DR BENJAMIN RICHARDS
The Maser Group
Prof Neil Alford
Prof Andrew Horsfield
Dr Ke Jie Tan
Prof Chris Kay
Prof Peter Haynes
Dr Mark Oxborrow
Dr Jonathan Breeze
Dr Benjamin Richards
Dr Juna Sathian
Dr Stuart Bogatko
Dr Enrico Salvatori
Prof Martin Heeney
2
OUTLINE
“Nothing stops naysayers like a working device”:
Charles H. Townes
What is a maser and history
Current applications and problems
The Idea… and uses
Three Components of a maser
1. Maser gain material
2. Maser Cavity Research
3. Illumination Source
 Improving the Maser
Future
3
What is a MASER?
Microwaves Amplification by Stimulated Emission of Radiation
A maser is a device that produces coherent electromagnetic waves
through amplification by stimulated emission.
Pre-dates the laser: First built by Townes, Gordon and Zeiger at
Columbia University in 1953.
Charles H. Townes (left) and James P. Gordon
in 1955 with the first MASER
Spontaneous Emission
Townes, Basov and Prokhorov were awarded the 1964 Nobel Prize
for Physics for their theoretical work leading to the maser.
Stimulated Emission
Seeding photon
A21 =
2pn
e0 me
- Spontaneous event
e g1
f12 - Incoherent
3
- Non-polarized
c g
2 2
2
- Stimulated event
- Coherent
- Polarized
4p 2 e2 g1
B21 =
f12
me hn c g2
4
The First maser: Ammonia- beam maser (NH3)
• The ammonia beam maser used is a two level maser using
vibrational states
H
H
H
H
H
• The nitrogen atom oscillates between two positions,
above and below the plane of the hydrogen atoms
N
H
N
• These two arrangements do not represent exactly the
same energy
• The wave functions of the hydrogen and nitrogen atoms
are not quite symmetrical
• Therefore the molecule exists in two energy states
• The difference in energy between the states corresponds
to a frequency difference of 23.87 GHz , or ~24 GHz
l=1.25cm
5
Simplicity of the ammonia maser
6
• 2 state system. Neither molecular state
influences the other state, so no need for
three level or metastable states
1
• No need for pumping. Thermally populated
1:1 at maser ΔE
2
3
5
4
DE for maser is ~10-5 eV
DE for a laser is a few eV
population ratio
1.00
0.90
0.80
0.70
0.60
n2/n1
• Negligible spontaneous emission as it
scales with frequency ν3
nE 2
0.50
0.40
A21 8h

3
B21
c
3
nE1
0.30
0.20
 e  DE / kT
0.10
0.00
0.00E+00
2.00E-01
4.00E-01
6.00E-01
8.00E-01
1.00E+00
1.20E+00
delta E (eV)
masers
1.40E+00
1.60E+00
1.80E+00
6
2.00E+00
lasers
Moving to solid-state to create an amplifier
• Gas molecules are not closely crowded together unlike the molecules
of a solid, thus the power output of gas-beam masers remains low
• A solid-state medium would have a higher molecular density and so
greater amplification and therefore more useful as an amplifier.
7
Pumped solid state maser
• Ruby (chromium-doped sapphire)
can be used as a solid state maser
• Moved to a three level system
ruby (chromium-doped
sapphire)
• Used Zeeman splitting to achieve the
necessary electronic splitting
• Requires cryogenics to overcome
spin-lattice relaxation rate that
scales with temperature as T9
T1
(Bloembergen and Basov & Prokhorov)
8
OUTLINE
“Nothing stops naysayers like a working device”:
Charles H. Townes
What is a maser and history
Current applications and problems
The Idea… and uses
Three Components of a maser
1. Maser gain material
2. Maser Cavity Research
3. Illumination Source
 Improving the Maser
Future
9
Current Applications
The maser has found limited applications in
• Astronomy use masers as a low noise amplifier
• Frequency standards (accurate clocks) driven by a hydrogen maser
The main reason for its relative obscurity has mainly been due to the
inconvenience of the operating conditions needed for its various realizations
• atomic and free-electron masers require vacuum chambers and
pumping
• solid-state masers, although they excel as low-noise amplifiers and are
occasionally incorporated in ultrastable oscillators, typically require
cryogenic refrigeration.
10
Illustration of problems: Goonhilly Maser (Mullard / GPO) circa 1962
Supplying cryogenic fluids to keep the maser cold
Rather inconvenient…
Cryogenic ruby maser
ruby
refridgerator
Amplifiers in reality (figures of merit):
power
gain  G
• noise –as characterized
by its “temperature” [T]
amplifier noise power
 G  kT  Bandwidth
electronic
Amplifier
technologies
thermionic
valves
charge carriers
transistors
1900s
“magnetic”
spin flips
masers
1950s
1990s
HEMTs
superconducting
(cooper pairs)
1960s
SQUIDs
1940s
OUTLINE
“Nothing stops naysayers like a working device”:
Charles H. Townes
What is a maser and history
Current applications and problems
The Idea… and uses
Three Components of a maser
1. Maser gain material
2. Maser Cavity Research
3. Illumination Source
 Improving the Maser
Future
16
Moving to room temperature and zero field
• A zero magnetic field solid-state maser using a high quality factor (Q) whisperinggallery mode in a single crystal of sapphire with trace paramagnetic Fe3þ impurities
was demonstrated a decade ago, but required cryogenic cooling to liquid helium
temperature.
• Efforts to avoid either cryogenic cooling or high magnetic fields have met with
limited success.
• The prospect of realizing a room-temperature maser using population inversions
within photoexcited triplet-state sublevels in organic paramagnetic molecules was
first proposed by Blank for a device operating in high magnetic fields.
• Although maser oscillation was not observed in the reported device, this work
paved the way for the recent discovery of maser operation at room temperature and
zero magnetic field
Blank, A., Kastner, R. & Levanon, H. Exploring new active materials for low noise room-temperature microwave amplifiers and
other devices. IEEE Trans. Microw. Theory Tech. 46, 2137–2144 (1998).
Blank, A. & Levanon, H. Applications of photo induced electron spin polarization at room temperature to microwave technology.
Appl. Phys. Lett. 79, 1694–1696 (2001).
Blank, A. & Levanon, H. Toward maser action at room temperature by triplet-radical interaction and its application to microwave
technology. RIKEN Rev. 44, 128–130 (2002).
17
OUTLINE
“Nothing stops naysayers like a working device”:
Charles H. Townes
What is a maser and history
Current applications and problems
The Idea… and uses
Three Components of a maser
1. Maser gain material
2. Maser Cavity Research
3. Illumination Source
 Improving the Maser
Future
18
What components do we need for a maser device?
1. Cavity
2. Masing Material Medium
3. Pump Source
LASER
MASER
Energy input by pumping
Gain Medium
Microwave Cavity
Output
Coupler
OUTLINE
“Nothing stops naysayers like a working device”:
Charles H. Townes
What is a maser and history
Current applications and problems
The Idea… and uses
Three Components of a maser
1.Maser gain material
2. Maser Cavity Research
3. Illumination Source
 Improving the Maser
Future
20
Molecular energy level structure: Triplet states of organic
molecules
J. H. van der Waals
Appl. Magn. Reson. 20, 545-561 (2001)
Materials were tested with EPR spectrometers
Photo excited EPR measurements of
candidate materials. Tell us of the
abundance of triplet states (internal
conversion yield) and the energy
splitting's under high field regimes.
We grew a single-crystal of pentance:p-terphenyl
After investigating a fullerene derivate, we stumbled upon a
pentacene which had a huge EPR signal
Pentacene
Growth of pentacene-doped p-terphenyl single crystals for the MASER
Good quality pentacene doped p-terphenyl crystals were grown in polyfluoroalkoxy
alkane (PFA) tubing from an open system zone melting method.
UV-vis absorption spectrum of pentacene doped p-terphenyl single
crystal. The major peaks were 590 nm, 545 nm, 507 nm and 474 nm.
Pumping scheme for pentacene-doped p-terphenyl:
“triplet mechanism” (TM)
Compact zero field EPR Spectrometer
A compact 100 MHz zero field EPR
spectrometer was created to study the
dynamics and decay rates of pentacene
X-Y triplet state transitions.
As well as associated microwave plumbing
and microwave Robinson’s oscillator
RF Cavity
Coupling loop
Pentacene
crystal
EPR response
Coupling loop
Coil resonator
Xenon flash
26
Alternative room temperature MASER materials
?
Current candidate: Pentacene doped p-terphenyl crystal
Key elements: S0-S1 transition energy
Intersystem crossing rate (ISC)
T1 population inversion
Favourable decay rates
Zero field splitting
Decay rates : Pentacene
T1
X ~ 4x10⁴
Y ~ 2x10⁴
Z~10³
27
Theoretical search for alternative room temperature MASER materials
Density Functional Theory and Time-Dependent Density Functional Theory study of
size and substitution effects in linear polyacenes (PBE/cc-pvqz) in gas phase.
Benzene
 Low lying singlet excited state
energies decrease with increasing
acene length.
Pentacene
Naphthalene
Tetracene
2.32
1.91
S0
0.76
T3
2.25
T2
T1
1.68
eV
2.33
S1
eV
 Nitrogen substitution induces
large changes in the excited
state structure of linear
polyacenes.
Anthracene
0.77
S0
1.67
T1
28
OUTLINE
“Nothing stops naysayers like a working device”:
Charles H. Townes
What is a maser and history
Current applications and problems
The Idea… and uses
Three Components of a maser
1. Maser gain material
2.Maser Cavity Research
3. Illumination Source
 Improving the Maser
Future
29
Basic Equation for MASER Threshold
æ m0g 2 ö
ç
÷ × k Popt l ×T1T2hISC DN × Fm >1
è p c0 ø optical
resonator
triplet
Where μ0 is the permeability of free-space, γ is the electron gyromagnetic ratio, κ is the optical coupling
efficiency, Popt is the optical pumping power with wavelength λ, T1 is the triplet inversion lifetime (spinlattice relaxation), T2 is the spin-spin relaxation time, ηISC is the intersystem crossing yield from the
excited S1 state, ΔN is the difference in normalised populations of the triplet inversion, Fm is the
magnetic Purcell factor (defined as Q/Vm, where Q is the Q-factor and Vm is the magnetic mode
volume) and c0 is the speed of light in vacuum.
Three factors contribute:
Magnetic Purcell factor
Characteristics of the triplet-states within the paramagnetic molecules
Optical pumping system
30
Magnetic Purcell Factor
The Purcell factor (normally defined for an electric dipole), is the
enhancement in the spontaneous emission rate of atoms placed within a
resonant cavity
3 ælö
Fm = 2 ç ÷
4p è n ø
3
æQö
ç ÷
è Vm ø
Unloaded Quality Factor, Q
Inversely proportional to the losses
within the entire cavity
Magnetic Mode Volume, Vm
Defined as:
Vm =
m H (r)max
2
m ò H (r) dV
2
V
Maximize Q-factor and minimize mode volume to
improve magnetic Purcell factor
31
Evolution of Q: High Q Cavities
Hollow Metallic Cavity
Q ~ 10,000 for Ag-plated
cavity in TE111 mode
Dielectric Puck inside
hollow metal cavity
Q ~ 50,000 for Sapphire in
TE01δ mode at 10 GHz
Large dielectric puck
Q ~ 130,000 for Sapphire in
TE01δ mode at 10 GHz
Rs
Q =
G
−1
Q 1 = pd tanδ +
Rs
G
Fields within Dielectric Resonator
The TE01δ mode of a ring resonator has distinct advantages:
• Highest Q-factors
• Magnetic dipole Hz along cylindrical axis
• Very low electric energy density inside central bore where
gain medium is located
Cross-section (r-z) of typical dielectric ring resonator
Electric energy-density
Magnetic Eigen mode frequency simulations
Magnetic energy-density
33
Optically pumped organic maser prototype
pentacene:p-terphenyl
sapphire ring
(part of high-Q
microwave cavity)
Optical pump
Microwave cavity
But only
pulsed
operation!!
Pentacene Masers Noise Properties
Operating
temperature
Technology
Amplifier noise
temperature
1960
cavity ruby maser
20 K
1974
travelling-wave ruby maser
2K
2003
semiconductor: InP HEMT
4K
2012
pentacene maser
Date
4K
ruby (chromium-doped sapphire)
293 K
pentacene-doped p-terphenyl
~ 70 mK
Maser Applications: Astronomy / Astrophysics
Noise down, resolution up!
38
Measurements, sensing, imaging applications:
uncertainty 
noise power

noise does matter
duration of measurement
NMR / EPR spectrometers and scanners
39
Long-distance communications
10 watts
transmitted
10-18
watts
received
space probe-outer solar system
Receiver dish on earth
40
Applications: The Wider Picture
Operating at 1.45GHz and 100MHz without the overheads of
cooling and electromagnetic infrastructure a device that could
amplify weak microwave signals would be useful in:
• Structural biology and exploration
• Radio telescopes
• Communications
• EPR spectroscopy and Drug detection
• Quantum information processing
• NMR imaging
41
OUTLINE
“Nothing stops naysayers like a working device”:
Charles H. Townes
What is a maser and history
Current applications and problems
The Idea… and uses
Three Components of a maser
1. Maser gain material
2. Maser Cavity Research
3.Illumination Source
 Improving the Maser
Future
42
Illumination sources and technology
• Nlight V Rhodamine pulsed dye
laser
• OPO tuneable 5ns pulsed laser
• Xenon flash lamp
• 100W LED
43
Luminescent concentrator: alternative light source for
optical pumping of room-temperature MASER
The yellow light coupled by the Ce:YAG Luminescent Concentrator (LC) via TIR is
out-coupled for use in various applications.
•
•
•
•
>10 Watts of optical power through 3 mm2
aperture.
Zero threshold: output linear with input
current.
Higher wall-plug efficiency than laser.
Scalable to higher (or lower) powers.
Efficient conversion of InGaN-LED generated blue light into higher
luminance yellow light.
44
OUTLINE
“Nothing stops naysayers like a working device”:
Charles H. Townes
What is a maser and history
Current applications and problems
The Idea… and uses
Three Components of a maser
1. Maser gain material
2. Maser Cavity Research
3. Illumination Source
Improving the Maser
Future
45
Using higher permittivity dielectrics
Prototype used single-crystal sapphire which has relative permittivity εr = 9.8
and loss tangent tanδ ~ 10-6 @ 1.45 GHz
Simulations of dielectric resonators optimized for magnetic Purcell factor
Dielectric
Permittivity
TE01δ mode
frequency (GHz)
Q-factor
Magnetic
Mode Volume
(cm3)
Purcell Factor
Al2O3
9.3
1.45
169,000
58.6
2.0×106
TiO2
85
1.48
37,000
1.3
2.0×107
SrTiO3
318
1.53
10,060
0.2
3.6×107
All simulations performed using in-house mode-matching software.
46
Strontium Titanate MASER
Due to high permittivity of Strontium Titanate (STO) ~320, the resonator
dimensions are reduced by a factor of 6 compared to the original
sapphire resonator.
Volume is reduced by factor of 200.
Sapphire resonator:
Mode volume Vm~ 50 cm3 and Q ~ 105 (Q/V ~ 2x103 cm-3)
STO resonator
Mode volume Vm~ 0.2 cm3 and Q ~ 104 (Q/V ~ 5x104 cm-3)
Purcell factor ~ Q/V is 25 times higher for STO resonator, so optical
pump power threshold should be lower, 8W.
STO maser
Instead of dye laser, illuminated
with Xenon flash lamp
Improved room temperature zero field maser
• The resonator resonates in the T01 delta
mode with an axial magnetic field at 1.45
GHz
Sapphire
Threshold 230W
STO
• This was placed in a copper cavity and
illuminated by a 585nm laser or xenon
flash lamp
• Great improvements in threshold and
miniaturisation have been made
Threshold 2W
49
MASER Threshold
Threshold measurements – MASER EFFICIENCY
THRESHOLD: Masing gain exactly balances the losses within the resonator.
Peak output power (a.u.)
Laser Energy
Time
Threshold
0
3.5
7
10.5
Laser Energy (mJ)
14
17.5
50
Strontium Titanate MASER
But still only pulsed!
Page 51
MASER output vs Frequency
Tuning screw
Coupling loop
SrTiO3 resonator
TE01δ magnetic field mode
Pentacene:p-terphenyl
Single crystal
- Measure the frequency of D+E transition
- Measure the line-width of the D+E transition
Off resonance the signal smaller and delayed in time
Peak output power (a.u.)
Copper can
Tuning position
1.4475
1.4483
1.449
1.4497
1.4505
Resonator frequency (GHz)
1.4512
52
Exploiting new possibilities at VHF
~1ms
100MhHz (VHF) Resonator Designs
Lumped air coil resonator
Recent discoveries : Worlds first VHF Room temperature Zero field maser
• Worlds first zero field VHF
room temperature maser
built on pentacene X-Y
triplet transition
• Simulations of maser
performance and rate
equations
55
Raw Waveform and Relaxation oscillations
0
5
10
15
20
Time (µs)
25
30
FFT
106.3
106.6
Frequency (MHz)
106.9
10
10.05
Time (µs)
10.1
CW Masing ??
Voltage (mV)
100 MHz maser signal from μsec laser pulse.
Time (μs)
OUTLINE
“Nothing stops naysayers like a working device”:
Charles H. Townes
What is a maser and history
Current applications and problems
The Idea… and uses
3 Components of a maser
1. Maser gain material
2. Maser Cavity Research
3. Illumination Source
 Improving the Maser
Future
58
Future Projects
• CW 100MHZ
• Oscillator below threshold
Building an amplifier
• Split mode (spin flushing)
• X band
Solving the Z problem
• Deuteration
• Diamond
• Florescent concentrator
Other improvments
59