Bandgap engineering, monolithic growth, and operation parameters of GaSb-based SESAMs in the 2–2.4 µm range

Abstract

We present the detailed growth and characterization of novel GaSb-based semiconductor saturable absorber mirrors (SESAMs) operating in the 2–2.4 µm spectral range. These SESAMs at different wavelengths are bandgap engineered using ternary material compositions and without strain compensation. We observe that even when the thickness of quantum wells (QWs) exceeds the critical thickness we obtain strain relaxed SESAMs that do not substantially increase nonsaturable losses. SESAMs have been fabricated using molecular beam epitaxy with a AlAs0.08Sb0.92/GaSb distributed Bragg reflector (DBR) and strained type-I InxGa1-xSb or type-II W-like AlSb/InAs/GaSb QWs in the absorber region. All the type-I SESAMs show excellent performance, which is suitable for modelocking of diode-pumped semiconductor, ion-doped solid-state, and thin-disk lasers. The recovery time of the type-II SESAM is too long which can be interesting for laser applications. The dependence of the SESAM design, based on its QW number, barrier material, and operation wavelength are investigated. A detailed characterization is conducted to draw conclusions from macroscopic nonlinear and transient absorption properties at different wavelengths in the 2–2.4 µm range for the corresponding devices.

50-W average power Ho:YAG SESAM-modelocked thin-disk oscillator at 2.1 µm

“…It is worth mentioning that the saturation fluence of the sample still remains significantly lower in comparison with SESAMs available for 1-μm TDLs [20], and thus requires very large spot sizes (>1.4-mm radius) for stable operation at high pulse energy. This poses special challenges in the resonator design, the tolerable flatness and lensing of the sample, and thus shows a clear path for further optimizing of such 2-µm SESAMs in the near future [22]. Furthermore, we note that the rollover occurring at high fluences is here enhanced by the short pulse duration of the laser used for SESAM characterization.…”

Low-Noise Femtosecond SESAM Modelocked Diode-Pumped Cr:ZnS Oscillator

“…The recently developed, high-quality SESAM is grown with molecular beam epitaxy at the FIRST cleanroom facility, ETH Zurich [26], [27]. It consists of an absorber section with three In0.33Ga0.67Sb quantum wells on top of a 24-pair AlAs0.08Sb0.92 / GaSb distributed Bragg reflector.…”

“…Compared to earlier work in the field [8], the device has substantially lower loss, which enables efficient operation with low output coupling rates. The temporal response is characterized using our pump-probe setup and reveals a very fast recovery within 1 ps due to its strain relaxed quantum well structure (details are given in [27], see SESAM 3). The achieved SESAM parameters are very suitable for fundamental soliton modelocking even for repetition rates in the gigahertz regime [25].…”

“…This also enables higher pulse repetition rates [23] and the ability of compact cavity designs together with low-noise operation [24]. Compared to previous results [8] we recently have demonstrated SESAM modelocked high-power (1 W, 250 MHz, 120 fs, and 0.8 W, 250 MHz, 79 fs) [18] and gigahertz repetition rate (0.8 W, 2 GHz, 155 fs) [25] Cr:ZnS lasers using a high-performance GaSb-based SESAM with strongly reduced nonsaturable losses and a few picosecond recovery time [26], [27]. These lasers have been pumped with a 1.55-µm Er-fiber laser amplifier system.…”

“…Here we demonstrate a diode-pumped Cr:ZnS oscillator using our recently developed SESAMs [18], [27]. We report on a directly diode-pumped SESAM modelocked Cr:ZnS laser achieving stable, self-starting modelocking with 550 mW output power, 189-fs pulses, a pulse repetition rate of 435 MHz at a center wavelength of 2371 nm.…”