Introduction

Non-linear optical phase conjugation by degenerate four-wave mixing (DFWM) is an important technique with applications in many fields of science and technology including image transmission, optical image processing, optical filtering, and laser resonators[14, 29, 8]. When two counter-propagating and intense light beams interact with a non-linear medium, together with a less intense third one, a fourth beam is generated from the medium, which will be the phase conjugation of the third beam. This technique is called four-wave mixing. The unique feature of a pair of phase conjugate beams is that the aberration influence imposed on the forward (signal) beam passed through an inhomogeneous or disturbing medium can be automatically removed from the backward (phase conjugated) beam passed through the same disturbing medium[11]. The main applications of degenerate DFWM techniques are non-linear spectroscopy, real time holography, and phase conjugation. Phase conjugation by DFWM has been demonstrated in many organic and inorganic materials using pulsed or continuous-wave (CW) lasers[27, 12].

The phase conjugate light has a variety of characteristics and properties not associated with normal light and they can be enumerated as follows:

• (1) Phase compenzation effect: Waves that pass through an aberrating medium are injected into a phase conjugate mirror. If the outgoing wave is re-propagated through the medium, the waves phase distortion is compensated.

• (2) Space domain multiplicative interaction: Since phase conjugate waves are produced by interaction in a non-linear medium, the spatial multiplicative effect among the light waves appears in the phase conjugate waves.

• (3) Intensity dependant phase shift: Due to the optical Kerr effect in a non-linear medium, the phase shift of the electric field depends on the wave intensity.

• (4) Detuning dependence: The reflectance of a phase conjugate mirror strongly depends on the detuning between the pump and probe wave frequencies.

• (5) Time inversion: A phase conjugate wave can be regarded as a time inverted wave since its direction of propagation is exactly opposite that of the probe wave and the wave front is identical to that of the probe wave.

• (6) Time domain multiplicative interaction: When pulsed waves are used in the production of phase conjugate light, the resulting polarization, that is, the generalized phase conjugate wave intensity depends on the product of the interacting waves in the waves’ temporal domains.

Introduction

Chemical oxygen−iodine laser (COIL) was first demonstrated by McDermott et al. in 1978 and is the only chemical laser based on electronic transition. It operates on 2P1/2 → 2P3/2 transition of iodine atom at 1.315 μm and is pumped by the following reaction scheme,

This laser due to its short wavelength, high efficiency and scalability has found application in various military and civilian scenarios in the last decade. The possibility of carrying high power beams via optical fibers makes it extremely useful for decommissioning and dismantling dangerous structures such as obsolete nuclear reactors through remotely operated mechanisms by Tei et al.

The pumping source, O2(1) is an excited form of oxygen molecule with energy level very close to that of atomic iodine enabling near resonant energy transfer as shown in Fig. 8.1. Although, different techniques such as chemical, RF discharge, photo-sensitizer, have been reported for the production of O2(1), the chemical method is the only one till date that has been successful for scaling up power. Therefore, amongst the variousmechanisms studied for the production of O2(1), the chemical method still remains at the forefront for the development of large-scale COIL systems. The chemical method is based on chemical reactions between chlorine gas and basic hydrogen per-oxide (BHP) solution resulting in production of singlet oxygen, which in turn dissociates iodine molecules into iodine atoms and also subsequently excites these atoms.

For an efficient production of singlet oxygen, jet type singlet oxygen generator (JSOG) by Rajesh et al. has proved its potential over the other techniques such as rotating disc or bubbler type. Other forthcoming generators such as twisted aerosol and centrifugal bubble/ flow type are yet to be proven for large-scale power levels.

Figure 8.2 shows the functional block diagram of chemical oxygen iodine laser[3, 9] which exhibits coupling of all subsystems. BHP solution is prepared in a separate preparation tank and supplied to the SOG reaction chamber in the form of jets. Chlorine is supplied to the SOG reaction chamber from the bottom in a counter-flow direction to the jets so that it reacts at the surface of the liquid jets to produce singlet oxygen molecules.

Introduction

The interaction between a movable mirror and the radiation field of an optical cavity has recently been the subject of extensive theoretical and experimental investigations. This field known as cavity opto-mechanics has played a vital role in the conceptual exploration of the boundaries between classical and quantum mechanical systems. These opto-mechanical systems couple the mechanical motion to an optical field directly via radiation pressure buildup in a cavity. The coupling of mechanical and optical degrees of freedomvia radiation pressure has been a subject of early research in the context of laser cooling[1, 2, 3] and gravitational-wave detectors[4]. The recent improvements in nanofabrication techniques suggest that in the near future, these devices will reach the regime in which their sensitivitywill be limited by the ultimate quantumlimits set by the Heisenberg principle. The importance of the limits imposed by quantum mechanics on the resonator motion was first pointed out by Braginsky and coworkers[5] in the completely different context of massive resonators employed in the detection of gravitational waves[6]. However, in recent years, the quest for the experimental demonstration of genuine quantum states of macroscopic mechanical resonators has spread well beyond the gravitational wave physics community and has attracted wide interest.

Recently, there has been a great surge of interest in the application of radiation forces to manipulate the center-of-mass motion of mechanical oscillators covering a huge range of scales from macroscopic mirrors in the laser interferometer gravitational wave observatory (LIGO) project[7, 8] to nano-mechanical cantilevers[9, 10, 11, 12], membranes[13] and Bose−Einstein condensates[14, 15, 16, 17]. The central accomplishment of the field of cavity opto-mechanics is the investigation of radiation pressure force which allows one to manipulate the motional state of micromechanical oscillators. In particular, it has become possible to substantially cool the thermal excitation of a single mechanical mode, down to a few tens of remaining phonons[18]. With these developments, micro- and nano-mechanical resonators now represent an important model system with the prospect of demonstrating quantum effects on a macroscopic scale.

The quantum optical properties of a mirror coupled via radiation pressure to a cavity field show interesting similarities to an intracavity Kerr-like interaction[19, 20]. Recently, in the context of classical investigations of non-linear regimes, the dynamical instability of a driven cavity having a movable mirror has been investigated[21].

Introduction

As of today, attosecond pulses are in the forefront of state-of-the-art technology[1, 2]. An attosecond pulse, due to its very short duration, can be sustained only bywavelengths of hundreds of angstroms. Such pulses are highly energetic and can cause molecular dissociation. Thus, attosecond pulses cannot be used to study molecular processes.

The ability to sustain only a few cycles in the pulse is a characteristic unique to the attosecond domain. To capture this quintessential feature on molecular processes, the idea of few-cycle pulses at more viable energy ranges, which can represent femtosecond or sub-femtosecond duration was used. In this chapter, we will observe what happens when a few-cycle pulse interacts with matter.

A few-cycle pulse, even on a near-IR wavelength, would be of a temporal duration of the order of femtoseconds. Hence, any general two-level system can, under the interaction of such pulses, be treated as ideal and non-relaxing, since normal molecular processes relax at much longer time scales. In this chapter, we, therefore, study the interaction of few-cycle pulses with ideal two-level systems, where the approximation of a smoothly varying envelope function breaks down. We also see that despite the overwhelming importance of the oscillatory nature of a few-cycle pulse, the signatures of the envelope profile still persists in the interactions.

Breakdown of Rotating Wave Approximation

For many-cycle pulses, the smoothly varying envelope function approximation is valid. The approximation states that the internal oscillations, because of their comparatively high number, can be ignored and only the envelope function plays a role. This is also called the rotating wave approximation (RWA)[3]. When we move away from the many-cycle pulses domain to the case of few-cycles, the smoothly varying function approximation is invalidated, and hence RWA breaks down[4, 5, 6, 7].

Let us consider an ideal 2-level system, with energy difference ωe Fig. 3.1. The central frequency of the exciting laser pulse is ωrad. The rotating wave approximation holds under the following condition [8]:

Therefore, keeping the x coordinate fixed, we see that the standard deviation (width) of the Fourier domain pulse is inversely proportional to . Thus, the shorter the temporal duration of the pulse, the more spreadout its spectrum would be.

Introduction

The creation of doubly charged ions upon the multiphoton ionization of atoms and molecules was observed for the first time in[1]. Discovery of the effect of the creation of doubly charged ions upon multiphoton atomic ionization laid the foundation of a new branch in the physics of nonlinear ionization processes that involved the creation of multicharged ions due to the multiphoton or tunneling ionization of atoms and molecules.

In later experiments, the creation of doubly charged ions was revealed in experiments with atomic barium, samarium, europium, and ytterbium[2−4]. The creation of doubly charged ions was also observed upon multiphoton ionization of inert gas atoms and other atoms and molecules[5]. Experimental results have demonstrated the general character of the creation of doubly charged ions upon multiphoton ionization of various groups of atoms.

In this chapter, we will discuss the creation of singly and doubly charged ions upon multiphoton atomic ionization of alkaline-earth elements using laser radiation with nanosecond pulse duration at laser field strengths ϵ = 5 x 106 − 109 V/cm, which are significantly less than the intra-atomic field ϵa = 5x109 V/cm. The results of using this field strength significantly differ from the recent results obtained at the laser field strength ϵ ˂ 5 x 109 V/cm or ϵ > 5 x 109 V/cm in experiments with femtoseconds and atto second laser pulses[6]. The differences include the domination of the tunneling ionization of atoms and molecules under such conditions and the creation of doubly charged ions. The main processes are electron rescattering, shaking, etc.[6].

The processes that lead to the creation of doubly charged ions upon multiphoton ionization of the alkaline-earth elements under the given conditions differ from the above processes. Two effects (cascade and two-electron)were proposed in[7, 8] to account for the creation of doubly charged ions upon the multiphoton ionization of alkaline-earth elements. In the cascade effect, doubly charged ions (A2+) are created owing to themultiphoton ionization of singly charged ions (A+) that appear in the presence of the same laser pulse due to themultiphoton ionization of atoms A : A+K0 ħω−A++ e and A+ +K1 ħω−A2+ +e.

Introduction

COIL is a chemical laser in which the required pumping energy for population inversion is released via a chemical reaction. This property makes the COIL attractive for defence application because it eliminates the need for electrical power supply at remote locations. Among other chemical lasers, COIL has the advantages of power scalability, short wavelength (1.315 μm) compatible with fiber (Grunewald et al.) for remote operation and also better laser material interaction.

In COIL, a gas−liquid phase reaction between basic hydrogen peroxide and chlorine gas at sub-atmospheric pressure (Azyazov et al.) produces the pumping source, singlet oxygen. This is diluted with sufficient nitrogen buffer gas to reduce the various loss mechanisms. Part of the pump energy contained in the singlet oxygen is used in the dissociation of iodine molecules into iodine atoms and the rest is used to excite these iodine atoms by near resonant energy transfer reaction. The interaction of singlet oxygen with atomic iodine at appropriate flow conditions results in the generation of laser gain medium inside the laser cavity from where the laser output power is extracted using an optical resonator.

To develop a high power COIL, it is important to study the gain characteristics, i.e., the small signal gain and the saturation intensity of the active medium under different flow conditions and to evaluate the optimum cavity coupling for achieving maximum output power. In this chapter, different COIL input parameters required for optimal gain medium formation in the laser cavity have been analyzed and gain characteristics using simplified saturation model (SSM) (Hager et al.) for the development of high power COIL have been estimated. The resonator parameters and output mirror coupling are evaluated keeping in view the resonator stability, diffraction loss, utilization of mode volume and laser beam divergence. On the basis of these parameters, the laser cavity and optical resonator for high power COIL have been developed and tested.

COIL gain medium formation

Introduction

A low intensity conflict is the most common form of warfare today and is likely to be so in foreseeable future. Data suggests thatmore than 75%of the armed conflicts sinceWorldWar II have been of low intensity variety. Low intensity conflict operation is a military term used to refer to the deployment and use of troops and/or assets in situations other than conventional war. As compared to a conventional war; in the case of low intensity conflict operations, armed forces engaged in the conflict operate at a greatly reduced tempo, perhaps with fewer soldiers, reduced range of tactical equipment and limited scope to operate in a military manner. Moreover, use of artillery is avoided in the case of conflicts in urban territories and use of air power is often restricted to surveillance and transportation of personnel and equipment. Low intensity conflicts pose an alarming threat to national security and is an area of concern for the whole of the international community today. Its scope extends from emergency preparedness and response to domestic intelligence activities to riot and mob control, from combating illegal drug trafficking to protection of critical infrastructure, fromhandling counter-insurgency and anti-terrorist operations to detection of nuclear and biological agents, from detection and identification of chemical, biological warfare and explosive agents to detection of concealed weapons. Laser and opto-electronics technologies play an important role in handling low intensity conflict situations. The key advantages of the use of laser technology in such applications are near zero collateral damage, speed of light delivery and potential for building non-lethal weapons. Some of the well established laser devices in low intensity conflict (LIC) applications include laser dazzlers for close combat operations, mob/riot control and protection of critical infrastructures from aerial threats; lidar sensors for detection of chemical, biological and explosive agents; Femto second lasers for imaging of concealed weapons; lasers for sniper and gun fire location identification and so on. Use of laser vibrometry and electron speckle interferometry techniques for detection of buriedmines and high power lasers for disposal of unexploded ordnances are emerging applications of laser technology for homeland security. This chapter briefly describes both established and emerging applications of laser and opto-electronics technologies in homeland security in terms of salient features, potential usage and international developments.