Mikael Siltanen

Strong enhancement of second-harmonic generation in all-dielectric resonant waveguide grating

The authors demonstrate that a dielectric resonant waveguide grating can enhance optical second-harmonic generation by a factor of 550 compared to a similar flat surface. Their structure, which consists of purely dielectric and thereby transparent materials, has a low index silicon dioxide grating covered by a high index titanium dioxide layer and it is optimized for the fundamental wavelength of 1064nm. Polarization dependence of the second-harmonic response suggests that the enhancement arises from the favorable interaction of the resonant waveguide mode and its strong local field with the inherent surface nonlinearity of the dielectric materials.

A regression technique to analyze the second-order nonlinear optical response of thin films.

We present a new technique, based on regression analysis, to determine the second-order nonlinear optical susceptibility tensor of thin films. The technique does not require the absolute levels or phases of measured signals to be mutually calibrated. In addition it yields indicators that address the quality of theoretical models describing the sample. We use the technique to determine the susceptibility tensor of samples of a nonracemic chiral material which have very low symmetry (both chiral and anisotropic) and have many independent tensor components. The results show the importance of using detailed theoretical models that account for the linear optical properties of the sample.

Giant enhancement of second-harmonic generation in multiple diffraction orders from sub-wavelength resonant waveguide grating

We demonstrate a purely dielectric resonant waveguide structure that enhances the efficiency of second-harmonic generation by a factor of at least 5500 compared to a flat reference surface in the same geometry. We also show that the structure emits second-harmonic radiation in four different directions when the sample is illuminated with fundamental radiation incident at the resonant angle of the sample.

Grating-cavity continuous-wave optical parametric oscillators for high-resolution mid-infrared spectroscopy

The use of grating as a spectral filter provides a simple way of improving wavelength tuning and stability of continuous-wave optical parametric oscillators (cw OPOs). In this paper, we discuss how to design and use such grating-cavity cw OPOs for high-resolution spectroscopy in the molecular fingerprint region at ∼3μm. The first design presented in the paper is based on a metal-coated diffraction grating, which produces fast and broad wavelength tuning and high wavelength stability. The second design uses a bulk Bragg grating for high optical power and good spectral purity. We report a new Bragg-grating OPO and demonstrate its use in a Doppler-free absorption spectroscopy of CH4 at ∼3.22μm. In addition, we describe a new balanced detection scheme, which can be used to improve the signal-to-noise ratio of absorption measurements if the measurement noise is limited by the intensity noise of the mid-infrared OPO.

Pump-tunable continuous-wave singly resonant optical parametric oscillator from 2.5 to 4.4 µm

We report a continuous-wave singly resonant optical parametric oscillator pumped by a widely tunable titanium-doped sapphire ring laser. It produces up to 0.8 W of mid-infrared power. The wavelength can be tuned in a few seconds from 2.5 to 3.5 microm or from 3.4 to 4.4 microm and scanned up to 40 GHz without mode-hops by only changing the pump beam wavelength. Spectroscopic capability is demonstrated by measuring parts of the photoacoustic absorption spectrum of NH(3) near 3196 cm(-1).

High sensitivity trace gas detection by cantilever-enhanced photoacoustic spectroscopy using a mid-infrared continuous-wave optical parametric oscillator

Highly sensitive cantilever-enhanced photoacoustic detection of hydrogen cyanide and methane in the mid-infrared region is demonstrated. A mid-infrared continuous-wave frequency tunable optical parametric oscillator was used as a light source in the experimental setup. Noise equivalent detection limits of 190 ppt (1 s) and 65 ppt (30 s) were achieved for HCN and CH(4), respectively. The normalized noise equivalent absorption coefficient is 1.8 × 10(-9) W cm(-1) Hz(-1/2).

Continuous-wave optical parametric oscillator based on a Bragg grating

A single-mode cw optical parametric oscillator (OPO) based on a bulk Bragg grating is reported. The OPO is capable of producing multiwatt output power both at the signal (1535 nm) and idler (3468 nm) wavelengths with a side-mode suppression ratio better than 29 dB. Coarse frequency tuning of 120 GHz(4 cm(-1)) and fast mode-hop-free tuning of 40 GHz(1.3 cm(-1)) have been achieved by scanning the grating temperature and the OPO pump laser, respectively.

Continuous-wave optical parametric oscillator tuned by a diffraction grating.

A new continuous-wave singly resonant optical parametric oscillator is described. It is capable of stable, widely tunable single-frequency output in the mid-infrared region. A magnesium oxide doped periodically poled LiNbO(3) crystal provides the parametric amplification. The signal beam is in resonance with a 4-mirror linear cavity, where a diffraction grating in the Littrow configuration replaces one end mirror. The output frequency is tunable by rotating the grating. No additional components are needed inside the resonator.

Nonlinear Optical and Structural Properties of Langmuir-Blodgett Films of Thiohelicenebisquinones

We provide a detailed investigation of the second-order nonlinear optical and structural properties of Langmuir-Blodgett (LB) films of nonracemic thiohelicenebisquinone (THBQ). We prepare both X- and Y-type films of different thicknesses and characterize them using optical second-harmonic generation and atomic-force microscopy (AFM). We find that the overall nonlinear properties of the samples are essentially independent of the film thickness and the deposition type and arise from susceptibility tensor components associated with chirality. Both X- and Y-type films can be described by D2 symmetry, which is a higher symmetry than the previously assumed C2 of LB films of THBQ and a similar helicenebisquinone (HBQ). However, the two types of films are shown to differ significantly with respect to the orientation of the in-plane axis. For Y type, the axis follows the direction of vertical sample deposition, but for X type, the direction of the axis varies randomly and significantly between different samples. The Y-type samples are therefore more ordered than the X-type samples. This was confirmed by AFM measurements in which the Y type exhibits uniform ordering into columnar structures. Similar structures in X type, on the other hand, are shorter and more randomly oriented, like those earlier observed for racemic samples of HBQ [Verbiest, T., et al. Science 1998, 282, 913]. The common nonlinear properties and different high-level ordering observed here for two different types of nonracemic samples reinforces that the nonlinearity of THBQ (and probably HBQ, as well) originates from the low-level columnar aggregation of the molecules with the higher-level structures playing a lesser role. In addition, within the columns, the molecules likely assume fairly random azimuthal orientations so that the columns themselves exhibit approximate Dinfinity symmetry.

Experimental observation and analysis of the 3ν1(Σg) stretching vibrational state of acetylene using continuous-wave infrared stimulated emission

We present a sensitive experimental method for molecular spectroscopy that can be used to determine ro-vibrational states using mid-infrared stimulated emission. Our infrared stimulated emission probing (IRSEP) experiment is based on using a narrow-line, continuous-wave Ti:sapphire laser beam (pump) to excite the molecules to an upper vibrational state and a continuous-wave, mid-infrared beam from an optical parametric oscillator (probe) to detect the stimulated emission by the excited molecules. Spectroscopic data are gathered by tuning the wavelengths of the beams. The molecules are probed before their velocity distribution is disturbed by collisions, which leads to a sub-Doppler resolution. The full width at half maximum of the emission peaks is below 10 MHz. The stimulated emission lines are measured with an accuracy of at least 0.005 cm(-1). We use the IRSEP experiment to observe and analyze the symmetric ro-vibrational state [21+] (3ν1(Σg)) of acetylene (C2H2). This state is not accessible via one photon transitions from the ground vibrational state. We use the least-squares method to determine that the band center is at 9991.9725 (12) cm(-1) and the rotational parameters are B = 1.156145 (22) and D = 1.608 (87) × 10(-6) cm(-1), where the uncertainties in parentheses are one-standard errors in the least significant digit.