© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

6.3.6 Photonic Energy Conversion Processes

Visible light falling on a blackened surface is quickly absorbed and thermalized, the simplest example of photothermal transduction. Such transduction is reversed in solid-state optical refrigerators, which achieve cooling with 2% energy efficiency by flooding a sample with infrared photons of a particular frequency that will dampen molecular vibrations, then allowing the object to shed energy as higher-energy fluorescent light.⁵⁴⁹

Optomechanical transduction has been demonstrated by using low-power laser light impinging upon an optofluidic convertor to create a pressure pulse of sufficient magnitude to operate a pneumatic actuator in a push-pull mode,⁵⁴⁴ and an optical thermal "Brownian ratchet" uses rotating laser light to rectify the Brownian motion of a micron-scale plastic bead.¹⁰⁴³ Laser-heated gas microactuators have been studied,^560,640 and scattering forces from optical tweezers have been used to rotate small particles and thus may provide a means to power nanomachines by nonmechanical means.⁷⁷⁵ Photoacoustic transduction has been induced directly in piezoelectric indium gallium arsenide optical signal delay devices⁷¹⁴ and in "photostriction" materials such as PLZT,¹⁰⁶⁵ and indirectly in aqueous suspensions of 30-nm carbon particles illuminated by a pulsed laser.¹⁰⁵⁶ A 200-micron long, 600-nm thick silicon nitride beam with a 250-nm thick coating of a glassy material made of arsenic and selenium (chalcogenide) contracts ~1 nm when illuminated by photons polarized along the length of the beam, and expands ~1 nm when illuminated by photons polarized across the width of the beam.⁸⁷⁷ A ~300-micron optomechanical motor with diffraction grating developed for the DARPA MOEMS program operates at 20 volts and spins up to ~80 Hz.¹⁶⁶³

Light-driven proton pumps achieve optochemical transduction when optical photons shining on a photosensitive minicell activate a proton pump that pumps hydrogen ions across a membrane, thus building up a large ionic charge differential across the membrane and chemically storing the energy of the incident light. Experimental photoelectrolysis cells producing hydrogen and oxygen from sunlight and water with 12.5% energy efficiency have been built.¹⁵¹⁹ Green plants are optochemical transducers, but only 13% of the photonic energy falling upon a green plant is converted to chemical energy because most of the incident radiation is lost by reflection, transmission, or absorption by nonchloroplast pigments. Of the photons falling directly upon active photosynthetic pigments, up to ~35% is converted to chemical energy. Certain bacteria also employ photochemical transduction. Activation of unimolecular chemical reactions by ambient blackbody radiation has been demonstrated,¹¹²² and 10-100 GHz rotation in nanotube gears with dipole charges using laser-generated alternating electric fields has been simulated computationally.^1235,1236

Photoelectric cells are a well-known technology for converting optical to electrical energy. Commercial solar panel arrays³⁰⁴⁷ currently achieve ~10⁵ watts/m³, although polymer thin-film (100-200 nm) photovoltaic cells may reach ~10⁶ watts/m³ at only 5.5% efficiency.¹⁰⁵⁹ Multilayer multi-band-gap photovoltaic cells can convert solar energy to electrical energy with efficiencies >30%,⁵⁶² and photocells and various natural photoreceptors such as the LH2 (light harvester) molecular complex of the Rhodopseudomonas acidophila bacterium can achieve efficiencies of 60%-90% at selected monochromatic wavelengths.^3532,3533 Photoelectric dendrimeric light-harvesting molecular antennas have been designed and synthesized.^2586-2591

In the 400-700 nm optical band only, direct (cloudless noontime) sunlight energy flux is 100-400 watts/m². Taking the 100 watts/m² figure and applying a 30% conversion efficiency, power supply P_n to a nanodevice of circular crosssection of radius R is P_n ~ 100 (30%) p R² watts in direct sunlight requiring a circular collector spot ~0.7 microns in diameter to power a 10 pW nanorobot. Fullerene sheets exhibit interesting optoelectronic effects: high-intensity light shined on one side of a sheet induces an electrical voltage potential on the other face. Progress is also being made in developing optically pumped (optophotonic) molecular lasers.⁹⁹⁰

Low-frequency photons such as radiofrequency (rf) signals are readily converted into electrical current using millimeter-scale loop antennas (Section 6.4.2). For example, a 2 mm x 10 mm device implanted subdermally with a hypodermic needle electrically stimulates muscle using rf telemetry for control and power, in order to restore human motor functions.⁵⁶³

Last updated on 18 February 2003