Background-free correlation spectroscopy using an infrared mode-locked laser.

The recent advances in infrared laser technology are expanding the capabilities and applications of vibrational spectroscopy. A promising approach utilizing broadband infrared mode-locked lasers is background-free (BF) absorption spectroscopy. This method captures the free-induction decay (FID) of excited molecules while suppressing the background light. It is unique in that the signal strength increases with input optical power but eventually struggles with detector noise when targeting fewer molecules. In this paper, we present a novel method of multiplexed background-free spectroscopy using a spectral mask whose transmittance has a strong correlation with the absorption spectrum of a target molecule. We successfully demonstrate an order of magnitude increase in the sensitivity due to multiplexing as well as a high molecular contrast due to the spectral correlation. The presented results indicate the promising potential of the method for sensitive and selective detection of trace molecules.

Generation of High-Lying Vibrational States in Carbon Dioxide through Coherent Ladder Climbing.

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.

Broadband dispersion spectroscopy using interferometric phase modulation under background light suppression.

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.

Broadband background-free vibrational spectroscopy using a mode-locked Cr:ZnS laser.

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.

Mode-locked laser oscillation with spectral peaks at molecular rovibrational transition lines.

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.

Vibrational Strong Coupling in Subwavelength Nanogap Patch Antenna at the Single Resonator Level.

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.

Ultrafast saturable absorption of large-diameter single-walled carbon nanotubes for passive mode locking in the mid-infrared.

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.

Molecular ground-state dissociation in the condensed phase employing plasmonic field enhancement of chirped mid-infrared pulses.

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.