Development of Ultrafast Surface Spectroscopic Techniques
To achieve surface selectivity, we utilize even-order nonlinear optical processes, such as Sum Frequency Generation (SFG) and Second Harmonic Generation (SHG), which are symmetry-forbidden in isotropic bulk media in the electric dipole approximation. One particularly interesting spectral region for us is the mid-infrared: vibrational spectroscopy provides a wealth of information on the molecular structure and organization at surfaces and interfaces using the intrinsic vibrational chromophores present in every molecule.
In addition to the standard frequency-domain spectroscopic measurements that characterize ensemble averaged structures, we are working on an array of ultrafast (femtosecond) time-domain techniques, such as SFG-FID (Free Induction Decay) for complementary studies of the molecular dynamics in real time and Spectrally- and Time-Resolved SFG (STiR-SFG), a mixed time-frequency domain technique capable of measuring spectral evolution of vibrational coherences at surfaces. Another recent development is the heterodyne-detected SFG (HD-SFG) capable of ultrasensitive detection on a few percent of a monolayer level while simultaneously providing phase information on the molecular vibrations.
In addition to the standard frequency-domain spectroscopic measurements that characterize ensemble averaged structures, we are working on an array of ultrafast (femtosecond) time-domain techniques, such as SFG-FID (Free Induction Decay) for complementary studies of the molecular dynamics in real time and Spectrally- and Time-Resolved SFG (STiR-SFG), a mixed time-frequency domain technique capable of measuring spectral evolution of vibrational coherences at surfaces. Another recent development is the heterodyne-detected SFG (HD-SFG) capable of ultrasensitive detection on a few percent of a monolayer level while simultaneously providing phase information on the molecular vibrations.
Highlights of Spectroscopic Development Methods
Relative Phase Change of Nearby Resonances in Temporally Delayed Sum Frequency Spectra
Surface-selective sum frequency generation (SFG) spectroscopy has been previously shown to benefit from a finite time delay between two input laser pulses, which suppresses the nonresonant background and improves spectral resolution. Here we demonstrate another consequence of the time delay in SFG: depending on the magnitude of the delay, nearby resonances (e.g., vibrational modes) can “flip” their relative phase, i.e., appear either in-phase or out-of-phase with one another, resulting in either constructive or destructive interference in SFG spectra. This is significant for interpretation of the SFG spectra, in particular because the sign of the resonant amplitude provides the absolute molecular orientation (up vs down) of the vibrational chromophore.
J. Phys. Chem. Lett.,3 (23), 3493–3497 (2012) |
Temporal effects on spectroscopic line shapes, resolution, and sensitivity of the broad-band SFG
This paper addresses the general issue of spectral resolution and sensitivity of the broad-band (BB) SFG that involves a spectrally narrow nonresonant (usually visible) and a BB resonant (usually infrared) laser pulses. We examine how the spectral width and temporal shape of the two pulses, and the time delay between them, relate to the spectroscopic line shape and signal level in the BB-SFG measurement. By combining experimental and model calculations, we show that the best spectral resolution and highest signal level are simultaneously achieved when the nonresonant narrow-band upconversion pulse arrives with a nonzero time delay after the resonant BB pulse. The nonzero time delay partially avoids the linear trade-off of improving spectral resolution at the expense of decreasing signal intensity, which is common in BB-SFG schemes utilizing spectral filtering to produce narrow-band visible pulses.
J. Chem. Phys. 132, 234503, 1-9 (2010) |
Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy
Here we present a new technique of broad-band heterodyne-detected sum frequency generation (HD-SFG) spectroscopy and demonstrate its high sensitivity allowing surface-selective measurements of vibrational spectra at submonolayer surface coverage, as low as a few percent of a monolayer. This was achieved without the help of surface enhancement phenomena, on a transparent dielectric substrate (water), and without introducing fluorescent labels, in fact, without utilizing any electronic resonances. Only the intrinsic vibrational transitions were employed for the detection of the analyte molecules (1-octanol). Unlike conventional (homodyne-detected) SFG spectroscopy, where the signal intensity decreases quadratically with decreasing surface coverage, in HD-SFG, the scaling is linear, and the signal is amplified by interference with a reference beam, significantly improving sensitivity and detection limits. At the same time, HD-SFG provides the phase as well as the amplitude of the signal and thus allows accurate subtraction of the non-resonant backgrounda common problem for surfaces with low concentrations of analyte molecules (i.e., weak resonant signals). The technology is patented under the US patent 8,451,442.
J. Am. Chem. Soc. 130, 2271-2275 (2008) |