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Time-resolved optical characterization of proximized nano-bilayers for ultrafast photodetector applications

Time-resolved transient pump-probe spectroscopy measurements on proximized ferromagnet/superconductor (F/S) hybrid structures are presented. We focused our attention on both low and high critical temperature superconductors such as Nb and YBCO, while for F the weak-ferromagnetic alloy Ni0.48Cu0.52 has been used. Dynamics of the electron-phonon relaxation process has been investigated as a function of both the temperature and the F-film thickness. In the case of NiCu/Nb bilayers the presence of a thin F overlayer reduces the slow bolometric component of the photoresponse, while in YBCO-based structures faster relaxation times were measured. F/S nanobilayers are very attractive toward the development of novel hybrid superconducting photodetectors.


Introduction
Superconductors offer unique potentials for many radiation detection applications, due to their highenergy sensitivity over a very wide frequency range (from X-ray to mm-millimeter wavelengths), ultra-fast electronic response times (down to few ps), and radiation hardness.
In a superconductor the response to the radiation interaction after the absorption of radiation quanta involves a complex dynamics which depends on the relaxation processes involving phonons, quasiparticles, and Cooper pairs. At the very early stage of the cascade, the absorbed energy E 0 is shared to a fast photoelectron, and the down-conversion process is dominated by electron-electron (ee) scattering. A strongly nonequilibrium hot electron-hole distribution takes place typically on a time scale ≤10 fs, when the electron-phonon (e-ph) scattering process becomes mostly effective leading to the production of a large number of phonons. The time evolution of both phonon and quasiparticle distribution functions can be described by a system of coupled microscopic kinetic equations [1]. However, at T ≤ T c , where T c is the superconductor critical temperature, and under weak perturbations, it can assume that the electron and phonon dynamics are described in terms of their energy distributions: two different temperatures T e and T ph can be assigned to the electron and phonon sub-systems. The nonequilibrium state is hence described by a system of two coupled differential timedependent energy-balance equations [2], commonly referred as the two-temperature model (2-T). The 2-T equations involve the e-ph coupling constant g e-ph , the external source term (e.g., the optical pump pulse), and the heat capacities of both electrons and phonons, C e and C ph , respectively. At low excitation fluencies and/or near T c , the temperature dependences of C e , C ph , and g e-ph can be neglected and the e-ph relaxation time can be simplified as τ e-ph = C/g e-ph , where 1/C = 1/C el + 1/C ph . The estimation of C e , C ph , and the experimental determination of τ e-ph as in fast optical reflectivity changes measurements allow us to solve numerically the 2-T equations obtaining information about the involved dynamics.
The role of the superconducting material is crucial, since it influences drastically the electronphonon scattering time, the heath capacity, and the escape rate for phonons. NbN and YBa 2 Cu 3 O 7-x (YBCO) have been demonstrated to be good choices as superconducting materials for bridge-type superconducting photodetectors, as they are characterized by the overall picosecond photoresponse [3,4].
Besides simple metals, heterogeneous structures formed by a superconductor (S) and a normal metal (N) in good electrical contact can represent an interesting field toward the optimization of the ultrafast carrier dynamics regime. According to the physics of the proximity effect, two materials that form a bilayer influence each other on a spatial scale of the order of their coherence lengths near their interface [5]. However, because of the small value of the optical penetration depth α < 50 nm for metals at visible-light wavelengths (400-900 nm), optical experiments should sense electronic properties of only one (N) of the proximized layers, far from the interface where the major effect takes place in terms of spatial modification of the order parameter. Weak-ferromagnet/superconductor (F/S) bilayers are very interesting, since for these hybrids the coherence length in F, ξ F , is of the order of few nm due to the presence of the magnetic exchange energy, and it can be modulated by varying the thickness of the F layer as low as α .
Time-resolved transient pump-probe spectroscopy allows to investigate the electron-phonon relaxation times of proximized F/S nanobilayers as a function of F thickness and temperature. This characterization is useful for selecting structures with ultra-fast carrier dynamic in the framework of fast photodetectors. In this paper we focused our attention on both low and high critical temperature superconductors (LTS and HTS) as Nb and YBCO, well investigated in terms of the dynamic of the relaxation process [6,7], while for F we have chosen weak-ferromagnetic alloys as Ni 0.48 Cu 0.52 .

Sample fabrication
Thin Nb films were deposited by using a dedicated DC magnetron sputtering system. 70-nm-thick, base Nb layers were sputtered at a base pressure of 1x10 -7 Torr on chemically cleaned Corning glass substrates. The sputtering power and the deposition rate were 700 W and 2.2 nm/s, respectively. For the Ni 0.48 Cu 0.52 deposition, samples were inserted inside another vacuum system, where their surface was firstly cleaned for 30 s using an ion beam etching at a rate of about 0.2 nm/s. Afterwards Ni 0.48 Cu 0.52 films were deposited at a rate of 1.5 nm/s and a sputtering power of 200 W. The thickness of Ni 0.48 Cu0 .52 ranges from 6nm to 21 nm. Nb films were polycrystalline, while Ni 0.48 Cu 0.52 either amorphous, or polycrystalline with the grain size < 1 nm. Further information about the physical characterization of Ni 0.48 Cu 0.52 films can be found elsewhere [8], while the coherence length ξ F = 24 nm was estimated [9]. 100-nm thick c-axis YBCO films were deposited by DC magnetron sputtering on a SrTiO 3 substrate at T=950 K. The sputtering power and the deposition rate were 100 W and 3 nm/s, respectively. Then, the film was annealed at T=500 K, and covered by an Au buffer layer (about 20 nm), deposited by sputtering technique to reduce the diffusion of O 2 outside the superconducting film. The roughness of the YBCO film was estimated to be lower than 2 nm. Afterwards, Ni 0.48 Cu 0.52 films were deposited in a

Pump and probe spectroscopy
Femtosecond pump-probe spectroscopy experiments were performed using a commercial, modelocked, Ti:sapphire laser, with 100 fs pulses at the 810 nm wavelength with a repetition rate of 80 MHz. The pump and probe beams are cross polarized to eliminate coherent artifacts due to the interference of the two beams. The samples are mounted on a cold finger in a temperature controlled liquid helium continuous flow optical cryostat, operating down to 5K. A schematic of the experimental set-up is shown in figure 1. Both pump and probe beams are focused down to 30 µm diameter onto the sample. The pump-probe average power ratios were set at 10:1 and 3:1 for Nb and YBCO, respectively. The fluences were about 100 µJ/cm 2 and 30 µJ/cm 2 , values which minimize the optical heating while assuring good signal-to-noise ratio. The reflectivity signal is measured from the probe beam time-delayed with respect to the pump. The reflectivity change as a function of the time delay between the pump and the probe beams is directly related to the relaxation processes of excited carriers. In particular, the hot electrons thermalize first with other electrons on a time scale shorter than the pump pulse width, through electron-electron relaxation. Then they relax their energy by electron-phonon scattering. Finally, the bolometric lattice cooling process takes place on a time scale of the order of ns. Therefore, measuring the reflectivity change in the ps range is a direct probe of the electron-phonon relaxation processes which tend to restore the equilibrium between the electron and phonon subsystems.
The optical penetration depth of Ni 0.48 Cu 0.52 films was measured to be α = 10.6 nm at 810-nm.

Results and discussion
Pump-probe experiments were performed in a temperature range between 5 K and room temperature. Figure 2 shows the time-resolved optical reflectivity normalized ∆R/R signals collected at 6 K for Ni 0.48 Cu0 .52 /Nb with different thickness of Ni 0.48 Cu 0.52 . The waveforms are representive for all temperature measurements. They were fitted by using a two-exponential fitting formula with two different decay channels characterized by τ fast and τ slow times respectively. The fast relaxation time, subpicosecond in duration, corresponds to electron-Debye-phonon interaction of highly excited electrons. In the superconducting state, it leads to the accumulation of quasiparticles just above the superconducting energy gap. The slow relaxation time of the order of a few picoseconds refers to inelastic electronacoustic-phonon scattering and Cooper pair recombination. Finally, we observed a plateau, which decayed on a time scale of hundreds of picoseconds, related to the bolometric phonon escape. Probably, the presence of a F overlayer at larger thickness reduces the breaking of Cooper pairs in the underlaying Nb layer, and it makes less effective the "phonon bottleneck" effect responsible for the long time decay in reflectivity signals for photoexcited homogeneous superconducting films. The strongly suppressed bolometric plateau in F/S bilayers makes them very attractive for fast optical superconducting detectors.
The low-temperature τ fast varies overall between ~0.18 ps (Nb film) to ~0.32 ps (Ni 0.48 Cu0 .52 (21nm)/Nb bilayer), but for each sample the dependence is rather flat and no special features can be observed. At room-temperatureτ fast is longer and more than twice that of the averaged lowtemperature value. The τ slow time varies from ~2.5 ps (Nb film) to ~5.2 ps (Ni 0.48 Cu0 .52 (12nm)/Nb bilayer) in the whole investigated temperature range. In the τ slow (T) case, we observe a rich structure with peak magnitudes well above our experimental error level, but we were unable to associate those features with any expected temperature dependences such as, e.g., divergence corresponding to the T c of the F/S bilayer.  Figure. 3 shows the reflectivity changes normalized ∆R/R as a function of time delay for a NiCu(20nm)/Au(20nm)/YBCO(100 nm), and it is compared to a pure YBCO (100nm) film photoresponse. The superconducting transition temperature was T c =45 K. The presence of the ferromagnetic layer causes faster relaxation times as evidenced by the fitted curves ( see red line in Figure 3) and summarized in values reported in Table 1.  Moreover the F layer slightly reduces also the bolometric component of the signalaccording to the physical mechanism above discussed. Measurements on new samples with different NiCu thicknesses are still in progress: they will allow to study also the role of the Au film, used as a buffer layer. On the other hand, the presence of the Au overlayer can help the fabrication process of nanosized YBCO based devices, that are yet a challenge.

Conclusions
We performed femtosecond optical pump-probe measurements of proximized F/S structures formed by a ferromagnet (Ni 0.5 Cu 0.5 ) and both LTS and HTS superconductor (Nb and YBCO, respectively). The presence of a thin F overlayer on the top of the S film reduces the slow bolometric component of the photoresponse. Moreover, a reduction of the fast decay time, related to the electron-phonon scattering, is also observed in the case of YBCO based heterostructures. These features are promising toward future development of custom-made, F/S-type, ultrafast superconducting optical detectors.