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Continuous-wave optical parametric
terahertz source
Rosita Sowade1 , Ingo Breunig1∗ , Iv´an C´amara Mayorga2 , Jens
Kiessling1 , Cristian Tulea1 , Volkmar Dierolf3 , and Karsten Buse1
1 Institute
2 Max
3 Physics
of Physics, University of Bonn, Wegelerstr. 8, D-53115 Bonn, Germany
Planck Institute for Radioastronomy, Auf dem H¨ugel 69, D-53121 Bonn, Germany
Department, Lehigh University, 16 Memorial Drive East, PA 18015 Bethlehem, USA
[email protected]
http://photonik.uni-bonn.de/
Abstract:
Here, we present a continuous-wave optical parametric
terahertz light source that does not require cooling. It coherently emits
a diffraction-limited terahertz beam that is tunable from 1.3 to 1.7 THz
with power levels exceeding 1 µ W. Simultaneous phase matching of two
nonlinear processes within one periodically-poled lithium niobate crystal,
situated in an optical resonator, is employed: The signal wave of a primary
parametric process is enhanced in this resonator. Therefore, its power is
sufficient for starting a second process, generating a backwards traveling
terahertz wave. Such a scheme of cascaded processes increases the output
power of a terahertz system by more than one order of magnitude compared
with non-resonant difference frequency generation due to high intracavity
powers. The existence of linearly polarized terahertz radiation at 1.35 THz
is confirmed by analyzing the terahertz light with metal grid polarizers and
a Fabry-P´erot interferometer.
© 2009 Optical Society of America
OCIS codes: (040.2235) Far infrared or terahertz; (190.4360) Nonlinear optics, devices;
(190.4410) Nonlinear optics, parametric processes; (190.4970) Parametric oscillators and amplifiers; (230.7405) Wavelength conversion devices; (260.3090) far Infrared
References and links
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1.
Introduction
Applications of terahertz radiation in spectroscopy [1, 2], astronomy [3] and communications
[1, 4] plus improved ways to transmit [5] and manipulate terahertz waves [6] – including microscopy with nanoscopic resolution [7] – have raised much interest in terahertz photonics. The
optimum light source is still the largest challenge, with continuous-wave (cw) operation being
most desired for many applications because of its small linewidth. Established techniques to
generate narrow-band terahertz radiation rely on electronic and opto-electronic systems, which
are limited in output power and maximum achievable frequency [8].
Several attempts have been made to overcome this hurdle. There are two approaches that
deserve special attention since they have the potential to outdate the traditional devices: Firstly,
there are quantum cascade lasers [9, 10] with remarkable recent improvements, regarding operation parameters and beam profile characteristics [11, 12]. However, they still need cryogenic
temperatures and they can hardly produce radiation with frequencies below 1 THz. Secondly,
nonlinear-crystal-based light sources fill the gaps in the electromagnetic spectrum where lasers
struggle to emit light, and this also applies to the terahertz range. But so far these systems could
not achieve more than some nanowatts [13].
Looking onto nonlinear-optical methods: to create monochromatic terahertz radiation so far
only difference frequency generation has been employed [14, 13]. Optical parametric oscillators (OPOs), however, are more versatile because of their tuning properties. Unfortunately, the
power threshold for the onset of such an oscillation generating terahertz waves is in the order
of several hundreds of watts because of the high absorption of terahertz radiation by vibrational
excitations [15]. Here, we report on an approach that utilizes intensity enhancement within an
optical cavity to overcome this threshold. For that we exploit a cascaded nonlinear process,
where in a first step a near-infrared pump wave generates a signal and an idler wave, the signal
field being trapped within the cavity. This signal wave can reach kilowatt power levels inside
the resonator [16] and acts, in a second step, as a pump wave for another simultaneously phase#119263 - $15.00 USD Received 29 Oct 2009; revised 17 Nov 2009; accepted 20 Nov 2009; published 23 Nov 2009
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matched process, generating the desired terahertz wave [17].
2.
Concept of cascaded nonlinear processes
For optical parametric oscillation, two requirements have to be fulfilled: the resonance condition, ωp = ωs + ωi , and the phase-matching condition
~kp =~ks +~ki + K
~ .
(1)
Here, ωp , ωs and ωi are the angular frequencies while~kp ,~ks and~ki are the wavevectors of pump,
~ is the grating vector of an alternating second-order
signal and idler waves, respectively, and K
~ properly, so-called quasinonlinearity induced by periodic poling of crystals. By selecting K
phase-matching can be achieved, i.e. the energy transfer from the pump wave to signal and idler
waves is optimized.
These requirements apply to difference frequency generation as well. In optical parametric
oscillators, however, the signal light is resonantly enhanced by the cavity. Such oscillation
starts once the pump threshold is overcome. For periodically-poled lithium niobate crystals,
the possibility of a cascaded, phase-matched process has been discovered [17]. Combined with
the high intra-cavity power, this should produce terahertz radiation, as depicted in Fig. 1. at
the beginning, the pump wave (p) is converted into a signal (s1) and an idler wave (i1) of the
primary process (Fig. 1(a)). In a second parametric process, this first signal wave (s1) serves
as a pump wave for the second, cascaded process, in which it is converted into a second signal
(s2) and a second idler wave ~ki2 =~kTHz , being the desired terahertz radiation (Fig. 1(b)).
Fig. 1. (a) A pump wave (wavevector ~kp ) is converted into a signal and an idler wave
(wavevectors ~ks1 , ~ki1 ). Quasi-phase-matching is obtained by a periodically-poled crystal
~ being the grating vector. (b) The signal wave of the primary process (~ks1 ) acts as
with K
a pump wave for the cascaded parametric process, generating the second signal and idler
~ to ensure phase-matching. The backwards
waves (~ks2 , ~ki2 ), taking benefit from the same K
propagating second idler wave is in the terahertz regime: ~ki2 = ~kTHz . – The lengths of the
wavevectors do not scale.
The cavity is resonant at the same time for both signal waves, since their frequencies are very
similar. All wavevectors are collinear to ensure a long interaction length. The terahertz wave
travels backwards (see Fig. 1(b)) because of the condition
~ .
|~kTHz | = |~ks2 | − |~ks1 | + |K|
(2)
The big advantage of our system is, that the high-power signal wave (s1), used for driving the
cascaded process, is generated within the cavity itself. This first nonlinear process automatically
#119263 - $15.00 USD Received 29 Oct 2009; revised 17 Nov 2009; accepted 20 Nov 2009; published 23 Nov 2009
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selects an existing mode and is hence self-adaptive. In contrast, any effort to feed high-power
light directly into the cavity would require an active stabilization of the resonator plus careful
impedance matching of the mirror reflectivities.
3.
3.1.
Experimental methods
Optical parametric oscillator
Our experimental setup comprises a singly-resonant optical parametric oscillator with a bow-tie
cavity pumped by a cw Yb:YAG laser at 1030 nm. The linewidths of pump and signal waves
are about 1 MHz. The OPO cavity consists of two concave mirrors (curvature radius 100 mm)
and two plane ones (see Fig. 2). All mirrors are highly reflecting (> 99.9 %). As the nonlinear
medium we use a periodically-poled, MgO-doped lithium niobate crystal with a thickness of
0.5 mm. The measurements presented in Figs. 3 – 5 were performed with a 2.5-cm-long crystal
with the period length 30.0 µ m which is kept at 125 ◦ C. We achieve tuning of the infrared and
the terahertz waves by using crystal sections with different phase-matching periods from 24.4
to 31.0 µ m [17].
Fig. 2. Terahertz optical parametric oscillator: Pump light generates signal and idler waves.
The signal light is trapped within a ring cavity, being able to serve as a pump wave for
another, cascaded optical parametric process that generates a backwards-traveling terahertz
wave. This terahertz wave is deflected out of the resonator by a parabolic mirror which
transmits pump and signal waves.
3.2.
Terahertz wave detection
To extract the terahertz wave from the OPO cavity, an off-axis parabolic aluminium mirror is
placed into the cavity directly after the first concave mirror (see Fig. 2). A hole of 1 mm diameter is drilled into this parabolic mirror to let the infrared waves pass through. The backwards
propagating terahertz wave is much more divergent than the pump and signal waves is therefore reflected almost entirely. This terahertz wave is then sent onto a second off-axis parabolic
mirror which focusses the beam onto a calibrated Golay cell, chopped with 10 Hz. Calibration
is specified by the manufacturer Tydex
√ Corp. to be 80 kV/W, while the noise-equivalent-powerlevel of this Golay cell is 100 pW/ Hz. To keep away visible and infrared light, the diamond
incidence window of the Golay cell is covered by a blackened high-density polyethylene foil
(provided by GSE Lining Technology Inc.). We measured the transmittance of this filter at
1.35 THz to be 25 %. The output power values were corrected by this amount.
#119263 - $15.00 USD Received 29 Oct 2009; revised 17 Nov 2009; accepted 20 Nov 2009; published 23 Nov 2009
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3.3.
Terahertz wave analysis
All interacting waves are extraordinarily polarized. Therefore, the terahertz wave should have
linear polarization as well. To test this, a metal grid polarizer, consisting of tungsten wires
(width 15 µ m, spacing 60 µ m), is inserted between the off-axis parabolic mirrors. For residual
infrared waves the polarizer just acts as a shadow mask. To determine the wavelength of the
terahertz radiation, a Fabry-P´erot interferometer (FPI) was assembled with crossbred meshes
of gold-plated tungsten wire (thickness 30 µ m, spacing 100 µ m) acting as mirrors with a size
of 4 × 4 cm2 . This FPI for terahertz waves has got a free spectral range of 10 GHz and a finesse
of approximately 4. For the FPI measurement the setup was slightly modified to get a parallel
beam passing through the interferometer: the second parabolic mirror was replaced by a plane
mirror, followed by the FPI, and afterwards the second parabolic mirror once more focussed
the beam onto the Golay cell.
4.
Results
We have built and tested the described terahertz source. Figure 3 shows the power of the signal
waves and that of the THz wave versus the power of the external pump wave (p). Three regions
can be distinguished: below 2.8 W of pump power no oscillations occur at all. Then, the primary
process sets in (p, s1 and i1). Starting at 4.7 W, the secondary process (s1, s2 and i2) is observed
directly by detecting the emitted terahertz radiation. At a pump power (p) of 12 W, we reach
remarkable 2.2 µ W of terahertz power.
The spectra of the signal waves, presented in Fig. 4, underline the onset of the second parametric process. The frequency of the terahertz wave is given by the frequency difference between the two signal waves: 1.35 THz as shown in Fig. 4. Thus, the signal-wave linewidths
∆νs1,s2 also determine the linewidth ∆νTHz of the terahertz wave. With a scanning Fabry-P´erot
interferometer for near-infrared radiation we measured ∆νs1,s2 ≈ 1 MHz, which implies that
∆νTHz is of the same order of magnitude as well. The absolute terahertz frequency has been
confirmed by analyzing the terahertz beam in a THz Fabry-P´erot interferometer (see section
3.3). As a further validation of the existence of terahertz radiation, we place the metal grid polarizer in front of the detector and rotate it. Figure 5 shows that the detected terahertz power
drops drastically for the grid wires being parallel to the light polarization.
By varying the period length of the poling structure and by changing the crystal temperature,
we are able to tune the terahertz frequency from 1.3 to 1.7 THz. This tuning range can be
widened easily with crystals having different poling periods. It should be noted, that, in addition
to the terahertz radiation, tunable near and mid infrared waves from 1.2 to 1.8 µ m and 2.3 to
5.3 µ m are generated. Optical parametric oscillators are the only light sources providing such
a broad wavelength variety in the near, mid and far infrared within one device.
5.
Discussion
At first glance, one would expect a disappointing terahertz output power, since the absorption is
40 cm−1 at 1.3 THz [15], giving a Lambert-Beer penetration depth of 0.25 mm only. However,
evaluating the coupled wave equations including losses [18, 19] shows that a build-up of the
terahertz wave over millimeters is possible [20]. With reasonable parameters (effective nonlinear coefficient 107 pm/V [21]; first signal wave at 1.56 µ m with a power of around 300 W;
power of the second signal wave one tenth of that of the first (see Fig. 4); diameters of all
waves 200 µ m), we expect a second idler wave with a frequency of 1.35 THz and the remarkable power of 10 µ W. This is higher than our measured powers, however some additional losses
have to be considered: The major loss occurs because of reflection at the crystal surface just
before the terahertz wave leaves the crystal. Furthermore, part of the terahertz wave is absorbed
#119263 - $15.00 USD Received 29 Oct 2009; revised 17 Nov 2009; accepted 20 Nov 2009; published 23 Nov 2009
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Fig. 3. Signal power and terahertz power. a) Signal power (red ) vs. pump power (p). b)
Power of the terahertz wave (blue ×) with respect to the pump power (p). - In both panels
the symbols are measured values, solid lines act as guides to the eye. A and B label points
at which spectra were taken (see also Fig. 4).
on its way through air to the detector. The distance between the OPO crystal and the Golay cell
is around 30 cm. No flooding with nitrogen was performed, i.e. the terahertz radiation has to
travel through normal laboratory air.
There are several opportunities to extend the concept presented here. With regard to the
power: one can change the nonlinear-optical material used since the process presented here
works in principle for any second-order nonlinear medium that can be periodically oriented.
Candidates are, for example, lithium tantalate, potassium titanyl phosphate, and gallium arsenide. One can also optimize the optical parameters, i.e. the diameter and the power of the
first pump wave (p). For the latter an upper limit is present because too high pump powers
cause multi-mode operation of the optical parametric oscillator [22]. With regard to the footprint of the setup: monolithic optical parametric oscillators have been reported [23], but the
large absorption of the terahertz waves makes their operation challenging.
#119263 - $15.00 USD Received 29 Oct 2009; revised 17 Nov 2009; accepted 20 Nov 2009; published 23 Nov 2009
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Fig. 4. Spectra of the signal waves. (A) Spectrum taken at a pump power of 4.3 W. Only
the signal wave of the primary parametric process, λs1 , is present at 1557 nm. (B) Spectrum taken at a pump power of 5.0 W. The signal waves of the primary and the cascaded
parametric processes, λs1 and λs2 , appear with a frequency separation of 1.35 THz.
Fig. 5. Polarization properties of the terahertz wave. Terahertz output power (blue ×) with
respect to the orientation of a metal grid polarizer. The same measurement was performed
twice back and forth. The black solid line shows a calculated sinusoidal shape while the
dashed lines mark the baseline. The insets illustrate the orientation of the wires with respect
to the polarization of terahertz wave (l).
6.
Conclusions
We have presented a continuous-wave optical parametric terahertz source based on a cascaded
nonlinear process. The generated terahertz radiation is tunable and reaches output powers exceeding 1 µ W at a frequency of 1.35 THz. This source is therefore ideally suited for applications such as, e.g., spectroscopy. Based on these insights, we foresee a complementary use of
quantum cascade lasers [9] and cascaded nonlinearities, as they are presented here. For those
applications where high terahertz powers are important, quantum cascade lasers will be used.
#119263 - $15.00 USD Received 29 Oct 2009; revised 17 Nov 2009; accepted 20 Nov 2009; published 23 Nov 2009
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If tunability, diffraction limited beam profiles and room-temperature operation matter, the cascaded nonlinear processes are the method of choice.
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft DFG (FOR 557 and BU 913/18)
and the Deutsche Telekom AG is gratefully acknowledged.
#119263 - $15.00 USD Received 29 Oct 2009; revised 17 Nov 2009; accepted 20 Nov 2009; published 23 Nov 2009
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