Light is modeled as both a wave and a particle, known as a photon in quantum mechanics. When light interacts with matter, the behavior of photons is governed by the laws of quantum mechanics.

Intense pulsed light (IPL) and lasers use a spectrum of wavelengths to selectively target certain chromophores in the skin, such as melanin or hemoglobin. This technique is employed in several cosmetic and medical applications, including hair removal, skin rejuvenation, and the treatment of vascular and pigmented lesions.

The interaction of lasers and light in IPL with matter is based on the absorption of photons by the target chromophores. The absorbed photons transfer energy to the chromophores, which causes them to heat up and break apart, and quantum mechanics can describe the behavior of photons and matter at the atomic and subatomic levels.

Wave particle duality

The absorption of light energy by atoms and molecules is a quantum mechanical process that depends on the energy levels of electrons in atoms or molecules. Different atoms and molecules have unique energy levels that determine the wavelengths of light that they can absorb. Because of the principles of quantum mechanics that govern the interactions between light and matter, laser and IPL technologies can target specific chromophores and deliver precise treatment to the desired area. The absorption of light energy by chromophores is a probabilistic process that can be described using quantum mechanical principles.

Light can be described as a wave that travels through space with a wavelength that determines its color. Furthermore, light can exist as a stream of particles called photons, each with a discrete amount of energy determined by its wavelength (Fig. 1) [1-3].

Figure 1. Wave particle duality of light.

The particle nature of light is particularly relevant because the absorption of light by chromophores is a probabilistic process that can be described using quantum mechanical principles.

Pauli exclusion principle

This states that no 2 identical fermions can occupy the same quantum state simultaneously, implying that no 2 electrons in an atom can have the same set of quantum numbers and that they cannot occupy the same energy state in the same atom. This principle has important implications for the behavior of matter and is relevant to the interaction of light with matter in laser and IPL technologies. In IPL, the light energy is absorbed by chromophores in the skin, such as melanin, hemoglobin, and water. The absorption of light energy by these chromophores is a quantum mechanical process that depends on the energy levels of the electrons in the atoms or molecules that constitute the chromophores. When an electron in a chromophore absorbs a photon of light and moves to a higher energy level, it must move to an unoccupied energy level with the correct spin state. This process of electron excitation and relaxation contributes to the therapeutic effects of laser and IPL therapies. Because of the Pauli exclusion principle, only an appropriate number of electrons are excited at any given time, which prevents excessive heating and tissue damage when used cautiously. Furthermore, the unique energy levels of electrons in chromophores determine the wavelengths of light that can be absorbed, which is critical for the design of lasers and IPL devices (Fig. 2) [4-6].

Figure 2. Pauli exclusion principle.

Superposition

Moreover, the coherence of the laser beam is a consequence of the quantum-mechanical principle of superposition, which allows the waves of the emitted photons to be added constructively in the direction of the laser cavity and destructively in other directions. The quantum-mechanical description of these phenomena involves the use of the Schrödinger equation, density matrix formalism, and other mathematical tools for quantum mechanics.

The Fourier transform is a mathematical tool that helps us understand complex patterns by breaking them down into simpler components (Fig. 3) [7]. For instance, imagine a box filled with beads of different colors. Beads were arranged in a pattern by shaking the box. Similarly, a complex signal can be considered a pattern. The Fourier transform converts this pattern into a series of simple sine and cosine waves at different frequencies (Fig. 4). These simpler waves can then be analyzed individually, allowing us to better understand the overall pattern. This transformation is essential for understanding the mechanisms of Q-switch lasers, various pulse formations of different lasers, and IPL [8-10].

Figure 3. Fourier Transform Equation [7]. F(k) represents the Fourier transform of the function f(x), and k is the frequency variable. The π in the Fourier transform equations represents the mathematical constant pi, which is approximately equal to 3.14159. The variable x represents the time domain variable, and k represents the frequency domain variable. The function f(x) is the original function in the time domain, and F(k) is its Fourier transform in the frequency domain. The imaginary unit is represented by i, which is defined as the square root of –1. The number e, also known as Euler’s number, is a mathematical constant approximately equal to 2.71828 that can be characterized in many ways. It is the base of natural logarithms. It is the limit of (1+1/n)ⁿ as n approaches infinity.

Figure 4. When a very large number of laser modes that all have a “zero” in the same position interfere, the resultant superposition is an extremely narrow pulse.

Spontaneous emission and stimulated emission

Einstein postulated that excited atoms may undergo a process known as stimulated emission, in which they release energy stored as light. Typically, an excited atom emits a photon via spontaneous emission to initiate a process. Another excited atom is stimulated to emit a second photon upon interaction. There are two crucial aspects to this approach. In the first step, 1 photon is multiplied into 2 photons. These 2 photons produce four more photons when they interact with 2 additional excited atoms. Second, the notable aspect is that the wavelengths, directions, phases, and polarizations of the 2 photons are identical. The laser’s operation is based on the ability to “amplify” light when there are enough excited atoms present, which gives rise to “optical gain” and explains why the word “laser” is an acronym for “light amplification (by) stimulated emission (of) radiation.” Many different materials in solid, liquid, and gas phases display gains when pumped under appropriate circumstances.

Quantum mechanics is essential for understanding how lasers (Light Amplification by Stimulated Emission of Radiation) work [11]. The basic principle of a laser is to produce a beam of intense and coherent light by stimulating atoms or molecules to emit photons (particles of light) in a specific direction and phase [12,13].

The process begins with the excitation of atoms or molecules in a gain medium such as a gas, solid, or semiconductor. This excitation can be achieved by various means, such as electrical discharge, optical pumping, or chemical reactions. Excited atoms or molecules, on returning to their ground state, emit photons by spontaneous emission. However, in a laser, the emitted photons are not random but rather stimulated by incoming photons of the same frequency and phase. This leads to a cascade of reinforced photons, resulting in a coherent beam of light. The quantum nature of the process can be observed because the emitted photons have discrete energies corresponding to the energy levels of the gain medium. Energy levels are determined by the quantum mechanical properties of atoms or molecules, such as their electronic structures and interactions with each other and external fields [14].

In stimulated emission, a photon stimulates an excited atom or molecule to emit a second photon of the same energy and phase. This process results in light amplification, which is critical in the design of laser machines (Fig. 5).

Figure 5. Spontaneous emission is a random process, whereas stimulated emission produces photons with identical properties.

Photochemical reaction

When a molecule absorbs light, it undergoes a chemical reaction. The formation of free radicals and other reactive species occurs due to the absorption of laser energy by the ink particles during laser tattoo removal. These species can break down ink particles further, making them easier for the body to remove. This reaction is different from the thermochemical reactions, which involve changes in the temperature and heat energy of the system. This occurs when chemical bonds are broken or formed, releasing or absorbing energy in the form of heat. In addition, photochemical reactions are involved in the treatment of acneiform eruptions by decreasing inflammation (Fig. 6-9) [15]. Thermochemical reactions can occur at any temperature and do not require light or photosensitizer (Fig. 10, 11) [16].

Figure 6. Application of photochemical reaction on intense pulsed light treatment of acne. Photograph taken in our clinic before the first session.

Figure 7. Application of photochemical reaction on intense pulsed light treatment of acne. Photograph taken after the 12th session.

Figure 8. Application of photochemical reaction on intense pulsed light treatment of skin whitening. Photograph taken in our clinic before the first session.

Figure 9. Application of photochemical reaction on intense pulsed light treatment of skin whitening. Photograph taken in our clinic before the 7th session.

Figure 10. Photochemical reaction.

Figure 11. Difference between photochemical and thermochemical reactions. ΔG refers to the instantaneous difference in free energy between the reactants and the products. It can predict the direction of the chemical reaction under two conditions: constant temperature and constant pressure.

Photothermal reaction

When light energy is absorbed by a material, it can lead to an increase in temperature, which can cause thermal or chemical reactions. In laser and IPL therapies, the absorption of light energy by melanin or hemoglobin molecules in the skin can lead to localized heating and damage, which can be used for cosmetic or medical purposes (Fig. 12, 13) [17].

Figure 12. Application of photothermal reaction on intense pulsed light treatment of hair removal. Photograph taken in our clinic before the first session.

Figure 13. Application of photothermal reaction on intense pulse light treatment of hair removal. Photograph taken in our clinic after the 7th session.

Photomechanical reaction

During laser tattoo removal, the laser emits high-intensity pulses of light that are absorbed by tattoo ink particles in the skin. When ink particles absorb laser energy, they rapidly heat and cool, which causes rapid expansion and contraction of the ink particles. This rapid thermal expansion and contraction generates shock waves that cause mechanical disruption of the ink particles, breaking them into smaller fragments. These smaller fragments are removed more easily by the immune system. The photomechanical reaction occurs within a short timeframe, typically less than a nanosecond, and requires high laser energy to achieve the necessary shock waves. The laser energy was carefully calibrated to be primarily absorbed by the tattoo ink particles to minimize damage to the surrounding skin tissue (Fig. 14, 15) [18].

Figure 14. Application of photomechanical reaction on pico laser treatment of lower back tattoo removal. Photograph taken in our clinic before the first session showing erythema surrounding the tattoo due to an erythematous reaction 45 minutes after the application of topical anesthetic cream.

Figure 15. Application of photomechanical reaction on pico laser treatment of lower back tattoo removal. Photograph taken in our clinic before the first session showing petechial hemorrhages adjacent petechial hemorrhages immediately after the procedure. Frosting can be seen inside the tattoo.

Photoemission or photoionization

The absorption of light energy by chromophores in the skin results in the emission of electrons, a process known as photoemission or photoionization. The absorption of light energy by the chromophores results in the excitation of electrons to higher energy levels, which can lead to the emission of electrons from the chromophores. The emission of electrons can result in a range of biological effects, including the stimulation of collagen production, reduction of inflammation, and breakdown of pigmented cells. Understanding the photoemission properties of chromophores is critical for designing IPL and laser devices that can deliver an appropriate amount of light energy to the desired area of the skin. By controlling the wavelength and pulse duration of light, IPL technology can be optimized to induce photoemission and deliver precise treatment to the desired area (Fig. 16) [19,20].

Figure 16. Photoionization.

Laser and IPL therapy limitations

Laser and IPL therapies have shown promising results in treating various dermatological conditions; however, certain existing limitations and challenges need to be addressed. One of the limitations is the difficulty of effectively targeting the intended cells or tissues without damaging the surrounding cells (the most obvious is photoepilation). Operators use the extended theory of selective photothermolysis, which states that the treatment pulse width for non-uniformly pigmented targets is significantly longer than the target thermal relaxation time [21]. This can result in adverse effects such as hyperpigmentation or scarring [22-24]. Additionally, certain skin types (high Fitzpatrick skin types) may not be suitable for laser or light therapy because of their higher risk of adverse effects.

Another challenge is the limited penetration depth of light, which may inhibit its therapeutic effect in deeper dermal layers. This can be overcome using longer wavelengths or by incorporating other techniques, such as microneedling [25].

Furthermore, the efficacy of laser and light therapies can be influenced by factors such as patient age, skin type, and type of treatment. Some conditions may require multiple sessions to obtain optimal results.

Regarding safety concerns, the possibility of eye damage due to lasers can lead to blindness if appropriate precautions are not taken. Adequate training and safety measures must be implemented to minimize this risk.

This article highlights the crucial role the principles of quantum mechanics and physics in laser and light therapy. Quantum mechanics provides the theoretical framework necessary to understand the behavior of photons and their interactions with tissues during these therapies. The principles of particle-wave duality, Pauli exclusion principle and superposition are fundamental in explaining the mechanisms behind laser-induced tissue heating, selective photothermolysis, and the emission and absorption of photons. These principles help us understand that laser and light therapies involve more than just pressing buttons or using a touchscreen. Further research and advancements in quantum mechanics will undoubtedly enhance the efficacy and safety of laser- and light-based cosmetic interventions.

The authors have nothing to disclose.