EPSRC Fellowships in Manufacturing - "Manufacturing Routes for Organic Room-Temperature MASER"

The aim of this fellowship is to take a recently reported discovery, namely a solid-state maser capable of operating at room temperature, and to carry out a programme of work based upon it that will enhance "manufacturability" for two key application areas.

Background:
The MASER, standing for Microwave Amplification by Stimulated Emission of Radiation, was invented some 60 years ago and exploits paramagnetic transitions in quantum systems to amplify rf/microwave signals. The LASER, which followed directly on from it a few years later, operates through exactly the same physical mechanism (namely stimulated emission) but at much higher frequencies, exploiting electronic transitions to amplify -and so generate- ultraviolet, visible, and infrared light. As devices, masers are useful because they can amplify extremely weak (often precious) electromagnetic signals without corrupting them through the addition of random noise, as would otherwise happen if noisier, conventional (semiconductor-based) amplifiers were used.

Lasers are now ubiquitous and have found their way into a plethora of applications. Masers, on the other hand, are used only in specialised, performance-critical applications such as atomic clocks or as low-noise amplifiers in the most sensitive radiofrequency telescopes and ground stations. The cause of their relative obscurity to date is that all maser systems offering useful performance have necessarily included pieces of kit that are either bulky or consume substantial amounts of power (often kilowatts), or both. Specifically, conventional solid-state masers, as developed and used by NASA, require both cryogenic temperatures and substantial applied d.c. magnetic fields to work, so in turn requiring bulky, power-consuming refrigerators and magnets. Atomic/molecular masers, though capable of room-temperature operation (after a fashion), still require bulky vacuum chambers and pumps, which are equally hard to miniaturize. The novel type of maser to be developed through this fellowship avoids these limitations: it works in air, at room temperature, in the Earth's ambient magnetic field. It is thus, in principle, vastly more amenable to cost-sensitive, portable applications.

How the Fellowship specifically adds value:

Preliminary work leading to a proof-of-principle demonstrator has now been published*, but there remain issues that substantially inhibit manufacturability:

1 The prototype works only in pulsed mode; continuous (CW) operation is sought though with thermal implications concerning the removal of waste heat.

2 Wall-plug power efficiency must to be improved, aiming to get the threshold for CW masing down to a few watts.

3 Physical miniaturisation is required for portable applications.

Substantial progress on 1, 2 and 3 can be made through engineering solutions alone (as opposed to further "breakthroughs"), whilst being ever mindful of:

4 Certain critical components are currently made out of expensive-to-grow single crystals that are equally expensive to form (by skilled hand grinding and polishing) into their required shapes. These build costs need to be extracted.

The purpose of the fellowship is to find solutions to all of the above impediments -focussing especially on the last of them 4 and allowing it to regulate the more engineering activities in 1, 2 and 3.

* Room temperature solid-state maser, M.O., J. D. Breeze & N. M. Alford, Nature, DOI 10.1038/nature11339, 16th August 2012; see also Aharon Blank's "New and Views" (ibid.): http://www.nature.com/nature/journal/v488/n7411/full/488285a.html

This fellowship will bring a new type of spintronic amplifier, namely an organic maser optically pumped via molecular intersystem crossing, to a manufacturing readiness level (MRL) of 4*. The attractiveness of the device in applications stems from its operational convenience (room-temperature, ambient magnetic field) in conjunction with its superior performance over existing semiconductor amplifiers -including cryogenically operated ones- in the areas of added white noise (i.e. the amplifier's noise figure/temperature), intermodulation distortion, peak-power reserve, and 1/f ("flicker") noise. Its resultant beneficiaries include:

(1) Biochemical analysis: specifically, improved spectrometers and scanners based on magnetic resonance, i.e. EPR, NMR, ENDOR. There are actually two distinct, yet potentially combinable, forms of maser-enhancement:

(a) As explained in ref. [1], the low-noise attribute of maser amplifiers and oscillators (potentially incorporated within the EPR/NMR cavity itself) can boost sensitivity, towards detecting extremely minute amounts of a particular molecular species/marker.

(b) In so-called "multi-quantum" (MQ) EPR spectroscopy [2], the sample is exposed to two tones of microwave radiation closely separated in frequency. This obviates the need for a.c. magnetic-field modulation leading to substantial hardware simplifications. The competiveness of MQ-EPR has, however, been hampered by parasitic IM distortion of the sample's bi-tonal response introduced by semiconductor amplifiers. The far lower IM distortion of a maser obviates this problem.

(2) Security: anti-stealth RADAR, electronic warfare (EW) and countermeasures. Basically, the absence of IM makes maser receivers more difficult to jam. Also, in stark contrast to micro/nano-scale semiconductor junctions and channels, "mesoscopic" maser preamplifiers are a great deal more difficult to destroy, electromagnetically, by projected-energy weaponry.

(3) Super-resolution is difficult to achieve with passive metamaterials at microwave frequencies due to Ohmic loss in the metallic elements (e.g. split rings) within the material. Distributed maser gain allows for the realizing of active hyperlenses [3] that can overcome this problem.

(4) Space: improved communication downlinks for both micro-satellites and deep-space probes (e.g. ESA's new JUICE mission), where transmit power is severely limited. [4]

(5) Astronomy: more sensitive radio telescopes -exploiting a maser's low noise temperature, low IM, and low 1/f noise.

(6) Particle accelerators: In contrast to semiconductor devices, a maser holds a stockpile of inverted-spin energy that can be released suddenly. This enables the realization of compact microwave cavities capable of accelerating particles via the mechanism of PASER (particle acceleration by stimulated emission of radiation), as is being explored by a group associated with Argonne National Laboratory (and its wakefield accelerator) [5].

The work proposed here will impact on all of the above applications by making them affordable.

* See (for example): http://en.wikipedia.org/wiki/Manufacturing_Readiness_Level

[1] "Theoretical and experimental senstitivities of ESR spectrometers using maser techniques", J.C. Mollier et al, Rev. Sci. Instrum. 44, 1763-71(1973).

[2] "A general purpose multi-quantum electron paramagnetic resonance spectrometer", R. A. Strangeway et al, Rev. Sci. Instum 66, 4516-28 (1995).

[3] "Gain-assisted hybrid-superlens hyperlens for nano imaging", Y.-T. Wang et al, Optics Express 20, 22954 (2012)

[4] "Ruby masers", R. C. Clauss & J. S. Shell, in Low-Noise Systems in the Deep Space Network, Caltech (2008); http://descanso.jpl.nasa.gov/Monograph/series10/03_Reid_chapt+3.pdf

[5] "Photoinduced spin polarization and microwave technology", Sergey Antipov et al, Nucl. Instr. & Methods in Phys. Res. A, online 22 October 22, http://dx.doi.org/10.1016/j.nima.2012.10.016