## 1 Introduction  
Currently, endodontic treatment aims to reduce the bacterial load well below a certain threshold level, followed by the provision of adequate seals to promote apical tissue healing or to maintain apical tissue health [1, 2]. Chemomechanical treatment has often been used as a standard approach. However, bacterial growth in complicated intracanal anatomy, such as isthmus and subsequent biofilm formation, has reduced the successful chemomechanical intervention in the root canal [3]. With the persistence of infection in the pulp space, microorganisms also occupy the canal space, further complicating the disinfection process [4]. _Enterococcus faecalis_ is often found in failed endodontic treatment. The ability of this species to form bacterial biofilm has been well established [5]. Biofilm formation is a complex adaptive manner that enables bacterial survival in harsh growth conditions as well as increased tolerance to antimicrobial agents [5, 6]. The removal of bacterial biofilm in the root canal has remained a challenge because chemical and/or mechanical actions alone are insufficient to cause biofilm removal. Hence, disruptive physical cleaning procedures are required that can eliminate biofilm using physical force at an adjacent site that is inaccessible with existing techniques [3]. There are several methods for disinfecting the root canal. The most basic method is the use of an ultrasonic device or endodontic irrigant. These irrigants include normal saline, sodium hypochlorite (NaOCI), ethylenediaminetetraacetic acid (EDTA), chlorhexidine gluconate, MTAD (a mixture of a tetracycline isomer, an acid, and a detergent), calcium hydroxide (Ca(OH)\({}_{2}\)), etc. The repeated irrigations are necessary to reach all areas of the root canal and remove debris. The most common cleaning agent is NaOCI. However, the pungent odor and strong cytotoxicity are among the disadvantages of this corrosive detergent [7; 8; 9]. Among the irrigation techniques, photodynamic inactivation (PDI) using hydrodynamic force generates a physical effect during root canal irrigation [3]. PDI is based on the interactions of a photosensitizer and a harmless light source. The excited photosensitizer in the presence of oxygen generates reactive oxygen species, which induces the death of microorganisms. This technique is promising to affect oral bacterial infections [10]. There have been several studies of PDI in endodontics using methylene blue (MB), with a peak wavelength absorption of 664 nm [11; 12; 13]. MB is a hydrophilic and positively charged photosensitizer with a low molecular weight [13]. Several investigators have reported that _E. faecalis_ is effectively reduced by MB-PDI [14; 15]. However, MB has some disadvantages for root canal treatment, including a tooth staining effect [16; 17]. Additionally, excessive concentration of MB can reduce the PDI effect due to light-induced quenching [12]. Furthermore, _E. faecalis_ can infect the deeper dentin tubules, where the absence of adequate oxygen can suppress the PDI- mediated bacterial killing. Therefore, the traditional PDI effect at a low oxygen concentration condition inside the root canals may be significantly reduced [18]. Hence, the development of novel efficient photosensitizers or new PDI protocols may be a solution to this issue [19]. High-level laser therapy is based on the combination of a penetrating laser with a stabilized HP [20]. Multiple studies have concluded improved antimicrobial efficacy when hydrogen peroxide (HP) was employed in PDI [19; 21; 22; 23]. [PERSON], et al., stated that HP has provided similar characteristics as a photosensitizer [23]. The combination of a high-frequency diode laser (980 nm) and HP, if compared with classic PDI accomplished with photosensitizers, allows deeper penetration into the biofilm [24]. This technique is based on hydroxyl radical formation that causes lethal oxidative stress to bacterial cells [25].