## 1 Introduction  
Diamond is an important material having various applications, ranging from cutting tools to heat spreaders in light emitting diodes (LEDs) [1; 2], due to its remarkable hardness, thermal conductivity and chemical inertness [3; 4; 5]. These properties, however, combined with its high brittle nature, make diamond incredibly difficult to machine by conventional mechanical methods. Diamond cannot be cut with a diamond tool, obviously. Grinding with a diamond wheel and polishing with diamond abrasives are possible but extremely time-consuming, especially for micro groove channel fabrication. Thus, discovering an efficient method to process diamond has been the goal of many interdisciplinary studies. Recent investigations have shown that laser irradiation is one such method [6]. Irradiation causes the appearance and continuous growth of a graphitized region at low fluences, which is easily and rapidly ablated at higher fluences [7]. Some researchers have utilized this phenomenon to machine graphitic wires in bulk diamond [8]. As a non-contact method, laser processing results in no tool wear and has high geometrical freedom. Recent studies have found that different types of structures can be machined. Through the use of short pulses, high precision and sharp profiles can be achieved [9; 7], as heat conduction into the bulk material is significantly reduced [10]. The machining of curved profiles with micron-sized curvatures has also been shown to be possible by the use of femtosecond pulses and an accelerating beam [11]. In addition, laser irradiation was found to result in laser-induced periodic surface structure on diamond film, explained by interference between the incident laser and surface scattered wave [12; 13]. In a previous study of the present authors, it was discovered that nanosecond pulsed laser irradiation was also useable for microgrooving of diamond and produces four types of groove formation mechanisms for chemical vapour deposition (CVD) diamond [14].  
Artificial diamond is produced mainly by two methods, chemical vapour deposition (CVD) and high pressure high temperature synthesis (HPHT). Different crystal growth methods may result in different lattice structures and defect distributions. Diamond crystals grown by CVD are bombarded by energetic particles during their synthesis, which results in the generation of different irradiation-induced defect centres or neutral lattice vacancies. These are identifiable by electron paramagnetic resonance (EPR). CVD diamond has several exclusive defect centres, labelled KUL3-6, which are not detected in HPHT samples [15]. In addition, CVD diamond is known to contain significant densities of hydrogen, oxygen, nitrogen, boron and silicon impurities. In contrast, HPHT rarely contains silicon impurities but contains high levels (10-100 ppm) of nitrogen impurities, causing it to appear yellow in colour [16]. The lattice structure also differs. CVD diamond contains a small amount of sp2 bonds which can usually be detected as a peak between 1350 and 1600 cm\({}^{-1}\) in Raman spectroscopy [17]. This peak is not observed in most HPHT diamond samples. The aforementioned differences affect the diamond's optical properties, which should affect the diamond's responses to laser irradiation and the machining phenomena. However, up to now, there is no available literature on the laser irradiation characteristics of diamond crystals grown by different methods.  
Thus, in this study, laser irradiation has been performed on two different types of diamonds, one created by CVD and another created by HPHT synthesis. The difference in irradiation marks and subsurface damage were observed, and the mechanisms which may have caused these differences were considered. In doing so, the process criteria and optimal conditions for laser machining of diamond can be established. This study is one of the first to investigate and compare, both experimentally and theoretically, the laser irradiation characteristics of the two types of diamonds.