Controlled laser delivery into biological tissue via thin-film optical tunneling and refraction

Abstract

Due to the often extreme energies employed, contemporary methods of laser delivery utilized in clinical dermatology allow for a dangerous amount of high-intensity laser light to reflect off a multitude of surfaces, including the patient's own skin. Such techniques consistently represent a clear and present threat to both patients and practitioners alike. The intention of this work was therefore to develop a technique that mitigates this problem by coupling the light directly into the tissue via physical contact with an optical waveguide. In this manner, planar waveguides cladded in silver with thin-film active areas were used to illuminate agar tissue phantoms with nanosecond-pulsed laser light at 532nm. The light then either refracted or optically tunneled through the active area, photoacoustically generating ultrasonic waves within the phantom, whose peak-to-peak intensity directly correlated to the internal reflection angle of the beam. Consequently, angular spectra for energy delivery were recorded for sub-wavelength silver and titanium films of variable thickness. Optimal energy delivery was achieved for internal reflection angles ranging from 43 to 50 degrees, depending on the active area and thin film geometries, with titanium films consistently delivering more energy across the entire angular spectrum due to their relatively high refractive index. The technique demonstrated herein therefore not only represents a viable method of energy delivery for biological tissue while minimizing the possibility for stray light, but also demonstrates the possibility for utilizing thin films of high refractive index metals to redirect light out of an optical waveguide.

Ultrasonic modulation of tissue optical properties in ex vivo porcine skin to improve transmitted transdermal laser intensity

“…As such, Q‐switched Nd:YAG laser, similar to those used in clinical dermatology, was used to transmit pulses of light at a wavelength of 532 nm into samples of ex vivo porcine skin by means of a silvered acrylic optical waveguide, based on designs established in prior investigations . The light was confined within the waveguide until reaching a predefined active area in contact with the tissue, whereafter the light refracted into the skin.…”

Techniques and Challenges for Characterizing Metal Thin Films with Applications in Photonics

“…Consequently, for this example, the accurate characterization of the metal film's thickness is of paramount importance in appropriately exerting control over the beam propagation, both for maintaining confinement within the waveguide and for transmitting the light into the external medium. When a beam of light undergoes total internal reflection, as occurs within optical waveguides, the incident electromagnetic energy slightly protrudes out into the external media at each reflection point; if the cladding layer is sufficiently thin, the light may optically tunnel through the film, resulting in a beam of light within the external medium whose intensity is proportional to the film thickness [8].…”

“…The substrates are clad in thin films of titanium and silver with a total thickness on the order of 200 nm. Using thin films of metal as a cladding layer guarantees that the waveguides can operate throughout a range of internal reflection angles from 6 • < θ i <90 • , which can be directly calculated using Snell's law [8,[22][23][24].…”

Paraxial application of auxiliary devices with waveguide mediated laser irradiation for applications in medical biophotonics

“…The method, originally referred to as photonic ablation via quantum tunneling (PAQT), departs from traditional free-space propagation in favor of a thin planar optical waveguide that delivers the light via direct physical contact with the tissue. This method encloses the optical system, which circumvents hazards associated with backscattered light and prevents occlusion of the light while allowing for the paraxial operation of a second technology on the rear face of the waveguide [66].…”

“…For example, in the field of biophotonics, of particular interest to this work, articulated mirror arms are utilized in clinical laser dermatology systems to redirect high-powered, short-pulsed laser light toward the clinical target; meanwhile, metal film-clad optical waveguides are also being developed for direct-contact coupling of laser light into tissue via optical tunneling, as depicted in Fig. 2.1 [40,66,67,83].…”