Dielectric properties of semi-insulating Fe-doped InP in the terahertz spectral region.

We report the values and the spectral dependence of the real and imaginary parts of the dielectric permittivity of semi-insulating Fe-doped InP crystalline wafers in the 2-700 cm-1 (0.06-21 THz) spectral region at room temperature. The data shows a number of absorption bands that are assigned to one- and two-phonon and impurity-related absorption processes. Unlike the previous studies of undoped or low-doped InP material, our data unveil the dielectric properties of InP that are not screened by strong free-carrier absorption and will be useful for designing a wide variety of InP-based electronic and photonic devices operating in the terahertz spectral range.

Experimental demonstration of the microscopic origin of circular dichroism in two-dimensional metamaterials.

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space. While chiral optical properties are weak in most of naturally occurring materials, they can be engineered and significantly enhanced in synthetic optical media known as chiral metamaterials, where the spatial symmetry of their building blocks is broken on a nanoscale. Although originally discovered in 3D structures, circular dichroism can also emerge in a two-dimensional (2D) metasurface. The origin of the resulting circular dichroism is rather subtle, and is related to non-radiative (Ohmic) dissipation of the constituent metamolecules. Because such dissipation occurs on a nanoscale, this effect has never been experimentally probed and visualized. Using a suite of recently developed nanoscale-measurement tools, we establish that the circular dichroism in a nanostructured metasurface occurs due to handedness-dependent Ohmic heating.

Twisted optical metamaterials for planarized ultrathin broadband circular polarizers.

Optical metamaterials are usually based on planarized, complex-shaped, resonant nano-inclusions. Three-dimensional geometries may provide a wider set of functionalities, including broadband chirality to manipulate circular polarization at the nanoscale, but their fabrication becomes challenging as their dimensions get smaller. Here we introduce a new paradigm for the realization of optical metamaterials, showing that three-dimensional effects may be obtained without complicated inclusions, but instead by tailoring the relative orientation within the lattice. We apply this concept to realize planarized, broadband bianisotropic metamaterials as stacked nanorod arrays with a tailored rotational twist. Because of the coupling among closely spaced twisted plasmonic metasurfaces, metamaterials realized with conventional lithography may effectively operate as three-dimensional helical structures with broadband bianisotropic optical response. The proposed concept is also shown to relax alignment requirements common in three-dimensional metamaterial designs. The realized sample constitutes an ultrathin, broadband circular polarizer that may be directly integrated within nanophotonic systems.

Dynamics of actively mode-locked Quantum Cascade Lasers.

The impact of upper state lifetime and spatial hole burning on pulse shape and stability in actively mode locked QCLs is investigated by numerical simulations. It is shown that an extended upper state lifetime is necessary to achieve stable isolated pulse formation per roundtrip. Spatial hole burning helps to reduce the pulse duration by supporting broadband multimode lasing, but introduces pulse instabilities which eventually lead to strongly structured pulse shapes that further degrade with increased pumping. At high pumping levels gain saturation and recovery between pulses leads to suppression of mode locking. In the absence of spatial hole burning the laser approaches single-mode lasing, while in the presence of spatial hole burning the mode locking becomes unstable and the laser dynamics does not reach a steady state anymore.

Mode-locked pulses from mid-infrared quantum cascade lasers.

In this study, we report the unequivocal demonstration of midinfrared mode-locked pulses from quantum cascade lasers. The train of short pulses was generated by actively modulating the current and hence the gain of an edge-emitting quantum cascade laser (QCL). Pulses with duration of about 3 ps at full-width-at-half-maxima and energy of 0.5 pJ were characterized using a second-order interferometric autocorrelation technique based on a nonlinear quantum well infrared photodetector. The mode-locking dynamics in the QCLs was modeled based on the Maxwell-Bloch equations in an open two-level system. Our model reproduces the overall shape of the measured autocorrelation traces and predicts that the short pulses are accompanied by substantial wings as a result of strong spatial hole burning. The range of parameters where short mode-locked pulses can be formed is found.

Sum-frequency vibrational spectroscopy of chiral liquids off and close to electronic resonance and the antisymmetric Raman tensor.

The strength of the chiral vibrational peaks in infrared-visible sum-frequency (SF) vibrational spectra from isotropic chiral liquids is proportional to the square of the corresponding antisymmetric Raman element. Under the Born-Oppenheimer adiabatic approximation with nonadiabatic corrections, the antisymmetric Raman tensor is much weaker than the symmetric counterpart, but becomes significantly stronger as the input frequency (or the sum-frequency in SF generation) approaches electronic resonance. We verify the theory with experimental results obtained from infrared-visible doubly resonant sum-frequency generation from an isotropic solution of chiral molecules.

Optically active second-harmonic generation from a uniaxial fluid medium.

We have shown that optically active second-harmonic generation is allowed in a uniaxial fluid medium. A homeotropically aligned chiral smectic-A liquid crystal was used as an example. Phase matching was achievable by angle tuning. Chiral nonlinear susceptibility for the liquid crystal was deduced. The signal dropped precipitously as the sample underwent the transition from smectic-A to isotropic.

Sum-frequency vibrational spectroscopy of a helically structured conjugated polymer.

Sum-frequency vibrational spectroscopy was used as a novel technique to probe the molecular chirality of thin polymer films. Chiral vibrational spectra of poly(bithienylene-phenylene) were obtained, and the two enantiomers were distinguished by an interference method. Vibration-electronic double resonance was responsible for the observation of unusually strong chiral spectra of the phenylene vibration modes.

Doubly resonant IR-UV sum-frequency vibrational spectroscopy on molecular chirality.

We show theoretically and experimentally that for sum-frequency vibrational spectroscopy near electronic transitions, resonant enhancement of the chiral response can be much stronger than that of the achiral response. The doubly resonant spectrum selectively enhances the vibrational modes through their different electron-vibration couplings. The unusually strong resonant enhancement significantly improves sensitivity of chiral spectroscopy and allows detection of the chiral vibrational spectrum of a molecular monolayer for the first time.

Sum-frequency generation in chiral liquids near electronic resonance.

We demonstrate experimentally that visible-visible sum-frequency generation in the bulk of a chiral liquid is observable near electronic resonant transitions. Although the process is electric dipole allowed, it is rather weak because the orientational average over molecules effectively reduces the bulk chiral nonlinearity.

Sum-frequency vibrational spectroscopy on chiral liquids: a novel technique to probe molecular chirality.

Optical activity in sum-frequency vibrational spectra has been observed for the first time in chiral liquids. The electric-dipole allowed chiral element of the nonlinear susceptibility appears to be 3 orders of magnitude smaller than typical allowed achiral elements. This is partly because the observed chirality requires a breakdown of the Born-Oppenheimer approximation.