Excitation of the polarization sensitive photocurrent

Figure 4.1 shows a generic scheme of experiment for the generation and registration polarization-sensitive photocurrent. In this work the current is generated under irradiation with Nd³⁺:Yag-laser operated at a wavelength of λ = 1064 nm, optical parametric oscillator (OPO) tunable in the wavelength range of 1350 - 4000 nm or femtosecond titanium-sapphire laser operating at the wavelength of 795 nm. The polarizer was used to obtain a linearly polarized beam.

Figure 4.1. The generic setup of the photocurrent measurement.

The frequency converters of the Nd:YAG laser pulses were used to obtain radiation at the wavelengths of 532 nm (second harmonic), 355 nm(third harmonic) and 266 nm (fourth harmonic). Since the frequency conversion requires control of the orientation of the polarization plane azimuth of the fundamental beam, a half-wave plate operating at λ = 1064 nm was used. To filter radiation of the second harmonic (λ = 532 nm) the light filter SZS-21 was used (Figure 4.2) to absorb the fundamental beam. To pick out UV radiation of the third and second harmonics of the Nd:YAG laser, a quartz prism combined with a diaphragm was used (Figure 4.3). The beam incident on the prism was s-polarized.

67 Figure 4.2. The scheme of obtaining the second harmonic of the Nd³⁺:YAG laser.

Figure 4.3. The scheme of obtaining the third (fourth) harmonic of the Nd³⁺:YAG laser.

The beam of the desired wavelength was transmitted through a semitransparent beam splitter, which reflected a small part of the radiation to the reference photodetector (see Figure 4.1) to control the intensity. As a result it is possible to significantly reduce the effect of fluctuations of the pulse energy on the experimental results. Since the laser pulses energy fluctuated in a wide range, the measured photocurrent should be normalized on the actual value of the excitation pulse energy. In experiment, the energy of the laser pulse 𝑊_𝑖𝑛 was obtained using the following equation:

𝑊𝑖𝑛=𝑊𝑛𝑜𝑟𝑚 𝜀_𝑖𝑛

𝜀𝑛𝑜𝑟𝑚, (4.1)

where εⁱⁿ and ε^norm are readings of the photodetector during film irradiation and photodetector for reference pulse having energy W^norm, respectively.

Energy W^norm was measured using Thorlabs ES 111C energy meter. The Thorlabs ES 111C energy meter is a pyroelectric sensor that directly converts light energy into a voltage pulse regardless of wavelength. The energy meter has an aperture of 11 mm, which was much larger than the diameter of the laser beam. The active area of the sensor is covered with a special absorbing layer with approximately the same absorption coefficient over a broad wavelength range spanning from 185 nm to 25 μm. The range of measured energy is from 10 μJ to

68

150 mJ. The measurement error is 100 nJ. The maximum pulse repetition rate is 40 Hz. The maximum measured power density is 8 MW/cm².

In order to maintain the linear polarization, we employed another linear polarizer (see Figure 4.1).The polarization of the excitation beam was determined by half-wave or quarter-wave plates depending of the experiment. These phase plates were made of crystalline quartz. The laser beam incidents normally on the plate surface, i.e. by rotating the plate around the beam one can change the polarization plane azimuth or the degree of circular polarization (depending on the type of plate). Thus by using half- and quarter-wave plates we can control polarization of the excitation beam.

In experiments the Thorlabs half-wave or quarter-wave plates were used.

However in practice the fast axis in some of the commercial products may have wrong marks. To avoid using “wrong” quarter-wave plates the checking technique reported in Ref. [107] was used (see Figure 4.4). It is known that at oblique incidence of beam on metal the reflected beam acquires phase shift between s- and p-polarization (see, for example [108]). For example the p-polarized laser beam at λ

= 800 nm reflected from the gold mirror at angle α = 80° will get phase shift of +π/2 but s-polarized beam will get phase shift of -π. In other words the phase shift between p- and s-polarized beams will be equal 3π/2. If incident laser beam has linear polarization with azimuth of Φ = 45°, the reflected beam will have circular polarization. When the reflected beam passes through quarter-wave plate, it becames linear polarized with polarization azimuth depending on position of fast axis of the wave plate. In our experimental conditions if axis coincides with plane of reflection from the mirror, the beam phase shift will be fully compensated to have the initial polarization azimuth, which was observed in our experiments. It is worth noting that if the plate at same position has wrong mark of optical axis, the polarization azimuth will be rotated on 90°. Thus we can conclude that marks on wave plates in our photoresponse experiments are credible.

Figure 4.4. Scheme of experiments for checking quarter wave plate.

69 The Ag/Pd film was placed on a special goniometric holder, which permits rotating the sample around the normal to its surface and changing the angle of radiation incidence on the film. In experiments the maximum diameter of laser beam was 4 mm and the electrodes were not irradiated. It should be noted that in all experiments, the dependence of the photocurrent on the polarization state of the excitation beam was measured at the angle of incidence of α = 45°.

The longitudinal Jx and transverse Jy photocurrents were measured by placing pair of electrodes onto the film surface perpendicular (see Figure 4.5a) and parallel (see Figure 4.5b) to the plane of incidence, respectively. To register the photocurrent, the electrodes were connected to an oscilloscope.

In the experiments, we used the digital oscilloscopes Tektronix TDS7704B and LeCroy 42Xs, which enable studying the temporal profile and measuring the amplitude of the photocurrent. The Tektronix TDS7704B oscilloscope has 4 input channels and a bandwidth from 0 to 7 GHz. The maximum sampling rate is 2×10¹⁰ unit/s. Input impedance is Rin = 50 ohms. The limit of permissible absolute error of voltage measurement is ±(2×10²×U+0.08div×A^d), where U is the voltage in volts, A^d is the deviation coefficient, which can vary in the range from 2 mV/div to 1 V/div. The limit of permissible absolute error of measuring time intervals is

±(0.3/νd+2.5×10^-6×t), where νd is the sampling frequency, t is the time interval in seconds. The minimum level of the external synchronization signal is 100 mV. The Tektronix TDS7704B oscilloscope is metrologically provided and approved for use as a measuring instrument.

Figure 4.5. Sketch of the measurement setup for registration of the a) longitudinal and b) transverse photocurrents in Ag/Pd films. The electrodes are orientated a) perpendicularly and b) parallel to the plane of the incidence, which coinsides with the XZ plane of the laboratory Cartesian frame. E is electric field vector of initial beam, which is parallel to plane of incidence, k is wave vector, ne is slow axis of wave plate, N is a unit vector of normal to film surface, α is angle of incidence, x' is axis in plane of incidence. The polarization of the exciting radiation was determined by the angle φ of the orientation of the quarter-wave plate (ϕ in the case of a half-wave plate). The figure is adapted from [109].

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The LeCroy 42Xs oscilloscope has 2 input channels and a bandwidth from 0 to 400 MHz. The maximum sampling rate is 2.5×10⁹ div/s. Input impedance is Rin = 50 ohms. The limit of permissible absolute error of voltage measurement is

±(1.5×10²×U+0.04div×A^d). The deviation coefficient can vary from 2 mV/div to 1 V/div. The limit of permissible absolute error of measuring time intervals is

±(10^-5×t). The LeCroy 42Xs oscilloscope is metrologically provided and approved for measurement.

During the experiments, the temporal profile and amplitude of the photocurrent pulses was recorded and extreme value (amplitude) were measured. Measurements include the pulse duration τfw, which corresponds to the full pulse width at the half maximum as well as the rise time τrise, and the decay time τdec of the pulse. The τrise

and τ^dec were determined by the levels of 0.1 and 0.9 of the pulse maximum.

The photocurrent pulses were measured by averaging of at least thirty pulses.

After each experiment, a Thorlabs ES 111C energy meter was placed in place of the film and energy of the incidence pulse was measured by averaging energies of at least thirty laser pulses.

To register shape of nanosecond exciting pulses Thorlabs SIR5-FS photosensor was used. Thorlabs SIR5-FS high-speed photosensor is designed to convert an optical signal into an electric one. The photosensor is based on InGaAs structures, the bandwidth is 5 GHz. Output impedance 50 ohms. The maximum safe output voltage can reach 1 V. The minimum rise time of the pulse generated by the photosensor is 70 ps. The dark current is 1.5 nA.