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We report on the ultraviolet to near-infrared supercontinuum generation in a 3-mm-long yttrium orthosilicate channel waveguide implanted by oxygen. The generated broadest spectrum spans 575 nm (at −30 dB points) from 430 to 1005 nm when pumping the waveguide at 800 nm and spans 115 nm (at −30 dB points) from 345 to 460 nm when pumping the waveguide at 400 nm, respectively. The nonlinear refractive index of the ion implanted region is quantified by fitting our spectra to nonlinear pulse propagation simulations around 800 nm. Our analysis shows that the nonlinear refractive index of yttrium orthosilicate waveguide after ion implantation and thermal treatment is about half the value of the bulk crystal. Our research findings indicate that ion implantation is a promising method to fabricate channel waveguides for low-threshold supercontinuum generation.
© 2017 Optical Society of America
1. Introduction
Broadband supercontinuum (SC) sources have become more and more important due to their valuable applications in optical frequency combs generation, frequency metrology, pulse compression, and wavelength division multiplexing (WDM) [1–4]. Additionally, due to their low absorption and scattering coefficients in tissue and cells, the SC in the ultraviolet (UV) to near-infrared spectral window can provide potential applications in bio-imaging [5–7], such as optical coherence tomography (OCT), coherent anti-Stoke Raman scattering (CARS) microscopy, and two-photon excited auto-fluorescence. Such SC sources can also be used for quantum optics and all-optical signal processing [8,9].
Recently various waveguide-based platforms have been studied for efficient SC generation in a number of materials, such as silicon, silicon nitride, InGaP, silica, chalcogenide, and lithium niobate [10–17]. Different from its bulk competitor, in a waveguide based SC system both the pump light and the generated SC are confined in the cross section of the waveguide structure to small dimensions of a few hundred of nanometers to a few microns. As a result, higher intensities could be achieved with relatively lower power. Also it is more convenient to integrate the waveguide SC devices with other elements to realize compact and functional photonic integrated circuits [11]. Until now, several techniques have been employed to fabricate waveguide structures for SC generation, such as film deposition method [10–13], proton exchange [16,17], and femtosecond (fs) laser inscription [14]. Among these, a mature and frequently-used technique is ion implantation. Due to its wide applicability and accurate control of the doping ions, the ion implantation technique has been successfully used to form waveguide structures in more than 100 materials [18,19], including single crystals, glasses, polycrystalline ceramics, and organic materials.
Yttrium orthosilicate (YSO) is a promising material for SC generation in the UV to near-infrared band owing to its large band-gap (6.14eV) [20], wide transparent range of wavelength, and high chemical and photochemical stabilities. The broadband waveguide-based SC source has been realized in an ion implanted YSO planar (one dimensional) waveguide [21]. Compared with planar waveguides, the channel (two dimensional) waveguides can restrict the light propagation in two dimensions, offer smaller cross section dimensions, higher beam quality, and easier fabrication of high density integrated devices, which are intriguing light sources for on-chip integration [22,23]. However, to date there have been no reports on the SC generation in ion implanted channel waveguide structures.
In this letter, we demonstrate SC generation in an O ions implanted YSO channel waveguide. The mode propagation properties of the waveguide are investigated. The broadest spectrum that we generated ranges from 430 to 1005 nm when pumping the waveguide at 800 nm and from 345 to 460 nm when pumping the waveguide at 400 nm. In comparison with the ion implanted YSO planar waveguide, the channel waveguide achieved a wider spectral broadening with an order of magnitude lower pump power. The coherence lengths of the generated SC are measured by a Michelson interferometer. The nonlinear refractive index of the waveguide after ion implantation and annealing treatment is quantified by fitting our measurements to simulations. Our research work shows the potential of ion-implanted channel waveguides for SC generation spanning from the UV to near-infrared band.
2. Experiment
The YSO crystal was cut to the dimensions of 8(a) × 1(b) × 3(c) mm3 and optically polished. Figure 1 shows the schematic of the fabrication process of channel waveguides in the YSO sample. A thick-film positive photoresist (AZ4620) was spin-coated onto the polished a-c facet of the crystal, forming a film with thickness of about 5 μm. A mask consisting of open stripes with width of 5 μm and a spacing of 40 μm between the adjacent channels was used for subsequent UV exposure in a standard mask aligner. In the next step 6 MeV O ions were implanted into the samples at a fluence of 5 × 1014 ions/cm2. The ion implantations were carried out with the 1.7 MV tandem accelerator at Peking University. Implantation uniformity was guaranteed by electrically scanning the ion beam. A 7° angle was deviated from the axis of the crystal in order to avoid the channeling effect. After the implantation, annealing treatment at 200 °C for 30 mins in an open oven was performed on the sample to improve the waveguide quality and the thermal stabilities. The end faces of the sample were polished to optical quality for direct end-face coupling of light to investigate the waveguide mode propagation properties.
The experimental setup for SC generation in YSO channel waveguides is shown in Fig. 2. A mode-locked Ti:Sapphire laser (Legend Elite, Coherent) was used with a center wavelength of 800 nm. The laser provides pulses with about 100 fs duration, a repetition rate of 1 kHz, and a maximum average output power of 4 W. A half-wave plate (P1), a polarizer (P2), and a variable neutral density filter (NDF) wheel were used to adjust the input polarizations and peak powers. The pump laser was coupled into the YSO channel waveguides by a 20 × objective lens. The output pulse spectra were collected using a 40 × objective and recorded using an optical spectrum analyzer (AvaSpec-3648). The coherence lengths of the generated SC spectra were measured with a Michelson interferometer. The SC spectra output from the waveguide were sent into the interferometer. One arm of the interferometer can be moved to provide a delay. The power evolution with different delay lengths was recorded by a silicon photodiode detector.
Fig. 2 Experimental setup of SC generation in YSO channel waveguides. P1: half-wave plate; P2: polarizer; NDF: neutral density filter; Obj1: objective; Waveguide: YSO channel waveguides; Obj2: objective; OSA: optical spectrum analyzer. The inset shows a microscope photograph (1000 × ) of the cross-section of a channel waveguide.
3. Result
A photograph of the channel waveguide in YSO crystal formed by ion implantation is collected by a metallographic microscope with 1000 × magnification, as shown in the inset in Fig. 2. The channel waveguides are constructed in the unshielded regions of the sample surface with trapezoid boundaries at their cross sections because of the block effects of the wedged edges of the photoresist mask.
The 2D refractive index distribution of na (TE mode) and nb (TM mode) in the channel waveguide can be reconstructed according to the refractive index profile of the planar waveguide (reconstructed using two half-Gaussian curves based on the m-line spectra) [21], as well as by taking the trapezoid shape of the cross section into account. Assuming that the dispersion of the waveguide region is the same as that of the substrate, the refractive index profile Nλ(x,y) of the channel waveguide at different wavelengths λ can be estimated with the Eq. (1) [17]:
The reconstructed index profiles of na and nb of the channel waveguide at the wavelength of 800 nm are shown in Fig. 3(a) and 3(b). Based on this profile, FD-BPM (finite-difference beam propagation method) is used to simulate the light propagation in the channel waveguide at 800 nm. Figure 3(c) and 3(d) display the simulated intensity profiles of quasi-TE00 (fundamental transverse electric) and TM00 (fundamental transverse magnetic) modes by FD-BPM in the channel waveguide. For comparison, an end-face coupling test is performed to investigate the near-field intensity distribution of the light carried in the quasi-TE00 and TM00 modes. The measured mode intensity profiles are presented in Fig. 3(e) and 3(f). One conclusion can be obtained that all excited intensity distributions are dominated by the fundamental mode. The propagation loss of the channel waveguide is evaluated with the Fabry-Perot resonator method using a single frequency laser (657.1 nm) [24]. Propagation losses as low as 0.45dB/cm for TE polarization and 0.52dB/cm for TM polarization are obtained.
Fig. 3 Refractive index profiles of (a) na and (b) nb at the wavelength of 800 nm, the simulated (c) (d) and measured (e) (f) intensity distributions of TE00 and TM00 modes in a YSO channel waveguide.
The experiments of SC generation in a 3-mm long YSO channel waveguide are performed using a mode-locked Ti:Sapphire laser (Legend Elite, Coherent) with a center wavelength of 800 nm. Figure 4 shows the spectra of 800 nm laser pulses after passing through a YSO channel waveguide as a function of the peak powers coupled into the waveguide. As shown in Fig. 4(a), at a low peak power of 10 kW, the shape of the input spectrum is preserved. At 60 kW, a broadened spectrum that is symmetrical around 800 nm is obtained; small ripples appear at the center of the spectrum, which is mainly attributed to self-phase modulation (SPM). At 370 kW, the spectral broadening becomes asymmetric, enhancing the propensity for generation of higher frequencies (anti-Stokes broadening). At the available maximal coupled power of 750 kW, the −30 dB bandwidth spans 575 nm from 430 to 1005 nm, which is over 1.2 octaves of bandwidth. The bandwidth can be further broadened by increasing the peak power coupled into the waveguide, but it is restricted by the damage occurring at the input facet. The SC spectra from the YSO channel waveguide of the TM mode are also measured at different incident intensities, as shown in Fig. 4(b). At the maximal available coupled power of 750 kW, the −30 dB bandwidth spans 545 nm from 440 to 985 nm, more than an octave. Compared to the SC generation in ion implanted YSO planar waveguides in ref [21], the channel waveguide generates a relatively broader bandwidth. This is mainly because the light spreads in the plane when it propagates in the planar waveguide, resulting in a reduction in the peak powers when the laser pulses pass through the waveguide, and it will limit the bandwidth of SC.
Fig. 4 Evolution of the spectra of TE and TM modes as a function of the peak powers coupled into the waveguide.
The dispersion curves of TE00 and TM00 modes in the channel waveguide are calculated by the FD-BPM, as shown in Fig. 5. The calculation shows that dispersion is normal over the entire concerned wavelengths for the TE00 and TM00 modes. Therefore, the primary process responsible for SC generation is SPM, which causes the strongly symmetrical peaks modulating the SC spectra. However, strong asymmetric broadening is observed in the SC spectra. To explain the asymmetric SC generation, some mechanisms, such as four-wave mixing [25], ionization-enhanced SPM [26], and SPM enhanced by self-steepening [27] have been invoked. We attribute the mechanism of the asymmetric SC generation to the enhancement of SPM by free electrons generated through multi-photon excitation (MPE) [28,29]. The appearance of free electrons contributes negatively to the refraction index and can ultimately stop self-focusing by canceling the Kerr index, which leads to the frequency deviations becoming positive and causes a large anti-Stokes broadening by SPM [28].
Visible to near-infrared SC spectra have been generated in the waveguide. However there is still a need to extend the achievable bandwidths further, especially towards the UV regions for use in spectroscopy [30]. To achieve this, the Ti:Sapphire laser with pulses that are frequency doubled by a β-BBO crystal is used as the pump laser. Figure 6 shows the spectral broadening of TE and TM modes from 400 nm laser pulses after passing through a YSO channel waveguide at a peak power of 750 kW. The spectrum expands from 345 nm to 460 nm at −30dB points for TE polarization. A similar spectrum is obtained for TM polarization. This result experimentally proves that spectral broadening from an ion implanted YSO waveguide can extend into the UV regime.
Fig. 6 Ultraviolet spectral broadening for a YSO channel waveguide with the second harmonic pump source.
As is widely known, the coherence property is important for many applications of SC, such as bio-imaging [5] and WDM [31]. Therefore, the coherence lengths of the generated SC spectra are measured with a Michelson interferometer. The interferometric signals of TE and TM modes from 800 nm laser pulses after passing through a 3-mm-long YSO channel waveguide at peak power of 750 kW are shown together with their envelopes in Fig. 7. The coherence lengths are measured to be 3.5μm and 4.5μm at TE and TM polarizations.
Fig. 7 Interferometric signals and envelopes of TE and TM modes from 800 nm laser pulses after passing through a YSO channel waveguide with peak power of 750 kW.
Nonlinear refractive index has an important influence on the SC generation and design of nonlinear integrated optical devices. To fully characterize the ion implanted waveguides, it is essential to study the nonlinear refractive index of the bulk material and ion implanted waveguide. To determine the magnitude and sign of the nonlinear refractive index of bulk YSO crystal, the typical closed-aperture (CA) Z-scan measurement [32] is carried out on a 1 mm thick sample using a 190 fs high-power femtosecond laser with a center wavelength of 800 nm. Figure 8(a) shows the measured CA z-scan result and fitted curve with pulse energies of 58 nJ. From Fig. 8(a) we find that the YSO bulk crystal has a valley–peak-type CA Z-scan curve, which indicates a self-focusing effect in the YSO crystal with a positive nonlinear refractive index n2 of 6.1 ± 0.8 × 10−20 m2/W.
Fig. 8 (a) Experimental closed-aperture z-scan result and fitted curve of the YSO crystal at 800 nm, (b) Measured (black lines) and simulated (red lines) spectral broadening for a YSO channel waveguide at λ = 800 nm at incident peak powers from 10 kW to 500 kW.
The nonlinear refractive index of the ion implanted YSO channel waveguide is quantified by fitting nonlinear pulse propagation simulations to the experimental data [9,33]. When input peak power Pin<Pth, where Pth is the threshold power that is required for self-focusing to occur within a given medium, spectral broadening is mainly attributed to SPM [28,34]. In this situation, simulation solves the generalized Nonlinear Schrödinger Equation (NLSE) with the split-step Fourier method and the 4th-order Runge–Kutta algorithm. The expression is given below in Eq. (2) [1]:
Here α is the transmission loss of the waveguide and βn is the n-th order group velocity dispersion (GVD). γ is the nonlinear coefficient of the waveguide and is connected to the nonlinear refractive index n2 by , where Aeff is the effective area of the fundamental mode. The Raman term is omitted in the equation due to the low influence on the shape of the propagating pulse [33]. The coupling efficiency of the laser mode into the channel waveguide is measured to be 25%. The starting spectrum is the input spectrum measured by an OSA. The nonlinear refractive index n2 is used as a parameter to fit the calculated spectra to the measured ones. Figure 8(b) shows measured and simulated spectra after propagation through a YSO channel waveguide at TE polarization. The shape of the experimental spectra and the theoretical fitted spectra are in good agreement for pulse peak powers up to 160 kW. We obtain n2 of 3.3 × 10−20 m2/W in the waveguide region, which is about half the value of the bulk YSO crystal. When the peak power is above 370 kW, there is an obvious distinction between experimental and theoretical results; the measured spectral broadening in the anti-Stokes side extends much more than that in the simulated results. This is mainly due to the enhancement of SPM by free electrons generated in the condition of input peak power Pin>Pth [28,29].
4. Summary
In conclusion we have demonstrated a SC generation in an ion implanted YSO channel waveguide pumped at 800 nm. The waveguides properties are investigated at both TE and TM polarizations. The generated broadest spectrum extends from 430 to 1005 nm at pump wavelength of 800 nm and extends from 345 to 460 nm at pump wavelength of 400 nm. The coherence length of the broadest SC is measured with a Michelson interferometer to be 3.5 μm. We have also investigated the nonlinear refractive index of the ion implanted waveguide by analyzing self-phase modulation and obtain a nonlinear refractive index of 3.3 × 10−20 m2/W in the ion implanted region. Research works reported in this paper show that ion implantation is an efficient method to fabricate channel waveguide structures in yttrium orthosilicate crystal for broadband supercontinuum generation.
Funding
National Natural Science Foundation of China (Grant No. 61575129); the China Postdoctoral Science Foundation (Grant Nos. 2016M602511 and 2016M602510); the Shenzhen Science and Technology Planning (Grant No. JCYJ20160422142912923); and the State Key Laboratory of Nuclear Physics and Technology, Peking University.
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