July 04, 2009

Laser Switched Optical Transistor Could Enable future generation of ultrafast light-based computers

An artist's impression of a molecule acting as a transistor that makes it possible to use one laser beam to tune the power of another (Image: Robert Lettow)

An optical transistor that uses one laser beam to control another could form the heart of a future generation of ultrafast light-based computers, say Swiss researchers.

Conventional computers are based on transistors, which allow one electrode to control the current moving through the device and are combined to form logic gates and processors. The new component achieves the same thing, but for laser beams, not electric currents.

A green laser beam is used to control the power of an orange laser beam passing through the device.

They suspended tetradecane, a hydrocarbon dye, in an organic liquid. They then froze the suspension to -272 °C using liquid helium – creating a crystalline matrix in which individual dye molecules could be targeted with lasers.

When a finely tuned orange laser beam is trained on a dye molecule, it efficiently soaks up most of it up – leaving a much weaker "output" beam to continue beyond the dye.

But when the molecule is also targeted with a green laser beam, it starts to produce strong orange light of its own and so boosts the power of the orange output beam.

This effect is down to the hydrocarbon molecule absorbing the green light, only to lose the equivalent energy in the form of orange light.

"That light constructively interferes with the incoming orange beam and makes it brighter," says Sandoghar's colleague Jaesuk Hwang.

Abstract at the journal Nature: A single-molecule optical transistor

a, Energy level scheme of a molecule with ground state (|1), and ground (|2) and first excited (|3) vibrational states of the first electronic excited state. Manifold |4 shows the vibronic levels of the electronic ground state, which decay rapidly to |1. Blue arrow, excitation by the gate beam; green double-headed arrow, coherent interaction of the CW source beam with the zero-phonon line (ZPL); red arrow, Stokes-shifted fluorescence; black dashed arrows, non-radiative internal conversion. b, Time-domain description of laser excitations and corresponding response of the molecule simulated by the Bloch equations with periodic boundary conditions. Blue spikes and red curve represent the pump laser pulses and the population of the excited state |2, respectively. Black curve shows the time trajectory of Im(21). Straight green line indicates the constant probe laser intensity that is on at all times. Inset, magnified view of curves during a laser pulse. c, Schematic diagram of the optical set-up. BS, beam splitter; LP, long-pass filter; BP, band-pass filter; HWP, half-wave plate; LPol, linear polarizer; S, sample; SIL, solid-immersion lens; PD1, PD2, avalanche photodiodes. Transmission of the probe beam (green) is monitored on PD1, and the Stokes-shifted fluorescence (red) is recorded on PD2.

The transistor is one of the most influential inventions of modern times and is ubiquitous in present-day technologies. In the continuing development of increasingly powerful computers as well as alternative technologies based on the prospects of quantum information processing, switching and amplification functionalities are being sought in ultrasmall objects, such as nanotubes, molecules or atoms. Among the possible choices of signal carriers, photons are particularly attractive because of their robustness against decoherence, but their control at the nanometre scale poses a significant challenge as conventional nonlinear materials become ineffective. To remedy this shortcoming, resonances in optical emitters can be exploited, and atomic ensembles have been successfully used to mediate weak light beams. However, single-emitter manipulation of photonic signals has remained elusive and has only been studied in high-finesse microcavities or waveguides. Here we demonstrate that a single dye molecule can operate as an optical transistor and coherently attenuate or amplify a tightly focused laser beam, depending on the power of a second 'gating' beam that controls the degree of population inversion. Such a quantum optical transistor has also the potential for manipulating non-classical light fields down to the single-photon level. We discuss some of the hurdles along the road towards practical implementations, and their possible solutions.

Nature's editor's summary of nanoptics and optical transistors.

Quantum information processing systems and related technologies are likely to involve switching and amplification functions in ultrasmall objects such as nanotubes. In today's electronic devices the transistor performs these functions. A 'quantum age' equivalent of the conventional transistor would, ideally, use photons rather than electrons as information carriers because of their speed and robustness against decoherence. But robustness also stops them being easily controlled. Now a team from optETH and ETH in Zurich demonstrates the realization of a single-molecule optical transistor. In it, a single dye molecule coherently attenuates or amplifies a tightly focused laser beam, depending on the power of a second 'gating' beam.

A single molecule, represented here as a rotating mirror, can in principle behave as an all-optical transistor — it can modulate the transmission of a beam of light (the source beam, blue) in response to another beam of light (the gate beam, red). The waist-shaped surface represents a beam of light, focused on the molecule. The diagrams under each of the transistors represent the electronic energy levels of the molecule. a, If the molecule is in its ground state (g) and the source photons are equivalent in energy to the electronic energy transition from g to an excited state (e), then the source photons are resonantly scattered (totally reflected) as electrons oscillate between the e and g states. b, A gate photon of appropriate energy (different from that of the source photons) excites the molecule to a long-lived excited state (s). c, The excited molecule no longer absorbs source photons, which are instead perfectly transmitted. Hwang et al.3 report the first all-optical transistor that works on similar principles.
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