Thinking of Grad School: Guest Lecture

1. Who are you and why are you here? 2. Graduate School . . .

. . . what have you gotten yourself into? 3. Chemical Physics . . . the worst of both worlds?

1. Theoretical Chem Phys, leaders in the field 2. Experimental Chem Phys, an insider’s scoop

4. Special Topics 1. Femtochemistry

- leaders in the field … what are they doing? 2. Quantum Biology

5. Some biased thoughts . . .

Who are you and why are you here?

A: Someone who was probably in your place 5-6 years ago.

I ended up studying experimental chemical physics at UC Berkeley.

I am now a Cornell postdoc with:

- Paul McEuen (Physics)

-Jiwoong Park (Chemistry)

- Kavli Institute for Nanoscience at Cornell

Who are you and why are you here?

Optics, Microscopy & Photonics lab

PSB B30A

Transient photocurrent

Waveguide spectrophotometry

Stimulated Microscopy

Graduate School: A video interpretation by The Simpsons

1) “ I heard an assistant professorship opened up…at the university of psych!” (Bart Simpson) Partly fact: To keep the population stable every P.I. should have only one student become a professor. But, some groups have 30+ students at any given time. Reality:

- There are a lot of teaching oriented jobs for PHD’s - Finding a postdoctoral position is easy (but not always wise) - Some group have close to 100% placement in academe - Industry will hire a PhD (virtually guaranteed), you may just

not like the job or location

Constructive Analysis

2) “ I am a grad student, I’m 30 and made $600 last year.” (Bart Simpson) Myth: $21 to $32K standard stipend, more possible on fellowships.

Also consider: - Tax/pay deduction are negligible

- University subsidized housing/ share an apartment - Most grad students are not supporting a family - Your adviser often pays ~$30K for your tuition

- i.e. actual costs can be $60K/year + your research costs

Conclusion: It is possible to save money during grad school. Compare this to medical, business or law school!

Constructive Analysis

3) “No food until you grade 3,000 papers!” (Professor) Partly Fact - MANY professors are too poor to pay you. - Grading/teaching provides not only pay, but tuition waivers. - Be wary of schools that are not clear about your teaching load:

- (translation) some students never teach, others do so every semester. Reality Check: - Teaching in moderation will be helpful for you. - Lots of free food.

Constructive Analysis

3) “Graduate school is a poor life choice” (Marge Simpson)

You decide!

Best case scenarios: - You become a famous scientist

- You get a (good) job lecturing; job security, money and free time - You work for government/National Labs/major corporation

- A few even become astronauts! (there are no limits to what you may achieve)

Constructive Analysis

3) “Graduate school is a poor life choice” (Marge Simpson)

You decide! Some unpredictable scenarios that often happen too: - You become a famous scientist - You work in scientific sales (always an option for PhDs) - You become a postdoc for life (low pay, little security) - You drop out after 2-3 years

“Surveyed 3,400 professionals in 29 countries, fewer than 1 out of 2 respondents are satisfied in their current jobs. However, nearly 3 out of 4 respondents have no plans to leave their current companies” Forbes 2011

Constructive Analysis

Chemical Physics . . . the worst of both worlds?

Major scientific gaps exists where disciplines collide.

Chemical Physics . . . the worst of both worlds?

Would you enjoy it?

Q1: Do you want to make or measure?

Q2: Are you driven by

fundamental understanding or realizing applications.

Q3: Can you handle working on

long-term projects? (i.e. results in years, not weeks)

Why bother?

What’s the Goal?

Motivation: highly efficient solar voltaics.

How do we get there . . . one way is better solar technology

hν

hν hν

. . . towards a better understanding of how light interacts with novel materials.

My Scientific Interests in Chemical Physics

1 Charles M. LIEBER Harvard University

2 Omar M. YAGHI

University of California Los Angeles 3 Michael O’KEEFFE

Arizona State University 4 K. Barry SHARPLESS

Scripps Research Institute 5 A. Paul ALIVISATOS

University of California Berkeley 6 Richard E. SMALLEY†

Formerly Rice University 7 Hongjie DAI

Stanford University 8 Xiaogang PENG

University of Arkansas 10 Peidong YANG

Who Should I work for? … top 100 Chemists by Citation

19 Michael S. STRANO Massachusetts Institute of Technology

21 Dmitri V. TALAPIN University of Chicago

22 Ryoji NOYORI Nagoya University

23 Chad A. MIRKIN Northwestern University

26 Robert H. GRUBBS California Institute of Technology

27 Carlos F. BARBAS Scripps Research Institute

28 James R. HEATH California Institute of Technology

29 Moungi G. BAWENDI

CONCLUSION: Chemical Physicists aren’t usually the highest cited

Who Should I work for? … top 100 Chemists by Citation

Case Study I: Theoretical Chemical Physics

B. Whaley, UC Berkeley Professor of Chemistry, Physics (X.apt) and Mathematics (X.apt)

Princeton: Herschel Rabitz

You can excite a chemical bond with the right wavelength, but the energy redistributes all around the molecule rapidly.

But exciting with an intense, shaped ultrashort pulse can control the molecule’s vibrations and produce the desired products.

Theoretician (some experiment): a founder of the field of quantum control.

M.I.T.: Troy Van Voorhis

Theoretical Chemical Physics.

Modeling electron motion and distribution in real time.

Berkeley: Martin Head-Gordon

Theoretical Chemical Physics.

Probably the leading electronic structure research group.

e.g. If you’ve ever read ab initio calculations in a paper, chances are they came from software developed by this group.

Case Study II: Experimental Chemical Physics

Harvard: Adam Cohen

Experimental Chemical Physics.

Single molecule spectroscopy, quantum biology, biophysics, and random weirdness.

Stanford: W. E. Moerner

Pioneer of single-molecule spectroscopy - Biophysics, quantum dots, optical trapping

Berkeley: Naomi Ginsberg

Experimental Chemical Physics.

Studying energy transfer through imaging.

1 minute 10 fs light pulse

10 -14 10 -9 10 -4 10 1 10 6 10 11 10 16

Age of universe

Time (seconds)

Computer clock cycle

Camera flash

Age of pyramids

One month Human existence

10 fs 1 minute as 1 minute age of the universe.

10 fs 1 sec as 5 cents US national debt.

Case Study II: Ultrafast Spectroscopy

1 minute 10 fs light pulse

10 -14 10 -9 10 -4 10 1 10 6 10 11 10 16

Age of universe

Time (seconds)

Computer clock cycle

Camera flash

Age of pyramids

One month Human existence

Case Study II: Ultrafast Spectroscopy

What we can intuitively understand

Start the clock ! one femtosippi, . . .

The Birth of Ultrafast Technology

Leland Stanford Eadweard Muybridge

Bet: Do all four hooves of a galloping horse ever simultaneously leave the ground?

Unsupported transport is impossible!

Unsupported transport is impossible!

Leland Stanford Eadweard Muybridge

Time Resolution: 1/60th of a second

Unsupported transport is impossible!

Unsupported transport is impossible!

L. Stanford, “The Horse in Motion”, Palo Alto, 1872

Bet: Do all four hooves of a galloping horse ever simultaneously leave the ground?

The Birth of Ultrafast Technology

Harold Edgerton MIT, 1942

“How to Make Apple sauce

at MIT” 1964

“Splash on a Glass” Curtis Hurley Junior High School student 1996

Time Resolution: a few microseconds

Harold Edgerton - Strobe Photography

1960 1970 1980 1990 2000

10 –6

10 –9

10 –12

10 –15 Ti

mes

cale

(sec

onds

)

Year

Electronics

Optics

No one expects electronics to ever catch up.

Optics vs. Electronics

Continuous vs. ultrashort pulses of light

A constant and a delta-function are a Fourier-Transform pair (i.e. ∆t∆ω~1)

Continuous beam: Ultrashort pulse:

Irradiance vs. time Spectrum

time

time

frequency

frequency

Long vs. short pulses of light

The uncertainty principle says that the product of the temporal and spectral pulse widths is greater than ~1.

Long pulse

Short pulse

Irradiance vs. time Spectrum

time

time

frequency

frequency

When a light hits matter: Maxwell's Equations

The induced polarization, P, contains the effect of the medium:

0 0 0

0

0

BE Et

E PB Bt t

µ ε µ

∂∇ ⋅ = ∇× = −

∂∂ ∂

∇ ⋅ = ∇× = +∂ ∂

0P Eε χ=

( )0 1ε ε χ= +

This has the effect of simply changing the dielectric constant:

The polarization is proportional to the field:

The effect of an induced polarization on a wave requires solving Maxwell’s Equations.

The induced polarization in Maxwell’s Equations yields another term in the wave equation: The polarization is the driving term for a new solution to this equation.

2 2 2

02 2 2 2

1E E Px c t dt

µ∂ ∂ ∂− =

∂ ∂ψµψ

Maxwell's Equations in a Nonlinear Medium

•What are the effects of such nonlinear terms? •Consider the second-order term: • 2ω = 2nd harmonic! •Harmonic generation is one of many exotic effects that can arise!

(1)0ε χ = P E

*

*2222

( ) ,

( )

exp( ) ex

2exp(2 ) exp(

p

2

(

)

)

E

E i t

i t E

E i t

i t

t

t E

ω ω

ω ω

∝

−∝

+ −

+ +

E

E

Since

(1) (2) 2 (3) 30 ...ε χ χ χ = + + + P E E E

Nonlinear optics is what happens when the polarization is the result of higher-order (nonlinear!) terms in the field:

...)3()2()1( +++= EEEEEEP χχχ

ψµψ

Nonlinear Spectroscopy: What do you see?

The Highest Intensities Imaginable

1 kHz “Chirped-Pulse Amplification (CPA)” system at the University of Colorado (Murnane and Kapteyn)

0.2 TW = 200,000,000,000 watts!

National Ignition Facility

192 shaped pulses 1.8 MJ total energy

Even Higher Intensities

Berkeley: Steve Leone

Experimental Chemical Physics.

Attosecond, X-Ray and gas phase ultrafast spectroscopy

Berkeley: Steve Leone

Experimental Chemical Physics.

Attosecond, X-Ray and gas phase ultrafast spectroscopy

A sequence of snapshots showing the oscillatory motion of a valence electron inside an atomic ion, as reconstructed from

attosecond measurements.

M.I.T. : Andrei Tokmakoff

Dynamics of water.

Dynamics of protein folding.

Experimental Chemical Physics.

Ultrafast Spectroscopy

Chicago: Gregory Engel

Experimental Chemical Physics.

Ultrafast Spectroscopy & Molecular Dynamics

Northwestern: Mark Ratner

Experimental Chemical Physics.

Molecular Spectroscopy

CALTECH: Ahmed Zewail Caltech: Ahmed Zewail

Experimental Chemical Physics.

Ultrafast Spectroscopy (Femtochemistry)

Case Study III: Quantum Biology

Photosynthesis Neural Activity

Bird Navigation

Solvay Conference, 1927 Quantum Mechanics Est.

Solvay Conference, 2010 Quantum Biology Est.???

Case Study III: “Quantum Biology”

B. Whaley, Berkeley Quantum Computation

G. Engel, Chicago Biophys Spectroscopy

Asparu-Guzik, Harvard Chem Phys (theory)

K. Nelson, M.I.T. Ultrafast Spectroscopy

M. Ratner, Northwestern Ultrafast Spectroscopy

R. Marcus, Caltech G. Fleming, Berkeley S. Mukamel, Irvine Spectroscopy, theory

Case Study III: “Quantum Biology”

Berkeley: Graham Fleming

Experimental (and some theory) Chemical Physics.

Condensed Phase Ultrafast Spectroscopy

|h〉

|gb〉

|b〉

Ener

gy

|gh〉

|H〉

|B〉

Excitonic Coupling

B b

H a

B a

H b

Fe 2 +

Qb Qa

P

~ 3 ps

< 1 ps

~ 1 0 0 fs

~ 1 5 0 fs

e- transfer

Energy transfer

Energy transfer

hν

New Insights in Quantum Biology from Ultrafast Spectroscopy

1. G.S. Engel et al., Nature, 446, 782 (2007)

Long lived coherent wave-like motion in energy transfer1

Random Thoughts

• You will get more sleep in graduate school than undergrad

• Scientific progress and understanding is highly nonlinear

• Everything meaningful in science takes (at least) a year to do

• Publish a paper no later than the middle of your 4th year

• Everyone feels the “imposter syndrome”; developing confidence in your field will be your greatest challenge

• Output is not proportional to effort. Get out of lab ocasionally !

These were some of my observations from graduate school:

time <0 τ (Coherence time)

T (waiting time)

t3 (Coherence time)

A single system

Ensemble System

Relative Phase of a

dipole oscillation

k1 k2 k3

q1

q2

q3

q4

<q>

T τ

k1

k2

k3

( ) megeg t δωωω +=

egω

ne

ng

Two Level System – Inhomogeneous Spectral Broadening

t

δω

time <0 τ (Coherence time)

T (waiting time)

t3 (Coherence time)

A single system

Ensemble System

Relative Phase of a

dipole oscillation

k1 k2 k3

( )tegω ( )tegω ( )tegω ( )tegω ( )tegω ( )tegω ( )tegω ( )tegω ( )tegω( )tegω ( )tegω

T τ

k1

k2

k3

( ) ( )tt egegeg δωωω +=

egω

q1

q2

q3

<q>

Two Level System – Homogeneous Spectral Broadening

ne

ng

t

δω

time <0 τ (Coherence time)

T (waiting time)

t3 (Coherence time)

A single system

Ensemble System

Feynman diagram

k1

k2

k3

ks

k1 k2 k3

Double Sided Feynman Diagrams

k1

k2

k3

kS

egee

ge

gg

ge

Rephasing Pathway

(i.e. photon echo)

time <0 τ (Coherence time)

T (waiting time)

t3 (Coherence time)

A single system

Ensemble System

Feynman diagram

k1 k2 k3

Double Sided Feynman Diagrams

k1

k2

k3

kS

ee

ge

gg

ge

ge

Non-Rephasing

(i.e. free induction decay)

Q: How can we measure how an optical excitation evolves?

A: Simultaneously measure phase and time to unambiguously track an excitation. 2-D Electronic FT Spectroscopy