By adapting a laser scanning microscope with a titanium sapphire femtosecond pulsed laser and transmission optics, we are able to produce live cell images based on the nonlinear optical phenomenon of second harmonic generation (SHG). Second harmonic imaging (SHIM) is an ideal method for probing membranes of living cells because it offers the high resolution of nonlinear optical microscopy with the potential for near-total avoidance of photobleaching and phototoxicity. The technique has been implemented on three cell lines labeled with membrane-staining dyes that have large nonlinear optical coefficients. The images can be obtained within physiologically relevant time scales. Both achiral and chiral dyes were used to compare image formation for the case of single- and double-leaflet staining, and it was found that chirality plays a significant role in the mechanism of contrast generation. It is also shown that SHIM is highly sensitive to membrane potential, with a depolarization of 25 mV resulting in an approximately twofold loss of signal intensity.

High-resolution nonlinear optical imaging of live cells by second harmonic generation

New fabrication methods are strongly demanded for the development of thin-film saturable absorbers with improved optical properties (absorption band, modulation depth, nonlinear optical response). In this sense, we investigate the performance of indium nitride (InN) epitaxial layers with low residual carrier concentration (<1018 cm−3), which results in improved performance at telecom wavelengths (1560 nm). These materials have demonstrated a huge modulation depth of 23% and a saturation fluence of 830 µJ/cm2, and a large saturable absorption around −3 × 104 cm/GW has been observed, attaining an enhanced, nonlinear change in transmittance. We have studied the use of such InN layers as semiconductor saturable absorber mirrors (SESAMs) for an erbium (Er)-doped fiber laser to perform mode-locking generation at 1560 nm. We demonstrate highly stable, ultrashort (134 fs) pulses with an energy of up to 5.6 nJ.

https://www.mdpi.com/2076-3417/10/21/7832/pdf

High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development

Indium nitride (InN)-based semiconductor saturable absorbers have previously shown advantages for application in near-IR fiber lasers due to their broad modulation depth, ultrafast nonlinear response and thermal stability. However, up to now all demonstrated saturable absorber elements based on InN (either transmissive or reflective) have shown limited performance due to poor coupling and insertion losses. We present here a simple mode-locking device based on a GRIN-rod lens in conjunction with an InN semiconductor saturable absorber mirror (SESAM) for its use in a passively mode-locked all-fiber laser system operating at telecom wavelengths. Our results demonstrate that this coupling element ensures not only a compact, turnkey and alignment-free design but also a highly-stable optical femtosecond pulse train. The reduction of insertion losses (3.5 dB) enables the generation of 90-fs ultrafast pulses with an average power of 40 mW and up to 7 nJ of pulse energy without the need for additional amplification.

Performance enhancement of an ultrafast all-fiber laser based on an InN saturable absorber using GRIN coupling

Supercontinuum generation in ultra-flattened near-zero dispersion PCF with C7H8 infiltration

: This document broadly outlines many military uses for lasers; for example, as weapons, in optical guidance, and for communication systems.

Military Applications of Lasers

We report on the improvement of performance of InN-based saturable absorbers in fiber lasers operating at 1.55 µm by reducing the residual doping, due to the lower Burstein-Moss effect. The improved tuning of the band-to-band transition with respect to the operation wavelength leads to an enhancement of nonlinear optical effects, resulting in $30$30 % of modulation depth. We introduce the development of an ultrafast mode-locked fiber laser using an improved InN-based saturable absorber that incorporates a buffer layer between the active layer and the substrate. The laser delivers output pulses with a temporal width of $\sim 220$∼220 fs, a repetition rate of $5.25$5.25 MHz, and high-pulse energy of $5.8$5.8 nJ.

Effect of the residual doping on the performance of InN epilayers as saturable absorbers for ultrafast lasers at 1.55µm

The application of a recently demonstrated mode-locked ultralong pulsed fibre ring laser architecture (Gallazzi et al., 2021) to the inexpensive and efficient generation of supercontinuum from a pulsed output at 1.56 μ m in standard single-mode telecommunication fibre is experimentally demonstrated. The impact of broadening fibre, dynamic switching between different harmonic modes and the influence of laser polarization on the properties of the generated supercontinuum are studied for three different kinds of telecommunication fibre. Flexible spectral broadening beyond 200 nm is obtained with good temporal correlation and pulse compression from 240 fs down to 50 fs.

Ultralong ring laser supercontinuum sources using standard telecommunication fibre

Semiconductor saturable absorbers are becoming a matter of interest since they are keystone elements of pulsed lasers, leading to ultrashort pulses with high peak intensities and wide optical spectra. This kind of ultrafast lasers are empowering new applications in the fields of optical telecommunications and nonlinear optics. In order to improve the radiation source with regard to pulse energy and temporal duration, new saturable absorbers are still under development.

L. Monroy

Papers

High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development

Performance enhancement of an ultrafast all-fiber laser based on an InN saturable absorber using GRIN coupling

Effect of the residual doping on the performance of InN epilayers as saturable absorbers for ultrafast lasers at 1.55µm

Ultralong ring laser supercontinuum sources using standard telecommunication fibre

Novel InN-Based SESAMs with Ultra-Short Time Response