High-Quality Brewster's Angle Polarizer for Broadband Infrared Application

Orthogonally-polarized excitation for improved two-photon and second-harmonic-generation microscopy, applied to neurotransmitter imaging with GPCR-based sensors

Fluorescent proteins are excited by light that is polarized parallel to the dipole axis of the chromophore. In two-photon microscopy, polarized light is used for excitation. Here we reveal surprisingly strong polarization sensitivity in a class of genetically encoded, GPCR-based neurotransmitter sensors. In tubular structures such as dendrites, this effect led to a complete loss of membrane signal in dendrites running parallel to the polarization direction of the excitation beam. To reduce the sensitivity to dendritic orientation, we designed an optical device that generates interleaved pulse trains of orthogonal polarization. The passive device, which we inserted in the beam path of an existing two-photon microscope, removed the strong direction bias from fluorescence and second-harmonic (SHG) images. We conclude that for optical measurements of transmitter concentration with GPCR-based sensors, orthogonally polarized excitation is essential.

Silicon Brewster plate wavelength separator for a mid-IR optical parametric source

The availability of optical elements for the mid-infrared wavelength range, such as polarizers and wavelength separators, is limited especially when a broadband wavelength range coverage is required. We propose a polarizer based on uncoated silicon Brewster plates. A detailed analysis of the polarizer's contrast and the influence of parasitic reflections, its dependence on wavelength, and the angular misalignment is shown. Two different arrangements of the two- and four-plate polarizers are discussed. With contrast including the influence of parasitic reflections of over 10³ for the whole transparency range of silicon (1.2-6.5 µm), the four-plate polarizer is an effective, low-cost, high-power compatible tool providing sufficient contrast for signal and idler beam separation of the broadband mid-infrared Type II optical parametric sources. The proposed polarizers can function as an attenuator assembly without any wave plate.

Infrared Polarizer Based on Direct Coupling to Surface Plasmon Polaritons

We propose a new type of reflective polarizer based on polarization-dependent coupling to surface plasmon polaritons (SPPs) from free space. This inexpensive polarizer is relatively narrowband but features an extinction ratio of up to 1000 with efficiency of up to 95% for the desired polarization (numbers from a calculation) and thus can be stacked to achieve extinction ratios of 10⁶ or more. As a proof of concept, we experimentally realized a polarizer based on nanoporous aluminum oxide that operates around a wavelength of 10.6 μm, corresponding to the output of a CO₂ laser, using aluminum anodization, a low-cost electrochemical process.

Optical and electronic solutions for power stabilization of CO2 lasers

High pressure-temperature conditions can be readily achieved through the laser-heated diamond anvil cell (LH-DAC). A stable laser source is required for reliable in situ measurements of the sample, as the sample is small with a thermal time constant of the order of microseconds. Here, we show that the power instabilities typical of CO₂ gas lasers used in LH-DAC's are ±5% at the second timescale and ∼±50% at the microsecond timescale. We also demonstrate that the pointing instability of the laser requires either a diffuser or an integrating sphere for reliable total power measurements with small sized detectors. We present a simple solution for stabilizing the power of a CO₂ gas laser on the second timescale by the direct modulation of the current across the tube and another solution that stabilizes the power to the microsecond timescale by externally modulating the CO₂ laser beam. Both solutions can achieve a ±0.3% power stability.

Optimized interferometric setup for chiral and achiral ultrafast IR spectroscopy

We report an actively stabilized interferometer-based set-up for the detection of vibrational circular dichroism (VCD) and optical rotatory dispersion (VORD) with femtosecond laser pulses. Our approach combines and improves elements of several previous measurement strategies, including signal amplification in a crossed polarizer configuration, precise control and modulation of polarization, phase stability, tight focusing, broad-band detection and spectral interferometry. Their importance for static and transient measurements is motivated by a signal analysis based on Jones matrices and response theory. Only depending on the pump-beam polarization, the set-up can selectively detect transient VCD and VORD or transient linear birefringence (LB) and linear dichroism (LD), which usually constitute the dominant artifacts in the chiral measurements. For illustration we present transient LB and LD data of an achiral Rhenium carbonyl complex, detected simultaneously by spectral interferometry, and we analyze residual background signals in the experimental configuration for transient chiral spectroscopy.

Linear dichroism amplification: adapting a long-known technique for ultrasensitive femtosecond IR spectroscopy

We demonstrate strong amplification of polarization-sensitive transient IR signals using a pseudo-null crossed polarizer technique first proposed by Keston and Lospalluto [Fed. Proc. 10, 207 (1951)] and applied for nanosecond flash photolysis in the visible by Che et al. [Chem. Phys. Lett. 224, 145 (1994)]. We adapted the technique to ultrafast pulsed laser spectroscopy in the infrared using photoelastic modulators, which allow us to measure amplified linear dichroism at kilohertz repetition rates. The method was applied to a photoswitch of the N-alkylated Schiff base family in order to demonstrate its potential of strongly enhancing sensitivity and signal to noise in ultrafast transient IR experiments, to simplify spectra and to determine intramolecular transition dipole orientations.

Infrared optical activity: electric field approaches in time domain

Vibrational circular dichroism (VCD) spectroscopy provides detailed information about the absolute configurations of chiral molecules including biomolecules and synthetic drugs. This method is the infrared (IR) analogue of the more popular electronic CD spectroscopy that uses the ultraviolet and visible ranges of the electromagnetic spectrum. Because conventional electronic CD spectroscopy measures the difference in signal intensity, problems such as weak signal and low time-resolution can limit its utility. To overcome the difficulties associated with that approach, we have recently developed femtosecond IR optical activity (IOA) spectrometry, which directly measures the IOA free-induction-decay (FID), the impulsive chiroptical IR response that occurs over time. In this Account, we review the time-domain electric field measurement and calculation methods used to simultaneously characterize VCD and related vibrational optical rotatory dispersion (VORD) spectra. Although conventional methods measure the electric field intensity, this vibrational technique is based on a direct phase-and-amplitude measurement of the electric field of the chiroptical signal over time. This method uses a cross-polarization analyzer to carry out heterodyned spectral interferometry. The cross-polarization scheme enables us to selectively remove the achiral background signal, which is the dominant noise component present in differential intensity measurement techniques. Because we can detect the IOA FID signal in a phase-amplitude-sensitive manner, we can directly characterize the time-dependent electric dipole/magnetic dipole response function and the complex chiral susceptibility that contain information about the angular oscillations of charged particles. These parameters yield information about the VCD and VORD spectra. In parallel with such experimental developments, we have also calculated the IOA FID signal and the resulting VCD spectrum. These simulations use a quantum mechanical/molecular mechanical molecular dynamics (QM/MM MD) method and calculate the electric dipole/magnetic dipole cross-correlation function in the time domain. Although many quantum chemistry calculation approaches can only consider a limited number of geometry-optimized conformations of chiral molecules in a gas phase, this computational method includes the solute-solvent interactions and the inhomogeneous distributions of solute conformers in condensed phases. A subsequent Fourier transformation of the chiral response function produced a theoretical VCD spectrum in the entire mid-IR frequency range. Directly comparing theory and experiment, we demonstrate quantitative agreement between frequency-tunable femtosecond IOA measurements and QM/MM MD simulations of (1S)-β-pinene in CCl(4) solution. We anticipate that these direct IOA measurement and calculation methods will be applied to the studies of equilibrium chiroptical properties and structure determinations. These methods provide tools to investigate ultrafast structural dynamics of chiral systems with unprecedented time resolution.