Vibrational optical activity: From discovery and development to future challenges.

The electronic structure of multiexcitons significantly impacts the performance of nanostructures in lasing and light-emitting applications. However, these multiexcitons remain poorly understood due to their complexity arising from many-body physics. Standard transient-absorption and photoluminescence spectroscopies are unable to unambiguously distinguish effects of sample inhomogeneity from exciton–biexciton interactions. Here, we exploit the energy and time resolution of two-dimensional electronic spectroscopy to access the electronic structure of the band-edge biexciton in colloidal CdSe quantum dots. By removing effects of inhomogeneities, we show that the band-edge biexciton structure must consist of a discrete manifold of electronic states. Furthermore, the biexciton states within the manifold feature distinctive binding energies. Our findings have direct implications for optical gain thresholds and efficiency droop in light-emitting devices and provide experimental measures of many-body physics in ...

Seeing Multiexcitons through Sample Inhomogeneity: Band-Edge Biexciton Structure in CdSe Nanocrystals Revealed by Two-Dimensional Electronic Spectroscopy.

We introduce a high-performance hyperspectral camera based on the Fourier-transform approach, where the two delayed images are generated by the Translating-Wedge-Based Identical Pulses eNcoding System (TWINS) [Opt. Lett.37, 3027 (2012)10.1364/OL.37.003027], a common-path birefringent interferometer that combines compactness, intrinsic interferometric delay precision, long-term stability and insensitivity to vibrations. In our imaging system, TWINS is employed as a time-scanning interferometer and generates high-contrast interferograms at the single-pixel level. The camera exhibits high throughput and provides hyperspectral images with spectral background level of −30dB and resolution of 3 THz in the visible spectral range. We show high-quality spectral measurements of absolute reflectance, fluorescence and transmission of artistic objects with various lateral sizes.

https://re.public.polimi.it/bitstream/11311/1095781/1/96%20-%20Perri%20Opt%20Express%202019%20%28Hyperspectral%20imaging%20with%20a%20TWINS%20birefringent%20interferometer%29.pdf

Hyperspectral imaging with a TWINS birefringent interferometer.

Enhanced charge separation of CuS and CdS quantum-dot-cosensitized porous TiO 2 -based photoanodes for photoelectrochemical water splitting

Recently, it was demonstrated that charge separation in hybrid metal-semiconductor nanoparticles (HNPs) can be obtained following photoexcitation of either the semiconductor or of the localized surface plasmon resonance (LSPR) of the metal. This suggests the intriguing possibility of photocatalytic systems benefiting from both plasmon and exciton excitation, the main challenge being to outcompete other ultrafast relaxation processes. Here we study CdSe-Au HNPs using ultrafast spectroscopy with high temporal resolution. We describe the complete pathways of electron transfer for both semiconductor and LSPR excitation. In the former, we distinguish hot and band gap electron transfer processes in the first few hundred fs. Excitation of the LSPR reveals an ultrafast (<30 fs) electron transfer to CdSe, followed by back-transfer from the semiconductor to the metal within 210 fs. This study establishes the requirements for utilization of the combined excitonic-plasmonic contribution in HNPs for diverse photocatalytic applications.

Visualizing Ultrafast Electron Transfer Processes in Semiconductor–Metal Hybrid Nanoparticles: Toward Excitonic–Plasmonic Light Harvesting

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.

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

Exciton relaxation dynamics of CdSe and quasi-type-II CdSe/CdS core/shell nanocrystals were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The use of 2DES allowed for determination of structure-specific and state-resolved carrier dynamics for CdSe nanocrystals formed with five, or fewer, CdS passivation monolayers (ML). For CdSe and CdSe/CdS nanocrystals formed with one through three MLs of CdS, excitation using broad bandwidth femtosecond visible laser pulses generated electron–hole pairs among the |X1〉 = 2.14 eV and |X2〉 = 2.27 eV exciton states. For both excitations, the electron is promoted to the lowest energy excited (1Se) conduction-band state and the hole is in the 1S3/2 (X1) or 2S3/2 (X2) valence-band state. Therefore, the relaxation dynamics of the hot hole were isolated by monitoring the-time-dependent amplitude of 2DES cross peaks. The time constant for hot hole relaxation within the CdSe valence band was 150 ± 45 fs. Upon passivation by CdS, this hole relaxation time constant increased to 170 ± 30 fs (CdSe/CdS-3ML). This small increase was attributed to the formation of a graded, or alloyed, interfacial region that precedes the growth of a uniform CdS capping layer. The small increase in hole relaxation time reflects the larger nanocrystal volume of the CdSe/CdS system with respect to the CdSe nanocrystal core. In contrast, the dynamics of larger core/shell nanocrystals (≥4ML CdS) exhibited a picosecond buildup in 2DES cross-peak amplitude. This time-dependent response was attributed to interfacial hole transfer from CdS to CdSe valence-band states. Importantly, the 2DES data distinguish CdSe exciton relaxation from interfacial carrier transfer dynamics. In combination, isolation of structurally well-defined nanocrystals and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices.

Dissecting charge relaxation pathways in CdSe/CdS nanocrystals using femtosecond two-dimensional electronic spectroscopy

Fourier transform (FT) spectroscopy [1] is a powerful technique to measure spectra in the time domain. With respect to frequency-domain spectrometers, FT spectrometers have the advantage of higher signal to noise ratio and optical throughput, and the possibility to adjust the frequency resolution. FT spectroscopy requires to generate two field replicas whose delay must be scanned with accuracy of a small fraction of the optical cycle. Typically the replicas are generated by a Michelson interferometer in which the stability of the delay requires very robust mechanical setups and optical feedbacks. This difficulty is overcome by birefringent interferometers [3], in which two cross-polarized field replicas travel a common optical path and hence have high long-term stability. FT spectroscopy is particularly advantageous in the mid-infrared (mid-IR) from 5 to 10 μm (corresponding to 1000–2000 cm−1), the so-called fingerprint region [2], where array detectors for spectrometers are noisy and very expensive. We recently demonstrated a birefringent interferometer for FT spectroscopy [4], based on the Translating-Wedge-based Identical pulses eNcoding System (TWINS) [5] with alpha barium borate and lithium niobate. However the transparency of these crystals does not extend to the mid-IR fingerprint region.

Linear and nonlinear fourier-transform spectroscopy in the vibrational fingerprint region with a birefringent interferometer

Continuous progress in the development of light sources for the generation of tunable ultrashort pulses from the near-infrared to all the visible range provides a very efficient tool for spectroscopic techniques able to follow ultrafast dynamical processes in matter. Second-order nonlinear optical processes like Optical Parametric Amplification (OPA) have demonstrated the capability of generating light pulses with durations down to few cycles of the carrier wave. The aim of this paper is to introduce the basic concepts for the generation of tunable ultrashort pulses and show their application to ultrafast spectroscopy. We will discuss the principles of parametric amplification and the main criteria for the design of broadband OPAs; we will also illustrate some of the schemes providing ultrashort pulses in the spectral ranges from the ultraviolet to the infrared, offering a comprehensive overview of the state of the art of the current research activity in this rapidly evolving field. An important requirement for the applications of ultrashort pulses is the capability to measure the amplitude and phase of their electric field; we will discuss a rich ensemble of spectrographic, interferometric or phase-scanning techniques for the temporal characterization of ultrashort pulses. Pump-probe techniques with broadband pulses and more recently two-dimensional spectroscopic techniques allow to follow in great detail complex dynamical processes. The few examples reported in this review just provide a sample of what is already a vast class of ultrafast phenomena, which can be studied nowadays in real time.

A. Oriana

Papers

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

Dissecting charge relaxation pathways in CdSe/CdS nanocrystals using femtosecond two-dimensional electronic spectroscopy

Linear and nonlinear fourier-transform spectroscopy in the vibrational fingerprint region with a birefringent interferometer

Tunable few-optical cycle pulses and advanced ultrafast spectroscopic techniques