Sinan Karaveli

Quantifying the magnetic nature of light emission

Tremendous advances in the study of magnetic light-matter interactions have recently been achieved using man-made nanostructures that exhibit and exploit an optical magnetic response. However, naturally occurring emitters can also exhibit magnetic resonances in the form of optical-frequency magnetic-dipole transitions. Here we quantify the magnetic nature of light emission using energy- and momentum-resolved spectroscopy, and leverage a pair of spectrally close electric- and magnetic-dipole transitions in trivalent europium to probe vacuum fluctuations in the electric and magnetic fields at the nanometre scale. These results reveal a new tool for nano-optics: an atomic-size quantum emitter that interacts with the magnetic component of light.

Orientation of luminescent excitons in layered nanomaterials

In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties. Directional optical properties can be exploited to enhance the performance of optoelectronic devices, optomechanical actuators and metamaterials. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling-MoS₂ and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS₂ mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations.

Efficient Photon Collection from a Nitrogen Vacancy Center in a Circular Bullseye Grating

Luozhou Li, 2 Edward H. Chen, 2 Jiabao Zheng, Sara L. Mouradian, Florian Dolde, Tim Schröder, Sinan Karaveli, Matthew L. Markham, Daniel J. Twitchen, and Dirk Englund ∗ These authors contributed equally. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Dept. of Electrical Engineering, Columbia University, New York, NY 10027, United States Element Six, 3901 Burton Drive, Santa Clara, CA 95054, USA (Dated: 11 Sept 2014)

Strong enhancement of magnetic dipole emission in a multilevel electronic system

The Purcell effect is commonly used to increase light emission by enhancing the radiative decay of electric dipole transitions. In this Letter, we demonstrate that the opposite effect, namely, the inhibition of electric dipole transitions, can be used to strongly enhance light emission via magnetic dipole transitions. Specifically, by exploiting the differing symmetries of competitive electric and magnetic dipole transitions in trivalent europium, we demonstrate a fourfold enhancement of the far-field emission from the (5)D(0)→(7)F(1) magnetic dipole transition in trivalent europium. We show that this strong enhancement is well predicted by a three-level model that couples the individual Purcell enhancement factors of competitive transitions from the same excited state.

Time-Resolved Energy-Momentum Spectroscopy of Electric and Magnetic Dipole Transitions in Cr3+:MgO

Due to the recent interest in magnetic light-matter interactions, the magnetic dipole (MD) transitions in lanthanide ions have been studied for potential applications in nano-optics. Similar to lanthanide ions, transition-metal ions also exhibit strong MD emission at room temperature, but their prominent MD zero-phonon lines are often accompanied by significant electric dipole (ED) sideband emission. Here, we extend energy-momentum spectroscopy to time-resolved measurements, and use this technique to quantify the ED and MD contributions to light emission from trivalent chromium doped magnesium oxide (Cr(3+):MgO). This allows us to differentiate the MD (2)E → (4)A2 zero-phonon line from phonon-assisted (2)E → (4)A2 and (4)T2 → (4)A2 ED sidebands. We also demonstrate how the relative intensities of the sharp MD zero-phonon line and the broad ED sidebands can be used as a qualitative measure of the MD and ED local density of optical states.

Direct modulation of lanthanide emission at sub-lifetime scales.

The long lifetime of lanthanide emitters can present a challenge for conventional pump-based modulation schemes, where the maximum switching speed is limited by the decay time of the excited state. However, spontaneous emission can also be controlled through the local optical environment. Here, we demonstrate a direct modulation scheme enabled by dynamic control of the local density of optical states (LDOS). Specifically, we exploit the LDOS differences between electric and magnetic dipole transitions near a metal mirror and demonstrate that rapid nanometer-scale mirror displacements can modulate the emission spectra of trivalent europium ions within their excited state lifetime. The dynamic LDOS modulation presented here can be readily extended to faster optical modulation schemes and applied to other long-lived emitters to control the direction, polarization, and spectrum of spontaneous emission at sublifetime scales.

Quantifying and controlling the magnetic dipole contribution to 1.5-μm light emission in erbium-doped yttrium oxide

demonstrate that the 4 I13/2 → 4 I15/2 emission near 1.5 μm originates from nearly equal contributions of electric dipole (ED) and MD transitions that exhibit distinct emission spectra. We then show how these distinct spectra, together with the differing local density of optical states for ED and MD transitions, can be leveraged to control Er 3+ emission in structured environments. We demonstrate that far-field emission spectra can be tuned to resemble almost pure emission from either ED or MD transitions and show that the observed spectral modifications can be accurately predicted from the measured ED and MD intrinsic emission rates.