Atom Chips at Sandia - Sandia National Laboratories

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1st Experiments: make mirror-. MOT and then ... Deposit gold mirror (Ti/Pt adhesion layer). ... Dielectric. Coatings. ~100 μm. Si. Cavity design. Chord = 33.9 μm.

Engineered high finesse micro optical cavities

Atom Chips at Sandia

Optimized fabrication gives smooth micro-optical cavity templates

K. Fortier, P. Schwindt, M. Blain, D. Stick, T. Loyd, M. Mangan, G. Bedermann, J. Hudgens

1 μm SiO2 aperture, 60 min F* chemical downstream etch, 100 μm SF6 plasma smoothing, 2 μm oxidation + strip

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DEAC04-94AL85000.

JM Geremia, Univ. of New Mexico Hideo Mabuchi, Stanford University

Failure current [A]

Properties of current atom chip • Wire material: Al, 0.5% Cu • Substrate material: Si • Wire thickness: 2.5 μm • Distance from wire to mirror: 1.5 μm

Fiber Hemisphere R = 62.7 μm

Low resitivity Si Chord = 70.5 μm

Chord = 57.1 μm ~100 μm

Hemisphere R = 68.6 μm

Optical Field

Si M. Trupke, E.A. Hinds, S. Eriksson, E.A. Curtis, Z. Moktadir, E. Kukharenka, M. Kraft, Appl. Phys. Lett. 87, 211106 (2005).

Spherical mirror fabrication Deposit SiO2 and photo resist. Pattern photo resist. SiO2: 2 μm

Surface Roughness

AFM measurements at bottom of mirror

Strip SiO2 with HF. Smoothing etch with SF6 and Ar plasma.

Photoresist Si

Si

1st plasma etch

2nd plasma etch

Oxidation

Anneal

Wet etch

1

1.13

0.67

-

-

0.30 cavity 0.30 field

2

0.98

0.62

0.55

0.48 well 0.66 field

0.46 cavity 0.58 field

Yes

0.216 cavity 0.218 field

3

Thermal oxidation smoothing.

SiO2: 2 μm

Units: nmrms; 1 µm SiO2 aperture Wafer

Si

Plasma Etch SiO2 SiO2 Si

0.94

0.85

Post 1st plasma etch AFM scan, Wafer 1

0.48

Scattering loss v. surface roughness

Strip SiO2 with HF.

SiO2: 2 μm

Si

(

− 4πσ

Loss = 1 − e

-4

10

)

2

λ

Cavity Measurements

2

Thermal loading of chip 50

100

150

200

Al wire sidewall

Dielectric Mirror R @ 780 nm = 99.97%

Wire Optical Field

Gold Mirror R @ 780 nm = 99.92%

0.1

SiO2

Cross section of the atom chip across a wire bonding pad

SiO2: 200 nm

Chemical mechanical polish

Al: 2500 nm

Al: 2500 nm

SiO2: 200 nm Si

Deposit photo resist and pattern. Etch SiO2 for contact pads or vias.

SiO2: 200 nm

SiO2: 4000 nm

Al: 2500 nm

SiO2: 200 nm Si

Si

Deposit SiO2 and photo resist. Pattern photo resist. Photo resist Al: 2500 nm

After this step the process can be repeated to add another layer of wires.

SiO2: 5000 nm

Au: 200 nm

SiO2: 200 nm Si

Al: 2500 nm

Atom Chip

• Enable photon mediated interactions between atoms trapped in optical cavities.

• Trap atoms in a remote mirror-MOT and magnetically transport to the individual cavities.

• Atom chip testing apparatus assembly including vacuum chamber and laser systems is complete Laser • 1st Experiments: make mirror- Cooling MOT and then magnetically trap Beams atoms on the atom chip • Chamber has good optical access • Atom chip is mounted on a polyimide circuit board Atom chip mounted in a • Rapid changing of the atom chip spherical octagon with a modular design. • DFB lasers with frequency offset locking for the MOT.

Atom Chip

Macro U

SiO2: 5000 nm SiO2: 200 nm Si

Al: 2500 nm

Atom Chip

SiO2: 4000 nm SiO2: 200 nm Si

SiO2: 4000 nm SiO2: 200 nm Si

Polyimide Board

Multi Pin Connector

Atom Chip Assembly

Lift off resist Au: 200 nm

4

Optical Fiber Fiber Connection

Deposit photo resist and pattern. Deposit gold mirror (Ti/Pt adhesion layer).

Block out plasma etch Al: 2500 nm

SiO2: 4000 nm

Si

Cl plasma etch the aluminum

10 1

Atom

0.24 nm

Cavity Length

Patterned Conductor Fabrication

Al: 2500 nm

5

-6

10

Sandia Atom Chip Testing Apparatus

Photo resist

10 -5

10

Electrostatically actuated mirror

Expected at 780 nm: Measured at 852 nm: strong coupling regime Experimental Finesse = 4200 Finesse = 1750 g0 = 3.3 x 109 s-1 Cavity Length = 40 μm Expected Finesse = 2400 κ = 6.7 x 109 s-1 g02/κΓ = 100 Cavity Length = 15 μm 2 g0 /κΓ = 36 Q = 60,000

SEM image

Deposit Al (0.5% Cu) and photo resist. Pattern photo resist.

6

Integrated Opto-Atomic Circuit

Si

SiO2

Si

Prior to SiO2 and Au coating Caltech conductor pattern

10

Surface Roughness, σ [nmRMS]

4

0

Al

Chord = 70.5 μm

Si

Conductor width [μm] Ideal scaling of maximum current: Imax α wh1/2

Al

Hemisphere R = 68.6 μm

6

Au Au

Chord = 106 μm

Data Linear fit first three points

8

0

The atom chip

Dielectric Coatings

In vacuum

10

High resistivity Si

Chord = 33.9 μm

Cavity design

Chemical downstream etch with fluorine radicals

Failure current of Al wires

• High current carrying capability. • Low sidewall roughness • Multi layer capability • Top surface mirror for mirror-MOT as close to the conductors as possible

Before plasma etch 1 μm SiO2 aperture

Reflected power

Goals

Magnetic Micro Traps

Open access Fabry-Perot cavity Finesse > 10,000 Low mode volume Process compatible with Al wire process

Cavity Finesse

1. H. Mabuchi, M. Armen, B. Lev, M. Loncar, J. Vuckovic, H. J. Kimble, J. Preskill, M. Roukes, and A. Scherer, Quantum Information and Computation, 1, 7 (2001). 2. Y.-J. Wang, D. Z. Anderson, V. M. Bright, E. A. Cornell, Qu. Diot, T. Kishimoto, M. Prentiss, R. A. Saravanan, S. R. Segal and S. Wu, Phys. Rev. Lett. 94, 090405 (2005). T. Schumm, S. Hofferberth, L. M. Andersson, S. Wildermuth, S. Groth, I. Bar-Joseph, J. Schmiedmayer, P. Kruger, Nature Physics, 1, 57 (2005).

• • • •

Fractional Scattering Loss

Much like laser cooling in the 1990's, atom chip technology today is rapidly gaining popularity as a convenient and powerful approach to achieving precise control over an atom’s motion and internal state. While great success has been achieved in magnetically manipulating the atoms, integrating optical elements onto the atom chip is an active area of research. Premier applications for these “optoatomic circuits” can be foreseen in both quantum information science1 and in quantum sensors.2 At Sandia, our efforts are focused on developing atom chips for quantum information processing applications in collaboration with researchers at the University of New Mexico and California Institute of Technology. Our atom chips will contain patterned conductors forming magnetic traps and guides integrated with open access optical cavities. Although at first, the magnetic trapping chips and the optical cavities will be developed separately, later efforts will be focused on integrating the fabrication process.

Collaborators:

Silicon substrate type

Goals

First Atom Chip shipped to Mabuchi Group March 2008

Sandia is a multiprogram laboratory operated by Sandia Corporation a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000