Non equilibrium optical properties in semiconductors from first--principles: a combined theoretical and experimental study of bulk silicon

The calculation of the equilibrium optical properties of bulk silicon by using the Bethe--Salpeter equation solved in the Kohn--Sham basis represents a cornerstone in the development of an ab--initio approach to the optical and electronic properties of materials. Nevertheless calculations of the {\em transient} optical spectrum using the same efficient and successful scheme are scarce. We report, here, a joint theoretical and experimental study of the transient reflectivity spectrum of bulk silicon. Femtosecond transient reflectivity is compared to a parameter--free calculation based on the non--equilibrium Bethe--Salpeter equation. By providing an accurate description of the experimental results we disclose the different phenomena that determine the transient optical response of a semiconductor. We give a parameter--free interpretation of concepts like bleaching, photo--induced absorption and stimulated emission, beyond the Fermi golden rule. We also introduce the concept of optical gap renormalization, as a generalization of the known mechanism of band gap renormalization. The present scheme successfully describes the case of bulk silicon, showing its universality and accuracy.Comment: 14 pages, 13 figure

Two-dimensional electronic spectroscopy in the ultraviolet by a birefringent delay line

We introduce a 2D electronic spectroscopy setup in the UV spectral range in the partially collinear pump-probe geometry. The required interferometrically phase-locked few-optical-cycle UV pulse pair is generated by combining a passive birefringent interferometer in the visible and nonlinear phase transfer. This is achieved by sum-frequency generation between the phase-locked visible pulse pair and narrowband infrared pulses. We demonstrate a pair of 16-fs, 330-nm pulses whose delay is interferometrically stable with an accuracy better than λ/450. 2DUV maps of pyrene solution probed in the UV and visible spectral ranges are demonstrated

Generation of 85-fs pulses at 13 μm for ultrabroadband pump-probe spectroscopy

We report on a near-infrared non-collinear optical parametric amplifier (NOPA) based on periodically poled stoichiometric lithium tantalate. The NOPA generates mu J-energy pulses with spectrum spanning the 1-1.7 mu m wavelength range, which are compressed to nearly transform-limited 8.5 fs duration by a deformable mirror. By synchronizing this source with a sub-10-fs visible NOPA, we demonstrate an unprecedented combination of temporal resolution and spectral coverage in two-colour pump-probe spectroscopy. (C) 2009 Optical Society of Americ

Fourier transform spectroscopy in the vibrational fingerprint region with a birefringent interferometer

We introduce a birefringent interferometer for Fourier transform (FT) spectroscopy in the mid-infrared, covering the vibrational fingerprint region (5-10 μm, 1000-2000 cm-1), which is crucial for molecular identification. Our interferometer employs the crystal calomel (Hg2Cl2), which combines high birefringence (ne-no≈0.55) with a broad transparency range (0.38-20 μm). We adopt a design based on birefringent wedges, which is simple and compact and guarantees excellent delay accuracy and long-term stability. We demonstrate FTIR spectroscopy, with a frequency resolution of 3 cm-1, as well as two-dimensional IR (2DIR) spectroscopy. Our setup can be extended to other spectroscopic modalities such as vibrational circular dichroism and step-scan FT spectroscopy

Hyperspectral microscopy of two-dimensional semiconductors

Here we present an interferometric wide field hyperspectral microscope based on a common-path birefringent interferometer with translating wedges, to measure photoluminescence emission from two-dimensional semiconductors. We show diffraction-limited hyperspectral photoluminescence microscopy from two-dimensional materials across millimeter areas, proving that our hyperspectral microscope is a compact, stable and fast tool to characterize the optical properties and the morphology of 2D materials across ultralarge areas

Balanced-detection Raman-induced Kerr-effect spectroscopy

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Ultrafast Excited-State Decay Mechanisms of 6-Thioguanine Followed by Sub-20 fs UV Transient Absorption Spectroscopy

: Understanding the primary steps following UV photoexcitation in sulphur-substituted DNA bases (thiobases) is fundamental for developing new phototherapeutic drugs. However, the investigation of the excited-state dynamics in sub-100 fs time scales has been elusive until now due to technical challenges. Here, we track the ultrafast decay mechanisms that lead to the electron trapping in the triplet manifold for 6-thioguanine in an aqueous solution, using broadband transient absorption spectroscopy with a sub-20 fs temporal resolution. We obtain experimental evidence of the fast internal conversion from the S2(ππ*) to the S1(nπ*) states, which takes place in about 80 fs and demonstrates that the S1(nπ*) state acts as a doorway to the triplet population in 522 fs. Our results are supported by MS-CASPT2 calculations, predicting a planar S2(ππ*) pseudo-minimum in agreement with the stimulated emission signal observed in the experiment

Interplay between Strong Coupling and Radiative Damping of Excitons and Surface Plasmon Polaritons in Hybrid Nanostructures

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