The development of laser‐based sources in the mid‐infrared opened up unprecedented possibilities in many fields including spectroscopy and trace‐gas detection, atmospheric science, greenhouse gas and pollution monitoring, homeland security, hyperspectral imaging, infrared countermeasures, free‐space optical communications, biomedical diagnostics, surgery and neurosurgery, industrial process control, the study of combustion dynamics, organic material processing, and investigation of metamaterials, among others. This chapter reviews the most important applications of the mid‐IR.

7.1 Spectroscopic Sensing and Imaging

Vibrational spectra provide fingerprints of molecular structures, and as such they are used extensively in science and technology. Coherent laser sources in the mid‐IR, especially in the 3–20 μm spectral range, have long been recognized as important tools for both fundamental and applied spectroscopy and sensing. The vast majority of gaseous chemical substances exhibit fundamental vibrational absorption bands in that region, and the absorption of light by these bands provides a nearly universal means for their detection. The main advantage of optical techniques is the nonintrusive in situ detection capability for trace gases and their isotopologues (molecules that contain isotopes). As an illustration, the fundamental absorption spectra, associated with transitions between rotational–vibrational states, for 11 small molecules are shown in Figure 7.1.

The mid‐IR region contains two important windows (3–5 and 8–14 μm) in which the Earth's atmosphere is relatively transparent. When the strongest rotational–vibrational (“ro‐vibrational”) molecular absorptions in the fingerprint region are targeted, one can detect small traces of vapors with sensitivities of parts‐per‐billion (ppb) to parts‐per‐trillion (ppt), in a variety of atmospheric, security, and industrial applications.

There is a plethora of literature on spectroscopic applications in the mid‐IR published to date. An outstanding review by Tittel et al. [2] provides an insight into different spectroscopic techniques using a variety of mid‐IR laser sources. In this chapter, we will focus on the most recent developments with emphasis on the new techniques and advanced laser sources.

7.1.1 QCLs for Spectroscopy and Trace‐gas Analysis

Quantum cascade lasers (QCLs) are almost ideal sources for trace‐gas monitoring, thanks to their small size and high spectral purity, when operated in a single‐longitudinal‐mode (SLM) regime. Recent progress in the development of QCLs has led to the availability of robust mid‐IR spectroscopic light sources capable of high output power and room temperature operation [3–7]. QCLs cover the mid‐IR region from 3.5 to 19 μm, where most molecules have their fundamental absorption bands. For accurate spectroscopic analysis, in most cases both single transverse and single longitudinal mode operation for QCLs are required. The latter can be achieved with distributed feedback (DFB) QCLs [8, 9] or by using an external‐cavity configuration [10]. QCLs can be used to carry out absorption spectroscopy with different techniques including multipass spectroscopy, wavelength‐ and frequency‐modulation spectroscopy, cavity‐enhanced spectroscopy, cavity ring‐down spectroscopy (CRDS), intracavity spectroscopy, magnetic field rotation spectroscopy, photoacoustic spectroscopy (PAS), and photothermal spectroscopy [2, 11].

QCL‐based trace‐gas sensors that rely upon direct optical absorption in the mid‐IR have wide applications for practical detection of trace gases in the atmosphere. Nelson et al. [12] reported measurements of nitric oxide (NO) at λ = 5.26 μm in outside air with a detection limit of less than 1 ppb using a thermoelectrically cooled QCL with a DFB grating that operated in pulsed (duration ~10 ns) frequency‐sweep mode with an “instantaneous” full‐width half‐maximum (FWHM) linewidth down to 0.02 cm⁻¹. With a 210‐m path‐length multiple‐pass absorption cell filled with NO gas at 50‐torr pressure, a detection precision of 0.12 ppb Hz^−1/2 was achieved using a liquid‐nitrogen‐cooled detector.

Daylight Solutions demonstrated a high‐resolution QCL spectrometer based on an external‐cavity QCL (EC‐QCL) that contains a diffraction grating and operates in a continuous‐wave (CW) single‐frequency regime. The instrument can be continuously tuned, without mode hops, over >100 cm⁻¹ of the optical frequency, with a linewidth that can be as narrow as 0.001 cm⁻¹. A closed‐loop servo with feedback was used to optimize the cavity length to select and support the desired single mode at every grating angle. Figure 7.2 shows a portion of the Doppler‐resolved spectrum of N₂O gas at 10‐torr (13‐mbar) pressure obtained with such an instrument [13]. Wysocki et al. performed high‐resolution background‐free magnetic field rotation (Faraday rotation) spectroscopy of NO molecules using room‐temperature CW mode‐hop‐free EC‐QCL with a single‐mode tuning range of 155 cm⁻¹ near λ = 5.3 μm (maximum power of 11 mW). The authors also used another mode‐hop‐free EC‐QCL with a single‐mode tuning range of 7.77–9.05 μm (span 182 cm⁻¹, maximum power 50 mW), to measure high‐resolution N₂O spectrum via direct absorption spectroscopy [14]. Chao et al. performed wavelength‐modulation‐spectroscopy (WMS) for real‐time in situ NO detection in combustion gases at exhaust temperature of 600 K [15]. The laser used for these experiments was a commercial EC‐QCL (Daylight Solutions) that could be tuned mode‐hop‐free from 1895 to 1951 cm⁻¹ (5.1256–5.2770 μm) with a nominal spectral linewidth <0.0015 cm⁻¹. This tuning region can cover 20 pairs of NO absorption transitions in the R‐branch of its fundamental vibrational band.

PAS is based, similar to the optical absorption spectroscopy, on resonant optical absorption process; however, it relies on a different physical principle [16]. When light at a specific wavelength is absorbed by the gas sample, the excited molecules subsequently relax to the ground state by means of non‐radiative processes. This produces localized heating in the gas, which in turn results in the production of a sound wave. The key advantages of this technique are that (i) no optical detector is required – the resulting sound waves can be detected by an inexpensive microphone, and (ii) it is a background‐free technique in the sense that the photoacoustic signal is present only in the presence of absorption and is proportional to the absorption coefficient of a measured sample. A recently developed novel approach to photoacoustic detection utilizes a quartz tuning fork that can be regarded as a resonant acoustic detector [11, 17, 18]. The basic idea of quartz‐enhanced photoacoustic spectroscopy (QEPAS) is to accumulate the acoustic energy in a high‐Q resonant system – a quartz tuning fork. The latter is used, for example, in electronic clocks as a frequency standard (its resonant frequency is ~32.77 kHz). If the incident light intensity is modulated at the same frequency, the photoacoustic signal from the quartz tuning fork can be dramatically enhanced. QEPAS is now widely used for detection of various chemical species, such as NH₃, H₂O, CO₂, N₂O, CO, CH₂O, and NO molecules. The best results in terms of minimum detectable gas concentration have been obtained by adding an acoustic micro‐resonator tube (Figure 7.3) to further enhance the acoustic signal. In such a way, ppb‐level concentrations can be measured for several small molecules [19, 20]. For example, Dong et al. achieved a sensitivity of detecting NO of 5 ppb at one‐second averaging time at an optimal gas pressure of 210 torr by targeting the 1900.08 cm⁻¹ (5.26‐μm) absorption line of NO. The authors utilized an EC‐QCL combined with QEPAS system with an added acoustic micro‐resonator [21].

The CRDS method is based on measurement of the decay time of the intensity of light that is trapped in a low‐loss optical cavity [22, 23]. The laser light is injected into an optical cavity formed by a pair of highly reflective (R > 99.9%) mirrors. (In the case of a narrow‐band CW laser, the laser frequency should match one of the cavity resonances.) The light that is trapped inside the cavity reflects back and forth between the two mirrors, with a small fraction transmitting through each mirror. The resultant leaking light is monitored as a function of time and this allows the decay time of the cavity to be determined. The apparatus is converted to an extremely sensitive absorption spectrometer by placing an absorbing medium (e.g. molecular gas) between the two mirrors and recording the frequency‐dependent decay time of the cavity. Measurement of the ring‐down time allows the absolute single‐pass transmission coefficient of the cavity to be determined with high accuracy. The absorption intensities are obtained by subtracting the baseline absorption of the cavity, which is measured when the laser wavelength is off molecular resonance.

CRDS methods are widely used with QCLs. Kosterev et al. [24] demonstrated a spectroscopic gas sensor for NO based on the cavity ring‐down technique by accessing absorption lines at 1921.599 and 1921.601 cm⁻¹. A CW DFB QCL operating near 5.2 μm was used as a tunable single‐frequency light source. Measurements of ppb NO concentrations in nitrogen with a 0.7‐ppb standard error for a data collection time of eight seconds have been performed.

Using CRDS, Galli et al. demonstrated mid‐IR detection of carbon dioxide containing ¹⁴C isotope (“radiocarbon dioxide,” ¹⁴CO₂) at 4.5‐μm wavelength with an unprecedented parts‐per‐quadrillion (10⁻¹⁵) sensitivity level [25]. The spectroscopic apparatus consisted of a high‐finesse cavity with a ring‐down time constant of 17.5 μs (effective interaction path of 5.2 km). The system employed a narrow‐linewidth (9‐kHz) frequency‐stabilized QCL with a tuning range 2208–2212 cm⁻¹ and an output power 100 mW. The sample gas was kept at 12 mbar pressure and 170 K temperature. The authors used a newly developed saturated‐absorption cavity ring‐down spectroscopic technique, where, at the beginning of the ring‐down process, the intracavity intensity is so high that it saturates the absorption of the sample gas, while at the end of the ring‐down process, the decay rate is determined by the linear gas absorption. This allowed the authors to decouple the linear gas absorption decay rate from that determined by the passive cavity losses. In such a way the authors reached a sensitivity of measurement of ¹⁴CO₂ concentration of 5 × 10⁻¹⁵ with respect to the main isotopologue [25].

7.1.4 Broadband Spectroscopy with Frequency Combs

Optical frequency combs (OFCs), initially conceived for precision spectroscopy of the hydrogen atom in the UV, are now commonly used for probing molecular ro‐vibrational transitions throughout the whole mid‐IR molecular fingerprint region [37]. Spectroscopic applications of broadband OFCs include several scenarios:

• A system where a frequency comb directly interrogates an absorbing sample after which the spectrum is dispersed in two dimensions and sensed with detector arrays [38–40].
• Michelson interferometer‐based Fourier‐transform spectroscopy [41–46] to enhance detection sensitivity; this technique may use a multipass cell [41] or an external high‐finesse optical cavity [45, 47], or it may perform intracavity spectroscopy inside the comb source [43].
• Dual‐comb spectroscopy (DCS). This is a form of high‐speed Fourier‐transform spectroscopy involving no moving parts and capable of combining vast spectral coverage with very high resolution [48–65]. The technique is capable of simultaneously probing molecular absorption on thousands of narrowly spaced wavelengths using a single high‐speed photodetector.

By using an OPO broadband comb (span ~300cm^–1) that operated in the 2.7–4.8‐μm spectral range, combined with a multipass gas cell and a Fourier transform spectrometer (FTS), Adler et al. demonstrated rapid trace‐gas detection with 0.0056‐cm⁻¹ resolution. The authors achieved ppb detection limits in 30 seconds of integration time for several important molecules including methane, ethane, isoprene, and nitrous oxide [41]. Figure 7.4 provides an example of detecting the Q‐branch spectrum of methane with such a system.

Based on a comb source centered at 8.5 μm created by difference frequency mixing in an orientation‐patterned GaP (OP‐GaP) crystal, and in combination with cavity‐enhanced direct frequency comb spectroscopy, Changala et al. obtained the first high‐resolution ro‐vibrational spectrum of the fullerene, C60, molecule [51].

In DCS, which is the most advanced of the comb techniques, a second frequency comb, with a small offset of mode spacing (e.g. pulse repetition frequency), effectively plays the role of the time‐delayed second arm in the Fourier‐transform spectrometer. DCS requires a high degree of coherence between the two combs, and most reports that simultaneously take full advantage of the dual‐comb technique (broad spectral coverage, comb‐tooth resolved spectra, and rapid scans) have till recently privileged the near‐IR domain [52–54]. The true promise of DCS for sensitive molecular detection lies in the mid‐IR and proof‐of‐principle demonstrations of molecular detection have been carried out in different spectral regions of the mid‐IR. These include: the 10‐μm wavelength region (instantaneous spectral span 250 cm⁻¹, spectral resolution 2 cm⁻¹) [49], the 2.4‐μm region (span 200 cm⁻¹/resolution 2 cm⁻¹) [55], the 3‐μm region (150 cm⁻¹/0.2 cm⁻¹ [56]; 250 cm⁻¹/0.8 cm⁻¹ [57]; 350 cm⁻¹/0.2 cm⁻¹ [58]; 250 cm⁻¹/0.07 cm⁻¹ [59]; and 160 cm⁻¹/0.5 cm⁻¹ [60]), and the 6.5‐to‐7.5‐μm region (span 85 cm⁻¹/resolution 0.3 cm⁻¹) [61]. A very high spectral resolution has been demonstrated with a DFG‐based dual comb in the 3.4‐μm region, although with reduced simultaneous spectral coverage (spectral span 30 cm⁻¹/resolution 10 kHz ≈ 3 × 10⁻⁷ cm⁻¹) [62], and in the 7‐μm region with the use of QCL‐based dual frequency comb (spectral span 16 cm⁻¹/resolution 0.003 cm⁻¹) [63]. A broadband (2.6–5.2 μm) DCS was reported in [64] using dual combs created via DFG. The above span was covered in several steps, by adjusting the proper poling period of the frequency‐mixing crystal for each step.

Most recently, using a pair of broad‐bandwidth subharmonic OP‐GaAs OPO combs (Figure 7.5) with a high degree of mutual coherence, the acquisition of 350 000 comb‐tooth resolved data points over the entire (no gaps) range of 3.1–5.5 μm (spectral span >1300 cm⁻¹, spectral resolution <0.0038 cm⁻¹) has been achieved [65]. With a measurement time varying from 7 ms to 1000 s, the authors demonstrated parallel detection of 22 molecular species (including isotopologues) in a mixture of gases and demonstrated the full potential of the dual‐comb technique in the mid‐IR (spectral coverage, spectral resolution, speed, and the absolute frequency referencing, e.g. to atomic clock). Molecular spectra obtained with a 76‐m multipass are shown in Figure 7.6. Also, an exceptional degree of coherence between the two subharmonic OPOs (residual linewidth 25 mHz, mutual coherence time 40 seconds) allowed resolving frequency comb modes with a finesse of 4000.

A paper by Vainio and Halonen gives a very good review of different methods for producing frequency combs based on nonlinear optics and gives examples of their use for molecular spectroscopy [66]. Another excellent paper, by Cossel et al. [67], provides an overview of broadband spectroscopy methods and applications, such as atmospheric measurements, chemical kinetics, breath analysis, astrochemistry, fundamental laboratory spectroscopy, and industrial applications.

7.1.5 Hyperspectral Imaging

The goal of hyperspectral imaging is to add spectral information for each pixel of the 2D image of a scene, with the purpose of finding hidden objects, identifying materials, detecting trace gases, etc. Stothard et al. demonstrated a scanning system for active, real‐time hyperspectral imaging in the mid‐IR [68]. The authors used a combination of a CW tunable OPO, an electromechanical scanning imager, and a single detector. The elegant design of a pump‐enhanced CW OPO, which used periodically poled RTA (PP‐RTA) crystal, and where the pump and the signal wave were both resonant, required only <500 mW of pump power from a 1064‐nm Nd:YVO₄ laser to achieve the OPO threshold. With 1 W of pump power, the OPO provided adequate mid‐IR idler‐wave power (~50 mW) for 2D scanning.

The geometry of the scanner was such that the radiation from the OPO was directed to the scene under surveillance via the polygon scanner with 10 facets and a tilting mirror. The backscattered radiation (that can be approximated as Lambertian reflection into the 2π solid angle) returning from the scene was collected via the same tilting mirror and the polygon scanner, and was then focused by the collection lens onto a mid‐IR detector with the sampling rate for digitization of 200 kHz. Due to the wide spectral coverage (3.18–3.5 μm) and high spectral resolution (<1 GHz), the system was capable of being selectively tuned into narrow (~5 GHz) absorption features of a wide variety of gaseous species in the atmosphere. As an example, the authors showed how a strong absorption peak exhibited by methane at 3057.7 cm⁻¹ (3.27 μm) can be accessed and gas plumes can be imaged in concentrations as low as 30 ppm from the distance of several meters (Figure 7.7).

Dam et al. [69] demonstrated a compact imaging system where the incoming mid‐IR light (coherent or incoherent) was upconverted in a single pass through a periodically poled lithium niobate (PPLN) nonlinear crystal, by mixing with a resonating 1064‐nm Nd:YVO₄ laser beam, so that the mid‐IR 2D image information was conserved in the process. The upconverted near‐IR image near 800‐nm wavelength, with quantum conversion efficiency 20%, was then detected by a charge‐coupled device (CCD) camera. The system can be tuned to any wavelength band in the 2.85–5 μm region with the bandwidth, determined by the phase matching, which varied from 5 nm at 2.9 μm to 25 nm at 3.8 μm. According to the authors, by combining image upconversion and modern low‐noise electron‐multiplying CCD cameras, one can obtain mid‐IR images with a sensitivity of about one photon per pixel.

7.2 Medical Applications

7.2.1 Laser Tissue Interactions

The laser was envisioned as a surgical tool very soon after its first demonstration. Laser surgery provides a noncontact approach capable of cutting tissue with up to single‐cell precision. Short‐pulsed, mid‐infrared lasers are promising tools for human surgical applications that complement other medical lasers, due to their ability to ablate tissue with relatively little damage to surrounding tissue and reduced photochemical side effects.

A residual tissue damage needs to be minimized, hence the most important laser parameters are wavelength, pulse duration, and pulse fluence (pulse energy per unit area). High optical absorption (and small penetration length) of the tissue, typical for the mid‐IR wavelengths, is beneficial since it reduces the pulse fluence needed to superheat the tissue material and drive ablation; also, high optical absorption combined with short pulse duration help to reduce the residual tissue damage. By using laser sources operating in the mid‐infrared, water‐rich, lipid‐rich, and protein‐containing tissues can be ablated with minimal collateral damage.

7.2.1.1 Holmium and Thulium Surgical Lasers

Holmium and thulium lasers operating near 2 μm are most widely used in surgery. Cited below is an extract from the National Institute of Health report regarding the wide use of multi‐watt pulsed Ho:YAG lasers (λ = 2.1 μm): “Ho:YAG lasers, like CO₂ lasers, offer precise cutting with minimal damage to adjacent tissue; however, unlike CO₂ lasers, they also offer fiber optic delivery (which is ideal for endoscopic use) and the ability to treat tissue in a liquid‐filled environment (e.g. saline, blood). The initial specialty for which the Ho:YAG laser was used was arthroscopic surgery, especially discectomy (the surgical removal of herniated disc material that presses on a nerve root or the spinal cord). Today, it is effectively used in many surgical specialties, including general surgery, urology, laparoscopy, neurosurgery, lithotripsy, angioplasty, orthopedic surgery (e.g. meniscectomy, bone sculpting, plastic surgery, and some experimental surgery, such as cartilage shrinking to tighten loose joints), and dentistry. Because of its broad range of potential applications, it has been called the Swiss Army Knife of lasers” [70].

Also, in soft‐tissue medicine, direct application of the Tm³⁺‐doped silicate fiber laser has been widespread, owing to the fact that its emission wavelength (1.94 μm) overlaps with the absorption wavelength of the combination vibration of the O─H bond in water. The high‐power thulium fiber laser is capable of vaporization of the prostate and incision of ureteral and bladder tissues. This laser has several potential advantages over the Ho:YAG laser, including smaller size, more efficient operation, more precise incision of tissues, and operation in either the pulsed or the CW mode. Thulium fiber‐based lasers of high average power and short pulse lengths are extremely beneficial for clinical use in rapid vaporization of the prostate and more precise incision of urethral/bladder neck strictures [71].

7.2.1.2 Er:YAG Lasers (λ = 2.9 μm)

Wavelengths near 2.9 μm, which are close to the fundamental absorption peak of stretching vibrations in water, have significant benefits for laser surgery. In tissue, which is typically 70% water, the optical penetration depth near 2.9 μm is approximately 1 μm (water is a natural chromophore in human tissue). This means that the zone of thermal damage is minimized, especially when short pulses are used.

Figure 7.8 displays the absorption coefficient of liquid water over a large (six decades) span of frequencies, from the UV to THz. For clinical use, the major advantage of mid‐IR 2.9‐μm radiation over UV excimer laser radiation is that there are no mutagenic or carcinogenic effects of infrared radiation [72, 73]. Kaufmann et al. have shown that pulsed 2.94‐μm Er:YAG laser surgery allows the extremely precise etching of superficial skin lesions and also has a potential for skin resurfacing [74]. Franjic et al. reported on the highly efficient ablation of tooth enamel using 55‐ps‐long laser pulses at λ ≈ 2.95 μm [75]. The 2.95‐μm wavelength used in the experiment was both in resonance with the OH‐stretch mode of water, and also near the absorption peak of hydroxyapatite crystals (hydroxyapatite is the predominant mineral of dental enamel). The reported fluence needed to achieve ablation was 0.75 J/cm², several times smaller than the typical ablation thresholds reported for longer laser pulses, with nanosecond and microsecond pulse durations.

Fiber lasers of the 3‐μm range (see Chapter 3) have the potential to make a significant impact in laser surgery. Pierce et al. performed preliminary studies of the interaction between a CW erbium‐doped fiber laser operating at around 3 μm and soft biological tissue [76]. The pulse‐periodic nanosecond regime of such lasers would be particularly valuable for laser surgery. Also, solid‐state pulse‐periodic Cr²⁺:ZnS/ZnSe lasers tuned to 2.9‐μm wavelength (see Chapter 2) can be a good alternative to Er:YAG lasers for tissue ablation, thanks to their high efficiency and ability to operate at high repetition rates.

7.2.1.3 Importance of the Spectral Band of 6–7 μm

The ultimate goal of medical laser ablation is to remove a defined amount of material in an efficient manner with the least amount of collateral damage. Investigations were centered on two specific mid‐IR wavelengths, 6.1 and 6.45 μm. These wavelengths coincide with, respectively, the amide I and amide II absorption bands of protein, and, in addition, with the bending mode of water at 6.1 μm [77]. At these wavelengths, energy is coupled into the protein matrix as well as the bound and unbound water within the tissue.

Edwards et al. reported that targeting laser radiation wavelength to the amide II band of proteins at 6.45 μm leads to the ablation of rat brain cortex tissue with minimal collateral damage while maintaining a substantial ablation rate [78]. The authors found that the collateral damage (coagulation necrosis) at λ = 6.45 μm was much smaller than at λ = 2.94 μm, at the same dosage. They used a free‐electron laser (FEL) in their experiments. FEL is a very flexible source of electromagnetic waves, in terms of tunability, from the UV to THz, although bulky and very expensive, since it involves an electron accelerator. FEL output was in the form of 5‐μs‐long macropulses consisting of a series of short, ~1 ps, pulses repeated at an electron beam repetition rate of 2.85 GHz; thus, there were ~15 000 micropulses per macropulse and each macropulse, with ~20 mJ energy, was emitted at a repetition rate of ~4 Hz.

Peavy et al. [79] made comparison of cortical bone ablations by using different FEL wavelengths, from λ = 2.9 to 9.2 μm. The main conclusion was that the use of wavelengths in the 6.1‐μm amide I to 6.45‐μm amide II region were the most efficient for cutting cortical bone, and produced less collateral thermal injury than cutting with a surgical bone saw.

Since FELs proved too costly and too complex for widespread surgical use, several alternative laser systems at 6–7 μm were developed and have demonstrated the ability to cleanly cut soft tissues with little collateral damage [80, 81]. Mackanos et al. [81] used a 2.8‐μm Er:YSGG laser pumped‐tunable (6–8 μm) ZGP OPO producing millijoule‐level 100‐ns pulses at 5 Hz repetition rate. Among several wavelengths studied, the best results were produced with the two wavelengths: λ = 6.1 μm (the range of high water and protein amide I band absorption), and λ = 6.45 μm (moderate water absorption and high protein amide II band absorption). When using porcine cornea, the average zone of thermal damage at λ = 6.1 μm was found to be 4.1 μm for the OPO and 5.4 μm for the FEL. At λ = 6.45 μm wavelength, the damaged zone was 7.2 μm for the OPO and about the same for the FEL. Thus, the OPO caused similar or less thermal damage when compared with the FEL while generating significantly deeper craters.

Edwards et al. developed a four‐stage laser system that starts from a Nd:YLF laser, uses a combination of stimulated Raman scattering and difference frequency mixing, and generates 6.45‐μm pulses with 3–5‐ns pulse duration, 2‐mJ pulse energy, and 1/2‐Hz repetition rate [82]. The laser system was used to ablate rat brain tissue, where both a collateral damage and an ablation rate compare favorably with those previously observed with an FEL. Kozub et al. demonstrated a Raman‐shifted alexandrite laser for soft tissue ablation (they used goat cornea, rat heart, rat skin, and rat kidney) in the 6–7‐μm wavelength range [83]. In this setup, a tunable alexandrite laser pumped a two‐stage Raman converter; the first‐order Stokes shifted light (near 1 μm) was generated in a deuterium (D₂) convertor; this output then generated the second‐order Stokes beam in a multipass hydrogen (H₂) convertor to yield mid‐IR light. Tunable output, from 6 to 7 μm, was achieved by tuning the alexandrite laser from 771 to 785 nm. The mid‐IR pulses had up to 9‐mJ pulse energy, 10–20 ns pulse duration, and 10‐Hz repetition rate.

7.3 Nano‐IR Imaging and Chemical Mapping

Infrared spectroscopy has been a benchmark technique in a broad range of sciences and industries to characterize and identify materials via vibrational resonances of chemical bonds on a macroscale. Because of the diffraction limit, traditional mid‐IR techniques, such as Fourier‐transform IR (FTIR) microscopy, cannot measure spectra on a scale that is below several wavelengths. The ability to identify material with nanoscale resolution under the tip of an atomic force microscope (AFM) has always been an ambitious goal in the AFM community. While AFM can measure mechanical, electrical, magnetic, thermal, and other properties of materials, it has lacked the robust ability to chemically characterize unknown materials. Keilmann and coworkers [116–119] introduced a new type of microscopy that provides simultaneous spectral identification and a spatial resolution that is far beyond the classical Abbe diffraction limit of one‐half wavelength (λ/2), and also beyond the practical limit (~λ/10) of aperture‐based scanning near‐field optical microscopy (SNOM). The operation principle is based on the fact that the illuminated tip of an AFM can exhibit enhanced optical fields in its neighborhood. Such near fields are modified by the presence of a sample. As a consequence of this near‐field interaction, the scattered light measured in the far field carries information on the sample's local optical properties. This near‐field scattering is the basis of what has been named scattering‐type scanning near‐field optical microscopy (s‐SNOM). In practice, the metallized tip of an AFM serves as a scatterer, and it is the radius of curvature at its apex (typically 20 nm or less) that determines both the mechanical and the optical resolution. Hence, there is no wavelength‐related resolution limit, and a resolution of λ/500 was obtained in the mid‐IR at λ = 10 μm [117]. The principle of s‐SNOM is illustrated in Figure 7.9. Here, a focused IR beam illuminates the tip region of an AFM, where a sample is approached and scanned to produce a topographic image. By recording the backscattered light, an optical image is simultaneously generated at different wavelengths. To improve the signal‐to‐noise ratio, the tip oscillates at the cantilever's mechanical resonance frequency Ω (tapping mode) so that the near‐field optical signal is modulated at Ω and its harmonics. This allows a synchronous lock‐in detection. The IR laser wavelength has a clear effect on the IR image (Figure 7.9c and e). As the wavelength is tuned to a vibrational resonance of polystyrene embedded in polymethylmethacrylate (PMMA), the IR image is darker. This effect disappears at absorption‐free wavelengths, giving clear evidence of near‐field vibrational image contrast [116].

Dazzi and colleagues introduced a different principle of nano‐chemical mapping with an AFM using the photoacoustic (photothermal) effect [120–123]. When the wavelength of a pulsed (nanosecond) laser is tuned to the absorption resonance, a fast heat deposition results in rapid thermal expansion of the absorbing region. This expansion pulse is then detected with the tip of an AFM through detecting the oscillations of the cantilever at its resonant modes. Thus, the system acts as a detector of extremely small motions induced by optical absorption. The detection scheme used here is analogous to PAS, except that it is the AFM tip and cantilever that are used to detect the thermal expansion, instead of a microphone in a gas cell. In life sciences, the AFM‐IR technique provides a label‐free method for mapping IR‐absorbing species in biological materials. For example, Dazzi et al. performed chemical spectroscopy at the subcellular level and demonstrated chemical mapping of the distribution of viruses in the infected bacteria Escherichia coli [123]. By tuning the laser to a DNA absorption peak around 1080 cm⁻¹ (9.26 μm), the authors were able to localize single viruses (bacteriophage T5) inside the bacteria. Figure 7.10 shows that while the topography performed by an AFM was not able to reveal the viruses, the AFM‐IR was able to do so. As a tunable source in their AFM‐IR experiments, the researchers used a pulsed mid‐IR FEL “CLIO” located in the Centre Laser Infrarouge d'Orsay, France [121–123]. In 2010, Anasys Instruments, Inc., commercialized a compact version of the AFM‐IR device that uses a broadly tunable benchtop IR source based on a nanosecond OPO [124].

Lu and Belkin reported a photothermal technique that allows obtaining AFM‐IR spectra under low‐average‐power illumination from a pulsed tunable QCL (energy per pulse 4 nJ, duration 40 ns) [125]. To detect minute thermal expansion in a polymer deposited on a silicon substrate, the authors tuned the repetition frequency of the QCL to be in resonance with the mechanical oscillation frequency of the AFM cantilever, which is in the range 10–200 kHz. The enhancement of the AFM response, as compared to a single pulse excitation, is equal to Q, where Q ≈ 100 is the quality factor of the cantilever oscillation. The achieved spatial resolution of 50 nm (λ/170) was determined by the thermal diffusion length in a sample during the laser pulse duration; it may be further improved using shorter QCL pulses [125]. This technique can be regarded as a nanoscale analog of the QEPAS [17].

7.4 Plasmonics in the Mid‐IR

Plasmons – charge oscillations at the interface between a dielectric and a metal – were first studied in the visible regime, with increased interest now extending to the mid‐IR [126]. Plasmons give rise to very strong local fields and can be guided along the interface in the form of a traveling wave, known as a surface plasmon polariton. The excitement generated by mid‐IR plasmonics comes from the interplay between circuit (antenna) theory and wave optics. To date, research in mid‐IR plasmonics is focused on emitters (in particular, related to QCLs), improved IR detectors, and chemical sensing surfaces. Another goal of mid‐IR plasmonics is to increase the absorption in a given volume of material: in mid‐IR detectors, smaller volumes provide lower noise, and higher material absorption results in a stronger output signal. One of the ways to achieve this goal is to use an antenna‐like structure to couple incident light to surface plasmons, which are focused by the antenna to a small detection region.

Based on mid‐IR plasmonics, Yu, Wang, and Capasso demonstrated a QCL with improved beam characteristics. This was achieved by monolithically integrating specially designed plasmonic structures on the cleaved substrate of a QCL, below the laser waveguide [127]. The plasmonic structure creates desirable far‐field radiation by coupling laser emission into surface waves propagating on the substrate and by coherently scattering the energy of the waves into the free space. This is illustrated in Figure 7.11a, which shows a scanning electron microscope (SEM) image of a λ = 9.9 μm QCL with an integrated 1D plasmonic collimator, which is made of gold and has the following geometry: aperture size = 2 × 25 μm, grating period = 8.9 μm, distance between aperture and the first grating groove = 7.3 μm, groove width = 0.8 μm, and groove depth = 1.5 μm. One can see from Figure 7.11c that the measured divergence of the beam is dramatically improved in the “fast” (vertical) direction when the plasmonic collimator is used.

The other avenue of plasmonics is spectroscopic chemical sensing with surface‐enhanced IR absorption using metamaterials. Metamaterials are artificially constructed materials that consist of subwavelength elements whose spatially averaged response can be treated as that of a homogeneous medium with a characteristic effective permittivity and permeability. This permits the optical properties to be engineered, enabling, for example, near‐unity absorbance at the surface [128, 129]. Chen et al. demonstrated a dual‐band perfect absorber for plasmon‐enhanced infrared spectroscopy using an asymmetric cross‐shaped antenna made of gold nanoparticles that were placed on top of a thick gold film and separated from it by a magnesium fluoride (MgF₂) spacer layer [129]. The authors demonstrated, by mid‐IR reflection spectroscopy, strong absorption enhancement for the two absorption bands: the C─H band at 2850 cm⁻¹ (3.5 μm) and C═O band at 1700 cm⁻¹ (5.9 μm) of a thin (4‐nm‐thick) polymer film. Even despite the fact that the two bands were well separated in wavelength, the authors demonstrated that both of them have shown a large field enhancement.

7.5 Infrared Countermeasures

Infrared countermeasure (IRCM) systems are used to defend the host aircraft from heat‐seeking attacking missiles by detecting the approaching threat and disabling it through the use of directed laser energy (Figure 7.12). The laser intensity should be higher than that of the target (usually blackbody radiation from a jet engine) in the appropriate mid‐IR range. When the mid‐IR radiation produced by an IRCM system is seen by a missile seeker, it overwhelms the signal from the aircraft jet and confuses the missile.¹ The missile will begin to deviate from the target, rapidly breaking lock. Once an infrared seeker breaks lock (a typical field of view of 1–2°), it rarely reacquires the target. Typically, high‐power broadband pulse‐periodic ZGP OPO systems with 10‐W average power at 3–5 μm (BAE systems) or high‐power QCL systems with 10‐W average power (the high‐power beam is obtained by combining outputs from few QCLs) at 4–5‐μm wavelength (Daylight Solutions, Pranalytica) are utilized. Northrop Grumman uses similar technologies (OPOs and QCLs) in their multiband IRCM laser systems.

7.6 Extreme Nonlinear Optics and Attosecond Science

When energetic few‐cycle laser pulses interact with an atomic gas (He, Ne, Ar, etc.), with peak intensities >10¹⁴ W/cm², one can generate high odd‐number optical harmonics reaching the extreme UV and even X‐ray spectral range. High‐harmonic generation (HHG) in this case can no longer be explained by perturbative nonlinear optics, which predicts decrease of the harmonic power with harmonic number. Rather, the process is initiated by electron tunneling and completed by the recollision of a highly energetic electron with its associated ion. Three decades of research into high‐order harmonic generation in gases has uncovered several practical routes for generating phase‐matched coherent X‐ray beams using tabletop femtosecond lasers [130, 131]. A bright supercontinuum that spans the entire spectrum from the UV to more than 1.6 keV with the harmonic number as high as >5001 can now be produced, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. This opens up two major classes of applications: (i) creating a tabletop source of coherent X‐rays that might be used for super‐resolution imaging in medicine and nanotechnology; (ii) in fundamental science – the attosecond time durations of high‐harmonic pulses can be applied to capture the fastest events in the natural world, for instance, for imaging molecular orbitals.

The highest energy for the HHG photon emitted is given by the microscopic single‐atom cutoff rule:

(7.1)

where I_p is the ionization potential of the gas atom and U_p is the ponderomotive energy of a free electron (the average energy of an electron driven by the oscillating laser field), which is proportional to the driving laser intensity and the wavelength squared. The quadratic wavelength dependence suggests a counterintuitive result that using a longer‐wavelength driving laser should result in a higher cutoff photon energy (and thus more energetic X‐rays). This prediction was verified by several experiments in different laboratories [131].

Figure 7.13 shows experimental HHG spectra emitted under phase‐matching conditions as a function of driving‐laser wavelength of 0.8, 1.3, 2, and 3.9 μm. One can see that for a longer‐wavelength driving laser, larger X‐ray cutoff photon energy was achieved. Popmintchev et al. used 80‐fs, 10‐mJ pulses at 20‐Hz repetition rate at λ = 3.9 μm from an optical parametric chirped‐pulse amplification (OPCPA) system. The phase‐matched HHG emission was produced by focusing the optical beam into a 200‐μm diameter in a 5‐cm‐long, high‐pressure He gas‐filled hollow waveguide, with the spectrum extending beyond 1.6 keV (λ < 7.7 Å). The broadband X‐ray supercontinuum could support (if properly compressed) a single‐cycle X‐ray pulse with 2.5‐attosecond duration (inset to Figure 7.13) [131].

Weisshaupt et al. have demonstrated the generation of hard X‐ray photons, consisting of the characteristic X‐ray lines of copper (Cu) at energies K_α ≈ 8.0 keV and K_β ≈ 8.9 keV and of a broad Bremsstrahlung component that extended to ~100 keV, by the interaction between a Cu target and intense 15‐mJ femtosecond mid‐IR pulses at 3.9 μm, generated by OPCPA [132]. X‐ray generation occurred in this case via inner‐shell ionization of copper followed by a radiative transition of an outer‐shell electron into the unoccupied inner shell. The flux of X‐ray photons was about 25 times higher than in the case when 800‐nm pulses were used. This enhancement of the X‐ray flux is caused by the much more efficient electron acceleration at a longer wavelength, which results in higher kinetic energies of the electrons accelerated in the optical field. This makes high‐energy, long‐wavelength (>3 μm) lasers with few‐cycle pulse duration and high repetition rate a highly desired driving source for the generation of coherent high‐harmonic X‐rays and isolated attosecond pulses in atomic gases (see Chapter 2 for the recent solid‐state laser development).

HHG with strong electromagnetic fields has also been demonstrated in bulk crystalline materials. Chin and coauthors have observed extreme nonlinear optical phenomena in bulk semiconductors, such as ZnTe, ZnS, ZnSe, and GaAs, produced by intense (10⁹–10¹¹ W/cm²) mid‐IR pulses at λ = 3.5–3.9 μm [133]. These phenomena include generation of multiple mid‐IR harmonics (up to the seventh harmonic in the case of ZnSe) below the bandgap edge of semiconductors, accompanied by significant broadening of mid‐IR harmonic spectra, which the authors attribute to the onset of self‐phase modulation (SPM) as well as cross‐phase modulation (XPM) at these high intensities.

Ghimire et al. demonstrated nonperturbative (i.e. that cannot be described in the frame of nonlinear optics based on perturbation theory) HHG in a strongly driven wide‐bandgap (3.2 eV) semiconductor ZnO using 3.2–3.7‐μm (0.34–0.38‐eV) few‐cycle laser pulses having peak intensity 5 × 10¹² W/cm² and field strength 60 MV/cm (corresponding to 0.6 V/Å, comparable to atomic field strength) [134]. The authors measured harmonics up to the 25th order (Figure 7.14), extending to the photon energy of 9.5 eV (λ = 130 nm) – more than 6 eV above the band edge. They also found that the high‐energy cutoff scaled linearly with the driving field (in contrast to the atomic case where the scaling is linear to the intensity). The spectrum comprised odd, or both odd and even, harmonics, depending on crystallographic orientation. Vampa et al. suggested, based on their high‐field HHG experiments in ZnO, that at similar field strengths and laser wavelengths, a recollision between an electron and its associated hole is the primary source of high harmonics [135].

Also, using few‐cycle mid‐IR laser excitation at the driving laser center wavelength of 10 μm (30 THz), with peak fields of up to 72 MV/cm, high harmonics have been generated in a 220‐μm‐thick semiconducting gallium selenide (GaSe) [136]. The authors observed the emission of phase‐stable transients, covering the entire terahertz‐to‐visible spectral domain between λ = 3 mm and λ = 440 nm (0.1–675 THz). Emission between 0.1 and 10 THz originated from the optical rectification in GaSe, while the shorter wavelengths were generated via HHG (up to order n = 23) in the non‐perturbative regime. The authors attributed the main mechanism of HHG to coherent interband polarization combined with dynamical Bloch oscillations [136].

Langer et al. studied the effects of strong optical field in a monolayer of tungsten diselenide (WSe₂). Electron–hole pairs were generated by a near‐IR pulse and were subsequently accelerated by a mid‐IR transient with center wavelength 7.5 μm (40 THz), ~3‐cycle pulse duration, and peak electric field (in air) of 18 MV/cm, followed by a recollision of the constituent electron and hole. Recolliding electrons and holes emitted their kinetic energy in high‐order sidebands of the near‐IR pump. Despite a low sample thickness, harmonic sidebands (spaced by mid‐IR laser frequency) up to order n = 12 (λ = 340 nm, 880 THz) were observed [137].

7.7 Other Applications

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