**2. Photochemical lasers**

The photochemical method of gaseous active media pumping by radiation from broadband optical sources originates from the development of the high‐power photodissociation laser in metastable iodine atoms (λ = 1.315 μm) (for example, see [6–8]), resulted in variety of remark‐ able results: 1 MJ of output energy in a single beam [9], 2 and 30 kJ in a short laser pulse obtained, respectively, with flash lamp [10] and surface discharge [11] optical pumping. The breakthrough results obtained in the course of the iodine photodissociation laser development stimulated extensive studies of the potentialities of the gaseous active media optical pumping and the search for new active media for high‐power optically driven lasers in many laboratories around the world.

laser systems providing pulses shorter than 100 fs are based on the Ti:sapphire and optical

Presented in this chapter are milestones and main results obtained in the course of the realization of a novel hybrid (solid/gas) approach to the development of femtosecond systems that, unlike the all‐solid‐state systems operating in the near‐infrared (NIR) region, allow for producing super‐intense optical pulses in the blue‐green spectral range. This approach aims to marry robust solid‐state laser technologies highly developed for femtosecond pulse generation and amplification in the NIR spectral range with advantages of photochemically

Most extensive development of the photochemical method of pumping gaseous active media dates back to the 1960s–1990s of the last century (see [3–5] and references cited therein). Being applied to optical excitation of gas lasers on electronic molecular transitions by radiation from such unconventional pump sources as high‐temperature electrical discharges and strong shock waves in gas, This method resulted in emerging a new class of gaseous active media for lasers emitting in the spectral range extending from the NIR to UV with a high output energy increasing in proportion to the active volume and pump energy. Among a variety of molecules lasing upon optical excitation, there are three broadband active media (XeF(C‐A), Kr2F, and Xe2Cl), which offer a number of characteristics extremely attractive for the amplification of femtosecond optical pulses up to ultrahigh peak powers. The gaseous nature and visible spectrum of emission of these media promise important virtues of the hybrid systems. First of all, the gaseous nature of these media is characterized by low nonlinear refractive index allowing amplification of optical pulses with much higher intensities as compared with solid media. Secondly, the visible spectrum of emission requires nonlinear frequency upconversion of a seed pulse generated in the NIR spectral region by a solid‐state front end, thereby providing efficient temporal cleaning of the ultrashort optical pulse and the high temporal contrast ratio of an output pulse, which is of primary importance for a number of high‐field

The main motivation for the development of hybrid systems in the visible is favorable drive frequency scaling of laser‐matter interaction in a number of high‐field applications. Of overriding importance is the dramatic improvement of the recombination soft X‐ray laser

The photochemical method of gaseous active media pumping by radiation from broadband optical sources originates from the development of the high‐power photodissociation laser in metastable iodine atoms (λ = 1.315 μm) (for example, see [6–8]), resulted in variety of remark‐ able results: 1 MJ of output energy in a single beam [9], 2 and 30 kJ in a short laser pulse obtained, respectively, with flash lamp [10] and surface discharge [11] optical pumping. The breakthrough results obtained in the course of the iodine photodissociation laser development stimulated extensive studies of the potentialities of the gaseous active media optical pumping

excitation in an optical field ionized (OFI) plasma with drive frequency.

parametric chirped‐pulse amplification (OPCPA) technologies.

driven gaseous gain media of the visible range.

1322 High Energy and Short Pulse Lasers

experiments.

**2. Photochemical lasers**

It is important to stress the major impact of untraditional pumping sources on the photo‐ chemical method development, such as high‐current open discharges initiated with exploding wires or sliding sparks, as well as strong shock waves driven by detonation of chemical explosives [4]. These pumping sources, initiated directly in laser working mixtures, had no shell separating them from an active medium that removes limitations on energy deposition into the pumping sources and makes it possible to utilize the radiation in any spectral range, including the UV and VUV, where the most intense absorption bands of the overwhelming majority of molecules are located. As compared with ordinary flash lamps, these pumping sources have much higher brightness temperature reaching 30–35 kK.

The main emphasis in these studies was placed on molecular transitions that, unlike atoms, do not require complete population inversion of the electronic states participating in the laser transition and enable a fairly simple depletion of the lower laser levels due to vibrational relaxation and/or dissociation of the lower state. The application of such optical sources has led to emerging new class of gas lasers emitting in the spectral range from the NIR to UV regions due to the development of a variety of photochemical excitation techniques relying on the photolysis and direct optical excitation of molecular gases, as well as secondary photo‐ chemical reactions (see [3–5] and references cited therein).

One of the most remarkable results obtained in the course of these studies is the optical excitation of lasing on broadband bound‐free Kr2F(4 <sup>2</sup> Γ‐1,2 <sup>2</sup> Γ), Xe2Cl(4 <sup>2</sup> Γ‐1,2 <sup>2</sup> Γ), and XeF(C‐ A) transitions in the visible. These active media are very sensitive to the internal losses because they have much lower small‐signal gain as compared with excimers emitting in the UV spectral range on the B‐X transition due to the large width of their luminescence spectra and long radiative lifetime of the excited states. For this reason, e‐beam or fast discharge pumping of these active media turned out to be ineffective since electron excitation technique is based on plasmochemical reactions involving ionized and highly excited atoms and molecules that are characterized by strong absorption in the visible [12]. For example, laser action from electron‐ beam‐excited Kr2F and Xe2Cl active media was observed only in the afterglow of the pump pulse when the transient absorption is significantly reduced [13]. The photochemical techni‐ que, relying on reactions of neutrals excited to low‐lying energetic states, free of the short‐ comings associated with transient absorption. This makes optical pumping to be superior to electron excitation in the efficiency of producing laser action on the aforementioned transitions.

Due to extremely broad gain bandwidth supporting pulses of shorter than 10 fs, these active media are of practical interest for amplifying femtosecond optical pulses to ultrahigh peak powers [4, 5, 14, 15]. The main spectroscopic characteristics of the transitions are listed in the **Table 1**. One of most important parameters is the saturation fluence accounting for the maximum energy extraction per surface unit of an amplifier output aperture. This parameter, ranging from 0.05 J/cm<sup>2</sup> for XeF(C‐A) to 0.2 J/cm<sup>2</sup> for Kr2F due to rather long radiative lifetime of the upper laser states, promises peak power, Iout, of up to ∼10 TW per square cm of an output aperture in a 25 fs pulse. At the same time, the gas active media are easily scalable to large volumes with wide aperture (several hundred cm<sup>2</sup> ) to reach PW level of output peak power. Moreover, the combined use of two media with spectrally shifted emission bands (Kr2F with XeF(C‐A) or Xe2Cl) in an amplifier chain makes promising for the amplification of even shorter optical pulses due to twofold broadening a gain bandwidth (**Figure 1**) and spectrally inho‐ mogeneous gain saturation.


λmax is the wavelength of the gain maximum.

Δλ is the gain bandwidth.

τFT is the transform‐limited pulse width (for Gaussian profile).

τsp is the spontaneous lifetime.

σst is the stimulated emission cross‐section.

εsat is the saturation fluence.

Iout is the estimated maximum peak power density related to a laser amplifier output aperture.

**Table 1.** Characteristics of broadband active media.

**Figure 1.** Emission spectrum for the mixed Kr2F/XeF(C‐A) system.

An important advantage of gaseous active media resides in the much lower value of the nonlinear index of refraction as compared with solid‐state materials, which allows amplifica‐ tion of optical pulses at much higher intensities than in condensed gain media. For the first time, hybrid architecture employing a gaseous active medium was realized in the system comprising a dye femtosecond oscillator and e‐beam or fast discharge‐driven boost amplifier on the UV B‐X transitions of ArF, XeCl, KrF, and XeF rare‐gas‐halide excimer molecules. The highest peak power obtained in the hybrid systems of this type reached ∼1 TW in a 310 fs pulse [16]. However, gain bandwidth on the B‐X transition does not exceed 2 nm in full width at half‐ maximum (FWHM) corresponding to the spectrally limited pulse width of ∼50 fs. With spectral gain narrowing in amplifiers, it is difficult to count on the possibility of producing pulses shorter than 0.1 ps at the TW level of peak power. Moreover, due to the short sponta‐ neous lifetime of the B state and rather narrow spectral bandwidth, these transitions exhibit as low as 1 mJ/cm2 saturation fluence, which corresponds to the output intensity of 10-2 TW/cm2 in a 0.1 ps pulse, requiring too large output aperture to produce multiterawatt output peak power.

Compared with the B‐X transition, the broadband photochemical media are characterized by more than an order of magnitude larger gain bandwidth and saturation fluence allowing for several TW/cm2 to be obtained in hybrid systems comprising these active media. In addition, unlike to the electron‐impact excitation, the optical pumping, which is practically free of transient absorption within the laser transition spectral band, makes the entire transition bandwidth to be accessible for the femtosecond pulse amplification. These active media are briefly reviewed in the subsequent sections.

#### **2.1. XeF(C‐A) active medium**

Moreover, the combined use of two media with spectrally shifted emission bands (Kr2F with XeF(C‐A) or Xe2Cl) in an amplifier chain makes promising for the amplification of even shorter optical pulses due to twofold broadening a gain bandwidth (**Figure 1**) and spectrally inho‐

**Γ‐1,2<sup>2</sup>**

**Γ) Xe2Cl(42**

**Γ‐1,2<sup>2</sup> Γ)**

mogeneous gain saturation.

1344 High Energy and Short Pulse Lasers

λmax is the wavelength of the gain maximum.

σst is the stimulated emission cross‐section.

**Table 1.** Characteristics of broadband active media.

τFT is the transform‐limited pulse width (for Gaussian profile).

**Figure 1.** Emission spectrum for the mixed Kr2F/XeF(C‐A) system.

Δλ is the gain bandwidth.

τsp is the spontaneous lifetime.

εsat is the saturation fluence.

**Transition XeF(C‐A) Kr2F(42**

λmax, nm 480 420 510 Δλ, nm 70 80 100 τFT, fs 5 3 4 τsp, ns 100 180 250 σst, cm2 10-17 2.3 × 10-18 2.8 × 10-18 εsat, J/cm2 0.05 0.2 0.15 Iout (τ = 25 fs), TW/cm<sup>2</sup> 2 8 6

Iout is the estimated maximum peak power density related to a laser amplifier output aperture.

An important advantage of gaseous active media resides in the much lower value of the nonlinear index of refraction as compared with solid‐state materials, which allows amplifica‐ tion of optical pulses at much higher intensities than in condensed gain media. For the first time, hybrid architecture employing a gaseous active medium was realized in the system comprising a dye femtosecond oscillator and e‐beam or fast discharge‐driven boost amplifier on the UV B‐X transitions of ArF, XeCl, KrF, and XeF rare‐gas‐halide excimer molecules. The A schematic energy diagram of the upper and lower XeF laser levels is shown in **Figure 2**. Behind the optical pumping XeF active medium is the photolysis of XeF2 vapor in the spectral range of <204 nm to produce XeF excimers mainly in the B state [17–19]. The C state, lying lower than the B state, is populated due to collisional relaxation of the latter in the presence of a buffer gas. Depending on composition of working mixture, laser action is observed on the B‐X (353 nm) or C‐A (480 nm) transition [20]. Broad gain bandwidth on the C‐A transition (Δλ  = 70 nm [21]) is accounted for by the repulsive nature of the A state. On the other hand, repulsive character of the lower A state provides its virtually instantaneous depopulation and ensures population inversion independently on the presence of buffer gas.

**Figure 2.** The potential curves of low‐lying energetic states of the XeF excimer molecule.

Lasing in the photochemically driven XeF active medium was obtained for the first time in 1977 on the *B‐X* transition with an exploding wire as a pump source [22]. Later on, in a series of papers, laser action on XeF(B‐X) and XeF(C‐A) was reported upon optical pumping by broadband VUV radiation from exploding wires [20, 23], surface discharges [24–27], formed‐ ferrite flash [28, 29], and strong shock waves [30], as well as by the Xe2 spontaneous emission at 172 nm excited by an electron beam [31–33]. These studies have led to a number of significant achievements. Among them are high output energies attaining as much as 1 kJ and 170 J in the UV region with the strong shock wave [30] and surface discharge optical pumping [24], respectively, as well as 120 J and 10 J obtained in the visible under optical pumping by radiation from surface discharges operating in the single shot [24] and 1 Hz repetitive rate [27] modes respectively.

Studies of the XeF(C‐A) laser showed that, besides minor transient absorption discussed above, the optical pumping XeF(C‐A) active medium has an additional advantage over the electron‐ impact excitation, which consists in the much weaker competition of the B‐X transition. The point is that upon optical excitation, relative populations of the closely lying B and C states are determined by the thermodynamic equilibrium at a buffer‐gas temperature that is close to the room temperature, while, upon e‐beam and fast discharge pumping, the main role in the energy exchange between these states is played by secondary electrons, which have a tem‐ perature of ∼1 eV characteristic of this pumping. In the optically driven active medium, the electron concentration is negligibly low, thereby providing an efficient laser action on the C‐ A transition.

From the viewpoint of femtosecond pulse amplification, the pump technique relying on the e‐beam‐driven spontaneous emission of Xe<sup>2</sup> excimers is of particular concern because, along with surface discharge optical pumping, it paved a new way for the development of the hybrid (solid/gas) femtosecond systems in the blue‐green region. With the use of this technique to pump the XeF(C‐A) laser, as high as 6 J of output energy was reported in [33].

Finally, it is essential to note that one of the most important conclusions gained from the optically driven XeF(C‐A) laser studies is that at a proper composition of the active medium its broad amplification band is not practically modified by transient absorption making the entire transition bandwidth to be accessible for the femtosecond pulse amplification. A drawback of the XeF(C‐A) active medium is that it must be replaced in a laser chamber after each shot because of the photodecomposition of the parent XeF2 molecules.

### **2.2. Xe2Cl active medium**

Among the broad bandwidth photochemically driven active media listed in **Table 1**, Xe2Cl is of special interest because initial working mixture is not consumed throughout pump flash allowing for operation in the repetitive mode without replacing the working gas mixture after each shot. Moreover, as compared with the XeF(C‐A) transition, this active medium has a much longer radiative lifetime of the upper state, which was measured to be 245 ns [34].

This active medium, as well as Kr2F discussed below, is currently studied to a much lesser extent as compared with XeF. First observation of lasing in the Xe2Cl triatomic excimer was reported in 1980 upon e‐beam pumping [35]. This was followed in 1985 by successful operation of the Xe2Cl laser optically pumped by the VUV radiation at 137 nm from the open discharge initiated by an exploding wire [36]. Behind the laser action is the reaction of optically excited molecular chlorine with Xe to form XeCl(B) excimers that then recombine with xenon into Xe2Cl\* emitting in the blue‐green region with a fluorescence quantum yield experimentally measured to be ∼75%, with respect to the absorbed pumping photons [37].

Lasing in the photochemically driven XeF active medium was obtained for the first time in 1977 on the *B‐X* transition with an exploding wire as a pump source [22]. Later on, in a series of papers, laser action on XeF(B‐X) and XeF(C‐A) was reported upon optical pumping by broadband VUV radiation from exploding wires [20, 23], surface discharges [24–27], formed‐ ferrite flash [28, 29], and strong shock waves [30], as well as by the Xe2 spontaneous emission at 172 nm excited by an electron beam [31–33]. These studies have led to a number of significant achievements. Among them are high output energies attaining as much as 1 kJ and 170 J in the UV region with the strong shock wave [30] and surface discharge optical pumping [24], respectively, as well as 120 J and 10 J obtained in the visible under optical pumping by radiation from surface discharges operating in the single shot [24] and 1 Hz repetitive rate [27] modes

Studies of the XeF(C‐A) laser showed that, besides minor transient absorption discussed above, the optical pumping XeF(C‐A) active medium has an additional advantage over the electron‐ impact excitation, which consists in the much weaker competition of the B‐X transition. The point is that upon optical excitation, relative populations of the closely lying B and C states are determined by the thermodynamic equilibrium at a buffer‐gas temperature that is close to the room temperature, while, upon e‐beam and fast discharge pumping, the main role in the energy exchange between these states is played by secondary electrons, which have a tem‐ perature of ∼1 eV characteristic of this pumping. In the optically driven active medium, the electron concentration is negligibly low, thereby providing an efficient laser action on the C‐

From the viewpoint of femtosecond pulse amplification, the pump technique relying on the e‐beam‐driven spontaneous emission of Xe<sup>2</sup> excimers is of particular concern because, along with surface discharge optical pumping, it paved a new way for the development of the hybrid (solid/gas) femtosecond systems in the blue‐green region. With the use of this technique to

Finally, it is essential to note that one of the most important conclusions gained from the optically driven XeF(C‐A) laser studies is that at a proper composition of the active medium its broad amplification band is not practically modified by transient absorption making the entire transition bandwidth to be accessible for the femtosecond pulse amplification. A drawback of the XeF(C‐A) active medium is that it must be replaced in a laser chamber after

Among the broad bandwidth photochemically driven active media listed in **Table 1**, Xe2Cl is of special interest because initial working mixture is not consumed throughout pump flash allowing for operation in the repetitive mode without replacing the working gas mixture after each shot. Moreover, as compared with the XeF(C‐A) transition, this active medium has a much

This active medium, as well as Kr2F discussed below, is currently studied to a much lesser extent as compared with XeF. First observation of lasing in the Xe2Cl triatomic excimer was

longer radiative lifetime of the upper state, which was measured to be 245 ns [34].

pump the XeF(C‐A) laser, as high as 6 J of output energy was reported in [33].

each shot because of the photodecomposition of the parent XeF2 molecules.

respectively.

1366 High Energy and Short Pulse Lasers

A transition.

**2.2. Xe2Cl active medium**

However, the application of this technique for the femtosecond pulse amplification is of little practical significance since, due to the narrow Cl2 absorption bandwidth at 137 nm, an efficiency of the Xe2Cl pumping by the thermal pump source is expected to be low. Moreover, to take advantage of repetitive pulse mode of operation, it requires the development of a powerful large‐area pumping source radiating in the short‐wave part of the VUV spectral range in a repetitive pulse regime.

On the other hand, as was shown in [38], more promising is the excitation of the active medium in mixtures of Xe and C12 vapor due to the photoassociation process

$$\text{Xe} + \text{Cl} + \text{hv} \rightarrow \text{XeCl} (\text{B,C})$$

at the wavelength of a XeCl laser (308 nm) followed by the three‐body XeCl(B,C) recombination with xenon to form Xe2Cl(4 <sup>2</sup> Γ). Production of Cl atoms is provided by the same pump pulse of the XeCl laser via photodissociation of C12. In the mixtures of Xe and Cl2 at pressures of 1– 2 bar and 1–2 Torr, respectively, the quantum efficiency for the energy transfer from XeCl(B,C) to Xe2Cl(4 <sup>2</sup> Γ) is close to 100% due to the extremely high rate constant (1.3 × 10-30 cm6  s-1 [39]) of the XeCl(B,C) recombination with xenon. The main loss process in the pump mechanism considered here is the C12 photodissociation to accumulate a sufficient number of Cl atoms and thereby ensure a high enough absorption of pump photons in the longitudinal geometry of excitation. Nevertheless, the overall efficiency for the conversion of the pump energy into the energy stored in the Xe2Cl active medium is estimated to be as high as 5% at the pump intensity of 5 MW/cm<sup>2</sup> in a 100 ns pump pulse. According to Ref. [38], this mechanism of pumping can be of great practical importance, since it enables to produce a small‐signal gain in the Xe2Cl active medium, which is expected to be even higher than that obtained in the XeF(C‐A) amplifier to be discussed later. Moreover, molecular chlorine is not consumed upon optical excitation because chlorine atoms, generated in the Cl2 photodissociation, recombine back to the molecular state at a time scale of 1 ms. This makes it far easier to operate a Xe2Cl femtosecond amplifier in the pulse repetition regime.

For the sake of completeness, it should be noted that the gain on the Xe2Cl laser transition excited due to photoassociation at 308 nm was observed for the first time in chlorine‐doped solid [40] and liquid [41] xenon. However, unlike gaseous active media, realization of the femtosecond pulse amplifier based on this technique is a serious technological problem.
