Femtosecond Circular Photon Drag Effect in the Ag/Pd Nanocomposite
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We report on the observation of the helicity-dependent photoresponse of the 20-μm-thick silver–palladium (Ag/Pd) nanocomposite films. In the experiment, 120 fs pulses of Ti:S laser induced in the film an electric current perpendicular to the incidence plane. The photoinduced current is a linear function of the incident beam power, and its sign depends on the beam polarization and angle of incidence. In particular, the current is zero for the p- and s-polarized beams, while its sign is opposite for the right- and left-circularly polarized beams. By comparing experimental results with theoretical analysis, we show that the photoresponse of the Ag/Pd nanocomposite originates from the photon drag effect.
KeywordsPhoton drag effect Circular polarization Circular photocurrent
Circular photon drag effect
Circular photogalvanic effect
Growing interest to the engineering of the surface or bulk spin-polarized photoinduced currents  has attracted attention to the circular photogalvanic (CPGE)  and circular photon drag (CPDE) effects . These phenomena, which manifest themselves as conversion of the photon angular momenta to momentum of charge carrier, were extensively studied in two-dimensional (2D) [1, 4, 5, 6, 7, 8, 9] and planar [10, 11, 12, 13, 14] materials during the last decade.
CPGE can be observed in gyrotropic media lacking inversion centre and mirror symmetry and originates from the imbalanced distribution in the momentum space of the carriers excited when an elliptically polarized beam hits the sample surface . CPGE provided information on the spin–orbit coupling and has been observed in crystalline bismuth silicate (Bi12SiO20) , lead germanate (Pb5Ge3O11) , lithium niobate , indium nitride (InN) films [10, 11], quantum wells [4, 7, 8, 18, 19, 20] and 2D heterostructures [9, 21].
The photon drag effect [22, 23, 24] originates from the transferring the momentum from photon to the charge carriers and manifests itself as light-induced dc current. In contrast to the photogalvanic effect, it is permitted in both noncentrosymmetric and centrosymmetric media . The CPDE manifests itself as a helicity-dependent current propagating perpendicular to the plane of incidence. It was observed at oblique incidence in quantum wells , graphene [6, 26], InSb  and bulk tellurium . At the nanosecond excitation, the CPDE has been recently observed in nanoporous gold thin film , 2D metallic photonic crystal slabs  and Ag/Pd nanocomposite [14, 29, 30]. However, to the best of our knowledge kinetics of the helicity-dependent photoinduced surface currents injected by a single femtosecond laser pulse in metallic nanocomposite has not been studied yet.
In this paper, we report on the excitation of the helicity-dependent photoinduced voltage (PIV) in a 20-μm-thick Ag/Pd nanocomposite film under irradiation with the femtosecond laser pulses at an oblique incidence. We reveal that the relaxation time of the photocurrent generated at the film surface is as long as several nanoseconds, i.e. the photoresponse of the film lasts much longer than the duration of the incident femtosecond pulse. We demonstrate that the polarity and magnitude of PIV can be controlled by the ellipticity of the laser beam as well as by the angle of incidence. By studying the dependence of the PIV on the polarization and the angle of incidence, we demonstrate that it originates from the CPDE.
In our experiments, we employ Ti:S laser operating at a wavelength of λ = 795 nm with a pulse repetition rate of 1 kHz. The duration and energy of the laser pulses are 120 fs and 2 mJ, respectively. The sketch of the experimental setup is shown in Fig. 2. The p-polarized laser beam with diameter of 4.5 mm passes through an achromatic quarter-wave plate and obliquely incidents onto the film surface. The polarization state of the beam that hits the surface of the Ag/Pd nanocomposite is determined by the angle φ between slow axis of the wave plate (n e) and the polarization azimuth of the incident beam (x′). In particular, the beam is left- and right-circularly polarized after quarter-wave plate at φ = 45° and −45°, respectively. The plane of incidence (xz) is parallel to the electrodes A and B, which are not irradiated by the laser beam.
The measurements of the PIV was performed by digital oscilloscope with a bandwidth of 200 MHz. The magnitude of the PIV measured at the exposure for 0.2, 1, 2 and 5 s was the same indicating that the heating of the film by the train of the femtosecond pulses with repetition rate of 1 KHz does not influence the photoinduced current. The chosen data acquisition time of as short as 200 ms provides enough data to perform statistical averaging of the signal and allows us to avoid overheating of the nanocomposite. It is worth adding that the PIV measured between electrodes A and B (see Fig. 2) does not depend on the position of the laser beam on the film surface (providing that electrodes are not irradiated, see also ). The large (25 × 25 mm2) surface area of the sample allowed us to carry out measurements in incidence angles range ±75° at a laser beam diameter of 4.5 mm.
where U 1 = 3.51 mV and U 2 = 0.49 mV represent magnitudes of the helicity-sensitive and helicity-insensitive contributions, respectively. It is worth noting that the ellipticity-insensitive part of the photoinduced signal changes the polarity at φ = 45°.
Under irradiation with nanosecond laser pulse, the temporal profile of the PIV arising due to the photon drag effect reproduces that of the excitation pulse because of the subpicosecond carrier momentum relaxation time (see e.g. [35, 37]). One can observe from Fig. 3 that in Ag/Pd nanocomposite, the 120-fs-long laser pulse generates PIV pulse with a sharp (subpicosecond) front and long (22.6 ns) tail. In the nanosecond experiment , the rise time of the PIV signal (3.2 ns) is determined by the excitation pulse, while in the femtosecond experiment, it was restricted by the oscilloscope bandwidth. This indicates that the response time of the nanocomposite lays in the picosecond time scale, i.e. corresponds to the carriers momentum relaxation time. It is worth noting that nanosecond decay time of the PIV signal at the femto- and nanosecond excitations indicates that relaxation time of the photoexcited carriers in Ag/Pd nanocomposite lays in the nanosecond range. Such a long decay time of the photogenerated current may originate from a slow relaxation of charge carriers in the electric field of the Schottky barriers at the interfaces between metallic Ag–Pd and semiconductor PdO crystallites.
Since the film is composed of centrosymmetric Ag–Pd and PdO nanocrystallites, the measured signal cannot be originated from the CPGE . It is worth noting that in the Ag–Pd nanocomposite, the PIV has not shown resonance features in the broad spectral range spanning from 266 to 2100 nm [14, 29]. This indicates that in our experiment, the measured PIV is not originated from plasmon polaritons, which gives rise to the pronounced wavelength dependence of the longitudinal and transversal PIV near plasmon resonance in one- and two-dimensional plasmonic structures [28, 36, 38]. That is in the Ag–Pd nanocomposite, the helicity-dependent photoinduced current directed perpendicular to the incidence plane originates from the CPDE.
where subscripts k and l label Cartesian coordinates (x, y) on the interface, E and H are complex amplitudes of electric and magnetic field in the light wave at frequency ω in the medium, ξ 1, ξ 2 and ξ 3 are the complex transport coefficients depending on the excitation wavelength. Equation (2) suggests that the light penetration depth does not exceed the electron mean free path of the medium. In our experimental conditions, the nanocomposite film consists of the Ag–Pd, PdO and Ag2O with a mass ratio of 80.3:18.7:1.0, respectively (see Fig. 1b), with 59% of total the Ag content. In silver, the electron mean free pass l σ = 57 nm , while the light penetration depth d = 12 nm . Therefore, we believe that in our experiment, condition l σ > d is satisfied. It is worth mentioning here that if the electron mean free path is much smaller than the electric field penetration depth, the surface current density should be obtained by integrating the bulk photon drag current over the light penetration depth. In an isotropic medium, the bulk photon drag current lays in the plane of the incidence  and it cannot contribute to the PIV measured in our experiment.
One can observe from Eqs. (3) and (4) that photon drag current generated in the plane of incidence (j x) does not depend on the helicity of the incident beam , while that generated in the perpendicular plane (j y ) does [13, 14, 29, 30].
n is the complex refractive index of the film. Equation (5) implies that the sign of the PIV is opposite for left-hand (φ = +45°) and right-hand (φ = −45°) circularly polarized excitation beams. It is also worth noting that the helicity-sensitive signal vanishes in purely dielectric medium with real refractive index. In Figs. 4 and 6, solid lines show fitting of the experimental data with Eq. (5) at n = 1.1 + i 1.59 and μ = 1.06.
The incident angle and helicity dependencies of the PIV induced by the femtosecond laser pulses shown in Figs. 4 and 6 resemble those measured at nanosecond excitation [14, 30]. We believe that the Eq. (5) can be also employed to elucidate results obtained in . It is worth noting that the high porosity of the Ag/Pd nanocomposite (see Fig. 1a) prevents direct ellipsometric measurement of the complex refractive index. However, one can observe from Figs. 4 and 6 that Eq. (5) fits well experimental data at complex refractive index n = 1.1 + i 1.59, which corresponds to the light penetration length of 40 nm. Although this value is higher than that for pure silver (13 nm), the light penetration depth in the composite is smaller than the electron mean free pass in silver (57 nm). This indicates that the condition d < l σ holds, i.e. the analysis based of Eq. (5) is correct.
It is necessary to mention that in  and , the origin of the observed helicity-dependent photoinduced currents in centrosymmetric strain-free InSb crystal and a porous gold film, respectively, has not been explained. In contrast, our theoretical and experimental results suggests that CPDE does explain the light-induced surface current in highly conductive porous films.
One can observe from Figs. 4, 5 and 6 that irradiation of the nanocomposite with femtosecond pulses results in the photoresponse with amplitude as high as several mV. By comparing this experimental finding with results obtained for the nanographite  and single-walled carbon nanotubes films  one may conclude that the magnitude of PIV generated in the Ag/Pd nanocomposite can be increased by suppressing short-circuit currents. This can be done by decreasing the area and thickness of the film and/or reducing the distance between the electrodes. Furthermore, PIV can be obviously increased by amplifying the signal , thus allowing one to observe photoresponse at much lower pulse energy. The conversion efficiency can be increased even further by accumulating and averaging the signal opening a way towards application of Ag/Pd nanocomposite in the beam helicity sensors.
We demonstrate for the first time that the helicity-sensitive transverse photocurrents in Ag/Pd nanocomposite can be generated by a femtosecond light pulse. By comparing results of nanosecond and femtosecond experiments we show that the shorter the laser pulse, the faster the rise time of the helicity-dependent PIV signal, while the fall time of the PIV pulses remains the same. This experimental finding, which evidences a long relaxation time of the photoexcited carriers in the Ag/Pd nanocomposite, allows us to revisit and clarify the results of the nanosecond experiment. The similarity in the angular and polarization dependence of the photoinduced currents clearly show that results of both nanosecond and femtosecond experiments can be explained by the photon drag effect in the metal-semiconductor nanocomposite. We developed a phenomenological theory, which describes results of measurements at the nano- and femtosecond excitation. This theory can also be employed for interpretation of the experiments on the helicity-dependent PIV in centrosymmetric conductive films. The opportunity to tune optoelectronic properties and pronounced polarization dependence of the PIV make Ag/Pd nanocomposite an interesting material for fabrication of helicity-sensitive photon drag photodetectors.
This work was supported by the RFBR (Grant Nos. 16-38-00552 and 16-02-00684), the Academy of Finland (Grant Nos. 288547 and 298298) and FP7 Marie Curie NANOCOM project (grant #269140).
GMM proposed the idea of the experiments, carried them out and drafted the manuscript. ASS characterized the films studied and processed the experimental data. VVV participated in the experiments on femtosecond laser. KGM measured and interpreted the films’ Raman spectra. YPS developed the theory of circular photon drag effect for nanocomposite films and participated in the preparation of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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