ABSTRACT:We address the sensitivity of Interferometric Cross-Polarization Microscopy by comparing scattering and absorption by spherical 10 nm nanoparticles through a combination of modeling and experiment. We show that orthogonality of light in the two polarization branches of Cross-Polarization Microscopy ensures that only light that has interacted with a nanoparticle is interferometrically enhanced. As a result background-free shot noise-limited detection is achieved for sub-μW optical powers at the sample. Our modeling in particular shows that in the near-infrared regime, above the plasmon resonance frequency of spherical nanoparticles, the cross-polarization approach is several orders of magnitude more sensitive than conventional extinction based detection. This enhanced near-infrared sensitivity for spherical nanoparticles is promising for applications requiring low absorption and low power imaging of nanoparticles in cells.

Optical properties of nanostructures depend on size, shape, material, and local environment. These characteristics can be probed interferometrically, given a broadband source. However, broadband supercontinuum sources are intrinsically noisy, limiting the measurement sensitivity. In this article we describe the application of an auto-balancing technique to reduce the noise in a broadband supercontinuum source, thus increasing the signal to noise ratio. We show a noise reduction of 41dB allowing optical powers as small as 0.01pW to be interferometrically detected with a 5ms integration time.

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Probing the optical properties of single nanostructures is becoming increasingly important in the context of nanoscience. The optical properties of nanostructures frequently occur over a range of wavelengths, and vary depending on medium and size. The spread of optical information over several wavelengths suggests that characterisation of nanostructures should be possible. However the optical properties of small particles are intrinsically difficult to investigate due to their small absorption and scattering cross section.Interferometry is a detection strategy that enables the detection of small particles with an excellent signal to noise ratio. In this approach the weak optical signal from the nano-structure is amplified through interference with a much stronger optical reference. Currently most of these interferometric approaches rely on using single, or a few, discrete wavelengths giving only limited optical information on the nano-structure of interest. Instead using a high power supercontinuum light source, could potentially determine the optical features and properties of the nanostructure over the full visible -NIR wavelength range (here: 450nm -2µm).The major drawback of using these supercontinuum sources for interferometry is that they are intrinsically very noisy -typical rms-noise of 5%. As interferometric detection relies on multiplying the signal by a reference from the same source, the noise will start to dominate very quickly over any signal from the nano-structures. The interference signal (in terms of the electric field) can be written as (Where E r and E s is the reference and signal fields, and ∆E r , and ∆E s is the noise in the reference and signal fields respectively. From the third term, it can be seen that the signal is multiplied by the noise in the reference. In order to extract the signal, it must be greater than the noise in the reference beam. In a conventional Mach-Zehnder interferometer the two outputs experience a different number of phase shifts due to reflections off mirrors (π) and either passing through (0) a or being reflected (π) by a beam splitter (under the assumption that both beam paths experience the same level of dispersion). It can be shown that the intensity signal of the interference is positive on one output and negative on the other [1]. Therefore using a subtractive circuit causes the interference signal to be doubled. Here, we used a balanced detection circuit suggested by Hobbs [2], where the amplitude fluctuations of the supercontinuum can be effectively reduced. To date we have shown, on average (fig 1) a 20dB reduction in noise, from a standard transimpedance photodiode (green) to the balanced circuit (red), improving the SNR. Fig. 1. a). The balanced, background, and single (used as a standard transimpedance amplifier) outputs of the detector are shown for the supercontinuum whist using a bandpass filter (centre = 633nm, FWHM = 10nm). A significant reduction in the noise is shown: the noise level becomes comparable to that of the background. b). Fourier tran...