Raman scattering is a ubiquitous phenomenon in light-matter interactions, which reveals a material's electronic, structural, and thermal properties. Controlling this process would enable new ways of studying and manipulating fundamental material properties. Here, we report a novel Raman scattering process at the interface between different van der Waals (vdW) materials as well as between a monolayer semiconductor and 3D crystalline substrates. We find that interfacing a WSe monolayer with materials such as SiO, sapphire, and hexagonal boron nitride (hBN) enables Raman transitions with phonons that are either traditionally inactive or weak. This Raman scattering can be amplified by nearly 2 orders of magnitude when a foreign phonon mode is resonantly coupled to the A exciton in WSe directly or via an A optical phonon from WSe. We further showed that the interfacial Raman scattering is distinct between hBN-encapsulated and hBN-sandwiched WSe sample geometries. This cross-platform electron-phonon coupling, as well as the sensitivity of 2D excitons to their phononic environments, will prove important in the understanding and engineering of optoelectronic devices based on vdW heterostructures.
In monolayer semiconductor transition metal dichalcogenides, the exciton-phonon interaction strongly affects the photocarrier dynamics. Here, we report on an unusual oscillatory enhancement of the neutral exciton photoluminescence with the excitation laser frequency in monolayer MoSe 2 . The frequency of oscillation matches that of the M-point longitudinal acoustic phonon, LA(M), suggesting the significance of zone-edge acoustic phonons and hence the deformation potential in exciton-phonon coupling in MoSe 2 . Moreover, oscillatory behavior is observed in the steady-state emission linewidth and in time-resolved PLE data, which reveals variation with excitation energy in the exciton lifetime. These results clearly expose the key role played by phonons in the exciton formation and relaxation dynamics of two-dimensional van der Waals semiconductors.
We demonstrate the suppression of nuclear spin fluctuations in an InAs quantum dot and measure the timescales of the spin narrowing effect. By initializing for tens of milliseconds with two continuous wave diode lasers, fluctuations of the nuclear spins are suppressed via the hole assisted dynamic nuclear polarization feedback mechanism. The fluctuation narrowed state persists in the dark (absent light illumination) for well over one second even in the presence of a varying electron charge and spin polarization. Enhancement of the electron spin coherence time (T2*) is directly measured using coherent dark state spectroscopy. By separating the calming of the nuclear spins in time from the spin qubit operations, this method is much simpler than the spin echo coherence recovery or dynamic decoupling schemes.Single electrons trapped inside self assembled quantum dots form a well defined and optically accessible qubit, and are featured as the central element of many proposed quantum logic devices [1][2][3][4][5][6][7]. However, the electron spin in a quantum dot is coupled to many (10 4 − 10 5 ) nuclear spins, primarily via the Fermi contact hyperfine interaction, whose fluctuations form the main contribution to electron spin dephasing at cryogenic temperatures [8][9][10][11]. Thus, there has been considerable interest in suppressing electron spin dephasing by manipulating the nuclear spin ensemble [12][13][14][15][16][17][18][19][20][21][22], and recent results have shown that it is possible to protect the coherence of an ensemble of dots [23,24] and even recover the electron spin coherence in a single quantum dot [25].In this Letter, we use the hole assisted nuclear feedback mechanism [22,26] to demonstrate the reproducible preparation of the nuclear magnetic field (Overhauser field) to a fluctuation suppressed state, considerably enhancing the electron spin coherence. This nuclear spin narrowing (NSN) [12][13][14][15][16][17][18][19][20][21][22] is accomplished without the creation of large nuclear spin polarizations. The spin narrowed state can be prepared in tens of milliseconds, and persists for well over a second even in the presence of a fluctuating electron charge and spin [27,28]. We directly measure the enhanced T2* using coherent dark state spectroscopy.The sample is an InAs self assembled quantum dot embedded in a Schottky diode structure. A DC bias voltage applied across the sample charges the dot with a single electron and Stark shift modulation spectroscopy a large amplitude AC component (0.08VAC) at 3.5Khz directly measure the absorption spectrum [29]. Applying a 2.64T magnetic field in the Voigt profile (perpendicular to the growth axis) turns on spin flip Raman transitions between the spin ground states (|X± >) and the charged exciton (trion) states (|T ± >) (Fig. 1a). We selectively excite a three-level lambda (Λ) subsystem (dashed outline in Fig. 1a) with narrow linewidth continuous wave lasers (Fig. 1b). The lasers are passed through acoustooptical shutters to individually gate the lasers on and...
Monolayer transition-metal dichalcogenides (TMDs) are the first truly two-dimensional (2D) semiconductor, providing an excellent platform to investigate light–matter interaction in the 2D limit. The inherently strong excitonic response in monolayer TMDs can be further enhanced by exploiting the temporal confinement of light in nanophotonic structures. Here, we demonstrate a 2D exciton–polariton system by strongly coupling atomically thin tungsten diselenide (WSe2) monolayer to a silicon nitride (SiN) metasurface. Via energy-momentum spectroscopy of the WSe2-metasurface system, we observed the characteristic anticrossing of the polariton dispersion both in the reflection and photoluminescence spectrum. A Rabi splitting of 18 meV was observed which matched well with our numerical simulation. Moreover, we showed that the Rabi splitting, the polariton dispersion, and the far-field emission pattern could be tailored with subwavelength-scale engineering of the optical meta-atoms. Our platform thus opens the door for the future development of novel, exotic exciton–polariton devices by advanced meta-optical engineering.
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