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Transient behaviour of quantum-dot saturable absorber mirrors at varying excitation fluence
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.ORCID iD: 0000-0002-4606-4865
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.ORCID iD: 0000-0002-2508-391X
2014 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 116, no 4, 919-927 p.Article in journal (Refereed) Published
Abstract [en]

We present results from studying the carrier dynamics in self-assembled InAs/GaAs quantum-dot saturable absorbers intended for mode-locking of solid-state lasers. Four samples are examined, featuring controlled variations in the resonance condition of the electric field inside the absorber, the number of quantum-dot (QD) layers and the thickness of the GaAs barriers between these QD layers. Pump-probe experiments are conducted at a wide range of excitation fluences and reveal a fast relaxation component of the initial bleaching at low excitation fluences, while a slowly relaxing induced transparency becomes dominant at higher excitation fluences. Time-resolved photoluminescence measurements reveal a large and slowly relaxing induced transparency due to a capture of excess carriers from the barrier bands into the QDs and a slow radiative recombination there. The resonance condition as well as the thickness of the barriers between the QD layers can be used to control the relaxation behaviour. The fastest response is obtained in a structure with an increased number of QD layers at each individual anti-node of the electric field, which is attributed to the appearance of efficient non-radiative recombination channels and capture centres. These centres are probably related to dislocations and other defects appearing in thick QD stacks.

Place, publisher, year, edition, pages
2014. Vol. 116, no 4, 919-927 p.
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-131454DOI: 10.1007/s00340-014-5778-1ISI: 000341369600017Scopus ID: 2-s2.0-84906951488OAI: oai:DiVA.org:kth-131454DiVA: diva2:656437
Note

Updated from submitted to published.

QC 20141009

Available from: 2013-10-15 Created: 2013-10-15 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Novel Technologies for Mode-Locking of Solid-State Lasers
Open this publication in new window or tab >>Novel Technologies for Mode-Locking of Solid-State Lasers
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The subject of this thesis is the investigation of novel technologies for mode-locking of diode-pumped, solid-state lasers. Novel saturable absorbers are used: quantum dots (QDs) and carbon nanotubes (CNTs), which both are low-dimensional nano-formations. In addition, mode-locking by cascaded nonlinearities is explored.

Absorber structures containing self-assembled InGaAs QDs are characterised in detail by pump-probe experiments, time-resolved photoluminescence spectroscopy, and measurement of the nonlinear reflectivity. The samples show sub-picosecond relaxation times of the reflectivity, modulation depths between 0.18% and 2.9%, as well as low saturation fluences on the order of 1–10μJ/cm2. The structures’ design parameters are related to their transient and nonlinear performance.

The characterised QD saturable absorbers are then used for mode-locking of diode-pumped, solid-state lasers, delivering picosecond pulses with optical spectra in the region of 1020–1040nm. In particular, a QD absorber with a saturation fluence of 4μJ/cm2 and a relaxation time <200fs is successfully employed for fundamental mode-locking of an Yb:KYW laser at a repetition rate of 1GHz. This laser emits pulses with a duration of 1.7ps at an output power of 339mW. Apart from this, an Yb-thin-disc laser is demonstrated, emitting pulses with a duration of 1.6ps at an output power of 13W, thereby showing, that the absorber withstands fluences of up to 2.4mJ/cm2 without being damaged.

An absorber with a linear loss of only 1% is obtained by embedding CNTs in a thin plastic film, coated onto a glass substrate. Using this absorber, mode-locking of an optically-pumped semiconductor disc-laser is achieved. The laser emits pulses with a duration of 1.12ps at a repetition rate of 613MHz and with an average output power of 136mW.

For cascaded mode-locking, a periodically-poled KTP crystal is placed inside a laser cavity and the two second-order nonlinearities from second-harmonic generation and back-conversion are used to emulate a third-order nonlinearity with an effective nonlinear refractive index of 2.33·10−17m2/W. For precise control of the nonlinearity, the laser’s spectrum is fixed to a wavelength of 1029.1nm by a volume Bragg grating. The laser emits pulses with a duration of 16ps at a repetition rate of 210MHz and with an output power of 690mW.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xiii, 131 p.
Series
Trita-FYS, ISSN 0280-316X ; 2013:48
Keyword
laser, mode-locked laser, pulsed laser, quantum dot, carbon nanotube, nonlinear optics, saturable absorber, Kerr-lens mode-locking, ultra-fast laser, picosecond laser, pulsed laser, semiconductor saturable absorber
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-131455 (URN)978-91-7501-840-9 (ISBN)
Public defence
2013-11-05, FA32, AlbaNova Universitetscentrum, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131016

Available from: 2013-10-16 Created: 2013-10-15 Last updated: 2013-10-16Bibliographically approved

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Marcinkevicius, SauliusPasiskevicius, Valdas

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