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Abstract
A unique design of our ultracompact microcavity wavelength conversion device exploits the simple principle that the wavelength conversion efficiency is proportional to the square of the electric field amplitude of enhanced pump light in the microcavity, and expands the range of suitable device materials to include crystals that do not exhibit birefringence or ferroelectricity. Here, as a first step toward practical applications of all-solid-state ultracompact deep-ultraviolet coherent light sources, we adopted a low-birefringence paraelectric SrB4O7 crystal with great potential for wavelength conversion and high transparency down to 130 nm as our device material, and demonstrated 234 nm deep-ultraviolet coherent light generation, whose wavelength band is expected to be used for on-demand disinfection tools that can irradiate the human body.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the outbreak of COVID-19, deep-ultraviolet (DUV) light has attracted considerable attention for virus inactivation and bacterial disinfection by suppressing the replication function of nucleic acid. DUV light of approximately 260 nm wavelength has the highest virus inactivation efficiency [1–6]. Low-pressure mercury lamps with a wavelength of 254 nm have been used in these applications, but ∼260 nm AlGaN light-emitting diodes (LEDs) are replacing them owing to their high efficiency, long lifetime and low environmental impact [7–9]. However, they cannot be used in the presence of humans since they also kill human cells. A DUV light source of approximately 230 nm wavelength, which also has high sterilization and disinfection efficiency comparable to those of ∼260 nm DUV light, can be used in the presence of humans, since it is absorbed by the nucleic-acid-free stratum corneum, which covers the surface of the human skin [10–12]. Excimer lamps of 222 nm wavelength are already in practical use, but they are not suitable for personal use in individual homes because of their large size, high cost, short lifetime, operational instability, and low efficiency [13]. AlGaN LEDs also have the potential to replace excimer lamps because the shortest emission wavelength of AlGaN is as low as 210 nm [14]. However, it is extremely difficult to further shorten the emission wavelength while maintaining high wall-plug efficiency owing to the insulating properties of wide-gap materials [15,16]. For the same reason as for AlGaN Fabry–Perot laser diodes (LDs), the shortest reported oscillation wavelength has been limited to 271.8 nm under pulsed operation [17]. Note that there is no need for coherent light for the limited applications of disinfection, in contrast to the mandatory properties of small size, low cost, long lifetime, high stability and efficiency. On the other hand, coherent DUV light with an extremely small spot diameter is essential for ultrafine lithography, wafer inspection, surgery, and other applications. Therefore, for the application of DUV light sources by the general public, the key prerequisite is an all-solid-state ultracompact coherent light source that does not require current injection into AlGaN.
Second-harmonic generation (SHG) devices are also candidates to fulfill the above requirement when they are utilized with high-efficiency pump light sources such as InGaN LDs. As summarized in Table 1, we discovered a microcavity phase matching method that can solve various problems of conventional SHG devices as follows. To realize high-efficiency SHG, phase matching between fundamental and second-harmonic (SH) waves is important. There are two typical phase matching methods: periodically polarity-inverted (PPI) quasi-phase matching (QPM) and birefringence phase matching (BPM). In PPI QPM SHG devices, the QPM period becomes extremely short as the conversion wavelength decreases toward the DUV region, and high-order QPM significantly reduces the efficiency [18]. As the SHG device in DUV region, twinned crystal quartz, stacked multiple quartz plates, and periodically poled LaBGeO5 (LBGO) were observed, but high-efficiency wavelength conversion cannot be expected owing to their low optical nonlinearity and the difficulty in fabricating short-period QPM structures with these materials [19–22]. In BPM SHG devices, owing to the limitation of the phase-matching angle, the shortest SHG wavelength does not match the absorption edge of the crystal. In addition, very large crystals are essential for high-efficiency wavelength conversion, and walk-off reduces the efficiency and degrades the beam quality of SH waves. CsLiB6O10 (CLBO), β-BaB2O4 (BBO), and KBe2BO3F2 (KBBF) can be used as BPM SHG devices in the DUV region. CLBO, with the shortest SHG wavelength of 237 nm, cannot realize high conversion efficiency without ultrashort-pulse laser excitation owing to its low optical nonlinearity, and its deliquescence hinders the device application [23–25]. BBO is an excellent nonlinear optical crystal with moderate nonlinearity and slight deliquescence, but it is not suitable for high power operation because of nonlinear absorption in the UV region [26]. To generate vacuum ultraviolet (VUV) SH waves using CLBO and BBO, sum frequency generation (SFG) must be performed, which makes the entire system very large and complicated. KBBF is a nonlinear optical crystal that can generate VUV light by SHG, and 177.3 nm SHG by pulsed excitation has been reported [27,28]. However, KBBF has low optical nonlinearity and has not been put into practical use since the crystal growth is difficult owing to the layered crystal structure. In other words, it is extremely difficult to realize a practical all-solid-state ultracompact DUV coherent light source covering the VUV region by the combined use of a conventional phase matching method and a nonlinear optical crystal.
Table 1. Comparison of properties of nonlinear optical crystals. Physical properties of typical nonlinear optical crystals in the DUV region and adaptable phase matching method. The wavelength conversion efficiency is proportional to the square of d. The top row refers to this work.
Therefore, we previously proposed a microcavity phase matching method that does not rely on ferroelectricity or birefringence, which greatly expanded the freedom of the device structure and material selection since this method can be applied to any nonlinear optical crystal with high optical damage tolerance [29]. In our previous work, we demonstrated 428 nm SHG from an ultracompact microcavity SHG device using the low-birefringence paraelectric material GaN. By confining the fundamental wave in a single-domain, small-volume wavelength conversion region with a length close to the coherence length [18] which was sandwiched with two distributed Bragg reflectors (DBRs), the wave intensity was significantly enhanced. The SH wave was generated efficiently and emitted outside by appropriate phase control. Although the previous GaN SHG device could not generate DUV light from the absorption of GaN, DUV SHG can be expected by replacing GaN with any nonlinear optical crystal transparent in the DUV region. Therefore, in this work, we focused on SrB4O7 (SBO). The crystal structure of SBO is shown in Fig. 1(a), in which all boron ions are in tetrahedral coordination and are densely packed [30]. SBO has superior properties of large optical nonlinearity comparable to that of ferroelectric materials, high transparency down to 130 nm, and superior optical damage tolerance to ferroelectric materials and other borate crystals [31–33]. However, conventional phase matching methods cannot be adapted owing to its paraelectricity and low birefringence. There is one report of VUV SHG using SBO with random QPM [34], but high-efficiency SHG cannot be expected unless an extremely large crystal is used since the generated SH waves are added together in a random phase relationship. In addition, the directivity and spatial profile of the generated SH wave are a major problem. A QPM structure of stacked multiple SBO crystals is possible, but the problem of a short coherence length makes it extremely difficult to fabricate a high-efficiency device. In this work, we adopted SBO and a surface-emitting microcavity as the device material and structure, respectively, and demonstrated DUV SHG with a wavelength of 234 nm. A light source of this wavelength can be used for sterilization and disinfection harmless to the human body. This demonstration is a major achievement in the pursuit of an ultrashort wavelength for ultracompact coherent light sources covering the VUV region.
Fig. 1. SBO crystal and SHG device structure. (a) Schematic of SBO crystal structure. (b) Schematic of SBO microcavity SHG device.
Figure 1(b) illustrates the proposed SBO microcavity SHG device. The device consists of a b-plane SBO SHG layer, a HfO2/SiO2 phase adjustment layer, and two DBRs, which are composed of multiple HfO2/SiO2 pairs. Since these pairs are transparent in the DUV region and provide a high refractive index contrast, the DBRs have broad stop bands. The operation principle is as follows. When a blue fundamental wave incident vertically from the top of the device satisfies the resonant condition with very strong confinement, its field amplitude is enhanced significantly in the SHG layer. DUV SH waves are efficiently generated and propagate from top to bottom and from bottom to top in the device. By inserting a phase adjustment layer between the SHG layer and the bottom DBR, the phase shift upon reflection is controlled so that the reflected SH wave at the bottom DBR is in phase with the SH wave generated toward the upper side. By controlling the SH wave reflectivities for both DBRs appropriately, the SH wave can be emitted only from the upper side. After the SH wave is emitted from the top of the device, it is reflected by a dichroic mirror that selectively reflects DUV light. The planar structure provides high stability against environmental disturbance and eliminates the need for the fine alignment of the excitation laser beam. Also, by adopting the surface-emitting microcavity, high-power operation can be realized, because the total amount of power can be increased while maintaining the power density. In the future, a two-dimensional array device integrated with vertical cavity surface emitting lasers (VCSELs) or photonic crystal surface emitting lasers (PCSELs) can be expected to achieve even higher power operation.
2. Method
2.1 Design of SHG device
Firstly, we designed the SBO microcavity SHG device, which operates with a 460 nm fundamental wave. The device structure was optimized to maximize the SHG efficiency ηSH by varying the thickness of each device layer, where the initial thickness of the SHG layer was set to the coherence length of the SHG process, 1.55 µm. [18]. ηSH was calculated by the same method as in our previous paper [29]. The transfer matrix method was employed to calculate the reflectivities of the DBRs and the electric field distributions along the propagation direction. To utilize the largest second-order nonlinear optical tensor component of d33 (=3.5 pm/V) of SBO, extraordinary polarizations (the electric field parallel to the c-axis of SBO) were employed for both the fundamental and SH waves [31]. To enhance the fundamental wave intensity within the resonator, both the top and bottom DBRs must have high fundamental wave reflectivity close to unity. On the other hand, to extract the generated SH wave from the only upper side of the device efficiently, the SH wave reflectivities of the top and bottom DBRs should be zero and unity, respectively. As a premise for optimization under no pump depletion approximation (NPDA) [18], the incident fundamental wave is 1 W with a beam waist 2w0 of 15 µm. In this case, the nonlinear coupling coefficient κ was estimated as 0.28 W−1/2cm−1 assuming an effective nonlinear optical coefficient of deff = (2/π)d33 and an effective cross section of Seff = πw02 [18]. The large κ compared with those of traditional microring SHG devices realizes high SHG efficiency and extremely small footprint [35–36]. Figure 2(a) shows the calculated dependence of the maximum ηSH and the wavelength tolerance of the fundamental wave which is the full width at half maximum (FWHM) Δλf, on the number of top DBR pairs. For bottom DBRs, 19 pairs with sufficient reflectivity for both fundamental and SH waves were adopted. Considering the trade-off relationship between ηSH and Δλf, we adopted 11 pairs of top DBRs to demonstrate our operating principle. ηSH and Δλf were estimated as ∼1% and ∼0.007 nm, respectively. The small Δλf originates from the high-finesse cavity for the fundamental wave. The total device thickness was estimated to be as small as ∼6 µm. Figure 2(b) shows the calculated dependence of ηSH and the resonant fundamental wavelength on the deviation from the designed thickness of the SBO SHG layer. Because of the wide stop band of the DBRs around the fundamental wave, fundamental wavelengths that realize high ηSH are periodically expressed. Even if the deviation of the SBO SHG layer is ∼30 nm, ηSH of more than 0.5% can be expected by changing the fundamental wavelength by ∼7 nm. The phase adjustment layer plays a very important role: if the phase adjustment is ineffective, the generated SH wave will be cancelled out and ηSH will drop to nearly 0%. Figure 2(c) shows the calculated dependence of ηSH and the resonant fundamental wavelength on the incident angle of the fundamental wave. Even if the wavelength of the fundamental wave deviates from the resonant wavelength by ∼3 nm, ηSH of more than 0.5% can be expected by tilting the incident angle by ∼12°, which enables stable device operation. This is also effective in compensating for the red shift of the resonant wavelength with the temperature rise due to the strong excitation. In addition, when a curved mirror is adopted as the top DBR to increase the power density of the fundamental wave, ηSH increases proportionally to the power density. Also, when the SBO crystal with reversed polarity in the thickness direction, which is fabricated by the surface-activated bonding of two SBO crystals with a thickness of approximately the coherence length, is adopted as the SHG layer, the efficiency increases by a factor of four. Thus, we have theoretically clarified that the microcavity phase matching method with the SBO crystal can realize an ultracompact and highly efficient DUV light source.
2.2 Fabrication of SHG device
Secondly, we fabricated an SBO microcavity SHG device. The fabrication process is summarized in Fig. 3. The device was fabricated using the b-plane SBO crystal grown by the top-seeded solution growth (TSSG) method [32]. After cutting the crystal into small pieces (Fig. 3(a)), the + b plane of the SBO crystal was polished using diamond slurry (Fig. 3(b)). Figure 4(a) shows an atomic force microscopy (AFM) image of the polished SBO plane with a dimension of 100 µm square. The root mean square (rms) roughness of the polished plane was measured as ∼0.21 nm. After confirming that extinction coefficients in the wavelength of 230 nm of the thin films of HfO2 and SiO2 are almost zero as a result of the analyses using a spectroscopic ellipsometer system, the phase adjustment layer and the bottom DBR consisting of HfO2/SiO2 were deposited by sputtering on the polished plane (Fig. 3(c)). After the sample was adhered to a sapphire substrate using epoxy resin with the DBR side down (Fig. 3(d)), the –b plane of the SBO crystal was polished to a thickness of several micrometers using diamond slurry (Fig. 3(e)). The SBO thickness was roughly adjusted using a confocal microscope. The SBO length is different from the coherence length, but in this device, it is more important to satisfy the resonance enhancement than to control the thickness of the SBO layer. For high-power operation of this device, we are considering direct bonding of the device and the support substrate by surface-activated or hydrophilic bonding instead of the epoxy resin in the future. The SBO thickness was roughly adjusted using a confocal microscope. Figure 4(b) shows an AFM image of the polished SBO crystal with a dimension of 100 µm square. The rms roughness of the polished plane was measured as ∼0.27 nm. Finally, the top DBR consisting of HfO2/SiO2 was deposited by sputtering on the polished plane (Fig. 3(f)). Figure 4(c) shows transmission spectra of the bottom and top DBRs, where the DBRs on synthetic quartz substrates were used for the measurement. Both the bottom and top DBRs have transmittances close to zero for the fundamental wave. Also, the bottom DBR has transmittance close to zero for the SH wave, and the top DBR has a transmittance of 0.45 for the SH wave. In other words, the length of the DBR and the phase adjustment layer is close to the designed thickness. Therefore, the generated SH wave was selectively emitted from the upper side of the device. A digital camera image of a fabricated SHG device is shown in Fig. 3(f).
Fig. 2. Device design. (a) Calculated dependence of maximum ηSH and wavelength tolerance of fundamental wave on number of top DBR pairs. (b) Calculated dependence of ηSH and resonant fundamental wavelength on deviation from designed thickness of SBO SHG layer. (c) Calculated dependence of ηSH and resonant fundamental wavelength on incident angle of fundamental wave.
Fig. 3. (a) Cutting out b-plane SBO crystal after TSSG. (b) Polishing of + b plane of SBO. (c) Fabrication of bottom DBR on the polished + b plane of SBO. (d) Bonding of SBO with bottom DBR and sapphire substrate via epoxy layer. (e) Polishing of –b plane of SBO. (f) Fabrication of top DBR on polished –b plane of SBO. A digital camera image of the fabricated SHG device is shown in the inset of (f).
Fig. 4. Surface flatness of SBO and DBR transmittance. AFM images of polished (a) lower- and (b) upper-side SBO planes. (c) Transmission spectra of bottom and top DBRs (blue: top side, red: bottom side).
3. Results and discussion
Finally, we demonstrated DUV light emission from the fabricated SHG device. The optical characterization of the device was performed using a picosecond laser with a tunable wavelength range. The output pulses had a spectral FWHM of 0.6 nm and a repetition rate of 1 kHz. The fundamental wave was set to extraordinary polarization and focused on the SHG device. To avoid the absorption of the SH wave by the objective lens, the dichroic mirror was placed between the objective lens and the SHG device to selectively reflect DUV light. After the cleaning of the reflected light by a bandpass filter, the SH wave was focused on an optical fiber connected to a spectrometer with wavelength resolution of ∼0.01 nm. Note that the spectra of both the fundamental and SH waves can be observed with the same setup. The optical experimental setup is illustrated in detail in Supplementary Information A. When the peak wavelength was set at 467.68 nm, a fine resonance dip with an FWHM of 0.01 nm or less was observed as shown by the blue line in Fig. 5(a). When the measurement wavelength of the spectroscope was changed to the SH wave, the SH wave spectrum with a wavelength of 233.84 nm also appeared as shown by the red line in Fig. 5(a). Although the FWHM of the resonance dip is very narrow, that of the SH wave is as wide as 0.18 nm. The main reason for this is still under investigation, but it is expected to the short coherent distance derived from the picosecond short pulse laser. The effect of Cherenkov radiation phase matching, in which SH waves are generated at arbitrary angles that satisfy phase matching, is also considered to be superimposed [37]. As shown in Fig. 5(b), the SH wave peak intensity was proportional to the square of the power of the fundamental wave, which is characteristic of the second-order nonlinear optical process. By varying the central wavelength of the fundamental wave, the SH wave intensity was significantly changed. As shown in Fig. 5(c), the SH peak intensity normalized by the square of the peak intensity of the fundamental wave has tuning characteristics with a sharp single peak centered at the resonance fundamental wavelength. The similarity of the graph shapes of Figs. 2(b) and 5(c) can be explained by the phase shift of the SH wave and the sharpness of the resonance condition. Finally, Fig. 5(d) shows that the extraordinary SH wave intensity is maximized under excitation by the extraordinary fundamental wave. This result indicates that SHG via the second-order nonlinear optical tensor component of d33 was achieved. When the ordinary fundamental wave was incident to the device from the same optical path, a weak extraordinary SH wave was also observed. This is SHG via the second-order nonlinear optical tensor component of d31, which is caused by the fact that SBO is a crystal belonging to the 2 mm point group [31]. Details of the polarization characteristics of SHG are discussed in Supplementary Information B. The benefit of resonant enhancement is not fully utilized, and the quantitative measurement of the power of the SH wave is difficult if we reduce the average pump power to avoid the destruction of the device owing to its high peak power density with a short pulse duration. Since a quality factor Q estimated from an FWHM of the fine resonance dip was almost equal to the theoretical Q of the designed SHG device, we expect that high-efficiency wavelength conversion can be achieved by continuous wave excitation. The quantitative evaluation of the efficiency with continuous wave excitation is under way and will be published elsewhere.
Fig. 5. Properties of SHG emission. (a) Spectra of fundamental and SH waves. (b) Excitation power dependence of 233.84 nm SH wave intensity ISH pumped by 467.68 nm fundamental wave. (c) Dependence of ISH normalized by square of excitation intensity IF on fundamental wave wavelength. (d) Spectra of ordinary and extraordinary SH waves pumped by extraordinary fundamental wave (blue: ordinary SH wave, red: extraordinary SH wave).
4. Conclusion
In conclusion, we succeeded in 234 nm DUV coherent light generation in an ultracompact microcavity SHG device using SBO crystal, which does not exhibit birefringence or ferroelectricity. Thus, this result will lead to the realization of ultracompact DUV coherent light sources with low cost, high efficiency, and long lifetime. In addition, by adopting the MgO/SiO2 DBR, VUV light generation can be expected.
Funding
Japan Society for the Promotion of Science KAKENHI (JP17K19078, JP17H05335, JP17H01063); Grant-in-Aid for JSPS Fellows (Grant Number JP22J10864).
Acknowledgments
We are grateful to Photonics Center, Osaka University for the help of the spectroscopic ellipsometry measurements.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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