Chirped-pulse Upconversion Research Articles

Broadband Two-Dimensional Infrared Spectroscopy with Signal Detection in Visible Range by Nonlinear Chirped-Pulse Upconversion

Broadband Two-Dimensional Infrared Spectroscopy with Signal Detection in Visible Range by Nonlinear Chirped-Pulse Upconversion

Chirped pulse upconversion for femtosecond mid-infrared spectroscopy at 100 kHz.

We demonstrate that chirped pulse up-conversion (CPU), a method routinely used with systems based on 1-kHz Titanium:Sapphire lasers, can be extended to a repetition rate of 100 kHz with an Ytterbium diode-pumped femtosecond amplifier. Individual mid-infrared spectra can thus be measured directly in the near infrared using a fast CMOS linescan camera. After an appropriate Fourier processing, a spectral resolution of 1.1 cm-1 is reported, currently limited by our spectrometer. Additionally, we demonstrate the application of CPU to a pump-probe measurement of the vibrational relaxation in carboxy-hemoglobin, and we show that the combination of fast scanning and fast acquisition enables a straightforward removal of pump scattering interference.

Ultrafast pump-probe spectroscopy via chirped-pulse up-conversion with dispersion compensation.

In this study, ultrafast transient signals were detected on a single-shot basis using chirped-pulse up-conversion spectroscopy with dispersion compensation. Unlike in the conventional time-encoding technique using chirped pulses, distortion of the ultrafast waveform was reduced by applying dispersion compensation to the chirped probe pulses and using sum-frequency generation with the chirped readout pulses. The method was applied to terahertz time-domain spectroscopy and near-infrared pump-probe spectroscopy, providing ultrafast observations with an improved temporal resolution comparable to the transform-limited pulse durations. Terahertz waveforms, Kerr rotation signals, and phonon-polariton oscillations were measured accurately with no significant waveform distortion, thereby showing the proposed scheme to be promising for single-shot pump-probe spectroscopy in a wide range of spectroscopic applications.

Polarization-sensitive femtosecond mid-infrared spectrometer with a tunable Fabry–Perot filter and chirped-pulse upconversion

Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular structure and vibrational dynamics. By using a polarized narrowband pump pulse in which center frequency is rapidly scanned with an electrically tunable Fabry-Perot filter, we have developed a femtosecond MIR spectrometer that is more sensitive to molecular structure. The pump energy and polarization dependences of transient MIR absorption spectra measured with the spectroscopic system revealed the appearance of a dark (infrared-inactive) mode due to a slight decrease in the molecular symmetry of a metal carbonyl complex, Mn2(CO)10, in solution. In addition, the rotational relaxation time of Mn2(CO)10 in solution was determined as approximately 20ps.

Development and Application of Sub-Cycle Mid-Infrared Source Based on Laser Filamentation

This paper is a perspective article which summarizes the development and application of sub-cycle mid-infrared (MIR) pulses generated through a laser filament. The generation scheme was published in Applied Sciences in 2013. The spectrum of the MIR pulse spreads from 2 to 50 μ m, corresponding to multiple octaves, and the pulse duration is 6.9 fs, namely, 0.63 times the period of the carrier wavelength, 3.3 μ m. The extremely broadband and highly coherent light source has potential for various applications. The light source has been applied for advanced ultrafast pump–probe spectroscopy by several research groups. As another application example, single-shot detection of absorption spectra in the entire MIR range by the use of chirped-pulse upconversion with a gas medium has been demonstrated. Although the measurement of the field oscillation of the sub-cycle MIR pulse was not trivial, the waveform of the sub-cycle pulse has been completely characterized with a newly developed method, frequency-resolved optical gating capable of carrier-envelope phase determination. A particular behavior of the spectral phase of the sub-cycle pulse has been revealed through the waveform characterization.

Wide-bandgap nonlinear crystal LiGaS2 for femtosecond mid-infrared spectroscopy with chirped-pulse upconversion.

Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular vibrational dynamics. A quantitative study on nonlinear media for upconversion is still essential for wide applications, particularly at the frequencies below 2000 cm-1. We evaluate wide-bandgap nonlinear crystals of Li-containing ternary chalcogenides based on their performance as the upconversion medium for femtosecond MIR spectroscopy. The upconversion efficiency is measured as a function of the MIR pulse frequency and the chirped pulse energy. LiGaS2 is found to be an efficient crystal for the upconversion of MIR pulses in a wide frequency range of 1100-2700 cm-1, especially below 2000 cm-1. By using LiGaS2 as an efficient upconversion crystal, we develop a MIR pump-probe spectroscopy system with a spectral resolution of 2.5 cm-1, a time resolution of 0.2 ps, and a probe window of 120 cm-1. Vibrational relaxation dynamics of CO stretching modes of Mn2(CO)10 in cyclohexane and bovine serum albumin in D2O are demonstrated with a high signal-to-noise ratio.

Ultrabroadband mid-infrared spectroscopy with four-wave difference frequency generation

Four-wave difference frequency generation (FWDFG) is a third-order optical parametric process, which is generally explained as or , where three input frequencies are , , and , and the output frequency is . Here we report the use of FWDFG for chirped-pulse upconversion (CPU) of an ultrabroadband mid-infrared (MIR) supercontinuum and the application of the technique for MIR spectroscopy. When the CPU technique is used for MIR spectroscopy, ultrashort MIR pulses are converted into visible ones. This way, the spectra can be recorded with a visible spectrometer, which has much higher performance than MIR spectrometers. In the previous experiments, the CPU has been performed by using sum-frequency generation (SFG) with a solid crystal, and the bandwidth has been limited to less than 1000 cm−1 due to the phase matching condition of the SFG. This limitation can be removed by using FWDFG, which is a third-order nonlinear process that allows us to use centrosymmetric nonlinear media such as gases for the upconversion. Since gaseous media have much less dispersion than solid media, the bandwidth of the phase-matching condition for the upconversion process becomes very broad. In our experiments, the entire spectrum of the MIR supercontinuum spanning from 200 to 5500 cm−1 was upconverted by using a 4.9 ps chirped pulse to visible wavelength radiation, which was detected with a conventional visible dispersive spectrometer. The technique has been applied to attenuated total reflectance MIR spectroscopy. Absorption spectra of liquids in the range from 200 to 5500 cm−1 were measured with a visible spectrometer on a single-shot basis.

Attenuated total reflectance spectroscopy with chirped-pulse upconversion.

Chirped-pulse upconversion technique has been applied to attenuated total reflectance (ATR) infrared spectroscopy. An extremely broadband infrared pulse was sent to an ATR diamond prism and the reflected pulse was converted to the visible by using four-wave mixing in krypton gas. Absorption spectra of liquids in the range from 200 to 5500 cm(-1) were measured with a visible spectrometer on a single-shot basis. The system was applied to observe the dynamics of exchanging process of two solvents, water and acetone, which give clear vibrational spectral contrast. We observed that the exchange was finished within ∼ 10 ms.

A comparative study on chirped-pulse upconversion and direct multichannel MCT detection

A comparative study is carried out on two spectroscopic techniques employed to detect ultrafast absorption changes in the mid-infrared spectral range, namely direct multichannel detection via HgCdTe (MCT) photodiode arrays and the newly established technique of chirped-pulse up-conversion (CPU). Whereas both methods are meanwhile individually used in a routine manner, we directly juxtapose their applicability in femtosecond pump-probe experiments based on 1 kHz shot-to-shot data acquisition. Additionally, we examine different phase-matching conditions in the CPU scheme for a given mid-infrared spectrum, thereby simultaneously detecting signals which are separated by more than 200 cm(-1).

Single-shot detection of mid-infrared spectra by chirped-pulse upconversion with four-wave difference frequency generation in gases

Single-shot detection of ultrabroadband mid-infrared spectra was demonstrated by using chirped-pulse upconversion technique with four-wave difference frequency generation in gases. Thanks to the low dispersion of the gas media, the bandwidth of the phase matching condition of the upconversion process becomes very broad and the entire mid-infrared spectrum spanning from 200 to 5500 cm(-1) was upconverted by using a 10 ps chirped pulse to visible wavelength radiation, which was detected with a conventional visible dispersive spectrometer. This method was demonstrated by the successful measurement of infrared absorption spectra of organic polymer films.

Lower frequency region mid-infrared spectroscopy by chirped pulse upconversion

UV/visible pump, mid-IR probe spectroscopy measurements based on the chirped upconversion method were expanded to the frequency region below 1800cm−1 with the nonlinear optical crystal AgGaGeS4 . Pump-probe experiments were demonstrated with GaAs and the photoreceptor protein Slr1694.

Ultrafast mid-infrared spectroscopy by chirped pulse upconversion in 1800-1000cm^−1 region

Broadband femtosecond mid-infrared pulses can be converted into the visible spectral region by chirped pulse upconversion. We report here the upconversion of pump probe transient signals in the frequency region below 1800cm(-1), using the nonlinear optical crystal AgGaGeS4, realizing an important expansion of the application range of this method. Experiments were demonstrated with a slab of GaAs, in which the upconverted signals cover a window of 120cm(-1), with 1.5cm(-1) resolution. In experiments on the BLUF photoreceptor Slr1694, signals below 1 milliOD were well resolved after baseline correction. Possibilities for further optimization of the method are discussed. We conclude that this method is an attractive alternative for the traditional MCT arrays used in most mid-infrared pump probe experiments.

Ultrabroadband detection of a mid-IR continuum by chirped-pulse upconversion

Ultrabroadband mid-IR continuum pulses can be detected on a single-shot basis using chirped-pulse upconversion. Converting the mid-IR pulse to the visible reduces the fractional bandwidth and enables use of a silicon CCD camera. Removing the cross-phase modulation of the chirped pulse results in 1 cm(-1) resolution over a 600 cm(-1) detected bandwidth.

Direct mid-infrared femtosecond pulse shaping with a calomel acousto-optic programmable dispersive filter

Direct amplitude and phase shaping of mid-infrared femtosecond pulses is realized with a calomel-based acousto-optic programmable dispersive filter transparent between 0.4 and 20 μm. The shaped pulse electric field is fully characterized with high accuracy, using chirped-pulse upconversion and time-encoded arrangement spectral phase interferometry for direct electric field reconstruction techniques. Complex mid-infrared pulse shapes at a center wavelength of 4.9 μm are generated with a spectral resolution of 14 cm(-1), which exceeds by a factor of 5 the reported experimental resolutions of calomel-based filters.

Multiply Excited Vibration of Carbon Monoxide in the Primary Docking Site of Hemoglobin Following Photolysis from the Heme

We investigate ultrafast vibrational ligand dynamics in carboxyhemoglobin using chirped pulse upconversion and demonstrate the formation of vibrationally multiply excited carbon monoxide trapped in the primary docking site of hemoglobin after photolysis. The bleach signal due to ligand dissociation and the incipient docking-site absorption signal are about 200 cm−1 apart and differ by more than an order of magnitude in absorbance. In conventional approaches, these signals are monitored individually. Our method allows simultaneous observation of these signals with both high spectral resolution and high sensitivity. The large amount of vibrationally hot CO in the docking site as observed under Soret band excitation of the heme is discussed in the context of excess energy provided by the pump photon and is shown to be in quantitative agreement with predictions based on changes in the CO equilibrium distance upon instantaneous dissociation.

Measuring absorptive two-dimensional infrared spectra using chirped-pulse upconversion detection

Chirped-pulse upconversion (CPU) enables the detection of mid-infrared spectra using a silicon CCD camera by sum-frequency mixing a mid-IR field with a highly chirped near-IR (800 nm) field. Although the substantial chirp limits the spectral broadening and phase distortions caused by the inherent cross-phase modulation, the exquisite phase fidelity needed to measure a fully absorptive two-dimensional IR spectrum demands the correction of the phase distortions. We demonstrate how to correct all of the phase distortions involved in recording an absorptive 2DIR spectrum including delay stage calibrations as well as the requisite nonlinear signals. Besides the extra experimental step of the upconversion process itself, the phase correction procedure requires only the knowledge of the spectral or temporal phase of the chirped pulse. The method is demonstrated in a metal carbonyl complex, Mn2(CO)10, in n-hexane and methanol solvents showing that the method operates well in cases of both homogeneous and inhomogeneous broadening.

Removing cross-phase modulation from midinfrared chirped-pulse upconversion spectra

We observe that narrow spectral features in mid-infrared spectra obtained by chirped-pulse up-conversion are strongly distorted by cross-phase modulation between the mid-infrared field and the chirped pulse. We discuss the consequences of this effect on spectral resolution, and introduce a correction method that recovers masked lines. This simple correction can be applied either when the upconverted field is fully characterized, such as in multidimensional spectroscopy, or when causality can be used, such as in absorption spectroscopy, which we demonstrate experimentally.

Direct observation of ligand transfer and bond formation in cytochrome c oxidase by using mid-infrared chirped-pulse upconversion.

We have implemented the recently demonstrated technique of chirped-pulse upconversion of midinfrared femtosecond pulses into the visible in a visible pump-midinfrared probe experiment for high-resolution, high-sensitivity measurements over a broad spectral range. We have succeeded in time-resolving the CO ligand transfer process from the heme Fe to the neighboring Cu(B) atom in the bimetallic active site of mammalian cytochrome c oxidase, which was known to proceed in <1 ps, using the full CO vibrational signature of Fe-CO bond breaking and Cu(B)-CO bond formation. Our differential transmission results show a delayed onset of the appearance of the Cu(B)-bound species (200 fs), followed by a 450-fs exponential rise. Trajectories calculated by using molecular-dynamics simulations with a Morse potential for the Cu(B)-C interaction display a similar behavior. Both experimental and calculated data strongly suggest a ballistic contribution to the transfer process.

Two-dimensional infrared spectroscopy detected by chirped pulse upconversion

A two-dimensional (2D) infrared spectrum of Mn2(CO)10 is measured by using chirped-pulse upconversion (CPU) of the nonlinear signal field plus a reference local oscillator. By converting the spectrum to the visible, a silicon CCD camera can be used. The method offers an attractive alternative to direct IR detection due to the technological maturity of silicon and its greater intrinsic detectivity over HgCdTe. Using CPU, we acquired a rephasing 2D IR spectrum in a few seconds.