Nonlinear Optics

(2)Spectroscopy : Surfaces and Interfaces

(2)spectroscopy selectively probes surfaces and interfaces. Surface selectivity arises due to the reduced symmetry of surfaces and interfaces compared with isotropic bulk materials such as liquids and many solids. Here, we study monolayer films of biomolecules at the air/water interface, which may be considered as models for biological membranes. By examining structural features of the monolayers with (2)spectroscopy, useful insights may be obtained into the nature of molecular assemblies at biological and other interfaces.

Two types of (2) spectroscopy are used: 1) second harmonic generation (SHG) including particularly second harmonic generation optical activity (SHG-OA); and 2) sum frequency generation (SFG) vibrational spectroscopy. SHG involves conversion of incident radiation at the fundamental frequency ω to reflected radiation at the second harmonic frequency 2ω. Achiral, isotropic surfaces (with full rotational symmetry about the surface normal and mirror planes perpendicular to the surface) exhibit SHG due to a net polar alignment of molecules at the interface (Figure 1). Reflected light from the interface includes a small component of second harmonic frequency 2ω.

Figure 1: Surface second harmonic generation (SHG)

By measuring the efficiency of SHG for plane-polarized states of fundamental and second harmonic radiation, information on the tilt angle of molecular chromophores at the interface may be obtained. Chiral, isotropic surfaces lack mirror planes perpendicular to the surface. The chirality of the interface gives rise to SHG-OA, which is generally observed as a differential response in SHG for left-and right-hand circularly polarized fundamental radiation incident on the surface (Figure 2). We have studied SHG-OA of several biomolecular interfaces in order to assess the potential of the effect as a tool for structural characterization of chiral interfaces.

Figure 2. Second harmonic generation optical activity (SHG-OA) of chiral interfaces.

A schematic diagram of the experimental setup for SHG-OA studies of air/water interfaces is shown in Figure 3. The measurements involve continuous variation of the polarization state of incident fundamental radiation by rotation of an achromatic quarter-wave retardation plate (AWP in Figure 3). SHG-OA results for a tryptophan derivative (BOC-Trp) at an air/water interface are shown in Figure 4. The presence of a chiral response is immediately apparent in equal and opposite effects in the quarter-wave plate rotation traces for L- and D-enantiomers of BOC-Trp. Note that no such effect is present for the racemic (1:1) mixture of L- and D-enantiomers. We have shown that the microscopic origin of this SHG-OA effect is chiral assembly of achiral indole chromophores present in BOC-Trp molecules partially oriented at the air/water interface. Chiral assembly is specifically associated with asymmetry in the orientational distribution of achiral chromophores. Chiral assembly is invariably present when chiral molecules composed of achiral chromophores are partially oriented at an interface.

Figure 3. Experimental arrangement for SHG-OA. OPA is optical parametric amplifier (TOPAS); BC is Berek's compensator; GP is Glan laser prism; BS is beamsplitter; REF is reference SHG channel (not shown); L is lens; M is mirror; AWP is achromatic quarter-wave retardation plate; F is filter; RP is Rochon prism polarizer; PB is Pellin-Broca prism; PMT is photomultiplier tube.

Figure 4. Quarter-wave plate rotation traces showing SHG-OA of tryptophan derivative BOC-Trp at air/water interface.

A second type of (2) probe, infrared sum frequency generation (IR-SFG), is a surface selective vibrational spectroscopic technique. As depicted in Figure 5, SFG involves conversion of two distinct incident frequencies to the sum frequency in reflection. In SFG vibrational spectroscopy, infrared and visible laser beams are incident on the surface, and reflected sum frequency ωir + ωis radiation is detected. If the infrared frequency is near a resonance due to vibrational excitation of interfacial molecules, then SFG is resonantly enhanced, and thus SFG vibrational spectroscopy is possible. The particular implementation of SFG vibrational spectroscopy that we are developing uses a combination of short-duration (~80 fs), broad bandwidth infrared laser pulses and narrow bandwidth visible laser pulses. The broad spectrum of the infrared pulse is selected to span a range of molecular vibrational frequencies, for example in the C-H region. Up-conversion with the narrow-band visible pulse produces a spectrum in the visible region that includes resonances due to excitation of molecular vibrations. The spectrum of the up-converted visible pulse is observed by using a CCD spectrometer.

A novel feature of our approach is the production of infrared and visible laser pulses by nonlinear conversion of synchronized pairs of signal and idler pulses from a TOPAS optical parametric amplifier (OPA). The OPA is pumped with 800 nm laser pulses with duration ~80 fs. Broadband infrared pulses are produced by difference frequency mixing of signal and idler pulses in a AgGaS2 crystal, and narrowband visible pulses with ~ 650 nm are produced as the second harmonic of the signal beam. An important feature of the setup is the use of a long (2 cm length) LiNbO3 crystal for second harmonic generation of the signal, which ensures that the second harmonic has a narrow spectral bandwidth as well as inverted pulse shape of Fabry-Perot etalon (Figure 6). Introduction of time delay between short infrared pulse and long inverted pulse suppress nonresonant background signal which affects the contrast of the spectrum (Figure 7). A schematic diagram of the setup is shown in Figure 8. The advantage of this approach is that both visible and infrared laser pulses are produced by using a single OPA in a compact and highly efficient setup.



Figure 5. Sum frequency generation (SFG).

Figure 6. The experimental cross correlation, shown as open blue circles, of the inverted time-asymmetric pulse from a 2 cm LiNbO3 crystal. The thin red line shows the independent result from a calculation (no adjustable parameters). The inset shows the measured visible pulse spectrum, as blue circles, and, as the red line, the Fourier transform power spectrum calculated from the computed cross correlation shown in the main figure.

Figure 7. IR-SFG spectra of model monolayer systems recorded in C-H stretch region.

Figure 8. Setup for production of broadband infrared and narrowband visible laser pulses for IR-SFG spectroscopy.