Stephan Rapp

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.

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.

Ultrafast pump-probe ellipsometric measurements of transient optical properties in metals during laser ablation

Ultrashort pulsed lasers offer a high potential for precise and efficient material processing. Thus, the fundamental understanding of the laser-material interaction aspects is of great importance. The reflectivity and absorption of material experience significant changes during and after being irradiated by an ultrashort laser pulse. These changes influence decisively the laser-material interaction of the processing pulse or of subsequent pulses in for example double pulse experiments. Absorption changes in metals induced by an ultrashort laser pulse near the ablation threshold are however insufficiently investigated to date. In this work, a unique pump-probe ellipsometry microscope is presented allowing the determination of the transient complex refractive index N = n − ik with a sub-ps temporal resolution (Fig. 1, left) [1]. The imaginary part k can directly be converted into the optical absorption coefficient α or into the optical penetration depth d. The setup enables measurements of laser irradiated optically thick samples below and above the threshold fluence for irreversible material modification. Thus, experiments above the ablation threshold fluence Fthr can be performed.

Optical dynamics in ultra-short laser pulse irradiated copper revealed by simulation and experimental measurement with time-resolved pump-probe ellipsometry

In the last decade, ultrashort pulsed lasers have been developing to a precise tool for material processing. Experimental and theoretical studies on the laser ablation efficiency of metals have demonstrated a high degree of dependency on the laser pulse duration. Within these studies, the transient energy deposition in the material defines the thermal and mechanical material response after laser irradiation.

Pump-probe reflectometric and ellipsometric investigation of femtosecond laser pulse induced ablation in molybdenum

Ultrashort pulsed laser sources offer new possibilities in precise and efficient material processing. Deep understanding of the fundamental laser-material interaction aspects is of great importance. We report on pump-probe reflectometric investigations of the ablation process on molybdenum over the complete temporal process range from the pulse impact to the final steady state. The ablation process can roughly be separated in three sections. In the first tens of picoseconds mainly the optical material properties are changed without significant material motion. Between 50 ps and a few ns the irradiated material is bulging in a spallation or phase explosion process. The actual ablation by material ejection is observed at delay times greater than 20 ns. The transient reflectivity during and in the first tens of ps after the laser irradiation in conjunction with the transient absorption influences decisively the laser-matter interaction for example when working with longer pulse durations or double pulse sequences. Direct measurements of the absorption properties by ultrafast time-resolved ellipsometry at fluences close to the ablation threshold fluence are missing to date. In this paper, pump-probe ellipsometric measurements on molybdenum – complementing the pump-probe reflectometric measurements – are presented showing ultrafast changes of the complex refractive index N = n – ik including additional information on the absorption. The imaginary part k is reduced already after 10 ps by 50% representing an increase of the optical penetration depth by a reduction of the material density. These extensive investigations pave the road towards a better understanding of pulse duration dependent laser ablation efficiency, double or burst mode laser ablation and lattice modifications in the first ps after the laser pulse impact.

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.