Technologies based on manipulation of individual quantum states are expected to soon become commonplace—the “second quantum revolution” is on everyone’s lips these days. At the heart of many envisioned device architectures are quantum systems generating exactly one photon per excitation event—so-called single-photon emitters.
A vast amount of experimental effort is thus concerned with identifying, characterizing, and manipulating such quantum emitters by optical means—that is, in practice, by employing suitable laser light sources. Yet, the demands are quite challenging. Recently developed continuous-wave optical parametric oscillators (OPOs), now also providing nearly gap-free coverage of the visible and the near-infrared spectral range, are expected to take quantum nanophotonics a decent leap forward.
Optical parametric oscillators vs. conventional lasers
Light sources based on OPO technology offer a remarkable versatility of operational wavelengths compared to conventional lasers. Essentially, this is because the OPO principle relies on a process referred to as parametric conversion in a nonlinear optical material—rather than on stimulated emission in a suitable laser gain medium.
The OPO process can be perceived as splitting of an incoming pump photon of high energy into two photons of lower energy, the latter usually referred to as signal and idler photons, respectively (see Fig. 1). It is subject to the conservation principles of photon energy and photon momentum (phase-matching condition), but otherwise—at least in theory—not limited by fundamental restrictions. In other words, as long as the two conditions are met, the operational wavelength of the signal (and respectively the idler) can be freely chosen.
In view of their flexibility, OPOs appear as the technology of choice not only for applications demanding laser light at “unconventional” wavelengths, but in particular when tunable laser light output is required. However, while the OPO concept was experimentally demonstrated already more than half a century ago,1 commercially available devices have been slow to come. This is especially true for systems operating in continuous-wave mode (CW OPOs), so that generation of widely tunable CW laser light mostly had to rely on conventional lasers until quite recently.
Exciting advances in CW OPO technology have been driven, on the one hand, by the emergence and increasingly sophisticated design of new nonlinear crystals, like periodically poled lithium niobate (PPLN). On the other hand, the increasing availability of suitable high-performance pump lasers, such as diode-pumped solid-state (DPSS) lasers and fiber lasers, has spurred the practical realization of widely tunable CW OPO devices with unprecedented characteristics.
Laser light tunable from 450 to 3500 nm
By the nature of the OPO process, any generated output wavelengths will be longer than those used for pumping. Consequently, OPO devices operating across the visible spectral range do either require UV pump sources or, alternatively, need to employ additional frequency conversion stages (to convert signal and idler wavelengths in the visible range). As of today, only the latter approach has been proven to be technically practicable for commercializing operationally stable turnkey systems.2
Figure 2 illustrates two examples of such a two-stage design concept, adapted to a 532 nm DPSS laser and a 780 nm fiber laser pump source, respectively. The operational principle relies on a cascaded sequence of nonlinear optical processes within two cavities (OPO and second-harmonic generation cavities). The OPO cavity scheme employed is commonly referred to as singly resonant OPO cavity design; that is, the cavity is operated on resonance at either a particular signal wavelength or a particular idler wavelength.
Thereby, the effective cavity length is actively stabilized to a multiple integer of the selected operational wavelength. While circulating one of the generated (signal or idler) waves resonantly inside the OPO cavity, its counterpart can be extracted for wavelength conversion into the visible spectral range by another nonlinear process. This wavelength conversion takes place in a second, separate cavity by frequency doubling of the primary OPO output, a process widely known as second-harmonic generation (SHG).
As of today, by adaptation of the design concept to suitable laser pump sources and exploiting the whole range of accessible signal and idler wavelengths, an unprecedented wavelength coverage ranging from 450 nm up to 3500 nm is realized. In combination with application-inspired wavelength tuning mechanisms, this allows experimenters to conveniently carry out measurements that would have been otherwise hampered by the technical complexity of suitable sources—or even the lack thereof.3,4 This statement is substantiated with real-world examples in the following.
Diamond photonics
Within the quantum-research community, implementing single-photon emitters in solid state is widely recognized to offer technologically appealing advantages, including the potentially available arsenal of existing (classical) chip fabrication techniques. Among the most-promising candidates are microscopically localized impurities in diamond crystals, or so-called color centers.
Early attention has focused on the nitrogen vacancy (N-V) center in diamond, where a nitrogen atom together with a vacant site replace two adjacent carbon atoms of the diamond lattice. While the N-V center arguably is the most extensively studied quantum emitter, another class of defects based on group-IV elements (Si-, Ge-, Sn-, Pb-V) has been attracting considerable interest recently, due to potentially further improved properties like lower susceptibility to external noise.
Though the inventory of optically active centers in diamond is remarkably diverse, their spectral characteristics are governed by similar physical principles (see Fig. 3). A crucial factor is the extent of coupling of the emitters’ electronic transitions to lattice vibrations of the surrounding crystal host.
The strength of this phonon coupling can be inferred by the appearance of phonon sidebands (PSBs) in absorption and/or emission spectra, in addition to the sharp zero-phonon-line (ZPL) of the purely electronic transition. For an ensemble of single-photon emitters, in turn, the spectrum is additionally broadened by inhomogeneous static variations in the local microscopic environments of individual sites.
Searching for needles in a haystack
It should be emphasized that a spectral characterization of single quantum systems by “traditional” direct absorptions measurements is hardly feasible. Researchers typically rather rely on photoluminescence excitation (PLE) spectroscopy, in which the photon emission intensity of a quantum emitter is measured while tuning the excitation (laser) frequency. Recalling the distinct spectral features outlined above, the identification and characterization of single emitters require a profound design of laser frequency tuning mechanisms.
On the one hand, due to spectrally broad and unstructured phonon sidebands, a thorough spectral characterization of a particular color center requires wavelength tuning over a wide spectral range, typically more than 100 nm (at room temperature). On the other hand, to identify and resonantly excite a zero-phonon transition peak—typically on the order of a few gigahertz width at cryogenic temperature conditions—requires a sufficiently narrow laser emission linewidth and tuning in the (sub-) picometer range. Along the same lines, the requirements in terms of absolute frequency accuracy as well as long-term frequency stability are typically demanding.
The good news is that OPOs offer all handles to be tailored to these demands. Figure 4 illustrates three different wavelength-tuning modes of commercially available CW OPOs, with vastly different step sizes and tuning speeds. For wavelength selection on the nanometer scale, the OPO/SHG nonlinear crystal temperature can be adjusted at the push of a button, according to an internal temperature-wavelength mapping scheme. Quasicontinuous step-wise tuning in the picometer range is accomplished through a piezo-driven high-finesse intracavity etalon. For mode-hop free and truly continuous tuning, in turn, the OPO cavity length can be scanned via a piezo element.
For applications with the highest of demands, the performance characteristics can be further improved by operating the system in a closed-loop configuration; that is, in conjunction with an external wavelength-measurement device (wavemeter). In this operation mode, the achievable long-term stability essentially approaches the measurement resolution of the external wavemeter device itself, which can be as low as on the order of a few megahertz.
Real-world performance: OPOs at work
The experimental data in Figure 5 is compiled from several studies concerned with the germanium-vacancy center in diamond (Ge-V).5,6 It has been recently proposed to offer a variety of attractive properties for quantum technology applications, such as single-photon emission under room-temperature conditions. Notably, these defects possess an inversion symmetry and therefore are not sensitive to local fluctuation in electric fields.
A PLE spectrum of an ensemble of Ge-V color centers (recorded at room-temperature conditions) is shown in Figure 5a. It has been recorded by exploiting the full coarse tuning range of the OPO scheme discussed above, straightforwardly accessible by automated crystal selection and temperature tuning. The measurement clearly reveals a maximum count rate when resonantly driving the ZPL at approximately 602 nm. The data is adapted from a comparative study on Si-V and Ge-V color centers.5![FIGURE 5. Photoluminescence excitation (PLE) spectrum of an ensemble of Ge-V centers (a); the spectrum is recorded at room temperature for excitation-wavelength tuning from 450 to 640 nm (adapted with permission from S. Häußler et al. [5]). Electronic level structure and transitions within the ground and the first excited state of a Ge-V center (b). Normalized photoluminescence spectrum of a single Ge-V center recorded at cryogenic temperature, revealing the electronic four-level fine structure (c). Photoluminescence excitation spectrum of the most prominent transition in (c), recorded under tuning resonant excitation (d); the orange line represents the signal when an additional 532 nm gating laser is switched on and blue lines represent the signal when the gating laser is off (adapted with permission from D. Chen et al. [6]).](https://img.laserfocusworld.com/files/base/ebm/lfw/image/2020/12/2012LFW_spe_5.5fc8fe1713aaa.png?auto=format,compress&fit=max&q=45&h=271&height=271&w=250&width=250 "FIGURE 5. Photoluminescence excitation (PLE) spectrum of an ensemble of Ge-V centers (a); the spectrum is recorded at room temperature for excitation-wavelength tuning from 450 to 640 nm (adapted with permission from S. Häußler et al. [5]). Electronic level structure and transitions within the ground and the first excited state of a Ge-V center (b). Normalized photoluminescence spectrum of a single Ge-V center recorded at cryogenic temperature, revealing the electronic four-level fine structure (c). Photoluminescence excitation spectrum of the most prominent transition in (c), recorded under tuning resonant excitation (d); the orange line represents the signal when an additional 532 nm gating laser is switched on and blue lines represent the signal when the gating laser is off (adapted with permission from D. Chen et al. [6]).")
Figure 5b shows the refined electronic level structure of a Ge-V center. Because of strong spin-orbit coupling, the ground state and the first excited state are split into a pair of energy levels with twofold spin degeneracy. In fact, by recording the photoluminescence spectrum of a single Ge-V center at cryogenic temperature conditions, the four-line fine structure of the ZPL emission can be experimentally revealed (see Fig. 5c). Note that the spectral window shown in Figure 5c is just a tiny cutout of Figure 5a around the ZPL emission, showing the signal of a single Ge-V emitter rather than an ensemble.
In Figure 5d, the resonant fluorescence characteristics of the most prominent feature in the spectrum of the single Ge-V emitter is precisely characterized. For this purpose, the laser excitation wavelength is tuned within an interval of approximately ±5 GHz across the spectral resonance by truly continuous frequency tuning in conjunction with a wavemeter device. It should be emphasized that the resonant PLE signal is only detectable at additional nonresonant excitation—that is, when the Ge-V center is additionally excited with a gating laser (at 532 nm). The role played by the 532 nm light is that of a switch controlling the onset and decrease of resonant fluorescence. The experimental findings are discussed in detail in D. Chen et al.,6 where the authors also quantitatively explain the observed dynamics by the presence of a dark state.
Outlook
The coming of age of widely tunable CW OPOs coincides with a variety of research initiatives in the realm of quantum technology. Since the OPO concept is quite general, it should allow for continuous adaption to new experimental requirements, like novel types of single-photon emitters and systems alike. As illustrated in this contribution, CW OPOs can be expected to mature into a recognized choice among laser light sources that accompany the rapidly evolving field of quantum research.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the support and fruitful discussions with Rudolf Bratschitsch, Weibo Gao, Fedor Jelezko, and Alexander Kubanek and their groups.
REFERENCES
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5. S. Häußler et al., New J. Phys., 19, 6, 063036 (2017).
6. D. Chen et al., Phys. Rev. Lett., 123, 033602 (2019).