Highly efficient and isotope selective
photo-ionization of barium atoms using
diode laser and LED light
B. Wang,1,2 J. W. Zhang,1,2 C. Gao,1,3 and L. J. Wang1,2,3,4,∗
1 Joint
Institute of Measurement Science, Tsinghua University, Beijing 100084, China
of Precision Instruments and Mechanology, Tsinghua University,
Beijing 100084, China
3 Department of Physics, Tsinghua University, Beijing 100084, China
4 Max-Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1, Bau 24, 91058
Erlangen, Germany
2 Department
∗ [email protected]
Abstract:
We demonstrated a simple method to photo-ionize barium
atoms using 791 nm diode laser together with 310 nm UV LED. It solved
the bottle-neck problem of previous method using 791 nm diode laser and
337 nm N2 laser, whose ionization rate was limited by the repetition rate
of N2 laser. Compared with previous method, it has advantages of high
efficiency together with simple and cheap setups. By tuning the frequency
of 791 nm laser to be resonant with the desired isotope, isotope selective
photo-ionization has been realized.
© 2011 Optical Society of America
OCIS codes: (260.5210) Photoionization; (020.4180) Multiphoton processes; (160.3220) Ionic
crystals.
References and links
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Received 16 Jun 2011; revised 29 Jul 2011; accepted 29 Jul 2011; published 11 Aug 2011
15 August 2011 / Vol. 19, No. 17 / OPTICS EXPRESS 16438
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1.
Introduction
Ion traps have become important tools for many areas of physics, including precision frequency
standard [1, 2], precision measurement of fundamental physical constants [3, 4], mass spectroscopy [5], and quantum information science [6, 7]. As an important, practical step of operating an ion trap, a simple ionization method will greatly simplify the experimental setup; a
highly efficient and isotope selective method will supply a pure ionic medium and a clean trap
environment. There are several methods to ionize atoms, including electron beam bombardment, two-photon ionization [8–18], and photoelectric ionization [12]. Electron beam bombardment method can be applied to ionize any atomic or molecular species, but it will cause
charge buildup on insulating surfaces of the trap which may degrade the trap performance over
time and enhance micromotion heating of the trapped ions. In addition, this method can not
realize isotope selective ionization of the target atoms. Photoelectric ionization process was
observed by A.V. Steele et al when they using UV lamp to ionize the barium atoms [12], and
it have the same disadvantages as electron beam bombardment method. With the virtue of isotope selection, high efficiency and no charge buildup, two-photon ionization method has been
adopted by many groups to load the particular ions, especially for applications of atomic clocks
and quantum computing. However, it also has the disadvantage of using a complicated and
expensive experimental setup compared with the other two methods. Normally, these two-step
excitation transitions always correspond to two laser systems, and sometimes need expensive
laser system such as the dye or diode pumped second-harmonic-generation (SHG) laser sys-
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tem [15, 17, 18], and even the frequency-quadrupled mode-locked Ti:sapphire laser [14]. Recently, some groups [11, 16, 18] used high power light emitting diode (LED) for the second excitation of photo-ionization process of calcium, and isotope-selectively trapped a small amount
of 48Ca+ , 43Ca+ and 40Ca+ ions, respectively.
c
337 nm
or 310 nm
340.1 nm
6s6p 3P1
2
6s5d 3D2
791.1 nm
1
2 1
6s S0
6s5d 3D1
Fig. 1. Energy level scheme of neutral barium and relevant wavelengths for photoionization. Solid lines indicate transitions interacted with relevant lasers in experiment,
dotted line indicates the excitation threshold wavelength from state |2 to continuum, wavy
lines indicate the spontaneous decay from state |2 to two D states (6s5d 3 D1 and 6s5d 3 D2 ).
For the case of barium, as shown in Fig. 1, A. V. Steele et al. [12] first demonstrated photoionization loading of a barium ion trap with 6s2 1 S0 → 6s6p 3 P1 transition at 791 nm followed
by the excitation into continuum by 337 nm pulsed laser, and shown it is more efficient than
electron bombardment ionization. In this scheme, the 791 nm laser was supplied by an extended
cavity diode laser (ECDL), and the 337 nm pulsed laser was supplied by a commercial N2 laser
(SRS NL100) which provided 170μ J pulses with a width of 3.5 ns and maximum repetition rate
of 20 Hz. We have also employed this photo-ionization scheme (scheme 1) in our experiment,
and found that the ionization rate was limited by the repetition rate of N2 laser, which will be
analyzed in detail in this paper. The ionization rate or loading rate is an important parameter
for ion traps, because a high loading rate is essential to trap a large number of particular ions.
High loading rate can reduce the running time of the atomic oven, which can greatly reduce
the quantity of materials sputtered onto the trap electrodes. Ref. [19] has shown that clean
electrode surfaces will reduce the heating rate of trapped ions from the motional ground state.
For isotope selective loading of rare isotopes, high loading rate is necessary to overcome the
charge exchange processes with the abundant isotope from the atomic beam [20] (for example,
138 Ba in barium atomic beam). In this paper, we analyze the bottleneck of the Ba ions’ loading
rate using 791 nm laser and 337 nm N2 laser, and demonstrate that using a cheap UV LED
operating at 310 nm to replace the N2 laser, a higher loading rate can be achieved. In our
experiment, using a 310 nm LED (SET, Inc., UVTOP310-TO39HS) with driving current of 19
mA (corresponding to the optical power density about 250μ W /cm2 at the trap center), a 434
ions/sec average loading rate for 138 Ba+ can be obtained. This loading rate is proportional to
the power of the 310 nm LED, and can be increased by using more UV LEDs, using high power
UV LED, and optimizing the imaging system of the 310 nm beam. With this photo-ionization
scheme (scheme 2), different barium isotopes can be loaded and cooled to crystallization.
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2.
Experimental Procedure
CCD
URFcos(ȍt)
2
1
10
6
7
3
5
4
791 nm
Laser
337 nm or
310 nm light
11
9
8
12
y
z
x
Cooling &
Repumping
(a)
Ba Oven
200ȝm
(b)
Fig. 2. (a) Schematic of the experimental setup to photo-ionize the barium atoms and cool
the barium ions. (b) 138 Ba+ Ion crystals we obtained using photo-ionization scheme 2,
when the 791 nm laser power is P791 = 22mW and the LED current is 19 mA.
The experiment was performed in a linear quadruple trap which was composed by 12 electrodes with gold coating. As shown in Fig. 2(a), all of the even numbered electrodes are applied
with a rf voltage of the form Vr f cos(Ωt). A typical rf driving frequency is Ω = 2π × 1.96
MHz, with amplitude Vr f = 100V . All odd numbered electrodes are ac grounded. On the eight
end electrodes (electrodes 1-4 and 9-12), a positive dc voltage (Uz = 120V ) is added to the rf
voltage (or ac ground) through the LC coupling to provide axial confinement. These eight end
electrodes serve as the end-caps. The length of the center electrodes and end-caps are 25 mm
and 17 mm, respectively. The diameter of all electrodes are 2R=8 mm, and the distance between
two diagonal electrodes’ inner surface is 2r0 = 7 mm. The barium oven with 3 mm diameter
points the trap center along ẑ − x̂ direction, and the distance between the oven exit and the trap
center is 40 mm. The 791 nm laser beam with 5 mm diameter is directed into the trap along
−x̂ − ẑ direction, which interacts with the transition 6s2 1 S0 → 6s6p 3 P1 of neutral barium.
The 337 nm pulse laser from N2 laser and 310 nm light from the UV LED can be switched
alternately by a flip mirror. They are directed into the ion trap along the x̂ + ẑ direction which
is perpendicular to the barium atomic beam to minimize the Doppler shifts . The 337 nm pulse
beam is focused down to a waist of 1 × 3 mm by a cylindrical lens with a focal length of 300
mm. The UV LED has a hemispherical output coupling lens, therefore its output beam has a
relatively small emission angle of 7◦ . After focusing by a lens of 75 mm focal length, it is focused down to spot of 4 × 4 mm, which is the image of the LED’s chip. At the typical operating
current of 19 mA, its emission spectrum, centered at 310 nm, has a width of ΔλFW HM = 30nm.
Considering the coupling loss of imaging system and the delectric coating of the vacuum window (AR coating for 493 nm+650 nm+791 nm +337 nm; reflectivity at 310 nm > 10% ), the
optical power of 310 nm light at the trap center is about 40 μ W (power density is 250μ W /cm2 ).
The cooling laser at 493 nm with power P493 = 4.5mW and repumping laser at 650 nm with
power P650 = 4mW are coupled into a single mode fiber and directed into the trap along the −ẑ
direction. The diameters of these two beams are 2 mm inside the trap.
We choose 138 Ba as an example to measure the loading rate of two photo-ionization schemes.
During the loading process, we first heat the barium oven at 7 A, which corresponding to the
oven temperature of approximately 550 ◦C. After 30 s heating, which ensures a uniform atomic
beam, we turn on all of the light beams simultaneously (493 nm + 650 nm + 791 nm + 337
nm, or 493 nm +650 nm +791 nm +310 nm ). The 493 nm cooling laser is about -100 MHz
detuning to the transition 138 Ba+ 6S1/2 → 6P1/2 ; the 650 nm repumping laser is resonant with
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the transition 138 Ba+ 5D3/2 → 6P1/2 ; and the 791 nm laser is resonant with the transition 138 Ba
6s2 1 S0 → 6s6p 3 P1 . After illuminating the barium atomic beam for a duration of τ =10 s, we
turn off the photo-ionization beams (791 nm + 337 nm or 791 nm + 310 nm) and the barium
oven heating current, simultaneously. Through slowly decreasing the frequency detuning of
cooling laser, we can obtain the 138 Ba+ ion crystals in the trap center. The ion crystals are
imaged onto a charge coupled device (CCD) camera (Olympus CC-12) with a 3× imaging
system. The entire imaging system is mounted on a three-axis transition stage with a motional
resolution of 0.1μ m. By moving the imaging system with the transition stage, we can measure
the size of whole ion crystals, and therefore the distance between two neighboring ions, a.
Figure 2(b) shows the ion crystals we obtained using photo-ionization scheme 2, when the 791
nm laser power is P791 = 22mW and the LED current is 19 mA. The trapped ion crystals have
a shape of ellipsoid with the long axis length of 2.18 mm and short axis length of 0.23 mm,
which corresponds to a spatial volume of about Vcry = 6.15 × 10−5 cm3 . The measured distance
a is about 30μ m, which corresponds to the unit ion volume of about Vunit = 1.41 × 10−8 cm3 .
Using the formula Vcry /(Vunit τ ) ,we obtain a loading rate for this ionization process of ∼434
ions/s. This ionization process is repeated several times, and the average loading rate can be
obtained.
500
Loading Rate (ions/s)
400
300
200
100
(a)
0
0
5
10
15
20
25
Optical power of 791 nm laser (mW)
500
Loading Rate (ions/s)
400
300
200
100
(b)
0
0
5
10
15
20
25
Optical Power of 791 nm laser (mW)
Fig. 3. Loading rate of 138 Ba+ ions as a function of 791 nm laser power using (a) photoionization scheme 1 and (b) photo-ionization scheme 2. The experimental parameters are:
repetition rate of N2 laser is 20 Hz, LED driving current is 19 mA, barium oven temperature
is 550 ◦C, and loading time is 10 s. The gray lines are the theoretical fitting results for these
two ionization schemes.
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Using this method, we have experimentally investigated the dependence of loading rate on
the optical power of 791 nm laser for these two different photo-ionization schemes, respectively.
The results are shown in Fig. 3. We can see that the loading rates of 138 Ba+ ions tend to be
saturated with the increase of 791 nm laser power for both ionization schemes. This means
that for our current experimental system, further increasing the 791 nm laser power will not
dramatically increase the loading rate of 138 Ba+ ions any more. We have also investigated
the dependence of loading rates of 138 Ba+ ions on the repetition rate of 337 nm N2 laser for
ionization scheme 1 and on driving current of UV LED for ionization scheme 2, for which,
the results are shown in Fig. 4. For ionization scheme 1 (Fig. 4(a)), the loading rate of 138 Ba+
ions increases linearly with the repetition rate of 337 nm pulse laser. While the problems is,
the maximum repetition rate of 337 nm N2 laser is 20 Hz, which means the 427 ions/s’ loading
rate (corresponding to 5.6 × 10−21 ionization probability per photon of 337 nm laser) is the
maximum loading rate for our current experimental system using ionization scheme 1. For
ionization scheme 2 (Fig. 4(b)), the loading rate of 138 Ba+ ions also increases linearly with the
driving current of UV LED. The 434 ions/s’ average loading rate (corresponding to 7.1 × 10−12
ionization probability per photon of 310 nm light) for 138 Ba+ has been achieved when the
driving current of UV LED is 19 mA, which is comparable to the maximum loading rate using
ionization scheme 1. As the UV LED is very small and easy to operate compared with the
N2 laser, the ionization scheme 2 can simplify the experimental setup for the photo-ionization
process; as UV LED is much cheaper than the N2 laser, we can easily add more UV LEDs to
irradiate the barium atomic beam from different directions or use high power UV LED, thus
further increasing the loading rate of barium ions.
3.
Theoretical Analysis
For the photon-ionization scheme 2, we can use rate equations [21] to describe each atom’s
ionization process.
dn1
= −n1
dt
dn2
= n1
dt
d νσa (ν )I(ν ) + Γ21 n2 + n2
d νσa (ν )I(ν ) − n2
d νσs (ν )I(ν ),
d νσs (ν )I(ν ) − Γ21 n2 − β n2 − σ310 F310 n2 ,
(1a)
(1b)
dnI310
= σ310 F310 n2 .
(1c)
dt
In these rate equations, n1 and n2 are the population probability on level |1 and |2, respectively. nI310 is the population probability of atom which is excited to the continuum state |3.
I(ν )d ν represents the number of photons from 791 nm laser beam per cm2 per second in the
frequency interval d ν , and
F791
(ν − ν 0 ) 2
I(ν ) = √
exp[−
]
2Δν 2
2π Δν 2
(2)
where F791 is the photon flux of 791nm laser beam, ν0 is the resonant frequency of the first
ionizing transition, Δν = 1.5MHz is the linewidth of 791 nm laser. In our case, the photon flux
of 791nm laser beam at 22 mW is about F791 = 4.5 × 1017 cm−2 s−1 . F310 is the photon flux of
310 nm beam at the trap center, in our case, it is about 3.8 × 1014 cm−2 s−1 at the optical power
density of 250μ W /cm2 . σa (ν ) is the cross-section of photon absorption from level |1 to |2,
σs (ν ) is the stimulated emission cross-section from level |2 to |1, and σ310 is the cross-section
for photon-ionization of state |2 at the wavelength of 310 nm. Reference [22] gives the photonionization cross-section value of σ310 = 3.7 × 10−17 cm2 . Γ21 is the spontaneous transition rate
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500
Loading Rate (ions/s)
400
300
200
100
(a)
0
0
5
10
15
20
Pulse repetition rate of N2 laser (Hz)
500
Loading Rate (ions/s)
400
300
200
100
(b)
0
0
4
8
12
16
20
Driving Current of UV-LED (mA)
Fig. 4. Loading rate of 138 Ba+ ions as a function of (a) repetition rate of 337 nm N2 laser,
and (b) driving current of UV LED. Here, the power of 791 nm laser is 22 mW, other
parameters are same as in Fig. 3. The gray lines are the theoretical fitting results for these
two ionization schemes.
from state |2 to state |1, which is about Γ21 = 2π × 47.6kHz; β is the spontaneous decay
rate from state |2 to two lower D states (6s5d 3 D1 and 6s5d 3 D2 ) of barium atoms (as shown
in Fig. 1), which is about β = 2π × 19.6kHz + 2π × 50.6kHz = 2π × 70.2kHz [23]. From
the perspective of atom, the photon-ionization beam can be seen as a pulse, and the pulse
duration time τL is the mean transition time of atoms through the light beam(τL = 6.4μ s at the
barium oven temperature of 550oC). With this assumption and the initial condition of n1 (0) =
1, n2 (0) = nI310 (0) = 0, the population probability nI310 can be solved. During the loading
process, we heat the barium oven at 550oC, which corresponds to the atomic beam flux rate of
about N0 = 1.96 × 1012 s−1 at the interaction region interacting with photon-ionization beam.
Thus, we can evaluate the loading rate through the formula NL310 = Aη N0 nI310 , where η is the
trapping and cooling efficiency of our ion trap system, A is the abundance of 138 Ba atom (Table
1).
For the ionization scheme 1, the ionization process is different with the ionization scheme 2.
First, the maximum interaction time of an atom with the 337 nm laser pulse will not exceed the
pulse width of τ337 = 3.5ns, which is far smaller than the mean interaction time of an atom with
791 nm laser beam. Second, not all barium atoms transmitted through the interaction region will
interact with the 337 nm laser pulse. And the interaction time window for an atom to see the
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337 nm laser pulse is only ρ = f τ337 1 at repetition rate of f . Consequently, the interaction
processes of atom with 791 nm laser beam and 337 nm laser pulse can be approximately seen as
two independent processes, and the rate equation of the process interacting with 791 nm laser
beam can be written as:
dn1
= −n1
dt
d νσa (ν )I(ν ) + Γ21 n2 + n2
d νσs (ν )I(ν ),
(3a)
dn2
= n1 d νσa (ν )I(ν ) − n2 d νσs (ν )I(ν ) − Γ21 n2 − β n2 .
(3b)
dt
The ionized population probability nI337 and the loading rate NL337 can be calculated through
equation
(4)
nI337 = σ337 F337 n2 ρτ337 ,
NL337 = Aη N0 nI337 ,
(5)
where σ337 = 8.8 × 10−17 cm2 is the cross-section for photon-ionization of state |2 at the wavelength of 337 nm [22], F337 is the photon flux of each 337 nm laser pulse at the trap center, in
our case, it is about 2.5 × 1024 cm−2 s−1 . Other parameters are same as those in Eq. (1).
Using Eqs. (1)–(5), we theoretically calculated the loading rates, and gave the relationship
of loading rates with the optical power of 791 nm laser (the gray lines in Fig. 3) for the two
ionization schemes, respectively. During the calculation, the only fitting parameter is η . Our
fitting results is η = 2.27%, and it is same for two ionization schemes. We also calculated the
dependence of loading rate on the repetition rate of 337 nm laser (gray line in Fig. 4(a)) and on
the driving current of UV LED (gray line in Fig. 4(b)), respectively. The theoretical results fit
the experimental results very well. From these results, we clearly see that the short laser pulse
and low repetition rate of the N2 laser are the limiting factors for ionization scheme one. Hence,
the UV LED with cw output light is an appropriate replacement to N2 pulse laser. As it is very
cheap and easy to use, one can easily use more LEDs to realize a higher loading rate of barium
ions.
4.
Isotope Selection
Table 1. The Abundances of Naturally Occurring Barium Isotopes and Their Isotope Shifts
Respecting to 138 Ba Atom and Ion
Mass
number
138
137
136
135
134
Natural
abundance
71.7%
11.23%
7.854%
6.592%
2.417%
Isotope shift (MHz)
Ba 6s2 1 S0 → 6s6p3 P1
0
183.4
109.2
219.9
122.3
Isotope shift (MHz)
Ba+ 6S1/2 → 6P1/2
0
271.1
179.4
348.6
222.6
Isotope shift (MHz)
Ba+ 6S1/2 → 5D3/2
0
13.0
-68.0
-82.7
-174.5
As the 310 nm light from the UV LED only need to supply enough energy to excite the
Ba atoms from the 6s6p 3 P1 state to continuum, isotope-selective loading can be achieved by
tuning the 791 nm laser to be resonant with the desired isotope in both ionization schemes.
The abundances of naturally occurring barium isotopes and their isotope shifts respecting to
138 Ba atom [24] and ion [25, 26] are shown in Table 1. For loading 136 Ba+ , and 134 Ba+ , we
tune the 791 nm laser frequency to 109.2 MHz and 122.3 MHz detuning to the 6s2 1 S0 →
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Received 16 Jun 2011; revised 29 Jul 2011; accepted 29 Jul 2011; published 11 Aug 2011
15 August 2011 / Vol. 19, No. 17 / OPTICS EXPRESS 16445
6s6p 3 P1 transition of 138 Ba atom, tune the 493 nm cooling laser frequency to 179.4 MHz and
222.6 MHz detuning to the cooling transition of 138 Ba+ ion, and tune the 650 nm repumping
laser frequency to -68.0 MHz and -174.5 MHz detuning to the repumping transition of 138 Ba+ .
Other processes are same with those of loading 138 Ba ions. Owing to a special properity of
barium ion, namely, the less abundant barium isotopes generally have higher cooling transition
frequencies than 138 Ba+ . When we load and cool down the rare abundant barium isotopic ions,
for example, 134 Ba+ , the 493 nm cooling laser will cool the 134 Ba+ and simultaneously heat the
more abundant isotopes (136 Ba+ and 138 Ba+ ). Thus we can trap the pure barium isotope easily,
even though its abundance is very small compare to that of 138 Ba. For the odd isotope 137 Ba,
it has a nuclear spin I = 3/2, giving several hyperfine levels. To ionize the 137 Ba atoms, we
tune the 791 nm laser to resonance with one of the hyperfine transitions of 6s2 1 S0 → 6s6p 3 P1 .
The cooling scheme of odd isotopes is different to that of the even isotopes. Using a technique
similar to that of Ref. [27], we modulated the frequencies of cooling and repumping lasers to
generate sidebands corresponding to the different hyperfine cooling and repumping transitions,
respectively. To overcome the “coherent population trapping” (CPT) effect and the “dark states”
between two hyperfine levels, we make the two-photon detuning of these two hyperfine cooling
transitions non-zero. This effectively eliminates the CPT effect and the details will be shown
elsewhere. Figure 5 shows the different barium isotope crystals which were loaded and cooled
down using ionization scheme 2. From top down, they show crystals of 138 Ba+ , 137 Ba+ , 136 Ba+ ,
and 134 Ba+ , respectively. From Fig. 5, we estimate that the number of ions in different crystals
are approximately, 4340, 780, 488, and 149, respectively. This means the loading rate ratio
of 138 Ba+ , 136 Ba+ , and 134 Ba+ are proportional to their natural abundance, yielding that their
ionization cross-sections are very similiar. In Fig. 5, we did not give the ionic crystal of 135 Ba+
because that it has different hyperfine splitting comparing with 137 Ba+ and needs different
experimental setups for cooling.
138
Ba+
137
Ba+
136
Ba+
134
Ba+
200ȝm
Fig. 5. Barium isotope crystals loaded and cooled down using photo-ionization scheme 2.
The experimental parameters are: 791 nm laser power is 22 mW, UV LED driving current
is 19 mA, barium oven temperature is 550 ◦C, and loading time is 10 s.
5.
Conclusion
We have demonstrated a highly efficient and isotope selective photo-ionization method for barium atoms using 791 nm diode laser and 310 nm UV LED. Compared with the method using
791 nm diode laser and 337 nm N2 laser, it has advantages of high loading efficiency, simple
and cheap setups. With a mere power density of 250μ W /cm2 of 310 nm light at the trap center,
∼434 ions/s average loading rate for 138 Ba+ has been achieved, and the loading rate is proportional to the power of 310 nm LED, which can be increased by using more LEDs, using
high power UV LED, and optimizing the imaging system of 310 nm beam. With this method,
different barium isotopes can be loaded and cooled to crystallization.
#149226 - $15.00 USD
(C) 2011 OSA
Received 16 Jun 2011; revised 29 Jul 2011; accepted 29 Jul 2011; published 11 Aug 2011
15 August 2011 / Vol. 19, No. 17 / OPTICS EXPRESS 16446
Acknowledgments
We acknowledge funding supports from the Major State Basic Research Development Program
of China (973 Program) (No. 2010CB922901) and the Tsinghua University Initiative Scientific
Research Program (No. 20091081474).
#149226 - $15.00 USD
(C) 2011 OSA
Received 16 Jun 2011; revised 29 Jul 2011; accepted 29 Jul 2011; published 11 Aug 2011
15 August 2011 / Vol. 19, No. 17 / OPTICS EXPRESS 16447