- Email: [email protected]
Fringe resolved autocorrelator for characterization of ultrashort laser pulses using second harmonics of ZnO nanorods
Optics Communications 402 (2017) 398–400
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Fringe resolved autocorrelator for characterization of ultrashort laser pulses using second harmonics of ZnO nanorods Rudrashish Panda, Susanta Kumar Das * Department of Physics, School of Applied Sciences, KIIT University, Bhubaneswar, Odisha, 751024, India
a r t i c l e
i n f o
Keywords: Autocorrelation ZnO nanorod Ultrashort laser Second harmonics
a b s t r a c t Use of second harmonics (SH) of Zinc oxide (ZnO) nanorods for ultrashort pulse characterization is reported here. The used ZnO nanorods are grown by chemical bath deposition method. The pulse characterization is done by the autocorrelator technique. From this, the pulse duration is estimated to be 14 femtosecond (fs). The spectral bandwidth of the pulse under probe is ∼85 nm with central wavelength at 820 nm. Theoretically, Fourier transformation limited pulse width corresponding to this spectral bandwidth is 12 fs. So experimentally measured pulse width closely matches with the theoretical prediction. The reported characterization system is cost effective and can be used for characterization of fs laser of broad wavelength range. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Ultrashort lasers are useful for monitoring various physical and chemical processes that occur in the time scale of picosecond to femtosecond. They are also useful for nonlinear optical microscopy, materials processing, frequency meteorology, THz generation, detection, imaging etc. [1–4]. For reproducibility in these applications; continuous, real time and accurate diagnostic or characterization of ultrashort pulses is highly essential. Various systems like autocorrelator, frequency resolved optical gating (FROG), Spectral phase interferometry for direct electric field reconstruction (SPIDER) etc. are generally used for this ultrashort pulse characterization. Second harmonic generation (SHG) is the most commonly used phenomena for these characterization systems. SHG in these systems is generally done by expensive nonlinear optical crystals. Reports indicate that ZnO nanorods can show very high second order optical nonlinearity and SHG behavior [5–8]. They also have other advantages like high chemical stability, high damage threshold and low cost. So, in this communication we report on utilization of second harmonics of ZnO nanorods for the ultrashort pulse characterization. Advantages of the developed pulse characterization system are discussed. 2. Experiment The field emission scanning electron microscope (FESEM) image of the ZnO nanorods used for this work is shown in Fig. 1. The procedure * Corresponding author.
E-mail address: [email protected] (S.K. Das). http://dx.doi.org/10.1016/j.optcom.2017.06.028 Received 27 April 2017; Received in revised form 30 May 2017; Accepted 6 June 2017 0030-4018/© 2017 Elsevier B.V. All rights reserved.
for the growth of these materials is reported previously [5]. Briefly, low-temperature seed assisted chemical bath deposition (CBD) method was used for the growth of these ZnO nanorods. An aqueous solution of Zinc nitrate hydrate (ZNH) and hexamethylenetetramine (HMT) in the molar ratio of 30 mM in distilled water was used for this purpose. A 10 nm thick seed structure was prepared on the glass substrate by sputtering technique which assisted the aligned growth of the nanorods. The growth temperature was maintained at 95 ◦ C. The average diameter and length of the nanorods were 56.30 nm and 1300 nm respectively. A Ti:sapphire laser oscillator (Femtosource) emitting linearly polarized fs pulses at a repetition rate of 75.3 MHz with 4 nJ pulse energy and a central wavelength near 820 nm has been used as the test pulse for its characterization. Fig. 2. shows the schematics of the experimental setup which is used for this purpose. Basically it is based on a SHG fringe resolved autocorrelator (FRAC) [9,10] with ZnO nanorods as second harmonic generating element. We have chosen this form of autocorrelator system because of its advantages like easier to align and its ability to give phase information and higher sensitivity etc. over the intensity autocorrelator. It contains a Michelson interferometer with one mirror driven by a piezoelectric motion controller in a step size of 35 nm. The delayed and re-combined pulses are made to incident on the samples by a 25 mm focal length concave mirror in order to generate the SH. The signal was separated from residual pump radiation by a BG39 filter and the same is recorded by a CCD camera. The motion of the interferometer mirror
R. Panda, S.K. Das
Optics Communications 402 (2017) 398–400
Fig. 3. SHG autocorrelation trace of the probe: 15 fs pulse.
Fig. 1. FESEM image of 𝐶-axis oriented ZnO nanorods grown on glass substrate.
Fig. 4. The spectrum of fs pulse under test.
Fig. 2. Schematics of the experimental setup used for pulse characterization of fs pulses using SHG of ZnO nanorods.
the constructive interferences of the pulses coming from two arms of the Michelson interferometer. If ‘∧’ represents the full width and half maximum (FWHM) of this envelope then for Gaussian pulse it relates to the pulse duration (𝑡𝑝 ) by [9].
and the second harmonic signal capture were synchronized by using an in-house developed Win32-GUI program to get the FRAC trace.
∧ (5) 1.72 From Fig. 3, ∧ is estimated to be 24 fs. Putting this value in Eq. (5) the time period 𝑡𝑝 is estimated to be 14 fs. The spectrum of the fs pulse under test is shown in the Fig. 4. From this, the spectral bandwidth in wavelength domain (Δ𝜆) of this pulse is found to be 85 nm. The corresponding spectral bandwidth in frequency (Δ𝛾 ) is estimated to be 3.77×1013 Hz. Theoretically, the duration (Δ𝑡) of a Fourier transformation limited pulse of Gaussian shape is given by [1]
𝑡𝑝 = 3. Results and discussion The SHG FRAC signal w.r.t delay (𝜏) can be given by the equation [9] 𝐼𝑡 =
| |2 |{𝜀 (𝑡) exp 𝑖 (𝜔𝑡 + 𝜙) + 𝜀 (𝑡 − 𝜏) exp 𝑖 [𝜔 (𝑡 − 𝜏) + 𝜙 (𝑡 − 𝜏)]}2 | 𝑑𝑡. (1) | ∫ |
From this equation it can be seen that at zero delay (𝜏 = 0), the FRAC signal is a peak and is represented as 𝐼𝑝 = 𝐼𝑡 (0) = 24
∫
𝜀4 (𝑡) 𝑑𝑡.
Δ𝑡 =
(2)
∫
𝜀4 (𝑡) 𝑑𝑡.
(3)
From Eqs. (2) and (3) we get, 𝐼𝑝 𝐼𝑏
=
8 1
(6)
Putting the aforementioned value of Δ𝛾 in Eq. (6), the duration of the pulse is estimated to be 12 fs. This value is found to be matched closely with the experimental value of the pulse duration. So, the pulse width measurement with the ZnO nanorods is found to be quite reliable. An independent measurement of the pulse width is done with a commercial autocorrelator (APE mini autocorrelator, model: Mini PD) & the pulse duration is found to be 13.5 fs thereby supporting our experiment resulting in ZnO nanorod based autocorrelator. It is to note here that some investigations have already been done to use ZnO thin films for development of SHG autocorrelators by other researchers [11,12]. In these investigations, molecular beam epitaxy (MBE) and metal–organic vapor phase epitaxy (MOVPE) technologies were used to grow the ZnO thin films. As these technologies are very expensive, the autocorrelator based on these films are not cost effective. On the other hand in our work the ZnO nanorods are grown by using CBD method. This method has various advantages like simplicity in fabrication, environment friendly reactions, low temperature growth,
From Eq. (1) it can also be found that the background FRAC signal (𝐼𝑏 ) i.e. the SHG signal measured by the detector for non-overlapping condition is 𝐼𝑏 = 2
0.441 Δ𝛾
(4)
i.e. theoretically peak to background ratio of FRAC signal should be 8:1. Experimentally observed FRAC signal is shown Fig. 3. From this figure, peak to background ratio is found to be almost same as that of the theoretical background, indicating the good capability of ZnO nanorods for FRAC application. Here it should be mentioned that by joining the maxima points of the FRAC signal, one can get the upper envelope. This envelope represents 399
R. Panda, S.K. Das
Optics Communications 402 (2017) 398–400
large-scale production and cost effectiveness [13]. Hence the reported autocorrelator is much cheaper. Beta Barium Borate (BBO) is one of the most commonly used NLO crystal for realization of commercial FRAC system because of its high second order nonlinear optical coefficient (𝑑ef f ). The 𝑑ef f value for BBO is 2 pm/V. In contrast to this, the nonlinearity of the ZnO nanorods are reported to be much higher [5–7]. Particularly the 𝑑ef f value for the type of ZnO nanorods used in this FRAC development is 15 pm/V [5] i.e. it is 7.5 times higher than that of BBO. Localization of field within the nanorods is predicted to be the cause for nonlinearity [12]. Nevertheless because of high nonlinearity, ZnO nanorod films of smaller thickness can generate enough signal for the FRAC application. We estimated that, the thickness of our sample (1.3 μm) is smaller than the coherence length (𝑙𝑐 ) for SHG conversion of fs pulses in broad wavelength range (750– 2200 nm). Here 𝑙𝑐 is given by 𝑙𝑐 =
𝜆 ( ) 4𝜋 𝑛2𝜔 − 𝑛𝜔
are grown by CBD method. The pulse characterization was done by the interferometric autocorrelator technique. From this, the pulse duration is estimated to be 14 fs. The spectral bandwidth of the pulse under probe is ∼85 nm with central wavelength at 820 nm. Theoretically, transformation limited pulse width corresponding to this spectral band width is 12 fs. So, experimentally measured pulse width closely matches with theoretically predicted pulse width. As the process of SHG in ZnO is not wavelength specific, the autocorrelator based on this can be used for fs pulses of broad wavelength range. Also because of low cost of CBD grown ZnO nanorods, the reported SHG FRAC is cost effective. Acknowledgment SKD thanks Science & Engineering Research Board (SERB), Govt. of India (Project File Number: EMR/2015/001175) for financial support.
(7)
References [1] A.M. Weinner, Ultrafast Optics, in: Willey Series in Pure and Applied Optics, 2009. [2] P.F. Curley, A.I. Ferguson, J.G. White, W.B. Amos, Application of a femtosecond self-sustaining mode-locked Ti:sapphire laser to the field of laser scanning confocal microscopy, Opt. Quantum Electron. 24 (1992) 851–859. [3] A.M. Streltsov, N.F. Borrelli, Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses, Opt. Lett. 26 (2001) 42–43. [4] A.G. Okhrimchuk, A.V. Shestakov, I. Khrushchev, J. Mitchell, Depressed cladding, buried waveguide laser formed in a YAG:Nd3+ crystal by femtosecond laser writing, Opt. Lett. 30 (2005) 2248–2250. [5] S.K. Das, M. Bock, C. O’Neill, R. Grunwald, K.M. Lee, H.W. Lee, S. Lee, F. Rotermund, Efficient second harmonic generation in ZnO nanorod arrays with broadband ultrashort pulses, Appl. Phys. Lett. 93 (2008) 181112. [6] M.C. Larciprete, M. Centini, Second harmonic generation from ZnO films and nanostructures, Appl. Phys. Rev. 2 (2015) 031302. [7] S.W. Chan, R. Barille, J.M. Nunzi1, K.H. Tam, Y.H. Leung, W.K. Chan, A.B. Djurisic, Second harmonic generation in zinc oxide nanorods, Appl. Phys. B 84 (2006) 351– 355. [8] R. Panda, S. Bhattacharya, R. Samal, A. Singh, P.K. Sahoo, P.K. Datta, S.K. Das, Second harmonic generation of femtosecond pulses using ZnO nanorods grown by chemical bath deposition with drop casted seed layer, J. Nonlinear Opt. Phys. Mater. 25 (2016) 1650029. [9] J.-C.M. Diels, J.J. Fontaine, I.C. McMichael, F. Simoni, Control and measurement of ultrashort pulse shapes (in amplitude and phase) with femtosecond accuracy, Appl. Opt. 24 (1985) 1270. [10] K. Yamane, T. Kito, R. Morita, M. Yamashita, Experimental and theoretical demonstration of validity and limitations in fringe-resolved autocorrelation measurements for pulses of few optical cycles, Opt. Express 12 (2004) 2762–2773. [11] Y. Kobayashi, D. Yoshitomi, K. Iwata, H. Takada, K. Torizuka, Ultrashort pulse characterization by ultra-thin ZnO, GaN, and AlN crystals, Opt. Express 15 (2007) 9748–9754. [12] M. Mascheck, S. Schmidt, M. Silies, T. Yatsui, K. Kitamura, M. Ohtsu, D. Leipold, E. Runge, C. Lienau, Observing the localization of light in space and time by ultrafast second-harmonic microscopy, Nat. Photonics 6 (2012) 293–298. [13] D. Byrne, E. McGlynn, M.O. Henry, K. Kumar, G. Hughes, A novel, substrate independent three-step process for the growth of uniform ZnO nanorod arrays, Thin Solid films 518 (2010) 4489–4492.
with 𝑛𝜔 and 𝑛2𝜔 are the refractive indices for fundamental and second harmonic wavelength respectively (Taking the transparency region of ZnO and sensitivity of our Si detector into account, we restricted our estimation of 𝑙𝑐 for fundamental wavelength within the range of 750–2200 nm). This indicates that ZnO nanorods can generate SHG efficiently for any fs pulses within the aforementioned wavelength range. The efficient SHG in BBO in the other hand takes place through birefringence phase matching process. This process is wavelength specific. Therefore a BBO crystal designed for a specific wavelength cannot generate efficient SHG of the aforementioned broad wavelength. So, unlike ZnO nanorods FRAC, the BBO based FRAC will be more wavelength specific. Further it is to note that, because of low cost fabrication process, the ZnO nanorods are quite cost effective. We estimated that for growth of ZnO nanorods on slide of size (2 × 3) cm2 it approximately costs only $10.00. Nonlinear crystals are on the other hand are quite expensive because of high cost of fabrication and cutting in desired angle for phase matching. A commercial BBO of area (1 × 1) cm2 for example can cost $600–$1000. So, the ZnO nanorods are also of better choice for FRAC application from cost point of view. Lastly, it is worth to mention here that the phase matching process for SHG in NLO crystal has the bandwidth limit. This phase matching bandwidth limit is inversely proportional to crystal thickness. So, to have the SHG of fs pulse of broad spectral bandwidth, one needs NLO crystal of few 10 s of μm thickness. Preparation of such thin crystal is generally very difficult for which they are much more expensive than the thicker crystals. From this point of view the inexpensive and efficient nanostructured nonlinear optical thin film containing ZnO nanorods are of much better choice for development of SHG FRAC for characterization of broadband ultrashort pulses. 4. Conclusion Characterization of broadband ultrashort laser pulses using second harmonics of ZnO nanorods is reported here. The used ZnO nanorods
400
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Fringe resolved autocorrelator for characterization of ultrashort laser pulses using second harmonics of ZnO nanorods Rudrashish Panda, Susanta Kumar Das * Department of Physics, School of Applied Sciences, KIIT University, Bhubaneswar, Odisha, 751024, India
a r t i c l e
i n f o
Keywords: Autocorrelation ZnO nanorod Ultrashort laser Second harmonics
a b s t r a c t Use of second harmonics (SH) of Zinc oxide (ZnO) nanorods for ultrashort pulse characterization is reported here. The used ZnO nanorods are grown by chemical bath deposition method. The pulse characterization is done by the autocorrelator technique. From this, the pulse duration is estimated to be 14 femtosecond (fs). The spectral bandwidth of the pulse under probe is ∼85 nm with central wavelength at 820 nm. Theoretically, Fourier transformation limited pulse width corresponding to this spectral bandwidth is 12 fs. So experimentally measured pulse width closely matches with the theoretical prediction. The reported characterization system is cost effective and can be used for characterization of fs laser of broad wavelength range. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Ultrashort lasers are useful for monitoring various physical and chemical processes that occur in the time scale of picosecond to femtosecond. They are also useful for nonlinear optical microscopy, materials processing, frequency meteorology, THz generation, detection, imaging etc. [1–4]. For reproducibility in these applications; continuous, real time and accurate diagnostic or characterization of ultrashort pulses is highly essential. Various systems like autocorrelator, frequency resolved optical gating (FROG), Spectral phase interferometry for direct electric field reconstruction (SPIDER) etc. are generally used for this ultrashort pulse characterization. Second harmonic generation (SHG) is the most commonly used phenomena for these characterization systems. SHG in these systems is generally done by expensive nonlinear optical crystals. Reports indicate that ZnO nanorods can show very high second order optical nonlinearity and SHG behavior [5–8]. They also have other advantages like high chemical stability, high damage threshold and low cost. So, in this communication we report on utilization of second harmonics of ZnO nanorods for the ultrashort pulse characterization. Advantages of the developed pulse characterization system are discussed. 2. Experiment The field emission scanning electron microscope (FESEM) image of the ZnO nanorods used for this work is shown in Fig. 1. The procedure * Corresponding author.
E-mail address: [email protected] (S.K. Das). http://dx.doi.org/10.1016/j.optcom.2017.06.028 Received 27 April 2017; Received in revised form 30 May 2017; Accepted 6 June 2017 0030-4018/© 2017 Elsevier B.V. All rights reserved.
for the growth of these materials is reported previously [5]. Briefly, low-temperature seed assisted chemical bath deposition (CBD) method was used for the growth of these ZnO nanorods. An aqueous solution of Zinc nitrate hydrate (ZNH) and hexamethylenetetramine (HMT) in the molar ratio of 30 mM in distilled water was used for this purpose. A 10 nm thick seed structure was prepared on the glass substrate by sputtering technique which assisted the aligned growth of the nanorods. The growth temperature was maintained at 95 ◦ C. The average diameter and length of the nanorods were 56.30 nm and 1300 nm respectively. A Ti:sapphire laser oscillator (Femtosource) emitting linearly polarized fs pulses at a repetition rate of 75.3 MHz with 4 nJ pulse energy and a central wavelength near 820 nm has been used as the test pulse for its characterization. Fig. 2. shows the schematics of the experimental setup which is used for this purpose. Basically it is based on a SHG fringe resolved autocorrelator (FRAC) [9,10] with ZnO nanorods as second harmonic generating element. We have chosen this form of autocorrelator system because of its advantages like easier to align and its ability to give phase information and higher sensitivity etc. over the intensity autocorrelator. It contains a Michelson interferometer with one mirror driven by a piezoelectric motion controller in a step size of 35 nm. The delayed and re-combined pulses are made to incident on the samples by a 25 mm focal length concave mirror in order to generate the SH. The signal was separated from residual pump radiation by a BG39 filter and the same is recorded by a CCD camera. The motion of the interferometer mirror
R. Panda, S.K. Das
Optics Communications 402 (2017) 398–400
Fig. 3. SHG autocorrelation trace of the probe: 15 fs pulse.
Fig. 1. FESEM image of 𝐶-axis oriented ZnO nanorods grown on glass substrate.
Fig. 4. The spectrum of fs pulse under test.
Fig. 2. Schematics of the experimental setup used for pulse characterization of fs pulses using SHG of ZnO nanorods.
the constructive interferences of the pulses coming from two arms of the Michelson interferometer. If ‘∧’ represents the full width and half maximum (FWHM) of this envelope then for Gaussian pulse it relates to the pulse duration (𝑡𝑝 ) by [9].
and the second harmonic signal capture were synchronized by using an in-house developed Win32-GUI program to get the FRAC trace.
∧ (5) 1.72 From Fig. 3, ∧ is estimated to be 24 fs. Putting this value in Eq. (5) the time period 𝑡𝑝 is estimated to be 14 fs. The spectrum of the fs pulse under test is shown in the Fig. 4. From this, the spectral bandwidth in wavelength domain (Δ𝜆) of this pulse is found to be 85 nm. The corresponding spectral bandwidth in frequency (Δ𝛾 ) is estimated to be 3.77×1013 Hz. Theoretically, the duration (Δ𝑡) of a Fourier transformation limited pulse of Gaussian shape is given by [1]
𝑡𝑝 = 3. Results and discussion The SHG FRAC signal w.r.t delay (𝜏) can be given by the equation [9] 𝐼𝑡 =
| |2 |{𝜀 (𝑡) exp 𝑖 (𝜔𝑡 + 𝜙) + 𝜀 (𝑡 − 𝜏) exp 𝑖 [𝜔 (𝑡 − 𝜏) + 𝜙 (𝑡 − 𝜏)]}2 | 𝑑𝑡. (1) | ∫ |
From this equation it can be seen that at zero delay (𝜏 = 0), the FRAC signal is a peak and is represented as 𝐼𝑝 = 𝐼𝑡 (0) = 24
∫
𝜀4 (𝑡) 𝑑𝑡.
Δ𝑡 =
(2)
∫
𝜀4 (𝑡) 𝑑𝑡.
(3)
From Eqs. (2) and (3) we get, 𝐼𝑝 𝐼𝑏
=
8 1
(6)
Putting the aforementioned value of Δ𝛾 in Eq. (6), the duration of the pulse is estimated to be 12 fs. This value is found to be matched closely with the experimental value of the pulse duration. So, the pulse width measurement with the ZnO nanorods is found to be quite reliable. An independent measurement of the pulse width is done with a commercial autocorrelator (APE mini autocorrelator, model: Mini PD) & the pulse duration is found to be 13.5 fs thereby supporting our experiment resulting in ZnO nanorod based autocorrelator. It is to note here that some investigations have already been done to use ZnO thin films for development of SHG autocorrelators by other researchers [11,12]. In these investigations, molecular beam epitaxy (MBE) and metal–organic vapor phase epitaxy (MOVPE) technologies were used to grow the ZnO thin films. As these technologies are very expensive, the autocorrelator based on these films are not cost effective. On the other hand in our work the ZnO nanorods are grown by using CBD method. This method has various advantages like simplicity in fabrication, environment friendly reactions, low temperature growth,
From Eq. (1) it can also be found that the background FRAC signal (𝐼𝑏 ) i.e. the SHG signal measured by the detector for non-overlapping condition is 𝐼𝑏 = 2
0.441 Δ𝛾
(4)
i.e. theoretically peak to background ratio of FRAC signal should be 8:1. Experimentally observed FRAC signal is shown Fig. 3. From this figure, peak to background ratio is found to be almost same as that of the theoretical background, indicating the good capability of ZnO nanorods for FRAC application. Here it should be mentioned that by joining the maxima points of the FRAC signal, one can get the upper envelope. This envelope represents 399
R. Panda, S.K. Das
Optics Communications 402 (2017) 398–400
large-scale production and cost effectiveness [13]. Hence the reported autocorrelator is much cheaper. Beta Barium Borate (BBO) is one of the most commonly used NLO crystal for realization of commercial FRAC system because of its high second order nonlinear optical coefficient (𝑑ef f ). The 𝑑ef f value for BBO is 2 pm/V. In contrast to this, the nonlinearity of the ZnO nanorods are reported to be much higher [5–7]. Particularly the 𝑑ef f value for the type of ZnO nanorods used in this FRAC development is 15 pm/V [5] i.e. it is 7.5 times higher than that of BBO. Localization of field within the nanorods is predicted to be the cause for nonlinearity [12]. Nevertheless because of high nonlinearity, ZnO nanorod films of smaller thickness can generate enough signal for the FRAC application. We estimated that, the thickness of our sample (1.3 μm) is smaller than the coherence length (𝑙𝑐 ) for SHG conversion of fs pulses in broad wavelength range (750– 2200 nm). Here 𝑙𝑐 is given by 𝑙𝑐 =
𝜆 ( ) 4𝜋 𝑛2𝜔 − 𝑛𝜔
are grown by CBD method. The pulse characterization was done by the interferometric autocorrelator technique. From this, the pulse duration is estimated to be 14 fs. The spectral bandwidth of the pulse under probe is ∼85 nm with central wavelength at 820 nm. Theoretically, transformation limited pulse width corresponding to this spectral band width is 12 fs. So, experimentally measured pulse width closely matches with theoretically predicted pulse width. As the process of SHG in ZnO is not wavelength specific, the autocorrelator based on this can be used for fs pulses of broad wavelength range. Also because of low cost of CBD grown ZnO nanorods, the reported SHG FRAC is cost effective. Acknowledgment SKD thanks Science & Engineering Research Board (SERB), Govt. of India (Project File Number: EMR/2015/001175) for financial support.
(7)
References [1] A.M. Weinner, Ultrafast Optics, in: Willey Series in Pure and Applied Optics, 2009. [2] P.F. Curley, A.I. Ferguson, J.G. White, W.B. Amos, Application of a femtosecond self-sustaining mode-locked Ti:sapphire laser to the field of laser scanning confocal microscopy, Opt. Quantum Electron. 24 (1992) 851–859. [3] A.M. Streltsov, N.F. Borrelli, Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses, Opt. Lett. 26 (2001) 42–43. [4] A.G. Okhrimchuk, A.V. Shestakov, I. Khrushchev, J. Mitchell, Depressed cladding, buried waveguide laser formed in a YAG:Nd3+ crystal by femtosecond laser writing, Opt. Lett. 30 (2005) 2248–2250. [5] S.K. Das, M. Bock, C. O’Neill, R. Grunwald, K.M. Lee, H.W. Lee, S. Lee, F. Rotermund, Efficient second harmonic generation in ZnO nanorod arrays with broadband ultrashort pulses, Appl. Phys. Lett. 93 (2008) 181112. [6] M.C. Larciprete, M. Centini, Second harmonic generation from ZnO films and nanostructures, Appl. Phys. Rev. 2 (2015) 031302. [7] S.W. Chan, R. Barille, J.M. Nunzi1, K.H. Tam, Y.H. Leung, W.K. Chan, A.B. Djurisic, Second harmonic generation in zinc oxide nanorods, Appl. Phys. B 84 (2006) 351– 355. [8] R. Panda, S. Bhattacharya, R. Samal, A. Singh, P.K. Sahoo, P.K. Datta, S.K. Das, Second harmonic generation of femtosecond pulses using ZnO nanorods grown by chemical bath deposition with drop casted seed layer, J. Nonlinear Opt. Phys. Mater. 25 (2016) 1650029. [9] J.-C.M. Diels, J.J. Fontaine, I.C. McMichael, F. Simoni, Control and measurement of ultrashort pulse shapes (in amplitude and phase) with femtosecond accuracy, Appl. Opt. 24 (1985) 1270. [10] K. Yamane, T. Kito, R. Morita, M. Yamashita, Experimental and theoretical demonstration of validity and limitations in fringe-resolved autocorrelation measurements for pulses of few optical cycles, Opt. Express 12 (2004) 2762–2773. [11] Y. Kobayashi, D. Yoshitomi, K. Iwata, H. Takada, K. Torizuka, Ultrashort pulse characterization by ultra-thin ZnO, GaN, and AlN crystals, Opt. Express 15 (2007) 9748–9754. [12] M. Mascheck, S. Schmidt, M. Silies, T. Yatsui, K. Kitamura, M. Ohtsu, D. Leipold, E. Runge, C. Lienau, Observing the localization of light in space and time by ultrafast second-harmonic microscopy, Nat. Photonics 6 (2012) 293–298. [13] D. Byrne, E. McGlynn, M.O. Henry, K. Kumar, G. Hughes, A novel, substrate independent three-step process for the growth of uniform ZnO nanorod arrays, Thin Solid films 518 (2010) 4489–4492.
with 𝑛𝜔 and 𝑛2𝜔 are the refractive indices for fundamental and second harmonic wavelength respectively (Taking the transparency region of ZnO and sensitivity of our Si detector into account, we restricted our estimation of 𝑙𝑐 for fundamental wavelength within the range of 750–2200 nm). This indicates that ZnO nanorods can generate SHG efficiently for any fs pulses within the aforementioned wavelength range. The efficient SHG in BBO in the other hand takes place through birefringence phase matching process. This process is wavelength specific. Therefore a BBO crystal designed for a specific wavelength cannot generate efficient SHG of the aforementioned broad wavelength. So, unlike ZnO nanorods FRAC, the BBO based FRAC will be more wavelength specific. Further it is to note that, because of low cost fabrication process, the ZnO nanorods are quite cost effective. We estimated that for growth of ZnO nanorods on slide of size (2 × 3) cm2 it approximately costs only $10.00. Nonlinear crystals are on the other hand are quite expensive because of high cost of fabrication and cutting in desired angle for phase matching. A commercial BBO of area (1 × 1) cm2 for example can cost $600–$1000. So, the ZnO nanorods are also of better choice for FRAC application from cost point of view. Lastly, it is worth to mention here that the phase matching process for SHG in NLO crystal has the bandwidth limit. This phase matching bandwidth limit is inversely proportional to crystal thickness. So, to have the SHG of fs pulse of broad spectral bandwidth, one needs NLO crystal of few 10 s of μm thickness. Preparation of such thin crystal is generally very difficult for which they are much more expensive than the thicker crystals. From this point of view the inexpensive and efficient nanostructured nonlinear optical thin film containing ZnO nanorods are of much better choice for development of SHG FRAC for characterization of broadband ultrashort pulses. 4. Conclusion Characterization of broadband ultrashort laser pulses using second harmonics of ZnO nanorods is reported here. The used ZnO nanorods
400