Surface-plasmon-enhanced strain-wave-induced optical diffraction changes from a segmented grating.

We report on surface-plasmon-polariton-enhanced (SPP-enhanced), strain-wave-induced reflection and diffraction changes on a Au-covered, segmented grating. The segmented grating has a 6020 nm period, and its lines are segmented into 7 periods of a 430 nm period grating, which allows the excitation of SPPs. This grating has three SPP resonances at different optical wavelengths, for the same incident angle. Pump-pulse-induced strain waves are probed by measuring reflection and diffraction of a tunable probe pulse in a wavelength range that includes all three SPP resonances. Surface Acoustic Waves (SAWs) and Longitudinal Waves (LWs) are identified. When probing close to SPP resonances, the reflection changes from SAWs and LWs are strongly enhanced by factors of 23 and 36, respectively, compared with reflection changes observed when probing at off-resonance wavelengths. The relative SAW- and LW-induced diffraction changes are larger by additional factors of up to 3.3 and 2.6, respectively, compared to the reflection changes.

Plasmonic enhancement of photoacoustic-induced reflection changes.

In this paper, we report on surface-plasmon-resonance enhancement of the time-dependent reflection changes caused by laser-induced acoustic waves. We measure an enhancement of the reflection changes induced by several acoustical modes, such as longitudinal, quasi-normal, and surface acoustic waves, by a factor of 10-20. We show that the reflection changes induced by the longitudinal and quasi-normal modes are enhanced in the wings of the surface plasmon polariton resonance. The surface acoustic wave-induced reflection changes are enhanced on the peak of this resonance. We attribute the enhanced reflection changes to the longitudinal wave and the quasi-normal mode to a shift in the surface plasmon polariton resonance via acoustically induced electron density changes and via grating geometry changes.

High-resolution microscopy through optically opaque media using ultrafast photoacoustics.

We present a high-resolution microscope capable of imaging buried structures through optically opaque materials with micrometer transverse resolution and a nanometer-scale depth sensitivity. The ability to image through such materials is made possible by the use of laser ultrasonic techniques, where an ultrafast laser pulse launches acoustic waves inside an opaque layer and subsequent acoustic echoes from buried interfaces are detected optically by a time-delayed probe pulse. We show that the high frequency of the generated ultrasound waves enables imaging with a transverse resolution only limited by the optical detection system. We present the imaging system and signal analysis and demonstrate its imaging capability on complex microstructured objects through 200 nm thick metal layers and gratings through 500 nm thickness. Furthermore, we characterize the obtained imaging performance, achieving a diffraction-limited transverse resolution of 1.2 µm and a depth sensitivity better than 10 nm.

Laser-induced ultrasonics for detection of low-amplitude grating through metal layers with finite roughness.

We report on the use of laser-induced ultrasonics for the detection of gratings with amplitudes as small as 0.5 nm, buried underneath an optically opaque nickel layer. In our experiments, we use gratings fabricated on top of a nickel layer on glass, and we optically pump and probe the sample from the glass side. The diffraction of the probe pulse from the acoustic echo from the buried grating is measured as a function of time. We use a numerical model to show how the various physical phenomena such as interface displacement, strain-optic effects, thermo-optic effects, and surface roughness influence the shape and strength of the time-dependent diffraction signal. More importantly, we use a Rayleigh-Rice scattering theory to quantify the amount of light scattering, which is then used as in input parameter in our numerical model to predict the time-dependent diffracted signal.

Thickness dependent terahertz emission from cobalt thin films.

When cobalt thin films are illuminated with femtosecond laser pulses, we observe the emission of terahertz pulses. For a cobalt film thickness less than about 40 nm, the THz electric field direction rotates when the sample is rotated about the surface normal. This azimuthal angle-dependent emission is consistent with the assumption that laser-induced changes in an in-plane magnetization are responsible for the emission. For thicker layers, however, we observe the development of an azimuthal angle-independent contribution to the THz emission which we attribute to laser-induced changes in an out-of-plane magnetization component. We show that the relative contribution of this component grows when the cobalt film thickness increases. Our observations are supported by magnetic force microscopy measurements which show that for film thicknesses below 40 nm, the magnetization is predominantly in-plane whereas for thicknesses larger than 40 nm, an out-of-plane magnetization component develops.

Plasmon enhanced terahertz emission from single layer graphene.

We show that surface plasmons, excited with femtosecond laser pulses on continuous or discontinuous gold substrates, strongly enhance the generation and emission of ultrashort, broadband terahertz pulses from single layer graphene. Without surface plasmon excitation, for graphene on glass, 'nonresonant laser-pulse-induced photon drag currents' appear to be responsible for the relatively weak emission of both s- and p-polarized terahertz pulses. For graphene on a discontinuous layer of gold, only the emission of the p-polarized terahertz electric field is enhanced, whereas the s-polarized component remains largely unaffected, suggesting the presence of an additional terahertz generation mechanism. We argue that in the latter case, 'surface-plasmon-enhanced optical rectification', made possible by the lack of inversion symmetry at the graphene on gold surface, is responsible for the strongly enhanced emission. The enhancement occurs because the electric field of surface plasmons is localized and enhanced where the graphene is located: at the surface of the metal. We believe that our results point the way to small, thin, and more efficient terahertz photonic devices.

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.

THz near-field Faraday imaging in hybrid metamaterials.

We report on direct measurements of the magnetic near-field of metamaterial split ring resonators at terahertz frequencies using a magnetic field sensitive material. Specifically, planar split ring resonators are fabricated on a single magneto-optically active terbium gallium garnet crystal. Normally incident terahertz radiation couples to the resonator inducing a magnetic dipole oscillating perpendicular to the crystal surface. Faraday rotation of the polarisation of a near-infrared probe beam directly measures the magnetic near-field with 100 femtosecond temporal resolution and (λ/200) spatial resolution. Numerical simulations suggest that the magnetic field can be enhanced in the plane of the resonator by as much as a factor of 200 compared to the incident field strength. Our results provide a route towards hybrid devices for dynamic magneto-active control of light such as isolators, and highlight the utility of split ring resonators as compact probes of magnetic phenomena in condensed matter.

Surface plasmon-enhanced terahertz emission from a hemicyanine self-assembled monolayer.

Emission of terahertz radiation is observed when surface plasmons are excited on a thin film of gold, in the Kretschmann geometry. When a hemicyanine-terminated alkanethiol self-assembled monolayer of thickness 1.2 nm is deposited on the gold film, stronger terahertz emission is observed. Our experimental results confirm that enhanced terahertz emission is possible from planar gold surfaces when surface plasmons are excited.

Percolation-enhanced generation of terahertz pulses by optical rectification on ultrathin gold films.

Emission of pulses of electromagnetic radiation in the terahertz range is observed when ultrathin gold films on glass are illuminated with femtosecond near-IR laser pulses. A distinct maximum is observed in the emitted terahertz amplitude from films of average thickness just above the percolation threshold. Our measurements suggest that the emission is through a second-order nonlinear optical rectification process, enhanced by the excitation of localized surface plasmon hot spots on the percolated metal film.

Terahertz spectroscopy to identify the polymorphs in freeze-dried Mannitol.

We show how terahertz time-domain spectroscopy (THz-TDS) in the range from 0.1 to 7.5 THz can be used to identify the polymorphs of Mannitol, a frequently used excipient in the freeze drying industry. The results are subsequently used to study the effect that different freeze drying techniques have on the formation of these polymorphs. We find that, depending on the freeze-drying technique, the Mannitol either crystallizes in the delta form, or in a mixture of both the delta form and the beta form. The results are in agreement with conventional X-ray diffraction measurements used to identify the polymorphs.

Terahertz generation from graphite.

Generation of subpicosecond terahertz pulses is observed when graphite surfaces are illuminated with femtosecond near-infrared laser pulses. The nonlinear optical generation of THz pulses from graphite is unexpected since, in principle, the material possesses a centre of inversion symmetry. Experiments with highly oriented pyrolytic graphite crystals suggest that the THz radiation is generated by a transient photocurrent in a direction normal to the graphene planes, along the c-axis of the crystal. This is supported by magnetic-field induced changes in the THz electric-field polarization, and consequently, the direction of the photocurrent. We show that other forms of graphite, such as a pencil drawing on paper, are also capable of emitting THz pulses.

Bendable, low-loss Topas fibers for the terahertz frequency range.

We report on a new class of polymer photonic crystal fibers for low-loss guidance of THz radiation. The use of the cyclic olefin copolymer Topas, in combination with advanced fabrication technology, results in bendable THz fibers with unprecedented low loss and low material dispersion in the THz regime.We demonstrate experimentally how the dispersion may be engineered by fabricating both high- and low-dispersion fibers with zero-dispersion frequency in the regime 0.5-0.6 THz. Near-field, frequency-resolved characterization with high spatial resolution of the amplitude and phase of the modal structure proves that the fiber is single-moded over a wide frequency range, and we see the onset of higher-order modes at high frequencies as well as indication of microporous guiding at low frequencies and high porosity of the fiber. Transmission spectroscopy demonstrates low-loss propagation (< 0.1 dB/cm loss at 0.6 THz) over a wide frequency range.

TeraHertz imaging of hidden paint layers on canvas.

We show terahertz reflection images of hidden paint layers in a painting on canvas and compare the results with X-ray Radiography and In-frared Reflectography. Our terahertz measurements show strong reflections from both the canvas/paint interface and from the raw umber/lead white interface, indicating sufficient refractive-index contrast. Our results show that X-rays cannot be used to image through the lead white pigment which effectively blocks the X-rays. Although Infrared Reflectography is capable of vaguely observing the hidden paint strokes from the canvas side, we show that only terahertz imaging is capable of providing information on the thickness of the hidden paint layers. Terahertz imaging is thus shown to be a powerful imaging method for art historians, conservators and conservation scientists.

Quasi-near field terahertz generation and detection.

We describe a simple terahertz (THz) time domain spectrometer with a bandwidth extending up to 7.5 THz. We show that by keeping the generation and detection crystals close to each other a high signal-to-noise ratio (SNR) can be achieved without using lock-in detection and dry nitrogen flushing. The observed spectra show very good agreement with the spectra calculated based on a simple model which includes phase matching and absorption in the generation and detection crystals. Using this set-up we have measured the absorption lines in D-tartaric acid from 0.5 THz up to 7 THz. We show that the high frequency region > 3 THz is the better choice to measure small changes in the water content of a hygroscopic sample compared to the low frequency region.

Frequency-dependent radiation patterns emitted by THz plasmons on finite length cylindrical metal wires.

We report on the emission patterns from THz plasmons propagating towards the end of cylindrical metal waveguides. Such waveguides exhibit low loss and dispersion, but little is known about the dynamics of the terahertz radiation at the end of the waveguide, specifically in the near- and intermediate-field. Our experimental results and numerical simulations show that the near- and intermediate-field terahertz spectra, measured at the end of the waveguide, vary with the position relative to the waveguide. This is explained by the frequency-dependent diffraction occurring at the end of the cylindrical waveguide. Our results show that near-field changes in the frequency content of THz pulses for increasing wire-detector distances must be taken into account when studying surface waves on cylindrical waveguides.

Terahertz polarization imaging.

We present a new method to measure the polarization state of a terahertz pulse by using a modified electro-optic sampling setup. To illustrate the power of this method, we show two examples in which the knowledge of the polarization of the terahertz pulse is essential for interpreting the results: spectroscopy measurements on polystyrene foam and terahertz images of a plastic coin. Both measurements show a sample-induced rotation of the terahertz electric field vector, which is surprisingly large and is a strong function of frequency. A promising aspect of our setup is the possibility of simultaneously measuring both transversal electric field components.