Optical spectroscopy in the mid-infrared (IR) region of the electromagnetic spectrum (mid-IR, 2–20 µm, 500–5000 cm-1) is a powerful tool for the detection, characterization, and quantification of molecules, compounds, and complex molecular systems. In combination with Fourier-transform spectroscopy (FTS), broadband mid-IR sources provide parallel access to the entire molecular content of a sample under study, enabling a plethora of applications in medicine, industry, and fundamental research.
Conventional Fourier-transform spectrometers, widely deployed as laboratory workhorses, have a limitation that arises from the low brightness of incoherent thermal radiation sources used for illumination. A typical Fourier-transform spectrometer is equipped with an electrically heated silicon carbide rod (globar) emitting a black body spectrum. For optically dense samples or instances where a long sample path is required, not enough signal reaches the detector.
Conversely, a coherent source offers laser-like brightness and provides a tremendous advantage for measurements that are normally signal-limited. The brilliance of ultrabroadband mid-IR femtosecond laser sources now exceeds the brilliance of third-generation synchrotrons, making the combination of FTS and laser-based sources very appealing. Furthermore, some femtosecond lasers are configurable as optical frequency combs with ultrahigh spatial and temporal coherence of the pulse trains. Frequency combs enable FTS with significantly lower detection limits, improved precision and sensitivity, as well as various regimes of dual-comb spectroscopy. Notable examples of FTS with laser-based IR sources include micro- and nanoscale IR spectroscopy,1 FTS with sub-nominal resolution beyond the Voigt profile,2 and fingerprinting of trace molecules with IR frequency combs.3,4
The conventional approach to mid-IR femtosecond laser radiation is based on the downconversion of radiation of available visible and near-IR femtosecond lasers in optical parametric oscillators and difference frequency generators. The significant gap between the initial near-IR wavelengths and the desired mid-IR wavelengths results in low efficiency of the downconversion process, which in turn increases the overall system complexity and imposes limitations on the achievable average power in the mid-IR. Starting with a femtosecond source directly in the mid-IR is much more efficient, enabling a much simpler, compact, and robust high-performance device at a fraction of legacy near-IR solution cost.
Emerging technologies
Quantum cascade lasers (QCLs) are another emerging and rapidly evolving mid-IR laser technology, offering many advantages of semiconductor lasers including compactness and ease of use. However, QCLs still suffer from the limited spectral bandwidth and low spatial coherence of the output beam. They also lack compatibility with the femtosecond laser regime.
Transition metal (TM)-doped II-VI (TM:II-VI) semiconductors directly access the 2–6 µm spectral range and represent an appealing alternative to downconversion of near-IR lasers. Zinc sulfide (ZnS) and zinc selenide (ZnSe) doped with chromium (Cr2+) are typical representatives of a large TM:II-VI family. These materials are often referred to as the titanium sapphires (Ti:sapphires) of the mid-IR due to their broad tuning range, from 1.8 to 3.4 µm, and room-temperature operation with high quantum efficiency. For example, Figure 2 illustrates that compared to Ti:sapphire, Cr:ZnS features higher absorption and emission cross-sections at a 3X longer central wavelength. Another convenient feature of Cr:ZnS is optical pumping by low-cost and reliable near-IR fiber lasers—for example, continuous-wave (CW) erbium (Er)-doped fiber lasers (EDFLs).
A unique blend of optical, physical, and laser parameters of Cr:ZnS attracts constant attention from the ultrafast laser community. Many important pioneering results on the generation of femtosecond pulses and optical frequency combs with single-crystal Cr:ZnS and Cr:ZnSe lasers were achieved by Irina Sorokina, Evgeni Sorokin, and Alphan Sennaroğlu in the early 2000s.5 Single-crystal Cr:ZnS and Cr:ZnSe of high optical quality and sufficiently high dopant concentrations are difficult to grow. Most recent progress in Cr-based laser technology has relied on ceramic materials fabricated with the post-growth thermal diffusion doping.6 This technology enabled mass production of large-size laser gain elements with pre-assigned parameters.
Commercially available turnkey, compact femtosecond Cr:ZnS lasers feature few-cycle output pulse trains with watt-level power in a broad range of pulse repetition rates. For instance, Figure 3 shows that the instantaneous spectrum of a commercial mode-locked Cr:ZnS laser oscillator approaches and even exceeds the limits imposed by the gain bandwidth of the laser medium.7
Another set of exciting opportunities arises from a combination of superb ultrafast laser capabilities of Cr:ZnS with high second- and third-order nonlinearities of II–VI semiconductors. Recent studies demonstrate that optically pumped polycrystalline Cr:ZnS supports the generation of supercontinua with exceptionally broad instantaneous spectra spanning the entire transparency window of the material, from the bandgap edge at 0.35 µm to the phonon cutoff at about 14 µm. Notably, the supercontinuum generation occurs in a bulk medium with a remarkably low threshold and at a high multi-megahertz repetition rate of femtosecond pulses. The schematic of ultrabroadband supercontinuum source that is also configurable as an optical frequency comb is illustrated in Figure 3.
The pulse train from a three-cycle Cr:ZnS oscillator is superimposed with a CW EDFL radiation and coupled to a bulk polycrystalline Cr:ZnS gain element with optimized microstructure. The supercontinuum generation in polycrystalline Cr:ZnS is complemented by the generation of optical harmonics (2f, 3f) and optical rectification (0f). The available optical spectra of the Cr:ZnS-based ultrafast sources are illustrated in Figure 4. Depending on the application, one can utilize a milliwatt-level longwave-infrared (LWIR) signal generated directly in the Cr:ZnS or couple the output of the Cr:ZnS laser to an external nonlinear medium. In the latter case, the LWIR power and spectral span can be extended to 0.25 W and 18 µm, respectively.
Importantly, all of the necessary optical signals for converting the supercontinuum source to the optical frequency comb are generated directly inside polycrystalline Cr:ZnS medium. The combs’ carrier-envelope offset frequency (f0) is measured in the 2f to 3f band of the continuum, while the 2f band of the comb with 0.3 W power is used for the measurement of a beating (fB) between a narrowband CW laser—for example, a standard 1.064 µm laser—and a spectral component of the comb. The f0 and fB signals are then phase-locked to a radio frequency (RF) standard. Thus, the spectral components of the LWIR part of the frequency comb are stabilized with the large “lever arm,” allowing it to achieve ultralow, attosecond-level timing jitter of the pulse train.
Femtosecond Cr:ZnS lasers generate broad instantaneous spectra spanning several optical octaves from 1 to 20 µm. This new laser regime is governed by a complex, yet well-controllable interplay between laser and nonlinear interactions in the polycrystalline Cr:ZnS medium. The key advantage of the Cr:ZnS ultrafast laser technology is a high conversion efficiency of low-cost CW EDFL lasers to few-cycle femtosecond pulses in the mid-IR: 20% and 2% optical-to-optical conversion from 1.5 to 2.4 µm and 10 µm, respectively. Because of this advantage, Cr:ZnS femtosecond lasers and combs are compact (125 × 225 × 425 mm3) and lightweight.
Historically, the size and complexity of traditional femtosecond mid-IR laser systems confined them to laser laboratories and required constant attention and deep laser expertise to operate. Cr:ZnS/Se lasers are cost-effective, field-deployable, and ready for real-world practical applications in spectroscopy, imaging, and sensing. The recent introduction of fully referenced mid-IR and LWIR optical frequency combs based on Cr:ZnS lasers provides exciting new opportunities in conventional Fourier-transform spectroscopy and dual-comb spectroscopy.
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