Vapor Phase Assembly of Molecular Emitter Crystals for Photonic Integrated Circuits

Abstract

Organic molecules embedded in an organic matrix exhibit lifetime-limited optical coherence and bright emission at cryogenic temperatures below 3 K. Here we present a simple vapor-phase growth method for synthesizing optically thin DBT-doped anthracene crystals that are compatible with integrated nanophotonics. The crystals are ~200 nm thick with sub-nm surface roughness and a tunable lateral dimension of up to 200 $μ$m. The molecular transitions remain narrow and spectrally stable, with inhomogeneous broadening below 100 GHz, comparable to DBT in bulk anthracene. The dopant density is tunable up to several hundred molecules per $μ$m$^2$, ensuring emitters within the near-field of nanophotonic structures. We demonstrate that the crystals can be micropositioned onto integrated photonic devices with the molecular dipole aligned to the optical mode. This approach opens a path toward on-chip single-photon sources and collective many-emitter effects.

Figures (14)

• Figure 1: Growth of optically thin and high emitter density anthracene crystals(a) Molecular structure of DBT in an anthracene lattice. (b) Jablonski diagram for DBT in anthracene. S$_{s,v}$ denotes a singlet state in the electronic level $s$ and vibronic sublevel $v$. $T_{1,0}$ denotes the long-lived triplet states. The excited state S$_{1,0}$ decays at a rate of $\Gamma=4$--5 ns with a branching ratio to the ground state S$_{0,0}$ given by $a_\mathrm{DWFC}=0.3$. The intersystem crossing (ISC) is negligible ($\sim 10^{-7}$). The vibrational states relax nonradiatively to the ground vibrational state in the order of picoseconds. (c) A tunable laser is focused onto a DBT-doped crystal in a cryostat at 2.9 K. Inset: single-molecule fluorescence. (d) PLE spectrum of DBT molecules in an anthracene crystal. There are approximately 200 molecules in a 1 $\mu \mathrm{m}^2$ region. (e) PLE spectrum of a nearly lifetime-limited DBT molecule fitted to a Lorentzian with $\mathrm{FWHM}=43(3)$ MHz.
• Figure 2: DBT-doped crystal growth(a) Crystal growth apparatus. A powder of DBT and anthracene sublimes in a hot zone. A piston moves the hot column of air through the cold zone, where crystals form, and deposit on a substrate. (b) AFM scan of the edge of a crystal. The height of the crystal is 188(1) nm with a surface roughness of 0.8(1) nm (RMS). (c) Mean crystal size versus temperature difference $\Delta T$ between the hot and cold zones of the tube furnace. The data are fit to an exponential $S(\Delta T) = ae^{-b\Delta T} + c$ with $a$ = 120(20) $\mu$m, $b = 0.013(7)\,{^\circ \mathrm{C}}^{-1}$, and $c$ = 8(16) $\mu$m. (d) Crystal lateral size distributions at different values of $\Delta T$. Each distribution is measured from a sample of 200 crystals.
• Figure 3: Optical properties of DBT emitters at 3 K(a) Level structure of fluorescence measurements. A CW laser drives resonantly to the S$_{0,0}\rightarrow$ S$_{1,0}$ transition. The fluorescence above 800 nm is collected while the background from the laser light is rejected. (b) PLE spectrum of a molecule of DBT fit to a Lorentzian curve with $\mathrm{FWHM}=49(2)$ MHz. (c) Scattering rate of a DBT molecule as a function of excitation intensity. The fit gives $I_\mathrm{sat}=125(7)~W\,cm^{-2}$ and $R_\infty=36.3(5)$ kcps. $R_\infty$ is denoted with the dashed line. (d) Fluorescence decay curve of a molecule of DBT. An exponential fit gives $\tau=4.73(8)$ ns.
• Figure 4: Inhomogeneous broadening and spectral stability. (a,b) PLE spectra of crystals at different doping densities. The density of molecules is $\sim450/\mu\mathrm{m}^2$ (a) and $\sim25/\mu\mathrm{m}^2$ (b). (c) Distribution of linewidths for the molecules in (a) fit to a gaussian distribution with mean 82(3) MHz with $\sigma=51(4)$ MHz. (d) Distribution of resonance frequencies for the molecules in (a) fit to a gaussian distribution centered at 779.54(1) nm with $\sigma=0.11(1)$ nm. A gaussian fit to the distribution of resonances in (b) gives a mean frequency of 779.96(5) nm and a standard deviation of 0.16(5) nm. (e) Spectral wandering of a single molecule over 1.5 hours fit to a Voigt profile with Lorentzian component with $\mathrm{FWHM}=54(3)$ MHz and gaussian component $\sigma=24(4)$ MHz. The saturation parameter is $I/I_\mathrm{sat}=0.1$.
• Figure 5: Emitter-Device Integration Strategy for Nanophotonicsa-d) Different stages in the micropositioning process. A tapered fiber picks up the crystal from a PVC substrate and stamps it onto a nanophotonic device made from etched Si$_3$N$_4$ on SiO$_2$. The dipole moment of DBT, which is aligned with the crystal's b-axis, is aligned with the TE mode of the device. The crystal adheres to the fiber and nanophotonic device through van der Waals forces. After integration, a layer of PVA is spin-coated over the crystal to prevent sublimation.