Hyper-Rayleigh scattering measurements
with single-photon sensitivity
J. Campo, at al., from the University of Antwerp used the 4 Quik E ICCD camera to setup a experiment for wavelength-dependent hyper-Rayleigh scattering measurements.
Experimental setup for wavelength-dependent hyper-Rayleigh scattering measurements. Reprinted figure with permission from J. Campo et al., Optics Express, Vol. 17, Issue 6, pp. 4587-4604 Copyright (2009) by The Optical Society.
Wavelength-dependent hyper-Rayleigh scattering measurements
Hyper-Rayleigh scattering (HRS) or second harmonic light scattering (SHS) is the main technique to measure the first hyperpolarizatizability β, which describes the second order nonlinear response of optical material at the molecular level. The HRS technique is based on incoherent second harmonic generation (SHG) by the randomly oriented molecules in a liquid solution. With HRS, no external field is required and therefore it is possible to measure hyperpolarizabilities from molecules having any structure. However, previous measurements are generally characterized by a very low signal-to-noise (S/N) ratio, which was attributed to the noise of the detector and the intensity fluctuations of the incoming laser pulses.
Experimental setup for hyper-Rayleigh scattering measurements
The laser system consists of a Ti:sapphire regenerative amplifier seeded by a mode-locked Ti:sapphire laser and optically pumped by a intracavity doubled Nd:YLF laser. The optical parametric amplifier enables tuning of the output laser pulses in the spectral range from 300nm to 3µm. A small fraction of the laser is split off, frequency doubled and detected with a silicon PIN-diode for laser power correction. The laser pulses (2ps width, 1.5kHz repetition rate) are focused with a set of cylindrical lenses into the sample cell. The generated hyper-Rayleigh light is collected and imaged to the entrance slit of a spectrograph orthogonal to the incoming laser pulses. The combination of spectrograph and intensified CCD camera (4 Quik E) provides a narrow spectral range around the second harmonic wavelength, with single-photon sensitivity.
Single photon detection with 4 Quik E camera
The 4 Quik E ICCD camera is equipped with a dual-stage microchannel plate image intensifier with an enhanced IR photocathode. The ICCD camera operates at room temperature and provides single-photon sensitivity. In order to achieve the single-photon sensitivity, the group around J. Campo implemented a thresholding scheme in the hardware simply by setting the signal voltage range of the input amplifier of the analog-to-digital converter. The lower signal is set to a value well above the dark noise of the CCD, but sufficiently low to register most actual (intensified) photon events, while the upper signal level is set as low as possible, corresponding to a maximum amplification.
Straylight repression by short gating times
By this thresholding scheme, the dark nose (including read-out noise) of the CCD sensor is completely eliminated and the S/N ratio in the limit of weak signals is improved by orders of magnitude. The only remaining source of dark noise is the actual dark electrons emitted by the photocathode and ion feedback. Furthermore, to suppress stray light, darkcounts and cosmic rays, a 20ns time gating is used, synchronized with the arrival of the laser pulses.
HRS measurement at the excitation wavelength of 1072nm, clearly illustration that correction for multi-photon fluorescence (red line) is needed. Blue: HRS signal of the solution, green: reference signal of the pure solvent. Reprinted figure with permission from J. Campo et al., Optics Express, Vol. 17, Issue 6, pp. 4587-4604 Copyright (2009) by The Optical Society.
High sensitivity enables background elimination
For HRS measurements, ICCD images are recorded alternatingly for the solution and the pure reference solvent. These images are integrated vertically, which yields to spectra such as shown in the figure above. The blue curve shows the signal of the solution and the green curve shows the reference signal of the pure solvent. The outer parts of the spectra allow reliable elimination of background signals, multi photon fluorescence (MPF) in particular. Moreover, thanks to the high detection sensitivity the reliable calibration against the pure solvent is reached and in combination with high stability of the laser systems this yields an excellent S/N ratio.
Title: Highly sensitive setup for tunable wavelength hyper-Rayleigh scattering with parallel detection and calibration data for various solvents
Author: Jochen Campo, Filip Desmet, Wim Wenseleers and Etienne Goovaerts
Institute: University of Antwerp, Antwerp, Belgium