1 © 2014 Astro Ltd Printed in the UK

1. Introduction

All-optical switching is necessary to achieve ultrafast and ultrahigh bandwidth information processing. The basic chal-lenge is to design an ultrafast energy-efficient all-optical switch that exhibits high contrast, low-power operation, along with photo and thermal stability and flexibility to reconfig-ure and tune its characteristics [1–3]. The prospect of using energy-efficient natural photoreceptors optimized over cen-turies of evolution to meet these requirements is extremely fascinating.

The unique photochromic retinal protein BR found in the purple membrane fragments of Halobacterium salinarum has emerged as an outstanding material for biomolecular photonic

applications due to its unique properties and advantages. It exhibits charge transport, photochromic and photoelectric responses, high quantum efficiency of converting light into a state change (~64%), large absorption cross-section and nonlinearities, ability to form thin films in polymers and gels, robustness to degeneration by environmental perturbations and flexibility to tune its kinetic and spectral properties by physical, chemical and genetic engineering techniques for device applications [4]. It also exhibits high photo and thermal stability over a wide pH range (0–12). BR molecules in gels and polymer thin films are extremely stable, maintaining their functional activity for several years without any degradation. No noticeable change has been observed in a BR film even after switching more than a million times [4]. In addition,

Laser Physics Letters

All-optical sub-ps switching and parallel logic gates with bacteriorhodopsin (BR) protein and BR-gold nanoparticles

Sukhdev Roy and Chandresh Yadav

Department of Physics and Computer Science, Dayalbagh Educational Institute, Dayalbagh, Agra 282 005, India

E -mail: sukhdevroy@dei.ac.in

Received 25 January 2014, revised 19 September 2014Accepted for publication 25 September 2014Published 23 October 2014

AbstractWe propose a model for the early sub-picosecond (sub-ps) transitions in the photochromic bacteriorhodopsin (BR) protein photocycle (B570 → H → I460 → J625 → B570) and present a detailed analysis of ultrafast all-optical switching for different pump–probe combinations. BR excitation with 120 fs pump pulses at 570 or 612 nm results in the switching of cw probe beams at 460 and 580 nm exhibiting reverse saturable absorption (RSA) and saturable absorption (SA) respectively. The effect of pump intensity, pump pulse width, lifetime of I460 state, thickness and concentration on switching has been studied in detail. It is shown that low intensity (MW cm−2), high contrast (100%), sub-ps all-optical switching can be achieved with BR-gold nanoparticle solutions. The validity of the proposed model is evident from the good agreement of theoretical simulations with reported experimental results. The switching characteristics have been optimized to design ultrafast all-optical parallel NOT, OR, AND and the universal NOR and NAND logic gates. High contrast, ultrafast switching at relatively lower pump intensities, compared to other organic molecules, opens up exciting prospects for ultrafast, all-optical information processing with BR and BR nano-biophotonic hybrid materials.

Keywords: ultrafast nonlinear optics, optical logic, all-optical devices, all-optical switching, bacteriorhodopsin, nanomaterials, ultrafast information processing

(Some figures may appear in colour only in the online journal)

S Roy and C Yadav

Printed in the UK

125901

lPl

© 2014 Astro ltd

2014

11

laser Phys. lett.

lPl

1612-2011

10.1088/1612-2011/11/12/125901

Letters

12

laser Physics letters

Astro Ltd

HC

1612-2011/14/125901+8$33.00

doi:10.1088/1612-2011/11/12/125901Laser Phys. Lett. 11 (2014) 125901 (8pp)

S Roy and C Yadav

2

BR can retain its folded native structures up to temperatures as high as 140 °C when incorporated in multilayer structures of self-assembled ordered dry films [5]. A wide range of applications has been proposed for BR that includes various aspects of information processing such as optical switching, 3D memories and ultrafast light detection, as well as desali-nation of sea water, energy conversion, artificial retinas and chemo-and biosensing [4]. Recently, efficient integration of BR with semiconductor quantum dots and metal nanoparticles has been shown to enhance nonlinear optical and photovol-taic responses for its applications in hybrid nano-biosystems based bioelectronic and biophotonic devices [6, 7].

By absorbing green–yellow light, the wild-type BR mol-ecule undergoes several conformational transformations in a complex photocycle that generates a number of intermediate states, spanning the entire visible spectrum: B570 + hν → J625 (~ps)  →  K610 (~μs)  →  L550 (~μs)  →  MI

410 (~μs)  →  MII410

(~ms) → N520 (~ms) → O640 (~ms) → B570, where subscripts denote the respective peak absorption wavelength in nm. All-optical switching with BR has been extensively studied based on the important feature that all the intermediates can be pho-tochemically switched back to the initial B-state by shining light at a wavelength near the absorption peak of the inter-mediates [4, 8–11]. Various designs of all-optical logic gates have been proposed with BR based on its photochromic prop-erties that include sequential photoexcitation, two-wave mix-ing, degenerate four-wave mixing (DWFM), photoinduced dichroism and birefringence, complementary modulated sup-pression transmission (CSMT), degenerate multiwave mix-ing (DMWM) and nonlinear intensity-induced excited-state absorption [12–16]. However, the main limitation so far has been the speed of operation (~ms–ns) due to switching involv-ing either the spectrally shifted longer lifetime M410 and O640 intermediates or the earlier K610 intermediate [4, 8–13, 17]. All-optical switching and arithmetic logic operations have been exhibited with BR-coated high-Q silica microresonators with μs switching response [10, 15]. Fabian et al (2011) have recently demonstrated ultrafast switching based on a shift in the frequency of a probe laser beam with a BR coated 1D pho-tonic crystal on a thin film waveguide [18].

BR is a model system to study the signal transduction path-ways on light absorption in natural photoreceptors. Recent femtosecond (fs) spectroscopic characterization of the ini-tial trans-cis isomerization in BR-doped polymer thin films at 640 nm and 790 nm at peak pump intensity ~700 GW cm−2 has revealed the formation of the F540 state due to simultaneous two-photon absorption (TPA) induced transition as the excitation pulse duration of 120–130 fs is much less than the formation time of the first intermediate J625 (~500 fs [19, 20]). This thermally sta-ble photoproduct exhibits strong anisotropic absorption that has also been shown to facilitate polarization based write-once-read-many (WORM) optical data storage [19, 20]. Recent studies on coherent control of the all-trans to 13-cis isomerization of retinal in BR liquid samples have shown that TPA induced transitions can occur at high peak intensities (≥ 200 GW cm−2 [21]).

Excitation with fs pulses around 570 nm has revealed an additional spectrally distinct I460 state before the J state: B570 + hν  →  HFC (~fs)  →  I460 (FS) (<ps)  →  J625 (<ps)  →  B570.

Although detailed experiments have been performed to ascer-tain the signal transduction pathways and characterization of these states, a complete picture is yet to emerge. HFC denotes the Franck–Condon excited state of BR with an all-trans retinal Schiff base and I460 denotes the fluorescent state (FS), which is characterized by excited-state absorption around 460 nm and a stimulated emission band around 860 nm [18, 22–26]. Preliminary results on all-optical switching based on a simplified three-state model using B570 ↔ I460 transitions of the BR photocycle were presented recently [27]. A 570–460 nm pump-probe combination was analyzed, which resulted in RSA that was used to design all-optical NOT, NOR, and NAND logic gates. In this paper, we present a more rigorous analysis of ultrafast all-optical switching with BR and BR-gold nanoparticle solutions based on all the ultrafast transitions (B570 → H → I460 → J625 → B570) in a four-state model, consid-ering both SA and RSA at different wavelength combinations and optimize it to design all-optical NOT, OR and AND, as well as the universal NOR and NAND logic gates.

2. Theoretical model

We introduce the simplified energy level diagram shown in figure  1 to represent the ultrafast early transitions B570 →H → I460 → J625 →B570 of the BR photocycle, which enables adoption of the simple rate-equation approach for the population densities in the various intermediate states. We consider BR molecules exposed to a pump beam of inten-sity Im, well below the level at which multiphoton processes arise. It modulates the population densities of different states through the excitation and de-excitation processes, described by the following rate equations,

σ ψ

νσ

νσ

ν τ τ= − + + + +dN

dt

N

h

N

h

N

h

N NI I IB B B BH J H

HB

I

IB

m m I m JI

(1)

σ ψ

ν τ τ= − −dN

dt

N

h

N NIH B B BH H

HB

H

HI

m

(2)

σ

ν τ τ τ= − + − −dN

dt

N

h

N N NII H

HI

I

IJ

I

IB

m II

(3)

σ

ν τ= − +dN

dt

N

h

NI I

IJ

J m JJ

(4)

Figure 1. Simplified energy-level diagram for sub-ps transitions in BR.

Laser Phys. Lett. 11 (2014) 125901

S Roy and C Yadav

3

where NB, NH, NI, and NJ are the population densities of the B, H, I and J states, respectively, σB, σI and σJ are the absorp-tion cross-sections of the B, I and J states at modulating pump wavelength, respectively, ψBH is the quantum efficiency for the B → H transition and τHB, τIB, τHI and τIJ are the relaxation times for non-radiative transitions.

Considering optically thin BR samples, the propaga-tion effects on the modulating light beams can be neglected. Gaussian modulating pump laser pulse is given by

⎜ ⎟⎛⎝⎜

⎛⎝

⎞⎠

⎞⎠⎟

Δ=   − −

I I ct t

texpm m

m0

2

(5)

where c = 4ln2 is the pulse profile parameter and ∆t is the pulse width, which modulates the propagation of a cw probe laser beam of intensity Ip (Ip <<  Im) through the sample governed by (dIp/dx) = −αp(Im)Ip, where x is the distance in the medium and αp is the absorption coefficient at probe wavelength, which is NI(Im)σIp for 460 nm and NB(Im)σBp + NJ(Im)σJp for 580 nm probe beams, with σIp, σBp and σJp as the absorption cross-sections at the probe wavelength.

3. Results and discussion

All-optical switching characteristics, namely the varia-tion in the normalized transmitted probe intensity (NTPI)

with time, have been computed using equations  (1)–(5), considering the spectroscopic data and experimental condi-tions, i.e., σB  =  2.4   ×  10−16 cm2, σI  =  0.22   ×  10−16 cm2 and σJ  =  1.8   ×  10−16 cm2 at 570 nm; σB  =  2.34   ×  10−16 cm2 and σJ  =  1.98   ×  10−16 cm2 at 580 nm; σB  =  1.22   ×  10−16 cm2, σI = 0.286  ×  10−16 cm2 and σJ = 1.91  ×  10−16 cm2 at 612 nm; and σI =1.8  ×  10−16 cm2 at 460 nm; τHB = 30 fs, τHI = 100 fs, τIJ  =  500 fs and ψBH  =  0.64, ∆t  =  120 fs; film thickness L  =  30 µm; and concentration C  =  28 µM, τIB  =  0.63τIJ and Im0 = 0.5–40 GW cm−2 [4, 22–26].

We consider three pump-probe combinations, correspond-ing to the experimental results reported earlier, namely (i) 570–460 nm [23–25], (ii) 612–460 nm [22] and (iii) 612–580 nm [22]. The variation in NTPI at 460 nm with time in BR thin films for cases (i) and (ii) is shown in figure 2(a). For case (i), NTPI decreases and the percentage modulation increases with increase in Im0, which results in increased absorption of the probe beam at 460 nm that saturates after a particular value. This is evident from the corresponding variation in the normal-ized population density in figure 2(b). At Im0 = 5.0 GW cm−2 and 40 GW cm−2, the probe beam gets modulated by 28.5% and 83%, respectively, with switch-off times of 0.2 and 0.23 ps and switch-on times of 0.86 and 1.1 ps, respectively. At the same intensity values, the percentage modulation for case (ii) is lower due to lower population build-up of I460 state by 612 nm pump pulse in comparison to the 570 nm (case (i)).

Figure 2. (a) Variation of NTPI with time at 460 nm for different Im0 values for cases (i) (solid lines) and (ii) (dashed lines), both with ∆t = 120 fs and C = 28 µM. (b) Variation of the normalized population densities for different states for case (i). (c) Variation of NTPI with time at 580 nm for different Im0 values for case (iii) at C = 5 µM. (d) Variation of the normalized population densities for case (iii).

Laser Phys. Lett. 11 (2014) 125901

S Roy and C Yadav

4

The variation of NTPI at 580 nm with time for case (iii) is shown in figure 2(c). In this case, the NTPI and the per-centage modulation increased with increase in Im0. For Im0 = 40 GW cm−2, the probe beam is modulated by 8.5% with switch off-on times of 0.7 and 0.16 ps, respectively. The cor-responding variation in the normalized population densities of different states with time is shown in figure 2(d). In this case, increase in Im0 leads to increased depletion of the B state, that results in less absorption at 580 nm and hence increased trans-mission. The switching contrast is lower in this case due to absorption by both B and J states. Theoretical simulations are in good agreement with experimental results reported earlier [22, 25].

The effect of the variation of Im0 on percentage modula-tion for each case (i, ii, and iii) is shown in figure 3. It is evi-dent from figure 3 that the percentage modulation increases much more rapidly for case (i) as compared to cases (ii) and (iii). For case (ii), the percentage modulation is less as the absorption of the initial B state is less at 612 nm. Hence, for the same percentage modulation, the pump power required for case (ii) is more in comparison to case (i). The varia-tion for case (iii) is very small. This is due to the pump and probe wavelengths being very near to each other as well as near to the peak value of the J state. The effect of variation of concentration on switching characteristics for cases (i, ii and ii) is shown in figure 3 (insets (a) and (b)). An increase in the concentration results in decreased transmission of the probe beam due to increased absorption by BR molecules in I460 state (case (i)). For case (ii), increase in the concentra-tion results in decreased transmission with lower percentage modulation due to lower population build-up of I460 state by 612 nm pump pulse in comparison to 570 nm. For case (iii), increase in the concentration also results in decreased trans-mission with lower percentage modulation in comparison to

cases (i) and (ii) due to absorption by both B and J states. At C = 380 µM, 100% modulation of the probe beam is achieved at Im0 = 5.0 GW cm−2 (case (i)). BR thin films can be easily fabricated with higher concentration. Increasing the concen-tration of the BR film leads to 100% switching at lower Im0 values. For instance, at C = 3 mM, the probe gets completely switched off at Im0 = 0.57 GW cm−2.

The effect of the variation of pump pulse widths (∆t) for cases (i) and (ii) on switching characteristics is shown in fig-ure 4 and for case (iii) in figure 4(a). The switch off-on time and percentage modulation increase with increase in ∆t, due to increased interaction time of the pump beam with BR mol-ecules. At ∆t = 120 fs, the switch off-on time is 0.23 and 1.0 ps with 28.5% modulation for case (i), whereas for case (iii), the switch off-on time is 0.56 and 0.16 ps with 5.4% modu-lation. As expected, the switching characteristics become more symmetric as ∆t ≥ τIB. The effect of variation of sample thickness on switching characteristics for cases (i) and (ii) is shown in figure 4 (inset (b)). An increase in thickness results in increased contrast of the transmitted probe beam due to increased absorption.

The life time of I460 state (τIJ) has been shown to vary from 500 fs to more than 1.1 ps by various techniques [23–25]. For instance, increasing pH results in an increase in τIJ from 0.45 to greater than 1.1 ps [23]. Recently, nano-bioengineering approaches employing colloidal semiconductor and metal nanoparticles conjugated with biosystems have opened up exciting possibilities for optimization of the photoresponse [6, 7, 24, 28]. The presence of quantum dots and metal nano-particles varies the life time of the BR photocycle, depending on the proportion of QDs/metal nanoparticles to BR. Variation in the photocycle can lead to optimized energy efficient high contrast all-optical switching with BR based hybrid nano-bio-systems. τIJ has been shown to increase to 800 fs by varying the concentration and size of gold nanoparticles (Au-NPs) due

Figure 3. Variation of percentage modulation with intensity for cases (i, ii and iii). Inset (a) shows the variation of NTPI at 460 nm with time for different concentrations at Im0 = 5.0 GW cm−2 for case (i) (solid lines) and for case (ii) (dashed lines). (b) Variation of NTPI at 580 nm with time for different concentrations at Im0 = 25 GW cm−2 for case (iii).

Figure 4. Variation of NTPI with time for different ∆t values for cases (i) (high modulation) and (ii) (low modulation) at Im0 = 5.0 GW cm−2. Inset (a) shows the variation of NTPI with time for different ∆t values for case (iii) at Im0 = 25 GW cm−2 and inset (b) shows the variation of NTPI with time for case (i) solid lines and case (ii) dotted lines at different thickness values (L) at Im0 = 0.5 GW cm−2, C = 28 μM and ∆t =120 fs.

Laser Phys. Lett. 11 (2014) 125901

S Roy and C Yadav

5

to the optically induced plasmonic field effect [24]. Recently, it has been reported that in the presence of the ionic sur-factants cetrimonium bromide (CTAB) and sodium dodecyl sulfate (SDS), the life time of the reactive excited-state I460 increases by up to 20% [25].

Au-NPs surface plasmon effects have been shown to affect the proton pumping process of the primary step of BR. Au-NPs absorb and scatter light strongly, which is useful in sens-ing, imaging and medical diagnostics. They rapidly convert the light absorbed into heat that is useful in many photother-mal applications, for instance, destroying cancer cells [28]. Excitation of localized surface plasmon oscillations generate plasmonic fields that decay with distance. For Au, Ag and Cu NPs, the resonance condition is fulfilled at visible frequencies. Plasmonic nanoparticles exhibit enhanced light scattering, absorption, luminescence and surface enhanced spectroscopy, and easy synthetic tunability makes them attractive for opti-cal applications. However, metal nanoparticles that provide plasmonic enhancement can lead to similar behaviour pro-vided their plasmonic fields have similar spatial and temporal response at desired frequencies. The surface plasmonic field is formed by both the pump and probing laser pulse intensity and decays in a few fs. The effect of the plasmonic field is to slow down the retinal isomerization in the primary step of BR photosynthesis. The plasmonic field affects both the protein and the retinal system. The concentration as well as size of the Au-NPs affect the life time of the fluorescent I460 state. A large charge distribution in the protonated Schiff base (PSB) takes place on the absorption of light by the retinal in BR, which induces large polarization on the protein cavity around the retinal. The potential energy surface of the retinal electronic system is thus affected by the oscillating plasmonic electric field. The large polarization change at the chromophore site also appears to affect protein structural changes. Increase in the number of NPs on the BR surface leads to enhancement of plasmon coupling and amplification of the effect. The aggre-gation of NPs and their size dependent accumulation on differ-ent sites on the protein lead to a stronger plasmonic field effect

[24]. The effect of increasing the concentration of BR-Au-NP solutions on switching characteristics considering experimen-tal values of Biesso et al (2008), i.e. with Imo = 1.2 MW cm−2 and ∆t = 100 fs, is shown in figure 5. The change in τIJ with concentration is shown in figure 5 (inset). Theoretical simu-lations match extremely well with experimental results that show 100% modulation and validate the accuracy of the pro-posed model [24].

To present the maximum bit rate possible with the switch-ing operation in BR, the effect of pump pulse frequency on the BR switching characteristics for case (i) (dashed lines) and for case (iii) (dotted lines), both with Imo = 40 GW cm−2 and τHI = 30 fs (considering the lower limit of τHI shown to vary from 30–100 fs [25]), is shown in figure 6. For case (i), the optimum pulse separation is 1.1 ps with 97% modula-tion, whereas for case (iii) it is 0.7 ps with 11.2% modulation, which leads to bit rates as high as 1.4 Tbits s−1. In the case of BR-Au-NPs solution (solid line), the optimum pulse separa-tion is ~1.0 ps with 100% modulation at Imo = 1.2 MW cm−2. This would be useful to enhance the speed and bit rate of the recently proposed all-optical computing circuits at very low power [15].

The switching characteristics corresponding to different probe beams can be used to design all-optical NOT and the universal NOR and NAND (figures 2(a) and 5), and OR and AND (figure 2(c)) logic gates, with multiple pulsed pump laser beams. Our simulations show that the percentage modu-lation increases with increase in Imo and saturates after a cer-tain value. Hence, there is an optimum value of Imo for which the difference between the maxima in the NTPI for single and double input pulses is maximum. Taking this into account, for OR and AND logic gates, amplitude modulation of the cw probe laser beam at 580 nm (case (iii)) is considered as output (figure 7(a)) and the two pulsed pump laser beams each with Imo = 40 GW cm−2, τHI = 30 fs and ∆t = 120 fs at 570 nm are considered as the two inputs, as shown in figure 7(b).

Increase in NTPI at 580 nm for single and multiple pulses correspond to the all-optical OR logic gate, as the output is high when either one or both the input pulses is present and is low only when none of the pulses are present, as shown in fig-ure 7(a). The same configuration also results in an all optical

Figure 5. Effect of the concentration of 40 nm Au-NPs on BR switching characteristics at Im0 = 1.2 MW cm−2, L = 1 mm and ∆t =100 fs. (i) C = 1.3  ×  10−11 M (τIJ = 515 fs); (ii) C = 2.5  ×  10−11 M (τIJ = 720 fs)’; and (iii) C = 3.8  ×  10−11 M (τIJ = 800 fs). Inset shows the effect of concentration of Au-NPs on the decay kinetics of τIJ [24].

Figure 6. Effect of pulse frequency for BR thin films for cases (i) (dashed lines) and (iii) (dotted lines), both with Im0 = 40 GW cm−2, τHI = 30 fs and ∆t = 120 fs and for BR-Au-NPs (C = 1.3  ×  10−11 M) solid line with Im0 = 1.2 MW cm−2 and ∆t = 100 fs.

Laser Phys. Lett. 11 (2014) 125901

S Roy and C Yadav

6

AND logic gate by considering a threshold level, shown by the solid line in figure  7(a). In this case, the output is low when either one or none of the pulses is present and high only when both the input pulses are present simultaneously.

Amplitude modulation of the cw probe laser beam at 460 nm for two input pulsed pump laser beams with Im0 = 5.0 GW cm−2 and 0.4 MW cm−2 for case (i) for BR thin films with ∆t = 120 fs and τHI = 30 fs and for BR-Au-NP solu-tions with ∆t = 100 fs and τHI = 30 fs is shown in figure 7(a) for realization of sub-ps all-optical NOT and the universal NAND and NOR logic operations. The inverted response to an input pump pulse results in the all-optical NOT logic gate. For the all-optical NOR logic gate, the output is low when either one or both the pulses is present and high when none of the two pulses are present. The same set up results in the all-opti-cal NAND logic operation by considering a threshold level shown by the dotted lines and solid line for BR thin films and BR-Au-NP solutions, respectively (figure 7(a)). The switch-ing time of these gates with the typical parameters used for BR and BR-Au-NPs is in the sub-ps range. It is evident from the analysis that BR gold nanoparticles open up prospects for tailoring the BR photoresponse for low power ultrafast all-optical devices.

It would be useful to obtain various logic operations in parallel simultaneously for higher optical computing circuits, as shown in figure 8(a). A schematic setup for realization of

parallel logic gates for cases (ii) and (iii) is shown in fig-ure 8(b) with input intensities Im1 and Im2 made to be colline-arly incident on the BR film with different probe beams Ip1 and Ip2. Different logic gates such as AND, OR and the uni-versal NOR and NAND can be simultaneously realized for combinations of incident pump and probe beams. The truth table corresponding to the different all-optical logic gates is shown in table 1.

As mentioned earlier, BR exhibits exceptional photo and thermal stability. A number of experimental investigations have demonstrated continuous switching of BR with no pho-todegradation [9–13]. The maximum intensity values in the range 0.5–40 GW cm−2 used in the present simulations are far below the values at which multiphoton effects arise and hence also the damage threshold, as is evident from the recent experi-mental studies on BR thin films at ~700 GW cm−2 and BR liq-uid samples ~200 GW cm−2 [19–21]. Excitation with 120 fs pulses at 570 nm and 612 nm in our simulations, near the peak absorption of the initial B state, also ensures that neither the direct TPA induced F540 state is formed nor the F620 state from J625 or K590 states is formed, as shown experimentally [19, 20].

Ultrafast switching in BR is at relatively lower pump intensities compared to other organic molecules, for instance, Cu-phthalocyanine thin films (~TW cm−2 [29]) and graphene oxide thin films (~1010 W cm−2 [30]). Although experimental ultrafast spectroscopic characterization studies have demon-strated only some features of switching in terms of the for-mation of the I460 state, the present study provides a detailed theoretical analysis in terms of an accurate model and gen-erates important insights necessary for designing devices for practical applications. All-optical ultrafast switching in BR-Au-NP solutions highlights the feasibility of BR based hybrid nano-bio-photonic integrated devices for ultrafast infor-mation processing at low pump intensities (~MW cm−2). Since the properties of BR can be tailored by physical, chemical and genetic engineering techniques, the switching characteristics and the operation of ultrafast logic gates can be optimized for desired applications. Switching a near-IR probe beam in 2  ×  2 BR-coated silica microresonator couplers with ultrashort pump pulses at 532 nm and 460 nm simultaneously can enhance the speed of operation and bit rates of the recently proposed all-optical computing and communication circuits [15].

4. Conclusion

The present analysis demonstrates the applicability of BR and BR-nanoparticle engineered materials for all-optical ultrafast operations in the simple pump-probe geometry with sub-ps switch off/on time and high switching contrast at relatively lower pump intensities compared to other organic molecules and opens up exciting prospects for its use in ultrafast and ultrahigh bandwidth information processing. Bio-inspired technologies based on efficient integration of biological materials with engineered nanomaterials hold tre-mendous potential for development of devices with unique electronic and photonic properties for novel nano-bio-pho-tonic applications.

Figure 7. All-optical logic operation (a) OR gate (without threshold) and AND gate (with threshold), both with the variation of NTPI at 580 nm for BR thin film (dashed lines) at Im0 = 40 GW cm−2, τHI = 30 fs and ∆t =120 fs; NOR gate (without threshold) and NAND gate (with threshold), both with the variation of NTPI at 460 nm dotted lines for BR thin film at Im0 = 5.0 GW cm−2,τHI = 30 fs and ∆t = 120 fs; and solid line for BR-gold nanoparticles (C = 1.3  ×  10−11 M) at Im0 = 0.4 MW cm−2, τHI = 30 fs and ∆t = 100 fs. (b) Combined normalized pulse profiles of the two inputs.

Laser Phys. Lett. 11 (2014) 125901

S Roy and C Yadav

7

Acknowledgments

The authors thank the reviewers for valuable suggestions. They are also grateful to the University Grants Commis-sion, Government of India for partial support of this work and a research fellowship to C Y under grant no. F.530/19/DRS/2009 (SAP - I).

References

[1] Wada O 2004 Femtosecond all-optical devices for ultrafast communication and signal processing New J. Phys. 6 1–35

[2] Haque S A and Nelson J 2010 Toward organic all-optical switching Science 327 1466–7

[3] Roy S 2011 Special issue on optical computing circuits, devices and systems IET Circuit Devices Syst. 5 73–5

[4] Hampp N 2000 Bacteriorhodopsin as a photochromic retinal protein for optical memories Chem. Rev. 100 1755–76

[5] Shen Y, Safinya C R, Liang K S, Ruppert A F and Roths-child K J 1993 Stabilization of the membrane protein bacte-riorhodopsin to 140 °C in the two dimensional films Nature 366 48–50

[6] Bouchonville N, Cigne A L, Sukhanova A, Molinari M and Nabiev I 2013 Nano-biophotonic hybrid materials with controlled FRET efficiency engineered from quantum dots and bacteriorhodopsin Laser Phys. Lett. 10 085901

[7] Rakovich A, Nabiev I, Sukhanova A, Lesnyak V, Gaponik N, Rakovich Y P and Donegan J F 2013 Large enhancement of nonlinear optical response in a hybrid nanobiomaterial consisting of bacteriorhodopsin and cadmium telluride quantum dots ACS Nano 7 2154–60

[8] Rao D V G L N, Aranda F J, Wiley B J, Akkara J A, Kaplan D L and Roach J F 1993 Mirrorless all-optical bistability in bacteriorhodopsin Appl. Phys. Lett. 63 1489–91

[9] Huang Y, Wu S and Zhao Y 2004 All-optical switching characteristics in bacteriorhodopsin and its applications in integrated optics Opt. Express 12 895–906

[10] Topolancik J and Vollmer F 2006 All-optical switching in the near infrared with bacteriorhodopsin-coated microcavities Appl. Phys. Lett. 89 184103

[11] Fabian L, Wolff E K, Oroszi L, Ormos P and Der A 2010 Fast integrated optical switching by the protein bacteriorhodop-sin Appl. Phys. Lett. 97 023305

[12] Huang Y, Wu S-T and Zhao Y 2004 Photonic switching based on the photoinduced birefringence in bacteriorhodopsin films Appl. Phys. Lett. 84 2028–30

Table 1. Truth table of all-optical logic gates (figure 8).

Input signal (612 nm) NTPI (Ip1out) at 460 nm

Im1 (GW cm−2) Im2 (GW cm−2) OR Gate AND Gate0 1(40 GW cm−2) 1 00 0 0 01(40 GW cm−2) 0 1 01(40 GW cm−2) 1(40 GW cm−2) 1 1

Input signal (612 nm) NTPI (Ip1out) at 580 nm

Im1 (GW cm−2) Im2 (GW cm−2) NOR Gate NAND Gate0 1(5 GW cm−2) 0 10 0 1 11(5 GW cm−2) 0 0 11(5 GW cm−2) 1(5 GW cm−2) 0 0

Figure 8. Schematic for realization of two-input parallel all-optical OR (without threshold), AND (with threshold) and the universal NOR (without threshold) and NAND (with threshold) logic gates. (a) Block diagram. (b) Circuit diagram with Im1 (45 GW cm−2) as the intensity for logic ‘1’ state. BS: beam splitter, M: mirror, C1 = 5 µM, and C2 = 28 µM.

Laser Phys. Lett. 11 (2014) 125901

S Roy and C Yadav

8

[13] Chen G, Lu W, Xu X, Tian J and Zhang C 2009 All-optical time-delay switch based on grating build-up time of two-wave mixing in a bacteriorhodopsin film Appl. Opt. 48 5205–11

[14] Han J, Yao B, Gao P, Chen L and Huang M 2012 All-optical logic gates based on photoinduced anisotropy of bacteri-orhodopsin film J. Mod. Opt. 59 636–42

[15] Roy S, Prasad M, Topolancik J and Vollmer F 2010 All-optical switching with bacteriorhodopsin protein coated microcavi-ties and its application to low power computing circuits J. Appl. Phys. 107 053115

[16] Roy S, Sethi P, Topolancik J and Vollmer F 2012 All-optical reversible logic gates with optically controlled bacteriorho-dopsin protein-coated microresonators Adv. Opt. Technol. 2012 727206–12

[17] Mathesz A, Fabian L, Valkai S, Alexandre D, Marques P V S, Ormos P, Wolff E K and Der A 2013 High-speed integrated optical logic based on the protein bacteriorhodopsin Biosen-sors Bioelectron. 46 48–52

[18] Fabian L, Heiner Z, Mero M, Kiss M, Wolff E K, Ormos P, Osvay K and Der A 2011 protein-based ultrafast photonic switching Opt. Express 19 18861–70

[19] Yao B, Lei M, Ren L, Menke N and Wang Y 2005 Polarization multiplexed write-once–read-many optical data storage in bacteriorhodopsin films Opt. Lett. 30 3060–2

[20] Yu X, Yao B, Lei M, Gao P and Ma B 2013 Femtosecond laser-induced permanent anisotropy in bacteriorhodopsin films and applications in optical data storage J. Mod. Opt. 60 309–14

[21] Prokhorenko V I, Halpin A, Johnson P J M, Miller R J D and Brown L S 2011 Coherent control of the isomerization of retinal in bacteriorhodopsin in the high intensity regime J. Chem. Phys. 134 085105

[22] Petrich J W, Breton J, Martin J L and Antonetti A 1987 Femtosecond absorption spectroscopy of light-adapted and dark-adapted bacteriorhodopsin Chem. Phys. Lett. 137 369–75

[23] Logunov S L, Song L and El-Sayed M A 1994 ph dependence of the rate and quantum yield of the retinal photoisomeriza-tion in bacteriorodopsin J. Phys. Chem. 98 10674–7

[24] Biesso A, Qian W and El-Sayed M A 2008 Gold nanoparti-cle plasmonic field effect on the primary step of the other photosynthetic system in nature, bacteriorhodopsin J. Am. Chem. Soc. 130 3258–9

[25] Cheng C, Lee Y and Chu L 2012 Study of the reactive excited-state dynamics of delipidated bacteriorhodopsin upon surfactant treatments Chem. Phys. Lett. 539-540 151–6

[26] Abramczyk H 2012 Mechanisms of energy dissipation and ultrafast primary events in photostable systems: H-bond, excess electron, biological photoreceptors Vib. Spectrosc. 58 1–11

[27] Roy S and Yadav C 2013 All-Optical Ultrafast Switching and Logic With Bacteriorhodopsin Protein (Lecture Notes in Computer Science) (Berlin: Springer) 7715 68–77

[28] Biesso A, Qian W, Huang X and El-Sayed M A 2009 Gold nanoparticles surface plasmon field effects on the proton pump process of the bacteriorhodopsin photosynthesis J. Am. Chem. Soc. 131 2442–3

[29] Roy S and Yadav C 2011 All-optical ultrafast logic gates based on saturable to reverse saturable absorption transition in CuPc-doped PMMA thin films Opt. Commun. 284 4435–40

[30] Roy S and Yadav C 2013 Femtosecond all-optical parallel logic gates based on tunable saturable to reverse saturable absorption in graphene oxide thin films Appl. Phys. Lett. 103 241113

Laser Phys. Lett. 11 (2014) 125901