Determination of the full scattering matrix using coherent Fourier scatterometry.

We demonstrate a method to obtain within an arbitrary numerical aperture (NA) the entire scattering matrix of a scatterer by using focused beam coherent Fourier scatterometry. The far-field intensities of all scattered angles within the NA of the optical system are obtained in one shot. The corresponding phases of the field are obtained by an interferometric configuration. This method enables the retrieval of the maximum available information about the scatterer from scattered far-field data contained in the given NA of the system.

Reconstruction of sub-wavelength features and nano-positioning of gratings using coherent Fourier scatterometry.

Optical scatterometry is the state of art optical inspection technique for quality control in lithographic process. As such, any boost in its performance carries very relevant potential in semiconductor industry. Recently we have shown that coherent Fourier scatterometry (CFS) can lead to a notably improved sensitivity in the reconstruction of the geometry of printed gratings. In this work, we report on implementation of a CFS instrument, which confirms the predicted performances. The system, although currently operating at a relatively low numerical aperture (NA = 0.4) and long wavelength (633 nm) allows already the reconstruction of the grating parameters with nanometer accuracy, which is comparable to that of AFM and SEM measurements on the same sample, used as reference measurements. Additionally, 1 nm accuracy in lateral positioning has been demonstrated, corresponding to 0.08% of the pitch of the grating used in the actual experiment.

Optical characterization of gold-cuprous oxide interfaces for terahertz emission applications.

We show that the interface between gold and thermally formed cuprous oxide, which emits terahertz radiation when illuminated with ultrafast femtosecond lasers, is in fact an AuCu/Cu2O interface due to the formation of the thermal diffusion alloy AuCu. The alloy enables the formation of a Schottky-barrier-like electric field near the interface which is essential to explain the THz emission from these samples. We confirm the formation of this AuCu layer by x-ray diffraction measurements, ellipsometry, and visual inspection. We determined the frequency-dependent complex refractive indices of the Cu2O and AuCu layer and verified them using reflection spectroscopy measurements. These refractive indices can be used for optimizing the thickness of Cu2O for maximum THz emission from these interfaces.

Enhanced terahertz emission by coherent optical absorption in ultrathin semiconductor films on metals.

We report on the surprisingly strong, broadband emission of coherent terahertz pulses from ultrathin layers of semiconductors such as amorphous silicon, germanium and polycrystalline cuprous oxide deposited on gold, upon illumination with femtosecond laser pulses. The strength of the emission is surprising because the materials are considered to be bad (amorphous silicon and polycrystalline cuprous oxide) or fair (amorphous germanium) terahertz emitters at best. We show that the strength of the emission is partly explained by cavity-enhanced optical absorption. This forces most of the light to be absorbed in the depletion region of the semiconductor/metal interface where terahertz generation occurs. For an excitation wavelength of 800 nm, the strongest terahertz emission is found for a 25 nm thick layer of amorphous germanium, a 40 nm thick layer of amorphous silicon and a 420 nm thick layer of cuprous oxide, all on gold. The emission from cuprous oxide is similar in strength to that obtained with optical rectification from a 300 µm thick gallium phosphide crystal. As an application of our findings we demonstrate how such thin films can be used to turn standard optical components, such as paraboloidal mirrors, into self-focusing terahertz emitters.

Terahertz emission from surface-immobilized gold nanospheres.

Electromagnetic wave emission based on optical rectification at terahertz (THz) wavelengths was observed from surface-immobilized gold nanospheres (SIGNs) above a gold surface. Although the excitation wavelength is off-resonant with the localized surface plasmons, the THz emission field was observed to be approximately 4.8 times greater than that from a percolated gold thin film of 10 nm thickness. A theoretical calculation predicts that the light electric field is enhanced in the SIGN system, even at off-resonance wavelengths. The observed THz field amplitude was quadratic with the illumination light field, suggesting that the THz generation is due to a second-order nonlinear optical process.

Bright visible emission from carbon nanotubes spatially constrained on a micro-bubble.

We report emission of broadband light in the spectral range 500 nm - 900 nm from single walled carbon nanotubes (SWNTs) in a liquid environment upon irradiance by a very low power (typically <5 mW), continuous-wave laser source in a tweezers setup. We show (i) formation of micro-bubbles upon irradiation of fluids containing bundles of SWNTs, (ii) optical trapping of such micro-bubbles, (iii) adhesion of SWNTs on the surface of such micro-bubbles, and (iv) bright emission of white light due to tweezer-induced localized heating of spatially-constrained SWNTs.