Matthias Domke

Ultrafast pump-probe microscopy with high temporal dynamic range

Ultrafast pump-probe microscopy is a common method for time and space resolved imaging of short and ultra-short pulse laser ablation. The temporal delay between the ablating pump pulse and the illuminating probe pulse is tuned either by an optical delay, resulting in several hundred femtoseconds temporal resolution for delay times up to a few ns, or by an electronic delay, resulting in several nanoseconds resolution for longer delay times. In this work we combine both delay types for temporally high resolved observations of complete ablation processes ranging from femtoseconds to microseconds, while ablation is initiated by an ultrafast 660 fs laser pump pulse. For this purpose, we also demonstrate the calibration of the delay time zero point, the synchronization of both probe sources, as well as a method for image quality enhancing. In addition, we present for the first time to our knowledge pump-probe microscopy investigations of the complete substrate side selective ablation process of molybdenum films on glass. The initiation of mechanical film deformation is observed at about 400 ps, continues until approximately 15 ns, whereupon a Mo disk is sheared off free from thermal effects due to a directly induced laser lift-off ablation process.

Laser lift-off initiated by direct induced ablation of different metal thin films with ultra-short laser pulses

Molybdenum thin films on glass substrates play an important role as contact layer for thin film solar cells. They can be ablated by picosecond laser pulses irradiated from the substrate side at low laser fluences of less than 1 J cm−2, while structured trenches remain free from thermal damage and residues. The fluence for that so-called direct induced ablation from the substrate side is in contrast to metal side ablation reduced by approximately one order of magnitude and is far below the thermodynamic limit for heating, melting and evaporating the complete layer. For an extended investigation of the direct induced laser ablation and the underlying mechanism, further thin film materials, chromium, titanium and platinum, with thicknesses between 200 nm and 1 µm were examined. Finally, a simple thermo-dynamical model is able to connect the observed ablation energetics with the mechanical ductility and stress limit of the metal thin films.

Transient temperature modeling and shock wave observation in confined laser ablation of thin molybdenum films

The transient behavior of the laser lift-off of thin molybdenum films, initiated by glass substrate side irradiation with a 660 fs laser pulse, is investigated in the picosecond range. For this purpose, a pump-probe microscopy setup is utilized to measure the transient relative reflectivity change in the center of the irradiated spot at the molybdenum/glass interface, which enables an interferometric observation of the shock wave propagation in the glass. In addition, a transient simulation of the electron and lattice temperature was performed. The results suggest that ultrafast heating initiates a shock wave in the molybdenum and the glass when the laser pulse has reached maximum intensity. At 10 ps, a confined phase explosion adds further momentum, and the Mo layer is caused to bulge.

Time and space resolved investigations of confined fs ablation of Ta2O5/Pt thin film systems

Maskless patterning of biocompatible sensor chips consisting of a Ta2O5/Pt/glass layer system can be realized by ultrashort laser pulse ablation allowing fast and precise structuring. Here, a 650 fs laser at a center wavelength of 1053 nm is used at a peak fluence of 5 J/cm2. It was observed, that a greater diameter of the Pt film (400 nm) is ablated when it’s coated with Ta2O5 (200 nm) compared to the uncoated Pt. One reason was found in the anti-reflective effect of the Ta2O5 layer causing an increase of energy deposition in the material. The underlying physical effects of the ablation reaction are investigated over the whole reaction time ranging temporally from fs to μs by ultrafast pump-probe microscopy. For the direct ablation of the uncoated Pt, results show ultrafast heating and melting after 2 ps, the creation of a gas-liquid mixture and plasma at 10 ps. At around 100 ns the actual ablation takes place indicated by the ejection of small particles. The results for the Ta2O5/Pt layer system reveal heating and electron excitation in the Ta2O5 layer during the first 2 ps. In the following the spot center behaves identical to the direct ablation of Pt. Here, the Ta2O5 is ablated with the Pt. A confined ablation where an additional amount of laser energy is deposited in the layer system or at the layer interface is assumed to take place. In the rim of the spot only the Ta2O5 is removed by indirectly-induced ablation at around 35 ns.

Time and space resolved microscopy of induced ablation with ultra-short laser pulses

Laser lift-off processes have been observed during structuring CIS thin film solar cells with ultra-short laser pulses, if a Mo film on glass is irradiated from the glass substrate side. To investigate the underlying physical effects, ultrafast pump-probe microscopy is used for time- and space resolved investigations. The setup utilizes a 660 fs-laser pulse at a wavelength of 1053 nm that is split up into a pump and a probe pulse. The pump pulse ablates the thin film, while the frequency doubled probe pulse illuminates the ablation area after an optically defined delay time of up to 4 ns. For longer delay times, a second electronically triggered 600 ps-laser is used for probing. Thus, the complete ultra fast pulse initiated ablation process can be observed in a delay time range from femtoseconds to microseconds. First experiments on the directly induced ablation of molybdenum films from the glass substrate side show that mechanical deformation is initiated at about 400 ps after the impact of the pump laser pulse. The deformation continues until approximately 15 ns, then a Mo disk shears and lifts-off with a velocity of above 70 m/s free from thermal effects.

Ultrafast movies of thin metal film ablation with ultra-short laser pulses

Laser lift-off processes have been observed during structuring CIS (copper-indium-diselenide) thin film solar cells with ultra-short laser pulses, if a thin molybdenum film is irradiated from the glass substrate side.To investigate the underlying physical effects, ultrafast pump-probe microscopy was used to record the transient behavior of the single pulse ablation process. The ablation itself is initiated by a 660 fs pump pulse at a wavelength of 1053u2005nm. The ultrafast dynamic of the ablation process in the femtosecond and picosecond range is captured by illuminating the laser-material interaction region with an optically delayed 510 fs probe pulse up to 4u2005ns. Delay times can be extended by a second electronically delayed 600 ps probe laser pulse to record events in the nanosecond and microsecond range, which reveal mechanical deformation.Time-resolved investigations show that glass side ablation generates a confined gas-liquid mixture after 10 ps. Expansion of the gas and a generated shock wave delaminate the film at about 400 ps. The film then bulges to a maximum at 20u2005ns, if the fluence is below 0.6u2005J/cm². At higher fluences an intact Mo disk shears and lifts-off, while at fluences above 0.75u2005J/cm² a steep reflectivity decrease occurs due to nonlinear absorption in the glass substrate after 1 ps.Laser lift-off processes have been observed during structuring CIS (copper-indium-diselenide) thin film solar cells with ultra-short laser pulses, if a thin molybdenum film is irradiated from the glass substrate side.To investigate the underlying physical effects, ultrafast pump-probe microscopy was used to record the transient behavior of the single pulse ablation process. The ablation itself is initiated by a 660 fs pump pulse at a wavelength of 1053u2005nm. The ultrafast dynamic of the ablation process in the femtosecond and picosecond range is captured by illuminating the laser-material interaction region with an optically delayed 510 fs probe pulse up to 4u2005ns. Delay times can be extended by a second electronically delayed 600 ps probe laser pulse to record events in the nanosecond and microsecond range, which reveal mechanical deformation.Time-resolved investigations show that glass side ablation generates a confined gas-liquid mixture after 10 ps. Expansion of the gas and a generated shock wave delamina...

Selective femtosecond laser structuring of a platinum/tantalum pentoxide thin film layer system by induced laser ablation investigated with pump-probe microscopy

A biocompatible sensor chip consists of a 200u2005nm conducting platinum layer between a glass substrate and a 200u2005nm tantalum pentoxide layer as insulator on top. The irradiation of the biochip from the Ta2O5 side with ultra-short laser pulses (660 fs) at a wavelength of 1053u2005nm leads to a selective ablation of the Ta2O5 layer from the platinum. The ablation threshold for the removal of the Ta2O5 layer is about 0.08u2005J/cm². Moreover, the Ta2O5 lifts-off in form of an intact disk at a fluence of 0.14u2005J/cm². For deeper understanding of the ablation mechanisms, a pump-probe microscopy setup was utilized to record a stop-motion movie of the lift-off process. The setup utilizes an optically combined with an electronically delayed probe pulse. For the first time to our knowledge the laser lift-off of a transparent oxide could be recorded over the complete temporal dynamic range. The movies reveal that an intact disk leaves the hole after a few 10u2005ns. At higher fluences of about 0.20u2005J/cm² the disk disrupts in several particles at about 40u2005ns.A biocompatible sensor chip consists of a 200u2005nm conducting platinum layer between a glass substrate and a 200u2005nm tantalum pentoxide layer as insulator on top. The irradiation of the biochip from the Ta2O5 side with ultra-short laser pulses (660 fs) at a wavelength of 1053u2005nm leads to a selective ablation of the Ta2O5 layer from the platinum. The ablation threshold for the removal of the Ta2O5 layer is about 0.08u2005J/cm². Moreover, the Ta2O5 lifts-off in form of an intact disk at a fluence of 0.14u2005J/cm². For deeper understanding of the ablation mechanisms, a pump-probe microscopy setup was utilized to record a stop-motion movie of the lift-off process. The setup utilizes an optically combined with an electronically delayed probe pulse. For the first time to our knowledge the laser lift-off of a transparent oxide could be recorded over the complete temporal dynamic range. The movies reveal that an intact disk leaves the hole after a few 10u2005ns. At higher fluences of about 0.20u2005J/cm² the disk disrupts in sever...