Chris Lehane

Interface effects on YBa2Cu3O7−δ ultrathin film growth monitored by in situ resistance measurement

The growth dynamics of pulsed laser deposited ultrathin YBa2Cu3O7−δ films on various substrates was explored using an in situ resistance measurement technique. The results indicate that an interface region exists before the onset of uniform growth. On MgO and YSZ substrates, the first 30–50 A has a small contribution to the conductivity of the film. This interface layer is less than one unit cell thick on better lattice‐matched substrates, such as SrTiO3, LaAlO3, and CeO2. The thinnest interface layer was found on PrBa2Cu3O7−δ . Misfit strain relaxation and island growth mechanism can be used to explain experimental observations. The superconductivity degradation for ultrathin YBa2Cu3O7−δ films is also discussed. In situ resistance measurement can be an excellent method to study the ultrathin film growth mechanism.

Generation and measurement of picosecond voltage pulses in YBa2Cu3O7 thin films

An autocorrelation method was proposed and applied to measure the temporal decay of ultrafast voltage pulses in the superconducting state. Using a 40 ps laser, voltage pulses with fast components <40 ps were obtained in a YBCO thin film which was biased within the superconducting state.

Study of photoresponse of high‐Tc Y‐Ba‐Cu‐O superconducting ultrathin films using a picosecond laser pulse train

We have systematically studied the photoresponse of Y‐Ba‐Cu‐O (YBCO) ultrathin films to a train of picosecond laser pulses. The onset and decay of nonequilibrium superconductivity due to different levels of optical excitation can be observed within the single pulse train. Thermal and nonthermal responses can be clearly identified. In addition, the superconducting film resistance was found to increase progressively with laser power above the onset threshold. The YBCO thin films were possibly excited into a nonequilibrium intermediate (resistive) state from the superconducting state. The laser intensity dependence indicated a nonthermal origin for these voltage pulses.

In and out‐diffusion of oxygen during YBa2Cu3O7−δ thin film deposition by in situ resistance measurement

The instantaneous oxygen content of YBa2Cu3O7−δ thin films was measured during pulsed laser deposition by in situ resistance measurements. The oxygen deficiency (δ) of the surface layer was found to be ∼0.835 during the laser plume and increase to 0.90 when the plume was turned off. Using a theoretical fit it was found that the thickness of the oxygen‐rich layer at the surface increased with increasing film thickness and saturated at ∼300 A for thick films. The out‐diffusion of oxygen in the absence of the laser plume can be well characterized by a simple diffusion equation, with a diffusivity of D=1.5×10−13 cm2/s. On the other hand, the in‐diffusion of oxygen during the laser plume occurs with a diffusivity of D≳10−8 cm2/s. The vastly different diffusivity is due to the presence of activated atomic oxygen in the laser plume.

Ultrafast optical response of ultrathin YBCO films

The interaction between ultrashort laser pulses and superconductors is reviewed. It is shown that the duration of the voltage pulse generated is limited by hot phonon decay. A two bridge autocorrelation method is proposed and demonstrated to measure the picosecond voltage pulses generated. The dependence of the voltage pulse duration on bias current, temperature, and laser intensity were investigated. A systematic film thickness dependence is observed which is not explainable by a simple hot phonon escape process.

Laser drilling at large standoff distances using dual pulse Nd laser

A new method which improves the laser drilling efficiency and allows for drilling at large laser-target standoff distances has been developed. A dual pulse laser was found to be capable of penetrating stainless steel targets with a thickness greater than 1/4” at standoff distances of 1 meter. The standoff distance is defined by the distance between the focusing lens and the target. The dual pulse laser produced two free-running laser pulses from a single Nd:glass or Nd:YAG laser head driven by integrated power supplies. The output consisted of the combination of a high energy, long duration pulse (pulse 1) with a high peak power laser pulse (pulse 2) fired at the end of pulse 1. Pulse 1 is used to produce a large molten pool of liquid which is expelled by the vapor recoil pressure generated by the interaction of the high intensity pulse 2 with the metal.A new method which improves the laser drilling efficiency and allows for drilling at large laser-target standoff distances has been developed. A dual pulse laser was found to be capable of penetrating stainless steel targets with a thickness greater than 1/4” at standoff distances of 1 meter. The standoff distance is defined by the distance between the focusing lens and the target. The dual pulse laser produced two free-running laser pulses from a single Nd:glass or Nd:YAG laser head driven by integrated power supplies. The output consisted of the combination of a high energy, long duration pulse (pulse 1) with a high peak power laser pulse (pulse 2) fired at the end of pulse 1. Pulse 1 is used to produce a large molten pool of liquid which is expelled by the vapor recoil pressure generated by the interaction of the high intensity pulse 2 with the metal.