Intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy-efficient display technology. Here, we propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic, directed photoluminescence. Analysis of the two-dimensional k-space distribution reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters.

The optical properties of atomically thin transition metal dichalcogenide (TMDC) semiconductors are shaped by the emergence of correlated many-body complexes due to strong Coulomb interaction. Exceptional electron-hole exchange predestines TMDCs to study fundamental and applied properties of Coulomb complexes such as valley depolarization of excitons and fine-structure splitting of trions. Biexcitons in these materials are less understood and it has been established only recently that they are spectrally located between exciton and trion.Here we show that biexcitons in monolayer TMDCs exhibit a distinct fine structure on the order of meV due to electron-hole exchange. Ultrafast pump-probe experiments on monolayer WSe2 reveal decisive biexciton signatures and a fine structure in excellent agreement with a microscopic theory. We provide a pathway to access biexciton spectra with unprecedented accuracy, which is valuable beyond the class of TMDCs, and to understand even higher Coulomb complexes under the influence of electron-hole exchange.The exchange interaction between electrons and holes in semiconductors has long been a major field of interest in both theory and experiment. It has been discussed in the context of longitudinal-transverse exciton splitting in bulk materials [1, 2] and of exciton fine-structure splitting in quantum dots, where it is a major limitation to the generation of entangled photons [3,4]. Biexcitons, also known as exciton molecules, are fundamental to the nonlinear optical response of semiconductors. Based on a many-body theory including biexcitonic effects, nonlinear pump-probe experiments in InGaAs quantum wells have been successfully described [5,6] without taking electron-hole exchange into account. On the other hand, several authors have recognized the relevance of electronhole exchange for biexcitons in materials like CuCl and CdS [7][8][9][10][11][12][13]. Although in some cases significant corrections of biexciton binding energies due to electron-hole exchange have been reported, a fine-structure splitting of biexcitons has not been discussed.Atomically thin TMDC semiconductors offer new possibilities for studying Coulomb correlation effects by introducing reciprocal-space valleys as a new degree of freedom selectively addressable by circularly polarized light. Electron-hole exchange has strong implications for the valley dynamics and is currently an object of intense research [14][15][16][17]. The theory of electron-hole exchange in TMDC semiconductors has been put on a microscopic basis by Qiu et al. [18], who discussed the resulting nonanalytic exciton dispersion and strong splitting between bright and dark excitons. Similarly strong effects are observed in the fine-structure splitting of inter-and intravalley trions in these materials [19][20][21]. It is worth speculating whether biexcitons could also have a fine structure with implications for nonlinear optical effects utilized in coherent control schemes [22] and for manipulation of exciton spin coherences [23]. However, even...

We show that CdSe nanoplatelets are a model system to investigate the tunability of trions and excitons in laterally finite 2D semiconductors.

We investigate valley dynamics associated with trions in monolayer tungsten diselenide (WSe2) using polarization resolved two-color pump-probe spectroscopy. When tuning the pump and probe energy across the trion resonance, distinct trion valley polarization dynamics are observed as a function of energy and attributed to the intra-valley and inter-valley trions in monolayer WSe2. We observe no decay of a near-unity valley polarization associated with the intra-valley trions during ~ 25 ps, while the valley polarization of the inter-valley trions exhibits a fast decay of ~ 4 ps. Furthermore, we show that resonant excitation is a prerequisite for observing the long-lived valley polarization associated with the intra-valley trion. The exceptionally robust valley polarization associated with resonantly created intra-valley trions discovered here may be explored for future valleytronic applications such as valley Hall effects.Keywords: atomically thin semiconductors, valley, trions, ultrafast dynamics PACS:The valley degree of freedom (DoF) indexes the crystal momentum of a local energy minimum within the electronic band structure, and has been proposed as an alternative information carrier, analogous to charge and spin [1]. In atomically thin transition metal dichalcogenides (TMDs), fundamental optical excitations, excitons (electron-hole pairs) and trions (charged excitons), are formed at the hexagonal Brillouin zone boundaries at the ( ′) points. As such, they inherit the valley index which is locked with electron spins in TMDs. Thus, exciton and trion resonances allow optical access and manipulation of the valley DoF in TMDs using circularly polarized light [2][3][4][5][6]. The exceptionally large binding energies of these quasiparticles (i.e. 200-500 meV for excitons and an additional binding energy of 20-40 meV for trions) further promise room temperature valleytronic applications [2,3,[7][8][9][10][11][12][13].High efficiency valley initialization and a long lifetime of valley polarization are preferred in valleytronic applications [14][15][16][17]. Initial experiments based on steadystate photoluminescence have shown the possibility of creating a near-unity valley polarization in MoS2 and WSe2 via exciton resonances [4,18]. Time-resolved measurements soon revealed that exciton valley polarization is quickly lost (~ 1 ps) due to intrinsic electron-hole exchange interaction [19]. The large initial exciton valley polarization observed in the steady-state PL results from the competition between the valley depolarization time (~ 1 ps) and the exciton population relaxation time (~ 100-200 fs) [13,20,21]. On the other hand, trions offer an interesting alternative route for optical manipulation of the valley index for a number of reasons. First, in contrast to the ultrafast exciton population relaxation time, trions exhibit an extended population relaxation time of tens of picoseconds in monolayer TMDs [22][23][24][25][26][27][28][29][30]. Secondly, trions as charged quasiparticles influence both transport and optical...

Optimized second-harmonic generation (SHG) in quantum cascade (QC) lasers with specially designed active regions is reported. Nonlinear optical cascades of resonantly coupled intersubband transitions with giant second-order nonlinearities were integrated with each QC-laser active region. QC lasers with three-coupled quantum-well (QW) active regions showed up to 2 W of SHG light at 3.75 m wavelength at a fundamental peak power and wavelength of 1 W and 7.5 m, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 W/W 2. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 and 4.55 m, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 W/W 2. Full theoretical treatment of nonlinear light generation in QC lasers is given, and excellent agreement with the experimental results is obtained. For the best structure, a second-order nonlinear susceptibility of 4 7 10 5 (2 10 4 pm/V) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.