M. Haiml

Low-loss GaInNAs saturable absorber mode locking a 1.3-μm solid-state laser

We have demonstrated stable self-starting passive cw mode locking of a solid-state laser at about 1.3 μm using a GaInNAs semiconductor saturable absorber mirror (SESAM). GaInNAs SESAMs show negligible nonsaturable losses, low saturation fluences (11u2009μJ/cm2) and picosecond decay times which make them well-suited for self-starting and stable cw mode locking. Sub-10-ps pulses were produced with a Nd:YLF laser at 1314 nm. The incorporation of about 2% nitrogen into InGaAs redshifts the absorption edge above 1330 nm and reduces the strain in the saturable absorber grown on a GaAs/AlAs Bragg mirror. Final absorption edge adjustments have been made with thermal annealing which blueshifts the absorption edge.

Output-coupling semiconductor saturable absorber mirror

We present a semiconductor saturable absorber mirror (SESAM), which also acts as an output coupler at the same time. The influence of the output coupler transmission onto the absorber parameters is investigated theoretically, as well as experimentally. A passively Q-switched Nd:YVO4 microchip laser is built using such a nonlinear output coupler, yielding clean pulses of 143 ps duration, 48 nJ energy, and 572 W peak power. This result is compared with the traditional approach, where the SESAM is not used as an output coupler.

Femtosecond response times and high optical nonlinearity in beryllium-doped low-temperature grown GaAs

We have investigated the effect of beryllium doping on the optical nonlinearity and on the carrier dynamics in low-temperature (LT) grown GaAs for various growth temperatures and doping levels. Pump–probe experiments with 20 fs pulses and quantitative measurements of the nonlinear absorption show that in undoped LT GaAs, ultrafast response times are only obtained at the expense of low absorption modulation. In contrast, in Be-doped LT GaAs, high absorption modulation is maintained for response times as short as 100 fs. These results are qualitatively explained accounting for the point-defect-related optical transitions in LT-GaAs.

Optical nonlinearity in low-temperature-grown GaAs: Microscopic limitations and optimization strategies

We have quantitatively measured the linear and the nonsaturable absorption as well as the absorption modulation and its recovery time in as-grown and annealed low-temperature (LT) GaAs. Correlation of the optical data with As antisite (AsGa) defect densities yields the absorption cross section and the saturation parameter of the dominant AsGa to the conduction-band defect transition. We show that this defect transition is mainly responsible for the large nonsaturable absorption in as-grown LT GaAs with fast recovery times. Reducing the AsGa density by annealing yields an optimized material with small nonsaturable absorption, high absorption modulation, and fast recovery times.

Nonlinear optical absorption and temporal response of arsenic- and oxygen-implanted GaAs

We have measured the nonlinear optical absorption of arsenic and oxygen implanted epitaxial GaAs for a range of ion doses and annealing temperatures. The response time, τA, and a parameter, Mmax, which characterizes the performance of the structures as modulators, are both reduced by implantation, and correspondingly the nonbleachable losses are increased. We show that similar combinations of (τA, Mmax) can be achieved using either ion species and various combinations of dose and annealing temperatures. Furthermore, the data were all located on a well-defined curve in the (τA, Mmax) plane, provided amorphization, which occurs at high implant doses, was avoided. We deduce that there exists a limit to the modulation if a specific response time is required.

Two-photon fluorescence excitation of macroscopic areas on planar waveguides.

In this paper, we report the first successful demonstration, to our knowledge, of two-photon fluorescence excitation (TPFE) using planar thin-film waveguide structures of macroscopic excitation dimensions (square millimeters to square centimeters in size). The high intensity of excitation light required for TPFE is available not only at a single focus point but along the whole trace of the beam guided in the waveguide structure. Line profiles of the fluorescence excited by TPFE show excellent correlation with the geometry of the launched laser beams. A clear second-order dependence of the fluorescence intensity on the excitation intensity confirms the two-photon character of fluorescence generation. Spectra of the emission generated by one-photon excitation and by two-photon excitation show only minor differences.

Role of implantation-induced defects on the response time of semiconductor saturable absorbers

Arsenic ion implantation with thermal annealing was used to shorten the response times of GaAs-based saturable absorber structures. Ultrafast absorption bleaching measurements indicated that the recovery time was decreased with increasing the implantation dose. However, above a certain dose the recovery time increased again. This behavior was correlated with the microstructure of the residual implantation defects.

Femtosecond nonlinear optics of low-temperature grown semiconductors

Abstract We investigate the nonlinear optical properties of low-temperature-grown (LT) GaAs. It is shown that the strength of the absorptive nonlinearity is weak in as-grown undoped LT-GaAs with a fast recovery time of the nonlinear absorption. The reason for this behavior is revealed by a quantitative study of the nonlinear optical properties of the most important defect transition from the neutral As antisite to the conduction band. Based on this analysis, we demonstrate two methods for the improvement of the ultrafast nonlinearity of LT-grown semiconductors: post-growth annealing or Be doping of LT-GaAs yields a material with a strong ultrafast nonlinear optical response.

Fluoride semiconductor saturable-absorber mirror for ultrashort pulse generation

We demonstrate what is to our knowledge the first ultrabroadband monolithically grown AlGaAs/CaF(2) semiconductor saturable-absorber mirror (SESAM) that covers nearly the entire gain spectrum of a Ti:sapphire laser. A large high-reflectivity bandwidth of more than 300 nm is provided by a device consisting of only six material layers. This fluoride SESAM had a modulation depth of 2.2%, a fast recovery time constant of less than 150 fs, and a slow recovery time constant of 1.2 ps. Using this SESAM inside a Ti:sapphire laser produced self-starting sub-10-femtosecond pulses.

GaAs absorber layer growth for broadband AlGaAs/fluoride SESAMs

Abstract Semiconductor saturable absorber mirrors (SESAMs) are used for ultrashort pulse generation. They are required to support self-starting sub-10-fs pulse generation with Ti:sapphire lasers. So far conventional AlGaAs/AlAs SESAMs have been limited by their small reflection bandwidth of about 60xa0nm. Broadband SESAMs with high reflection bandwidth were needed. In this paper, we report for the first time on the successful growth and operation of a broadband AlGaAs/CaF 2 SESAM with a high reflection bandwidth of more than 300xa0nm for ultrashort pulse generation with GaAs used as saturable absorber. We will demonstrate a pulse spectrum supporting sub-6-fs pulses obtained from an AlGaAs/CaF 2 SESAM in a Ti:sapphire laser. Furthermore, we have studied the formation of twin defects depending on the growth mode for the GaAs absorber layer heteroepitaxially grown on CaF 2 . Defects in the GaAs absorber are necessary to achieve fast response times. However, they can generate surface roughness causing nonsaturable losses by light scattering. A GaAs saturable absorber was grown on a two pair AlGaAs/CaF 2 Bragg mirror with CaF 2 spacer layer without significant degradation of the saturable absorber parameters relevant for passive mode-locked solid-state lasers.