We will present how to fabricate nanoantennas and metasurfaces in van der Waarls (vdW) materials in a variety of geometries and a range of photonic applications. We observed Mie resonances as well as strong coupling between the excitonic features and anapole modes in the vdW nanoantenna. Due to the weak vdW interactions of the nanoresonators and the substrate, we were able to use an atomic force microscopy cantilever in the repositioning of double-pillar nanoantennas to achieve ultra-small gaps of 10 nm. By employing a monolayer of WS2 as the gain material, we observe room temperature Purcell enhancement of emission as well as low temperature formation of single photon emitters with enhanced quantum efficiencies. More recently, we have also achieved bound states in the continuum ultra-low threshold lasing with these materials, highlighting the vdW materials as a promising platform for optoelectronic devices.
Two-dimensional semiconductors offer a compelling platform for excitons with robust interaction with light, owing to their confined nature and their numerous manipulable degrees of freedom. In bilayers, interlayer excitons (IX) combine these degrees of freedom with high interactions due to their out-of-plane alignment. However, their oscillator strength is often negligible. Interlayer hybridization provides IX with a significant oscillator strength. Here, we examine the ultrafast dynamics of these hybrid IX in bilayer and trilayer MoSe2. We find that IX are particularly strong in trilayers. These unexplored excitonic species exhibit fundamentally different dynamics from IX in bilayers, with delayed rise times of over 2 ps and significantly longer lifetimes. We attribute this to the origin of this excitonic species and confirm it with theory. Our findings offer insights into high oscillator strength, long-living interlayer excitons in trilayers, superior to their bilayer counterparts.
Hybridization between inter- and intralayer excitons can occur in Transition Metal Dichalcogenide (TMD) bilayers, giving rise to dipolar excitons with high oscillator strength. Such excitons can be exploited to achieve high optical nonlinearities, when TMDs are strongly coupled to light confined in optical microcavities. However, observations of TMD polaritons ultrafast temporal dynamics and their exploitation remain elusive. We performed pump-probe spectroscopy experiments at 8K in a custom-made microscope to study hBN-encapsulated monolayers and bilayers of MoS2 placed in optical microcavities. We probe the ultrafast dynamics of exciton-polaritons in such systems by resonantly exciting the cavities with femtosecond pulses and measuring the transient differential reflectivity. Our experiments revealed an ultrafast sub-picosecond switching from strong to weak coupling regime with a fast reversible recovery, and we demonstrated its high frequency operation (250 GHz) as an optical switch. The rich dynamics of TMD polaritons explored in our work give access to extreme nonlinear phenomena in TMD systems on ultrafast time scales for future optical logic gates.
While high index dielectrics and plasmonics offer many opportunities for research and techonology in the field of nanophotonics, 2D materials can expand this potential in the visible and near-infrared due to high refractive indices, a large range of transparency windows, and new fabrication possibilities due to van der Waals adhesion to any substrate. We extract dielectric constants of 11 layered materials including TMDs, III-VI semiconductors, and magnetics. We fabricate nanoantennas and observe Mie resonances as well as strong coupling of TMD excitons and anapole modes with Rabi splittings of 140 meV. We also observe room temperature Purcell enhancement of WSe2 monolayer emission and low temperature formation of single photon emitters with enhanced quantum efficiencies. Due to weak adhesion to the substrate, we employed an AFM tip in the repositioning of dimer nanoantennas to form ultra-small hotspots enabling optical trapping of quantum emitters with Purcell factors above 150.
Strong coupling between light and excitations of a two-dimensional electron gas (2DEG) are important to both pure physics and to the development of future photonic nanotechnologies. Studying the relationship between spin polarisation of a 2DEG in monolayer semiconductor MoSe2, and resultant light-matter interactions modified by a zero-dimensional optical microcavity, finds the robust spin-susceptibility of the 2DEG simultaneously enhances/supresses trion-polariton formation in opposite photon helicities. This leads to optical non-linearities arising from the highly non-linear behaviour of the valley-specific strong light-matter coupling regime and allowing all-optical tuning of the enhanced polaritonic Zeeman splitting from 4 to more than 10 meV.
https://www.nature.com/articles/s41566-022-01025-8
The locking of the electron spin to the valley degree of freedom in transition metal dichalcogenide (TMD) monolayers has seen them emerge as a promising platform in valleytronics. When embedded in optical microcavities the large oscillator strength of excitonic transitions in TMDs allows the formation of hybrid quasiparticles which are a superposition of the exciton and cavity photon states called polaritons. Here, we report the valley addressability of MoSe2 cavity polaritons under non-resonant helical excitation with a polarisation degree observed in photoluminescence that can be controlled by the exciton-cavity frequency detuning. Moreover, we observe both strong trion-cavity and exciton-cavity coupling demonstrating the formation of valley-polarised polaritons which are a linear superposition of excitons, trions and photons. In contrast to the very low circular polarisation degrees seen in MoSe2 exciton and trion resonances, we observe a significant enhancement of up to 20% when in the polaritonic regime. We further extend this work to the control of valley coherence in magnetic field in the polariton regime in WSe2 observed as rotation of linear polarisation imprinted by the pump laser. An unexpected feature here is the very different rotation angles of the linear polarisation in the lower and upper polariton branches, that is in addition controlled by the exciton-cavity-mode detuning. The effect originates from the modified exciton dynamics of the high-momentum excitons in the presence of the low-momentum polariton states.
In this work, we demonstrate semiconductor quantum dots weakly coupled to photonic crystal cavity modes operating in
the visible spectrum. We present the design, fabrication and characterization of two dimensional photonic crystal cavities
in GaInP and measure quality factors in excess of 7,500. We demonstrate control over the spontaneous emission rate of
InP quantum dots and by spectrally tuning the exciton emission energy into resonance with the fundamental cavity mode
we observe a Purcell enhancement of ~8.
An ultrafast pump-probe method based on differing polarization properties of neutral and charged excitons in
semiconductor quantum dots (QDs) is employed to study carrier dynamics in InGaAs QDs grown in nominally
undoped, modulation doped and p-i-n structures. We find that at low temperature even in the nominally undoped
samples there are large fractions of charged dots. It is also demonstrated that for bipolar electrical injection there
is a high probability of the independent capture of electrons or holes into the dots, resulting in dot charging.
Voltage-control of the charged exciton population, created via a combination of electrical and optical excitation,
which exhibits a long lived spin-polarization (or spin-memory) is demonstrated.
We report the selective area molecular beam epitaxial (SAMBE) growth of quantum dot (QD) structures. The formation of polycrystalline deposits on dielectric masks is shown to be controlled by the growth rate and growth temperature. Furthermore, we report the size, areal density and energy control of QDs in the region of the dielectric mask. We show that for SAMBE, a reduction in InAs QD size and areal density is obtained close to a polycrstal covered dielectric mask, and that this effect is dependent upon the amount of polycrystalline GaAs coverage of the mask. We attribute this effect to the transport of indium from neighboring epitaxial areas to the polycrystalline GaAs covered mask.
Non-linear carrier-photon dynamics are studied for optically pumped InAs quantum dot (QD) laser structures, using excitation into the GaAs barrier by two degenerate pump and probe laser pulses. The non-linear emission from QDs excited by the pump pulse is further amplified by the probe excitation. By varying the delay between the two pulses a very fast decay of the QD excited state emission is measured. Notably slower dynamics for the QD ground state are observed, governed by state filling phenomena, which result in gain saturation.
In semiconductor microcavities (MCs) with embedded quantum wells (QW) a strong two-dimensional (2D) confinement of the light in the growth direction leads to an enhanced exciton-photon interaction, which results in the formation of mixed exciton-photon states described in terms of quasi 2D-polaritons. The density of these states is strongly reduced compared to exciton one, due to a very small in-plane mass. As a result, one can hope that the high filling of the polariton states near the polariton band bottom can be achieved at relatively small total density without destroying the strong coupling regime. However such a filling never was reached at low excitation density range where the polariton relaxation to the polariton branch bottom is determined by the emission of acoustic phonons. The reason is in the fact that the polariton lifetime is comparable with phonon scattering time. As a result, the energy distribution of polaritons clearly demonstrates 'bottleneck effect' both under the above band gap excitation and the resonant excitation below free exciton level. However the high occupation of polariton states near the band bottom is relatively easy reached under conditions of a strong resonant excitation into the lower polariton (LP) branch at particular wavenumbers close to the inflection point of the LP dispersion. It is explained as the result of stimulated hyper-Raman scattering (or four-wave mixing) of pumped polaritons with (Ep, kp) to states with the energy and momentum [E1, k1 approximately 0] and [E2 = 2Ep - E1, k2 = 2kp], which referred to as the 'signal' and 'idler,' respectively. Here we investigate the influence of a temperature and an additional above band gap excitation on the stimulated scattering in MCs. A GaAs/AlAs MC containing 6 InGaAs quantum wells in the active layer (Rabi splitting Ω of 6 - 7 meV) has been investigated in a wide range of detunings between free exciton level and bottom of the photonic mode from δ = 0 to -3 meV. The resonance excitation into an LP branch was carried out with a tunable Ti-sapphire laser. The HeNe laser was used for the additional above GaAs band gap excitation. The sample was mounted in a helium thermostat with the temperature control at T = 5 - 30 K.
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