZnS laser. Interferometric subtraction and multichannel detection across the broad laser spectrum realize simultaneous background-free detection of multiple vibrational modes over a spectral span of >380 cm-1. Importantly, we achieve detection of small absorbance on the order of 10-4, which is well below the detection limit of conventional absorption spectroscopy set by the detector dynamic range. The results indicate the promising potential of the background-free method for ultrasensitive and rapid detection of trace gases and chemicals.
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We developed a diode-pumped, mode-locked polycrystalline Cr:ZnS oscillator using single-walled carbon nanotubes as a saturable absorber. The oscillator exhibits self-start mode-locking operation, generating sub-100 fs pulses with an average power of 300â mW. We found a unique feature in which the intensity noise originating from relaxation oscillation is suppressed by inherent second harmonic generation in polycrystalline Cr:ZnS. The observed noise suppression is reproduced by a theoretical model that includes an instantaneous nonlinear loss.
RESUMEN
Amid the increasing potential of ultrafast mid-infrared (mid-IR) laser sources based on transition metal doped chalcogenides such as Cr:ZnS, Cr:ZnSe, and Fe:ZnSe lasers, there is a need for direct and sensitive characterization of mid-IR mode-locked laser pulses that work in the nanojoule energy range. We developed a two-dimensional spectral shearing interferometry (2DSI) setup to successfully demonstrate the direct electric-field reconstruction of Cr:ZnS mode-locked laser pulses with a central wavelength of 2.3 µm, temporal duration of 30.3 fs, and energies of 3 nJ. The reconstructed electric field is in reasonable agreement with an independently measured intensity autocorrelation trace, and the quantitative reliability of the 2DSI measurement is verified from a material dispersion evaluation. The presented implementation of 2DSI, including a choice of nonlinear crystal as well as the use of high-throughput dispersive elements and a high signal-to-noise ratio near-IR spectrometer, would benefit future development of ultrafast mid-IR lasers and their applications.
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We demonstrate spectral peak formation in a mode-locked solid-state laser that contains a gas cell inside the cavity. Symmetric spectral peaks appear in the course of sequential spectral shaping through resonant interaction with molecular rovibrational transitions and nonlinear phase modulation in the gain medium. The spectral peak formation is explained as that narrowband molecular emissions triggered by an impulsive rovibrational excitation are superposed on the broadband spectrum of the soliton pulse by constructive interference. The demonstrated laser, which exhibits comb-like spectral peaks at molecular resonances, potentially provides novel tools for ultrasensitive molecular detection, vibration-mediated chemical reaction control, and infrared frequency standards.
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Vibrational strong coupling (VSC) between a vacuum field and molecules in a cavity offers promising applications in cavity-modified chemical reactions and ultrasensitive vibrational spectroscopy. At present, in order to realize VSC, bulky microcavities with large mode volume are utilized, which limits their potential applications at the nanoscale. Here, we report on the experimental realization of strong coupling between molecular vibrations and infrared photons confined within a deeply subwavelength nanogap patch antenna cavity. Our system exhibits a characteristic anticrossing dispersion, indicating a Rabi splitting of 108 cm-1 at the single resonator level with excellent angular insensitivity. The numerical simulations and theoretical analyses quantitatively reveal that the strength of coupling depends on the cavity field-molecule overlap integral and the image charge effect. VSC at the single nanogap patch antenna level paves the way for molecular-scale chemistry, ultrasensitive biosensors, and the development of ultralow-power all-optical devices in the mid-infrared spectral range.
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We study the saturable absorption properties of single-walled carbon nanotubes (SWCNTs) with a large diameter of 2.2 nm and the corresponding exciton resonance at a wavelength of 2.4 µm. At resonant excitation, a large modulation depth of approximately 30 % and a small saturation fluence of a few tens of µJ/cm2 are evaluated. The temporal response is characterized by an instantaneous rise and a subpicosecond recovery. We also utilize the SWCNTs to realize sub-50 fs, self-start mode locking in a Cr:ZnS laser, revealing that the film thickness is an important parameter that affects the possible pulse energy and duration. The results prove that semiconductor SWCNTs with tailored diameters exceeding 2 nm are useful for passive mode locking in the mid-infrared range.
RESUMEN
Selective bond cleavage via vibrational excitation is the key to active control over molecular reactions. Despite its great potential, the practical implementation in condensed phases have been hampered to date by poor excitation efficiency due to fast vibrational relaxation. Here we demonstrate vibrationally mediated, condensed-phase molecular dissociation by employing intense plasmonic near-fields of temporally-shaped mid-infrared (mid-IR) pulses. Both down-chirping and substantial field enhancement contribute to efficient ladder climbing of the carbonyl stretch vibration of W(CO)6 in n-hexane solution and to the resulting CO dissociation. We observe an absorption band emerging with laser irradiation at the excitation beam area, which indicates that the dissociation is followed by adsorption onto metal surfaces. This successful demonstration proves that the combination of ultrafast optics and nano-plasmonics in the mid-IR range is useful for mode-selective vibrational ladder climbing, paving the way toward controlled ground-state chemistry.
RESUMEN
We develop a mode-locked Cr:ZnS polycrystalline laser using single-walled carbon nanotubes (SWCNTs) that have resonant absorption at the wavelength of 2.4 µm. The laser generates ultrashort pulses of 49 fs duration, a 2.4 µm center wavelength, and a 9.2 THz (176 nm) spectral span at a repetition rate of 76 MHz. We also confirm self-starting of the mode-locked operation. SWCNTs, if appropriately controlled in terms of their diameters, prove to be useful as ultrafast saturable absorbers in the mid-infrared region.
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Infrared pump-probe measurement for the OD stretching mode in KTaO3 has been performed. We observed the induced absorption signal corresponding to the 1-2 transition for the first time in oxide crystals in addition to the bleaching signal of the 0-1 transition. Both signals show long decay time of â¼200 ps, and the potential anharmonicity is found to be 3.6%. We also investigated temperature dependence of OH/OD absorption spectra to explore how a proton/deuteron interacts with the surrounding lattice vibrations, and identify the phonon mode which causes the potential fluctuations as a O-Ta-O bending motion. On the basis of the results obtained, we calculate the potential energy surface on which a proton/deuteron moves and discuss the proton conduction mechanism in solid oxides.
RESUMEN
High harmonic generation (HHG) in solids has great potential for coherent extreme ultraviolet (EUV) sources, all-optical band-structure reconstruction, and electron dynamics metrology. Solid HHG driven by plasmonic near-fields will open a new paradigm, enabling high repetition-rate HHG with a compact laser, HHG manipulation with meta-surfaces, and precise control over carrier trajectory. In this paper, we demonstrate antenna-enhanced HHG in a wide-bandgap semiconductor ZnO. By exploiting gold nano-antennas resonating at the driver wavelength of 2 µm, we successfully trigger HHG at input intensity of ~0.02 TW/cm2 and observe harmonic radiations up to 9th-order. Orders-of-magnitude enhanced conversion efficiency at the hot-spots brings about ten-fold enhancement in the total yield. The spectral selection rule is found to reflect crystal symmetry, suggesting the possibility of nano-scaled EUV sources and band-structure reconstruction.
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The polarization property of high harmonics from gallium selenide is investigated using linearly polarized midinfrared laser pulses. With a high electric field, the perpendicular polarization component of the odd harmonics emerges, which is not present with a low electric field and cannot be explained by the perturbative nonlinear optics. A two-dimensional single-band model is developed to show that the anisotropic curvature of an energy band of solids, which is pronounced in an outer part of the Brillouin zone, induces the generation of the perpendicular odd harmonics. This model is validated by three-dimensional quantum mechanical simulations, which reproduce the orientation dependence of the odd-order harmonics. The quantum mechanical simulations also reveal that the odd- and even-order harmonics are produced predominantly by the intraband current and interband polarization, respectively. These experimental and theoretical demonstrations clearly show a strong link between the band structure of a solid and the polarization property of the odd-order harmonics.
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We conducted a numerical study on the excitation of a two-colored temporal soliton in a segmented quasi-phase-matching (QPM) structure. The device has three parts: a periodic QPM grating for second-harmonic generation, a single domain for phase shift, and a periodic QPM grating for soliton evolution. The second harmonic pulse generated in the first grating works as a seed in the cascaded up-and-down conversions in the second grating. The numerical results showed that the second harmonic seeding enables the excitation of soliton pulses with an improved spatio-temporal intensity profile in a broad bandwidth of the wave-vector mismatch.
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We present simultaneous separation of polydisperse particles driven by an optical gradient force in the absence of microfluidic flow. The separation mechanism involves particle-size dependence of the potential landscape generated by a one-dimensional asymmetric optical stripe pattern. The outcome is that the particles align in different stacks according to their sizes. The dynamics of Brownian particles inside the optical potential landscapes are investigated theoretically and experimentally for various optical intensities and particle sizes. By introducing sequential changes in the optical profile, we also show that this technique allows semipassive arrangement of particles in arbitrary configurations.
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In the liquid phase, water molecules form a disordered fluctuating network of intermolecular hydrogen bonds. Using both inter- and intramolecular vibrations as structural probes in ultrafast infrared spectroscopy, we demonstrate a two-stage structural response of this network to energy disposal: vibrational energy from individually excited water molecules is transferred to intermolecular modes, resulting in a sub-100 fs nuclear rearrangement that leaves the local hydrogen bonds weakened but unbroken. Subsequent energy delocalization over many molecules occurs on an approximately 1 ps time scale and is connected with the breaking of hydrogen bonds, resulting in a macroscopically heated liquid.
RESUMEN
The ultrafast relaxation of the excited O-H stretching vibration is studied by ultrafast infrared-pump/infrared-probe and infrared-pump/Raman-probe spectroscopy. We demonstrate a 200 fs lifetime of the hydrogen-bonded O-H stretching mode in 2-(2'-hydroxy-5'-methyl-phenyl)benzotriazole (TINUVIN P). O-H stretching relaxation occurs through a few major channels that all involve combination and overtone bands of modes with considerable in-plane O-H bending character. In particular, the mode, which contains the largest O-H bending contribution, plays a prominent role for primary processes of intramolecular vibrational energy redistribution. Theoretical calculations of vibrational energy transfer rates based on a Fermi golden rule approach account for the experimental findings.
RESUMEN
A novel scheme to achieve broadband cascaded third-harmonic generation by use of two-dimensional quasi-phase-matching gratings is proposed. Intrinsic group-velocity mismatch is compensated by the pulse front tilt in noncollinear interaction geometry. The presented scheme enables broadband third-harmonic generation in a single device of long interaction length, which reduces the operation intensity dramatically.
RESUMEN
We numerically show that it is possible to achieve adiabatic compression of femtosecond quadratic solitons in aperiodically poled lithium niobate device. Two-colored solitons of the fundamental wavelength of 1560 nm can be adiabatically shaped by using group-velocity matching schemes available in quasi-phase-matching (QPM) devices. We investigate the performance of the adiabatic compression based on two different group-velocity matching schemes: type-I (e: o + o) collinear QPM geometry and type-0 (e: e + e) non-collinear QPM geometry. Two-colored temporal solitons with pulse duration of 35 fs are generated without visible pedestals from 100-fs fundamental pulse. We also show that walking solitons with shorter pulse durations are adiabatically excited under small group-velocity mismatch condition. The walking solitons experience deceleration or acceleration during compression, depending on the sign of the groupvelocity- mismatch. The demonstrated adiabatic pulse shaping is useful for generation of shorter pulses with clean temporal profiles, efficient femtosecond second harmonic generation and group-velocity control.
