Single molecules quantum dynamics and quantum optics

肖连团量子光学与光量子器件国家重点实验室

山西大学激光光谱实验室

激光光谱实验室

Dalian 6, Aug, 2010

1111 光缔合制备超冷分子及其在量子信息中的应用

2222 超冷里德堡原子的实验研究

3333

激光光谱实验室

单分子量子光学

超冷分子

光缔合超冷铯分子与超冷铷铯分子的制备、测量与调控。

Ti:Sa laser

Dye laser

超冷里德堡原子

Cs+ Cs2+

探测装置示意图

时序控制示意图

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• Introduction - Why single-molecule optics detection?

- How to detect single molecules?• Single molecules probe - Long memory time effect in soft matter - Probing surface dynamics

• Single molecules fluorescence manipulation - Electric-current modulation

- Enhancement and suppression of single- molecule fluorescence• Single molecules quantum optics

- Interaction between single molecule and single photons - Quantum states

• Discussion and Outlook

Outline

• Single molecules probe

In molecular physics texts, single molecule detection has proven to be a unique method to investigate the behavior of complex condensed systems.

In chemistry texts, molecular interactions and chemical reactions are generally described on a single-molecule basis.

The single molecule approach has changed the way problems are addressed in biophysics or even biochemistry.

1.1 Single molecule optics

Single molecule observation• In 1952, Schrodinger said that “ We would never experiment

with just one electron, one atom or one molecule ”.• In 1960, Feynman anticipated the future and then said “

There are no physical limitations to arranging atoms the way we want ”.

• By 1980s, Scanning Tunneling Microscopy and observations of fluorescence from a single molecule allowed us manipulate single molecules, atoms and bonds.

Scanning Tunneling Microscopy

Can we really catch hold of a single molecule?

Single molecule studies do not mean taking one molecule and analyzing it.

Its only that we are detecting one molecule at a time.

Detection volume is 1 m3. When a fluorophore traverses the laser excitation

volume, a fluorescence photon burst is generated. The bursts are analyzed in terms of number of

photons emitted. It doesn’t mean only one experiment on one

molecule but millions of experiments on one molecule.

Histograms are made out of these large number of experimental results.

Single-Molecule Optics

Prof. Dr. Michel Orrit

• " Single pentacene molecules detected by fluorescence excitation " Phys. Rev. Lett., 65 (1990) 2716-19.•"Molecular entanglements" Science 297 (2002) 1160-1163 •"Photothermal imaging of nano- metal particles among scatterers" Science 298 (2002) 369-370 •"The motions of an enzyme soloist" Science 302 (2003) 239-240 •"Single-Molecule Optics" Annu. Rev. Phys. Chem. 55 (2004) 585 •"Single-photon sources" Rep. Prog. Phys. 68 (2005) •"Quantum light switch" Nature. Phys 3 (2007) 755-756

1. Single molecules spectroscopy

2. Organic field effect transistors

3. Rheology

Organic field effect transistors

Figure 1: (a) Source-drain current versus gate voltage for different source-drain voltages of an Ac-crystal in a FET structure.

(b) Stack of fluorescence-excitation spectra.

The conducting layer of an organic crystal is doped with a very small concentration of DBT dye molecules.

单分子纠缠

1.2 Single molecule fluorescence detection

Molecules randomly dispersed inside a matrix host :Transparent liquid or polymer thin film ,"spin-coated" on a glass cover-plate

7.6 m

Photobleaching

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Surface density ~ 1 molecule / 10 m2

100 nm

Glass cover-plate

Set up – Gaussian Beam

Sample

Dichroic Mirror

Lens

laser : λ1

Focal Volume

Fluorescence

Fluorescence light : λ2

Molecules are Fluorescent

λ2

λ1

Confocal Microscopy

O.M Resolution limit ~ 200nm

But we want to see single molecule!

-> Confocal Microscopy

• Single molecules probe

2.1 Motivation

• In the particular case of glass-forming systems, rotational diffusion will probe the relaxation of the host because the rotation of the dopant molecules is subject to this relaxation.

• Since the single-molecule approach not only yields the average but also the distribution of rotation times, it directly probes the extent of spatially inhomogeneous dynamics in the host.

Characteristic timescales

Absorption 10-15sVibrational relaxation 10-12sLifetime of S1 10-10 – 10-7s Intersystem crossing 10-10 – 10-8s Internal conversion 10-11 – 10-9sLifetime of T1 10-6 – 1sDiffusion 10-2s - minutes

Molecular Fluorescence by Bernard Valeur, Wiley-VCH

Fluorescence Correlation Spectroscopy

Probing viscosity with fluorescence

• Fluorescence Anisotropy (during emission)

• Polarization fluctuations (small ensembles)

( )

B

TV

k T

Our Method:

Single-molecule orientation

Rotational diffusion time:

V is the hydrodynamic molecular volume of the fluorophore, η is the viscosity of glycerol.

As a supercooled liquid is cooled to lower temperatures, itsviscosity increases and the molecules which comprise it move more and more slowly.

supercooled liquid

Supercooled liquid

• Liquids at temperatures below their melting points are called supercooled liquids.

Polarized single-molecule fluorescence

单分子荧光光子计数

两个偏振方向的光子计数是跳跃改变的，当一个方向的光子计数减少时，另一个方向的光子计数就会相应的增大 。

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水平偏振光子数

垂直偏振光子数

Single-molecule tumbling at variable T

T-dep. of tumbling rates for 69 molecules

Long memory time of local tumbling rate

2.2 The dynamics of polymerglasses surface

SPCM

Pulse Laser

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实验装置图

SR dye molecules

Orientation imaging of single SRmolecules on polymer glasses surface

•分子 a ：偶极取向对应水平方向，

•分子 b ：偶极取向对应垂直方向，

分子 c ：表现出了三重态的影响，荧光出现中断

•分子 d ：在探测过程中被光漂白了

•分子 e ：偶极取向在两个方向之间跳动

区域面积 10×10 m2 ，成像像素为150×150 ，采样积分时间为 10 ms

Single molecule orientational states

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Three typical jumping patterns of orientation are obtained statist

ically from reorientational molecules on PMMA surface.

• Our experiments yield the characteristic timescale of the SM's rotational diffusion and thereby probe the relaxation dynamics.

• Our results indicate the presence of extremely long-lived spatial inhomogeneities in supercooled glycerol probably related to very slow, larger-scale dynamics.

• The rotational correlation time quantified the time scales for the dynamics of the polymer glasses surface.

Solid-solid interaction -Friction: local pressure and temperature, third body,...-Adhesion: role of a soft layer in between two solids, probed with different dyes

SM probe

Outlook: SM’s and nano-probing for soft matter studies

3. Single molecules fluorescence manipulations

3.1 Preparation of samples

3 、 PMMA polymer are spin-coated onto ITO film.

1 、 The single SR dyes molecules are prepared by spin-coating onto the a silica glass substrate.

2 、 ITO films are spin-coated onto dyes molecules.

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Electric-current modulation of single-molecule emission intensity.

Suppression of single-molecule fluorescence by different electric-current.

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Enhancement of single-molecule fluorescence by some electric-current, sporadically.

Results

Enhancement and suppression of single-molecule fluorescence by different electric-current.

Results

Results

Results

Polymer chain

Singlet excited state

Photo-excitation

Relax

SR + acceptor kf

kB

SR + + acceptor-

Possible mechanism

Radiative or nonradiative decay to the ground state

Formation of long living triplet state

Charge separation e

Electron acceptor

4. Single molecules quantum optics

• By focusing the excitation light near to the extinction cross-section of a molecule, it is possible to explore resonance fluorescence over nine orders of magnitude of excitation intensity and to separate its coherent and incoherent parts.

• Under weak excitation, the detection of a single molecule with an incident power as faint as 600aW, paving the way for studying nonlinear effects with only a few photons.

单分子与单光子强耦合：

Introduction

• In view of quantum optical operations with photons and emitters, it would be highly desirable for as many of the incident photons as possible to interact with a single emitter.

• Such a regime would open the door to a wealth of nonlinear interactions between single emitters and single or few photons, which have been so far only achieved using sophisticated high-finesse microcavities.

• Coherent preparation of a single molecule electronic state.

4.1 EFFICIENT DETECTION OF A SINGLE EMITTER IN TRANSMISSION

• In the weak excitation regime, the effect of a molecule on an incident plane wave can be formulated as

• is the extinction cross-section of the transition between two levels.

• A key technological hurdle in resonant spectroscopy of single molecules in the condensed phase is to achieve a tight focus.

单分子能级与吸收

a, The energy-level scheme of a molecule with ground state and excited state. Manifold 3 shows the vibrational levels of the electronic ground state. b, The arrangement of the lenses in the cryostat. c, Schematic diagrams of the optical set-up. d, A laser scan image of a single molecule, showing a FWHM spot of 370 nm.

T =1.4 K

单分子与光子强耦合

Here, solid-immersion lens technology is used to reduce the focus area F close to the diffraction limit, enabling us to achieve an efficient coupling between a single molecule and a freely propagating laser beam in a single-pass encounter.

单分子透射谱

An example of a raw transmission spectrum, revealing a 11.5% dip determined by the lorentzian fit. The integration time per pixel was 160 ms and the noise amounts to 0.7%.

• The excitation spectrum registered on PD1 has to take into account the interference between the excitation field Ee and the field Em scattered by the molecule according to

The first term is simply the part of the incident intensity that arrives at the detector. The second term proportional to A represents the molecular emission and is always positive. The third term in this equation, also called extinction, is proportional to B and denotes the interference between Ee and the coherent part of Em.

Two examples of the recorded spectra. The blue and the green lines show the contributions of the molecular fluorescenceintensity and extinction components respectively.

SINGLE-MOLECULE DETECTION WITH ULTRAFAINT LIGHT SOURCES

Figure5 Few photons exciting a molecule. An extinction spectrum recorded from a single molecule under an ultrafaint detected power of 550 photons per second,equivalent to an incident power of 600aW.

a) Maximum Rabi frequency of Ω=370 MHz and a pulse area of A=5.7π .

b) displays the result of an experiment with fixed the pulse duration to 4 ns and increased the laser intensity.

Raw Stokes-shifted fluorescence of a single molecule red curve and a theoretical fit black curve.

Outlook

• Triggered Single Photon Sources 1. Quantum Cryptography 2. Quantum computing 3. Single photons interact with single molecule

• Coherent state preparation based on low temperature single molecules

Thank you for your Thank you for your attention!attention!