This PDF file contains the front matter associated with SPIE Proceedings Volume 12875, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Tissue-diagnostic techniques such as spontaneous Raman scattering suffer from low signal intensities and large fluorescent background. Nonlinear techniques such as Coherent Anti-Stokes Raman Scattering (CARS) may overcome these drawbacks, yet traditional CARS requires prior knowledge about the Raman transition to be probed. Ultrabroadband two-beam CARS combines the multiplex capability of spontaneous Raman scattering with laser-like signal detection. In our setup, compressed approximately 7 fs pulses act as pump and Stokes pulses which allow exciting both the fingerprint region (⪅ 1900 cm^-1) and Raman transitions up to approximately 4000 cm^-1. A probe pulse at approximately 515 nm is used to acquire broadband CARS spectra and to probe the temporal Raman decay. Apart from the diagnostic use, we demonstrate that such ultrashort pulses can be used for defined tissue ablation in human and animal tissue samples. Thus, a combined diagnostic with tissue removal is envisioned.

With the advent of femtosecond lasers, non-linear optical microscopy techniques have become powerful tools to image and study biological tissue. Ultra-fast Titanium Sapphire (Ti:sapphire) lasers have emerged as the gold standard in this field due their broad emission spectrum. Historically, the wider adoption of these imaging techniques outside of laboratory settings has been slowed by the complexity, cost and big footprint of femtosecond Ti:sapphire oscillators. However, in recent years, the new development of high power Gallium Nitride (GaN) based laser diodes has enabled direct diode pumping of Ti:Sapphire lasers, paving the way for drastic reductions in complexity, footprint and price. We present a compact, direct diode pumped, ultra-fast Kerr-lens Mode-Locked (KLM) Ti:sapphire oscillator with a novel tuning scheme based on a combination of a razor-edge slit and a single prism to minimize intracavity dispersion. This scheme allows for a 150 nm continuous tuning range while permitting sub-70 fs pulse duration at the sample plane over the full wavelength range including all optical components including microscope objective. The slit aperture and position are adjustable which allows for tuning of the central wavelength and bandwidth of the emission spectrum. To demonstrate the versatility of the laser for biological imaging, we show two-photon excited fluorescence microscopy and second harmonic generation images of various human tumor biopsies and ex-vivo sheep myocardium slices acquired at different wavelengths ranging from 750 nm to 900 nm.

Burst-mode ultrafast laser treatments in biological tissues or in materials-processing use high-repetition-rate (⪆MHz) delivery of femtosecond laser pulses. This takes advantage of characteristically tiny residual heat left in a substrate through individual femtosecond-laser-matter interaction. At the same time, the approach opens the door to manipulating the accumulation of that same tiny heat during rapid pulse-repetition. This mode of fluence-delivery may, for instance, be able to denature the protein in the walls of a laser-cut wound and possibly improve infection rates in ultrashort-pulse laser surgery in certain contexts. Isolated intense sub-picosecond laser pulses typically do not rely on intrinsic chromophores for absorption, instead they first create a limited plasma via nonlinear ionization, then increase that plasma through collisional ionization. Used in burst-mode, plasma-mediated ablation can exploit some residual ionization which persists for a few nanoseconds, meaning that subsequent pulses need not re-initiate dielectric breakdown. In effect, the plasma is ‘simmered’ continuously throughout a burst, controlling the mode and amount of absorption and opening the door to particularly gentle laser cutting of tissues and dielectric materials. We describe pulse-by-pulse studies of the persistence of the plasma state within a burst of approximately 60 pulses, each of 300 fs duration, arriving with an intra-burst repetition rate of 200 MHz (5 ns separation). We also present the impact of these burst-mode treatments on cellular necrosis in a phantom of rat-glioma cells suspended in hydrogels and in porcine cartilage samples.

The major advancements in ultrafast laser ablation technology are revolutionizing surgical precision and minimizing thermal impact compared to traditional methods. However, the primary challenge hindering widespread clinical adoption has been the slow material removal rate (MRR). Towards this gap, a compact fiber-based laser delivery system has been developed, boasting an impressive 82-fold increase in MRR over the previous femtosecond laser surgical probes. This benchtop setup utilizes a hollow-core Kagome fiber (NA≈0.02) coupled to a high-power Yb-doped fiber laser (λ=1035 nm) to deliver laser pulses onto the sample. Employing a piezo-scanned Lissajous-based beam steering mechanism, the system achieves efficient distribution of ultrashort pulses onto the target surface. Remarkably, the system maintains a high transmission efficiency of 74% while operating at peak intensities, with no components exhibiting nonlinear behavior. For a FOV scan width of 550 µm, the logarithmic relationship between the ablation depth and laser fluence was determined for two different translational velocities. The system achieved material removal rates of ~10.7 mm3 /min for the maximum applied laser fluence of 9.3 J/cm², without initiating carbonization. Moreover, by fine-tuning laser parameters, the system can swiftly create clean-cut trenches of significant dimensions, 3 x 3 mm2 size and ~1 mm deep, mimicking conventional surgical procedures such as spinal decompression within a minute, all without carbonization or tissue damage. This remarkable achievement underscores the reliability and potential of ultrashort-laser ablation techniques for a wide array of surgical interventions.

This study introduces a Bessel-like Beam Generator (BBG) array for optical trapping systems, exploring its application in manipulating dielectric particles in the aqueous medium. The BBG array generates Bessel-like beams through multimode interference. By utilizing heavy water(D₂O) with low laser absorption, the photothermal effect induced due to the water convection is reduced, enhancing the trapping performance of D₂O. We could trap the polystyrene dietetic particles around the non-diffractive length of BBG. The study analyses the beam profile in D₂O and water revealing a significant increase in the non-diffractive length of D₂O. These findings have implications for biophysics and medical research, enabling precise manipulation and observation of biological particles in aqueous environments.

Ultrashort pulse sources are ubiquitous in scientific research as well as industrial applications. Delivering ultrashort pulses with high fidelity over a fiber-optic network to multiple target locations on a time-sharing basis can potentially overcome their complexity in operation and reduce overhead. We previously demonstrated a mechanism to deliver dispersion-compensated sub-400fs pulses in the C-band to different satellite locations using standard telecom-fiber links, as well as characterize them using a compact detector module at the delivery location assisted by a pulse shaper at the source, both controlled remotely via the cloud. The measurement procedure relied on creating a pulse pair with varying delays before launching them into the delivery fiber and measuring first and second-order autocorrelations at the remote location. However, this method proved inadequate to detect the optical nonlinearities as the spectral broadening seen by a pulse pair with varying delays differs from that of a pair of pulses undergoing nonlinear broadening separately since the degree of overlap between the pulses varies with the delay. To overcome this drawback, we propose to launch the variable-delay pulse pair with no temporal overlap to avoid combined nonlinear distortions and measure the autocorrelation at the output by adding a fixed delay interferometer to our detector module. The in-house fabricated fixed delay element consisted of a quartz plate, which provided a delay ≈ 11ps between the reflections from the front and back surfaces. Both surfaces were coated by custom-engineered partially reflecting semiconductor coatings to give ≈ 40% power in both reflections. The addition of the fixed delay element enables us to detect the spectral changes to the sub 400 fs pulses in the presence of nonlinearities in the delivery links using a compact detector module with no movable parts.

Single-photon time-resolved measurements are of great importance in broad application fields, such as ultrafast phenomena, sensing, and quantum information science. Single-photon detectors have limited, temporal resolution, hence, there is need for novel approaches. In this study, we developed an asynchronous optical sampling technique for single-photon time-resolved cross-correlation measurements using a dual-wavelength comb. Employing slightly different repetition frequencies, high-speed and high-time resolution detection was achieved without the need for a mechanical delay stage. Using distinct-color combs for the signal and pump pulses, highly sensitive detection was achieved by efficiently suppressing the strong background caused by the high-power pump. Furthermore, we experimentally demonstrated femtosecond time-resolved measurements at the single-photon level. The signal and pump pulses were derived from the Er and Yb fiber combs. The center wavelengths of the comb were 1560 and 1050 nm, and their repetition frequencies were 107 and 750 MHz. Signal pulses were attenuated to the single-photon level, and the pump pulses were amplified to 1.3 W. The high power and high repetition frequency of the pump enabled highly efficient nonlinear time gating. Temporal characteristics of a weak signal pulse is obtained by photon counting of the generated sum frequency light of the signal and pump using a nonlinear crystal. We obtained the temporal profiles of the single-photon Er comb pulses as a cross-correlation waveform with a half-width of 173 fs and measured the higher-order chirp of a single-photon femtosecond pulse. The developed technique is promising for single-photon-level ultrafast optical applications.

We propose a novel approach employing structured sampling and a single-pixel detector to measure, simultaneously, the spatial profile and the spatially resolved temporal profile of a femtosecond laser beam with an autocorrelation method. The experimental system integrates a Digital Mirror Device (DMD) into a conventional autocorrelation setup. An experimental comparison with a raster scanning method illustrates the advantages of this setup, achieving comparable accuracy with reduced energy levels. This study introduces a promising technique for the precise characterization of ultrafast laser pulses, with potential applications in various fields that demand accurate spatial and temporal measurements as material processing or imaging applications.

Recently, we developed a simple, sensitive, and single-beam method for measuring the local nonlinear refraction using a tightly focused laser beam and the Nonlinear Ellipse Rotation (NER) signal. Such configuration allows mapping the nonlinear refraction as a function of depth. The NER signal for pure instantaneous nonlinearity is inversely proportional to the pulse width, which can be shortened or lengthened depending on the sign of the sample’s dispersion and the input pulse chirp and pulse propagation length. However, in materials exhibiting non-instantaneous molecular orientation effects, such as liquids, we have to take into account the effective nonlinear refraction. Hence, here we propose to perform NER measurements in thick and highly dispersive samples, with instantaneous and non-instantaneous refractive nonlinearities, to obtain both the pulse and sample’s parameters, including the nonlinear refraction as a function of the pulse width.

When multiplexing ultrashort laser beams of different wavelengths into a single beam, the pulse shape of each color must be managed individually due to the various accumulated GDD. This presents a challenge to the industry in terms of making the process compact and flexible. To address this issue, a thin film-based beam combiner has been designed and developed that not only redirects the channels by wavelength but also manages the reflected GDD independently. This opens the door to more practical applications in both industry and research labs.

This paper provides an overview of the thermal stability of volume nanogratings (NGs) inscribed in more than 20 laboratory and commercial oxide glasses using femtosecond laser pulses. The role of glass composition and its viscosity-temperature dependence are particularly investigated. Other parameters, such as nanostructure morphology (e.g., porosity size) can play a role in it. Although it has become established that high viscosity glasses typically yield improved thermal stability, recent results highlight deviations from this trend. This is particularly pronounced in low SiO₂ containing glasses with large Al₂O₃ content (typ. > 50 mol%), where their NGs thermal stability is comparatively higher than what is known for pure silica, which may prove useful for optical devices operating in extreme environments.

Femtosecond laser irradiation followed by chemical etching in NaOH is a material processing technique capable of creating hollow channels with submicrometric cross-sections, extending up to a millimeter in length. In this study, we unveil the fabrication conditions leading to an effective nano-structuring of bulk fused silica, overcoming the limitations of traditional machining techniques in terms of both minimum achievable dimensions and high aspect ratios.

Cleaving of glass substrates with shaped edges using a laser-only concept is presented. In a first laser process shaped ultrashort laser pulses modify in a single pass the entire material thickness with arbitrary edge shape geometries. Afterwards in a second pass CO₂ laser radiation is absorbed in the modified area and resulting stresses lead to the separation of the glass. We investigate the quality of the achieved edges and corresponding mechanical properties. The cutting strategy, so far conducted on straight contours, is successfully transferred to curved contours maintaining edge qualities.

This manuscript introduces a comprehensive overview in the field of Direct Laser Interference Patterning (DLIP) with a focus on its evolution over the last years, its current status, and the emerging challenges it faces. Starting with the emerging stages of DLIP, attention will be directed towards the role played by the development of innovative optical systems, which have been instrumental in pushing the boundaries of precision and control. Examples of surface functionalization achieved through DLIP techniques are also showcased, emphasizing the transformative impact that DLIP has had across various industries, from the enhancement of material properties to the facilitation of new functionalities. Furthermore, consideration will be given to the evolving landscape of inline monitoring approaches within DLIP. These monitoring techniques are poised to address the intricate challenges associated with real-time quality control and process optimization in DLIP applications.

We demonstrate ultrafast high-power laser operation, both at multi-kW average power in ultrashort-pulsed operation over extended bursts with hundreds of MHz intra-burst repetition rate from a modified TruMicro 6020 industrial laser, as well as uninterrupted, quasi-CW operation at an average power beyond 1 kW obtained with a TEM00 multipass thin-disk laser booster amplifier. The pulse repetition rate can be varied from 50 MHz to beyond 1 GHz, with single-pulse energies well above 10 μJ and single-pulse peak powers far beyond 10 MW without a post-compressor. These systems are attractive, e.g., for high-throughput materials processing or for driving nonlinear processes.

Sustainable and cost-effective long-term storage remains an unsolved problem. The most widely used storage technologies today are magnetic (hard disk drives and tape). They use media that degrades over time and has a limited lifetime, which leads to inefficient, wasteful, and costly solutions for storing long-lived data. We are building Silica: the first cloud storage system for archival data underpinned by quartz glass, an extremely resilient media with virtually unlimited lifetime. Data is written using ultrafast laser nano-structuring in the bulk of the glass, creating permanent modifications to the media that allows data to be left in situ indefinitely. Designing and building a new storage technology solely for the cloud affords us with a tremendous opportunity to completely re-think how storage systems are built, free from the legacy constraints of existing technologies. In Silica, we are co-designing and co-optimizing the entire system from the media & write/read processes all the way up to the cloud service level with sustainability and low-cost as primary objectives. Our design follows a cloud-first, data-driven approach underpinned by principles derived from analysing a real public cloud archival service. Here we discuss how these principles have shaped the Silica technology down to the laser nano-structuring process, ushering in a new era of sustainable, cost-effective storage.

Mastigocladopsis repens Rhodopsin (MastR), the newest member of a family of retinal-binding anionic transmembrane pumps, can trap light energy to allow inward chloride transport across a cellular membrane. The pumping action is driven by retinal isomerization, which is thermodynamically unfavorable in the ground state but occurs at a higher rate under photo-excitation in the host protein environment. Particularly interesting is MastR’s conversion to an outward proton pump (MastR-T74D) due to single-point mutation of an amino acid located close to the retinal chromophore. Although flash photolysis studies have characterized the MastR/MastR-T74D photocycle on a microsecond timescale, its ultrafast dynamics involving retinal isomerization have not yet been investigated. We performed ultrafast transient absorption studies on MastR and MastR-T74D to look at the femtosecond to picosecond dynamics that lead to retinal isomerization and compared them to the well-studied ultrafast dynamics of bacteriorhodopsin.

Modern USPL (Ultra Short Pulse Laser) development is trending towards higher repetition rates and higher average power systems. High peak power, low repetition rate USPLs have long been used to generate laser filaments, which consist of a plasma channel and region of focused high intensity propagation. Filamentation leads to heat deposition in the air from linear and nonlinear effects, producing a gas density depression that persists over hydrodynamic timescales (milliseconds). This is long after the femtosecond pulse has passed. In the “single shot” (approximately 10 Hz) regime of filamentation, the time between pulses allows the air density to return to equilibrium before the next pulse arrives. Prior work has experimentally measured the single shot gas density depression via interferometry and demonstrated that high repetition rate filamentation leads to deflection of subsequent pulses due to residual heating from the prior pulses. This work experimentally examines USPL thermal blooming as a function of laser repetition rate. Residual heating effects between pulses are demonstrated through measurements of the energy deposition by the laser filament. The temporally and spatially resolved energy deposition is extracted from interferometric measurements of the phase shift due to the gas density depression. Comparison is made between experimentation and modeling, as well as verification of past results. This work demonstrates how atmospheric propagation of modern high average power, high repetition rate USPL pulses differ from traditional single shot USPL systems.

Publisher's Note: This paper, originally published on 12 March 2024, was replaced with a corrected/revised version on 17 April 2024. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.