Femtosecond / attosecond all-optical switches based on photonic crystal defect states and their design methods

Abstract

This invention belongs to the field of all-optical information technology, specifically a femtosecond / attosecond-level all-optical switch based on photonic crystal defect states and its design method. The design method includes: setting a defective photonic crystal structure with nonlinear materials as the structure of the ultrafast all-optical switch; determining the time of the pump pulse with the highest reflectivity during the transmission and reflection process of a signal pulse incident on the all-optical switch structure, and recording this state as a standard on state, while recording the low reflectivity state without a pump pulse as a standard off state. The key feature of this invention is that the duration of the dielectric constant change caused by the pump pulse is extremely short, much shorter than the residence time of the center frequency in the photonic crystal, and that the high and low reflectivity states of the defect states are used as the basis for determining the on and off states. This invention also provides a femtosecond / attosecond-level ultrafast all-optical switch based on the reflectivity changes of photonic crystal defect states, utilizing a dynamic modulation mechanism.

Description

Technical Field

[0001] This invention belongs to the field of all-optical information technology, specifically relating to a femtosecond / attosecond all-optical switch and its design method. Background Technology

[0002] Optical switches are a crucial area of ​​research in controlling microscopic optical currents, playing a vital role in optical communication and optical sensing. They come in various types, such as magneto-optical switches, thermo-optical switches, and all-optical switches, with all-optical switches boasting the fastest response speed, a trend that continues to evolve. Simultaneously, with advancements in research and manufacturing processes, the performance (design principles, size, and efficiency) of all-optical switches is constantly being optimized and improved, indicating immense potential for the future of all-optical switches and integrated optical circuits. Photonic crystal devices, with their small size and high integration, can flexibly control microscopic optical currents at the wavelength scale, naturally meeting the requirements of all-optical switches. There are multiple approaches to realizing photonic crystal all-optical switches, the most important being the utilization of material nonlinearity. The most frequently used method in designing photonic crystal all-optical switches is the optical Kerr effect, which offers the fastest response speed. This involves the change in the dielectric constant of a dielectric material when a high-intensity pump pulse is applied, resulting in a corresponding change in its reflectivity. Typical picosecond-level ultrafast switches are based on a "bandgap drift" mechanism, while this invention is based on a "dynamic modulation" mechanism. This means the duration of the nonlinear effect is only on the femtosecond / attosecond scale, much shorter than the residence time of light at the operating frequency in the switch structure (typically tens to hundreds of femtoseconds). This novel mechanism provides a femtosecond / attosecond-level ultrafast all-optical switch based on the reflectivity variation of photonic crystal defect states. Summary of the Invention

[0003] Unlike the "bandgap drift mechanism" of picosecond ultrafast switches, the present invention aims to provide a femtosecond / attosecond ultrafast all-optical switch based on the reflectivity change of photonic crystal defect states and its design method for femtosecond ultrafast switches based on dynamic modulation mechanisms.

[0004] The present invention provides a design method for femtosecond / attosecond-level ultrafast all-optical switches based on the reflectivity variation of photonic crystal defect states. The specific steps are as follows:

[0005] Step 1: Select the photonic crystal lattice structure and material parameters; the length of the photonic crystal structure is generally from a dozen to several hundred cells (for example, between ten and one thousand). Calculate or measure the transmittance and reflectance within a certain frequency range (selected according to the band characteristics) to determine the photonic bandgap of the photonic crystal, i.e., the low-transmittance radio frequency band or the high-reflection frequency band. The photonic bandgap can be selected in the visible light band, ultraviolet and infrared electromagnetic wave bands.

[0006] Step 2: By adjusting the structural or material parameters of a single cell or multiple cells in the middle of the photonic crystal, defects are formed in the photonic crystal. Defect states are found within the photonic bandgap of the photonic crystal, that is, high transmittance peaks in the transmission spectrum or low reflectance valleys in the reflection spectrum. The frequency corresponding to the defect state is selected as the working frequency. At this time, the reflectance R should be a small value, generally less than 10%.

[0007] Step 3: Assume that the material at the defect, or the material at the defect and part / all of the photonic crystal, is a nonlinear material. This nonlinear material will change its dielectric constant or magnetic permeability constant under external optical pumping. The nonlinear relaxation time of this nonlinear material is in the femtosecond or sub-femtosecond range, for example, hundreds of attoseconds to twenty femtoseconds.

[0008] Step 4: Define the defective photonic crystal structure with nonlinear material as the structure of the ultrafast all-optical switch;

[0009] Step 5: Determine the parameters of the signal pulse and pump pulse. The duration of the signal pulse and pump pulse can be from hundreds of attoseconds to tens of femtoseconds. Combining the nonlinear relaxation time in step 3, it can be deduced that the duration of the nonlinear effect caused by the pump pulse is on the order of hundreds of attoseconds to twenty femtoseconds. The duration of this nonlinear effect is much shorter than the dwell time of the working frequency light in the switching structure (generally, the duration is on the order of fifty femtoseconds to one picosecond).

[0010] Step 6: When the signal pulse is incident on the all-optical switch structure and during transmission and reflection, assuming that a pump pulse is introduced at a selected time point to irradiate the defect and the part of the photonic crystal containing nonlinear material, record the corresponding time-domain reflection field and obtain the reflectivity at the working frequency under this condition.

[0011] Step 7: Change the time point of the pump pulse introduction. Repeat the operation of step 6 for each time point τ of the pump pulse introduction. A reflectivity R is obtained at the working frequency. Therefore, a curve can be drawn with the pump introduction time τ as the x-axis coordinate and the reflectivity R as the y-axis coordinate.

[0012] Step 8: According to the curve, it can be found that at "certain time points when pump pulses are introduced", the reflectivity of the working frequency increases significantly, and the reflectivity R can increase to 50% or higher. Determine the time when the pump pulse with the highest reflectivity is introduced, and record this state as a standard "on" state. Record the low reflectivity state "without pump pulse" as a standard "off" state.

[0013] Furthermore:

[0014] The photonic crystal lattice type mentioned in step 1 can be, but is not limited to, a square lattice.

[0015] The unit cell of the photonic crystal structure described in step 1 can be, but is not limited to, an air-hole structure with the medium as the background material.

[0016] The photonic crystal material parameters mentioned in step 1 also include the dielectric constant.

[0017] The reflectivity described in steps 1, 6, and 7 is determined by R = |Er(ω)Ei(ω)| 2 The reflectivity at a certain frequency is calculated. Ei(ω) and Er(ω) are the complex amplitudes of the frequency domain field, which can be obtained from the Fourier transforms of the incident field Ei(t) and the reflected field Er(t) in the time domain.

[0018] As described in step 7, for each time point τ when the pump pulse is introduced, a corresponding time-domain reflection field Er(t) can be obtained. The complex amplitude Er(ω) of the reflection field can be obtained through Fourier transform, thereby obtaining the reflectivity R(τ) of the working frequency under the condition of "time point τ when the pump pulse is introduced".

[0019] As described in steps 2 and 5, the duration of the nonlinear effect caused by the pump pulse is approximately on the order of several hundred attoseconds to twenty femtoseconds. This duration is much shorter than the light residence time at the operating frequency (typically on the order of fifty femtoseconds to one picosecond). Therefore, the mechanism of the ultrafast all-optical switch designed in this invention is a dynamic mechanism, completely different from the traditional nonlinear "band drift" mechanism (which requires the nonlinear pump time to be longer than the light residence time). Thus, this invention also provides a femtosecond / attosecond-level ultrafast all-optical switch based on the reflectivity variation of photonic crystal defect states, obtained according to the above design method.

[0020] This invention realizes femtosecond / attosecond-level ultrafast all-optical switching based on the reflectivity change of defect states in photonic crystals. This mechanism is completely different from that of picosecond-level ultrafast switching and is a dynamic modulation mechanism. Attached Figure Description

[0021] Figure 1 This is a diagram of the original two-dimensional photonic crystal structure provided according to the present invention.

[0022] Figure 2 It is the transmission / reflection spectrum within a certain range of the original two-dimensional photonic crystal structure.

[0023] Figure 3 This is a diagram of a defective photonic crystal structure.

[0024] Figure 4 It is the transmission / reflection spectrum of an ultrafast all-optical switch structure.

[0025] Figure 5 This is the reflection spectrum of an ultrafast all-optical switch structure. The operating frequency point is marked in the figure, at which the reflectivity R is approximately 7%.

[0026] Figure 6 It is the time-domain reflection field of an ultrafast all-optical switch structure.

[0027] Figures 7-11 The time-domain field of the ultrafast all-optical switch structure is compared with that without pump pulses when pump pulses are added at different time points τ, and a local magnified image is shown. The reflectivity at the operating frequency point and the complex amplitude of the pulses before and after time point τ without pump pulses are shown.

[0028] Figure 7 The value is τ = 54fs. Among them, (a) is a comparison of the time-domain reflection field (red) of the structure with the pump pulse added at τ = 54fs and the time-domain reflection field (blue) without the pump pulse added; (b) is a magnified view of the pulse; (c) is the corresponding reflectivity; and (d) is the complex amplitude of the pulse before and after τ = 54fs without the pump pulse added.

[0029] Figure 8 Let τ = 65fs. Among them, (a) is the comparison between the time-domain reflection field (red) of the structure with the pump pulse added at τ = 65fs and the time-domain reflection field (blue) without the pump pulse added; (b) is a magnified view of the pulse; (c) is the corresponding reflectivity; and (d) is the complex amplitude of the pulse before and after τ = 65fs without the pump pulse added.

[0030] Figure 9 Let τ = 78fs. Among them, (a) is the comparison between the time-domain reflection field (red) of the structure with the pump pulse added and the time-domain reflection field (blue) without the pump pulse added when τ = 78fs, (b) is a magnified view of the pulse, (c) is the corresponding reflectivity, and (d) is the complex amplitude of the pulse before and after τ = 78fs without the pump pulse added.

[0031] Figure 10 Let τ = 94fs. Among them, (a) is the comparison between the time-domain reflection field (red) of the structure with the pump pulse added and the time-domain reflection field (blue) without the pump pulse added when τ = 94fs, (b) is a magnified view of the pulse, (c) is the corresponding reflectivity, and (d) is the complex amplitude of the pulse before and after τ = 94fs without the pump pulse added.

[0032] Figure 11 The value is τ = 109fs. Among them, (a) is a comparison of the time-domain reflection field (red) of the structure with the pump pulse added at τ = 109fs and the time-domain reflection field (blue) without the pump pulse added; (b) is a magnified view of the pulse; (c) is the corresponding reflectivity; and (d) is the complex amplitude of the pulse before and after τ = 109fs without the pump pulse added.

[0033] Figure 12The curve is a fitted curve of reflectivity R as a function of pump introduction time τ, with the pump introduction time τ as the x-axis coordinate and reflectivity R as the y-axis coordinate. The dashed line corresponds to the time point when the pump pulse is introduced. Detailed Implementation

[0034] The present invention will be further described below with reference to specific embodiments.

[0035] In a specific example of this invention, a two-dimensional photonic crystal is used, such as... Figure 1 As shown, this includes 20 unit cells in the x-direction with a lattice constant of a = 270 nm. The blue portion represents the nonlinear background material with a dielectric constant of ε = ε. l +χ (3) |E| 2 +iε i , where ε l =3.4 is the linear dielectric constant, χ 3 =-10 -12 cm 2 W -1 These are third-order nonlinear coefficients, with the imaginary part ε. i ≤0.0015, the material absorption is negligible, the white part represents air pores with radius r = 0.2a, the signal pulse is a Gaussian pulse with center frequency f0 = 3 × 10⁻⁶. 14 The pulse is a Gaussian pulse with a full width at half maximum (FWHM) of 15 fs, incident from the left interface of the photonic crystal. The pump pulse is also a Gaussian pulse with a full FWHM of 5 fs, uniformly irradiating the photonic crystal from the z direction.

[0036] Measure a certain frequency range (2.5*10) 14 Hz to 3.5*10 14 Transmittance and reflectance within Hz, such as Figure 2 As shown, the middle curve (red) represents the transmission spectrum, the lower curve (blue) represents the reflection spectrum, and the upper curve (blue) represents the sum of the two.

[0037] By adjusting the air hole radius of the 10th and 11th unit cells in the x-direction of the photonic crystal to 0.5a, photonic crystal defects are formed, serving as an ultrafast all-optical switch structure, such as... Figure 3 As shown.

[0038] Figure 4 The graph shows the transmission / reflection spectra of the ultrafast all-optical switch structure. The middle curve (red) represents the transmission spectrum, the lower curve (blue) represents the reflection spectrum, and the upper curve (blue) represents the sum of the two. The operating frequency is marked in the figure. A high transmittance peak appears within the bandgap in the transmission spectrum (a low reflection valley appears in the reflection spectrum), and the defect state corresponds to a frequency of 2.96 × 10⁻⁶. 14 Hz, set as the operating frequency.

[0039] Figure 5 This is the reflection spectrum of an ultrafast all-optical switch structure. The operating frequency point is marked in the figure, at which the reflectivity R is approximately 7%; this state is denoted as a standard "off" state.

[0040] Figure 6 This is the time-domain reflection field of the ultrafast all-optical switch structure. The dashed lines indicate the five selected time points τ for introducing the pump pulses, which are 54fs, 65fs, 8fs, 94fs, and 109fs.

[0041] Figures 7-11 The time-domain field of the ultrafast all-optical switch structure is compared with that without a pump pulse when a pump pulse is applied at different time points τ, along with magnified local images. The reflectivity at the operating frequency and the complex amplitude of the pulses before and after time point τ without a pump pulse are also presented.

[0042] Figure 7 (a) is a comparison of the time-domain reflection field (red) of the structure with the pump pulse added at τ=54fs and the time-domain reflection field (blue) without the pump pulse added. (b) is a magnified view of the pulse. (c) is the corresponding reflectivity. (d) is the complex amplitude of the pulse before and after τ=54fs without the pump pulse added.

[0043] Figure 8 (a) is a comparison of the time-domain reflection field (red) of the structure with the pump pulse added at τ=65fs and the time-domain reflection field (blue) without the pump pulse added. (b) is a magnified view of the pulse. (c) is the corresponding reflectivity. (d) is the complex amplitude of the pulse before and after τ=65fs without the pump pulse added.

[0044] Figure 9 (a) is a comparison of the time-domain reflection field (red) of the structure with the pump pulse added at τ=78fs and the time-domain reflection field (blue) without the pump pulse added. (b) is a magnified view of the pulse. (c) is the corresponding reflectivity. (d) is the complex amplitude of the pulse before and after τ=78fs without the pump pulse added.

[0045] Figure 10 (a) is a comparison of the time-domain reflection field (red) of the structure with the pump pulse added at τ=94fs and the time-domain reflection field (blue) without the pump pulse added. (b) is a magnified view of the pulse. (c) is the corresponding reflectivity. (d) is the complex amplitude of the pulse before and after τ=94fs without the pump pulse added.

[0046] Figure 11 (a) is a comparison of the time-domain reflection field (red) of the structure with the pump pulse added at τ=109fs and the time-domain reflection field (blue) without the pump pulse added. (b) is a magnified view of the pulse. (c) is the corresponding reflectivity. (d) is the complex amplitude of the pulse before and after τ=109fs without the pump pulse added.

[0047] according to Figure 7 , Figure 8 , Figure 9 , Figure 10 , Figure 11 The results were plotted with the pump introduction time τ as the x-axis and the reflectivity R as the y-axis, as shown below. Figure 12 As shown, the dashed line in the curve corresponds to the time point when the pump pulse is introduced. It is found that when the pump pulse is added at τ = 78fs, the reflectivity R at the operating frequency is 70%. This state is recorded as a standard "on" state.

Claims

1. A design method for a femtosecond / attosecond all-optical switch based on photonic crystal defect states, characterized in that, The specific steps are as follows: Step 1: Select the photonic crystal lattice structure and material parameters; the length of the photonic crystal structure is from a dozen to several hundred cells. Calculate or measure the transmittance and reflectance within a certain frequency range to determine the photonic bandgap of the photonic crystal, i.e., the low-transmittance radio frequency band or the high-reflection frequency band. The photonic bandgap is selected in the visible light band, ultraviolet and infrared electromagnetic wave bands. Step 2: By adjusting the structural or material parameters of a single cell or multiple cells in the photonic crystal, defects are formed in the photonic crystal. Defect states are found within the photonic bandgap of the photonic crystal, i.e., high transmittance peaks in the transmission spectrum or low reflectance valleys in the reflection spectrum. The frequency corresponding to the defect state is selected as the working frequency. At this time, the reflectance R is a small value, i.e., R is less than 10%. Step 3: Assume that the material at the defect, or the material at the defect and part / all of the photonic crystal, is a nonlinear material. This nonlinear material will change its dielectric constant or magnetic permeability constant under external optical pumping. The nonlinear relaxation time of this nonlinear material is in the femtosecond or sub-femtosecond range. Step 4: Use the defective photonic crystal structure with nonlinear material as the structure of an ultrafast all-optical switch; Step 5: Determine the parameters of the signal pulse and pump pulse; the duration of the signal pulse and pump pulse is from several hundred attoseconds to ten femtoseconds. Combining the nonlinear relaxation time in Step 3, it is inferred that the duration of the nonlinear effect caused by the pump pulse is on the order of several hundred attoseconds to twenty femtoseconds. The duration of this nonlinear effect is much shorter than the dwell time of the working frequency light in the switch structure. Step 6: When the signal pulse is incident on the all-optical switch structure and during transmission and reflection, assuming that a pump pulse is introduced at a selected time point to irradiate the defect and the part of the photonic crystal containing nonlinear material, record the corresponding time-domain reflection field and obtain the reflectivity at the working frequency under this condition. Step 7: Change the time point of the pump pulse introduction. For each time point τ of the pump pulse introduction, repeat the operation of step 6 to obtain a reflectivity R at the working frequency. Plot a curve with the pump introduction time τ as the x-axis coordinate and the reflectivity R as the y-axis coordinate. Step 8: According to the curve, it can be found that at "certain time points when pump pulses are introduced", the reflectivity of the working frequency increases significantly. Determine the time when the pump pulse with the highest reflectivity is added, and record this state as a standard "on" state. Record the low reflectivity state "without pump pulses" as a standard "off" state. The reflectivity described in steps 1, 6, and 7 is determined by R = |Er(ω) Ei(ω)| 2 The calculations show that Ei(ω) and Er(ω) are the complex amplitudes of the frequency domain field, obtained from the Fourier transforms of the incident field Ei(t) and the reflected field Er(t) in the time domain.

2. The design method according to claim 1, characterized in that, The photonic crystal lattice type mentioned in step 1 is a square lattice.

3. The design method according to claim 1, characterized in that, The unit cell of the photonic crystal structure described in step 1 is an air-pore structure with the medium as the background material.

4. The design method according to claim 1, characterized in that, The photonic crystal material parameters mentioned in step 1 also include the dielectric constant.

5. The design method according to claim 1, characterized in that, For each time point τ when the pump pulse is introduced as described in step 7, a corresponding time-domain reflection field Er(t) can be obtained. The complex amplitude Er(ω) of the reflection field is obtained by Fourier transform, thereby obtaining the reflectivity R(τ) of the working frequency under the condition of "introducing pump pulse time point τ".

6. A femtosecond / attosecond all-optical switch based on photonic crystal defect states, obtained by the design method described in any one of claims 1-5.

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