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OPTICS LETTERS / Vol. 38, No. 13 / July 1, 2013
Laser cooling of beryllium ions using a frequency-doubled
626 nm diode laser
F. M. J. Cozijn,1 J. Biesheuvel,1 A. S. Flores,1 W. Ubachs,1 G. Blume,2 A. Wicht,2,3 K. Paschke,2
G. Erbert,2 and J. C. J. Koelemeij1,*
1
LaserLaB, VU University, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
2
3
Ferdinand-Braun-Institut, Gustav-Kirchhoff-Straße 4, 12489 Berlin, Germany
Optical Metrology, Institut für Physik, Humboldt-Universität zu Berlin, 10117 Berlin, Germany
*Corresponding author: [email protected]
Received April 22, 2013; revised June 5, 2013; accepted June 5, 2013;
posted June 5, 2013 (Doc. ID 189316); published June 28, 2013
We demonstrate laser cooling of trapped beryllium ions at 313 nm using a frequency-doubled extended cavity diode
laser operated at 626 nm, obtained by cooling a ridge waveguide diode laser chip to −31°C. Up to 32 mW of narrowband 626 nm laser radiation is obtained. After passage through an optical isolator and beam shaping optics, 14 mW
of 626 nm power remains of which 70% is coupled into an external enhancement cavity containing a nonlinear
crystal for second-harmonic generation. We produce up to 35 μW of 313 nm radiation, which is subsequently used
to laser cool and detect 6 × 102 beryllium ions, stored in a linear Paul trap, to a temperature of about 10 mK, as
evidenced by the formation of Coulomb crystals. Our setup offers a simple and affordable alternative for Doppler
cooling, optical pumping, and detection to presently used laser systems. © 2013 Optical Society of America
OCIS codes: (140.2020) Diode lasers; (140.3320) Laser cooling; (140.3610) Lasers, ultraviolet; (140.7300) Visible
lasers; (270.5585) Quantum information and processing; (300.6520) Spectroscopy, trapped ion.
http://dx.doi.org/10.1364/OL.38.002370
Laser cooling of trapped beryllium (Be ) ions has played
an instrumental role in the realization of quantum-logic
optical clocks [1], quantum information processing [2],
coherent optical control of individual quantum systems
[3], and precision spectroscopy of light molecular
ions [4]. Doppler cooling of Be typically requires a
single laser beam with 0.01–1 mW of power at
313.133 nm, focused to a waist of 0.02–0.2 mm, which
is used to drive the 2 S1∕2 F 2 − 2 P3∕2 F 0 3 cycling
transition with a small red detuning from the resonance.
In addition, the 313 nm fluorescence photons emitted
during laser cooling can be collected to detect Be ions
as well as their internal state [3]. Both the required power
level and linewidth (∼1 MHz) of the cooling laser source
are in reach of extended-cavity diode laser (ECDL)
systems, which combine a low-complexity setup with
good spectral properties and high affordability. However,
semiconductor gain materials emitting at 313 nm near
room temperature are not generally available, and a
similar hiatus exists for laser diodes emitting at the first
subharmonic (626 nm) near room temperature. Therefore, 313 nm radiation for laser cooling of Be has previously been obtained by second-harmonic generation
(SHG) of dye lasers operated at 626 nm [3], resonant
sum-frequency generation of a laser source at 760 nm
(either a Ti:sapphire or a semiconductor laser) and a
532 nm source obtained by SHG of a 1064 nm Nd:YAG
laser [5], SHG of 626 nm obtained by sum-frequency
generation of amplified fiber laser systems at 1550 nm
and 1051 nm [6], and frequency quintupling of an amplified fiber laser system at 1565 nm [7].
In this Letter, we report laser cooling of 6 × 102 trapped
Be ions using 313 nm radiation obtained through SHG
of the output of a single ECDL operated at 626 nm. The
ECDL is based on a ridge waveguide diode laser chip
with a tensile-strained GaInP single quantum well [8].
The reflectivity of the diode front facet is about 1%,
allowing the diode laser chip to operate both with and
0146-9592/13/132370-03$15.00/0
without external feedback. In the latter case, a maximum
optical output power of 100 mW is obtained for a bias
current of 150 mA. The diode chip is housed in a hermetically sealed TO-3 package, and the chip substrate is
mounted onto a Peltier element for thermoelectric cooling. This allows tuning the center of the gain spectrum
from 635 nm at 15°C to 626.266 nm at −31°C, similar
to the observation of [9] (obtained with broad area diode
lasers), requiring 4.2 W of electrical power from the
Peltier driver (Fig. 1).
The output of the diode laser is directed through an
aspheric lens to collimate the beam [Fig. 2(a)]. The
polarization state of the laser field is approximately
linear and corresponds to the TM mode of the ridge
waveguide for which the electric field vector is
perpendicular to the epitaxial layers. To reduce the
free-running linewidth of the laser, a grating with line
density 2400∕mm is used in Littrow configuration to provide frequency-selective feedback. The grating lines are
oriented vertically so that the TM and TE modes receive
28% and 3% feedback, respectively. The grating holder is
based on a three-pivot mirror mount, which allows optical alignment and coarse frequency selection by tuning
Fig. 1. Unseeded diode laser emission spectra at 100 mA and a
temperature of 15°C (red dotted curve), 0°C (orange dashed–
dotted curve), −15°C (green dashed curve), and −30°C (blue
solid curve). 626.266 nm is achieved for a temperature of −31°C.
© 2013 Optical Society of America
July 1, 2013 / Vol. 38, No. 13 / OPTICS LETTERS
Fig. 2. (a) Schematic top view of the ECDL. (b) Threedimensional computer-aided design drawing of the ECDL.
(c) Top and side views of the ECDL including cooling stages
and enclosing box. See text for details.
the grating angle. Fine frequency adjustments and 1 GHz
mode-hop-free sweeps are possible through a small
piezoceramic actuator (PZT), mounted in series with
the horizontal adjustment screw. As (too) high intracavity laser power may damage the diode gain medium, we
limit the diode bias current to 100 mA, and we obtain
up to 32 mW of 626 nm behind the grating. To avoid
feedback by other optical elements, the ECDL output
is sent through an optical isolator exhibiting −33 dB
return loss and −2 dB insertion loss.
To effectively remove the heat produced by the diode
laser and Peltier cooler, the diode laser package is firmly
attached to an aluminum mount, using thermo conductive paste between the mating surfaces to improve the
thermal conductance, while the ECDL assembly is kept
at a constant temperature of approximately 5°C using a
second Peltier cooling stage [Fig. 2(c)]. To enhance the
thermal stability and mechanical stiffness of the setup,
the aluminum mount is connected to the grating holder
assembly through a 10 mm thick aluminum plate
[Fig. 2(b)]. The second Peltier element is placed on a
water-cooled aluminum breadboard at 16°C–18°C. The
breadboard is supported by four rubber (Viton) O-rings
for passive vibrational and thermal isolation with the
O-rings resting on the bottom of a metal box. The metal
box encloses the entire ECDL setup and is purged with
dry nitrogen to avoid condensation of water on the parts
below 15°C. The inside of the box is furthermore lined
with foam rubber to reduce disturbances of the ECDL
due to acoustical noise.
A small portion of the 626 nm beam is picked off and
coupled into a wavelength meter (0.3 GHz resolution).
The main 626 nm beam (20 mW) is further shaped
by a series of four cylindrical lenses to match the
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fundamental transversal mode of a bowtie-shaped enhancement cavity, similar to that described by Koelemeij
et al. [10]. The cavity contains a 10 mm long, Brewster-cut
beta-barium borate (BBO) nonlinear crystal for type I
critically phase matched SHG. The cavity length is stabilized using the Hänsch–Coulliaud scheme [11], applying
feedback to a PZT-mounted cavity mirror. The cavity
free-spectral range and linewidth are 0.55 GHz and
1.8 MHz, respectively. In principle, the highest circulating
(intracavity) power is achieved when the input coupler
transmission is matched to the cavity round-trip losses.
Having tested input couplers with reflectivities of 98.0%,
98.5%, and 99.3%, we find that the highest circulating
power is obtained with the 98.5% reflectivity mirror.
By monitoring the power reflected off the input coupler
when the cavity is in lock, we estimate that about 70% of
the incident light is coupled into the SHG cavity. With
9.7 mW of power coupled into the SHG cavity, we obtain
35 μW of power at 313 nm (as measured behind the
output coupler of the cavity). We note that the actual
produced power at 313 nm is larger: the exit face of
the BBO crystal introduces about 16% Fresnel loss, and
the output coupler transmission is 99.4%.
To determine the linewidth of the ECDL, we observe
the power of the 626 nm light transmitted by the SHG
cavity (while blocking the 313 nm light with a spectral
filter) using a sufficiently fast photodiode. Using the
SHG cavity as a scanning interferometer, we verify that
the ECDL spectrum contains a single longitudinal mode
only. When the ECDL wavelength is steered to a halfmaximum transmission point, fast frequency fluctuations
are converted linearly to voltage fluctuations in the
photodiode output, which is recorded using a digital
oscilloscope. Intrinsic ECDL power fluctuations (<1%)
are negligible compared to the frequency noise-induced
power fluctuations. From the known linewidth of the
SHG cavity we obtain an intensity-to-frequency conversion factor. This is used together with the recorded
photodiode signal (1 ms duration) to obtain the power
spectral density of frequency fluctuations from which
we obtain an upper limit for the ECDL linewidth of
0.35 MHz.
The apparatus containing the linear Paul trap for storage of Be ions is described in detail in [12], and is
equipped with an electron-multiplied charge-coupled device (EM-CCD) camera for imaging of 313 nm fluorescence photons. A 0.3 mT static magnetic field parallel
to the laser beam direction of propagation provides
a quantization axis for the F 2, M F 2 − F 0 3,
M 0F 3 cycling transition. The 313 nm beam is first
passed through a Glan–Taylor polarizer and a zero-order
quarter-wave plate to achieve a circular state of polarization, and subsequently focused to a waist of 0.14 mm by a
lens and overlapped with the symmetry axis of the ion
trap. The 313 nm power at the ion trap is 20 μW, leading
to an intensity equal to 0.8 I sat , with I sat as the saturation
intensity of the cycling transition.
Be ions are loaded into the 1 eV deep trap by electronimpact ionization of an effusive beam of Be atoms,
produced by a resistively heated oven, and subsequently
laser cooled. To optimize the cooling rate by the ECDL,
optical pumping out the cycling transition must be minimized, which is achieved by aligning the bias magnetic
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OPTICS LETTERS / Vol. 38, No. 13 / July 1, 2013
This compares well with previous results obtained for laser cooling of Be Coulomb crystals of similar size [14].
We note that the power available at 313 nm is also sufficient for optical repumping, as required for Be cooling
and manipulation through two-photon stimulated Raman
transitions [3].
In conclusion, we report the demonstration of laser
cooling of Be ions at 313 nm obtained through SHG
of the output of an ECDL operated at 626 nm. A key ingredient of this method is the ability to cool down the
laser diode chip to −31°C, which is achieved through a
two-stage Peltier cooler. A 626 nm ECDL exhibiting a
similar wavelength reduction was reported in the parallel
work by Ball et al. [15]. We anticipate that the small form
factor, low complexity, high affordability, relatively low
power consumption, and good spectral properties of this
setup will be beneficial for future developments toward
compact and practical setups for quantum information
processing, quantum sensors, and optical clocks based
on trapped ions.
Fig. 3. EM-CCD fluorescence images of 6 × 102 trapped and
laser-cooled Be ions, acquired with an exposure time of
3.5 s. Image width corresponds to 1.2 mm, and the trap symmetry axis coincides with the major axis of the ion ensemble.
313 nm intensity is equal to 0.8 I sat . Looking at (a) and (b) from
top to bottom, the 313 nm frequency is stepped from about
−0.6 GHz from resonance to a few tens of megahertz below resonance. (a) Gaseous state. (b) Coulomb crystals at various laser
detunings. Single ions are clearly visible in the region to the
right of the image center. Only this part is imaged sharply onto
the EM-CCD by the imaging optics, which view the ions at a
small angle with respect to the direction normal to the trap axis.
(c) Images as obtained from MD simulations (integration time
10 ms) for 590 Be and 5 BeH impurity ions at 6 mK (top),
10 mK (middle), and 15 mK (bottom) respectively, indicating
∼10 mK temperatures for the Coulomb crystals shown in (b).
field along the laser propagation direction (using two
pairs of orthogonal shim coils) and adjustment of the
quarter-wave plate. During this procedure, the ECDL
frequency should not drift. Instead of stabilizing the
frequency of the ECDL, we choose to perform the optimization using a few milliwatts from a frequency-doubled
and frequency-stabilized dye laser already available in
our setup [12]. After this procedure, the dye laser is exchanged with the ECDL within one minute while the ions
are either kept (as is the case for the images in Fig. 3) or
reloaded after emptying the trap.
Laser cooling at 313 nm is achieved by sweeping the
frequency at 0.1 GHz∕s from about −0.6 GHz to resonance, during which we observe the usual phase transition in the ensemble of Be ions from a gaseous state to a
crystalline state, also known as a Coulomb crystal [13].
This is evidenced by changes in both the fluorescence
level and the structure of the Be ensembles, as visible
in the EM-CCD images shown in Fig. 3. From the EMCCD images of Coulomb crystals, we are able to estimate
the number of trapped ions as well as the temperature of
the ions, by comparing them to the results of molecular
dynamics (MD) simulations [14]. Thus, we find a number
of 6 × 102 Be ions, and a temperature of about 10 mK.
This work was supported by FOM Program 125. J. C. J.
K. acknowledges support from NWO/STW (Vidi 12346).
The development of the laser diode was supported
by the German ministry of education and research
(BMBF) in the framework of the InnoProfile initiative
No. 03IP613.
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