Single-LED-pumped, room-temperature, solid-state maser

TL;DR

This work demonstrates a chip-scale, LED-pumped, room-temperature solid-state maser using an invasively driven waveguide to excite a pentacene-doped para-terphenyl crystal inside a SrTiO3-dielectric resonator. Optical–RF simulations and experiments show that invasive pumping substantially increases the overlap between the pump and the cavity magnetic field, yielding a cooperativity around $Γ ≈ 2$ and a magnetic quality factor $Q_m ≈ 3{,}000$, with quasi-CW maser oscillations. Compared with end-on pumping, the invasive geometry provides roughly a 2× improvement in pumping efficiency and enables a much smaller pump source without compromising performance, supporting progress toward commercially practical RT masers.

Abstract

Through their ability to achieve `cryogenic' levels of noise performance while operating at room temperature, optically-pumped, solid-state (OPSS) masers show great promise as quantum sensors, oscillators, and amplifiers. We here demonstrate maser oscillation in a microwave cavity containing a crystal of pentacene-doped \textit{para}-terphenyl (ptc:ptp) pumped by a single, chip-scale LED. Here, unlike previous work, the size of the pump source no longer dominates the size of the maser system as a whole. This miniaturization is achieved through invasive optical pumping in the form of a waveguide, the tip of which is embedded into the maser crystal. Combining experimental measurements with ray-tracing analysis, we find that our approach offers at least a factor of 2 enhancement in the cooperativity over end-on optical excitation.

Figures (6)

• Figure 1: Geometry of invasively pumped LED maser, including the LED chip, microwave cavity, waveguide and ptc:ptp crystal. The gray-dashed inset box on the then left details the geometry of the waveguide's 3-faceted tip. Displayed on the right is a photo of the invasively pumped ptc:ptp sample used in this work.
• Figure 2: Approximate optical pump volume reported for key publications on ptc:ptp masers vs year published. The inset photo displays the model of LED used in this work --beside a ruler for scale. References in ascending year are: Oxborrow et al.Oxborrow2012Room-temperatureMaser, Breeze et al.Breeze2015EnhancedMasers, Breeze et al.Breeze2017Room-temperatureStates, Wu et al.Wu2020InvasiveCoupling, Ng et al.Ng2023MoveElectrodynamics, Ng et al.Ng2024Maser-in-a-shoebox:Field.
• Figure 3: Meridional cross-sections through the optically-pumped maser's core for the two different pumping geometries considered. In both cases, the STO ring rests on a PTFE washer that itself rests on the cavity's copper base. Yellow shading indicates the extent of pump light penetration.
• Figure 4: Detected LED output power vs (maximal) drive current for 150-$\mu$s pulses.
• Figure 5: Maser oscillations produced by the invasively pumped ptc:ptp maser under two different pump intensities. The left column shows the maser bursts recorded at peak LED output power (6 mJ per pulse), whilst the right column shows the maser burst produced when the pump power was reduced to $3.3$ mJ per pulse. This was found to be the threshold optical pump power for this sample. The top row shows the power spectral density (PSD, in dBm.Hz$^{-1}$), derived from the short-time-Fourier transform of each maser oscillation, using Hamming windows containing $2.5\times10^4$ samples with 50 % overlap, generated in MATLAB R2025B. The middle row shows the voltage trace as detected by the oscilloscope in blue, and the corresponding RMS power envelope in red, taken as an envelope over windows of 3000 samples. The lower rows shows the time profile of the instantaneous optical power supplied by the LED in each case, with the inset indicating the integrated pulse energy.