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A nonlinear shift in the dispersion relation of a surface phonon-polariton (SPhP) is observed with grating-coupled pump–probe reflection spectroscopy. Upon excitation of an SPhP on a 4H-SiC surface...

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 300 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.

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.

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.

The outstanding laser properties of mode-locked Cr:ZnS and Cr:ZnSe lasers in the mid-infrared (mid-IR) are the reason for their recent emergence in the field of ultrafast optical science [1] . Their emitted ultrashort mid-IR pulses find potential applications in many industrial and scientific fields such as vibrational spectroscopy and strong-field phenomena. For the progressive development of the laser system and its applications, the development of a pulse characterization setup tailored for ultrashort pulses around 2.3 µm is strongly demanded. Here, we report on the pulse characterization for sub-50 i fs Cr:ZnS oscillator pulses [2] , [3] based on two-dimensional spectral shearing interferometry (2DSI) [4] .

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.

A marriage of ultrashort-pulsed laser and plasmonics in the mid-infrared range creates a novel platform for light–molecule interactions. Resonant gold nanoantennas are exploited to realize ultrasensitive nonlinear vibrational spectroscopy and vibrationally mediated chemical reaction control. Arbitrary shaping of spatiotemporal field distribution both in nanometer scale and in femtosecond scale has a great promise for advanced vibrational spectroscopy and controlled ground-state chemistry.

We demonstrate high harmonic generation from GaSe in the deep-UV range with its photon energy reaching three times the bandgap energy. The harmonic spectrum represents a clear dependence on the crystal orientation with respect to the incident field, reflecting the 6 ¯ 2 m point group. This reflection indicates the possibilities of high harmonic spectroscopy and high harmonic waveform control at frequencies well above the bandgap. The 3D time-dependent density matrix simulations reveal that the interband polarization mechanism dominates over the intraband current mechanism at frequencies above the bandgap and can generate attosecond bursts with characteristic sine-like waveforms.

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.

We demonstrate vibrationally-mediated dissociation of condensed-phase molecules by employing intense plasmonic near-fields of temporally-shaped mid-infrared pulses. Furthermore, we numerically investigate the impact of down-chirping on the plasmon-enhanced vibrational ladder climbing. © 2020 The Author(s)

We demonstrate the excellent saturable absorption of single-walled carbon nanotubes with resonance around 2.4 µm, realizing 36-fs oscillation in a Cr:ZnS oscillator. Introducing a two-stage single pass amplifier, the output exceeds 100 nJ.

Plasmonic nanoantennas provide powerful tools in enhancing light–matter interactions by linking propagating far fields with localized near fields. Microscopic understanding on the enhancements of light–matter interactions requires precise knowledges on the near-field distributions upon nanoantenna excitations. In this article, we study near-field distributions of nanoantennas under oblique illumination. We acquire the amplitude- and phase-resolved images of the near fields on Au nanorods in the infrared range, by use of the scattering-type scanning near-field optical microscopy and the interferometric homodyne detection. Asymmetric near-field distributions are experimentally visualized and theoretically reproduced. The asymmetry in the near-field distributions is attributed to the preferential excitation of the surface plasmon polariton in one direction and the finite loss of the surface plasmon polariton inside the cavity. Furthermore, we organize the excitation conditions of the nanoantenna modes under ...

We propose and demonstrate antenna-enhanced nonlinear infrared (IR) spectroscopy in reflection geometry. Our approach uses resonant metal nanoantennas to enhance near-fields and to amplify the interaction between molecular vibrations and IR light. We successfully obtain amplified nonlinear vibrational signals in backscattering light of nanoantennas using IR pump energy of 10 nJ, with local signal enhancement of more than 7 orders of magnitude. This ultrasensitive and reflection-type method is useful for characterizing the structure and dynamics of minute volumes of molecules, monolayer materials, and biomolecules in aqueous environments, with additional benefit of surface sensitivity.

Ultrashort pulses in the mid-infrared (mid-IR) region have a great potential in the applications of advanced vibrational spectroscopy and strong field phenomena. Traditionally, mid-IR ultrashort pulses are generated by frequency conversion of near-infrared pulses from Ti:S or Yb-based lasers. Laser oscillators which directly emit mid-IR femtosecond pulses are advantageous from the perspectives of simplicity and energy extraction efficiency. Cr:ZnS is an attractive gain medium for mid-IR ultrafast lasers because it has a broad fluorescence spectrum, large stimulated-emission cross-section and high thermal conductivity [1]. So far, passive mode-locking has been realized for Cr:ZnS laser, by employing Kerr-lensing, or by using saturable absorbers based on semiconductors or graphenes [2]. A single walled carbon nanotube (SWCNT) film is also known as a good candidate for saturable absorbers, possessing tunable resonance and large modulation depth. However, SWCNTs have rarely been applied to this laser [3]. Here, we demonstrate self-starting mode-locked oscillation of Cr:ZnS laser by using a SWCNT film which has resonant absorption at the laser emission wavelength of 2.4 μm.

We performed pump-probe spectroscopy on OD stretching mode in KTaO3 to explore how a proton/deuteron interacts with the surroundings. We identified the potential anharmonicity and phonon modes which interact with a proton/deuteron, considering the temperature effect.

We report on strong coupling between molecular vibrational resonances of polymethyl methacrylate (PMMA) molecules and gap plasmon resonance of an ultrathin plasmonic cavity in the midinfrared range. The strong coupling is achieved when the molecular vibrational mode and plasmonic cavity exchange energy faster than their relaxation rates and it is maximum when two relaxation rates are equal [1]. In this work, we designed, fabricated and characterized a composite medium consisting of a thin PMMA layer sandwiched between the nanoantenna array and a continuous metallic thin film to achieve vibration strong coupling. The spectral position and the relaxation rate of gap plasmonic resonance are tuned through the molecular resonance of the PMMA molecules (at 1730 cm−1) to go from weak to strong coupling regime. Strong coupling between vibrational modes and gap plasmon mode leads to the formation of new hybrid light-matter states called polaritonic states (@ 1690 cm−1 & 1810 cm−1), separated by the vacuum Rabi splitting (120 cm−1). Thin film coupled nanoantennas with sub-wavelength gaps have shown great potential in nanophotonic applications because they offer the ultimate electric field confinement in the gap. Our work is complementary to earlier work using microcavities and provides a new approach to achieve strong coupling with a nanoscale plasmonic cavity (λ/25) and the possibility to modulate the strong coupling regime by changing the gap thickness of the cavity and the lattice period of the nanoantenna array.

We demonstrate surface-enhanced ultrafast vibrational spectroscopy employing periodic arrays of infrared-resonant gold nanoantennas. The antenna-enhancements of molecular vibrational responses are analytically formulated with a simple coupled-dipole model, and the linear/nonlinear local signal enhancements are evaluated to be ~104and 107, respectively.

Infrared pump-probe measurement for the OD stretching mode in KTaO

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.

High harmonic generation (HHG) in a ZnO single crystal is driven by the enhanced near-fields of resonant nanoantennas. Non-perturbative HHG into deep-UV range is successfully induced by optical excitation of as low as 20 GW/cm2. The spectral selection rule is found to reflect crystal symmetry, suggesting the possibility of nano-scaled EUV sources and band-structure reconstruction.

Nonlinear infrared (IR) spectroscopy, including pump-probe IR spectroscopy and two-dimensional IR spectroscopy, is a powerful tool to study detailed molecular structures and dynamics [1]. In particular, these techniques have great potential to characterize protein conformation and its dynamics [2]. However, small oscillator strengths of molecular vibrations and strong IR absorption bands of liquid water limit the application of nonlinear IR spectroscopy to biomolecules in aqueous environments. One promising approach is to amplify the interaction of molecular vibrations with IR light by using IR-resonant metal nanoantennas and detect the vibrational signals in reflection from nanoantennas [3]. Here, we apply this approach to ‘nonlinear’ IR spectroscopy for the first time. We obtain a dramatic signal amplification of ∼107 and show that the enhanced nonlinear signals reflect the intrinsic vibrational dynamics of sample molecules.

Plasmonic enhancements of optical near-fields with metal nanostructures offer extensive potential for amplifying lightmatter interactions. We analytically formulate the enhancement of linear and nonlinear optical responses of molecular vibrations through resonant nanoantennas, based on a coupled-dipole model. We apply the formulae to evaluation of signal enhancement factors in the antenna-enhanced vibrational spectroscopy.

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.

Pumping a quadratic nonlinear crystal in the mid-IR we observe octave-spanning mid-IR supercontinua. A self-acting cascaded process leads to the formation of a self-defocusing nonlinearity, allowing formation of filament-free octave-spanning supercontinua in the 2.0–7.0 μm range with 10s of μJ pulse energies, much higher than filament-based techniques. This allows to use the supercontinuum as ultra-broadband excitation pulses in nonlinear optical applications.

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 ∼200 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.

We experimentally demonstrate engineerable compression of two-colored pulses in a linearly-chirped quasi-phase-matching grating. Quadratic solitons generated from fundamental input are reshaped through cascaded parametric processes of second-harmonic generation (SHG) and the back-conversion. We use type-I (e: o + o) SHG geometry in a 50-mm-long aperiodically-poled MgO:LiNbO 3 device to satisfy the group-velocity matching condition. Simultaneously compressed fundamental and SH pulses of about 55-fs duration with small pedestal are generated from the fundamental input pulses of 95-fs duration.

We studied the sorting of micro-scale objects by use of an optically induced asymmetric double-well potential. Migration events, driven by Brownian motion, were experimentally investigated for various particle sizes, inter-well distances, and optical intensities. Temporal evolution of the measured sorting efficiency qualitatively agreed well with theoretical predictions. We found that the sorting performance was improved by introducing temporal modulation on the potential landscape.

Nonlinear vibrational spectroscopy by use of mid-infrared femtosecond pulses enables the observation of ultrafast vibrational/structural dynamics of molecular systems. This article presents the recent experimental studies on vibrational energy relaxations and energy transfer in liquid water. In pure liquid water, vibrational energy relaxation of OH-stretching and OH-bending excitation occurs at about 200fs and 170fs, respectively. When vibrational energy from individually excited water molecules is transferred to intermolecular modes, a sub-100fs nuclear rearrangement occurs, leaving the local hydrogen bonds weakened but unbroken. Subsequent energy delocalization over many molecules proceeds with the breaking of hydrogen bonds on an 1-ps time scales. The observed vibrational/structral response should be relevant for chemical reactions and structural changes in aqueous environments.

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.

The femtosecond dynamics of O–H bending excitations are studied in pump–probe experiments at frequencies from 1000 to 3500 cm −1 . Upon resonant O–H bending excitation, one finds a v = 1 → 2 absorption extending over ∼200 cm −1 and a v = 1 lifetime of 170 fs. O–H bending relaxation transfers excess energy to intermolecular modes, giving rise to a red-shifted librational absorption. Upon O–H stretching excitation, the O–H bending oscillator shows an instantaneous response due to a Fermi resonance between the v = 1 stretching and v = 2 bending states. The coupled states decay with a 200 fs lifetime via the O–H bending and librational modes.

We have investigated photo-induced absorption changes for In 0.10 Ga 0.90 N multiple quantum wells and a thin film induced by a continuous-wave (cw) beam and a femtosecond pulse in the violet region. For the thin-film sample, nearly the same absorption changes were observed in cw pumping and pulse pumping measurements. Screening of the piezoelectric field in InGaN heterostructures causes absorption changes around the band edge. This screening results from spatial separation of electron-hole pairs to the opposite sides of the InGaN epilayer. These spatially separated carriers are trapped in deep levels and maintain the absorption changes. The trapped charges are considered to be thermally excited to the conduction band and diffuse to attain the equilibrium state. This process takes on the order of 100 ps and such a long decay time of absorption changes allows us to have large absorption changes even by low-intensity cw pumping.

Cascaded processes of energy exchanges between optical beams via second-order nonlinearities lead to various phenomena, including Kerr-like nonlinear phase shifts and optical solitons. The cascading nonlinearity, combined with the developments in quasi-phase-matching devices, is becoming more promising for all-optical signal processing and ultrashort pulse generation. Here we review the basic properties of the cascading nonlinearity and present our recent experiments on femtosecond cascading with quasi-phase-matching devices.

We studied the electric-field-induced birefringence, the photoconductivity and the photorefractive properties of photorefractive polymer composites based on N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine (TPD). Photorefractive response rate is improved in TPD-based photorefractive polymer, composed of TPD, polystyrene (PS), 4-butoxy-3-propyl-1-(4'-nitrophenylazo)benzene (m3p4) and C 6 0 . We obtained the photorefractive response time of as fast as 3.2 ms (@I = 837 mW/cm 2 , E = 40 V/μm). The photoconductivity of TPD-based photorefractive polymer is found to be greater than that of the PVK-based one.

Nondegenerate femtosecond pump and probe spectroscopy was performed in In0.10Ga0.90N/In0.03Ga0.97N multiple quantum wells and epilayers at room temperature. The structural dependence of the photoinduced differential absorption (Δα) spectrum was studied on four In0.10Ga0.90N layers of different thickness (5.5, 17, 34, and 66 nm). A single bulge was clearly observed in Δα spectra except for the 5.5 nm thick sample. The bulges are sharper in thicker In0.10Ga0.90N layer samples. These spectrum changes are caused by excitonic absorption quenching and screening of internal piezoelectric fields by photoinduced carriers. Both rapid and slow temporal behaviors were observed in transient absorption spectra. The results of time-resolved photoluminescence measurements and existence of saturation pump fluences indicate that trapped carriers in potential minima originate from In-rich regions and/or midgap carrier traps are responsible for the observed lasting screening of the internal electric field.

The adaptive measurement of nanometre-scale vibrations of diffuse surfaces using the polarization self-modulation effect in a photorefractive GaP crystal is described. The sensitivity is investigated as a function of the applied electric field, the intensity of the incident light, and the mean size of the speckles. Numerical estimation of the sensitivity dependence on the mean size of the speckles shows a good agreement with experimental results.

Cascaded quadratic nonlinearities in periodically poled lithium tantalate are studied with femtosecond pulses by using the spectrally resolved two-beam coupling technique. The nonlinear phase shifts show obvious dependence on the phase mismatch conditions. Relatively large intrinsic nonlinear refraction and its anisotropy are also measured.

Photorefractive response rate in a polymer composite of poly(N-vinylcarbazole)/2,4,7-trinitro-9-fluorenone, doped with 2,5-dimethyl-4-(4'-nitrophenylazo)anisole (DMNPAA) is improved by modifying the structure of DMNPAA. We found that four kinds of modified DMNPAA with alkyl substituents have fast orientational response to an external electric field and keep large anisotropy in polarizability. Among them, 4-butoxy-3-propyl-1-(4'-nitorophenylazo)benzene has the shortest reorientation time constant of 19 ms (E=54 V/µm) and photorefractive time constant of 80 ms (I=2 W/cm2, E=54 V/µm) that are 2,300 times and 63 times faster than those of the DMNPAA composite. The mechanism of the fast reorientation in new polymer composites is investigated. The fast reorientational response is achieved by the improvement of the dispersivity in the polymer composites and the decrease of the glass transition temperature.

Surface-enhanced nonlinear vibrational spectroscopy using periodic gold nanoantenna arrays is demonstrated. The dipolar coupling among arrayed nanoantennas is shown to have striking impact on near-field enhancements of femtosecond pulsed-fields and on nonlinear signal enhancements. The condition near the collective-resonance achieves averaged signal enhancement of 850 times and local signal enhancement of 1.8 × 106 times, substantially reducing the required pump energy from micro-joule to nano-joule level. The scheme is useful for characterizing structure and dynamics of minute-volume molecular samples, monolayers, and interfaces, as well as paves the way to nonlinear vibrational spectroscopy with compact light sources of oscillator-level.