In this section the discovery of a new poling technique is reported. Differently from Fujiwara’s experiments a high intensity UV femtosecond laser source and un-doped fused silica samples, excited by a two-photon mechanism were used. After irradiation, a strong second-harmonic signal in the areas of the sample that had been subjected to both the electric field and UV light, was observed. To the best of our knowledge, it is the first experimental demonstration of direct UV-poling in pure fused silica.
Samples Preparation
The experimental set-up is shown in Fig.5.1. The fused silica samples used for poling were made from Herasil 1 glass (Heraeus Quarzglas GmbH) and had dimensions 37.2×
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#
Figure 5.2: Schematic of the UV-assisted poling experiment: details of the beam propagation.
11×5 mm2, were deposited on both plane sides of the samples in such a way that the
edges of each electrode were at a distance of about 3 mm from the end of the plate and side edges (see Fig.5.1).
For sample irradiation, we used the UV pulses from a femtosecond Nd:glass laser (Twin- kle, Light Conversion Ltd., Vilnius, Lithuania). It is based on an active-passive mode- locked negative-feedback controlled Nd:glass master oscillator, a stretcher, a Nd:glass regenerative amplifier, and a compressor. It delivers 0.9 ps pulses with energy of as much as 4 mJ at 1055 nm with a repetition rate of 27 Hz. Further compression to 200 fs pulse width is accomplished in the course of second-harmonic generation with a potassium dihydrogen phosphate type II (KDP) crystal with time predelay between pulses with orthogonal polarization. The subsequent generation of third (351 nm), fourth (264 nm), and fifth (211 nm) harmonics of fundamental radiation is carried out in KDP and β- barium borate (BBO) crystals. The energy stability of pulses at 264 nm is∼10% [163]. The laser parameters used for the UV femtosecond poling were the following: the pulse energy, εp, is around 200µJ; the wavelength,λ, is 264 nm; the pulse duration,τFWHM, is 220 fs (τ =τFWHM/√ln2 at 1/e); the beam radius,wFWHM, is 0.15 cm (w=wFWHM/√ln2 at 1/e); and the repetition rate, Rr, is 27 Hz. The UV radiation was focussed into the narrow side of the sample by a cylindrical lens with focal length 21.8 cm. The sample was placed at a distance of 21 cm from the lens (Fig. 5.2).
The peak beam intensity, I0, is calculated assuming that the pulses generated by the laser have a Gaussian temporal and spatial profile. The following representation was chosen for the Gaussian beam:
I(r, t) =I0exp " − r2 w0/ √ 22 # exp − t2 (τ /2)2 (5.1) where w = w0/ √
2 is the beam radius when the intensity has decreased by 1/e and τ the pulse width at 1/e. Eq.( 5.1) has to be integrated over time and space in order to obtain the pulse energy,εp. This leads to the following result:
εp = Z +∞ −∞ dt Z +∞ 0 I(r, t)2πrdr= π √ π 4 I0τ w 2 0 (5.2)
From the above expression the peak beam intensity is calculated to be: I0= π√πεp
4 τ w20
(5.3)
corresponding to 8.7 GW/cm2 on the lens. Due to the focusing the beam cross sectional
area at the sample edge is reduced by approximately 27 times, as calculated using Gaus- sian beam propagation. As a result the peak intensity incident on the sample is about 236 GW/cm2. The incident fluence was calculated from the following equation [28]:
E= εpN πw2 0
(5.4) where N is the total number of pulses incident on the sample. A high voltage block FC30P4 (Glassman High Voltage, Inc.) was used to create an electric field of about 200 kV/cm through the sample. The sample, with the high voltage applied across it, was irradiated with the high-intensity UV laser light for approximately 3 hours (total incident fluence of around 8 kJ/cm2). Using the experimental values for the two-photon
absorption coefficient for Herasil glass reported in [164], we can estimate the light intensity value entering the volume inside the sample under high voltage as 40 GW/cm2
(the pulse energy, entering the poling volume, is about 40µJ). The total irradiation fluence regarding the poling volume was 2 kJ/cm2.
Current Measurements
A digital oscilloscope LT372 (LeCroy) was used to measure the photoelectric current induced in the sample while applying the high voltage and exposing it to the UV- femtosecond pulses. No current was observed in the absence of high-intensity UV light. Immediately, at the start of irradiation, a strong photocurrent pulse with an amplitude of up to 20 V (measured across an 100 kΩ resistor) appeared, which corresponded to a con- ductivity value ofσ = 2×10−9cm−1Ω−1(resistivity valueρ = 5×108cm·Ω, which is ten
-2 0 2 4 6 8 10 -6 -4 -2 0 2 4 Time[ms] V o l t s [ V ] PhotocurrentPulse LaserPulse
Figure 5.3: Laser pulse and simultaneous current pulse
orders less than the standard reference value for room temperatureρ = 1018cm·Ω[165]).
The calculated initial quantum yield of charged particles between the electrodes was about 2×10−6, if we presume that the time resolution of our oscilloscope was 50 ns.
During the irradiation, the pulse amplitude value degraded rather quickly (during some seconds) to a level of some volts (Fig.5.3). A horizontal movement of the sample in respect to the laser beam led to a revival of the photocurrent signal. After switching off the applied voltage, the photocurrent pulse changed its polarity and continued to degrade. The observed photocurrent behavior is qualitatively consistent with the one induced in earlier experiments on two-step ionization S0 → S1 → SN of oxygen defi- cient fused silica by KrF excimer laser radiation with λ = 248 nm and t = 25ns[166]. However, a side effect, which is difficult to isolate in this type of experiments, is the photoelectric emission of electrons from the electrodes, when illuminated by high energy UV photons. In order to estimate the importance of the contribution of photoelectrons to the measured current, future experiments will be performed using various metal elec- trodes, having different work-functions. Another possibility is to shine the UV laser beam alternatively on the anode electrode, inside the glass and on the cathode electrode and see how the measured current is affected.
Measurements of the Second-Order Nonlinearity
After the poling and removal of the electrodes (by etching), the sample was tested for evidence of second-harmonic generation. As a pump source, a mode-locked and Q- switched Nd:YAG laser was used (λ= 1064 nm). For the Maker’s fringe measurement the sample was placed on a rotation stage and the laser beam was focused on the poled area, with polarization parallel to the plane of incidence. The generated SH light was detected with a photomultiplier. The fit to the experimental data reveals that the region with an induced χ(2) corresponds to the whole bulk volume between the two electrodes
0 20 40 60 80 100 120 140 160 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 Incidentangle(deg) S H s i g n a l ( a . u . )
Figure 5.4: Maker’s fringe measurement on the UV poled sample
(Fig.5.4).
The comparison of the SHG signal value with that obtained from a reference sample (crystallineα-quartz) with a knownχ(2) value of 0.6 pm/V, allowed us to estimate the
maximum value of the induced second-order nonlinearity as about 0.02 pm/V. A recti- fication model,χ(2) = 3χ(3)E
dc withχ(3), being the third-order nonlinear susceptibility,
equal to 2×10−22m2V−2 for silica and the applied electric fieldE
dc equal to 200 kV/cm, gives a predictedχ(2)of 0.01 pm/V in remarkable agreement with the measured value. If the same experiment is performed in twin-hole fibres, where the maximum applied field is only limited by the dielectric strength of silica at room temperature (2×109V/m [75])
we can tentatively predict χ(2) exceeding 1 pm/V. The pump beam was also scanned across the surface of the sample to test the distribution of the second-order nonlinearity (Fig.5.5). The geometry was the same as for the Maker’s fringe measurements described above. The angle of incidence was chosen to be close to normal incidence (10 degrees) to reduce the effects of non-uniformities. A stronger SH signal is observed near the side where the laser beam entered the sample which could be explained by the decrease of UV intensity caused by two-photon absorption in the glass. During the inscription procedure we poled several regions under the electrodes by translating the sample. It is clearly seen that the second-harmonic signal is observed only in the area under the elec- trodes illuminated by the UV light beam, indicating that the poling takes place under the simultaneous action of both UV light and the applied electric field.