Room-Temperature Quasi-Continuous-Wave Pentacene Maser Pumped by an Invasive $\mathrm{Ce}:\mathrm{YAG}$ Luminescent Concentrator

We present in this work a quasi-continuous-wave (cw) pentacene maser operating at 1.45 GHz in the Earth’s magnetic field at room temperature with a duration of approximately 4 ms and an output power of up to $-25$ dBm. The maser is optically pumped by a cerium-doped yttrium-aluminum-garnet (YAG) ($\mathrm{Ce}:\mathrm{YAG}$) luminescent concentrator (LC), the wedge-shaped output of which is embedded inside a 0.1% pentacene-doped para-terphenyl (Pc:Ptp) crystal. The pumped crystal is located inside a ring of strontium titanate (STO) that supports a ${\mathrm{TE}}_{01\delta }$ mode of high magnetic Purcell factor. Combined with simulations, our results indicate that cw operation of pentacene masers at room temperature is perfectly feasible so long as excessive heating of the crystal is avoided.

Figure 1

A Jablonski diagram showing the optical-pumping scheme that prepares highly polarized pentacene triplets (blue) at zero applied magnetic field. The maser transition with a frequency of 1.4495 GHz occurs between the $|X⟩$ and $|Z⟩$ triplet sublevels. $\Gamma$, pump rate; $k_{\mathrm{sp}}$, spontaneous emission rate; $k_{\mathrm{ISC}}$, intersystem crossing rate; $k_{\mathrm{IC}}$, internal conversion rate; ${\gamma }_{XY}$, spin-lattice relaxation rate between sublevels $|X⟩$ and $|Y⟩$ and similarly for ${\gamma }_{XZ}$ and ${\gamma }_{YZ}$; $k_X$, decay rate from sublevel $|X⟩$ back to ground state $S_0$ and similarly for $k_Y$ and $k_Z$.

Figure 2

The setup of the $\mathrm{Ce}:\mathrm{YAG}$ LC-pumped pentacene maser. (a) A schematic diagram of the resonator and optical setup. The inset shows the ultraviolet-to-visible (UV-vis) spectrum of a filter installed at the output of the xenon flash lamp. (b) A photograph of the setup (without showing the copper enclosure of the STO cavity, the optical filter, and the xenon flash lamp).

Figure 3

The concept and design of the invasive optical pumping with the wedged $\mathrm{Ce}:\mathrm{YAG}$ LC. (a) A photograph highlighting the original and wedged output faces of the $\mathrm{Ce}:\mathrm{YAG}$ LC under room light. (b) A schematic of the wedged $\mathrm{Ce}:\mathrm{YAG}$ LC with its dimensions. With optical pumping, the induced luminescence (green lines) escapes the LC from its wedged output faces.

Figure 4

UV-vis spectra of $\mathrm{Ce}:\mathrm{YAG}$ (gray) and Pc:Ptp (purple) and emission spectra of $\mathrm{Ce}:\mathrm{YAG}$ (orange) and xenon flash lamp (cyan).

Figure 5

(a) A three-dimensionally rendered image of the crystal-growth assembly (the upper empty section of the outer glass vial is not shown). (b) A computer-generated image of the cylindrical Pc:Ptp maser crystal (pink) grown around the wedged end of the $\mathrm{Ce}:\mathrm{YAG}$ LC (lemon yellow), (c) An actual photograph of the red-fluorescing 0.1% Pc:Ptp crystal pumped with yellow light from the $\mathrm{Ce}:\mathrm{YAG}$ (lemon yellow), which itself is pumped by pseudowhite light from a mobile phone’s torch.

Figure 6

A two-dimensional (2D) axisymmetric simulation of the magnetic energy density (false-color “heat” map) and magnetic field vector (white arrows) of the ${\mathrm{TE}}_{01\delta }$ mode of the STO-loaded microwave cavity. The geometries of the STO ring, the Pc:Ptp crystal, and the cross-linked polystyrene support in medial cross section are highlighted in blue, magenta, and red, respectively. The width of the entire region of the simulation (14 mm) corresponds to the radius of the copper enclosure of the resonator, while the height of the region (15 mm) corresponds to the height between the tuning plate and the copper sheet at the bottom of the resonator.

Figure 7

The experimental (pink) and simulated (blue) 4-ms maser response evolving with the optical pumping of the $\mathrm{Ce}:\mathrm{YAG}$ LC. The inset shows an enlargement of the Rabi oscillations that occur at the beginning of the maser burst.

Figure 8

The determination of the optical-pump threshold of the 4-ms pentacene maser.