Multifunctional Logic Gates based on Resonant Transmission at Atomic-Plasmonic structure

doi:10.21203/rs.3.rs-1639147/v1

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Multifunctional Logic Gates based on Resonant Transmission at Atomic-Plasmonic structure

https://doi.org/10.21203/rs.3.rs-1639147/v1

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Regarding confinement of light at nanoscale dimensions in plasmonic structures, we try to show the impact of hot atomic vapor spectroscopy in a miniaturized scale. In such a combined structure, resonant coupling of atom to plasmonic mode provides diverse ways to control the optical response of the system. We fabricate an atomic plasmonic cell based on Rubidium atomic vapor and gold plasmonic thin film onto Krestchman setup to introduce resonant coupling (EIT-like) of atom- plasmons as a tunable all optical bandpass filter, switch or logic gates. These all-optical devices such as as NOR and XNOR logic gates well done based on the filter by incidence angle of light, temperature as well as external magnetic field. We believe the possibility of easy modulation of atomic susceptibility, not only through direct alteration on atoms but also through common methods available for modulation of plasmonic mode, has the potential to design and fabricate modern all-optical devices.

Resonant transmission (EIT-like)

Atomic-plasmonic

Fano resonance

Bandpass filter

Logic gate.

The idea of integration of atom as lab on a chip in miniaturized scale has attracted much interest over recent years to use them in appliances such as quantum computing, miniaturized quantum sensors, atomic clocks, etc. [1–5]. These studies cover effective control and optimal readout of atoms in micro or nano structures by in decreased volumes by optical near field of miniaturized structures [6][7][8]. Emergence of plasmonics with the possibility of confinement of light in conductive nano structures makes different intended and achievable photonic modes [9][10]. Photonic plasmonic modes have revealed many different properties, including complicated polarization state of near field as well as drastic enhancement of electromagnetic field near the surface. This makes surface plasmons (SP) a good candidate to all optically control and read out of atoms, thanks to large gradient potential of plasmonic modes in nano structures [11][12]. Touch of atoms by near field of SP’s gives rise to coupling of narrow discrete states of cooled and trapped or hot vapor of atoms with continuum state plasmonic resonator with different damping rates. The optical response of hybrid atomic-plasmonic system such as Fano resonance is strongly dependent on properties of both coupled oscillators [13][14].

Fano resonance has been well studied in optical systems with multiple resonant modes and found more useful applications in sensing and switching fields and especially in atomic plasmonic area [15][16]. In this study, we investigate the tunability of Fano resonance of atomic-plasmonic system and introduce this coupled system as a paradigm for actively controllable bandpass line filter and atomic multichannel logic gates and switches.

Furthermore, in our atomic plasmonic systems, narrow optical atomic bandpass filters work based on induced transmission in atomic transition lines [17]. These narrow line filters are required in atmospheric LIDAR [18], laser communication [19], quantum optics [20] or logic processing which is not possible without dynamic switching [21]. This means that having a filter with continuous and fast tunability with the possibility for integration on miniaturized solid-state devices will be especially important in the above-mentioned applications. Numerous types of physical mechanisms would be used to make narrow line atomic band pass filters. Well-known atomic line filters and switches are made based on magneto optical Faraday and Voigt atomic filters [22] or electromagnetically induced transparency (EIT) [23][24]. Most reported results of EIT in atomic vapors have been acquired using free space pump and probe optical fields accurately tuned to be coupled to specified quantum states of atoms. Here, we deduce physical similarities between EIT and atom-plasmon coupled resonant transmission (EIT-like) phenomena to propose a tunable bandpass filter and possibility to use a hybrid structure as a logic gate. In our designed setup, a change of plasmonic mode energy will control the transmittivity of filter and this technique would be extended rapidly considering many tunable plasmonic structure designs available [25]. Also, as advantage of hybrid structure disturbance on atoms such as strength and frequency of ac external magnetic field could be manipulated to resonantly control the transmitted light between on and off states.

To study the reflection spectrum of coupled atomic-plasmonic modes, a combined structure of an Rb vapor cell and plasmonic substrate was fabricated. We designed a vapor cell with a coated prism as the optical window to guarantee direct touch between plasmonic near field and Rb atoms. For this purpose, glass prism was sputter-coated 35nm of Au and to avoid direct contact and chemical reaction between Rb vapor and Au layer, 5 nm TiO₂ was deposited on top of Au thin film. The combined atomic vapor cell was made by epoxy glue bonding of the finely polished ended borosilicate pipe on top of the prism, such that the coated part would be in the interior part of the cell. Then, hot vapor of Rb metal was injected into the cell and the end of the cell was terminated by a fire torch. To achieve the proper density of atomic vapor, the cell heated up to 85 ^oC. The heater was designed in a way to make a temperature gradient on the cell to prevent the solidification of Rb metal on the prism.

To observe the resonant coupled transmission, we used the setup displayed in Fig. 1. Initially (part 1 as brown colored), we adjusted the incidence angle of distributed feedback (DFB) diode laser light onto the coated prism windows of Rb cell to excite SP wave. The DFB diode laser at the D₁ line of Rb was frequency-modulated through driving current modulation. The laser light was reflected from the gold thin film-Rb atoms interface after passing through the Glen-Taylor prism. Then, the laser wavelength was finely scanned across the hyperfine structure to make a highly coherent plasmonic field interacting with the quantum states of rubidium atoms (As shown in Fig. 1).

The reflected light was captured by a photodetector connected to a lock-in amplifier. The lock-in amplifier was locked on the frequency of laser driving current and the intensity variation of the reflected light was recorded on a computer through a LabVIEW program while sweeping the laser wavelength. During part 1 of measurements and to make the hybrid atomic-plasmonic structure tunable, a rotation stage was employed to rotate the incidence angle corresponding to the SP resonance angle. We had sweep from − 0.5 degree (before resonance) to 0 degree (SP resonance) and 0.5 degree (after resonance).

In part 2 (green colored devices) of our experiment, we tried to investigate the effect of ac magnetic field on coupled structure, a 500-turn coil with a diameter of 13 cm, and ac magnetic field of about 2.2 mT were applied perpendicular to the SPP propagation direction.

Figure 2 reveals the reflection spectrum of our hybrid structure while SP’s touched the Rb atoms with modal energy equal to Rb’s hyperfine transitions energy. Based on the large damping rate of plasmons, we neglect changes of SP’s energy in narrow range of Rb D1 hyperfine transitions. The results clearly show transmission of incoming light in wavelength detuning equal to any atomic hyperfine transition. The fact is, all incoming photons after passing the prism windows couple to plasmonic mode and get damped in different channels especially by ohmic damping in metal film. The transmitted photons of our structure scape plasmonic damping by resonant coupling to hyperfine transitions of atoms. The physical behavior of resonance in coupled atom-SP system proves resonant transmission (EIT-like) regime of our proposed hybrid structure acts on EIT based filters platform for a bandpass filter with FWHM of line, equal to atomic transitions. Also, we believe any physical change leading to sensible decrease in line width of atomic transmissions, such as selective reflection, or plasmonic saturation absorption would be utilized alongside SP’s to achieve a narrower bandpass filters, while taking advantage of coupling paradigm.

To analyze tunability of the combined behavior of coupled atom-SPP transmittivity, we performed spectroscopic measurements in two steps; first under a shift of SPP mode energy and second by modulating quantum state of Rb atoms by applying an ac external magnetic field. To change the energy of SP’s formed at metal-dielectric interface in Kretschmann configuration, the easiest way is altering the angle of incidence of laser light (supporting information S1-S2). Figure 2)a) represents measurements done in an all fixed experimental conditions where only angles of incidence of photons changed around SP resonance angle. Figure 1(b) clearly shows a sharp change of results of coupled system by changes in incidence angle for wavelength equal to 5²S_1/2 -5²P_1/2 atomic transitions of ⁸⁵Rb isotope. While a change of angle of incidence of laser light alters energy of SP, the coupling regime turns from Fano to EIT-like and vice versa. We would conclude that behavior of filter is changing from a band pass filter to an absorbing line filter. It happens because a major change in incidence angle causes overthrow of plasmonic mode and optical nearfield turns from SP to evanescent field of dielectric prism, so commonly atomic absorbance take place as is obvious in the last row of data.

SP modulating mechanisms would be used to tune atom plasmon coupling to exploit a hybrid device like the proposed filter. This includes variety of engineered plasmonic structures available in published papers [26]. In addition to the effect of change in angle as presented, we examined the effect of increase in temperature of measurement examinate possible ways on single Au thin film. We compare the result of measurement in 85 and 105 degree Celsius. Harsh increase in temperature actually eliminates condition of coupling of atom-SP’s due to increases in Ohmic damping of Au film. In response, the rate of energy damping of photons (polaritons) at SP mode overtakes the rate of energy exchange between atom and SP. With elimination of associated optical susceptibility of coupled structure, optical response of atoms to evanescent wave of total reflection from prism became dominant and resonant transmission behavior disappeared due to breakdown of coupling condition. (Supporting information, page S-3)

As explained in the experimental part, incidence angle to atom-plasmonic cell clearly shows resonant transmission of photons from coupled structure by all possible Rb atom transitions. Transmittivity is directly proportional to amplitude of both resonances specially, atomic hyperfine transition probability. By considering multichannel lines with different levels of transmittivity caused by Rb atoms, we propose logic behavior of atom-SP coupling.

We have sweep from − 0.5, -0.27, -0.17 degree (before resonance) to 0 degree (SP resonance) and 0.08, 0.17, and 0.29 degree (off SP resonance). The results show EIT-like transmission at SP resonance angle and asymmetric transmission line shape known as Fano resonance at off resonance angles. Figure 3(a) depicts the normalized reflectance at off resonance to the SP resonance results for different incidence angles. To investigate possibility of multichannel switching, we consider any single D₁ hyperfine absorbance lines of Rb atom as a distinct gate. Once can see four blue and red wavelength windows defined on results (Fig. 3(a)), we would assign 0 or 1 logic values depending on reflected intensity value in each of windows. Comparison of our results in Fig. 3(b) shows 1010 logic stream for after resonance and 0101 logic value before resonance angles. This change drifts us to introduce atom-SP hybrid structure as an optical switch which would alter the state of channels only by a change on incidence angle about SP resonance angle.

We strongly believe SP based logic switching paradigm have the potential to be easily controlled by external commands especially, electric current or voltage if photonic mode characteristics get mixed with available controllable plasmonic material designs. Here to investigate our proposed gate performance, we define systems inputs as incidence angles and the wavelength detuning. Figure 4 reveals a result of Heat Map, in which, normalized angle and wavelength are the first and second inputs, and reflectance intensity in four windows are output of any channel. We supposed incidence angles in range of [-0.4, 0.4] degree as ( 0 ) logic number and all other off range angles as (1) logic number.

In addition, the frequency detuning lines ν < 2.3 GHz, supposed to be (0) and distinct lines as (1) logic numbers. In this way, we would construct four 00,01,10, and 11 logic input states, as is done in Fig. 4. The results show that our hybrid structure would act as XNOR or NOR logic operator only by switch on frequency of incoming light and apply a change on SP mode energy.

As is expected from coupled structures behavior of both sides is effective in behavior of hybrid structure. Thus far, we have shown the effect of changes on SP mode in coupled susceptibility of atom- SP coupled media. Any disturbance in quantum states of Rb atom alters the operation of the coupled behavior too.

Applying a magnetic field breaks up the degeneracy of hyperfine levels and split them into Zeeman sublevels according to their quantum number, the strength and frequency of the applied magnetic field can play the role of the control signal in the proposed structure.

In our case, a linearly polarized light was optically pumped the Zeeman sublevels, while the magnetic field was applied perpendicularly to the propagation direction of the SP’s. Implementing frequency modulation technique, Rb hyperfine transitions for Rb D₁ line were completely resolved. However, due to the Doppler broadening, at this strength of the magnetic field, the allowed transitions between large numbers of magnetic sublevels could not be spectrally distinguished. So, instead of applying an external DC magnetic field, we have studied the transmittivity of the coupled atomic-plasmonic system, under the application of ac magnetic field, at a fixed wavelength of incoming light.

As the Zeeman splitting of each hyperfine line was related to the strength of the applied magnetic field, the transmitted light would be modulated and followed the changes of the ac magnetic field by the same frequency, but with a small phase shift. Our results also show the possibility of the change in transmittivity of our proposed gate by a variation in the amplitude of the applied ac magnetic field. As displayed in Fig .5(a), by a red dot, 0 logic number could be switched to 1 logic number by variation of the strength of the magnetic field. The results exhibited a linear change in the modulated light intensity related to the ac applied magnetic field strength (as shown in Fig. 5(b)). This figure indicated the sensitivity of magnetic field measurement in the order of 0.62 V/T.

Considering the impact of integration of atomic-photonic operations on solid state devices we tried to show advantage of coupling of atomic-plasmonic structures. Our experimental results show tunability of a combined atomic-plasmonic susceptibility is an in-hand candidate to define switchable atomic line filters. Coupled narrow bandpass line filters would be easily turned on and off by change of plasmonic mode energy. Also, we show our proposed filters take advantage of modulation of transmittivity by applying an external magnetic field. In completion of studies on bandpass filter, we introduced multifunctional logic operation based on atomic-plasmonic transmittivity. We believe the mix of fascinating applications of atomic hot vapor spectroscopy with widespread human knowledge on plasmonics would be strongly beneficial.

Acknowledgments

Authors would like to acknowledge Photonics, Laser, Advanced Materials and Manufacturing Technologies Development Council of Iran for valuable supporting. M. Ranjbaran would like to acknowledge Iran National Science Foundation (INSF) under grant number 97023023.

The authors declare no conflicts of interest.

Data availability statement

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.

Author contributions:

M. Mosleh did the optical measurement, analyzed the results and wrote the main text of the manuscript. M. Ranjbaran and S. M. Hamidi supervise the measurement part and writing the results of the study. All authors read and approved the final manuscript.

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No competing interests reported.

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Multifunctional Logic Gates based on Resonant Transmission at Atomic-Plasmonic structure

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Abstract

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I. Introduction

Ii. Material And Methods

Iii. Results And Discussion

Iv. Conclusion

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