Patent Application
20130016412
The present invention relates to optical processing and/or, in particular but not exclusively, to optical memories and logic gates.
In the recent years, different groups have developed a variety of optical memories—often referred to as all-optical flip-flops—and of logic gates. A selection of those projects is briefly presented in this chapter.
There are several realizations of optical polarization switches and AOFF working with holding beam and writing pulses. The use of holding beam is important to maintain the polarization state for durations longer than the lifetime of the spin carriers in the device. However the existing solutions using semiconductor heterostructures often rely on the relative phase between the switching pulses (see for instance EP 0809128 and EP 1128204 A1), which is difficult to control, and prevent to write information by sending pulses with arbitrary time delays.
Other switching solutions provide devices which are based on the competition between two different wavelengths: see for instance U.S. Pat. No. 5,151,589 A, EP 1255157 A1, U.S. Pat. No. 6,456,417 B1 and references 1 and 2 as follows:
The use of different wavelengths for the different polarization states prevents the coupling of polarization keying with WDM and for cascading several devices.
iii) XGM/XPM
Cross gain modulation (XGM) devices, like semiconductor optical amplifiers (SOA), also allow for AOFF operations but were only demonstrated with low contrast (3, 5 dB) and slow switching speed (˜1 ns) (U.S. Pat. No. 6,456,417 B1, US 2009 067300 A1).
XGM uses input beams at different wavelengths than the main lasing mode for creating injection locking and causing lasing on a side-mode; in this way the lasing of the main mode can be suppressed if the gain losses induced by the side modes are high enough. One problem with this approach is that different wavelengths with specific requirements are involved. As a consequence there is no cascadability, meaning that output of such a gate cannot be used as input for an identical gate, preventing for building arrays.
More generally XGM is also used to obtain switches with spatial effect, like the change of the lasing direction of a disk laser-see references 3-5 as follows:
These techniques also prevent from cascading devices or to use them in optical fibers.
Recent peer reviewed publications proposed to use polarization multistability using semiconductor microcavities in the strong coupling regime to realize AOFFs-see references 6 and 7 as follows:
The advantage of polarization multistability with respect to the prior art is that it discretizes the polarization states into only 2 or 3 available states. This ensures a stabilization of the polarization states and drastically increases the possibilities for designing logical components for optical processing.
However the proposal of a RAM device (see Ref 7) relies on the control of the relative phase between the switching pulses, which has previously been shown to be problematic.
However, for most all-optical logic circuits schemes, fluctuations and losses are considered as an important drawback for communication between optical stages.
There is therefore a need to improve the state-of-the-art devices.
In a first aspect the invention provides an all-optical spin device based on spin multistability of trapped microcavity polaritons.
In a first preferred embodiment the all-optical spin device according to the invention comprises a pulse laser for fast switching between an upper and a lower intensity branch.
In a second preferred embodiment the all-optical spin device according to the invention comprises a continuous wave laser as a power supply for polariton spin populations.
In a third preferred embodiment of the all-optical spin device according to the invention, the continuous wave laser is linearly polarized.
In a second aspect the all-optical spin device according to the invention is used for storing information (optical memory).
In a third aspect the all-optical device according to the invention is used for logic operations (logic gating).
The invention is described with reference to figures, which illustrate various aspects thereof.
An elegant way of solving the problems mentioned in the previous sections is to encode logic levels independently from the optical intensity. Optical spinor systems are therefore excellent candidates to develop such devices since the spin polarization can be used for storing information as well as for logic operations.
The invention therefore relates to an all-optical spin device based on spin multistability of trapped microcavity polaritons.
The device according to the present invention may admit two or more stable spin states for a given single optical excitation condition. The device is preferably driven by a single wavelength continuous wave (cw) excitation laser. This optical cw may be replaced for instance by electrical pumping through resonant tunneling. The switching between the different states can be achieved at constant excitation power by changing the excitation polarization (using a quarter-wave plate). The light emitted by the device has preferably the same wavelength as the input light. The polarization of the light emitted by the device is advantageously in one-to-one correspondence with the internal spin state of the device (see
Ultrafast, selective and reversible switching is achieved by keeping the continuous wave beam in the multistability region (with constant power and polarization) as a holding beam and by sending circularly (left, right) or linearly polarized sub-picosecond pulses to write the internal spin state of the device. After the pulse is gone, the information on the spin polarization is conserved as long as the holding beam is exciting the device. This may serves as an all-optical RAM where the information is encoded in the polarization state. The operation device does neither depend on the relative phase between the pulses nor on the phase between the pulses and the holding beam (see
Fundamental logic operations (e.g. NOR) are realized with a single device using polarized optical inputs. The preferable monochromatism of the device allows for cascadability by using the output of one device as the input of another device. More complex logic operations may be obtained using arrays of several of these devices (see
The present invention takes advantage from a situation called “cross dissipation modulation (XDM)”, i.e. the dissipation of a spin population in the device increases with the density of the other spin population. This leads to population competition.
XDM can be modulated in the vicinity of a Feshbach resonance for instance, like the biexciton resonance. It is important to stress the fact that XDM is a physical process which is substantially different from XGM.
Polarization switching is realized with ultrafast pulses independently from their relative phase. The output of the device has preferably the same wavelength as the input.
The invention preferably works with narrow linewidth polaritons, by using for instance lateral confinement in patterned structures and by using polariton energies close to the biexciton resonance. The very narrow polariton linewidth makes the behavior much more sensitive to energy variations close to the resonance. Bistability cycles are realized with excitation powers which are preferably more than two orders of magnitude lower than in other microcavity structures (see reference ref 8=Baas et al., PRA 69 023809, (2004)).
Because of the narrow linewidths the effect of XDM is much more significant on the upper bistability branches (for a high polariton density), causing a separation of lower bistability thresholds of spin-up and spin-down polaritons. The independence of the lower thresholds is advantageous to decrease the power consumption (<500 μW). Multistability can be realized in the region of independent lower thresholds.
XDM is responsible for the buildup of a reservoir and provides higher contrast (e.g. 20 dB) and more robust output polarization states. The device output is circular s+(1), s−(−1) or linear (0). It is possible to design the device to obtain a 0-state which is elliptical and to modify the symmetry of the multistability cycle.
The device according to the present invention is a versatile, multi-valued, optical polarization device which may be advantageously used in optical communication, optical processing and fiber optics technology. Some of those applications are briefly discussed below.
Telecom industry is facing two important challenges: on the one hand, there is an exponential increase of the number of users and on the other hand, the economic and climatic situations are calling for a significant reduction of the energetic costs.
In order to address the increase of traffic in the communication networks, wavelength-division-multiplexing (WDM) technology imposed itself as the most suitable technology in optical communication. Until very recently, the signal modulation was only encoded into the amplitude of the optical signals of different wavelengths (ON-OFF-Keyed, OOK). This format however suffers from poor spectral efficiency, limiting the increase of information fluxes. Since these fluxes will soon reach the Tbit/s regime, new modulation methods are nowadays developed. These new formats include phase modulation (PSK, Phase-Shift-Keyed) or polarization modulation (POLSK) and appear to be promising alternative solutions. This is discussed in references 9-12 as follows:
There are two main issues about polarization keying. The first one is the polarization sensitivity of opto-electronic devices such as semiconductor spin amplifiers (SOA). With the development of specific semiconductor nanostructures (OD columnar stacks) this problem has recently been solved [Akiyama et al. Proceedings of the IEEE 95, 1757 (2007)]. The second issue is the conservation of polarization through optical fibers, which is weak. The problem of polarization conservation in fibers is still being investigated.
It is a general feature in fiber optics technologies that the polarization degree of freedom is not exploited. As said previously, it is in general very difficult to maintain a given polarization state along a fiber. This is problematic not only for fiber optics communication but also for other applications, like for instance, fiber sensing, where fluctuations of the polarization is critical and can introduce artifacts in temperature/strain measurements. Polarization maintaining fibers are very expensive and their use have a lot of restrictions (ways to enter the fiber, short-length fibers only).
A possible solution consists in the amplification of the signal at certain positions only and at the same time to correct the polarization. The device according to the present invention provides a solution for polarization correction in optical networks.
iii) Optical Processing
The main obstacle to the development of optical processing is the lack of a simple, transistor-like, optical-component that can be used to control light flow as well as to realize logic operations. A proper storage element like an optical RAM is also difficult to design (see ref 13=D. A. B. Miller, Nature Photonics 4, 3-5 (2010)) when using state-of-the-art devices.
Ternary logic has for long been proposed as a powerful solution to compute complex algorithm.
However the difficulty to design a standalone multistable component compromises the development of ternary circuits. Instead, ternary functions are implemented using binary functions, which makes them even more complicated.
The present invention provides a solution to achieve polarization multistability devices that can be addressed reversibly with ultrafast pulses while being insensitive to the phase of the pulses.
The invention was in particular tested and confirmed in a semiconductor microcavity in the strong-coupling regime with patterned structures used to trap exciton-photon mixed states (polaritons).
In this example designed for two memory values, the device comprises a cw and a pulsed laser with femtosecond pulses for optically creating and controlling the polariton populations in the sample. The cw laser is linearly polarized for being able to pump the spin up and spin down polariton populations and has an energy slightly above the polariton eigenenergy. The laser pulses are split in two and the two pulses are then counter circularly polarized (+ and −). A delay line controls the time difference between the two pulses. The experiments are performed in transmission with the sample at liquid helium temperature. On the detection side the signals coming from the spin up and spin down polaritons are separated and detected with a spectrometer and a streak camera for time resolved measurements. The linearly polarized cw laser can serve as a power supply for both polariton populations and the laser pulses allow us to control for which polariton population the supply is ON or OFF (write operation). A − pulse for example brings the spin down population up and hence in resonance with the power supply. Due to the feedback created by the self-induced blueshift the supply will remain on after the pulse. The spin down pupulation surplus created by the − pulse induces at the same time non-linear losses which bring down the spin up population and hence cut it off from the power supply. For obtaining a high contrast we use the polarization degree of the emitted signal as value of the memory. The readout values can be −1 (if memory written with −), +1 (if memory written with +) or 0 (memory not yet written).
For three memory values the same operation can be achieved in exactly the same conditions using a pulse and a linear pulse. Hence, the system can reversibly and selectively be switched between right-circular, left-circular, and linearly polarized. 3-state polarization modulation can be achieved at a rate higher than 0.2 THz.
In summary:
| Number | Date | Country | |
|---|---|---|---|
| 61480413 | Apr 2011 | US |
