Preface

Lasers enable the modification of living and non-living matter with submicron precision in a contact-free manner. They are wonderful devices with enormous application potential in medicine and industry. This book presents the latest research in laser technology.

Terahertz quantum cascade lasers (THz-QCLs) have attracted tremendous interest in recent years due to their coherent and compact wave features. They can be developed using semiconductor quantum structures and the THz radiation obtained is attributed to inter-sub-band transitions, which rely on quantum transport between discrete sub-bands. Electron injection plays a significant role to achieve population inversion in a QCL. Chapter 1 investigates the effect of an indirect injection scheme in terahertz QCL designs. For the scattering-assisted (SA) injection method-based QCLs with two wells, it is found that the population inversion increases when additional high-lying sub-bands are included, owing to the activation of more depopulation channels. However, an increase in the optical gain does not correspondingly occur if the lasing frequency exceeds 3 THz; instead, the peak gain undergoes a significant decrease. This finding indicates that the high-lying sub-bands play an important role in the development of an efficient SA-QCL design. THz-QCLs can be used for many medical applications including imaging, biochemical label-free sensing, pharmacology, and more.

The continuous wave (CW) laser has been around since 1960 and, as its name suggests, it emits a constant laser beam. The advancement in the laser technology has given birth to pulsed lasers, which emit short 'pulses' of light at regular intervals, rather than continuously. For instance, femtosecond lasers release extremely brief pulses of light at time periods on the order of one quadrillionth of a second. One of the key advantages of femtosecond lasers is that they can generate huge amounts of peak power, while also massively reducing the heat produced during this process. Chapter 2 describes the basic principles of a femtosecond laser and its application in ophthalmology. Since femtosecond lasers produce lower levels of heat compared to CW or other longer pulsed lasers, they are suitable for implementing various ophthalmic procedures, such as corneal surgery, laser eye or 'refractive' surgery, and operations to remove cataracts from the eye. This is because minimal heat is especially important when operating on sensitive areas like the eye to prevent damage to the surrounding tissue. Femtosecond lasers promote safe surgery and fast healing times during ophthalmic procedures because they can process tissue materials within a 3D volume without altering its surface.

Pulsed lasers can also be used in other medical applications such as photo-thermal therapy for cancer treatment. A short-pulsed laser is generally used in laser-based photo-thermal therapy to destroy cancerous cells. The major challenge in this therapy is to destroy the cancerous cells without damaging the surrounding healthy tissue. Thus, it is essential to understand the thermal characteristics of the laser-irradiated

biological tissue to improve the efficacy of laser-based photo-thermal therapy. Chapter 3 discusses the modelling of laser-irradiated biological tissue to understand its thermal behavior, which may help to improve the efficacy of laser-based photothermal therapy. In this study, the light propagation through the biological tissue is mathematically modelled using the heat transfer equation (RTE). RTE is solved using the discrete ordinate method (DOM) to determine the intensity inside the laser-irradiated biological tissue. Consequently, the absorbed photon energy acts as the source term in the Fourier/non-Fourier model-based bio-heat transfer equation to determine the temperature distribution inside the biological tissue subjected to short-pulse laser irradiation.

Laser surface texturing is a top-down method for generating surface patterns on polymers, metals, ceramics, glasses, and alloys. This technique allows large-scale surface patterning. Chapter 4 describes the use of a femtosecond laser in micro-/ nano-texturing for fabricating coated and surface-treated dies with tailored textures. Through the femtosecond laser micro−/nano-texturing and CNC-imprinting, the metal, polymer, and glass product surfaces were optically decorated to have color grading and plasmonic brilliance and functionally controlled to be hydrophobic. The proposed approach can be used for micro−/nano-texturing of various industrial and medical products.

The interactions of concentrated energy fluxes such as femtosecond lasers and highenergy electron beams with absorbing substances have facilitated new discoveries and excitement in various scientific and technological areas. For instance, femtosecond laser ablation is an effective technique to functionalize surfaces. Due to the ultrashort pulse width and high light intensity (1012 W/cm2), it is possible for the laser to ablate or irreversibly modify the materials with negligible damage outside the focal volume, thereby allowing treatment of biological samples like live cells, membranes, and removal of thin films as well as bulk materials for many applications in diverse fields including micro-optics, electronics, and even biology under extremely high precision. Chapter 5 discusses the ablation of materials using femtosecond lasers and electron beams. Both femtosecond laser and e-beam ablations are being investigated for a range of medical applications.

This book is written by experts in the field and is a useful resource for researchers, engineers, and advanced students in the field of photonics, lasers, ultrafast optics, material processing, and medical physics.

> **Sulaiman Wadi Harun** Department of Electrical Engineering, University of Malaya, Kuala Lumpur, Malaysia

**Chapter 1**

Lasers

*Li Wang*

**Abstract**

channels, optical gain

**1. Introduction**

High-Lying Confined Subbands

in Terahertz Quantum Cascade

In designing the terahertz quantum cascade lasers, electron injection manner indeed plays a significant role to achieve the population inversion. The resonant tunneling process is commonly employed for this injection process but waste more than 50% fraction of populations out of the active region owing to resonance

alignment, and the injection efficiency is obviously degraded due to thermal incoherence. An alternative approach is to consider the phonon-assisted injection process that basically contributes to most of the populations to the upper lasing level. However, this manner is still not realized in experiments if a short-period design only containing two quantum wells is used. In this work, it is found in this design that the population inversion is indeed well improved; however, the optical gain is inherently low even at a low temperature. Those two opposite trends are ascribed to a strong parasitic absorption overlapping the gain. The magnitude of this overlap is closely related to the

lasing frequency, where frequencies below 3 THz suffer from fewer effects.

**Keywords:** intersubband transition, terahertz, quantum cascade lasers, parasitic

Thus far, the profusion of terahertz wave applications, including high-speed communications, industrial quality control, non-destructive cross-sectional imaging, gas and pollution sensing, biochemical label-free sensing, pharmacology, and security screening, has been demonstrated [1–3]. Moreover, the development of terahertz quantum cascade lasers (THz-QCLs) based on semiconductor quantum structures affords an attractive THz radiation source with coherent and compact wave features [4]. The basic radiation mechanism in this type of laser is intersubband transitions relying on quantum transport between discrete subbands. This method prevents the semiconductor bandgap limit at significantly low THz photon energies. The subbands can be freely tailored via engineering the thickness of quantum layers; therefore, the THz radiation frequency coverage is broad. However, THz-QCLs always suffer from temperature-triggered lasing quenching; consequently, the maximum operating temperature (*T*max) is still limited to below room temperature and thus requires additional
