Scaleable LED-pumped Room-temperature Maser using a Multi-blade Optical Injector

TL;DR

This work tackles the pumping optics bottleneck in room-temperature Pc:PTP masers by introducing a multi-blade invasive optical injector that distributes pump light across the gain medium. It couples microscopic absorption modelling with three-dimensional ray-tracing to decouple material nonlinearity from geometry and demonstrates the injector's practical viability through a hand-fabricated injector– crystal assembly that achieves masing at ~1.45 GHz under LED pumping. Key contributions include quantitative optical cross-sections for Pc:PTP, a scalable injector fabrication workflow, and a 3D geometric analysis showing superior uniformity and robustness of the multi-blade design under realistic absorption conditions. The findings indicate that multi-path optical pumping enhances scalability and performance, enabling high-power, portable, room-temperature masers for low-noise microwave applications, with the co-operativity concept $oldsymbol{ar{oldsymbol{ exteta}}}_{ extrm{maser}}$ guiding optimization through dependencies on $Q_ extrm{L}/V_ extrm{mode}$ and absorbed pump power $P_ extrm{abs}$.

Abstract

Though the performance of room-temperature masers has improved over the last decade, relatively little attention has been paid to the optics used to pump the maser's gain medium. In this work, we investigate a novel multi-blade optical ``injector'' that permits more effective and more scaleable pumping. The reported work encompasses an interdisciplinary mix of conceptualization, simulation, crystal growth, fabrication, and microwave engineering. Our gain medium is pentacene dissolved as a solid solution with para-terphenyl (Pc:PTP) molecular crystal. We accurately determine this pentacene's molecular absorption cross-section as a function of wavelength. Ray-tracing is then used to assess how different designs of waveguide inject light into the Pc:PTP crystal. A multi-blade injector made of high-refractive-index glass (namely Ohara S-TIH6) is predicted to pump it more completely and uniformly than previous designs. Upon hand-fabricating such an injector and Bridgman-growing a crystal of 0.1% Pc:PTP over it, an experimental maser oscillator using this combined injector-crystal assembly is demonstrated. The performance and scaleability of multiblade injection vis-a-vis alternative strategies is analyzed.

Figures (11)

• Figure 1: Device set-up, simplified Jablonski diagram for Pc:PTP maser and UV-vis spectrum of Pc dissolved in PTP. (a) A Pc:PTP crystal grown onto an "aloe vera" structured multi-blade waveguide under polychromatic LED-light illumination. (b) LED light pumps the Pc molecules into the excited singlet $S_3$ state. These molecules then decay back to the $S_1$ state via (rapid) internal conversion. This is followed by intersystem crossing to the $T_2$ triplet state. The maser transition then occurs at 1.45 GHz from $T_x$ to $T_z$ within the $T_1$ state at ambient conditions. (c) UV-vis absorption spectrum of Pc:PTP (mauve-pink) and emission spectrum of pump LED (mint green).
• Figure 2: Geometry of an "aloe vera"-type invasive waveguide. The golden region identifies where the maser crystal is illuminated by the LED. Note that the shanks of the waveguides in these two sketches are too short. In the left sketch (2D cross section), four different ray-trace scenarios are displayed (from left to right): (i) not captured by waveguide (angle of trajectory too acute) (ii) oblique-angled propagation, (iii) acute-angled propagation, (iv) failure to be absorbed by maser crystal. Both (ii) and (iii) end in absorption.
• Figure 3: Optical set-up for determining absorption through Pc:PTP sample.
• Figure 4: Optical absorption cross-section of 0.1% pentacene dissolved in solid para-terphenyl. For unpolarized light traveling perpendicularly to the crystal's primary growth facet ($a$-$b$ cleavage plane).
• Figure 5: Workflow for preparing multi-blade waveguides for maser experiments. Starting from a 25 mm × 76 mm glass slide (step 1), the substrate is measured and sectioned into narrow strips of defined widths (2.5–4.5 mm), accounting for kerf loss during cutting (step 2). The resulting strips resemble uniform ribbons. Each strip is then mounted in the polishing machine and processed to achieve a controlled wedge angle on one face (step 3). In step 4, multiple polished strips are aligned and temporarily stacked by using optical glue, lashing and clips to facilitate handling and further processing.