Surface Plasmon Amplification by Stimulated Emission of Radiation By: Jonathan Massey-Allard Graham Zell Justin Lau
Surface Plasmons (SPs) Quanta of electron oscillations in a plasma. o Electron gas in metals. Exist at the interface between two materials where the dielectric constant changes sign. o Metal sheet in air. Incident fields on material surface excite plasmons. o Need negative dielectric constant for absorption. o Skin depth = depth for which material absorbs field. SPs are bosons like photons. o Interact very weakly with one another. o Many SPs may accumulate in a single mode. o Can get stimulated emission with SPs.
Lasers vs. Spasers Similarities Propagating modes are created by feedback between the gain medium and the resonant cavity. Both work on the principles of stimulated emission and population inversion. Made possible by the fact that bosons (photons and SPs) can exist in the same mode in large numbers. Differences Lasers amplify photons while spasers amplify surface plasmons. Spasers can function in "dark" modes that don't couple to the far field. The cavity size of a spaser is determined by a metal's skin depth rather than the emission wavelength.
Resonator Need metals good at supporting SPs. o Noble metals like Au and Ag. Can use metal nanoparticles. o Shell thickness less than skin depths of ~25 nm. Resonant lifetime of SP modes characterized by their Q factor. o Want high-q modes for greatest field enhancement. M.I. Stockman, J. Opt., 12 (2010)
Gain Medium Need spectral and spatial overlap with SP resonant modes. Can use ordinary dye molecules or nano quantum dots (NQDs). NQDs better because they have a tuneable emission spectrum and don't suffer from excited state quenching and photobleaching like dyes do. M.I. Stockman, J. Opt., 12 (2010)
Basic Spasing Mechanism M.I. Stockman, N. Photonics, 2 (2008).
QM of Stimulated Emission The Spaser Hamiltonian: Matrix notation of Hamiltonian: Spontaneous emission is described by Fermi's Golden Rule: Perturbing Hamiltonian has odd parity Integral vanishes if spatial part is a pure eigenstate Dark modes exist if system is fully symmetric If asymmetric, spatial part is superposition of eigenstates
Of Lasers and Spasers Density matrix statistical description of ensemble: Resulting undamped equations of motion: Include population relaxation: Recognize these? Hint: PHYS 460
Conditions for Effective Spasing Spasing frequency: For spasing: Order of magnitude approximation: Desired conditions for spasing: Strong dipole transition µ 12 Low decay rate Γ 12 High chromophore density N c /V n High quality factor Q Q
Near-field Scanning Optical Microscopy Latest work in field concentrates light by shining laser beams on pointed tips, to generate strong evanescent fields. Laser sprays energy everywhere and heats up the surface, causing noise. Spasers could generate these fields via dark modes, without transferring extra energy to the sample. L. Novotny and S. Stranick, Annu. Rev. Phys. Chem., 57, (2006).
Nanolasers An array of weakly antisymmetric split ring currents may radiatively couple to the far field. Radiation is coherent, making a laser, but how do we generate the currents efficiently?
Nanolasers with Spasers Combine split ring array with supporting gain medium Tune resonant frequency of rings to the emission of the gain medium. Surface plasmons set up currents and amplify pump light coherently. Theoretical gain of ~35 db in near-infrared. N. Zheludev et al., N. Photonics, 2 (2008).
Plasmonic Circuits Can we use plasmons in nanocircuits to do ultrafast computation? Would need: 1.useful source of local fields 2.plasmon waveguides 3.nanoamplifiers? 4.nanoswitches? The spaser acts like a laser: it cannot amplify by definition! o Can we get around that? o What about achieving bistable operation like transistors?
Spaser Switch Bistable application <=> MOSFET transistor. o Two stable states can be attained. Logic "1" = fixed SP population Logic "0" = no SP population Use saturable absorbers in the gain medium. o Absorption over the spasing line but not with the pumping frequency (introduce losses). M.I. Stockman, J. Opt., 12 (2010)
Nanoamplifier In steady-state operation, the surface plasmon gain exactly offsets the loss in the metal, so the spaser has exactly zero amplification. When pumping is turned on, the initial population of SPs affects how quickly CW operation is reached. For ~100 fs, the spaser achieves positive amplification of SPs. Time delay before steady state may have uses in circuits. M.I. Stockman, J. Opt., 12 (2010)
Nanoamplifier When the spaser is hit with a pulse of pump light to induce a sudden inversion in the gain medium, the response is a series of oscillations in the SP population. Magnitude depends on the initial population of surface plasmons. Amplification! First pulse is always above a given level (here, 100) while the later ones fall below. M.I. Stockman, J. Opt., 12 (2010)
Successful Weaponization Larks, with frickin' spaser beams on their heads. D. Evil, J. App. Evil, 17 (2003)
Artist's rendering of a spaser in future applications. Questions? http://media.photobucket.com/image/spaser/yvan33/photos91.jpg
Giovanni di Simone, Pisa, Italy, 1 (1173).